Circuit failure judgment: LED failure mode analysis and discussion

　　1 LED drive circuit principle

　　LED is a light-emitting diode made of semiconductor material. It can only conduct in one direction, and its conduction voltage is not high, and the forward current cannot be too large. Therefore, there are certain requirements for the power supply of LED. At this time, LED The drive circuit came into being. In actual use, most LED products use alternating power supply as the power input of the LED drive circuit, and use the drive circuit to convert it into a circuit with a regulated output form or a constant current output form. LED drive circuits can be divided into many types according to different classification standards. Currently, based on the driving principle of the circuit, they can be divided into two categories: one is linear drive circuit, and the other is switching drive circuit.

　　1.1 Linear drive circuit

　　The schematic diagram of the linear drive circuit is shown in Figure 1. The structure generally includes the following parts, rectifier circuit, filter circuit, and voltage stabilizing circuit.

　　In the figure, full-wave bridge rectification is used to rectify the alternating power supply into a unidirectional pulsating voltage. The filter circuit adopts RC filtering, and the voltage value filtered by the filter circuit is already relatively close to the DC power supply. However, due to voltage fluctuations on the power grid, the output voltage of the drive circuit fluctuates, which is fatal for LEDs. Therefore, the filtered voltage needs to be added with a voltage stabilizing circuit. So that the linear drive circuit can maintain a relatively stable voltage to drive the LED.

　　In a linear drive circuit, the brightness of the LED is a function of the passing current and has nothing to do with the voltage drop imposed on the LED. As can be seen from the circuit schematic diagram above, the linear LED drive circuit has a simple structure, is easy to implement, has a short research and development cycle, low production cost, and is compact. Moreover, because it does not use many large-capacity capacitors and inductors, there is no need for circuit design. Consider EMI issues. Can be applied to low current lighting systems.

　　1.2 Switch type drive circuit

　　The schematic diagram of the switching drive circuit is shown in Figure 2. After the input alternating voltage is rectified, filtered and stabilized by the rectifier circuit, the current or voltage of the LED is controlled through the switching state, so that the LED can emit light smoothly. A typical switching drive circuit is given below to gradually analyze the working status of the switching drive circuit.

　　As can be seen from Figure 2, the switching LED drive circuit can be divided into the following parts: low-frequency rectification and filter circuit, self-oscillation circuit, voltage stabilization circuit, sampling pulse width adjustment circuit and high-frequency rectification and filter circuit, etc.

　　The mains AC 220V is stepped down by a 12V transformer, and then converted into a quasi-DC power supply through a bridge rectifier diode 3N258 and capacitor C2 to form a low-frequency rectifier filter circuit. Power transistors Q1, Q2, Q3 and capacitor C5 and resistor R2 form a self-excited oscillation circuit. Q2 is a PNP tube and a pulse width adjustment tube. One of Q1 and Q3 is a PNP tube and the other is an N P N tube. The two tubes are combined to form a switch. Adjuster, C5, R2 can set the oscillation frequency by adjusting parameters. Using this self-oscillation circuit, the DC-like power supply can be converted into a high-frequency pulse signal. The frequency of high-frequency signals can be calculated through frequency selection characteristics. The duty cycle of the high-frequency pulse can be adjusted to adjust the energy output by the device. When the current flows through the inductor, an induced electromotive force will be generated at both ends of L. When the current disappears, the induced electromotive force will generate a reverse voltage at both ends of the circuit. If this reverse voltage is greater than the reverse breakdown of some components voltage will damage these devices. A freewheeling diode D2 is connected in parallel to both ends of the inductor, and the reverse induced electromotive force has a leakage loop through the loop composed of R4 and C6.

　　The sampling circuit composed of R 6, R 7, R 8 and the reference source circuit composed of R 5 and D3 are used to adjust the pulse width of the high-frequency signal to adjust the saturation conduction time of the switching tube, thereby adjusting the output voltage of the power supply . Among them, R7 is an adjustable resistor to facilitate the adjustment of this voltage.

　　From the above analysis, we can see that compared with linear drive circuits, switching LED drive circuits have high efficiency and large output current. The current can also be adjusted by adjusting the pulse width. The output current accuracy is very high, so that the LED brightness can be Controlled, suitable for large-scale lighting situations and situations where the current output is relatively high.

　　2 Analysis of LED driver failure mechanism

　　2.1 Analysis of causes of LED drive circuit failure

　　(1) Surge current and surge voltage

　　Due to the moment when the drive circuit is turned on, the capacitor charging requires a large current, and the charging time is short, resulting in an instantaneous large current; due to voltage fluctuations on the power grid and the impact of surge voltage, the diodes and resistors on the drive circuit are damaged. momentary high voltage. This may cause permanent damage to the components on the LED driver circuit.

　　(2)Electrostatic discharge

　　Electrostatic discharge, or ESD phenomenon. Since the electricity is discharged in a very short period of time, the electrostatic discharge voltage can often reach several thousand volts. This is fatal for semiconductor devices. ESD may damage the internal structure of the LED lamp or driver IC.

