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Old 17th May 2006, 07:10 AM   #1
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Default Drive current for MOSFet's

Well I have made (partially) a SMPS, I haven't hooked up the output diodes or regulation yet because of a few issuse I have...

Anyways, I'm currently driving the MOSFet's (2 per half of the transformer (push pull)) and I'm getting very good square waves on the gates and drains but I have about ~1V sitting on the gate of the set that should be off but isn't when the next set comes on (do you understand what I mean? if not ask for pictures).

So, I see a lot of designes using a bipoler stage before the fets for higher drive current.

One of my questions is how much current is required to turn a MOSFet on and off fast enough? The ones I'm using are FKV550 they have a 2.7nF(2700pF) gate charge.

Second will one of these work 2SC945.pdf or 2SC3421?

Third, does ~1V at the gate really matter?

P.S each set of MOSFets are running at 32KHz +/-5%, over all frequency is 64KHz.

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Old 17th May 2006, 11:30 AM   #2
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Assuming this is something for powering a car amp. You don't say how you are driving the mosfets right now but if you are using a xx3525 then it will drive the fets directly at 32kHz no problem at all. Is there a choke between the secondary side bridge rectifier and the output filter caps, or does the bridge feed the caps directly? If the latter then the mosfets have a *very* easy life at turnoff so the gate drive can be very primitive and still work ok. Of course, the no-choke setup also means you can't use PWM to get voltage regulation of the output. Need the secondary side choke for that to integrate the pulse width to a dc value instead of simply the peak voltage.
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Old 17th May 2006, 05:20 PM   #3
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Sorry about that, I thought I had all the info in there.

I am driving the MOSFet's right off the SG3525 with 22 ohm gate resistors and I am planning on using output filters and voltage regulation.
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Old 18th May 2006, 10:41 AM   #4
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If your question is whether a "low" level of 1V on the gate will do any harm, you cn read the answer from the MOSFET data sheet: Threshold voltage of the FKV550T is min 1.0V, max. 2.5V for Id=0.25 mA(!).

If your'e rather the visual type, look at the Id=f(Vgs) graph (2nd on first row) - nothing at all happening under 2V there.

As for the other question - at 32kHz, the setup is plenty fast enough: a 3nF Cgs will discharge from 10V in 150ns with I=200mA (1/2 the 3525's output rating); your 22 Ohm Rg (one per gate, I hope) * Cgs yields a time constant of 66 ns.
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Old 19th May 2006, 03:28 AM   #5
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Quote:
Originally posted by wine&dine
If your question is whether a "low" level of 1V on the gate will do any harm, you cn read the answer from the MOSFET data sheet: Threshold voltage of the FKV550T is min 1.0V, max. 2.5V for Id=0.25 mA(!).

If your'e rather the visual type, look at the Id=f(Vgs) graph (2nd on first row) - nothing at all happening under 2V there.

As for the other question - at 32kHz, the setup is plenty fast enough: a 3nF Cgs will discharge from 10V in 150ns with I=200mA (1/2 the 3525's output rating); your 22 Ohm Rg (one per gate, I hope) * Cgs yields a time constant of 66 ns.

Wow, I can freeze these bastards to -55C and it's still over 2V turn on

I have a 22 ohms res for each mosfet (4 in total), so if I have this right, I'm just worring about nothing and the sg3525 has enough power?
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Old 19th May 2006, 07:36 PM   #6
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> the sg3525 has enough power?

Since it switches fast enough ... QED.
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