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From: Mike Poulton
Subject: Re: Problems with MOSFET drivers
Date: 16 Nov 2002 18:43:15 GMT
Organization: MTP Technologies
References: <3DD53525.FDFAE50E@ieee.org> <3DD6840C.289DCA3C@ieee.org>
On 16 Nov 2002, analog said:
> Are you taking any measures to deal with the reactive currents in the
> gate circuits? Consider that gate capacitance is a few nano Farads.
> This is being driven through the leakage inductance of the gate drive
> transformer which could be anywhere from a fraction to several micro
> Henrys. Taken together, these numbers imply that the series gate
> resistance required to absorb the reactive energy and dampen gate
> circuit ringing would be on the order of 10 to 40 ohms (square root
> of L/C). An added benefit of including such resistors is that they
> end up taking most of the fCV^2 losses, leading to cooler, more
> reliable operation of the driver ICs. -- analog
A quick peek with the scope shows significant ringing at the gates, but
the peaks are about 20V and the valleys are at about 12V. Consequently,
it doesn't actually affect the switching. I'm seriosuly considering
getting rid of the gate zeners (since they don't do anything) and adding
10 ohms in series with each gate. I would be leery of using more than
10 ohms because of the reduced rise time. This would really be a good
application for a resonant driver, but I'm not sure I want to get into
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