Pulse-Width Modulation (PWM) is a technique widely used in power electronics to control the amount of power delivered to a load by rapidly switching a power signal on and off. This method is commonly employed in various applications, such as motor control, LED dimming, voltage regulation, and more. PWM allows for efficient control of power levels without relying solely on linear voltage or current regulation, which can be inefficient and result in excessive heat dissipation.
The basic idea behind PWM is to modulate the width (duration) of the ON time of a periodic signal while keeping the frequency constant. This periodic signal is typically referred to as the "carrier signal" or "PWM signal." By adjusting the ratio of the ON time to the total time of each cycle, the effective power delivered to the load can be controlled.
Here's a step-by-step explanation of how PWM works:
Carrier Signal: Start with a high-frequency carrier signal, usually a square wave, generated by a pulse generator or microcontroller. The frequency of this carrier signal is much higher than the response time of the load (e.g., motor or LED).
Reference Signal: Generate a reference signal that represents the desired power level or control value. This reference signal is often an analog voltage or digital value that needs to be converted into a continuous power level.
Comparison: Compare the reference signal to the carrier signal at every cycle. The comparison results in a digital "high" or "low" signal, indicating whether the reference value is higher or lower than the carrier signal at that moment.
Modulation: The digital comparison signal is used to control a switch (usually a transistor or MOSFET) placed between the power source and the load. When the comparison signal is "high," the switch is turned on, allowing current to flow to the load. When the signal is "low," the switch is turned off, cutting off the current flow.
Average Power Control: The overall power delivered to the load is determined by the duty cycle, which is the ratio of the ON time to the total time of a single cycle of the carrier signal. A higher duty cycle leads to more power being delivered to the load, while a lower duty cycle reduces the power. By adjusting the duty cycle based on the reference signal, the effective power to the load can be controlled.
Smoothness and Filtering: The rapid switching of the PWM signal can result in unwanted high-frequency components that could cause interference or instability in the load. To mitigate this, filters and smoothing components may be added to the circuit to convert the pulsed output into a smoother, continuous signal.
By using PWM, power electronics systems can achieve precise and efficient control over power delivery to loads. It offers advantages like higher efficiency, reduced heat dissipation, and the ability to achieve various power levels without resorting to lossy linear regulators.