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From: Mike Poulton
Subject: Problems with MOSFET drivers
Date: 15 Nov 2002 14:07:09 GMT
Organization: MTP Technologies
I'm in the process of building a big (4kW) solid state tesla coil. The
basic idea is to use a mosfet H-bridge to drive the coil's primary from
a 300VDC source. There is a small feedback winding on the coil as well,
which provides a signal to switch the mosfets. The mosfets are driven
with a single toroidal transformer with one primary and four
secondaries. Each seconday goes to a fet, and they are phased to turn
on one pair on the positive cycle and the other pair on the negative
cycle. Therefore, the primary needs to be driven with a full bridge.
To do this, I am using a pair of drivers from Microchip, the TC4421 and
TC4422. These are supposedly bulletproof. They come in 5-pin TO-220
cases and are designed for 9A output. I have the pair (one inverting,
the other noninverting) in parallel pin-for-pin -- except the outputs,
of course. The output has a .1uF DC blocking capacitor in series with
the primary of the gate drive transformer. There is a 5.1V zener from
the inputs to ground, and a 4.7K resistor between the feedback winding
and the inputs. A .001uF cap goes from the inputs to ground as a low
pass filter in order to ensure that the coil oscillates in 1/4 wave
mode, not a higher mode. I have a .1uF bypassing capacitor right across
the IC power leads. In order to start oscillation, I have to give the
input a 5V pulse through a 330 ohm resistor in order to trigger a state
change and get things going.
Here's the problem: the drivers explode! Loudly and reliably. The top
of the package disappears and you get to see the die. If the driver
board is powered but the H-bridge is not, nothing happens. As soon as
power is applied to the bridge (only 12VDC, not the full 300) and you
give the input a 5V pulse, it goes nuts and the drivers explode. There
is no output from the coil during the brief time it operates. I tried
using a current limited supply and fresh ICs, and the driver board drew
the full 3A. That current should not damage the ICs, but it did -- they
show a short from Vdd to ground now. I've gone through six pairs of
drivers now, at $3.80 per chip. Seems clear that it's oscillating in a
bad way, but I can't tell how! I cannot comprehend what abnormal
operation would cause it to destroy itself like this -- they are
supposed to be super-tough devices. Any ideas?
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