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Sep 11 2020

LED Drivers Tutorial: Failure Analysis and Maintenance


LED lights cannot directly use the conventional mains grid voltage, because of the characteristics of LED lighting. In order to meet the special voltage and current requirements of LEDs, a specially designed voltage conversion device must be used to make LEDs work normally. This device is an LED driver. LED drivers are usually switching mode devices that convert the input voltage (Typically 120-220 VAC or 12 VDC) into a voltage at which the current drawn by the LED's is equal to its drive current. The drive current is regulated for optimum brightness, led service life, and battery life. A drive current lower than the maximum drive current of an LED can greatly prolong service life. As a key part of LED lighting, the quality of LED drivers directly affects the performance of LED lighting.

Choose the Correct LED Drivers For LED Lights

No matter how good the quality of the LED driver is, failure and maintenance are inevitable. This article will analyze the 10 failures in LED lighting design and its application based on the relevant technology and practical experience of the LED driver.

LED Driver For Panel Light



Ⅰ LED Driver Failure Analysis

1.1 Forward Voltage Drop (Vf) Range

1.2 Power Margin and Derating Requirements

1.3 LED Working Characteristics

1.4 Test Session

1.5 Different Load with Different Test Results

1.6 LED Driver Circuit Problem

1.7 Wrong Phase Wring

1.8 Grid Fluctuation

1.9 Frequent Line Trips

1.10 Drive Cooling

Ⅱ LED Driver Maintenance

2.1 Multimeter to Detect LED Driver

2.2 Identify LED Power Supply

Ⅲ LED Driver Circuit Modulation

3.1 Pulse Width Modulation (PWM)

3.2 Pulse Frequency Modulation (PFM)

3.3 Sliding-Mode Modulation

Ⅳ One Question Related to LED Driver

Ⅰ LED Driver Failure Analysis

The LED driver is measuring current passing through LEDs using sense resistor and then increase or decrease the voltage to maintain constant current continuously. LEDs are kind of diode so they need DC voltage to operate so most of the LED drivers are boost and can vary output supply in wide range (example, 16V to 38V). They also have dimming control by PWM signal from microcontroller OR by having a manual potentiometer to change sense resistor. According to them, LED driver failures are complex, but we can follow the steps below to analyse.

1.1 Forward Voltage Drop (Vf) Range

LED lamp load end is generally composed of a number of LEDs connected in series, and its working voltage Vo=Vf×Ns, where Ns represents the number of LEDs. And the Vf of an LED varies with temperature. Generally, at a constant current, Vf becomes lower at high temperatures and becomes higher at low temperatures. The LED lamp load working voltage is VoL at high temperature, and VoH represents a value at low temperature. When selecting an LED driver, consider that the driver output voltage range is greater than VoL~VoH.

If the maximum output voltage of the LED driver is lower than VoH, the maximum power of the lamp may not reach the actual power required at low temperature. If the minimum voltage of the selected LED driver is higher than VoL, the output of the driver may exceed the working range at high temperature. And LED driver will work unstable, making the lights flicker.

Considering the overall cost and lamp efficiency, don’t blindly pursue the ultra-wide output voltage range of the LED driver. Because the driver voltage is only in a certain range, its efficiency is the highest. When the range is exceeded, the efficiency and power factor (PF) will deteriorate. In addition, if the design of the driver output voltage range is too wide, high costs and unoptimized efficiency will be made.


1.2 Power Margin and Derating Requirements

In general, the nominal power of the LED driver refers to the data measured under the rated environment and rated voltage. Taking into account different applications, most LED driver suppliers will provide power derating curves in their product specifications (common load vs. ambient temperature derating curves and load vs. input voltage derating curves).

As shown in Figure 1, the red curve represents the power derating curve when the input is 120Vac, and its load varies with the ambient temperature. When the ambient temperature is lower than 50℃, the LED driver is allowed to be 100% full load. When the ambient temperature is as high as 70℃, the LED driver can only be derated to 60% of the load. When the ambient temperature changes between 50℃~70℃, the driver load varies with the temperature linearly.

Power Derating Curve (Load vs Ambient Temperature)

Figure 1. Power Derating Curve (Load vs Ambient Temperature)

The blue curve represents the power derating curve when the input is 230Vac or 277Vac, and its load varies with the ambient temperature. The principle is similar to the above mentioning.

As shown in Figure 2, the blue curve represents the derating curve of the LED driver when the ambient temperature is 55°C, its output power varies with the input voltage. When the input voltage is 140Vac, the load of the driver is allowed to be 100%, and the input voltage will be adjusted downward. If the output power remains the same, the input current will rise, resulting in input terminal loss and lower efficiency. When the device temperature rises, exceeding the rated temperature, which may cause the device to fail.

