Motor NotesMotor Current and Regeneration Current When Using a Single MOSFET in PWM Driving

This article concerns motor currents and regeneration currents when using one MOSFET in PWM driving of a brushed DC motor. There are a number of methods that can be used for PWM driving of a motor; among these is a simple method in which the MOSFET is inserted between the motor and ground and is switched. In this case, a regeneration current flows in the diode connected in parallel with the motor. Below is explained the relationship between the motor current flowing in the motor and this regeneration current.

Motor Current and Regeneration Current When Using a Single MOSFET in PWM Driving

We begin by reviewing the circuit and current paths when a single MOSFET is used in PWM driving of a brushed DC motor.

The diagram on the left is the circuit when one MOSFET Q1 is used in PWM driving. The MOSFET is inserted between the motor and ground, and is switched to perform PWM driving.
The center diagram shows the path on which the motor current flows, from the power supply through the MOSFET Q1, when Q1 is on.
The diagram on the right shows the path of the regeneration current, through the diode connected parallel to the motor, when the MOSFET Q1 is off.

The relationship between the motor current and the regeneration current in this PWM driving is as follows.

• ・When Q1 turns on, the motor current flows through Q1 and increases, reaching its maximum just before Q1 turns off.
• ・When Q1 turns off, the motor acts as if to continue passing a current, and so a regeneration current flows through the diode connected in parallel with the motor.
• ・The regeneration current is maximum at the time Q1 turns off; in other words, it is the same as the motor current, which is at maximum just before Q1 shuts off. Thereafter, the regeneration current gradually decreases.
• ・When Q1 turns on again, the motor current begins to increase, and the above-described series of actions repeats.

Thus in PWM driving using a single MOSFET, the maximum value of the regeneration current flowing in the diode is equal to the maximum value of the current flowing in the motor.

Please refer to the following waveform diagram.

From these facts, we see that the absolute maximum rating for the forward current of the diode connected in parallel must be chosen so as to be higher than the largest motor current. Also, power is consumed by the regeneration current flowing in the diode and the forward voltage (V_F) of the diode, and so a diode must be selected having a package with an allowable power dissipation greater than this power consumption.