Electronic – Mosfets burning in H-Bridge

dc motorh-bridgemosfetmosfet-driver

I'm having a problem with a H-bridge, that drives a motor.

Schematic

This is the schematic for the H-bridge.
Signals H[4:1] are generated by a microcontroller.
mX and mY are the outputs for a DC motor.

H-Bridge Schematic

How it works

The microcontroller receives an external message via UART.
When the message is received, the microcontroller switches de H-bridge, reversing the motor direction.
This messages are received around once at every 4 seconds. So, in practice, the motor should be changing its rotation direction at every 4 seconds.

Microcontroller firmware

while(TRUE) {
    if(receivedCmd == 0x01) {   //direction1
        GPIO_WriteLow(GPIOD, (GPIO_Pin_TypeDef) (GPIO_PIN_2 | GPIO_PIN_3));    //H1-H4
        GPIO_WriteHigh(GPIOC, (GPIO_Pin_TypeDef) (GPIO_PIN_3 | GPIO_PIN_4));   //H2-H3
    }
    else if(receivedCmd == 0x02) {  //direction2
        GPIO_WriteLow(GPIOC, (GPIO_Pin_TypeDef) (GPIO_PIN_3 | GPIO_PIN_4));    //H2-H3
        GPIO_WriteHigh(GPIOD, (GPIO_Pin_TypeDef) (GPIO_PIN_2 | GPIO_PIN_3));   //H1-H4
    }
    else {    //do not activate
        GPIO_WriteLow(GPIOC, (GPIO_Pin_TypeDef) (GPIO_PIN_3 | GPIO_PIN_4));    //H2-H3
        GPIO_WriteLow(GPIOD, (GPIO_Pin_TypeDef) (GPIO_PIN_2 | GPIO_PIN_3));    //H1-H4
    }
}

The problem

At first, the circuit is working as expected, but after working for 1 minute or so, the mosfets Q3 and Q4 will endup burning.

Questions

I only have 2 spare mosfets left with me right now, so I decided to ask for help here before trying to implement some changes.

  1. I did not add flyback diodes in parallel with the mosfets. Are they really necessary? Should I add them? Could this be the problem with my circuit?
  2. Is there something wrong with my firmware? Should I add a deadtime before reversing the motor direction?
  3. Could breaking the motor (activating Q1 and Q3, and deactivating Q2 and Q4) before reversing it be a solution?
  4. Any other ideas on what may be happening?

Best Answer

As I interpret the code:

if(receivedCmd == 0x01) {   //direction1
    GPIO_WriteLow(GPIOD, (GPIO_Pin_TypeDef) (GPIO_PIN_2 | GPIO_PIN_3));    //H1-H4
    GPIO_WriteHigh(GPIOC, (GPIO_Pin_TypeDef) (GPIO_PIN_3 | GPIO_PIN_4));   //H2-H3
}

In the code above, in direction 1, pins 3 and 4 are on.

else if(receivedCmd == 0x02) {  //direction2
    GPIO_WriteLow(GPIOC, (GPIO_Pin_TypeDef) (GPIO_PIN_3 | GPIO_PIN_4));    //H2-H3
    GPIO_WriteHigh(GPIOD, (GPIO_Pin_TypeDef) (GPIO_PIN_2 | GPIO_PIN_3));   //H1-H4
}

Now, in direction 2, pins 2 and 3 are on. Pin 1 is never used.

This means that in both directions pin 3 is on. This would mean a short through Q3 and Q4 during direction 2. Your comments for the lines "H2-H3" and "H1-H4" are correct, but I think the pins not implemented like this.

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