LED Driver Ultimate Guide - uPowerTek

LED lighting penetrates everybody’s life, becoming more intelligent and more sophisticated. When designing high quality and excellent luminaire, the design engineer must carefully understand the LED Driver since it is the heart of a light fixture. This article describes the most frequently used LED driver concepts and provides the methods to select a suitable and qualified LED driver.

Read more

1. What is LED driver

LED Driver, also named as LED power supply, converts typically Alternating Current (AC) into a regulated Direct Current (DC) output because the light-emitting diodes (LED) is a unique component that only accepts Direct Current input.

Don’t know what AC and DC are? This article explains everything.

2. Dimensions to Describe LED Driver

a. External vs. Interior LED driver

LED drivers can be built inside a lamp (Interior), put on the surface of a light fixture, or even put outside a fixture (External). Most of the low power indoor lights especially bulbs adopt interior LED drivers to make a lower cost and better-looking product but the external LED drivers are frequently used for downlights and panel lights.

And as the power further increases, the thermal situation inside the lights becomes worse, thus external LED drivers are more adopted in high power applications like street lights, floodlights, stadium lights and grow lights. The other advantage external LED Driver has is an easy replacement for maintenance.

b. Switching Power Supply vs. Linear Regulator

Linear LED drivers are often seen in AC LED, signage and strip applications and it is so simple that a resistor or a regulated MOSFET or IC can finish the job of making a constant current for LED. So it is very easy for power supplies to adapt and allows a super wider range of choices of constant voltage power supplies such as 12V, 24V LED drivers. The drawback of a linear regulator is that the power loss is high thus the light efficacy can not be as high as switching power supplies.

And obviously, the great advantage of switching supply is the high efficiency which results in high light efficacy which is the key parameter for most light applications. And compared with AC LED, switching power supply has a higher power factor, surge immunity and less flickering.

c. Isolated vs. Non-isolated LED Driver

When we compare those two items, both are referred to as switching power supplies. The isolated design has adequate voltage insulation between input and output, and normally it is 4Vin+V according to UL and CE and Vac according to 3C standards. The insulation prevents the high input voltage from penetrating to output thus improving safety and sacrificing the efficiency (~-5%) and cost (~+50%) by using a highly insulated transformer rather than an inductor as the manpower transferring component. Non-isolated design is just the opposite, and it is mainly adopted in low power built-in designs.

d. Constant current vs. Constant voltage LED Driver

There is no doubt that LED should be driven by a constant current source due to LED special V-I characteristic, but when there is a linear regulator or resistor in series with the LED to provide the current limitation, a constant voltage LED driver can be used. We also prepared another article for you if you want to learn how to dim your LED strips. Due to the much higher efficiency, constant current LED Driver is the mainstream for general lighting like bulbs, linear lights, downlights, street lights etc. while constant voltage LED drivers with 12V, 24V even 48V are used for signage and strip as the main solution. By using the constant voltage solution, it is very easy for users to configure the amount of light as long as the total power does not exceed the power supply rating thus it provides a lot of flexibility for field installation.  We also have another article to explain the Difference Between Constant Voltage And Constant Current LED Drivers.

e. Class I vs. Class II LED Driver

Here I and II are written in Roman numerals, rather than 1 and 2, which has totally different meaning shown in the next item. Class I and class II are the concepts from IEC (International Electro-technical Commission) standards while they both define the internal construction and electrical insulation of a power supply so as to provide safety against electric shock. IEC Class I input LED drivers have basic insulation and shall have protective earth (ground) connection to eliminate the electric shock. IEC Class II input models feature additional safety precautions such as double insulation or reinforced insulation, thereby there is no need for a protective earth (ground) connection. In general, class I LED Driver has ground cable at the input side and class II does not have but has a higher insulation level from input to either enclosure or output. And here is the normally used symbols for class I and class II.

f. Class 1 vs. Class 2 LED Driver

Using the Arabian numerals, class 1 and class 2 are NEC (National Electric Code) concepts describing the output feature of a power supply output with less than 60Vdc in dry location/30Vdc in a wet location, less than 5A current, and less than 100W power, as well as the detail requirement for the circuit design feature. UL Class 2 LED driver is regulated by UL and UL, and there is quite a lot of benefits using class 2 LED driver whose output is considered as safe terminal and no extra protection is required at the LED modules or light fixtures thus it saves cost for insulation and safety test. However, these limitations pose restrictions on the number of LEDs a Class 2 LED driver can operate.

UL Class 1 include all the LED drivers out of the range of class 2 and is regulated by UL and UL. Although class 2 LED drivers have good advantages of simplifying the safety design of the light fixture, class I LED drivers are still widely used because of higher efficiency and more uniform light output due to the lower output current and having more LEDs in series. In real applications, class 2 LED drivers are more used in the lights which are easy to be touched by users, such as grow lights, while class 1 LED drivers are more adopted in highly mounted lights such as stadium and pole lights.

g. Dimmable vs. Non-dimmable LED Driver

Each light is born to be dimmed in this new era. This is a big topic since there are quite a few dimmable schemes and let’s introduce them one by one.

1) 0-10V/1-10V dimming LED Driver

It is also called analog dimming and is most widely used. It was derived from the era of fluorescent and defined by IEC Annex E.

The drawback of this control scheme is that the dimming cable can have a voltage drop if the cable is long thus the consistency of the lights can not be ideal. Also, each LED Driver may need 100-500uA dimming control current from the master controller thus the maximum quantity of a lighting system is always limited. More about 0-10V dimming.

2) PWM dimming LED Driver

To overcome the drawback of 0-10V dimming, PWM (Pulse Width Modulation) dimming is utilized for more and more projects though the popularity is still far less than 0-10V. PWM signed is generated by the master as a digital signal, thus the signal on the dimmable cable can be very consistent. PWM duty cycle is detected by the LED Driver to determine the output current. Now there are two methods to realize PWM dimming LED driver in the market, one is “fake” PWM dimming, there is RC (resistor-capacitor) filter inside the LED Driver and the PWM dimming signal is filtered to a DC voltage which is proportional to PWM duty cycle. The drawback of this method is that the peak value of the PWM signal must be 10V, otherwise, the accuracy is very bad. Also, the frequency of the PWM signal is limited by the RC parameter. The typical application is Meanwell HLG/ELG/XLG series LED drivers. The other one is real PWM dimming and there is MCU inside the LED Driver thus PWM signal with any peak voltage can be detected, also the allowable PWM frequency range can be much wider than RC way. uPowerTek LED drivers are all MCU integrated to work with PWM dimming. And there are two different things easily mixed when we talk about PWM dimming, PWM signal dimming and PWM output dimming and the figure below shows the detail of the difference. In this section, PWM dimming means PWM signal dimming, while PWM output dimming circuitry chops the DC LED current between the on/off state in a high frequency thus the human eye is not able to perceive the flicker thus changing the light output of the LED.

Still don’t understand PWM dimming, we have more words and images to explain this topic, What is PWM dimming for LED Driver?

