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Impedance Transforms of the PWM Switch
Over the last 30 years, much progress has been made in modeling power switching circuits. In the 1980s, especially, the basic concepts were established for linearizing the single-pole, double-throw (SPDT) PWM switch having its common terminal in series with an inductance, a circuit “cell” that is found in the three basic converter configurations and was recognized by Richard Tymerski while a student at VPI. He first published the basic linear SPDT switch model and the circuit relationships were derived based on terminal quantities of the switch itself. In this TechNote, the basic equations are partially repeated and used to derive a more general model with voltages referred to an external ground node instead. By extending the basic switch equations in this way, the switch model can be viewed as a three-port device and impedances of each port referred to the others through transformations involving the duty ratio, D.
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