Abstract: This paper discusses a principle and method based on the failure mechanism of LED lighting driver circuit. LED lamp failures are caused by failures in the power supply circuit and failure of the LED device itself. In the research of this paper, the principle of typical LED power supply driving circuit is discussed, and the failure analysis is carried out by measuring the various parameters of the LED driving circuit from the simulation experiment under the surge voltage, so as to predict various practical work. The impact of sensitive parameters on failure. Finally, a theoretical solution to the failure of the LED device itself is given.
1 Introduction
In recent years, the number of people engaged in LED manufacturing and R&D has increased significantly. LED companies are also growing like mushrooms. Due to the large number of enterprises and people engaged in the research and development of LED drivers, their technical level is uneven, and the quality of LED driver circuits developed is mixed. The failure of LED lamps often occurs, which hinders the frequent promotion of LED lighting. LED lamp failure is the failure of the power supply and drive, and the failure of the LED device itself. This paper tries to analyze the working principle of the circuit from the actual LED power supply circuit, and then tries to analyze the failure effect of various working sensitive parameters from the LED driving circuit under different environments to perform the failure mode. The analysis, finally, verified the results by simulation. And theoretically give a solution to failure.
2. LED drive circuit principle
LED is a kind of semiconductor material made of light-emitting diode, which can only be single-pass, and its conduction voltage is not high, and the forward conduction current can not be too large, so there is a certain requirement for the LED power supply, then LED The drive circuit came into being. In actual use, most of the LED products use an alternating current power supply as the power input of the LED driving circuit, and the driving circuit becomes a circuit of a regulated output form or a constant current output form. The LED driver circuit can be divided into many types according to different division criteria. At present, according to the driving principle of the circuit, it can be divided into two categories: one is a linear driving circuit, and the other is a switching driving circuit.
2.1 linear drive circuit
The schematic diagram of the linear drive circuit is shown in Figure 1. The structure generally includes the following parts, the rectifier circuit, the filter circuit, and the voltage regulator circuit.
The figure uses full-wave bridge rectification to rectify the alternating power supply into a unidirectional ripple voltage. The filter circuit adopts RC filtering, and the voltage value filtered by the filter circuit is relatively close to the DC power supply. However, due to the voltage fluctuation on the power grid, the output voltage of the driving circuit fluctuates, which is fatal for the LED. Therefore, the voltage after filtering needs to be added with a voltage regulator circuit. So that the linear drive circuit can maintain a relatively smooth voltage to drive the LED.
In a linear drive circuit, the brightness of the LED is a function of the pass current, independent of the voltage drop applied across the LED. As can be seen from the above circuit schematic, the linear LED driver circuit is simple in structure, easy to implement, short in development cycle, low in production cost, small in size, and, because no large-capacity capacitors and inductors are used, circuit design is not required. Consider EMI issues. Can be applied to low current lighting systems.
2.2 Switching drive circuit
The schematic diagram of the switch-type drive circuit is shown in Figure 2. After the input alternating voltage is rectified and filtered by the rectifier circuit, the current or voltage of the LED is controlled by the switch state, so that the LED can illuminate smoothly. A typical switch-type drive circuit is given below to gradually analyze the operating state of the switch-type drive circuit.
As can be seen from FIG. 2, the switch type LED driving circuit can be divided into the following parts: a low frequency rectification filter circuit, a self-oscillation circuit, a voltage stabilization circuit, a sampling pulse width adjustment circuit, and a high frequency rectification filter circuit.
The mains AC 220V is stepped down by a 12V transformer, and then a low-frequency rectification and filtering circuit is formed by a bridge rectifier diode 3N258 and a capacitor C2, and converted into a DC-like power source. The power transistors Q1, Q2, Q3 and the capacitor C5 resistor R2 form a self-excited oscillation circuit, wherein Q2 is a PNP tube, which is a pulse width adjustment tube, one of Q1 and Q3 is a PNP tube, and one is an NPN tube, and two tubes are combined to form a switch. Adjuster, C5, R2 can set the oscillator frequency by adjusting the parameters. With this self-oscillating circuit, a DC-like power supply can be converted into a high-frequency pulse signal. The frequency of the high frequency signal can be calculated by the frequency selection characteristic. The duty cycle of the high frequency pulse can be adjusted to adjust the energy output from the device. When the current flows through the inductor, an induced electromotive force is generated at both ends of L. When the current disappears, the induced electromotive force generates a reverse voltage across the circuit, if the reverse voltage is greater than the reverse breakdown of some components. These voltages will damage these devices. A freewheeling diode D2 is connected in parallel across the inductor, and a loop consisting of R4 and C6 provides a venting loop for this reverse induced electromotive force.
The sampling circuit composed of R 6, R 7 and R 8 and the reference source circuit composed of R 5 and D3 are used for adjusting 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, it can be seen that the switching type LED driving circuit has higher efficiency than the linear driving circuit, and the output current is large, and the current can be adjusted by adjusting the pulse width, and the output current precision is very high, so that the LED brightness can be Controlled, suitable for use in large lighting situations and current output.
3. LED drive failure mechanism analysis
3.1 LED drive circuit failure analysis
(1) Surge current and surge voltage
Due to the instant of opening of the driving circuit, the capacitor charging requires a large current, and its charging time is short, resulting in a large instantaneous current; due to voltage fluctuations on the power grid and surge voltage, the diode and resistors on the driving circuit are caused. The moment of large voltage. This can cause permanent damage to devices on the LED driver circuit.
(2) Electrostatic discharge
Electrostatic discharge, the ESD phenomenon. Since the electricity is discharged in a very short period of time, the static bleeder voltage can often reach several thousand volts. This is fatal for semiconductor devices. ESD may damage the internal structure of the LED lamp or the driver IC.