Power Derating Curve (Load vs Input Voltage)

Figure 2. Power Derating Curve (Load vs Input Voltage)

Therefore, when the input voltage is less than 140Vac, the output load of the LED driver is required to linearly decrease as the input voltage decreases. According to the above derating curve and corresponding requirements, when choosing a LED driver, actual applying needs are important, as well as the derating margin.


1.3 LED Working Characteristics

When the required input power is a fixed value, such as a fixed error of 5%, the output current can only be adjusted to the specified power for each lamp. Due to different working ambient temperature and different lighting time, the power of each lamp will vary greatly.

Although there are considerations for marketing and business factors. However, the volt-ampere characteristic of the LED lamp determines that the LED driver is a constant current source, and its output voltage varies with the series voltage Vo of the LED load. When the efficiency of the driver is basically unchanged, its input power changes with Vo. And meanwhile, the overall efficiency of the LED driver will increase after thermal equilibrium. Under the same output power, the input power will decrease compared to the boot time.

Therefore, when formulating requirements, LED driver users should first understand the operating characteristics of LEDs. Avoid suggesting indicators that do not meet the principles of operating characteristics, and indicators that far exceed actual requirements, resulting in excess quality and cost waste.


1.4 Test Session

Sample test problems, for example, multi-brand LED driver samples all failed during the test. The reason is that a self-dual voltage regulator is used to directly power the LED driver for testing. After power on, the voltage regulator is gradually adjusted from 0Vac to the rated operating voltage of the LED driver. This kind of test operation can easily make the LED driver start and work with the load when the input voltage is very small, but this situation will cause the input current to be far greater than the rated value. And the internal input terminal related components, such as fuses, rectifier bridges, thermistors, etc. will fail due to excessive current or overheating, damaging the LED driver.

The correct test method is to adjust the voltage regulator to the rated operating voltage range of the LED driver, and then connect the driver to power-on test. Of course, technically improving the design can also avoid the failure caused by this kind of test misoperation. That is, a starting voltage limit circuit and an input undervoltage protection circuit are set at the input of the driver. When the input does not reach the start-up voltage set by the driver, the driver does not work. When the input voltage drops to the input undervoltage protection point, the driver enters the protection state. Although the driver has a self-protection function and will not fail, you must carefully understand whether the purchased LED driver product has this protection before testing (considering the actual application environment of the LED driver, most LED drivers currently do not have this set).


1.5 Different Load with Different Test Results

On the one hand, when the LED driver is tested with LED lights, the result is normal; on the other hand, when driver tested with an electronic load, the result may be abnormal. Usually this phenomenon has the following reasons:

1) The output instantaneous voltage or power of the driver exceeds the working range of the electronic load instrument. (Especially in CV mode, the maximum test power should not exceed 70% of the maximum power of the load. Otherwise, the load may instantaneously have overpower protection when loading, causing the driver to fail to work.)

2) The characteristics of the electronic load instrument used are not suitable for measuring constant-current device. And the load voltage gear jumping, result in the drive to fail to work.

3) Because there is a large capacitor inside the input of the electronic load meter. The test is equivalent to connecting a large capacitor in parallel with the driver output, which may cause the driver's current sampling work to be unstable. As we all known, the LED driver is designed to meet the working characteristics of LED lamps. The most practical test method is to use the LED lamp as a load, and connects an ammeter and a voltmeter in series to test.


1.6 LED Driver Circuit Problem

The following conditions often cause damage to the LED driver:

Connect AC to the DC output terminal of the driver, causing the driver to fail.

Connect AC to the DC/DC output or input of the driver, causing the driver to fail.

Connect the output terminal of the constant current to the dimming light, causing the driver to fail.

Connect the phase wire to the ground wire causing the driver has no output and the housing is charged.


1.7 Wrong Phase Wring

Take an international example: the rated working voltage between each phase line and the neutral line is 220V, and the voltage between the phase line and the phase line is 380V. If the driver is connected to two phase wires, after power on, the LED driver input voltage exceeds the rated range, which cause the product to fail.

