Browse Prior Art Database

SATURATING MAGNETIC CONTROL OF SWITCHING POWER SUPPLY

IP.com Disclosure Number: IPCOM000005497D
Original Publication Date: 1983-Mar-01
Included in the Prior Art Database: 2001-Oct-09
Document File: 2 page(s) / 92K

Publishing Venue

Motorola

Related People

Michael P. Bafaro: AUTHOR

Abstract

In a self-oscillating switch mode power supply, 100 volts AC is bridge rectified to supply the DC operating power to a power supply oscillator. The oscillator drive feedback is supplied by a control transformer T2 which samples the primary winding current in a power transformer Tl. A center tapped secondary winding of Tl comprising a pair of 17 turn windings provides an AC signal which is rectified to provide a DC output to a load.

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Volume 3 March 1983

SATURATING MAGNETIC CONTROL OF SWITCHING POWER SUPPLY

By Michael P. Bafaro

   In a self-oscillating switch mode power supply, 100 volts AC is bridge rectified to supply the DC operating power to a power supply oscillator. The oscillator drive feedback is supplied by a control transformer T2 which samples the primary winding current in a power transformer Tl. A center tapped secondary winding of Tl comprising a pair of 17 turn windings provides an AC signal which is rectified to provide a DC output to a load.

   The transformer T2 acts as a saturable reactor with its degree of saturation depending upon the amount of DC excitation of a 200 turn control winding. A DC error amplifier compares the DC output of the power supply to an effective reference voltage and controls the saturation of T2 so as to maintain the DC output constant and at a predetermined desired magnitude. This is accomplished in the follow- ing manner.

   Since T2 has a 25 turn primary winding in series with a 30 turn primary of Tl, Tl has effectively a large leakage inductance and this causes current in the collectors of oscillator transistors Ql and Q2, and therefore the Tl primary current, to be ramps of current. Since the currents are in the form of ramps, the amount of energy delivered to the load by the center tapped Tl secondary winding is propor- tional to the length of time current is permitted to ramp up. The energy delivered to the load is also an inverse function of the combined impedance of the 30 turn Tl primary and the 25 turn T2 primary since this combined impedance also determines the maximums of the collector currents.

   The degree of saturation of T2 controls the inductance and therefore also the impedance of the 25 turn T2 primaty winding. As T2 saturation increases, the inductance of the T2 primary decreases and this results in increasing the oscillation frequency which tends t...