(3) Failure of component use
Since the switch-type drive circuit requires a large capacitor to store power and voltage, a large capacitor generally uses an aluminum electrolytic capacitor. The failure rate of aluminum electrolytic capacitors is higher than that of other components, and because the transformers and LEDs generate heat when they are used, these heats increase the electrolyte movement of the electrolytic capacitors and shorten the normal service life of the aluminum electrolytic capacitors.
(4) Working environment leads to
At present, the mainstream LED driver uses an alternating power source as the power input. For some high-power LED driver circuits, the transformer coil generates a large amount of heat, and the temperature stress of the LED failure is generated by the heat. The time model of temperature stress is shown in the following formula:
Where M is the temperature stress, T is the temperature, and t is the time.
It can be seen that the temperature stress rises exponentially with time and temperature. The longer the electrical appliance is used, the greater the temperature stress and the higher the failure rate caused by heat.
3.2 Linear adjustment LED driver circuit failure analysis Figure 1 linear adjustment row LED driver circuit for failure analysis, linear LED drive a power-on instant, AC power supply needs to charge the capacitor inductance inside the drive circuit, so, there will be a power-on A relatively large current flows through the fuse and the rectifier bridge. Since the Multisim simulation software can only simulate analog quantities, it cannot be simulated for ambient heat and humidity. So this simulation can only be simulated from the aspect of electrical parameters. Here, two failure factors, surge voltage and surge current are added to simulate the failure of the linear LED driver circuit described above, and the various instrument parameters of the circuit operation after the surge voltage is added.
From the contents shown in Fig. 1, the values ​​of the respective meters can be read.
Vi=250V; Vo=29.934V; Vled=8.415V;
Iled=34.606mA.
After several simulation tests, the comparison of the electrical parameters of the LED driver 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 grid fluctuates by 10%, the working state of the linear LED driving circuit changes relatively. From the above figure, it can be found that the voltage amplitude on the grid has little effect on the operating voltage of the linear LED driver circuit due to the adoption of a suitable voltage regulator circuit. However, its drive current has changed greatly, and the drive current has increased by 40% compared with the normal input voltage. This will cause the LED to work overloaded, which will reduce the life of the LED lamp bead and may even directly damage the lamp bead.
3.3 Failure Analysis of Switching LED Driver Circuit
The failure analysis is performed by linearly adjusting the row LED driving circuit in Fig. 2. In the simulation diagram of Fig. 2, XSC2 represents the input AC power supply and the voltage comparison after rectification. The sinusoidal wave type is AC220V. After full-wave rectification, its voltage value Vimax ≈ 311V is relatively stable after rectification. After the rectification voltage, the voltage read from the mark point in the figure is 11.368V. After the low frequency rectification, after the voltage passes through the self-oscillation circuit and the high-frequency rectification circuit and the voltage stabilization circuit, the output voltage of the LED is output.
Since the Zener diode 1 N 4 7 3 5 A is used, the voltage regulation value is 6.6V, so the theoretical value of the LED driving voltage is 6.6V.
The actual simulated LED driving voltage is read from Figure 2 as a 6.64V DC voltage. In line with the theory.
After several simulation tests, the parameters of several instruments can be read as follows:
Vi=250V; Vo=12.3V; Vled=6.64V;
Iled=47.416mA;
See Table 2 for comparison with the electrical parameters of the LED driver circuit under normal conditions.
It can be seen from Table 2 that when there is surge voltage input on the grid, the switching LED driver circuit, due to the use of good voltage regulation measures, the voltage parameters of the driver circuit have not changed greatly, but the LED driver The circuit changes and the increase is 100%. This will cause the LED power to rise and the LED to fail.
3.4 Failure Solution
After the simulation analysis of the LED driver circuit in the previous sections, the following effective LED failure solutions can be summarized:
(1) For inrush current and voltage, a fuse and a PTC resistor can be added to the power supply input. The PTC resistor is a positive temperature coefficient resistor. When the primary current of the power input has a surge current or surge voltage input, according to the heating formula of the resistor Q=R*I^2*T, the increase of the PTC current or voltage will increase the heat of the PTC resistor. Therefore, the resistance of the PTC resistor rises, so that the primary power to the power input is partially reduced at the PTC resistor to ensure that the output power of the secondary side of the power supply is constant, and the voltage and current are kept stable. For outdoor use LED drivers, lightning protection should also be added.
(2) For the selection of the driving device, in the cost range, a better device, especially a capacitor, should be selected.
Moreover, the maximum current and voltage parameters of the device must be 2~3 times of the rated value of the normal operation of the circuit, and the specific parameters are specifically selected. In order to ensure that the circuit components have sufficient redundancy to cope with the sudden changes in electrical parameters.
(3) At the same time, attention should be paid to the layout of the circuit board. The heat generation should be separated from the layout to reduce the influence of the heat board. The circuit board should be protected from moisture and moisture.
In some specific environments, some measures should be taken to prevent moisture.
(4) For the switching type LED driver circuit, it should also prevent the failure caused by EMI. EMI can be reduced by adding X capacitors, common mode inductors, differential mode inductors, low pass filter circuits, shields, and more.
4 Conclusion
Based on the analysis of the principle of two typical LED driving circuits, the failure simulation of the linear LED driving circuit and the switching LED driving circuit under the influence of surge voltage is carried out. It can be seen from the simulation results that the surge voltage has a relatively large influence on the LED driving power supply. Especially the part of the drive circuit. The LED drive current is increased beyond its maximum forward conduction current to disable the LED lamp bead. According to the simulation analysis results, a reasonable failure scheme of the LED driver circuit is finally given. (Author: Wen Jinbao, Peng Jie)
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