　　(3) Components fail to use

　　Since the switching drive circuit requires large capacitors to store electrical energy and stabilize voltage, aluminum electrolytic capacitors are generally used for large capacitors. The failure rate of aluminum electrolytic capacitors is higher than that of other components, and because transformers and LEDs generate heat when used, this heat intensifies the movement of the electrolyte in the electrolytic capacitors, shortening the normal service life of aluminum electrolytic capacitors.

　　(4) Caused by working environment

　　At present, the mainstream LED driver uses alternating power supply as the power input. For some high-power LED driver circuits, the transformer coil will generate a large amount of heat, and the heat will cause temperature stress that causes LED failure. The time model of temperature stress is shown in the following formula:

　　Where M is temperature stress, T is temperature, and t is time.

　　It can be seen that the temperature stress increases exponentially with time and temperature. The longer the appliance is used, the greater the temperature stress and the higher the failure rate caused by heat.

　　2.2 Linear adjustment type

　　Failure analysis of the LED drive circuit is based on the linear adjustment LED drive circuit in Figure 1. As soon as the linear LED drive is powered on, the AC power supply needs to charge the capacitance and inductance inside the drive circuit. Therefore, there will be a relatively large voltage at the moment of power on. The current passes through the fuse and the rectifier bridge. Since the Multisim simulation software can only simulate analog quantities, it cannot simulate environmental heat and humidity. Therefore, this simulation can only be simulated from the aspect of electrical parameters. Here, two failure factors, surge voltage and surge current, are added to perform failure simulation on the linear LED drive circuit described above, and various instrument parameters of the circuit operation are added after the surge voltage is added.

　　From the contents shown in Figure 1, the values of each instrument can be read.

　　Vi=250V; Vo=29.934V; Vled=8.415V;

　　Iled=34.606mA.

　　After many simulation tests, the comparison of the electrical parameters of the LED drive circuit under normal conditions is shown in Table 1.

　　It can be seen from the data in Table 1 that when the voltage on the power grid fluctuates by 10%, the working state of the linear LED drive circuit changes significantly. From the figure above, it can be found that due to the use of a suitable voltage stabilizing circuit, the voltage amplitude on the power grid has almost no impact on the operating voltage of the linear LED drive circuit. However, its driving current has changed dramatically, and the driving current has increased by 40% compared with the normal input voltage. This will cause the LED to overload, reduce the life of the LED lamp bead, and may even directly damage the lamp bead.

　　2.3 Failure analysis of switching LED drive circuit

　　Failure analysis is performed using the linearly adjusted row LED drive circuit in Figure 2. In the simulation diagram of Figure 2, XSC2 represents the comparison of the input AC power supply and the voltage after rectification. The sine wave type is AC220V. After full-wave rectification, its voltage value Vimax≈311V, which is relatively stable after rectification. After rectification, the voltage read from the mark point in the figure is 11.368V. After the low-frequency rectification, the voltage passes through the self-oscillation circuit, the high-frequency rectification circuit and the voltage stabilizing circuit, and the output is the LED driving voltage.

　　Since the voltage regulator tube 1N4735A is used, the voltage regulation value is 6.6V, so the theoretical value of the LED driving voltage is 6.6V.

　　After many simulation tests, the parameters of several instruments can be read out as follows:

　　Vi=250V; Vo=12.3V; Vled=6.64V;

　　Iled=47.416mA;

　　A comparison of the electrical parameters of the LED drive circuit under normal conditions is shown in Table 2.

　　As can be seen from Table 2, when there is a surge voltage input on the power grid, the switching LED drive circuit adopts good voltage stabilization measures, so the voltage parameters of the drive circuit do not change greatly, but the LED drive circuit The change is huge, with an increase of 100%. This will cause the LED power to rise, causing the LED to fail.

　　2.4 Failure solutions

　　After the simulation analysis of the LED drive circuit in the previous sections, we can summarize the following effective solutions to LED failure:

　　(1) For surge current and voltage, a fuse and a PTC resistor can be added to the power input. PTC resistor is a positive temperature coefficient resistor. When the primary current of the power supply input has a surge current or surge voltage input, according to the heating formula of the resistor Q=R*I^2*T, the increase in the current or voltage flowing through the PTC will inevitably increase the heat generation of the PTC resistor. As a result, the resistance of the PTC resistor increases, causing part of the primary power input to the power supply to be reduced at the PTC resistor to ensure that the secondary output power of the power supply remains unchanged and maintains the stability of voltage and current. For LED drivers used outdoors, lightning protection measures should also be added.

　　(2) For the selection of driving devices, better devices should be selected within the cost range, especially capacitors.

　　Moreover, the maximum current and voltage parameters of the device must be guaranteed to be more than 2 to 3 times the rated value of the circuit for normal operation, and the specific parameters must be selected specifically. To ensure that circuit components have sufficient redundancy to cope with sudden changes in electrical parameters.

　　(3) At the same time, attention should be paid to the layout of the circuit board. Those that generate large amounts of heat should be separated to reduce the impact of heat on the board. Circuit boards should be protected against moisture and humidity.

　　In some specific environments, some insulation and moisture-proof measures should also be taken.

　　(4) For switching LED drive circuits, failures caused by EMI should also be prevented. Problems caused by EMI can be reduced by adding X capacitors, common mode inductors, differential mode inductors, low-pass filter circuits, shields, etc.

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