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.
if links don't work try this!
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.
if links don't work try this!
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.
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.
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.
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.
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.
wine&dine said:
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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