I am trying to design an H-bridge circuit and I am only given the following components:
I know that it is easy to design an H-bridge using 2 P-MOSFETs and 2 N-MOSFETs, but I am not given P-MOSFETs.
Is there an equivalent circuit for only N-MOSFETs?
Since this is a school assignment, I suspect the solution is expected to be simple, not industry grade. The simplest bootstrap that works for the high-side N-channel MOSFETs looks like this (half bridge shown):
This is from a textbook by Grant and Gowar.
Now since you're not given any PNPs, you can't have the complementary pair drive Q3 there. Instead you have to use a two-NPN totem pole in their place.
You seem to be given all the logic gates you want, so this looks like will satisfy your assignment requirements.
A bit more searching even found (in an article referenced by Grant and Gowar) something more economical along the lines I was talking about:
I have no idea why you're given "Infra-red Detector" and "Infra-red Emitter" but if by any chance that means you're given optocouplers, you could also do this:
You're not saying what kind of motor you need this H-bridge to drive. The above circuits are suitable for BLDCs or motors where the on-time of the high-side switches cannot be arbitrarily long because the capacitor can only recharge when the high-side is off (it's shorted while high-side drive is on). If you need arbitrarily long on-time, then a slightly more complex circuit called a charge pump is needed to continuously top off the boostrap/tank capacitor. The textbook-level circuit is (only the high-side driver is shown):
Again you'll have to substitute the complementary pair to meet your design requirement of no-PNPs-allowed. This circuit also requires a clock signal (square wave generator) to drive the pump, which you can make in many ways with what you've got, e.g. op amp. If you don't know how to do that bit (and you need it), you should search the site and/or ask a separate question.
Without even considering the concept of bootstrapping it is totally possible to build an H-Bridge with N-Channel MOSFETs in both the upper and lower switch positions. This is done all the time in high power drivers due to the much larger selection of high performance N-Channel FETs over that for P-Channel FETs.
The key parameter to keep in mind is that the gate drive voltage for the upper FETs needs to be driven above the output voltage positive level by a good amount to ensure that the upper FETs are turned on fully. A starter way to achieve this is to provide a higher voltage supply rail for the gate drivers than what is used for output positive voltage rail.
Once you have learned and understood the concepts of what it takes to drive the upper N-Channel FETs with sufficient gate voltage you can then begin to investigate how to get that higher voltage without having to provide an additional higher voltage power supply. One way to do that is to build a charge pump circuit, with some of your other components, to "double" the positive voltage rail up to near 2X the voltage. To utilize this higher voltage will require a circuit that can swing the upper FET gate up to this 2X voltage and then low. The Op-Amp would be a good candidate to do that for you.
In the industry there is a tendency for engineers to be tasked with designing circuits that cost less but give desired performance and function. There is technique called boot strapping that uses the switching action of the H-Bridge itself to drive a capacitor in a simpler form than the completely separate charge pump circuit. The bootstrap capacitor can achieve almost twice the positive voltage rail level of voltage and again provide the necessary gate drive bias for the upper N-Channel FET. We never get anything for free however and in this case the simpler bootstrap circuit has the requirement that the H-Bridge needs to keep switching all the time to provide the pumping action. This means that a motor drive would need to be switched PWM type drive and never get to full 100% ON drive level.
If you have a single-output power supply and only NMOS and NPN transistors available, you'll certainly be able to build an "H" bridge, using either, (or both) but you'll not be able to saturate the high side devices so, depending on what your load looks like, they (the high side transistors) may get too hot for comfort and your load probably won't perform the way you want it to.
However, with all the stuff you have available you could easily build a charge pump and use it generate a voltage higher than your supply voltage.
Once you did that you'd be able to drive either the gates of the high-side MOSFETs or the bases of the high-side BJTs at higher than your supply voltage and get the H bridge to work properly.
So I was dealing with the same issue: how to drive the high side N-channel MOSFETs in my H-Bridge. The references were really helpful. I was contemplating the best approach (charge pump, separate gate supply) but now 6 years down the road and I stumbled upon this nifty little chip from Infineon: https://www.infineon.com/cms/en/product/power/gate-driver-ics/irs2104/#!order
Due to the current chip shortage, looks like Digikey is sold out of the surf mount packages, but have plenty of the old-fashioned 8-pin dip style, which works way better for me as I'm at the breadboarding stage.
