What is Pulse Width Modulation?

How does pulse width modulation work? Referring to Fig. 1, the classical method of voltage mode PWM is to compare an error signal to a reference ramp oscillator waveform as shown in Fig. 1. In the figure, as the error signal crosses the rising edge of the ramp, the comparator output goes to its high output level, and conversely when the error signal crosses the falling edge of the ramp, the comparator switches to its low level output.

Also, if we wish the duty cycle to increase when the error signal falls, to correct the system output, or increase system output, such as the voltage of a regulated converter, or the speed of a motor, for example, the example of Fig. 1 shows exactly that effect. As the error signal drops, the pulse width increases, thereby applying more power, or voltage, or more signal level to the load.

Why do we convert the error signal to a PWM signal? Because the PWM signal can operate an electronic switch such as a mosfet or a transistor that will rapidly switch on and off and transfer power more efficiently. The comparator output, if it is 0 to 5 volts or higher, can drive the gate of a power mosfet directly thereby controlling a lot of power with a small signal. If needed, the output signal of the comparator can be buffered with a high current buffer pulse amplifier to allow the ability to drive very large transistor switches and even paralleled transistor switches for high power.