H-bridge circuit using only NMOSFET

Question

I have understood that I have problem with driving High Side MOSFETs as their Gate Voltage must be bigger in respect to their Source voltage. How do I provide a higher voltage supply rail for the gate drivers than what is used for output positive voltage rail so as to turn on the MOSFETs?

Doesn't anyone care about all those unused PMOSFETs that also want to be used and loved ? ;-) — Bimpelrekkie, Oct 20 '15 at 11:40
I'm guessing you didn't bother reading any of the refs I suggested to you in your previous (and nearly identical) question. Because they cover exactly the same material as the answers below. — the gods from engineering, Oct 23 '15 at 06:29

Andy aka · Answer 1 · 2015-10-20T12:19:14.200

1

Use a H bridge driver: -

It steals a bit of energy from the output to provide a higher voltage inside the chip to drive the top FET's gate at the correct level.

There are plenty of chips around and this picture shows the internals with a bit more detail: -

If you want a half-decent version that uses discrete components, try this: -

It's not guaranteed to be that great but does use the same techniques as the chips. Stolen from here

edited Oct 20 '15 at 12:19

answered Oct 20 '15 at 11:29

Andy aka

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But in my project , I am not given such chips to work with. – LearningLaSo Oct 20 '15 at 11:50
True, but you're given all the pieces to build such a thing. That's probably you're best bet to drive n channel fets on the top of the bridge. – michaelyoyo Oct 20 '15 at 11:52
by the way what is the triangle thing inside the chip? – LearningLaSo Oct 20 '15 at 11:56
It's a schmitt trigger but beware, the 2nd circuit has to bias PWM1 up to where its schmitt trigger is located - it's "0V" connection is now the source connection of the top transistor. If you are building this I'd be tempted to use Spehro's method - it's a bit clunkier but easier to test and get working. Me, I'd use a chip like the above every time. – Andy aka Oct 20 '15 at 12:15
Discreet version added. – Andy aka Oct 20 '15 at 12:22
He's not allowed to use PNP transistors either. :D – the gods from engineering Oct 23 '15 at 08:02

score 1 · Answer 2 · answered Oct 20 '15 at 12:11

There are two basic methods- bootstrap, which Andy has shown you. You cannot quite achieve 100% time high with that kind of circuit- it has to be driven low every once in a while to charge the reservoir capacitor.

The other method is to generate a supply voltage relative to the positive rail with a different kind of switching (the bootstrap circuit requires the low-side MOSFET to switch). For example, a transformer-based (isolated) DC-DC converter or a charge pump may be suitable, depending on the requirements.

H-bridge circuit using only NMOSFET

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