Overvoltage protection circuit

[vaibhavwaman]

Hello,

In mentioned circuit there is overvoltage protection is implemented at input using P channel MOSFET and transistor. When there is voltage higher than D2 it starts conducting and then respectively Q2 also starts conducting making Q1 turn off. What will be the voltage across Vgs in case of overvoltage ? Is shorting gate and source of P channel making them turn off ? Or Q2 goes into active mode and this voltage drop is turning of Q1 ?

 

[KlausST]

Hi,

I expect it to be close to zero. Just an expectation.

But you posting a riddle by hiding Q2 and Q3 informations.
You already know that Q2 is responsible for shutting down V_GS ...
So what´s the idea of hiding the most important part to answer your question?

Klaus

 

[Easy peasy]

It is a poor ckt, there is too much linear variance in when Q2, Q3 will turn on - there should be rather a snap action with hysteresis

 

[vaibhavwaman]

Hi,

I expect it to be close to zero. Just an expectation.

But you posting a riddle by hiding Q2 and Q3 informations.
You already know that Q2 is responsible for shutting down V_GS ...
So what´s the idea of hiding the most important part to answer your question?

Klaus

Sorry It was not mentioned in schematic. As per BOM Q2 is PDTA124EU,115 and Q3 is PDTC114EU,115. my intention is to use low cost descrete based overvoltage protection ciruit. Do you recommend to use this ciruit for overvoltage protection ?

 

[KlausST]

Do you recommend to use this ciruit for overvoltage protection ?

Hi,

I´m an electronics designer for decades now. Mainly highly reliable industrial equippment.

And what I´ve learned: First one needs requirements. numbers.
In your case: what´s the highest voltage you want to protect against, how fast does it need to react ... and so on.
What´s the nominal Q1 current, what´s the allowed voltage drop at Q1? What happens when the overvoltage is just at the edge and Q1 voltage is high and current is high resulting in high power dissipation. What happens when there was an overvoltage situation and voltage comes (slowly) back to normal level?

What´s unclear to me:
In case of overvoltage, you disable the SMPS. Why? It disables C1, C3, C4 from being discharged. So they stay at a high voltage level.

Regarding protection:
There are several devices involved: I guess you want to protect the SMPS IC and maybe the Cs against overvoltage, using Q1, Q2, Q3,
If you want us to validate in detail ... you should give links to the datasheets, so we and you have the same sources of informations.

And btw: nowadays there are free and easy to use circuit simulators. Then you can play around and see what happens.

Klaus