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The Fundamentals of PWM Switching Power Supplies

The operation of switching power supplies can be relatively easy to understand. Unlike linear regulators which operate the power transistor in the linear mode, the PWM switching power supply operates the power transistors in both the saturated and cutoff states. In these states, the volt-ampere product across the power transistor is always kept low (saturated, low-V/high-I; and cutoff, Hi-V/No-I). This EI product within the power device is the loss within all the power semiconductors.
This more efficient operation of the PWM switching power supply is done by “chopping” the direct current (dc) input voltage into pulses whose amplitude is the magnitude of the input voltage and whose duty cycle is controlled by a switching regulator controller. Once the input voltage is converted to an ac rectangular waveform, the amplitude can be stepped up or down by a transformer.
Additional output voltages can be derived by adding secondaries to the transformer. Ultimately these ac waveforms are then filtered to provide the dc output voltages. The controller, whose main purpose is to maintain a regulated output voltage, operates very much like a linear style controller. That is, the functional blocks, voltage reference, and error amplifier are arranged identical to the linear regulator’s. The difference is, the output of the error amplifier (the error voltage) is then placed into a voltage-to-pulsewidth converter stage prior to driving the power switches.
There are two major operational types of switching power supplies
: the forward-mode converter and the boost-mode converter. Although their arrangements
of parts are subtly different, their operation is very different and each
has advantages in certain areas of application.


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