Newsgroups: comp.robotics
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From: jkottas@world.std.com (James A Kottas)
Subject: Oscillations in MOSFET H-bridge amplifier
Message-ID: <CI05ur.4J2@world.std.com>
Keywords: MOSFET, H-bridge, Motorola, oscillation, PWM, brushless DC motor ampli
Organization: The World Public Access UNIX, Brookline, MA
Date: Tue, 14 Dec 1993 02:06:27 GMT
Lines: 26

Problem with Oscillations in a Motorola MPM3003 MOSFET H-bridge

We are trying to develop a brushless DC motor control amplifier(PWM)
for use with small high-torque motors (in a Y field winding
configuration).  We are using a Motorola MPM3003 MOSFET H-bridge
amplifier and followed the Motorola guidelines for designing the
circuit.  However, we are plagued with high frequency (18MHz)
oscillations on the output of the H-bridge that also can be seen on
the gate drives.  Motorola's guidelines briefly describe the
oscillation problem and say the cure is to increase the impedance of
the N-channel gate drives.  However, we are at the limit of their
suggested impedance range (1k).

Without going into the details of our current drive circuit, does
anyone (1) have any insights as to the physics of what is causing the
oscillations and how can they be eliminated, or (2) have any
references on the subject?  We realize the potential for oscillations
is there given the inductive loads, the junction capacitances of the
MOSFETs, and the high gain of the MOSFETs.  It's the equivalent AC
circuit of the H-bridge/motor combination and an explanation of what
is going on that we're interested in.

Thanks in advance.  Responses can be either posted to the net or sent
via e-mail to jkottas@world.std.com.

Jim Kottas