3) Triac dimming LED Driver

It is also called phase-cut dimming or leading/trailing edge dimming and was the popular way in the incandescent lamp era. Leading-edge has a major role in Triac dimming application. Triac dimming is the old and bad way to dim the LED lights where the “noise” is high on both the human ear and cable

4) DALI dimming LED Driver

DALI stands for Digital Addressable Lighting Interface. It is illustrated by the international standard IEC series as the first lighting digital protocol with bi-directional communication. As the first generation, DALI 1 system consists of a controller and a maximum of 64 ballasts or LED drivers given independent addresses. More about DALI dimming. In the year , the DiiA Digital Illumination Interface Alliance announced the second generation DALI 2 which supports 128 max devices and has much greater compatibility between the devices from different brands. DALI 2 also support sensors. Both DALI 1 and DALI 2 devices have to be tested by the professional DALI tester Probit, then certified and shown on the DiiA website. Want to Know the Difference Between DALI and DALI-2, please read this article. At the same time, the D4i concept was released to indicate the devices which have not only DALI 2 compatibility but also the functions of energy report, data transmission, diagnose&maintenance and memory bank.

5) DMX dimming LED Driver

Also named DMX512 (“Digital Multiplex with 512 pieces of information”), it is a standard for digital communication networks that are commonly used to control stage lighting and effects. For general lighting, DMX512 protocol is mostly used for stadium lights and architectural applications and is not very popular for other than those. This is a broadcasting way like PWM rather than DALI which is able to feedback, and the difference from PWM is that DMX512 devices have individual addresses so as to control one by one.

Want to know more about DMX dimming? Please read this article, What Does DMX Mean in Lighting?

6) Other Protocols of LED Driver

There are quite a few other protocols that are often adopted in lighting systems, such as wired solutions like PLC, KNX, RS485, CAN and wireless ones like LoRa, Bluetooth, Zigbee, but none of them are designed for lighting application only. The lighting industry is making efforts to make sure a dedicated lighting protocol will be developed on one of them especially the wireless solution.

h. Waterproof vs. Non-waterproof LED Driver

IP (ingress protection) rating is the only way to describe the waterproof level of LED drivers and is regulated in IEC. The IP code is composed of two numerals, the first numeral refers to the protection against solid objects using a scale from 0 (no protection) to 6 (no ingress of dust) while the second numeral rates the protection against liquids using a scale from 0 (no protection) to 7 (8 and 9 are rarely seen in lighting industry). It is obvious that waterproof LED drivers are used for outdoor applications and IP20 or other low IP rated LED drivers are used for indoor. But it is not always true, some indoor application adopts waterproof LED drivers only because they can deliver much higher power than low IP ones without an active cooling system which has a shorter life than IP rated LED drivers.

3. How to Choose a Suitable LED Driver

Choosing a suitable LED driver is one of the basic steps for designing a great luminaire, and let’s see how to make it.

a. Position the LED Driver

Determine the design is a performance competing one or a cost-effective one. Understand who the competitors are and what the cons and pros they have.

b. Go through [section 2: dimensions to describe a LED driver]

and find the answers for your luminaire design.

c. Understand the LED Driver input voltage

You should know where the target market is and thus decide the input voltage range. Here it is the global mains electricity voltage map.

This map only listed the single-phase voltage, and there is a lot of 3 phase application thus the voltage has to be multiplied by √3 or 1.732 for 3 phase usage. Designing narrow input voltage helps to decrease the cost, but increase the models for different areas of the world. But too wide an input voltage range increases the cost and reduce the performance. Thus the most balanced input voltage range in the industry is 100-277Vac (uPowerTek BLD series) and 200-480Vac (TLD series).

d. Find the right LED Driver output voltage

current and power. It is easier for constant voltage type LED drivers to decide the model because of fewer choices. The typical outputs are 12V, 24V and maybe 48V so users only have to decide the power. For constant current LED drivers, there are so many choices of output current and voltage thus making the LED driver industry very diversified. Once the lumen output is determined when making a fixture design, the power needed to drive the LED is clear by judging the LED light efficacy. Then design has to decide if high voltage/low current or high current/low voltage LED should be employed. There are a lot of considerations and no “always correct” answers to this question. High voltage low current can give the light fixture higher efficacy because of the higher LED driver efficiency, and better LED consistency without worrying about the imbalance of different LED strings, however, there is extra cost generated due to the higher insulation cost. And the low voltage high current design is just the opposite. And different light designers have different thinking of how to choose, but there are some special lights that do not have too many choices. For example, low bay grow lights which are installed at a height that people are easy to touch, have to use low voltage high current for safety consideration. Also for some high bay or pole lights that use the remotely mounted LED drivers for weight and maintenance, high voltage low current LED drivers are the mainstream to save the output cable cost.

e. LED Driver Form factor

There are different shapes of LED drivers and the form factor is especially important when LED drivers are fixed inside the luminaire.

f. Environment level

For most of the indoor application luminaires, 0-40C ambient temperature operating range is enough. For the outdoor LED drivers, -40-+70C ambient is preferred. For special applications like steelworks, the LED Driver should be able to work at 80C ambient while for the street light in some cold areas like Siberia and Alaska, the LED Driver should be able to start at the temperature of -55C. uPowerTek BLD series are able to meet those tough conditions.

Eaglerise contains other products and information you need, so please check it out.

4. How to Find a Better LED Driver

There are a lot of factors that make us be able to discriminate between a high-quality LED driver and common products. Understanding those factors can help a light designer a lot to win the market.

a. LED Driver Efficiency

Higher efficiency not only improves the whole light efficacy but also makes the LED Driver generate less heat thus having a better lifetime. In the LED era, efficiency is more and more important for saving more energy. Currently, the highest efficiency level in the LED driver industry is 96% that uPowerTek BLD-800 and TLD-800 series have achieved.

We also have another article to introduce what is efficiency, you can read if you would like to know more.

b. LED Driver PF and THD

Like efficiency, PF (Power Factor) and THD (Total Harmonic Distortion) are also the concepts that describe the energy converting efficiency and the difference is that Efficiency refers to the energy converting capability from LED driver input to output, while PF and THD refer to the energy converting from power grid to LED driver input. Higher PF (>0.9 according to DLC) and less THD (<20% according to DLC) are always preferred for a high-quality design. (DLC is Design Lights Consortium, a regional group that focuses on energy efficiency specifically in the lighting industry. It is a part of the Northeast Energy Efficiency Partnerships and was originally focused on the Northeast and Mid-Atlantic areas of the United States)

c. LED Driver Inrush current

Almost all of the luminaires are installed together with MCB (miniature circuit breaker) for safety consideration. And if there are multiple LED drivers connected with a single MCB and the overall LED driver inrush current can possibly trigger the MCB so as to result in startup failure. The inrush current issue first appeared when electrical ballast was widely used since either ballast or LED Driver are capacitive devices with big bulk electrolytic capacitor inside generating high peak inrush current during the AC power on. The most popular standard regulating the inrush current is NEMA410 which defines the concept and limits of inrush current.

