Hey all,

I'm attempting to create a half bridge for use with PWM from an AVR micro to run some 24v motors.

My Problem is that whenever I run the chip at full it's fine and the motor will run for forever. However if I try to use PWM I run into an issue whereby the motor seems to be producing too much reverse voltage and the power supply is struggling to cope.

Attached is schematic, sorry it's a bit untidy.

Any help would be appreciated

 

On a quick look at your schematic, it looks like you have both the high and low side switches(mosfets) wired to one side of the motor. If this is what you have done, it means that the motor ill not run when the high side switch is on. Actually when the high switch is on it acts as a 'dynamic brake' to the motor.

To use the circuit as you have it now, you have to do away with the high side mosfet connection to the motor and all of the connecting circuitry parts. This will then let the motor 'free wheel' when the low side mosfet is switched off. This keeps the motor spinning in stead of braking when the low side switches off.

You are actually using a half bridge not a H-bridge. A H-bridge is two half bridges. A H-bridge is used to run a motor in forward and reverse.

Instead of the resistor and capacitor on the mosfet connection you need a fast diode from the motor to the 24V supply connection. This is called a 'free wheel diode' and prevents voltage spikes on the mosfet. The spike can blow the mosfet if not used.

Hope this helps.

 

Hey, thanks for your reply.

It's not entirely obvious but D4 is in parallel with the RC network; it should be across the motor as you describe. The RC network is just extra protection which doesn't seem to be doing very much right about now.

As I understand it, if I want to PWM I have to not utilise the lower Mosfet until I require braking? This would then allow the motor to free wheel during the PWM downtime?

Not sure how standard Half Bridges deal with this on their PWM cycles. Also not sure whether the IR2110 will allow it since we need to cycle the lower Mosfet to charge the bootstrap for the upper Mosfet.

 

No you have it backward, the upper mosfet and circuit is the one not needed. Including the boot strap. You have a +24V supply hooked to one side of the motor, the other side of motor should be hooked to the low side mosfet. Then you PWM the gate of the low side mosfet to control the motor speed.

Now if you need to reverse you need a H-bridge, 2 upper and 2 lower mosfets and drivers. This guy Chuck McManis, has some real easy to understand tutorials on his site. Give it a read - http://www.mcmanis.com/chuck/robotics/projects/esc2/hbridge_spiking.html

http://www.mcmanis.com/chuck/robotics/index.html

http://www.mcmanis.com/chuck/robotics/projects/index.html