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2022.08.24 motor

Driving Brushed DC Motors Using PWM Output: Driving in the Form of BTL Amplifier Input

Brushed DC Motor

On the subject of methods for PWM driving of a brushed DC motor, in the previous article an example of constant-current driving was explained. This time, PWM driving in the form of BTL amplifier input is explained.

Driving Brushed DC Motors Using PWM Output:
Driving in the Form of BTL Amplifier Input

Again we begin with a circuit example. This is an example of a circuit that performs PWM driving in the form of BTL amplifier input. The operation of the BTL amp input circuit is explained here as well, and should be referenced.

This driver controls an H-bridge using the same differential amplifier configuration in the input circuit as a BTL amplifier. It is provided with a differential-input amplifier having Vref as the reference bias point and a triangular-wave oscillator having output of amplitude Vph-Vpl and with Vref at the center point between Vph and Vpl. The PWM signal is generated by using a comparator to compare the amplifier output with the triangular wave so as to obtain a square wave. One of the two square-wave outputs is the inverse of the other (see the waveform diagram).

When the input (VIN1-VIN2) is zero, the amplifier output (AmpOut) is the same voltage as Vref, and the two outputs each result in 50% duty driving; the average current flowing in the motor is zero (see the graph).

When the input (VIN1-VIN2) is positive, the on-duty of OUT1 is greater than 50%, the on-duty of OUT2 is less than 50%, and current flows from OUT1 to OUT2. And, when the input (VIN1-VIN2) is negative, the on-duty of OUT1 is less than 50%, the on-duty of OUT2 is greater than 50%, and current flows from OUT2 to OUT1.

If the initial-stage amplifier voltage gain is Gv, then VIN1-VIN2 = (Vph-Vref)/Gv, OUT1 is at 100% on-duty (with the H side always on), and OUT2 is at 0% on-duty (with the L side always on).

With VIN1-VIN2 = (Vpl-Vref)/Gv, the on-duty of OUT1 is 0% (with the L side always on), and the on-duty of OUT2 is 100% (with the H side always on).

The following should be noted regarding this duty control. This configuration also requires a simultaneous-on prevention circuit to ensure that a state in which transistors on the high and the low sides are turned on simultaneously does not occur. When a simultaneous-on prevention circuit is present, it may not be possible to preserve linearity of control at duties near 0% and near 100%, and so the characteristics must be examined. Also, because there is variation in the offsets of initial-stage amplifiers, triangular-wave oscillators, and comparators, it is important to note that there are shifts in duty values relative to the input voltage.

Key Points:

・In H-bridge PWM driving in the form of BTL amplifier input, the difference in VIN voltages is used to control the duty.

・When a simultaneous-on prevention circuit is present, it may not be possible to maintain linearity near duties of 0% and 100%.

・It should also be noted that, because there is variation in the offsets of the initial-stage amplifier, the triangular-wave oscillator, and the comparator, the duty shifts with respect to the input voltage.

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