But the limit in the NEMA410 standard is still not enough when tens of lights or even hundreds of lights are paralleled such as the application of grow lights. And there are a lot of ways to limit the inrush current such as using a soft start circuit and using the current limiting resistor inside LED drivers thus the low inrush current LED driver cost is a little bit higher. Now more and more standard LED drivers offer the feature of low inrush current without adding extra cost.

d. LED Driver Surge protection

Due to the more complicated electrical design, LED Driver is more susceptible to surge compared with magnetic ballasts. There are 2 major standards regulating the surge protection level of LED drivers, IEC-4-5 (Testing and measurement techniques – Surge immunity test) and IEEE Std C62.41.2 (IEEE Recommended Practice on Characterization of Surges in Low-Voltage ( V and Less) AC Power Circuit). LED Driver should be heavily protected especially in the outdoor usage by special surge protection circuitry composed by MOV (metal oxide varistor) and GDT (gas discharge tube). And the surge usually comes from two ways: one is high power machine nearby on and off operation, or heavy load-light load sudden switches which lead to surges between line and neutral, which is named as differential mode surge; and the other is from the lightning which makes earth voltage level fluctuate greatly thus creates the surge between either line or neutral and earth, which is named as common mode surge.

The most commonly adopted specification for outdoor LED driver surge level is 6kV for differential mode and 10kV for common mode, according to the test standard of IEC-4-5. Generally speaking, the surge protection level is essential to ensure the long term operation of outdoor luminaires thus the designers have to pay great attention to this parameter.

e. LED Driver Output ripple and flickering

Output ripple is related to the stability and quality of the LED Driver. Lower output ripple means less flickering of LED according to the curve below.

It shows that the LED lumen output is generally quite proportional to LED current, thus lower current ripple can result in lower flickering which is essential for indoor applications which only allow <=5% flickering in most of the countries in the world. There is only one exception that output current can be high even 100%, which is PWM dimming with adequate frequency. The curve below shows that only if the frequency is higher than 1.25kHz, the light can be regarded as flicker-free.

f. LED Driver Line and load regulation

This is a key concept for all the switching power supply including LED drivers. Line regulation describes the output stability versus input voltage while load regulation shows the output stability versus load. The high-quality LED driver can always control the line and load regulation to <=1% value.

g. Programmability of LED Driver

This function is one of the key features the LED lighting era brought because of the too many combinations of LED chips for different design purposes. The ability to adjust the output current is a key demand for LED drivers. In the early stage, a pot-meter was used for adjusting but gradually users found the small device is not reliable and low IP rated. Then programming by infrared controller appeared for a very short period of time because the LED drivers has to be powered on when doing the programming. And from the year , cable programming without powering the LED Driver became the mainstream way to program and the drawback of using the extra cable and doing extra wiring was overcome by NFC programming which is employed by Signify and uPowerTek.

There are quite a few extra programmable functions that can be added to LED Drivers such as time dimming, constant lumen output (lumen decay compensation) and luminaire over-temperature protection which is described below.

1) Time dimming

This is frequently adopted in street lights as the most convenient way to realize smart control and energy saving.

2) CLO (constant lumen output)

The LED light efficacy decreases with the operation time, designers want to keep the light output constant for their luminaires thus the output current of the LED Driver has to be increased accordingly to overcome the decrease.

Through the PC interface, users can set the customized compensation curve according to the LED lumen decay characteristics.

Want to know more about CLO, please check this article. What is CLO in lighting?

3) Thermal protection by NTC thermistor

Many quality-oriented designs have luminaire temperature sensing function so has to protect the product from being too hot or even damaged. So the LED drivers decrease the output current once the NTC (negative temperature coefficient) thermistor which carries the temperature information gets to a certain value indicating the overheating.

Through the programming interface, users can set the thermal foldback resistance threshold and protect the status current value.

h. Tc, max case temperature of LED Driver

This is marked on the LED driver label usually to indicate the hottest point on the LED driver surface.

The definition according to the lamp control gear standard IEC is: “highest permissible temperature which may occur on the outer surface (at the indicated place, if marked) under normal conditions and at the rated voltage or the maximum of the rated voltage range”. According to LED driver standards either UL or IEC system, the maximum case temperature should not exceed 90C. Tc is one of the key parameters fixtures designers have to check carefully because it relates to reliability and life greatly.

Higher Tc means better thermal performance and higher endurance to high ambient temperature. Though Ta (ambient temperature) rating is always shown in the LED driver datasheet, Ta is not that important compared with the Tc range because the case is much closer to the LED driver internal components compared with air thus reflecting the real operation situation of the LED drivers. In the uPowerTek datasheet, the Ta range is even not shown.

Want to learn more about what is a programmable LED driver, please read this article.

i. Standby power of LED Driver

Now more and more LED drivers support dim to off function to make the whole light go into standby mode. Both Energy Star from North America and ErP from Europe regulate that the standby power loss should be less than 0.5W. The standby power is usually composed of 2 parts, one is the energy from the AC side to keep the LED driver control circuitry still alive to receive the wakeup signal from the controller, and the other is the 12V auxiliary power which is powering the external controller. uPowerTek LED drivers to comply with Directive /125/EC, the requirements of Commission Regulation (EU) / (known as single lighting regulation) effective on September 1st, .

j. LED Driver 12V or 24V auxiliary power

There are a lot of luminaires integrated controllers or sensors so as to provide smart systems or functions to the end-users and the 12V/2~4W power from LED Driver can make the design much easier compared with using an AC adapter to create the 12V. Also, the 12V from LED Driver is safer and more reliable compared to a common adapter thanks to the high internal built-in surge protection circuitry. 24V power is proposed by DiiA and utilized for powering D4i devices like sensors, and the D4i standard is more and more popular with Signify and Osram strong promotion.

k. LED Driver Lifetime and MTBF

It is important to understand that product lifetime and product reliability are two very different, though not unrelated concepts. Unfortunately, because they are both often expressed in hours they are frequently confused. Lifetime refers to the length of time a user can expect a single product to work properly before a known wear-out mechanism renders the product unfit for use. Reliability deals with the random failure rate of a population of products and it may be expressed as a failure rate such as FITs (failures in 109 hours) or as the inverse, MTBF (Mean Time Between Failures). A lifetime of 50,000 hours implies that one would expect any given product to last up to 50,000 hours before failing. An MTBF of 50,000 hours implies that for a population of units, one could expect to see a random failure every 50 hours (i.e. every 50,000 hours of unit operation). Both concepts are important to understand and manage for a successful implementation of LED lighting.
The typical equation for the life of an electrolytic capacitor takes the following form:

Where,
Lx is the lifetime result,
k is the factor determined by the capacitor’s RMS ripple current and operating voltage, it is provided as either a value or a function,
L0 is the lifetime value tested in the standard condition provided in the datasheet,
Ts is the rated case temperature,
Ta is the operating case temperature.
In general reliability deals with the failure rate of a population of products operating within their rated conditions and within their operational life. A common way of expressing the reliability of a product is a metric known as MTBF. The following equation expresses the very simple concept of MTBF. It is the total operational time in hours of a population of products divided by the number of failures.

