CET Power Supplies

SWITCHED MODE SUPPLIES

Switched Mode Power Supplies use pulse width modulation (PWM) to regulate their output voltage.  They are highly efficient, lightweight, and less costly than their predecessor supplies known as linear power supplies.  They switch at much higher frequencies than line voltage and while this does introduce a little high frequency noise into the system; compared with the increase in efficiency and decrease in size this is seen as minor and tolerable.

First, the line voltage is rectified using diodes and filtered with capacitors.  This sets the input voltage to the PRI of a small ferrite core switching transformer.  The output of the transformer is a function of the turns ratio, frequency and the duty cycle of the switched DC.  By pulsing the DC into the PRI at a high frequency only a small transformer is needed because the PRI turns need be less to avoid core saturation.  It also means the core area is much less as well in avoiding the same core saturation issues.

Ferrite cores do have a lower MAX flux density than do silicon steel cores but since the applied volt-sec’s is so much less given the high frequencies used (200kHz to 500kHz typically), compared with 50/60Hz, not only can this be lower, but the turns and core area can all be lower and still satisfy the smaller required volt-sec’s.

The output of the switching transformer is then routed through diodes and filtered with capacitors to give a DC output.  Feedback is employed that increases and decreases the duty cycle as the load demand changes.

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