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Few DRSSTC questions

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Marko

Wed Apr 19 2006, 02:05AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Since I was unable to get UCC37322 and 21 chips I needed to make gate drive stage using only ICL7667s wich have no enable pin.

I improvised AND gates using fast diodes, trying to leave out additional ICs if possible.
One inverter will also need to be replaced by mosfet and pullup since I ran out of 74HC14 'not' gates.

I need to know if this is a good approach or I should use something more robust like 74LS08 AND gate.

Secondly, I intend to leave out overcurrent protection in such low-power coil (at least for now),
or is there some good reasonwhy should I keep it?

I do not plan to use doubler, so each IGBT will switch 150V maximum and I won't stress them too much.

schematic (part of it):

EDIT: bug in schematic - invertr must alse be put at the input of second ICL.
There is also a possibility of stacking multiple IC's.

HV Enthusiast

Wed Apr 19 2006, 11:41AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Firkragg,

Those ICL7667 MOSFET drivers are not going provide enough current to charge up the gates on the IGBTs you have there. I believe the ICL7667 has an output ON resistance of 4-12 ohms and can provide only 1.5A peak current.

Marko

Wed Apr 19 2006, 12:35PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Sorry all, I wrote this about 4 in the morning, and I suceeded to mess up forum format with that pic (fixed now).

You can see that I paralelled two drivers in each chip (so I have about 3A peak current) but it still seems diminutive to 18A peak in original schematic.

Could I use 7667's to drive mosfets (like IRFZ44) and switch GDT with them?

vasil

Wed Apr 19 2006, 07:42PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

would it possible to make a enable like function by pulsing the power to the chip drivers?

http://www.geocities.com/gemaria_ro/ppd.html

If yes, it would be simplier to use some Tc44xx

Marko

Wed Apr 19 2006, 09:16PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Definitely no, as you would again need to switch power supply quickly with high peak current (pointless since driver does it) and even if you could do that it is not recommended as output of IC (depending on construction) may hang in undefined (high impedance) state when power is switched off, not swinging down to GND.

You need to use external logic to emulate enable pin.

I hope that a bridge of mosfets will do the trick in my design, rather than stacking bunches of 7667's on top of each other.

Avalanche

Wed Apr 19 2006, 09:54PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I've driven a fullbridge of IRF740 MOSFETs at a fairly high frequency (800Khz) with just 2 ICL7667s, so I would say it is perfectly possible. The small gate capacitance of those MOSFETs is not much of a challenge for the ICL7667s, although they do need a heatsink.

I would say you could drive a monster of a GDT with a fullbridge of IRF740s or something. Only limitation is they are .5R on resistance...

HV Enthusiast

Wed Apr 19 2006, 11:17PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

I'd recommend getting a version (student perhaps) of PSPICE and simulating your circuit with given gate loads just to see how it works before building it. PSPICE can give you wonderful insight into the operation of your circuit before solder one wire together. Use the K_Linear model for the gate transformer and a coupling of 0.99 to 0.998 usually works well for tightly coupled toroidal gate transformers.

vasil

Thu Apr 20 2006, 09:25PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

I found a schema with external logics on Jason's page . Maibe it is useful for you.

Marko

Fri Apr 21 2006, 03:04PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I rather did some more real tests. Breadboard may be a bit stray(ish) but that didn't make much trouble.

The dode thing works, and even better than I expected. There is also a bug in schematic, i forgot to put inverter on input of second ICL.

I pulsed the whole thing at 400kHz with NE555 at 50% duty cycle and hooked it to my interrupter,
I get nice square wave at outputs of ICL's as expected, with about 50us on time.

I used 22nF cap as load (30N60 has some 18nF) to test driver's capabilities.
Its not even that bad without GDT, but when I try the same with GDT, something very strange happens:

IC goes into some kind of 'state' where he attempts self destruct, drawing enormous current, after output of GDT was temporarly overloaded/shorted. Output GDT waveform on scope starts to decrease slowly and GDT buzzes loudly at interrupter frequency.
Even when I remove all the load IC still seems shorted, drawing a lot more current and would probably blow up probably blow up if I didn't disconnect it form power supply.

IC wasn't damaged, just heated up quite a bit. I repeated this several times, and I still have no clue what is actually happening.

This happens with any capacitive load bigger than about 2,2nF no matter how much chips are stacked.

It may be some of 'protection' mchanisms in IC; some strange parasisic oscillation, I don't know.

I didnt try to debug this but I just used a full bridge of complementary bipolar transistors (two totem poles).
Thex were BD711 and 712, with bases directly conncted to ICL driver for max peak base current.

H bridge could itself drive 100nF load, with very little waveform deformation.

GDT ruins it a bit, this is a GDT primary waveform, when I load GDT with 22nF change is barely visible. I figured otu that I can filter the ridges with bigger decoupling caps (for now they are just 100nF). I have to find some good tantalum ones.
I use 3,3uF DC blocking cap in series with GDT primary.

Unloaded secondary, with zeners gives almost exact signal of about twice primary voltage, again nice as expected.

And big dissapointment in the end: I connected the 22nF cap, with its resistor, diode and zeners to output of GDT. Scoped, it shows some crappy sinusoidal waveform on it.

Primary signal is almost untouched so transistors obivously aren't overloaded.

I dont know what could I do more, I could try to rewind GDT (this one has primary somewhat separated from secondary, and I could wind both windings together for instance, maybe to reduce.

Also, I tried a mosfet driver using GDT with two primaries. Results were bad, each primary would caouse huge voltage spike when turned ott, and I couldn't do anything with such a signal.

ragnar

Fri Apr 21 2006, 11:12PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Sounds like the IC is latching.

Can you build multiple gate driver stages in parallel?

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