The most common method to evaluate MTBF is given by MIL-HDBK-217. And the figure below shows the famous bathtub curve which well illustrates the relation between life and MTBF.

l. LED Driver Certificate

It is important that a high-quality LED driver is certified by 3rd party with a high reputation like UL and TUV. The most essential certificates are UL (North America), ENEC (Europe) and CB (Global other than North America) which can be converted to PSE (Japan), KC (Korea), RCM (Australia), SASO (Mid-east), CCC (China) and etc. We also have another article about global LED Driver certificates, you can read if you want to know more.

5. LED driver manufacturing process

This video is a simple introduction of uPowerTek factory, you can learn the manufacturing process of LED drivers and how we control the quality

6. Summary

LED Driver is the key part working as the heart for lighting fixtures thus it is important that users should choose a suitable and reliable product. We have to consider a balance among the factors like performance, functions, form factors, certificates, price and lead time to the market. So it is not an easy job to find a LED driver or design it well into the light fixture. We are ready to help the designers out from solving the LED Driver related issues and optimize the cost for the whole luminaire design.

For more information, please visit Isolated led drivers.

An LED Light Is Only As Good As Its Driver​

The development and deployment of light emitting diode (LED) technology across the entire range of lighting applications have been breathtaking over the past few years. Despite the inherent high electro-optical conversion efficiency of LEDs, an LED light is only as good as its driver. The potential of this revolutionary lighting technology can only be unlocked when the performance metrics of LED drivers are consistently matched to the electrical characteristics of the LED light source. An LED lighting system is a synergistic combination of the light source, LED drivers, thermal management systems, and optics. Being the only component that characteristically influences the photometric performance and light quality of the LEDs in a lighting system, drivers play a critical role in more extensive and intensive applications of LED technology.

What Is an LED Driver?​

An LED Driver is an electronic device which regulates the power to an LED or a string (or strings) of LEDs. LEDs are solid state semiconductor devices impregnated, or doped, with layers to create a p-n junction. When the current flows across the doped layers, holes from the p-region and electrons from the n-region are injected into the p-n junction. They recombine to generate photons which we perceive as visible light. The conversion from current to light output is nearly linear, increasing the input current allows more electrons and holes recombining in the p-n junction and thus more photons are generated.

In contrast to conventional light sources that runs directly from an alternating current (AC) power supply, LEDs operate on DC input or modulated square wave input because the diodes have polarity. An input of the AC signal will cause an LED to only light up approximately half of the time when the AC signal is the correct polarity and immediately go out under negative bias. Hence, a constant supply of DC electrical current at a fixed output or a variable output within an allowed range must be applied to an LED array for stable, non-flickering lighting.

LED drivers provide an interface between the power supply (line) and the LED (load), converting the incoming 50 Hz or 60 Hz AC line power at voltages such as 120 Volts, 220 Volts, 240 Volts, 277 Volts or 480 Volts to the regulated DC output current. There are drivers designed to accept other types of power sources as well, e.g., DC power from DC micro-grids or Power over Ethernet (PoE). An LED driver circuit should have immunity against voltage spikes and other noise on the AC line within a predetermined design range while also filtering out harmonics in the output current to prevent them from affecting the output quality of the LED light source. The driver is not merely a power converter. Some types of LED drivers have additional electronics to enable precise control of the light output or to support smart lighting.

Constant Current or Constant Voltage?​

An electrical circuit that regulates the incoming power to provide a constant-voltage output has typically been referred to as a power supply, whereas an LED driver in the strict sense refers to an electrical circuit that provides a constant current output. Today, "LED driver" and "LED power supply" are very ambiguous terms that are being used interchangeably. Despite the terminological ambiguity, we can't afford to neglect the intrinsic differences between the constant current (CC) and constant voltage (CV) circuit schemes for LED load regulation.

Constant current LED drivers provide a constant current (e.g., 50mA, 100mA, 175mA, 350mA, 525mA, 700mA, or 1A), regardless of the voltage load, to an LED module within a specific voltage range. The driver may power a single module with LEDs connected in series or multiple LED modules connected in parallel. Series connection is preferred in CC circuit architectures because it ensures all the LEDs have the same current flowing across their semiconductor junctions and the light output is uniform across the LEDs. Driving multiple LED modules in parallel requires a resistor in each LED module, which leads to lower efficiency and poor current matching. Most CC drivers can be programmed to operate over an output current range for precise pairing between the driver and a specific LED module. Constant current LED drivers are used when light output should be independent of the input voltage fluctuation. They are found in many types of general lighting products, such as downlights, troffers, table/floor lamps, street lights and high bay lights, for which high current quality and precise output control are the priority. CC drivers support both pulse-width modulation (PWM) and constant-current reduction (CCR) dimming. Operating a power supply in a CC mode usually requires overvoltage protection just in case an excessive load resistance is encountered or when the load is disconnected.

Constant voltage LED drivers are designed to operate LED modules at a fixed voltage, typically 12V or 24V. Each LED module has its own linear or switching current regulator to limit the current in order to maintain a constant output. It is generally preferred to provide a constant voltage supply to multiple LED modules or fixtures connected in parallel. The maximum number of LEDs or LED modules and the forward voltages across them must not exceed the DC electrical energy power supply. The CV circuit must tolerate the power dissipation when the load goes short circuit. The current limiters typically have thermal shutdown to protect the circuit when a voltage higher than the maximum allowable voltage is placed across the current limiter. CV drivers are often used in low voltage LED lighting applications that demand ease of group connection in parallel control, e.g., driving LED strip lights, LED sign modules for lightboxes. Constant voltage drivers can only be PWM dimmed.

Switched-mode Power Supply (SMPS)​

As LEDs are very sensitive to current and voltage fluctuations, one of the most important roles of an LED driver is to reduce variations in forward voltage across the semiconductor junction of the LEDs. Switched-mode power supplies operate by modulating an electrical signal using one or more switching elements such as power MOSFETs at a high frequency thereby generating the predetermined magnitude of DC power under supply voltage or load variations. Switch-mode converters used in LED drivers require energy to be stored as current using inductors and/or as voltage using capacitors so as to maintain the output current or voltage on the load during the on/off switching cycle. An AC-DC SMPS LED driver rectifies AC power into DC power which is then converted into DC power capable of driving the LEDs properly.

For the switched-mode power conversion in LED drivers, various circuit topologies are available to support the LED load requirements. Among all SMPS topologies, buck, boost, buck-boost, and flyback are the most commonly used types.

