Hi Robert,

Thanks for your reply. Please see my responses below.

On 3/26/07, Robert Rolf <Robert.Rolf@ualberta.ca> wrote:
> Why not break your FETs into multiple banks and drive then with a
> multiple phase low off duty cycle signal?
> You could then use bootstrap drive mode. (clamp diode to give fast
> recharge recovery)
> If you have 8 Fets, you'd have 1 of 8 off for a few milliseconds
> every second so at most you'd need 1 extra Fet to cover the load
> of the one that is off.l

That's not all that bad an idea but I had a feeling that there was a
simpler way to do this which was a standard "easy way" to do this kind
of thing. Brent seems to have such an answer.

> Huh? The spec sheet says it does DC.
>
> "To ensure static DC operation an integrated recharge path charges the
> bootstrap cap while the driver is switched off."
>

That's what I thought, too. However, read further and you will see
that they are never clear about how that would work. There is no
charge pump or other boosting supply present. Also, they talk about
the bootstrap cap being charged "when the driver is off", which sounds
suspiciously like a normal bootstrap driver. The "recharge path" they
show would only help when the driver was in fact off. It shows no way
of charging while the FET is on. My tests confirmed this - after turn
on the gate drive slowly fell off until the source was lower than the
gate drive bias supply.

I think it must be poorly translated or misunderstood by the datasheet
writer. I think what they actually mean is that it is like a normal
bootstrapped driver except that they provide an internal path to
recharge the cap so you do not need a low-side switch (which would
normally ground the source node and charge the cap).

> And you can't use a relay because???
> Or bridge the Fets with a relay after their initial operation?

We looked at relays and couldn't find anything which could break 80VDC
and also carry 100A while closed for anything less than $90 USD in
large quantity. Most power relays are only spec'd to 32VDC.

Sean


>
> Robert
>
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