Pulse Width Modulation (PWM) is a basic concept that is employed in many areas of electronics. PWM is a simple averaging method that is used in everything from microwave power percentages to LED dimming. The methods of implementation are also simple yet there are aspects that are quite complicated.
One of the simplest ways to explain PWM is with LED lighting. LED’s are turned on at a recommended current primarily for optimal brightness. Secondary parameters for operating current are heat generation and long term reliability. Instead of dimming the operating current, the LED on-time is pulsed using a duty cycle. In this manner, the LED appears to be dimmed while still operating in a happy spot from a current standpoint.
When I operate my microwave on a percentage of power, I can hear the fan running continuously however the speed cycles as the power surges on and off. In a way, this operation is similar to the LED dimming where the 100% power during the ON time is averaged to get the percentage power setting.
Variable speed motor drive is another area where PWM methods are used to control motor speed by averaging the on-time.
PWM has been a favorite method of operating switched mode power supplies (SMPS) for the last 30 years or so. The development of adequate semiconductors and especially switching transistors has enabled PWM to become the established method for energy transfer. A typical buck regulator input (Vin) to output voltage (Vout) ratio is the duty cycle (D) of the PWM where
Thus the buck or step down regulator creates an output voltage that is a percentage (average) of the input voltage.
Making a PWM waveform is rather easy. All that is needed is a triangular or sawtooth wave of a given frequency fed into the negative side of a comparator with a modulation signal fed into the positive side of the comparator. The peak to peak value of the waveform has to exceed the range of the modulation signal for achieving full 100% duty cycle pulse widths.
Thus far, PWM is a straight forward averaging signal based on a constant frequency and trailing edge modulation. In other words, a clock initiates the PWM signal and the trailing edge termination is the duty cycle of the pulse. There are however other PWM methods including trailing edge modulation. This method was explored in power electronics research at Virginia Tech and found to have advantages in offsetting the Right Half Plane Zero in boost as well as flyback converters. An excerpt from similar work states that, “It is shown how a fixed-frequency, leading-edge modulated PWM can eliminate the undesirable positive zero in practical boost and flyback converters. This allows a substantial improvement in the closed-loop characteristics. Several techniques are employed to predict this result. The design procedure for elimination of the positive zero is presented. Experimental verification is provided.”
However, it appears as though leading edge modulation can introduce problems in addition to solving them according to Dr. Ray Ridley whose website states, “Note: this is an analysis of an existing satellite power system which had succeeded in eliminating the RHP zero by clever and intuitive design. It is not recommended that you build your boost or flyback converters this way, but you may run into this phenomenon at some time. Interesting paper if you are into control theory of converters. It's not practical in most cases since it requires the use of high ESR caps which leads to elevated loss and noise.”
Other forms of PWM include modulating both the front edge and the trailing edge of a waveform. Wikipedia’s explanation of PWM includes a figure that shows the three types (lead, center, trail) of modulation as follows:
Three types of PWM signals (blue): leading edge modulation (top), trailing edge modulation (middle) and centered pulses (both edges are modulated, bottom). The green lines are the sawtooth waveform (first and second cases) and a triangle waveform (third case) used to generate the PWM waveforms using the intersective method. (Image courtesy of Wikipedia)
Analog generation of PWM is not the only method used. As digital control adds more and more intelligence to electronics, digital PWM methods are becoming more common place. Once again, our friends at the University of Colorado have addressed digital PWM in their power electronics research. I found this work to be quite interesting as not only did it address sampling and A/D conversion, there were references to the associated state equations as well as the loop gain equations for the control aspect. Bode plots were also presented to show the loop gain and phase performance.
In closing, there is a lot more to PWM than just basic averaging. When implemented properly into a system control loop stability and response time can be optimized. I enjoyed this subject and the reading. I hope that you do as well. There is much more to learn on the subject of PWM beyond this introductory blog.
- “Pulse Width Modulation – DC Motor Drives”
- “Use of Leading-Edge Modulation to Transform Boost and Flyback Converters Into minimum-Phase-Zero Systems” Sable, Dan M.; Cho, Bo H.; Ridley, Ray B., AA(Virginia Polytechnic Institute and State University, Blacksburg), AB(Virginia Polytechnic Institute and State University, Blacksburg), AC(Virginia Polytechnic Institute and State University, Blacksburg),IEEE, Applied Power Electronics Conference, Los Angeles, CA, Mar. 12-16, 1990 IEEE Transactions on Power Electronics (ISSN 0885-8993), vol. 6, Oct. 1991, p. 704-711.10/1991
- “Minimum Phase Response in Digitally Controlled Boost and Flyback Converters” Vahid Yousefzadeh, Mariko Shirazi and Dragan Maksimovic, Colorado Power Electronics Center, ECE Department, University of Colorado, Boulder, CO 80309-0425
- Sable, D. M., B. H. Cho and R. B. Ridley, Elimination of the Positive Zero in Fixed Frequency Boost and Flyback Converters, IEEE Applied Power Electronics Conference Proceedings, Los Angeles, California, March 1990, pp. 205-211. Search among older Ridley Engineering publications here
- “Pulse-width modulation” From Wikipedia, the free encyclopedia