13.25.4 MOSFET Body Diode Reverse Recovery Snubber

When motor current is flowing through the external MOSFET body diodes and the complimentary MOSFET of the phase pair turns on, the body diode reverse recovery creates a momentary short circuit until the reverse recovery time is complete. When the body diode reverse recovery is complete, the current path is opened, causing the phase node voltage to slew rapidly towards ground or HVDD levels. The rapid slew rate may cause an inversion of the gate-to-source voltage on the MOSFET that is turning on and result in that MOSFET turning off.

The fast slew rate may also cause ringing on the phase node and the sense resistor if the turn-off is too fast.

The first remedy for the low-side turn-off is to slow down the MOSFET gate-to-source turn-off. That causes the RDSON of the low-side MOSFET to gradually increase as the gate voltage drops and the low-side MOSFET slowly turns off. The slow turn-off allows the phase voltage, generated by the motor current flowing through the low-side MOSFET RDSON, to slowly rise towards the positive motor supply level.

The same scenario is also valid for turning on the low-side MOSFET when the high-side MOSFET has just been turned off and current was flowing from the high side into the motor.

The MOSFET body diode reverse recovery situation occurs when the low-side MOSFETs are turned on while the motor current is flowing to the positive source through the high-side MOSFET body diode. The diode reverse recovery time allows a short circuit to exist between the positive supply and the low-side MOSFET drain until the high-side diode is reverse biased and the reverse recovery time has elapsed. The first remedies above should be used to slow the switching speeds of the MOSFETs. Then, a snubber is added to each MOSFET to fine-tune the phase node slew rate and eliminate any further transients. Adding a drain-to-source snubber slows down the slew rate of the phase node and results in a more controlled excursion of the phase node voltage. The snubber consists of a resistor and a capacitor connected in series between the drain and source of the MOSFET. The resistor is chosen to keep the initial snubber voltage below a few volts when peak motor current is flowing through the body diode. The capacitor is then chosen to provide an RC time constant longer than the MOSFET body diode reverse recovery time. A 0.1Ω resistor is typically used, along with a 0.1 μF capacitor to provide an RC of 10 ns.

The power dissipated by the capacitor is calculated by applying Equation 13-5.

The capacitor and resistor form factors are chosen to handle the dissipated power.