TL431 is a precision voltage regulator, and PC817 is a photocoupler. In switching power supplies, the design of the voltage regulator feedback circuit is usually carried out with the cooperation of TL431 and PC817. In the flyback power supply design, the feedback circuit often uses them as a reference. Therefore, the cooperation between the two is always a topic that engineers like to talk about. This article comes from the technical expert of the forum and uses the typical application of TOPSwicth to illustrate the cooperation between TL431 and PC817. First of all, let’s take a look at TOPSwicth-based, TL431, and PC8

TL431 and PC817 use the circuit diagram Next, with reference to Figure 1, the parameters in the circuit diagram will be analyzed and explained. If you want to understand the relationship between the two, you must first determine the values ​​of the four parameters R1, R3, R5, and R6 in the TL431 part of Figure 1. Let the output voltage be Vo, and the rectified output voltage of the auxiliary winding be 12V. The circuit compares the output voltage with the reference voltage formed by TL431, and controls the C pole of the TOP tube through the current change of the photocoupler PC817, so as to change the PWM width and achieve the purpose of stabilizing the output voltage. Because the controlled object is the TOP tube, it is first necessary to understand the control characteristics of the TOP tube.

From the technical manual of TOPSwicth, it can be seen that the current Ic flowing into the control pin C is inversely proportional to the duty cycle D, as shown in Figure 2

Figure2-Relationship between TOPSwicth duty cycle and control current

The current of Ic should be between 2-6mA, and PWM will change linearly, so the current Ice of the PC817 triode should also change in this range. And Ice is controlled by the diode current If, through the relationship curve between Vce and If of PC817 (as shown in Figure 3), the forward current If of the PC817 diode can be correctly determined. It can be seen from Figure 3 that when the forward current If of the PC817 diode is about 3mA, the collector-emitter current Ice of the triode changes at about 4mA, and the collector-emitter voltage Vce changes linearly in a wide range, which meets the control requirements of the TOP tube.

Figure 3 The relationship between PC817 collector-emitter voltage Vce and forward current IF

Therefore, it can be determined that the forward current If of the PC817 diode is 3mA. Looking at the requirements of TL431, it can be known from the technical parameters of TL431 that when Vka changes from 2.5V to 37V, Ika can vary in a wide range from 1mA to 100mA. Generally, 20mA is enough, which can not only work stably, but also provide some dead load. However, for TOP devices, because the dead load is very small, only about 3-5mA can be selected.

The above relationships are very important. With their foreshadowing, the resistance values we mentioned at the beginning of the article are easier to determine. According to the performance of TL431, R5, R6, Vo, and Vr have a fixed relationship: Vo=(1+ R5/R6) Vr

In the formula, Vo is the output voltage, Vr is the reference voltage, and Vr=2.50V, first take a value of R6, for example, R6=10k, and R5 can be calculated according to the value of Vo.

Then determine R1 and R3. As mentioned above, the If of PC817 is 3mA, and the value of R1 is 470Ω, then its voltage drop is Vr1=If* R1. According to the PC817 technical manual, the typical value of the forward voltage drop Vf of the diode is 1.2V, then The voltage drop Vr3=Vr1+Vf on R3 can be determined, and the current Ir3=Ika-If flowing through R3 can be determined, so the value of R3 can be calculated: R3=Vr3/Ir3= (Vr1+Vf)/(Ika-If )

According to the above calculation, the cathode voltage value Vka of TL431 can be known, as Vka=Vo’-Vr3, where Vo’ is 0.1-0.2V larger than Vo, for example, Vo=15V, take R6=10k. R5=(Vo/Vr-1)R6=(12/2.5-1)10=50K; Take R1=470Ω, If=3mA, Vr1=If R1=0.003*470=1.41V, Vr3=Vr1+Vf=1.41 +1.2=2.61V.

Take Ika=20mA, Ir3=Ika-If=20-3=17, R3= Vr3/ Ir3=2.61/17=153Ω.

The cathode voltage value Vka of TL431, Vka=Vo'-Vr3=15.2-2.61=12.59V.

Results: R1=470Ω, R3=150Ω, R5=10KΩ, R6=50K.

In this way, the resistance values of several key resistors are successfully obtained. However, some friends may not fully understand it, and more detailed supplements from technical experts will be attached below.

Regarding the value of R6, the resistance value of this parameter is not determined arbitrarily. There are two factors to consider, first, the current at the TL431 reference input. Generally, this current is about 2uA. In order to avoid the current at this terminal from affecting the voltage division ratio and avoiding the influence of noise, generally the current flowing through the resistor R6 is more than 100 times the reference segment current, so this resistance should be less than 2.5V/200uA=12.5 K. Second, the requirement of standby power consumption. If there is such a requirement, try to take a large value under the condition that it is less than 12.5K.

TL431 requires a working current of 1mA, that is, when the current of R1 is close to zero, it is also necessary to ensure that TL431 has 1mA, so R3≤1.2V/1mA=1.2K. In addition, there are also power consumption considerations. The value of R1 should ensure that the TOP control terminal can obtain the required current. Assuming PC817A, it's CTR=0.8-1.6, the lower limit is 0.8, and the maximum current flowing through the photodiode is required to be 6/0.8=7.5mA, so the R1 Value≤(15-2.5-1.2)/7.5=1.5K, the maximum current that the photodiode can withstand is about 50mA, TL431 is 100mA, so we take the maximum current flowing through R1 as 50mA, R1>(15-2.5-1.3 )/50=226 ohms.

In order to increase the gain at low frequencies and suppress low-frequency ripples, R5C4 is required to create a pole at the origin. That is the static error. R4C4 forms a zero point to increase the phase. It should be placed in front of the bandwidth frequency to increase the phase margin. The specific position depends on the phase of the rest of the power part at the design bandwidth. The lower the frequency of R4C4, the lower the frequency. The higher the boosted phase, of course, the maximum is only 90 degrees, but the low-frequency gain will also be reduced when the frequency is very low, generally placed at 1/5 of the bandwidth, and the boosted phase is about 78 degrees.