Switch mode power supply repair #8: Pulse Width Modulation explained (PWM)
1,483In this video we talk about pulse width modulation (PWM), which is a basic technique used in many electronic circuits like LED drivers, servo and motor controllers, switch mode power supplies, etc. PWM is used to regulate the amount of power flowing through the circuit, but without the disadvantages of linear regulators, where a large fraction of the power is being dissipated as heat.
The waveform of the PWM signal is being explained in combination with the on time, off time, rise time and fall time due to the switching delay in transistors, and this is compared to mechanical switches. Namely, transistors as switching elements are rather fast, but there is still a short time when the device is neither fully switched to the on or to the off state. In the case of PWM controllers we are referring to the switching element being "hard switched", which means that we force the switching element via the controller signal to fully turn on or to fully turn off.
In addition to the pulse width, we also discuss the Duty Cycle ration, which can be approximated as the ration between the on time and the full period of the oscillation of the driver signal at a given fixed frequency. In datasheets most of the time the Duty Cycle ratio is presented in form of a percentage value.
Pulse Width Modulation is nothing else but gradually chaning the on time of the switching transistor via a PWM controller chip such, that on the output the average power flowing to the consumer is being stabilized. In practice, for a switch mode power supply one will select a
switching frequency and a PWM controller and a specific topology. The PWM controller integrated circuit will keep track and ajust the duty cycle in a desired range to keep the output stable. This continuous adjustment of the duty cycle is what we refer to as pulse width modulation. For practical use in so called "hard switched" PWM switch mode power supplies we will keep the duty cycle below 50%, otherwise the magnetic core could go into magnetic saturation.