[FAQ] How to control voltage spikes on motor driver outputs?

In motor systems, voltage spikes and ringing noise can appear on the output terminals. These spikes can degrade the performance of the motor system, increase EMI noise, and even cause damage if not controlled.

Switch-node ringing noise is the results of fast slew rates at the output terminals of the motor system. Having a fast slew rate effective means having large dV/dt which can cause significant LC oscillations between the parasitic inductance of the PCB traces and the parasitic on the MOSFET package. The diagram below shows the typical parasitic components of a half-bridge.

This switch-node ringing not only creates voltage undershoot and overshoot which can be damaging to the motor system but can also cause high EMI noise. Controlling these switch-node oscillation is important to improve the performance of the system. Below are a couple solutions for reducing switch-node ringing:

• Reducing the output slew rate which can be done by adding a resistor on the gate of the MOSFET. Alternatively, TI offers gate drivers with adjustable IDRIVE that can adjust the output slew rate. Read this app-note to learn more about Smart Gate Drive.
• Placing snubbers across the MOSFETs. Read this blog to learn about designing RC snubbers.
• Improving the PCB layout to minimize trace parasitics and high current loops. Read this app-note to learn about best practices for board layout of motor drivers.

Large current flowing back to the power supply can also cause voltage spikes at the output terminals of the motor systems as well as on the power supply. The excess current can be produced by the motor acting as a generator or by the stored energy on the motor flowing back to the supply during fast decay (read this app-note to learn about the different decay modes). There can also be other unexpected sources that can cause excess current to flow back to the supply. The best way to minimize these voltage spikes is to control and absorb all of the excess energy. Below are some techniques to minimize these unwanted voltage spikes:

• Place enough bulk capacitance near the power supply pin of the motor driver such that the excess energy is absorbed by the capacitor. Read section 5 of this app-note to learn best placement of bulk and bypass capacitors.
• Place a clamping diode, such as a TVS or zener diode, from the power supply to GND. Make sure the clamping voltage of the diode is the same as the power supply voltage. Read this app-note to learn more about TVS diodes.