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From: firstname.lastname@example.org (Adam Seychell)
Subject: Re: Problems with MOSFET drivers
Date: 16 Nov 2002 03:10:21 -0800
NNTP-Posting-Date: 16 Nov 2002 11:10:22 GMT
Mike Poulton wrote in message news:...
> 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?
I'm impressed in your dedication and willingness to attempt such a
dangerous and high power project with the amount of understanding you
have as it appears in your posts. My hat is off to you.
* Study all the MOSFET technical litrature at www.irf.com. And get a
good understanding of power switches and how to drive them correctly.
There are many app notes on MOSFET drivers, also visit sites Telcom
* buy your semi's in quantity from a distributor since you'll end up
saving money after smoking more than you anticipated.
* get access to >= 60MHz CRO if you hanv't done so already.
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