Issues With MOSFET bridge, DC motor 24V 200W

Question

Hello everyone,

I have been working on a project to drive a 200W 24V motor that gets a continuous drive current of about 7-8A. The idea is to run the motor in forward and reverse direction interchangeably much like a car viper. The time for forward and reverse is about 20seconds each.

I want to use 4-N channel MOSFETs for this and use a discrete driver.

Mosfets used - STP55N i.e. 60V 50A

Input supply - 24V

PWM input - In-1 and In-2, a signal on one of the inputs will run the motor in one direction and vice versa.

Bootstrap Capacitor - 10uF 50V

Issues -

1. The circuit is working i.e. motor is getting turned on in both the direction but upper MOSFETs are getting heated upto 70degree within few minutes.

2. Is there an issue with the bootstrap circuitry?

dougw · Answer

R1 may be too high of a value. It will take time to charge the gate capacitance, which will turn the FET on slowly, causing heat losses. If R1 is made smaller, it may discharge the capacitor too fast to keep the FET on. You may need to drive the gate with a push pull transistor pair so it turns on and off quickly without discharging the capacitor. You also need to ensure that Q1 and Q2 are never turned on at the same time. If Q1 is slow to turn off, Q2 will pull current through it, heating it up. The same problem happens to Q2 if it is slow to turn off. You need a delay between turning each FET off and turning its counterpart on.

robogary · Answer

Maybe take a look with a scope to see if you are getting shoot thru, i.e. forward and reverse bridges firing at the same time or with some overlap. Do bridges have solid interlocking and deadband ? Are you running no load, or with load ? Try running motor uncoupled to see if heating changes significantly.

Issues With MOSFET bridge, DC motor 24V 200W

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