BEWARE of dead gate drive IC that still works

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Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

Hi All. 

(this post is SSTC/SMPS related, and is longwinded)

Maybe not just Maxim. Possibly other manufacturers also, but I've
seen this a LOT with Maxim specifically.

I had a circuit set up as follows:

PWM controller (NTE1729) that puts out a 100kHz to 800khz square wave
that is 12V. This was voltage divided with 2 resistors to get 5V, and
fed straight into the input pin of a MAX4420 gate drive IC. The
MAX4420 and MAX4429 IC's are simple but very powerful. They put out a
high-current high voltage output that will drive big power MOSFET's
directly, with logical inputs. (a typical gate drive IC, but read
futher)

Maxim's awesome IC:

The datasheet says it will slam 18 volts into 2500pF (MOSFET gate) in
20nS! And claims the output impedance is 1.5 ohms! I got datasheet
performance easily even on a breadboard, it is a wonderful device but
unfortunately, like the other chips, the output is ground referenced.
So no good for half or H-bridge SSTC's unless you do something about
that (opto, pulse xfmr, PVI, etc.) 

The Supertex HT0440 IC has ISOLATED OUTPUTS to 400 volts!! I/we need
to play with that IMHO. I think it works to 1Mhz.

To my main point:

Say like, for instance, just speculating, this didn't really happen
to me now ; ), that your gate isolation transformer kicked back into
the chip and fried it. Oops. I swear some 12V zener diodes literally
jumped out of my parts drawers and soldered themselves onto my board
after that happened ; )

Failure mode:

The failure mode is typical I've heard, but strange nonetheless. It
seems the output stage of my chip failed......the sucker still works,
but it won't put out any current! In other words, when I take the
MOSFET load off of the output of the chip, the chip's output looks
like a beautiful 20nS risetime, 50nS falltime square wave (along with
a little ringing). But as soon as the MOSFET is back in, the output
waveform looks like a triangle wave riding on 12VDC ; ) Loads it down
like a ______.

This would lead the user to believe that the chip doesn't have enough
output current, when actually, the chip is fried.

After destructively testing several more of the IC's (which included
MAX4420's and MAX4429's), I determined that the chip fails when the
steady-state supply to the primary of the isolation transformer is
disturbed. This is when:

1). The chip's supply voltage is applied.
2). The chip's supply voltage is removed.
3). Connection to the gate drive transformer is disturbed. (bump it)

If the chip fails upon power-up, it will look like the chip doesn't
have enough juice to drive your isolation transfomer. Thats what got
me the first time, so beware.

Obviously, applying Vcc and removing it is a must. So something has
to be added to protect the chip. I'm thinking a couple of
back-to-back zeners. Other methods slow down the rise and fall times
of the output waveform (which is ultra-critical 'cause it controls my
H-bridge running at 1.5kW!) 

I'm looking for a maximum 100nS for rise and fall times. To keep the
MOSFETs biased out of their linear regions as much as possible. To
increase efficiency and make the heatsinks smaller ; ) (even though,
in SMPS terms, typical SSTC H-bridges operate as ZVS (zero voltage
switched) power supplies, where the current/voltage at switching time
is zero.) But.......that's only when it's perfectly tuned........so
unless you've got a PLL going on you're hot!

Didn't mean to ramble so much. 

Anyway, thoughts?

Justin Hays
KC5PNP
Email: justin-at-hvguy-dot-com
Website: www.hvguy-dot-com


















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