MOSFETS in parallel

[bryce10110]

Hello, I want to build this PWM flyback driver, but I have a few questions about it. Instead of using a 2N2222 can I swap it for a 2N3055, and instead of using one MOSFET can I use four connected in parallel.

[indulis]

The 2N2222 is there to turn the MOSFET off. The 2N3055 gain is low. The 330 ohm will "slow down" the MOSFET turn-on. Why 4 MOSFETS in parallel? The IRF510 is a 100V 5.6A device with an Rds on of .54 ohms.

You will need a snubber across the primary winding or MOSFET (maybe both). You show a 3A source... what do you think 4 MOSFET's will do for you besides lower "Rds on"? Better devices are out there.

[Hero999]

I don't see what the 2N2222A is adding to the circuit, if anything it's making it worse not better, I'd get rid of it and drive the MOSFET directly from the 555.

MOSFETs can be paralleled but why not use a single better MOSFET such as the IRL540, rather than loads the obsolete IRF510.

[indulis]

The 2N2222 can sink a lot more current than the 555, so the "turn off" will be faster... and thats a good thing. I'd even put a diode across the 10 ohm gate resistor to speed it up even more.

[KevinIV]

One MOSFET is a better idea. Puting MOSFETS in parallel that all do the same thing only serves one good purpose and might be marginally beneficial. Replacing the transistor with one that is not designed for may work if it's close.

[Hero999]

The 2N2222 can sink a lot more current than the 555, so the "turn off" will be faster... and thats a good thing.

Unfortunately, the turn on will be slower due to the 330R.

I doubt there's a significant difference between driving it directly from the 555 and via the 2N2222A.

[indulis]

With the values shown, it could switch at up to ~48KHz... your not going to drive that MOSFET with a 555 directly at that frequency.  A 555 can only sink ~200mA whereas the 2N2222 is good for 1A.

Unfortunately, the turn on will be slower due to the 330R.

Yes, exactly what I said in my original reply.

[Hero999]

With the values shown, it could switch at up to ~48KHz... your not going to drive that MOSFET with a 555 directly at that frequency. 

Why not? The 555 can work all the way up to 500kHz.

A 555 can only sink ~200mA whereas the 2N2222 is good for 1A.

0.8A.

Would using a push-pull emitter follower configuration be any better?

[indulis]

Would using a push-pull emitter follower configuration be any better?

YES!!!

Put the gate resistor in the collector of the NPN not to "over-drive" the MOSFET and the PNP will be able to turn it off faster.

Most of our designs have PWM's capable of 1 or 2 amp's and if they don't, a driver is used.

[KevinIV]

That transistor is also used to create the DC bias for the MOSFET. To bias the MOSFET with only a 555 timer creates a lower impedance and higher current for the 555 timer. It's common to buffer IC's with higher current transistor's with high current loads, and in this case the load is the MOSFET biasing.

[indulis]

That transistor has absolutely nothing to do with the biasing of the MOSFET!!! If you take it out of the circuit, the MOSFET is still "biased" on. It's sole purpose is to turn the MOSFET off. You could run the MOSFET directly from the 555... the problem is that the 555 doesn't have the current driving capability to switch it fast. Anytime you have a MOSFET switching a high current load, you have to switch it fast to minimize "switching losses"... that's the I