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2022.08.03 motor

Driving Brushed DC Motors Using PWM Output:
PWM Driving with an H-Bridge Circuit

Brushed DC Motor

We begin explanations of specific methods for PWM driving of brushed DC motors.

Driving Brushed DC Motors Using PWM Output:
PWM Driving with an H-Bridge

Below, two representative examples of PWM driving using an H-bridge circuit are introduced.

① Direct driving in which a PWM signal is input to one of two input terminals
In motor driving using an H-bridge circuit, using two logic inputs to switch between four states (stopped, forward rotation, reverse rotation, braking) was explained in "Driving Brushed DC Motors with an H-Bridge: Switching Output States". The first PWM driving method involves inputting the PWM signal into one of these two input terminals.

This block diagram is of a certain driver IC for a brushed DC motor. The H-bridge is switched by the logic signals at IN1 and IN2.

Of the two truth tables, that on the left is standard switching logic. Therein, the H inputs for forward rotation (H/L) and reverse rotation (L/H) are PWM inputs. This is shown in the truth table on the right. Here the four states are stopped (OPEN), forward-rotation PWM driving, reverse-rotation PWM driving, and braking. For this control, a microcomputer or other device must send logic signals and PWM signals to IN1 and IN2. Of course, the driver IC must support this driving method.

In this method, the product of the duty cycle for the PWM input signal and the power supply voltage VM is the equivalent average applied voltage.

② Motor driver equipped with a Vref PWM control function
There are motor driver ICs that are equipped with PWM functions. This example is of a driver, equipped with an internal triangular wave generator and a comparator, that is capable of PWM driving of the output and enables PWM duty cycle adjustment.

The block diagram and waveform diagram indicate the design. The comparator outputs the result of comparison of the triangular wave from the triangular wave generator and an external reference voltage Vref. The output duty cycle of the comparator is determined by Vref; when Vref is 0 V, which is the minimum amplitude of the triangular wave, the duty cycle is 0%, and when it is Vph, the maximum amplitude of the triangular wave, the duty cycle is 100%. This comparator output (PWM signal) is combined with the IN1/IN2 logic, and passes through the H-bridge to be output to OUT1 and OUT2.

① and ② are both in principle as explained above, and PWM driving with an H-bridge is possible; but depending on the incorporated simultaneous on prevention circuit, and because in ② an analog circuit is used for PWM signal generation, the results may deviate from theory, and some adjustment may be necessary due to linearity, errors and the like.

Key Points:

・There are a number of types of motor driver ICs with H-bridges which support PWM driving of brushed DC motors.
 -There are driver ICs for which PWM signals can be input directly to IN1/IN2.
 -There are driver IC types provided with a PWM signal generation circuit
  having a Vref pin, a triangular wave generator, and a comparator.

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