Bidirectional PWM Motor Control

Once you understand unidirectional PWM of an inductive motor load, we can expand the concept to a bidirectional h-bridge PWM circuit. 

Let’s start by using our h-bridge circuit to mimic the unidirectional PWM circuit in the previous article.  For clarity, we’ll leave out the freewheeling diodes for the moment. 

First, we turn on S1 and keep it on to connect one motor lead to the positive supply. Then we alternately modulate S3 and S4. Look familiar?  Since the circuit is now symmetrical, we can hold S3 on and modulate S1 and S2 to drive the motor in the opposite direction.

This technique is called sign-magnitude modulation. The sign of the signal determines which side of the bridge is modulated and the magnitude determines the duty cycle of the modulation.  Sign-magnitude modulation is often used for electric vehicles and other applications where the motor spends most of its time turning in one direction. 

Note that we can also just as easily choose to hold a low-side switch on instead of a high-side switch. As mentioned briefly in the H-bridge basics article, when both high side (or both low side) switches are on, this is known as “recirculating” or “slow decay” mode.  

Now let’s put the diodes back in and imagine that we start with S1 and S4 closed as before, causing current to flow through the motor.  What if we suddenly open all the switches?  Current must continue flowing through the motor, at least for a moment, so it takes the only path available, through the freewheeling diodes. Current is actually flowing the “wrong” way, pumping power back into the power supply rails.  Since the current is flowing against the full force of the power supply voltage, it decays more rapidly that the slow decay recirculation.  For this reason it is known as “fast decay”.  We might as well turn on S3 and S2 as a kind of synchronous rectification like we did for sign-magnitude modulation. 

This leads to the a second strategy of bidirectional PWM, called locked-antiphase. With this method, both sides of the bridge are modulated simultaneously. When S1 is on, S4 is on and alternately when S2 is on, S3 is on. If the system spends the same amount of time in both states, the net effective voltage applied to the motor is zero. If the time spent in one state is increased and/or the other is decreased, the motor will be driven in the corresponding direction. 

Locked-antiphase modulation is often used for industrial servo motor controls where the motor is frequently accelerating, decelerating and reversing.