Also known as a step-down converter, a buck circuit regulates input DC voltage down to a desired DC voltage using a number of current control methods, including synchronous switching, hysteretic control, peak current control, and average current control. The buck topology is designed for mains-powered LED drivers which are required to drive a long string of LEDs, with the load voltage kept under the supply voltage. Buck circuits are also frequently found in low voltage applications where the input supply voltage is relatively low (e.g., 12 VDC for automotive lighting) and just one LED is being driven. The buck topology allows for circuit design with fewer component counts while maintaining a high efficiency (90–95%). However, the load voltage of a buck circuit must be less than 85% of the supply voltage. Moreover, buck LED drivers do not offer isolation between the input and output circuits.

A boost converter is designed to step up the input voltage to a higher output voltage by about 20% or more. Boost circuits generally require one inductor and operates in either the continuous conduction mode (CCM) or discontinuous conduction mode (DCM), as determined by the waveform of the inductor current. Low-power boost converters can use a charge pump, rather than an inductor, which uses capacitors and switches to raise the output voltage above the supply voltage. Inductor-based converters offer the advantage of low component counts and high operational efficiencies (greater than 90%). The disadvantage of this topology is that it offers no isolation between the input and output circuits. Boost converter outputs a pulsed waveform and thus requires a large output capacitor to reduce the current ripple. PWM dimming is challenging with the large output capacitor as well as the closed-loop control which demands a large bandwidth to stabilize the converter.

Buck-boost converters can provide an output higher or lower than the input voltage, making them ideal for applications where the input voltage rises and falls with a large variation (no more than 20%). Input voltage fluctuations of this type usually occur in battery-powered lighting applications, e.g., vehicle-mounted lighting for construction and agricultural machinery (forklifts, tractors, harvesters, diggers, snow ploughs, etc.) as well as trucks and buses. Two types of converters often found in buck-boost applications are known as SEPIC (single-ended primary inductance converter) and Cuk. The SEPIC converter is characterized by the use of two inductors, preferably a dual-winding inductor which has a small footprint, low leakage inductance, and the ability to increases the coupling of the windings for improved circuit efficiency. In a SEPIC architecture, the boost section provides power factor correction (PFC) and the buck section produces a voltage to be the same as, lower, or higher than the input voltage, while output polarity of both sections remains the same. The Cuk topology combines the continuous output current of a buck and the continuous input current of a boost, which gives the Cuk the best EMI performance and allows the capacitance to be reduced as needed. The buck-boost converter is a non-isolated driver circuit. Like boost converters, buck-boost converters require overvoltage protection to prevent damages from excessively high voltage in case of an open-load condition.

A flyback switching circuit is a discontinuous conduction mode converter which provides AC mains isolation, energy storage, and voltage scaling. It is very much like a buck-boost converter, but with the inductor split to form a transformer. The flyback transformer with at least two windings not only provides complete isolation between its input and output circuits, but also allows for more than one output voltage in different polarities. The primary winding is connected to the input power supply, the secondary winding is connected to the load. Magnetic energy is stored in the transformer while switch is on and at the same time the diode is reverse-biased (i.e., blocked). When the switch is off, the diode is forward-biased and magnetic energy is released by current flowing out of the secondary winding. Some flyback circuits use a third winding, called a bootstrap or auxiliary winding, to power the control IC. More accurate control of the average voltage across the capacitor, which is used to maintain current flow in the LED load when the converter is on the first step, requires isolated feedback, usually via an optocoupler. Flyback swiching circuits can be designed for a very wide range of supply and output voltages, with isolation from dangerously high voltages. However, these circuits are less efficient (75 - 85%, higher efficiency is possible by using expensive parts).

Linear Power Supply​

A linear power supply uses a control element (such as a resistive load) which operates in its linear region to regulate the output. In this type of LED driving circuits, the voltage flowing through a current-sensing resistor is compared to the voltage reference in a feedback loop to produce the control signal. A controller which is operated in a linear region of the closed loop feedback system adjusts the output voltage until the current flowing through the sensing resistor matches the feedback voltage. The current delivered to an LED string is thus maintained as long as the forward voltage does not exceed the dropout-limited output voltage. Linear drivers provides only step-down conversion, which means the load voltage must be kept lower than the supply voltage. If the load voltage is higher than the supply voltage or the supply voltage has a wide variation, a switching regulator is needed.

AC mains-powered applications, which has demanding requirement for voltage regulation, typically employ switched linear regulators to drive LED lamps with a long string of LEDs wired in series. Switched linear regulators are combinations of multiple linear regulators which are either integrated or cascaded in a modular form. Typically designed in surface-mount IC packages, these linear regulators are used to intelligently adjust the number of load connected LEDs in a string during a power line cycle so that the load voltage matches the instantaneous AC mains voltage.

Linear LED drivers provide an extremely simplified solution which eliminates the need for bulky and costly coils, capacitors, and the reactive (e.g., inductive and/or capacitive) input EMI/EMC filter elements. A significantly low parts count and the use of solid state components allow the switched linear regulator to be downsized to a compact IC chip. This makes linear drivers a competitive candidate for LED lamps of which cost and physical size are important design considerations. With the ability to generate a substantially resistive dimmer load which is similar to an incandescent lamp, linear LED drivers have generic compatibility with legacy phase-cut (TRIAC) dimmers that were designed for dimming resistive loads.

Characterized by its cost competitiveness, EMI/EMC immunity, small footprint and design simplicity, the linear driving topology is gaining a rising interest in the industry. However, linear drivers are struggling with their inherent disadvantages that hold them back from entering mainstream applications in quite a few product categories.

1. A linear LED driver can be of low efficiency, when the supply voltage runs substantially higher than the load voltage.
2. The excess power is released as heat energy, resulting increased thermal stress on the driver circuit and very likely on the LEDs as well if heat is not efficiently dissipated.
3. The limitation of having to keep the load voltage lower than the supply voltage within a certain range leads to a further disadvantage of only allowing a restricted supply voltage range.
4. Linear drivers available on the market are dominantly low cost circuits which give no special consideration on flicker elimination.
5. The non-isolated topology provides no electrical isolation from the AC mains supply.

Switched Vs. Linear​

The design of an LED driver involves many compromises. The selection between SMPS and linear drivers has to take cost, efficiency, control, lifespan, dimming, size, power factor, flicker, input/output, AC mains isolation, and various other factors into consideration.

Switching power supplies are obviously more efficient than the linear ones because of their "0/1" (ON/OFF switching) modulation. They can be designed to deliver high power efficiency as well as flicker-free illumination while maintaining a high power factor and low total harmonic distortion (THD). While linear LED drivers have been envisioned to be a prospective LED driving solution, SMPS is, for the foreseeable future, still the preferred LED driving solution for applications where efficiency, lighting control, light quality, and electrical safety are of paramount concern. In particular, the digital controllability of SMPS drivers, which are equipped with smart sensor technology and wireless connectivity, promises to enable a variety of Internet of Things (IoT) applications. Digital modulation allows encoding the data in binary for high-speed optical wireless communication (LiFi), which vastly expands the application potential of SMPS drivers.

