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Bipolar gate driving

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ben106

Tue Dec 09 2008, 11:23AM

Registered Member #1374 Joined: Tue Mar 04 2008, 04:14PM
Location: Switzerland
Posts: 10

Hi,
i've a question about bipolar gate driving (+12V=on, -12V=off). Does this work with a gate drive transformer ?

Maybe the real question is : does unipolar gate driving work with a GDT ? What does the gate see if I feed the GDT (1:1) with 0-12V ? I feel it will see +/-6V, so it's a bit too low to be reliable. Of course, one has to put a dc blocking cap before the GDT, so 0-12V is +/- 6V in reality.

What does experience say ?

Ben

Steve Conner

Tue Dec 09 2008, 12:53PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Experience says:

The average voltage across any inductor or transformer winding must be zero. If you try to force it otherwise, smoke will come out until it's equal to zero again.

ben106

Tue Dec 09 2008, 01:11PM

Registered Member #1374 Joined: Tue Mar 04 2008, 04:14PM
Location: Switzerland
Posts: 10

So, with a GDT you will for sure be driving the gate bipolar --> have to have +/- 12V at the outputs of the GDT.

Thanks a lot for the answer, now it's clear to me !

Arcstarter

Tue Dec 09 2008, 04:17PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Oh, and as a side note, for a gdt for typical smaller mosfets, you can use one gate driver ic, or a totem pole transistor pair and a 1:2:2 for half bridge, 1:2:2:2:2 for fullbridge and 1:2 for single switch(of course :P). But, if you don't want to have to wind all of that, or don't have enough space on your ferrite toroid or some other reason, you can use an inverting and non-inverting gate drivers, which will work in opposition, making a 24 volt peak to peak voltage across the primary. That will give the extra voltage you need to turn full on. An example would be ucc37322 and ucc37321 9 amp gate drivers, or tc4422, and tc4421(i believe that is it). You would hook inverting driver to one side of the gdt primary, and the non-inverting up to a dc blocking capacitor(i have used .39uf, Steve Wards mini sstc uses .1uf) and then to the other side of the primary of the gdt primary.

If you already knew this, i just typed that for nothing. Hehe..

LithiumLord

Tue Dec 09 2008, 05:44PM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

I'd not try the single-driver concept. First, the drive current is twice higher, so if you want to pull each fet with 2 amps, the overall drive current will be 2 * 4 * 2 = 16A. Also you can't make a driver-side off-state of the bridge with only one driver IC.

Arcstarter

Tue Dec 09 2008, 08:06PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Also you can't make a driver-side off-state of the bridge with only one driver IC.

Ah, yea that might help alot to use two gate drivers :P. It really is not any harder to use two instead of one, and if made good can take up like 1 inch x 2 inches, or less. I and rather inexperienced(very may be a better word) and i made a pc board with the two drivers and it only takes up (goes to measure) 1 1/2 by 1/ 1/2. So, a one and a half inch square. Works well, small, looks alright, other than the solder joints. Best of all, EASY! CHEAP! Now i just need some F'ing 74hc14's and make a half bridge and i have a sstc.

jovica

Tue Dec 09 2008, 08:49PM

Registered Member #1790 Joined: Fri Oct 31 2008, 10:10AM
Location: Sydney, Australia
Posts: 40

Arcstarter wrote ...

Now I just need some F'ing 74hc14's and make a half bridge and i have a sstc.

I dont think 74hc14 are hard to get, Couldnt you just go to your local radio shack and get a bunch?

Anyway if they are not availble you can get them at electronics goldmine for 40 cent each I beleive.

HV Enthusiast

Wed Dec 10 2008, 12:01AM

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

wrote ...

Now i just need some F'ing 74hc14's and make a half bridge and i have a sstc.

Is it really necessary to include swearing as part of your reply? Sure, its abbreviated, but we all know
what it means and it reflects poorly on you.

Please try to refrain from using profanity in the posts and keep it clean.

Thank you

Arcstarter

Wed Dec 10 2008, 01:49AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Dr. GigaVolt wrote ...

wrote ...

Now i just need some F'ing 74hc14's and make a half bridge and i have a sstc.

Yea, sorry.

ben106

Wed Dec 10 2008, 07:45AM

Registered Member #1374 Joined: Tue Mar 04 2008, 04:14PM
Location: Switzerland
Posts: 10

Arcstarter wrote ...

If you already knew this, i just typed that for nothing. Hehe..

I would say, I knew a part of what you wrote ;) didn't have the idea feeding the GDT with more voltage to reduce secondary windings so that it takes less space. But anyways, it's always helpful information, also for others.

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