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Coupled Capacitor Filter for Reduced Voltage Ripple on Multiple Output Switched Mode Power Supplies

IP.com Disclosure Number: IPCOM000042688D
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 3 page(s) / 52K

Publishing Venue

IBM

Related People

Spreen, JH: AUTHOR

Abstract

Capacitive coupling of output filters in some varieties of switched- mode power supplies can be used to significantly reduce the capacitor voltage ripple in one output, while another output coupled thereto has voltage ripple no worse then would be the case for conventional uncoupled filters. Further, less total filter capacitance is needed for the capacitively coupled filters than for the uncoupled filters. An example of a power supply in which this approach would be advantageous is the two-output flyback DC-DC converter shown schematically in conventional form in Fig. 1. The diodes D1 and D2 conduct during Toff, the time the power transistor Q is off; the diodes are off (non- conducting) during Ton, the time the transistor is on.

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Coupled Capacitor Filter for Reduced Voltage Ripple on Multiple Output Switched Mode Power Supplies

Capacitive coupling of output filters in some varieties of switched- mode power supplies can be used to significantly reduce the capacitor voltage ripple in one output, while another output coupled thereto has voltage ripple no worse then would be the case for conventional uncoupled filters. Further, less total filter capacitance is needed for the capacitively coupled filters than for the uncoupled filters. An example of a power supply in which this approach would be advantageous is the two-output flyback DC-DC converter shown schematically in conventional form in Fig. 1. The diodes D1 and D2 conduct during Toff, the time the power transistor Q is off; the diodes are off (non- conducting) during Ton, the time the transistor is on. Thus, during Ton the energy stored in the transformer is increased by energy drawn from the bulk supply. During T off, the secondary windings carry current, extracting stored energy from the transformer to be delivered to the loads. Regulation is provided by adjustment of the on/off time ratio of Q through the pulse-width modulation (PWM) feedback control circuitry. The capacitor filters smooth the pulsed current so that the load currents IR1 and IR2 flowing through the loads R1 and R2 are nearly constant direct currents with corresponding nearly constant output voltages Vo1 and Vo2 . In this illustration, the outputs are arranged to provide one negative and one positive output. Fig. 2a is a plot of filter capacitor current versus time for one switch period for an ideal square-wave flyback converter. Fig. 2b shows the corresponding output voltage waveforms, with ripple magnitudes exaggerated for clarity. For the idealized waveforms shown, with parasitic circuit elements neglected, the ripple voltages are: The capacitively coupled output filter, shown in Fig. 3, has two advantages over the conventional filter of Fig. 1. First, with the capacitor values shown and the simplifications assumed in the discussion above, output #2 has no voltage ripple: WVo2 = 0. Further, this ripple is eliminated with no change in the voltage ripple in output #1; it is still given by (1) above...