I've built a H-bridge circuit to control a brushed DC motor. A schematic of the H-bridge is shown below:
When designing the H-bridge I didn't add any diodes in parallel to the MOSFETs as I had assumed that the intrinsic diodes of the MOSFETs would suffice to control the back EMF of the motor for the following reasons:
- The maximum forward current of the diode is larger than the motor current.
- The turn-on time of the diode is negligible (the data sheet states that the turn-on time of the diode is limited by the pin inductance, not the diode itself). This means that the diodes can immediately begin to provide a current path when the MOSFETs are off.
When testing my board, I force Q1 to be on all of the time and Q4 switched on/off at the PWM frequency. I'm assuming that when Q4 is turned off, the motor current will continue to flow through the diode of Q3 and back through the channel of Q1. I therefore expected that the back EMF of the motor to be clamped at one intrinsic diode drop and no more. However, I can see on a scope trace that the actual back EMF is more than 50 V (see below) and I'm not sure that I understand why.
I'm hoping that someone can explain why I'm seeing such a high back EMF and perhaps suggest a way of improving this circuit (for example, was it a mistake not to add some very fast didoes in parallel to each MOSFET?).