# Things to consider when making a MOSFET driver

I would like to make my own MOSFET driver, as it is pretty hard to find specifically what I am looking I am making my own.

simulate this circuit – Schematic created using CircuitLab

The stars of my driver are the NSS40300 (PNP) and the NSS40301 (NPN).

Will I be able to switch a 1MHz square wave with this driver alone (not considering the MOSFET capacitance?) The NPN is rated at 215MHz while the PNP is rated at 160MHz so am I right to assume that 1MHZ switching should be no problem for these transistors?

Is it overkill to use a 2W transistor for this application? Is 1W to much? R1 will also be taking many pulses. Will a 2W R1 be sufficient?

Resistor selection, R1 is easy to calculate, I just have to limit the current to 3 A which my transistors are rated at the highest possible voltage, R = V/I = 30/3 = 10 ohms. R3 on the other hand I'm not sure. I would like to not waste much power as possible so I would like to know the highest possible resistance for R3 at 3A output.

Lastly am I missing something? Is this a robust design? Are there other things I should consider?

• NPN is rated at 215MHz That is probably the $f_t$ which is a number that is measured under very ideal conditions. In typical circuits it depends on the circuit what bandwidth you get. not considering the mosfet capacitance So you want to ignore the main speed-limiting factor? Why? I suggest building this circuit in a simulator (like LTSpice) and simulate to see what speed can be achieved. My guess is that 1 MHz will be possible but the switching might not be as ideal as you would like. If you want a robust design, use a gate driver chip. Jun 29, 2021 at 17:46
• I'll second a gate driver ic, it will work much better with less power wasted than this circuit. Consider that 30V is getting pretty high for the gate-source voltage of the MOSFET. You may need to rethink parts of the design.
– W5VO
Jun 29, 2021 at 18:09
• @Bimpelrekkie I want to focus on the driver itself first before it will be limited by the mosfet. i did try to find for spice models of the transistor but there are none and looking into how to make a model from a datasheet is just a rabbit hole. Jun 29, 2021 at 18:21
• When switching, it's charge storage time that matters more than transition frequency. You'd want to run the BJTs in forward-active mode (which would waste a lot of power!) if you want to get that high of a frequency, and even then you probably won't. But I would also like to ask, why not just use a driver IC? It makes things so much easier to just slap in an IXDI614 or something. Jun 29, 2021 at 18:25
• I want to focus on the driver itself first before it will be limited by the mosfet The input capacitance of the MOSFET IS the main limitation of this circuit. Suppose that without the MOSFET the driver could easily do for example 10 MHz. There's no point in making that 20 MHz when adding the MOSFET limits the speed to 1 MHz. It is fine that you're looking for ways to make problems smaller (I do this all the time) but those have to be independent problems. If you split an "unsplittable" problem, you're only fooling yourself. Fine be me but not a good design strategy. Jun 29, 2021 at 18:35