H-Bridge, N Mosfets and the like...

[somenick]

Hello to everyone.

I don't know if this is the correct subforum for my question, but its for a hobby project of mine so it will have to do!!

I want to make a (dual) H-bridge to control DC motors. I don't know yet which motors, but they will be 24V or 36V (or maybe 48V).

I'm getting fed up with the costs and poor performance of suitable P-mosfets for this project (especially ones that can withstand Vds of 48V) so I decided I will use only N-mosfets for this.

I don't want to use bootstrap drivers, because I want the high side mosfets to be active for however long i want, and as I understand it with such controllers I must disable the high side and enable the low side of each half-bridge in order for the bootstrap capacitor to charge.

So, my next idea was to use some sort of boost converter, to generate a voltage around 10 volts higher than what the motors will use and use that voltage to drive the high side mosfets.

Let's say for example I use 36V motors and I get a nice 45V voltage from the boost converter to drive the high side mosfets' gates.

I will post an image to help explain myself:



Suppose mosfet A is active, and D is pulsed (PWM). When D is in the off state, won't mosfet B see a Vgs of -36V ?
If that's the case it will for sure destroy the mosfet, as most claim maximum absolute Vgs of ±20V

How can I prevent this? Is it possible at all?


PS If you have any suggestions for a completely different way to do this please do tell! I just want to be able to have any high side mosfet constantly on.

 

[ArticCynda]

Typical MOSFETs can't stand more than 15V Vgs in either polarity, so you'll have to protect the gates. For a hobby project, I would recommend 12V zener diodes for this, with their anode connected to source and cathode to gate. Vgs will the be clipped to +12V or clamped to -0.6V, both well within common Vgs ranges. Of course, you'll need a series gate resistor to limit the current flow when the zener conducts! This series resistor will hinder the speed with which you can charge or discharge the gate, hence limiting the switching speed of your application. So this technique isn't recommendable for high speed switching, but up to a few kHz for PWM shouldn't be a problem.

Regarding the high side MOSFETs, I'd recommend driving their gates 10V above the supply voltage. Use an integrated monolithic boost converter IC with external inductor, flyback diode and caps to boost the voltage. You'll have to add some analog logic to trick the boost converter to boost the voltage 10V higher than the supply voltage.

 

[somenick]

@ArticCynda thanks for the input but I can't exactly picture the setup you propose. I am talking about the zeners part!

I understand the purpose of the zener, to keep Vgs within acceptable limits, but I can't figure how to control the mosfets after the zener has been added parallel to GS.

Can you post a rough sketch of what you have in mind?

My first thought was something like the image below, but then I realized that in the same case as I mentioned above, (A active and D in the LOW state of pulse), then the zener would short the motor to ground!




PS I don't really care about the switching speed, the high sides won't be pulsed for motor speed control. They will only be on/off for direction control.