As shown in Figure 3, V1 represents the first phase voltage, V2 represents the second phase voltage, and R1 and R2 respectively represent the drivers normally installed on the line. When the neutral line (N) on the circuit is disconnected, the drivers R1 and R2 on the two branches are connected to the 380V voltage after being connected in series. Because of the difference in input internal resistance, when one of the drivers is charged to start, the internal resistance becomes smaller. Most of the voltage may be applied to another driver, causing the overvoltage damage. Therefore, it is recommended that switches or short-circuiters on the same distribution branch should be disconnected together, not just cut off the neutral line. What’s more, do not put the power distribution fuse on the neutral line to avoid bad effect of the neutral line on the circuit.

Neutral Line Open Circuit Diagram

Figure 3. Neutral Line Open Circuit Diagram


1.8 Grid Fluctuation

When wires of a transformer grid branch is too long and there is large power equipment on the branch, the grid voltage will fluctuate sharply when the large equipment starts and stops. It even causes the grid to be unstable. When the grid voltage exceeds 310V, the drive may be damaged (even if there is an LED lightning protection device, it is useless. Because the lightning protection device is to deal with pulse spikes of tens of uS level, and the fluctuation of the grid may reach tens of mS, or even hundreds of mS) . Therefore, special attention should be paid when there is large electric machinery on the street lighting branch power grid. It is best to monitor the fluctuation range of the power grid or to supply power to the grid transformer separately.


1.9 Frequent Line Trips

Too many lights are connected on the same branch, which leads to overload on a certain phase and uneven power distribution among the phases, resulting in frequent line trips.


1.10 Drive Cooling

Although the LED has high luminous efficiency, only a small part of the energy flowing through the LED is radiated in the form of visible light. And most of the remaining energy is consumed in the LED in the form of heat, so the LED generates more serious heat. When the driver is installed in a non-ventilated environment, the driver housing should be in contact with the lamp housing as much as possible. If possible, apply thermal glue or a thermal pad on the contact surface between the housing and the lamp housing to improve the heat dissipation of the LED driver and ensure the reliability of the driver.

Constant Current LED Driver

Ⅱ LED Driver Maintenance

2.1 Multimeter to Detect LED Driver

Measuring the output voltage of the no-load LED driver with a multimeter, if the output voltage is not detected, does it mean that the driver is broken? Look at the following steps:

1) The voltage of the non-isolated LED power supply in the no-load state is about 300V tested with a multimeter, and it is about 220V with a PFC.

2) Isolating the LED power supply, the voltage in the no-load state, tested with a multimeter, is about 3-5V more than the total voltage of the rated LED series. However, although the output voltage can be tested under no load, it does not mean that it can be normal under load. At this time, it is necessary to connect the corresponding LED light board to see the performance of the LED lighting. If there is no flicker, the output voltage is also equal to the total voltage of LED lights in series connection. This situation can be considered normal, otherwise it fails. If there is no output voltage at no load, the power supply must be broken.


2.2 Identify LED Power Supply

The LED power supply is widely used in many applications. So how to distinguish the quality of LED power supply is particularly important. A few methods are briefly introduced below.


  • LED Driver IC

The power of IC drive, the quality of IC directly affects the whole power supply. The lighting manufacturer should understand the IC design solution and calculate the cost of the driver, so as to purchase power products at a reasonable price.


  • Transformers

The control chip can be regarded as the brain center of the power supply, while the transformers determine the power and temperature resistance. The transformer is responsible for the transfer of "AC to DC". However, the energy overload will damage the device. The core of the transformer is the magnetic core and the wire package.


  • Electrolytic Capacitors and Ceramic Capacitors

The quality and life requirements of input electrolytic capacitors are important. However, people tend to ignore the quality requirements of the output capacitor. In fact, the life of the output capacitor also has a great impact on the life of the power supply. The output end has a switching frequency of up to 60,000 times per second, which causes the parasitic resistance of the capacitor to heat up and produce substances similar to scale. Finally, the electrolyte heats up and bursts. Ceramic capacitors: The materials are divided into X7R, X5R and Y5V, and the actual capacitance value of Y5V can only reach 1/10 of the actual value. In addition, the nominal capacitance value only refers to when a capacitor works at 0V. Therefore, this tiny chip with poor options will also lead to a price difference in cost and greatly shorten the life of the power supply.


  • Circuit Design and Welding Process

Judgment of the pros and cons of the design: Aside from the professional point of view, it can be distinguished by some intuitive methods, such as the neat layout of the components, and soldering points. As for flying leads and manually adding components, it is a serious lack of techniques and efficiency. As we all know, the quality of mechanized production of wave soldering process is definitely better than manual welding. Because the machining process is more neat and uniform. Identification method: whether there is red glue on the back board.

