PWM driven motor constant current operation

Typical motor drive methods include voltage drive, current drive and PWM drive. This article will introduce the constant current operation using PWM drive. First, it introduces what PWM drive motor constant current operation is, and then the working principle of the circuit when PWM drive motor constant current operation is performed.

What is constant current operation of a PWM driven motor?

How does a motor work when it is driven with a constant current? When driven with a constant current, the motor can rotate with a constant torque. The torque of the motor is the value obtained by multiplying the torque constant by the motor current. In other words, the motor torque is proportional to the current, so if the current is constant, the torque is also constant. First, let's look at Figure 1, which is an example of a circuit that operates with a constant current through PWM drive. It is a motor driver circuit with 4 switches (in this example, an H-bridge using MOSFETs as the output stage). For more information about the H-bridge, please refer to the basics of Tech Web Motor. In addition, PWM drive is a method of sending the required power by turning on/off pulses in principle. The size (voltage) and period of the pulse are constant, and the power to be sent is controlled by adjusting the pulse width (time) when it is on. For more details, please refer to the basics of Tech Web Motor here.

The actual operation of the circuit shown is explained below. Figure 1 is based on the premise of forward rotation. In this case, in the pair of MOSFETs Q1 and Q2, Q1 is turned on, Q2 is turned off, OUT1 is connected to the power supply voltage Ea, and current flows through the positive pole of the motor. At the same time, in the pair of Q3 and Q4, Q3 is turned off, Q4 is turned on, and OUT2 is connected to GND via Rs of the driver RNF pin. In this way, current flows from the power supply to the motor, and the motor is in the energized state. Since the purpose here is constant current operation, the current needs to be controlled to a constant current, which is done by Rs and the comparator. Rs is the current detection resistor. The comparator is responsible for comparing the voltage generated by Rs×motor current with the reference voltage applied to the reference voltage pin Vref. The reference voltage is set to the desired constant current value×Rs. When the motor current gradually increases due to energization until the detection voltage of Rs exceeds Vref, the comparator turns off Q1 (Q2 can remain off or turned on) and stops energizing the motor. When the energization stops, the motor current will try to continue to flow, but it will gradually decrease. Then, after a certain period of time, when Q1 is turned on again and the motor is powered, the motor current begins to increase again. When the detection voltage of Rs exceeds Vref, Q1 is turned off again and the power supply stops. This action is repeated. The driver in Figure 1 counts the frequency of OSC (oscillator) and has set an arbitrary off time (toff). The operating waveform is shown in Figure 2.

Through this repetitive action, a triangular wave current with a peak value obtained by dividing Vref by Rs flows. If the off time (toff) of Q1 is set small enough, it can operate with an almost constant current, that is, constant current operation. The above is an explanation of the constant current operation of the motor driven by PWM, but the actual PWM drive requires more precise control. For example, when the power is stopped and the regenerative current flows, no current flows through Rs, so the current change in Rs becomes larger when the power is turned on again. Since parasitic inductance is inevitable, the conduction and cutoff of this current may cause a large voltage noise as shown in the waveform of the RNF pin (as shown in Figure 2), or a current that charges the parasitic capacitance of the MOSFET may flow and cause the Vref voltage to exceed. In order to prevent the erroneous shutdown action caused by these voltage noises, it is necessary to take measures such as ignoring the short-term peak current and setting the non-reaction time (tblnk) on the PWM drive, or using a filter to filter out the PWM drive noise.