pulse width modulation


Posted on Feb 5, 2014

PWM is a highly efficient control scheme used to manipulate the flow of power through various electronic devices. I have seen it most commonly used in switched-mode power supplies, LED dimming circuits, and variable speed motor controls, but there are several more applications. The concept of PWM draws from the most basic elements of a binary system:


pulse width modulation
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on (1) or off (0). At any given moment, switches are turned on or off depending on the feedback signal from the load receiving power. This process allows power to be taken from the source in doses and supplied to the output such that the output remains relatively constant. Exactly how much power is supplied to the load depends on the duty cycle of the switching system. The duty cycle of a PWM waveform refers to the amount of time relative to an entire switching period that the pulse stays high. It is usually expressed as a percentage or decimal and can vary anywhere between 0% (completely off) to 100% (completely on). Figures 1-3 show some examples of various duty cycles used for voltage regulation and the math explaining how the load sees an average voltage depending on the switching conditions. The figures assume the switches are operating at 50 kHz (being turned on and off 50, 000 times/second). In practice, PWM can be implemented in power supplies if the source power is switched at a sufficiently high frequency such that the output inductor can supply current without letting it fall to zero. Current passing through an inductor cannot change instantaneously which means even if the switch from the source is turned off the inductor current will drop steadily instead of falling to zero immediately. As long as the switch comes back on before the inductor current falls too low to be useful, the load sees a continuous flow of...




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