My first guess was going to be shoot-through currents, even before I saw =
the=20
schematic.  It is best to drive the complementary fets with dedicated=20
drivers that are interlocked in some way.  You can get driver chips desig=
ned=20
to do this, or you can do it with coordinated sofware pulses, or you can =
use=20
a simple totem-pole bipolar transistor network to drive them designed so =
one=20
is always off when the other is on.  As long as they are never on at the=20
same time, it is OK, but if tehy are both on, even for a fraction, things=
=20
start to get hot.

--Lawrence Lile
Electronic Solutions
Project Solutions Engineering
www.projsolco.com

>From: davedilatush@comcast.net (Dave Dilatush)
>Reply-To: "Microcontroller discussion list - Public." <piclist@mit.edu>
>To: "Microcontroller discussion list - Public." <piclist@mit.edu>
>Subject: Re: [EE] H-bridge problem
>Date: Sat, 26 Feb 2005 19:35:09 GMT
each PWM cycle where you turn off the P-channel
>device and turn on the N-channel device, the P-channel device
>stays ON for a little while; the resulting shoot-through current
>is what's causing the P-channel MOSFETs to get hot.
>
>Replacing the 4K7 with a 2K2 speeds up the P-channel devices'
>turn off, reducing the problem somewhat.  To eliminate it
>altogether I think you need to drive the P-channel devices with
>something that will turn them off quickly-- maybe a complementary
>PNP/NPN follower or something like that.
>
>Dave D.
>
>--
>http://www.piclist.com PIC/SX FAQ & list archive
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