Switch mode power supply repair #7: Basics of Pulse Width Modulation (PWM)

In the seventh video in the Switch Mode Power Supply (SMPS) repair series we introduce the basic concept of power pulsing. Through power pulsing we use a switching element instead of a series resistive element to limit and regulate the average value of the power flowing through the circuit. This is shown through a simple experiment using a push switch and an incandescent lamp. In contrast to linear power supplies, pulsing supplies are more efficient, since there is no power being dissipated in form of heat on the series resistive element (whether that is a resistor or a transistor). Next, we introduce some basic concepts related to periodic functions, so that it will be easier to understand the on/off time during a period for PWM. We look into the period of a function, which is the amount of time it takes to finish a full cycle. The frequency will give us the number of full periods over a given amount of time base. The frequency expressed in Hz (Hertz) is also discussed, which practically shows us the number of full periods over a second as time base. These terms are presented using the 50Hz AC line voltage as example, and compared to audio and radio frequency signals. Since switch mode power supplies operate in a frequency range located at the top of the audible audio frequency range, and at the bottom of the radio frequency range, the periods will be expressed in datasheets either in ms (miliseconds) or us (microseconds).