In this paper, a low-power LED driving circuit is designed by using the high-performance off-line PWM controller ob2532. The circuit adopts the primary side feedback mode. Compared with the traditional secondary side feedback, the primary side feedback driving circuit eliminates the optocoupler and TL431 chip, reduces the cost and improves the reliability of the system. The designed LED driving circuit has the characteristics of constant voltage and constant current control. The measured results show that four 1 w White LEDs work normally, the brightness is very high, and the test parameters meet the design requirements.
LED lighting has become the development trend of lighting in the future because of its characteristics of energy saving, environmental protection and long service life. Unlike ordinary incandescent bulbs, LED lamps can be directly connected to 220V AC mains power. They need low-voltage DC drive, so complex power conversion circuits must be designed. The service life of LED has a great relationship with the driving power supply. The unstable power supply of the driving power supply leads to the reduction of LED luminous efficiency, the shortening of service life, the change of color and even burning. In addition, conversion efficiency, constant current / constant voltage accuracy, power supply life, electromagnetic compatibility and other requirements are also factors that must be considered in the design of LED driving power supply. In this paper, an AC / DC LED constant current source driving power supply is designed, which adopts the primary side feedback mode. Compared with the traditional secondary side feedback optocoupler and TL431 structure, its biggest advantage is to reduce the cost and improve the reliability of the system.
1. Working principle of LED driving power supply
PWM (pulse width modulation) modulation mode is the most commonly used mode in switching power converters. The difference between the feedback signal and the reference signal at the feedback end is compared with the sawtooth wave generated internally, and then the square wave signal with constant frequency and wide width is output to control the power switch. The on time of the switch can be quickly adjusted according to the load, so as to stabilize the output voltage.
Switching power converters can be divided into two types from the control mode: voltage control mode and current control mode. The basic principle of voltage control mode is to generate PWM signal for control by comparing the output voltage of error amplifier with a sawtooth wave. The control principle diagram of PWM voltage mode is shown in Figure 1. Its principle is that the sampling resistors R1 and R2 detect the output voltage Vo, compare its input error amplifier EA with the reference voltage Vref, and input the amplified error voltage VEA to the PWM voltage comparator (pulse width regulator). Another input of PWM voltage comparator is sawtooth wave with period T. VEA is compared with sawtooth wave. When the sawtooth wave voltage is higher than VEA, the output of PWM voltage comparator changes from high level to low level, and Q1 is turned off, so as to adjust the on time of Q1 and ensure that the output voltage is constant.
Current control mode adds a current negative feedback link on the basis of voltage control mode. Figure 2 is the principle block diagram of PWM peak current control mode. The input of the PWM voltage comparator is changed from the sawtooth signal in the voltage control mode to the voltage vs (= iq1xrs) converted from the current sampling value. The other end of the comparator is still the error amplification value VEA between the output voltage sampling value and the reference reference. At the beginning of each cycle, the pulse signal control turns on the switch, and the current flowing through the switch and inductance increases. When the current increases to vs exceeding VEA, the trigger r end is set to high potential, and the switch is turned off. If the input Voltage VDC increases, the rising speed of VS is accelerated when the switch is on, and the time required for vs to exceed VEA is shortened, so the on time ton of the switch is shortened; On the contrary, when the input Voltage VDC decreases, vs exceeds VEA, the time required to turn over the PWM control signal is longer, the on time ton of the switch tube increases, and the energy provided to the load is maintained.
2. Selection of scheme
The volt ampere characteristics of LED are very similar to those of general diode, and the current increases exponentially, so a small change in power supply voltage will cause a large change in forward current. At present, the luminous efficiency of LED is still relatively low. Most of the input electric power is converted into heat energy, so its heating is very high. Because the temperature coefficient of the volt ampere characteristic of LED is negative, the increase of junction temperature causes the volt ampere characteristic curve to shift to the left, and the result is the increase of positive current. After the forward current increases, the input power of the LED increases when the power supply voltage is the same. However, after the junction temperature increases, the optical output will decrease, which means that more input power is converted into heat energy, that is, increasing the forward current, its optical output does not increase, but decreases, which leads to a vicious cycle of junction temperature increase. Therefore, the use of constant voltage power supply will increase the junction temperature, increase the light attenuation and shorten the service life. To sum up, the design adopts the constant current source driving scheme.
