Electronics > Beginners

Understanding P Mosfet drive

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Zero999:

https://www.eevblog.com/forum/beginners/mosfet-gate-breakdown-survivable/msg1826480/#msg1826480

ZeroResistance:

--- Quote from: Psi on March 25, 2019, 01:46:01 pm ---What kind of current do you need?
If it's low (under ~100mA) then optocouplers exists in your 10us specs at 100V.
Using something with isolated control gets around the interface problems of a fet and its gate voltage limits

--- End quote ---

Load current upto 5A.

ZeroResistance:
@Zero999 and @Ian.M

Thanks for the awesome circuits and sim's.
Are discretes the only way to go here? I mean aren't there any single chip driver for the P-channel fets?

Peabody:
Zero999, I'm tryng to understand your circuit.  If V2 is 5V, what are the collector and emitter voltages on Q3?

magic:
Zero999's circuit is roughly what I wanted to suggest.
With the Q4/Q2 totem pole, I think it might even be feasible to increase R2 and R3 10x to reduce power dissipation on Q3 which is currently 1W at 100V.


--- Quote from: Peabody on March 25, 2019, 05:18:58 pm ---If V2 is 5V, what are the collector and emitter voltages on Q3?

--- End quote ---
That's a standard BJT current source. B-E junction is basically a diode, so driving the base at 5V pulls the emitter up to about 4.3V. Then there is ~11mA current through R2. Due to the current gain of the BJT, only a small bit actually comes from the base and majority from the collector. Collector voltage dynamically adjusts to maintain that 11mA, here it will be 11V below the top rail.

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