What do you mean by "large H bridges"?  Current, voltage, or... ?

Cheers,
-Neil

On 12/16/2014 2:36 PM, peter green wrote:
> Using P-Fets for the high side switching is simple because the required
> gate drive voltages sit within the main power rails, So as you indicate
> you can easilly use a simple transistor/resistor combination to produce
> the gate drive voltages at least for small H bridges*.
>
> Due to the asymetric nature of semiconductor processes P-Fets have worse
> 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
> FET fully on it's gate voltage must be taken above the main supply
> voltage. That means a seperate power supply merely for gate driving.
> Furthermore it introduces new failure modes, if the gate drive supply
> drops to an intermediate level (say because the switching transistor in
> a boost coverter failed) then the FET will start acting as a voltage
> follower. Having what was meant to be a switch start acting as a voltage
> follower is likely to lead to overheating.
>
> For large H bridges the costs of correctly driving the gate on a N
> channel high side drive mosfet are worthwhile. For small H bridges they
> generally are not.
>
> * For large H bridges gate transistion time can become a consideration.

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