Which PWM technique is best for your motor control application? In a previous article, we looked at single-quadrant PWM technique, which is ideal for extremely cost-sensitive Duty ratio to control the speed of the motor. But a motor can only spin in one direction and generate torque in that same direction. We also introduce the “H-bridge” as a springboard to study other PWM topologies. In this article, let’s take a look at how to build a bi-directional speed control power stage using an H-bridge. In particular, we will build a 2-quadrant drive because it can produce forward motion with positive torque (quadrant 1), or reverse motion with negative torque (quadrant 3). We will again choose DC motors for our discussion because the concepts are easier to understand using DC motors. How to use an H-bridge configuration to create a unipolar two-quadrant driver
For unipolar PWM operation in Quadrant 1, Q1 is continuously on when we apply a PWM signal to Q4. You can watch an animation of unipolar PWM operation in Quadrant 1 here. When Q4 turns on, a current path is created from the V bus, through Q1, through the motor, through Q4, and back through ground. At the end of this PWM state, Q4 is turned off. Due to the inductance of the motor windings, it will try to keep the motor current flowing in the same direction. An inductor protects its current flow like a mother protects her child. It’s essentially saying, “Don’t mess with my current! If you do, I’ll generate whatever voltage is necessary to keep my current flowing.” As a result, the inductor forces Q3’s back-body diode to conduct. But since Q1 is always on, the motor current will return through Q1 instead of the DC supply. When you think about it, you realize that since Q1 is constantly on, the circuit behaves exactly like the one-quadrant driver discussed earlier, with one exception…if you want the motor to spin in the other direction, just keep Q3 on and Change to PWM Q2. This causes the motor to run in reverse and produce quadrant 3 operation with negative torque. You can view an animation of this process by clicking here.
Interestingly, in quadrant 1 and quadrant 3 operation, the bus current is either positive or zero regardless of which direction the current flows in the motor! In other words, this PWM technique cannot regenerate energy. This is because the sense flyback current is “trapped” in the upper half of the H-bridge and never flows back to the DC bus. Depending on your application, this can be an advantage or a disadvantage. If you never have to worry about renewable energy, then you don’t have to add expense to your design to handle it. On the other hand, if you want to recover load energy, then this PWM technique is not a good choice for you.
Another advantage of this technique is that it requires only one PWM signal at any given time. This means you can control more motors from one processor than some other PWM topologies. Also, only one transistor is switching at any given time, so your switching losses are minimized. Finally, there is only one diode transient event per PWM cycle (when Q4 turns on again after the Q3 backbody diode turns on). Therefore, this technique does not generate any more switching noise than the single-quadrant technique we discussed earlier.
The main disadvantage of this technique is that even if you have four transistors, you still can’t operate in all four quadrants. It’s like a car with no brakes! If you want to slow down, you have two options; take your foot off the accelerator and coast (lowering the PWM duty cycle), or suddenly reverse the car (immediately from the first Quadrant transitions to quadrant 3!) By the way, I don’t recommend trying this, or you could end up with pieces of your transmission all over the highway! The latter case is called PLUGGING. While it can cause the motor to decelerate super fast, it’s generally not a good idea, as the resulting high currents can litter your drive components all over your lab bench!
You should be aware that in one case, this PWM technique (even the single-quadrant circuit from the previous article) could cause energy to be regenerated back into your DC supply. When the load accelerates the motor in either direction, there is nothing to stop it from spinning out because this PWM technique cannot provide any braking. The motor will continue to accelerate until its back EMF voltage magnitude equals the DC supply voltage. If the speed exceeds this point, the backbody diode in the FET will conduct and negative current will flow into the DC bus. We’ll discuss ways to deal with this in a future article.
In summary, this PWM technique is popular in applications that require bi-directional motor speed control, but it doesn’t matter if the motor coasts on its own when you want to slow it down. In the next blog post, we’ll see that by changing just one signal on one of the transistors, we can use energy regeneration to slow down the motor at the same time.