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Capacitive Start-Up Circuit for Switching Power Supply

IP.com Disclosure Number: IPCOM000043662D
Original Publication Date: 1984-Sep-01
Included in the Prior Art Database: 2005-Feb-05
Document File: 2 page(s) / 30K

Publishing Venue

IBM

Related People

Groves, JO: AUTHOR [+2]

Abstract

A voltage divider circuit has capacitors to provide start-up power to the controller of a switching power supply. The capacitors do not consume power during subsequent operation as a prior resistor circuit did. An AC voltage supplied over lines 1 and 2 is rectified by a diode bridge 3 and filtered by a capacitor 4 so that a DC voltage is supplied to a line 5. At start-up, a high voltage transient occurs to cause a substantial current to pass through capacitors 6 and 7, which are in series between the line 5 and a line 8. By selecting the capacitance of the capacitor 6 to be smaller than the capacitance of the capacitor 7, the low voltage required to turn on a pulse-width modulator 9 is provided. When the modulator 9 is turned on, a transistor 10 repeatedly switches on and off.

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Capacitive Start-Up Circuit for Switching Power Supply

A voltage divider circuit has capacitors to provide start-up power to the controller of a switching power supply. The capacitors do not consume power during subsequent operation as a prior resistor circuit did. An AC voltage supplied over lines 1 and 2 is rectified by a diode bridge 3 and filtered by a capacitor 4 so that a DC voltage is supplied to a line 5. At start-up, a high voltage transient occurs to cause a substantial current to pass through capacitors 6 and 7, which are in series between the line 5 and a line 8. By selecting the capacitance of the capacitor 6 to be smaller than the capacitance of the capacitor 7, the low voltage required to turn on a pulse-width modulator 9 is provided. When the modulator 9 is turned on, a transistor 10 repeatedly switches on and off. This results in power being supplied to a load 11 through an isolating transformer 12. The rendering of the transistor 10 conductive also causes the isolating transformer 12 to feed power through a diode 13 in a feedback circuit to sustain operation of the modulator 9. A diode 14 protects the modulator 9 when power is shut off. Because the capacitor 6 charges initially from the transient voltage at start-up, no further current flows therethrough. As a result, no power is drawn along this path after start-up.

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