Neil wrote:
> Hi all,
>
> I'm designing an H-Bridge to drive a 12V motor from a PIC, and I=20
> understand most of the basics and math, but not clear on when to use=20
> P-channel and N-channel FETs vs all 4 N-ch FETS.  I expected I'd use=20
> PFETs & NFETs, with smaller transistors on the gates of the P-FETs to=20
> get the gate up to 12V to switch off properly, but I've see examples to=20
> the contrary (using all N-FETs).  Is there a specific reason?
>  =20
Using P-Fets for the high side switching is simple because the required=20
gate drive voltages sit within the main power rails, So as you indicate=20
you can easilly use a simple transistor/resistor combination to produce=20
the gate drive voltages at least for small H bridges*.

Due to the asymetric nature of semiconductor processes P-Fets have worse=20
performance than N-Fets. So people started trying to use all N-Fets.

However using N-Fets for high side switching is problematic. To turn the=20
FET fully on it's gate voltage must be taken above the main supply=20
voltage. That means a seperate power supply merely for gate driving.=20
Furthermore it introduces new failure modes, if the gate drive supply=20
drops to an intermediate level (say because the switching transistor in=20
a boost coverter failed) then the FET will start acting as a voltage=20
follower. Having what was meant to be a switch start acting as a voltage=20
follower is likely to lead to overheating.

For large H bridges the costs of correctly driving the gate on a N=20
channel high side drive mosfet are worthwhile. For small H bridges they=20
generally are not.

* For large H bridges gate transistion time can become a consideration.
--=20
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