Nonetheless, the captivating features of SMPS drivers are achieved at the expense of their dependence on bulky, expensive and unreliable reactive components, such as transformers, inductors, and capacitors. High-speed switching operation causes much noise, thus leading to a relative high level of electromagnetic interference which has to be filtered and screened using additional circuits. These additional circuits can tremendously increase the physical sizes and double the overall cost of the LED driver.

The largest disadvantage of SMPS drivers, which is also the most attractive feature of linear drivers, is their reliability. An SMPS driving circuit uses a large number of components including filters, rectifiers, power factor corrector (PFC) circuits, etc. The complex design may degrade circuit reliability. Widespread use of aluminum electrolytic capacitors in the PFC as an energy-storage component introduces the biggest concern about the reliability of an SMPS driver. Electrolytic capacitors are known for their high-capacitance value and high-voltage rating. Nevertheless, the electrolyte in the capacitor will evaporate over time. The evaporation rate linearly correlates to temperature. High temperature will accelerate electrolyte evaporation, which causes a decrease in capacitance and an increase in ESR (equivalent series resistance). Increased ESR translates to high output voltage ripple and noise. And the capacitor eventually fails when electrolyte dries out, leading to the premature failure of the entire lighting system. High-speed switching operation can produce electromagnetic interference (EMI) which adversely affects the surrounding circuit elements. This poses an additional design challenge to overcome. The uses of a noise filter leads to an increase in volume and weight as well as manufacturing cost.

On the other side, linear drivers do hold a great potential owing to the previously mentioned advantages. They typically outlive SMPS drivers, simplifies lamp design, and delivers cost, and reduce the BOM significantly. However it's challenging to design a linear driver with conversion efficiency and flicker mitigation comparable to SMPS circuits. This technology is currently being abusively used. The majority of the lighting manufacturers take it as a low cost driving solution only. While it's acceptable to employ linear drivers in LED luminaires for applications where high quality light and AC mains isolation aren't a top priority (e.g. outdoor lighting), some manufacturers are attempting to incorporate this low cost LED driving solution in the visually demanding, safety sensitive indoor lighting applications without improving the driver’s output quality (flicker control) and enhancing the electrical safety and heat dissipation of the lighting system.

DOB​

Driver-on-board (DOB) is a typical implementation of the linear driving topology. Also called an AC LED light engine, the DoB LED module accommodates the LEDs and all the driver electronics on a metal-core printed circuited board (MCPCB). DOB technology takes advantage of the MCPCB-mountability of the high voltage driver ICs (switched linear regulators). Unlike the SMPS driver circuitry which has to be mounted on a routed FR4 PCB, these surface-mount driver ICs can be soldered to the LED-mounted MCPCB without circuit routing. This eliminates entirely the need for a dedicated driver assembly and thus allows for a compact form factor. Another benefit of DOB design is that the excellent thermal conductivity of the MCPCB can facilitate rapid dissipation of heat generated due to the inefficient conversion of a linear driver.

Power Utilization​

The power processing that goes on inside an SMPS usually causes its power drain to be uneven due to current pulse modulation. The way that switching regulators draw pulses of current from the utility power grid can produce kinks and distortions in the power line current waveform as well as trip fuses and circuit breakers at power levels lower than power line capability. The presence of these harmonic distortion and nonlinear loads can lead to various problems such as overheating of neutral conductors and distribution transformers, failure or malfunction of the power generation and distribution equipment, and interference with communication circuits, etc. From a utility point of view, these damaging disturbances from downdream electrical equipment must be prohibited. Therefore utility companies have regulatory requirements on the power factor (PF) and total harmonic distortion (THD) of electrical equipment, including line-powered LED luminaires.

Power factor is the ratio of power utilized to power delivered and is expressed as a number between 0 and 1. A purely resistive loads has a power factor of 1 because draws current exactly in phase with the line voltage. Nevertheless, the reactive elements such as capacitors and inductors of an LED driver draw an additional reactive current which is difficult to measure and therefore impossible for the utility companies to collect revenue from. Most importantly, this reactive power will cause the delivered power (apparent power) larger than the power actually required by the LED luminaire. This can cause the utility's infrastructure to operate above capacity and can incur damage potential if no measure is taken to protect the infrastructure from being overloaded by the additional reactive power. The closer the PF is to 1, the more closely matched the current and voltage waveforms are. As the PF decreases, more power is wasted in the form of reactive power. In the commercial and industrial sectors, utilities will often surcharge end-users who operate with low-PF electrical equipment to compensate increased generation and transmission cost.

The power factor of an LED lamp or luminaire has become a specification requirement in many markets. The EU Directive requires an LED product with a power consumption of more than 25 W to have a PF higher than 0.9. In the U.S., both Design Light Consortium (DLC) and Energy Star have PF regulations similar to Europe. The State of California has clear regulation for PF value which has to be greater than 0.9 for all power levels of residential and commercial LED lighting. In order to meet regulatory PF values, line-powered LED drivers designed for AC mains applications must employ some form of power factor correction to maintain a high power factor over a wide input voltage range. A power factor correction (PFC) circuit is typically used to minimize the reactive power and maximize the available power from the source and distribution cabling. PFC circuits, which include active and passive PFCs, shape and time-align the input current into a sinusoidal waveform that is in phase with the line voltage.

Total harmonic distortion (THD) is often brought up in the same breath with the issue of a low PF. THD is a measurement of distortion in the current waveform caused by non-linear electrical loads such as rectifier loads. Distorted current waveforms can reduce the PF and create harmonic distortion as well. Harmonic distortion also occurs when the load draws a current that does not resemble a true sinusoid. THD is represented as a percentage. The lower the value, the better. High THD can cause issues within the power distribution equipment. So it's important that LED drivers meet regulatory THD values (typically less than 20%) over the entire input voltage range. THD is suppressed by the power factor correction circuitry which must effectively shape the input current to ensure minimal energy at higher frequencies is generated.

Both PF and THD can be affected by dimming. Therefore, it is necessary to have PF and THD measured at full and dimmed outputs.

Dimming Control​

The transition from traditional lighting technology to solid state lighting is driven by the need for greater efficiency, control and interaction. At the heart of lighting control is dimming technology which is an integral functionality of light management systems. One of the advantages to LEDs is the ability to respond instantaneously to changes in power input which is regulated by the LED driver. The dimming performance of an LED driver is increasingly important as lighting becomes more connected and adaptive to user needs and preferences. The most commonly used dimmer-to-driver controls include triac (triode for alternating current), 0-10V, and DALI (Digital Addressable Lighting Interface). Pulse width modulation (PWM) and constant current reduction (CCR) are the most common methods used to dim LED loads from the driver.

Phase control dimmers operates by chopping-out portions of the AC voltage cycle to control the light output. Phase control circuits include 2-wire forward phase (leading edge) control, 2-wire reverse phase (trailing edge) control and 3-wire forward phase (leading edge) control. Phase control dimming is often used in retrofit applications where pulling new or additional branch circuit wiring or back-end control wiring can be complicated and expensive. However, the LED driver must be designed to recognize and respond to the voltage signals from the dimming circuit. Failing to interpret a variable phase angle output from the phase control dimming is likely to produce flicker and reduce the dimming range.

