H-Bridge PWM Duty Cycle in Motor Control

Which PWM technology is best suited for your motor control application? In previous articles, we studied single-quadrant PWM technology, which is very suitable for cost-sensitive motor control applications. In these applications, you need to control the engine speed by changing the obligation cycle of the PWM flag. In any case, the engine can as it were pivot in one course and produce torque within the same course. We also introduced the "H-bridge" as a stepping stone to explore other PWM topologies. In this article, let's look at how to use the H-bridge to build a bidirectional speed control power stage. In particular, we will build a two-quadrant driver because it can produce forward motion with positive torque (1st quadrant) or reverse motion with negative torque (3rd quadrant). We will again choose a DC motor for discussion because using a DC motor makes it easier to understand these concepts.

Single phase H-bridge inverter

1. Creating a Unipolar Two-Quadrant Driver with H-Bridge Configuration

For unipolar PWM operation in the 1st quadrant, when we apply a PWM signal to Q4, Q1 is continuously on. You can click here to watch an animation of unipolar PWM operation in the 1st quadrant. When Q4 is on, a current path is created from Vbus, through Q1, through the motor, through Q4, and back to ground. At the end of this PWM state, Q4 turns off. Since the motor winding has inductance, it will try to maintain the motor current flowing in the same direction. The inductor protects its current just like a mother protects her child, actually saying, "Don't mess with my current! If you do, I will generate whatever voltage is necessary to maintain my current flow." As a result, the inductor forces the body diode of Q3 to conduct. However, since Q1 is always on, the motor current will return through Q1 instead of the DC power supply. When you think about it, you realize that since Q1 remains on, the behavior of this circuit is exactly the same as the single-quadrant driver discussed earlier, with one exception... If you want the motor to rotate in the other direction, simply keep Q3 continuously on and PWM Q2 instead. This will cause the motor to run in reverse and produce a negative torque, operating in the 3rd quadrant. You can click here to view an animation of this process.

Interestingly, in both 1st quadrant and 3rd quadrant operations, regardless of the direction of current flow in the motor, the bus current is either positive or zero! In other words, this PWM technology cannot regenerate energy. This is because the inductive flyback current is "trapped" in the upper half of the H-bridge and never flows back to the DC bus.This may be either an advantage or a drawback, depending on your application. On the off chance that you never ought to stress almost regenerative vitality, you do not have to be include the cost to handle it in your plan. On the other hand, if you want to recover load energy, then this PWM technology is not a good choice for you.

Another advantage of this technology is that it only requires one PWM signal at any given time. This means you can control more motors from one processor compared to some other PWM topologies. Additionally, only one transistor is switching at any given time, minimizing your switching losses. Finally, there is only one diode transient event per PWM cycle (when Q3's body diode conducts and Q4 turns on again). Therefore, this technology does not generate more switching noise compared to the single-quadrant technology we discussed earlier.

The main disadvantage of this technology is that even if you have four transistors, you still cannot operate in all four quadrants. It's like a car without brakes! If you want to slow down, you have two options: lift your foot off the accelerator and coast (reduce the PWM duty cycle), or suddenly reverse the car (immediately transition from the 1st quadrant to the 3rd quadrant!) By the way, I don't recommend trying this, or you may leave your transmission scattered across the highway! The latter situation is called PLUGGING. While it will cause the motor to decelerate very quickly, it is generally not a good idea because the resulting high current may cause your driver components to scatter across your lab bench.

2. Motor Control Applications of PWM Technology

In some cases, this PWM technology (even the single-quadrant circuit from the previous article) may cause energy regeneration back to your DC power supply. When the load causes the motor to accelerate in either direction, there is nothing to prevent it from running away, as this PWM technology cannot provide any braking. The motor will continue to accelerate until its back EMF voltage amplitude equals the DC supply voltage. If the speed exceeds this point, the body diodes in the FETs will start to conduct, allowing negative current to flow into the DC bus. We will discuss how to address this issue in detail in future articles.

3. Summary and Future Outlook

In summary, this PWM technology is popular in applications that require bidirectional motor speed control, but it doesn't matter if the motor coasts when you want to slow down.