Reducing MOSFET Vgs

Forum 2005-2010 (read only) Hardware Interfacing
#1

Hi guys,

I’ve got the following circuit that is supposed to switch a battery voltage in the range 7-30v with a p-ch MOSFET.

So far so good and this basic circuit got the thumbs up here a few weeks ago. But at the time I was talking about a VBAT of 12v.

Now at max 30v I realise I’ll be exceeding the Vgs of probably any MOSFET, however I have found some with a 25v Vgs (FDS6675BZ) which is close.

My first thought was to put a 5v1 zener in place as shown, but this would probably not allow the P-ch device to turn on if VBAT was at the 7v end of the range as the GS threshold voltage is typically 2v.

So how about a resistor, R3 that forms a divider with R1 and is set to cause Q1’s gate to be say a 4-5v below the source when VBAT is 7v, and whatever at higher VBATs.

Rob

#2

I would replace R1 with a 12V zener and make R3 10k or so. That will work over a broader range than a voltage divider.

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#3

Thanks again RC, I'll do that.

Rob

#4