TL431: The work of TL431

Hi Guo,

Could you please answer the following questions so I can better understand your question. When you say "with the rising of Vo" are you referring to adjusting the resistor divider on the reference pin to achieve a larger output? Also are you trying to understand how a change at the reference pin effects the cathode? In which case I would like to point out the "Ratio of change in reference voltage to the change in cathode voltage" parameter found in the TL431 data sheet. 

Regards, 

Oscar Ambriz

Hello Oscar

The Vo is connected to the load. With the change of load current going down , I think the Vo will be pulled upper  momently.

So the divider on the reference pin(REF) achieves a larger input. Then the REF-Vref will also be plus , so that the cathode current will be increased than before. But I think Vo will finally return to the voltage before  the load began to change. Can you tell me the reason. 

Hi Guo,

I apologize but I have another question. When you speak about load current going down are you referring to a transient event where load current decreases and then returns? or are you referring to a situation where load current decreases and enough time has passed that steady state is reached?

Thank you,

Oscar Ambriz

Hi Oscar

I am talking about a transient event where load current decreases and then returns.

Hi Guo,

Thank you for your response. As you mentioned in your initial post feedback is used to control the value of Vo. When the change in load is sensed the internal resistance of the device is adjusted and the appropriate current is shunted resulting in a correct Vo output. Please let me know if this helps.

Regards,

Oscar Ambriz

Hi Oscar

I wanna know how the device including TL431 and external PNP transistor adjust the output.

Hi Guo,

We are looking into this and will get back shortly. Really appreciate your patience here!

Hi Guo,

Thank you for your continued patience. Could you please inform me what the output load and voltage of your application is?

Regards,

Oscar Ambriz

The device is used in my flyback output side to make the voltage stable.

Pls see the circuit adapted:

Hi Guo,

Thank you for providing the circuit capture. 

From the circuit you provided I can see that you are using two references at the output. Is there a reason for this? SZBZX84C10 from my understanding is a 10V Zener Voltage regulator and TL431 appears to be set up as a 8V reference. Both of these references appear to be attached to the same node. Is there a reason for this? 

Regards,

Oscar Ambriz

Hello Oscar

 SZBZX84C10 is not mounted in our case.

Hi Oscar

Thank you for your response.

What I wanna know is how the PNP and TL431 work during the transient.

I think the TL431 and PNP work together to keep the output voltage stable while the load is fluctuating.