The primary side feedback AC / DC control technology is a new switching power supply control technology developed in recent 10 years. Compared with the traditional secondary side feedback optocoupler and TL431 structure, its biggest advantage is that it eliminates the two chips and a group of components working with them, which saves the space on the system board, reduces the cost and improves the reliability of the system. Ob2532 is a high-performance off-line PWM controller with primary side feedback. The primary side feedback technology is used to replace the feedback loop composed of chips PC817 and TL431 to reduce the circuit volume. At the same time, the chip integrates a proprietary constant voltage and constant current control, and its pin description is shown in Figure 3 and table 1.
3 circuit design
The LED driving circuit based on ob2532 is shown in Figure 4. After the input 220 V AC voltage is rectified by vd1-vd4, filtered by inductance L1 and capacitors C1 and C2, it is changed into DC voltage, added to the primary coil of the transformer, and then connected to the drain of MOS tube vt. The auxiliary winding N2 of the transformer provides power for ob2532 after passing through the rectifier diode vd5 and the filter capacitor C3 (initially, the higher DC voltage at the primary side ⑥ of the transformer is reduced by resistors R1 and R2). At the same time, the auxiliary winding N2 and resistors R3 and R4 also provide sampling feedback voltage for inv at the inverting end of ob2532. The resistance Rs connected to the source of the VT tube adds the detected current signal to the current detection input pin CS. Under the control of inv terminal feedback voltage signal and CS current signal, the pulse width of the gate drive signal of VT tube is adjusted, that is, the power supply and current are kept constant by PWM. The buffer network composed of vd6, R5 and C5 can make the reverse peak voltage consume its energy through diode vd6 and resistor R5 and reduce the reverse peak voltage, so as to avoid excessive voltage on winding N1 and damage VT tube during switching. At the secondary side of the transformer, the AC voltage is rectified by the diode vd7 and filtered by the capacitor C6 to obtain a constant voltage and current to drive the LED. The advantage of this circuit is that the primary side feedback control does not need to add optocoupler and voltage stabilizing source TL431, which reduces the cost. This design drives four 1 W LED diodes.
4 circuit test
4.1 primary side feedback characteristic test
In Fig. 4, N2 supplies power to IC through vd5 rectification and C3 filtering, and N2 also supplies sampling feedback voltage to inv at inverting terminal. Therefore, to test the primary side feedback characteristics, it is necessary to test the inv end of OB 2532, that is, test point ③. The waveform of point ③ of the secondary coil on the primary side of the transformer is shown in Figure 5. Although there are some burrs in the waveform, it well reflects the on and off of the switch tube.
As can be seen from the circuit diagram 4, the test point ⑥ is the DC voltage after rectification and filtering, which is measured with the 1000 V DC voltage gear of the multimeter. The test point ⑦ is connected with the VDD pin of ob2532, that is, the power supply voltage of the chip, which is measured with the 20 V DC voltage gear of the multimeter. In actual operation, pay attention to the selection of grounding point. There are 9 grounding points in Figure 4, including 7 on the primary side and 2 on the secondary side of the transformer. The secondary side directly drives the LED as an output, so the grounding points of the primary side and the secondary side are different, so they are represented by different symbols in Fig. 4. However, the grounding points of each side are the same. Here, the grounding point of capacitor C3 is selected. The specific measurement results are shown in Table 2.
4.2 secondary side output signal waveform
The test point ② in Figure 4, that is, the signal output from the secondary side of the transformer before diode filtering, is the key test point. Conduct waveform test with Tektronix tds21060 MHz 1gs / s digital oscilloscope, and connect 4×1 w LED as load. The obtained waveform is shown in Figure 6. It can be seen that the waveform is regular and meets the design requirements.
4.3 output parameter measurement
Firstly, the electronic load in constant resistance (CR) mode is used to test the current and voltage parameters. The specific operations are as follows: first measure the VO voltage at the output terminal with the DC voltage gear of the multimeter, that is, the voltage of the load is infinite (open circuit), which is 11.70 v. consult the relevant data, and the nominal current of the LED is 370 ma. Therefore, the load can be calculated to be about 32, so the electronic load can be debugged on this basis. The test results are shown in Table 3.
From the measured results, the brightness of four 1 w White LEDs is not very high after normal operation, and their luminous efficiency does not reach the highest. The circuit works under light load, so it should be further debugged to improve the luminous brightness. Here, the electronic load is still in the constant resistance mode, and the resistance is gradually changed. The recorded data are shown in Table 4.
The LED constant current driving power supply designed in this paper adopts the current mode PWM control of primary side feedback mode, which has the characteristics of small volume and low cost. It is suitable for low-power LED driving power supply, such as table lamp, ground lamp, small spotlight and so on. The test shows that the designed white LED driving circuit meets the design requirements.