MOSFET driver killed mysteriously and need help

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w2kwong's Avatar (by Gravatar)w2kwong posted on
April 18, 2007, 12:02 pm
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hi to everyone. I mysteriously killed 5 MOSFET half bridge drivers and
wish to see if anyone can help.

I'm making a N-MOSFET H bridge circuit that switch a sine wave output
using unipolar PWM for an inverter project. The gate driver I choose
was IR21834 and I began testing half of the bridge on breadboard
according to the datasheet circuit. Connecting input lines manually to
V+ or GND to produce the output, the lower MOSFET would first work,
but it always happened that somehow after changing the circuit such as
connecting driver input here and there or switching the value of
boostrap capacitor, at a random time the chip would go crazy . The
chip would feel hot touch, followed by increase in current drawn, all
of which signifying an internal short circuit. It seems the hide side
circuit is causing problem, since the low side output would usually
still functioning afterwards.

I believe such short circuit behavior can only be caused by shorting
high side output to ground or to V+, and I swear I didn't, nor did I
connect bootstrap diode and capacitor wrongly (I redid the circuit
several times and same error can't happen over and over again). I
thought it might be chip design problem but as I tried chip from other
manufacture, L6388 from ST, the same happened. Now I'm really
threatened as I don't want to kill any more chips. Although I might
find out the reason myself but that would probably take another 10
chips dedicated to destroy, and this is why I ask for help here.

While not expecting a direct reason to the problem, I am REALLY REALLY
glad if someone who have killed MOSFET driver before (for whatever
reasons) to share his/her story, and also tell me what to avoid when
working with drivers, thousands thanks in advance!


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e c kern's Avatar (by Gravatar)e c kern posted on
April 18, 2007, 3:11 pm
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Re: MOSFET driver killed mysteriously and need help

1. Connecting the input manually to vcc actually should not work.  The
driver you describe is a bootstrap-only type (as opposed to hybrid
bootstrap+chargepump) and so the output line must periodically go to
ground to recharge the bootstrap capacitor.

2. Are you just connecting HIN and /LIN?  Many IRF datasheets for
single-input drivers discuss having specific circuitry to prevent
shoot-through.  You should install a protective resistor on the rail
and use an oscilloscope to see if there are current spikes during
switching.  If there are, you must redesign your drive circuit to have
a deadband where both mosfets are open between high and low.

3. High-side drivers usually have a Vbs threshold to prevent
destroying mosfets by applying an insufficient drive voltage.  To some
extent this is more of a sanity check -- depending on your mosfets and
load, this threshold may not be high enough to fully protect your
circuit.  Verify that Vbs never goes below the (practical) "full-on"
Vds of your mosfet.

-chris.



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w2kwong's Avatar (by Gravatar)w2kwong posted on
April 18, 2007, 11:50 pm
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Re: MOSFET driver killed mysteriously and need help


yes I know. and that's why I first played around with low side output,
then pump in a low frequency square wave to test high side with
multimeter. Because of low frequency I used bootstrap cap 100uF. At
12V supply I checked HG output voltage is 20V when is high justfying
bootstrap was working, but then somehow it failed.


I thought the internal anti shoot through circuit which generates
500uS dead time is a bullet proof. Even if cross conduction occurs the
one who dies should be MOSFET, not driver.


If Vbs goes low, at most the chip shut down and deactivate MOSFET, but
this should not kill driver itself.

Thanks for your helping. This is the circuit I test with breadboard:
http://www.geocities.com/w2kwong/MOSFET.jpg


BRW's Avatar (by Gravatar)BRW posted on
April 18, 2007, 5:05 pm
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Re: MOSFET driver killed mysteriously and need help


First of all, let me say that an H-bridge PWM driver should not be
driven by a sine wave.  These circuits are designed to switch a square
wave PWM signal.  This may be the cause of your problem.

I will, however, assume that you meant square wave and typed sine
wave.

In my experience, here are a couple causes of MOSFET driver death.
Improperly sized gate resistor (the resistor connected between the
driver and the gate lead).  This is typically about 51 ohms.
Undersizing will overheat your driver.  Oversizing will overheat your
MOSFETs.
No 15V zener diode connected between the gate and the MOSFET source
lead.  This diode protects the driver.  Good drivers like the HIP4081A
have these built in.
Supply voltage sag - do not go below the minimum driver voltage from a
dying battery, power supply overload, etc.

Other words of advice:
I probably don't need to say this, but you must have flyback
suppression diodes across your MOSFET Drain/Source.  Many MOSFETs have
these built in.
Use about 100 uF per amp capacitor across your load supply lines. This
helps suppress flyback.

Overall, improper flyback suppression has been the biggest killer of
MOSFETs and drivers for me when I was new to this.  I recommend
looking at your driver output on a scope to make sure the signal is
clean.  You can expect some ringing and spikes of about 6V or so, but
no big spikes should be seen.

Hope this helps. I know how frustrating H-bridges can be.

BRW



w2kwong's Avatar (by Gravatar)w2kwong posted on
April 18, 2007, 11:42 pm
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Re: MOSFET driver killed mysteriously and need help


Sorry for my bad English and indeed it is square wave modulated with
sine signal @40kHz



An app note from fairchild suggested use of 4.7ohm as gate resistor,
and value too high will undermine efficiency.
http://www.fairchildsemi.com/an/AB/AB-9.pdf
Would 51 ohm being too high as it will slow down rise/fall time by too
much?


The gate is insulated from D and S and can be considered as floating
and there should not be any current flowing back to driver from gate.
why is it necessary to have zener diode in place?


I suppose low voltage may cause insufficient Vgs that result in MOSFET
overheat, but this should only kill the MOSFET not the driver.


I hadn't used freewheel diode as I haven't yet connect the bridge with
inductive load, for decoupling cap I used 10uF. Since I'm not loading
it with large current I think the cap value should be enough.


I thought driver+MOSFET  bridge is pretty straight forward stuff.
Although I've heard a lot how ppl blowed MOSFET, but very few
mentioned destroying the driver. As this is the 6th chip I killed, I'm
really frustrated. Thanks for your kind help!!!


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BRW's Avatar (by Gravatar)BRW posted on
April 19, 2007, 2:58 pm
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Re: MOSFET driver killed mysteriously and need help


efficiency.http://www.fairchildsemi.com/an/AB/AB-9.pdf

That's right. Use the recommended resistor for your component
selection.


Current is not the problem.  High voltages are the problem.  Don't
confuse theory and the real world.  It is highly recommended to use
15V zeners.


I think that about everything until I actually try to do it.  Golf
seems pretty straightforward, too :-)

BTW, how much current are you supplying?  If it's 3A or less you can
just use the LM18201 H-bridge.  They work great.

BRW



Peter Wallace's Avatar (by Gravatar)Peter Wallace posted on
April 21, 2007, 2:12 pm
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Re: MOSFET driver killed mysteriously and need help




A couple of things:

HBridge circuits with MOSFETS are liable to fail when breadboarded the
way you have done, --> way to much lead inductance for the high switching
speeds/currents involved.

Also 100 uF is too big for the bootstrap, this by itself could destroy
the driver on the first negative output swing if the Hbridge output
voltage is high enough

Peter Wallace

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