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# Delayed Power On Circuit with Soft Start

IP.com Disclosure Number: IPCOM000110557D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 2 page(s) / 47K

IBM

## Related People

Nakano, M: AUTHOR

## Abstract

This article describes a delayed power-on circuit with soft start. A switching power supply is almost required soft start function and delayed power-on function. A conventional circuit of switching power supply has those function as two independent blocks of circuit. The disclosed circuit consists of those two function in one circuit block shown in Fig. 1.

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Delayed Power On Circuit with Soft Start

This article describes a delayed power-on circuit with
soft start.  A switching power supply is almost required soft start
function and delayed power-on function.  A conventional circuit of
switching power supply has those function as two independent blocks
of circuit.  The disclosed circuit consists of those two function in
one circuit block shown in Fig. 1.

A is an operational amplifier.  Its output is connected to a
PWM (Pulse Width Modulation) comparator to control a switching power
supply.  Resistors R1,R2,R3 and a capacitor C make a time constant to
define both delayed power-on time and soft start time.  When power-on
voltage +V is applied the capacitor C is charging up and also its
voltage VC is going up.  When VC exceeds VT which is divided +V by R4
and R5, the operational amplifier A, whose output voltage is
saturated into +V, starts to work into active operation area and the
output voltage is going down.

The time which is from the power-on time to the crossing time
of VC and VT is delayed power-on time (Fig. 2).

Next, the output voltage (VO) of the operational amplifier A is
given by VO=(VT-VC)*R3/R2 because the operational amplifier A makes
an inverting amplifier with R2 and R3.  VO slowly decreases at a
defined rate according to the product of the rate of VC change and
the gain of the inverting amplifier(-R3/R2).  When the operational
amplifier A is saturated to ground...