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# Variable Frequency-Duty Cycle Slow Start Control for Switched-Mode Power Supplies

IP.com Disclosure Number: IPCOM000034577D
Original Publication Date: 1989-Mar-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 2 page(s) / 51K

IBM

## Related People

Bolash, JP: AUTHOR

## Abstract

Disclosed is a circuit that eliminates damaging charging currents from a switched-mode power supply to its secondary filter capacitor4s during power on without using relays, large inductors, or very fast switching components. Referring to Figs. 1 and 2, switching supplies generally have a current sense circuit to limit the current charging capacitor C during power on by forcing the duty cycle of the switching frequency fo to a minimum Dmin. Dmin is a function of the feedback loop delay td divided by the period of fo or Dmin=(td x fo).

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Variable Frequency-Duty Cycle Slow Start Control for Switched-Mode Power Supplies

Disclosed is a circuit that eliminates damaging charging currents from a switched-mode power supply to its secondary filter capacitor4s during power on without using relays, large inductors, or very fast switching components. Referring to Figs. 1 and 2, switching supplies generally have a current sense circuit to limit the current charging capacitor C during power on by forcing the duty cycle of the switching frequency fo to a minimum Dmin. Dmin is a function of the feedback loop delay td divided by the period of fo or Dmin=(td x fo).

However, if the output voltage the power supply tries to drive the capacitor to, estimated by 4the equation Vo=(Vin/turns ratio) x Dmin, is much greater than the voltage stored across the capacitor at that instant in time plus a diode drop, there is virtually no limit on the output current Io after three LC time constants other than parasitic resistances. The dotted waveforms of Fig. 2 show the result. td is too large a proportion of the period of fo for the switch control to further limit the current drive capability of the power supply.

(Image Omitted)

Shown in Fig. 1 is a standard switching power supply where the fixed frequency oscillator which generated a constant switching frequency fo is replaced by a voltage-controlled variable frequency-variable duty cycle oscillator.

When the power supply is turned on, fo is at a frequency much lower than the final running frequency....