H Bridge PWM function question

Other Hardware Motors, Mechanics, Power and CNC
1

An H-Bridge consisting of 4 MOSFETS for example, we'll forget about the snubber circuit for now... Let's say we're spinning the motor forward by turning on 2 MOSFETS while leaving the other 2 off. When using PWM to control speed, are both MOSFETS being switched at the same rate or is only one being switched and the other simply being left open? If so which one?

Basically, if you used analogWrite on one of the PWM pins going to the first MOSFET, and picked your value (0-255), could you simply digitalWrite(HIGH) to the other MOSFET? It also seems to me it wouldn't matter which of the 2 MOSFETS were getting the PWM signal and which was left in the HIGH state.. Am i correct here or way off?

2

Turn on / Enable the high side and PWM the corresponding low side. The other pair should be biased Off (in non-conducting states.)

3

Ok that's what I thought.

I'm using a 20V DC motor that draws up to 30A. Any help with snubber circuit design appreciated.

4

The Quencharc makes a pretty good snubber. Basically you design it as an RC circuit and choose your tradeoff between fast switch-on and snubbed switch-off.

5

lowandslow:
An H-Bridge consisting of 4 MOSFETS for example, we'll forget about the snubber circuit for now... Let's say we're spinning the motor forward by turning on 2 MOSFETS while leaving the other 2 off. When using PWM to control speed, are both MOSFETS being switched at the same rate or is only one being switched and the other simply being left open? If so which one?

Basically, if you used analogWrite on one of the PWM pins going to the first MOSFET, and picked your value (0-255), could you simply digitalWrite(HIGH) to the other MOSFET? It also seems to me it wouldn't matter which of the 2 MOSFETS were getting the PWM signal and which was left in the HIGH state.. Am i correct here or way off?

You reading topic for today is "decay modes". In particular fast decay mode v. slow decay mode.
You can also switch all 4 devices (rather like synchronous rectification), which means you have
no hiccups passing through zero speed at full torque.

6

MorganS:
The Quencharc makes a pretty good snubber. Basically you design it as an RC circuit and choose your tradeoff between fast switch-on and snubbed switch-off.

Snubbers are usually reserved for AC circuits where free-wheel diodes cannot work. H-bridges use
free-wheel diodes (often the built-in body diode if a MOSFET H-bridge).

7

Thanks for replies..

MarkT:
You reading topic for today is "decay modes". In particular fast decay mode v. slow decay mode.
You can also switch all 4 devices (rather like synchronous rectification), which means you have
no hiccups passing through zero speed at full torque.

Perfect. I'll be back. Thanks