Questions about 2N3904 BJT maximum Vbe and Ib

[eem2am]

BJT max Vbe and Ib

Hello,

Regarding inrush into offline SMPS DC Bus caps………

I have an inrush limiter whereby a FET (M2) is switched on from a 12V power supply , in order to short out the inrush limiting resistor (R22) , after the inrush event at switch-on has taken place.

-the point is that the 2N3904 BJT Q4 gets a base current of 43mA peak during the inrush event.
-is this too much for it?
-the datasheet says nothing of maximum Ib.

also, during the inrush event, the BJT Q4 suffers a Vbe of 1.6V peak.
--is this too high a Vbe for a BJT like 2N3904, -the datasheet says nothing.

here is the schematic:
https://i53.tinypic.com/s14wv8.jpg


…by the way, , Q4 prevents M2 from spuriously turning on when the voltage across R22 suddenly rises.

 

[fg]

Re: BJT max Vbe and Ib

-the point is that the 2N3904 BJT Q4 gets a base current of 43mA peak during the inrush event.
-is this too much for it?
-the datasheet says nothing of maximum Ib.

For reliability, I would limit the base current to less than 10...20mA. I think 43mA is a bit on the high side.

also, during the inrush event, the BJT Q4 suffers a Vbe of 1.6V peak.
--is this too high a Vbe for a BJT like 2N3904, -the datasheet says nothinug.

If that's a forward voltage then it's too high. The reverse voltage can be 6 volts according to the datasheet (https://www.fairchildsemi.com/ds/2N/2N3904.pdf).

here is the schematic:
https://i53.tinypic.com/s14wv8.jpg

I wasn't able to view this for some reason (401 Authorization Required).

 

[eem2am]

Re: BJT max Vbe and Ib

Hi,

though given that this high Ibe and high Vbe only last for around 10ms do you still think it's excessive?

 

[FvM]

Re: BJT max Vbe and Ib

I don't want to add more speculations about unspecified maximum ratings.

But I don't see an actual need for Q4 and the ensemble of parts around it. A simple capacitor in parallel
to R23 would do the same ("prevents M2 from spuriously turning").