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Switching Regulator Soft Start Circuit

IP.com Disclosure Number: IPCOM000062075D
Original Publication Date: 1986-Oct-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 36K

Publishing Venue

IBM

Related People

Radcliffe, JK: AUTHOR

Abstract

Switching regulators require a means of turning on the power supply gradually. In power supplies that use pulse-width modulators (PWM) a common method is to allow the pulses out of the PWM to gradually increase until the desired output voltage is reached and then allow the control loop to take over. A typical circuit that accomplishes this is seen in Fig. 1. Prior to time T1, the switching regulator is turned off. The error voltage is high as a result of the output voltage being low. Transistor Q1 is on, holding V1 low. The comparator arrangement shown (Fig. 1) causes the output voltage to respond to the lower of either V1 or the error voltage. At time T1, transistor Q1 is turned off and V1 begins to rise. At T2, the output voltage reaches its desired value and the error voltage begins to fall.

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Switching Regulator Soft Start Circuit

Switching regulators require a means of turning on the power supply gradually. In power supplies that use pulse-width modulators (PWM) a common method is to allow the pulses out of the PWM to gradually increase until the desired output voltage is reached and then allow the control loop to take over. A typical circuit that accomplishes this is seen in Fig. 1. Prior to time T1, the switching regulator is turned off. The error voltage is high as a result of the output voltage being low. Transistor Q1 is on, holding V1 low. The comparator arrangement shown (Fig. 1) causes the output voltage to respond to the lower of either V1 or the error voltage. At time T1, transistor Q1 is turned off and V1 begins to rise. At T2, the output voltage reaches its desired value and the error voltage begins to fall. However, the output voltage continues to rise until the error voltage becomes lower than V1 at time T3. The overshoot from T2 to T3 is unlikely to be eliminated with this method. A circuit that resolves this shortcoming is described in the following. The circuit is seen in Fig. 2. As in previous circuit (Fig. 1), when the regulator is turned off, the error voltage is high and V1 is low. At T1, transistor Q1 is turned off and V1 begins to rise as capacitor C discharges through resistor R. At T2, the output voltage reaches its final value and the error voltage begins to fall. The reduction in the error voltage is capacitively coupled to...