TL431: Does the different positions of resistor R5 in the feedback circuit composed of TL431 and PC817 in a switching power supply have different effects on the loop? The position of resistor R5 in Figure 1 and Figure 2 is different

liujun

Prodigy 190 points

Part Number: TL431

The position of resistor R5 in Figure 1 and Figure 2 is different

Figure 1

Figure 2

What kind of switching power supply loop is suitable for the two different circuits in Figure 1 and Figure 2?

Does the different positions of resistor R5 in the feedback circuit composed of TL431 and PC817 in a switching power supply have different effects on the loop?

7 months ago

0 Walter Zollinger 7 months ago

TI__Expert 6660 points

Hello Liujun,

Please use the position of R5 in Figure 1. Due to the operation of TL431, R5 needs to be between Vout and the anode of the optocoupler and the cathode of the optocoupler needs to be connected to the cathode of the TL431 for the circuit to work correctly.

Thanks,

Walter

0 liujun 7 months ago in reply to Walter Zollinger

Prodigy 190 points

Hi，

Thank you very much for your answer

Question 1

May I ask if there are any relevant technical articles explaining or discussing the two different connection methods of Figure 1 and Figure 2?

At present, I have seen a 4-cell lead-acid battery charger that adopts the connection method shown in Figure 2. The TL431 of the charger adopts Type III loop compensation. Why does this circuit have problems when charging 2-cell batteries (the charger for 2-cell batteries only changes the voltage divider of TL431, which means the highest output voltage of the charger is 27.6V, while the other circuit parameters remain unchanged)?

Question 2

Why does changing the value of R or C in the RC network shown in Figure 1 cause the reference terminal voltage of TL431 to be lower than 2.5V (1.8V), as if the optocoupler PC817 is conducting prematurely? May I ask if there are any relevant articles or materials that mention that different parameters of the RC network can lead to early or delayed conduction of PC817?

Thank you!

+1 Walter Zollinger 7 months ago in reply to liujun

TI__Expert 6660 points

Hello,

I've never seen the connection configuration in figure 2 in any article that's why I don't recommend it. Changing the resistor divider values can affect the compensation and the RC network needs to be recalculated.

The TL431 has some leakage where is begins to conduct before vref = 2.5V. ATL431 has the least leakage.