13.25.2 Bootstrap Voltage Suppression

The pins which handle the highest voltage during motor operation are the bootstrap pins (VBx). The bootstrap pin voltage is typically V_BOOT (12V) higher than the associated phase voltage. When the high-side MOSFET is conducting, the phase pin voltage is typically at HV_DD and the bootstrap pin voltage is typically at HV_DD + 12V. When the phase MOSFETs switch, current induced voltage transients occur on the phase pins. These currents are caused by the MOSFET body diode reverse recovery and MOSFET turn-on/turn-off times. Those induced voltages cause the bootstrap pin voltages to also increase. Depending on the magnitude of the phase pin voltage, the bootstrap pin voltage may exceed the safe operating voltage of the device. The current induced transients may be reduced by slowing down the turn-on and turn-off times of the MOSFETs. The external MOSFETs may be slowed down by adding a 10 to 100 ohm resistor in series with the gate drive. A 3.3 nF to 10 nF ceramic capacitor may be added that connects each MOSFET gate and source terminal. The added capacitance slows down the switching times of the MOSFET, while allowing the gate resistance to remain small enough to keep the gate clamped off. The added capacitance also results in a lower slew rate of the phase node and limits the shoot-through current caused by the body diode reverse recovery.

The high-side MOSFETs may also be slowed down by inserting a 10Ω to 25Ω resistor between each bootstrap pin and the associated bootstrap diode capacitor junction. Another 25Ω to 50Ω resistor is then added between the gate drive and the MOSFET gate. This results in a high-side turn-on resistance of 25Ω plus the series gate resistor. The high-side turn-off resistance only consists of the series gate resistance and allows for a faster shut-off time. Care must be taken to make sure the voltage drop across the bootstrap pin resistor does not cause an external MOSFET Undervoltage Fault.

When a system motor power supply voltage clamp is not used, 33V or 36V transzorbs may be connected from each bootstrap pin (VBx) to the ground. This will ensure that the bootstrap voltage does not exceed the absolute maximum voltage allowed on the pins. The resistors connected between the bootstrap pins and the bootstrap diode/capacitor junctions, mentioned in the previous paragraph, may also be used in order to limit the transzorb current and reduce the transzorb package size.