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Old 28th May 2007, 09:03 AM   #1
Pierre is offline Pierre  France
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Default Driving large mosfets: active PNP turn off

What characteristics should I look for the PNP transistor in an active PNP turn-off circuit for driving large TO247 mosfets? It is intended to be after the IR211x driver output.

-I guess VCEmax around 25V is more than enough, as Vce=Vgs.
-A high max. collector current (2 to 10 amps) is desired for really fast turn off, right?
-What about Hfe min, transition frequency, etc?

I want to run it at 350KHz approx.

I have found some candidates such as :

MJD2955
ZXTP2012ZTA
MJD45H11TM

Do you think they are suitable? Would you recommend any other part number?

Thanks!
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Old 28th May 2007, 09:56 AM   #2
alfsch is offline alfsch  Germany
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my favorite:
zetex fmmt717 , 10 A peak !!!! , sot23 , hfe > 200, 110Mhz
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Old 28th May 2007, 10:07 AM   #3
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Don't the IR211x gate driver IC already have active gate turn off?

Also they can switch at 120ns..???
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Old 28th May 2007, 01:42 PM   #4
fredos is offline fredos  Canada
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A 2n3906 is enought! The peak to discharge a gate is about fews ns....Except if you use a tick trace on your PCB, IR2110 is enought to discharge it. The only draw back, is the dissipation. Here you need a PNP discharge help...

Fred
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Old 28th May 2007, 02:13 PM   #5
Pierre is offline Pierre  France
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Well, I have seen some peak before turning on in the Vgs voltage that I don't like, it grows (up to 6-7V or more) as duty cycle is increased. I believe it can be due to reverse transfer capacitance turn on, so I think that a PNP turn off will help keeping the Vgs voltage down. Right?
I also want my IR2110 working more relaxed...
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Old 29th May 2007, 03:57 PM   #6
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if you change the gate driver to ir2010 isnt easier?
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Old 30th May 2007, 10:52 PM   #7
Gyula is offline Gyula  Hungary
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Hi! Why don't you use TC4422 and ADuM isolator close to GS for a few ns?
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Old 31st May 2007, 09:39 AM   #8
Pierre is offline Pierre  France
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Well, about IR2010, nice, but "only" 200V max. Vcc. A bit tight if you want +/-90V rails, isn't it?
Adding an optoisolator (or similar) and mosfet driver is a good idea, but I want to keep it as similar to the original design as possible, by the moment (using only IR2110).
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Old 31st May 2007, 11:09 AM   #9
IVX is offline IVX  Russian Federation
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Follower using it's good idea indeed, ir2110 almost have not chance to live, after the mosfet blew up, bc807/817 will have much more chance even. On the other hand, why you need 2A($2) driver, if followers in use anyway? So, you can get more reliability for less money.
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Old 31st May 2007, 12:00 PM   #10
Pierre is offline Pierre  France
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The idea is simply to update the existing design for faster turn-off, by the use of a typical active-turn off PNP transistor.
The turn-on is still driven by the IR2110, so you need 2A. This way the chip will work more relaxed, too, as it doesn't have to sink the high turn off current.
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