0-10V is a 4-wire (Hot and Neutral, plus 2 low-voltage control wires) dimming method and is sometimes referred to as 1-10V dimming as most typical 0-10V dimmable drivers can only be dimmed from 100% (10V) down to 10% (1V), and 0V turns the lamp off. In this method, the driver is the current source for the DC signal and is thus is reliable with dimming occurring in the driver. The control circuit sends low voltage control signals to adjust an input to the driver by varying the voltage between 1V and 10V DC. Since the control signal is a small analog voltage, long wire runs can cause a voltage drop and produce a drop in the signal level. 0-10V is a universal control protocol in the lighting industry and finds its popularity in commercial lighting applications. However, the 0-10V dimming standards for architectural applications in the US do not define the value of the minimum light output and address the shape of the dimming curve. This is likely to cause incompatibility between controls and devices from different manufacturers.

DALI, with the ability to provide addressing of individual fixtures and status feedback from the loads, provides great flexibility in lighting control through a 4-wire (Hot and Neutral, plus 2 low-voltage data link topology-free wires) system. DALI is typically used where the control strategy requires the light fixture to respond to more than one controller (e.g., a manual control switch and an occupancy sensor). DALI is a bidirectional protocol and a DALI lighting system can operate up to 64 control points (drivers, dimmers, relays) without using a central control unit. The DALI protocol uses logarithmic dimming which provides 256 steps of brightness with a standardized dimming curve in the range of 0.1% to 100%.

PWM controls the brightness of an LED by varying the duty cycle of a constant current at a pulse rate high enough to be imperceptible to the human eye. The ratio of on time to off time determines the perceived light intensity. Pulse width modulation keeps the forward current constant, which eliminates the concern of color shift and is thus advantageous for applications that requires a consistent CCT over a wide dimming range. PWM dimming is commonly used for both static and dynamic intensity adjustment with white light sources as well as RGB LEDs. In RGB color mixing applications, PWM dimming allows the brightness of the individual sources to be precisely adjusted to deliver the desired color. However, high speed switching can generate electromagnetic interference. PWM drivers cannot be mounted remotely from the light source because the increased transmission distance from the driver to the light source may interfere with the high frequency, time sensitive duty cycles.

CCR or analog dimming adjusts light intensity by changing the DC drive current flowing through an LED. Because the current is changed linearly, CCR is essentially flicker-free. Constant current dimming can also operate over a wider range of light output than typical phase-cut dimming. The disadvantages of CCR include poor performance at low currents (below 10%), color shift of LEDs when dimming the LEDs to 20% of the rated output, and asynchronous response at higher currents due to the droop effect. CCR dimming circuitry can be controlled through a variety of protocols, such as 0-10V, DALI, and ZigBee. CCR and PWM can be combined to provide hybrid dimming so the advantages of both techniques can be harvested.

Flicker Mitigation​

Flicker is amplitude modulation of the light output that can be induced by voltage fluctuations in AC mains, residual ripples in the output current provided to the LED load, or incompatible interaction between the dimming circuits and LED power supplies. Flicker can cause other temporal light artifacts (TLAs) which include stroboscopic effect (the misperception of motion) and phantom array (pattern appears when eyes move). TLAs come in both visible and invisible forms. Flicker that occurs at frequencies of 80 Hz and lower is directly visible to the eye, and invisible flicker is the temporal variations occurring at frequencies of 100 Hz or higher. The stroboscopic effect and phantom array will typically occur within a frequency range of between 80 Hz and 2 kHz, their visibility varies in populations. While invisible TLAs is not perceptible to the human eye they can still have a number of negative consequences.

Flicker and other TLAs are undesired temporal patterns of light output that can cause eye strain, blurred vision, visual discomfort, reduced visual performance and, in some cases, even migraines and photosensitive epileptic seizures. Therefore they're one of the key considerations in light quality assessment. The intended use of artificial lighting plays a role. Different lighting scenarios may tolerate different level of temporal light artifacts. TLAs may be less of a concern for roadway, parking lot, and outdoor architectural lighting, or other applications where the duration of exposure to artificial light is limited. Artificial light with a high percentage of flicker should not use for both ambient lighting and task lighting in homes, offices, classrooms, hotels, laboratories and industrial spaces. Flicker-free lighting is not only critical for visual tasks that demand precise positioning of the eyes and environments where susceptible populations spend considerable time, it's high desired for HDTV broadcasting, digital photography and slow-motion recording in studios, stadiums and gymnasiums. Video cameras can pick up TLAs the way like the human eye detects these effects.

The key to mitigating flicker lies in the LED driver which is designed to rectify commercial AC power into DC power and filter out any undesirable current ripple. Sufficiently large ripples, which typically occurs at twice the frequency of the AC mains voltage, in the DC current provided to the LED load result in flicker and other visual anomalies at a frequency of 100/120 Hz. Thus the allowed level of ripple current in the LEDs, such as ±15% ripple (a total of 30%), must be defined in LED drivers for various applications where flicker matters. The ripples may be smoothed out by using a filter capacitor. One of the major challenges in driver design is to filter out ripples and harmonics without using bulky, short-lived high voltage electrolytic capacitor on primary side. AC LED engines are inherently susceptible to the flicker phenomenon because the LEDs in fact run from what is essentially the intermediary DC voltage that would be in an SMPS-based LED lighting system. Rapid alteration in polarity gives rise to a flicker in the intensity at a frequency twice the AC sinusoidal frequency. Despite the simplicity in circuit design, additional circuitry is required to effectively reduce the temporal variation in the power supply.

Standards for limiting flicker for different applications are yet to be established. Two metrics were established by IES to quantify flicker. Percent flicker measures the relative change in the light modulation (the depth of modulation). Flicker index is a metric that characterizes the intensity variation over the entire periodic waveform (or duty cycle, for square waveforms). Percent flicker is better known to general consumers. In general, 10 percent flicker or less at 120 Hz or 8 percent flicker or less at 100 Hz is tolerable for most people except for the at-risk populations, 4 percent flicker or less at 120 Hz or 3 percent flicker or less at 100 Hz is considered safe for all populations and highly desired in visually intensive applications. Unfortunately, a large number of LED lamps and luminaires currently supplied on the market have a high flicker percentage. AC LED lights, in particular, come with flicker typically higher than 30 percent at 120 Hz.

Circuit Protection​

Depending on the driver topology, circuit design and application environments, LED drivers can run up against load anomalies and abnormal operating conditions such as overcurrent, overvoltage, undervoltage, short circuit, open circuit, improper polarity, loss of neutral, and overheating, etc. Therefore, LED drivers should incorporate protection mechanisms in order to address these challenges.

