Discussion on Coordination Circuit of PC817 and TL431

• Posted by Yang crystal April 27, 2023 - Filed in Other - #Circuit  - 182 views

  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, PWM will change linearly, so the current Ice of 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.

   

  

   

  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.