So, Which PWM Technique is Best? (Part 2)

https://e2e.ti.com/blogs_/b/motordrivecontrol/archive/2012/03/26/so-which-pwm-technique-is-best-part-2

So, Which PWM Technique is Best? (part 2)

Dave Wilson, Motion Products Evangelist, Texas Instruments

So, which PWM technique is best for your motor control application? In the previous blog, we examined the single-quadrant PWM technique, which is a good fit for extremely cost sensitive motor control applications where you want to control the motor’s speed by varying the duty-cycle of a PWM signal. But the motor can only spin in one direction, and generate torque in that same direction. We also introduced the “H-Bridge” as a springboard to investigate other PWM topologies. In this blog, let’s take a look at how to build a bi-directional speed control power stage by using an H-Bridge. In particular, we will construct a 2-Quadrant Drive since it can produce forward motion with positive torque (quadrant 1), or reverse motion with negative torque (quadrant 3). Again we will choose a DC motor for this discussion, since the concepts are more easily understood with a DC motor.



For Unipolar PWM operation in quadrant 1, Q1 is turned ON continuously while we apply a PWM signal to Q4. You can watch an animation of Unipolar PWM operation in quadrant 1 by clicking here. When Q4 is switched ON, a current path is created from Vbus, through Q1, through the motor, through Q4, and returning through ground. At the end of this PWM state, Q4 is switched OFF. Since the motor winding has inductance, it will fight to keep the motor current flowing in the same direction. An inductor protects its current just like a mother protects her child. It effectively says, “Don’t mess with my current! If you do, I will generate whatever voltage is necessary to keep my current flowing.” As a result, the inductor forces the back-body diode of Q3 to conduct. But since Q1 is always ON, the motor current will return through Q1, not the DC supply. When you think about it, you realize that since Q1 is ON continuously, this circuit behaves exactly like the single quadrant drive discussed earlier with one exception…if you want the motor to spin in the other direction, simply turn Q3 ON all the time and PWM Q2 instead. This results in quadrant 3 operation where the motor is running in reverse, and generating negative torque. You can see an animation of this process by clicking here.

It’s interesting to note that in both quadrant one and quadrant three operation, the bus current is either positive or zero, regardless of which direction the current is flowing in the motor! In other words, this PWM technique cannot regenerate energy. The reason for this is because the inductive flyback current is “trapped” in the top half of the H-Bridge, and never flows back into the DC bus. This can either be an advantage or a disadvantage, depending on your application. If you never have to worry about regenerated energy, then you don’t have to add expense to your design to deal with it. On the other hand, if you want to recover load energy, then this PWM technique is not a good choice for you.