The flashing phenomenon of the lamp after a period of use is basically caused by the power supply or the weak welding of the lamp beads. However, it is extremely difficult to detect the virtual welding of products through aging, so AOI must be used to detect the quality of the power supply.


  • Batch Inspection of Aging Racks and High Temperature Aging Rooms

No matter how good the power products are controlled by materials and production processes, they still need to be tested for aging. Because the incoming inspection of electronic components and transformers is difficult to control. Only through the aging of the entire batch of power supplies and the high temperature sampling inspection of the high temperature room. This is a wide-ranging screening to determine whether the materials have safety hazards.


Ⅲ LED Driver Circuit Modulation

The LED driver circuit is divided into constant-voltage type and constant-current type according to the power supply to the LED. Constant-current switch type LED driver circuit samples the current flowing through the LED lamp, and gives the output control signal to control the on and off of the switching power tube, which aims to adjust the output current as the set value. The dimming control circuit mainly includes SCR dimming circuit, pulse width modulation (PWM), pulse frequency modulation (PFM), sliding mode control, PWM_PFM, PSM, etc. Let's take pulse width modulation (PWM), pulse frequency modulation (PFM), and sliding mode modulation to introduce in detail below.


3.1 Pulse Width Modulation (PWM)

Pulse width modulation, shown in the figure below, refers to the stability of the output voltage by changing the on-time of the switching power tube in each cycle at a specific frequency. That is adjusting the duty cycle to obtain stable output voltage. When the output voltage changes due to the working environment, noise and other factors, the error amplifier samples the voltage change and sends the signal to the control circuit. The control circuit adjusts the duty cycle of the switching power tube signal to maintain the stability of the output voltage.

Figure 4. PWM Modulation Based on BUCK Structure

Voltage Mode

Figure 4 (a). Voltage Mode

Peak Current Mode

Figure 4 (b). Peak Current Mode

Average Current Mode

Figure 4 (c). Average Current Mode

3.1.1 Advantages of PWM

(1) The PWM modulation method has high efficiency under heavy load, and has a good dynamic response to load changes.

(2) The output ripple voltage is small and the linearity is high.

(3) The frequency is stable, the duty cycle adjustment is not restricted, the control is simple, and both the current control mode and the voltage control mode are applicable.

3.1.2 Disadvantages of PWM

(1) The efficiency of PWM modulation method decreases at light load.

(2) The transient response is slow during constant-voltage driving, and a more complicated compensation circuit is required.

(3) Accurate current detection circuit is required for constant-current driving.


3.2 Pulse Frequency Modulation (PFM)

The pulse frequency modulation is shown in the figure below. Under the condition of a certain on-time of the switching power tube, the output voltage can be controlled by adjusting the off time. When the output voltage changes, the error amplifier samples the feedback signal and sends the output signal compared with the reference signal to the control circuit. The control circuit analyzes the error signal and generates a square wave signal with constant pulse width and varying frequency to control the switch power tube to maintain the stability of the output voltage.

Pulse Frequency Modulation Based on BUCK Structure

Figure 5. Pulse Frequency Modulation Based on BUCK Structure

3.2.1 Advantages of PFM

(1) The PFM modulation has very high efficiency, better frequency characteristics and higher voltage regulation rate at light load.

(2) The PFM modulation has a relatively high transmission signal-to-noise ratio and a good anti-interference ability.

(3) The output voltage has a large adjustable range and low power consumption.

3.2.2 Disadvantages of PFM

(1) The efficiency of the PFM modulation will decrease under heavy load.

(2) The frequency spectrum of the output ripple is scattered and irregular.

(3) The load adjustment range is very small, resulting in high filtering costs.


3.3 Sliding-Mode Modulation

Sliding-mode modulation mode, the full name is sliding mode variable structure control,  is a discontinuous control. As shown in Figure 6, the sliding mode makes the system structure change purposefully according to its current state, which force the system to make small amplitude and high frequency up and down movements along the designed trajectory under response conditions. That is, sliding mode movement. Reduce the system's sensitivity to disturbances and load jumps.

Sliding Mode Control Based on BUCK Structure

Figure 6. Sliding Mode Control Based on BUCK Structure

3.3.1 Advantages and Disadvantage of Sliding Mode

It has the advantages of fast dynamic response, strong robustness and wide stability range, but it also has a problem that the operating frequency is not fixed.


4.1 Question

How long do LED drivers last?

4.2 Answer

While the light function of an LED may last for years, drivers can give out much sooner. This is why we recommend name brand LED bulbs for the home, especially those with 25,000 hour rated lives. In general, high power white LEDs use much more current, and need of more complicated drivers.

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