The output voltage of some constant current drivers, especially switching boost converters, can rise too much above the nominal drive voltage due to load disconnection or excessive load resistance. Open circuit protection or output overvoltage protection (OOVP) provides a shutdown mechanism which uses a Zener diode to give feedback and conduct the output current to ground when the output voltage exceeds a certain limit. A more preferred method of open circuit protection is to utilize an active voltage feedback scheme to shut down the supply when the overvoltage trip point is reached.

Input overvoltage protection (IOVP) is designed to relieve the driving circuit from overvoltage stress as a consequence of switching operations/load change on the power grid, lightning strikes nearby, lightning strikes directly on the lighting system, or electrostatic discharge. In AC line applications, slight but sustained overvoltage can cause high currents (energy impulses) in the LED driver and LEDs, which may lead to failure of the LED driver and control interfaces, and the premature aging of the LEDs. A metal oxide varistor (MOV) or transient voltage suppressor (TVS) can be placed across the input to absorb energy by clamping the voltage. A plastic film capacitor, which is typically connected across the AC line to reduce EMI emissions, also helps absorb some of the energy in surge pulses.

LED drivers usually come with a limited level of surge protection from the built-in overvoltage protection circuits. In some applications such as street lighting, additional surge protection devices capable of surviving multiple surges or strikes should be added to the driver to protect downstream components from high surges. The SPD should be rated reduce or discharge high pulse energy of a minimum 10 kV and 10 kA, as per ANSI C136.2.

A short circuit at the load of a linear power supply can lead to overheating but makes no difference to the current supplied to each LED because the current limiting circuits provide automatic short circuit protection. However, in a switching buck regulator, a short circuit will lead to a failure of an LED or the entire module depending on the circuit design. The failure of a single LED usually has minimal impact on the total light output. The change in voltage can be balanced out using a self-adjusting current sharing circuit which still distributes the current equally. On the other side, a short circuit at the load of an LED string can significant affect the total light output. The failure detection mechanism of short circuit protection can be implemented by monitoring the duty cycle. A short circuit typically results in a very short duty cycle.

Overtemperature protection for LED systems include Module Temperature Protection (MTP) and Driver Temperature Limit (DTL). DTC uses an NTC (negative temperature coefficient) resistor to cut back output current when the maximum driver case point temperature in the application exceeds a predefined limit. MTC monitors the temperature of the LED module and is interfaced with the driver which automatically reduce the current to the LEDs when a threshold temperature is detected by the MTC. DTL can also be used as alternative to MTP if the driver TC point and LED module temperature can be correlated.

EMI and EMC​

Electromagnetic interference (EMI), also referred to as radio frequency interference (RFI), affects other electrical circuit as a consequence of either electromagnetic conduction or electromagnetic radiation emitted by electronics such as those in LED drivers, CB radios and cell phones. Any LED driver connected to AC mains supply has to meet the radiated emissions standards such as defined in IEC -6-3. In an LED driving circuit, MOSFET switching is usually the main source of EMI. A PCB layout with paths for the switching currents kept short and compact is also important to limit EMI. In some applications an input filter is required to reduce high frequency harmonics and the design of this circuit is critical to maintain a low EMI. The ground plane on the circuit board must remain continuous so as to avoid creating a current loop that causes high levels of EMI to be emitted. A metal screen may be mounted over the switching area to provide an enclosure that stops EMI radiation.

Electromagnetic compatibility (EMC) is the ability of a device or system to operate in its electromagnetic environment without yielding EMI that disturbs neighboring equipment or being disturbed by the EMI radiated by neighboring equipment. The EMC performance of the LED driver is often automatically assured by a good EMI design. However, electrostatic discharge (ESD) and surge immunity which are not taken into account in EMI practices also affects the EMC performance.

Safety Considerations​

Safety should always remain the number one priority when evaluating a driver and the lighting system it operates. A line-powered LED driver with dielectric isolation, e.g., V RMS (50 or 60 Hz), from input to output is highly desired. The input/output circuit isolation can only be accomplished with a transformer which has primary and secondary windings with good galvanic isolation. The output voltage must be kept below the 60 VDC safety extra low voltage (SELV) limit as per IEC . However, there's an increasing number of LED lighting products which implement a non-isolated topology for the purpose of cutting cost. The risk of electric shock is a serious concern in LED products driven by low cost linear regulators. These circuits offer no isolation between the input and output circuits, and the electrical insulation of the lighting systems may have not been adequately tested.

The issues of creepage and clearance distances must be considered for AC powered products. The creepage distance between primary and secondary circuits must meet the spacing requirements otherwise electrocution or fire can occur. Clearance, which is defined as the shortest distance between two conductive parts, must be factored in to prevent arcing between electrodes caused by the ionization of air. As the sizes of electronic circuits continue to shrink, a good PCB design is essential for a driver circuit to not only reduce EMI emissions, but also reduces creepage and clearance problems.

All electrically-conductive and touchable parts of a line-powered Protection Class I LED driver must be connected to earth. LED drivers designed to operate LED lighting systems for residential and commercial applications are typically listed as Class II. There's no enclosure grounding for class II LED drivers, but all the conductor inside a class II drivers must be dual or reinforced insulated to ensure good insulation between the mains power circuit and the output side or the metal casing of the driver.

Thermal Considerations​

An LED driver is configured to convert the AC line voltage into DC output as efficiently as possible, and any energy lost in the conversion process will be converted into heat. This means an LED driver with 90% efficiency requires an input power of 100W/0.9 = 111 W to drive a 100W load. Among the input power 11W is the power loss that escapes in the form of heat. This places a high thermal stress on the LED driver circuit. When the driver is co-located within the luminaire housing, the thermal load from the LEDs will end up in additional increase in the driver temperature. In addition to utilizing components that are rated for high temperatures, the driver has to be designed to pull heat away from thermally-sensitive components. Excess heat buildup will cause reliability issues with components, including electrolytic capacitors which will dry out when exposed to heat. Therefore the temperature at which an LED driver is running is fundamentally important in defining its lifetime. To facilitate heat dissipation, LED drivers for high wattage LED luminaires use aluminum enclosures which can come with high density fins and thermally conductive potting.

Ingress Protection​

LED drivers for roadway, street, exterior and landscape lighting applications must be sealed to protect against ingress of dust, moisture, water and other objects that may pass through into the products. A high degree of ingress protection (IP) for LED drivers is critical for indoor applications such as carwashes, cleanrooms, bottling and canning plants, food processing facilities, pharmaceutical plants or any industrial application requiring exposure to daily high-pressure wash downs. Self-contained LED drivers for wet locations are usually potted in silicone to enhance enclosure integrity while also facilitating electrical insulation and thermal management. These drivers typically come with IP65, IP66 or IP67 level ingress protection.

Location Impact​

LED drivers can be remote mounted or co-located within lamp or luminaire housings. In co-located, non-DOB systems, the driver must be thermally isolated from LEDs which generate a huge amount of heat. Driver maintenance should be taken into consideration when designing a luminaire housing. In remote-mounted systems, PWM drivers can experience performance losses over a long distance. As such, CCR is the preferred dimming technique for remote-mounted systems.