Pulse width modulation (PWM) is a power control technique that adjusts the effective output of electronic signals by rapidly switching them at a fixed frequency. By adjusting the ratio of the "conduction" time to the total cycle, the digital signal source can simulate constantly changing analog voltage levels, thereby controlling the average energy provided to the load.
More broadly speaking, modulation technology refers to changing the electrical waveform or encoding information into an electrical waveform to influence the behavior of circuits or systems. In practical electronic products, this means shaping the signal to enable it to transmit data or manage the magnitude of the voltage or current reaching the device. This principle has been widely applied in motor drives, lighting dimming, audio systems, as well as power conversion or battery charging circuits.
Although PWM, amplitude modulation (AM), and frequency modulation (FM) are the main strategies for controlling signal perception of amplitude or frequency, this article will specifically discuss PWM.
PWM Fundamentals - Duty Cycle and Switching Frequency
As mentioned earlier, PWM forms waveforms by adjusting the effective voltage and current delivered to the load. This is achieved by quickly driving switching devices (usually transistors) to switch between fully on and fully off states. By changing the holding time of the switching device in each state, the system encodes information through the relative duration of the high-level and low-level intervals.
In fact, PWM limits its net electrical power by changing the time it takes for the device to obtain full power supply voltage in each switching cycle. Increasing the 'conduction time' will increase the average output voltage, while reducing the 'conduction time' will lower the effective voltage level of the load. This behavior can be described by two main parameters: duty cycle and switching frequency.
The duty cycle represents the proportion of time a signal is in an active or high-level state within a complete waveform cycle. This ratio is usually expressed as a percentage (%), indicating how long the output remains in the on (effective) state during each cycle. For example, if the digital waveform maintains a high level for 3 milliseconds and a low level for 1 millisecond, the total period is 4 milliseconds, the duty cycle is 75%, and the corresponding switching frequency is 250 Hz.
Since the duty cycle directly determines the duration of each pulse energized section, modifying the duty cycle can control the effective power delivered to the load by changing the ratio of high-level time to low-level time without changing the actual power supply voltage. In many systems, voltage and frequency are fixed parameters, and duty cycle is the main adjustable control variable. In applications such as PWM driven heating elements, monitoring the duty cycle can also serve as a reliable indicator for determining the expected power level provided by the system.
The switching frequency describes the number of times an event repeats within a given time period. Here, it refers to the number of "on-off" cycles performed per second by the switching device that drives the PWM signal. This frequency is measured in Hertz (Hz) and represents the cycling speed of the power level throughout the entire operating cycle.
To ensure the expected performance of the load, it is necessary to choose an appropriate PWM switching frequency. If the frequency set for a specific application is too high, mechanical components such as relays or certain types of actuators may not be able to achieve fast switching speed, resulting in premature failure. On the contrary, a low switching frequency may have adverse effects such as noise, vibration, or instability of controlled devices. For example, although relatively low frequencies are acceptable for driving motors, solid-state loads such as LEDs typically require significantly higher switching frequencies to operate smoothly without flicker.