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Power Supply Status Circuit with Isolation for Flyback Switching Power Supplies

IP.com Disclosure Number: IPCOM000051137D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 42K

Publishing Venue

IBM

Related People

Groves, JO: AUTHOR [+2]

Abstract

A power on reset (POR) signal and a power down indicator (PDI) signal are achieved using four comparators and voltage signals available at or near the output of the power supply. Such isolation from high voltage points in the power supply makes the circuit particularly useful when isolation is critical, such as with certain switching power supplies.

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Power Supply Status Circuit with Isolation for Flyback Switching Power Supplies

A power on reset (POR) signal and a power down indicator (PDI) signal are achieved using four comparators and voltage signals available at or near the output of the power supply. Such isolation from high voltage points in the power supply makes the circuit particularly useful when isolation is critical, such as with certain switching power supplies.

In the circuit shown, the output stages of a flyback switching power supply 1 are suggested at the left of the dotted line. Such a power supply rectifies and filters AC line voltage, chops the resulting DC voltage, scales the amplitude through an isolating transformer, and rectifies the pulses to produce DC outputs. The +5 and +25 volt sources used by this circuit are outputs of the standard power supply 1.

The power supply 1 has a silicon rectifier diode 3 connected to a secondary of the flyback transformer. Lead 5 is connected from the side of diode 3 away from the output through 200,000 ohm (200K) resistor 7 to the junction of the input of comparator U1A, 39 pf capacitor 9, and 41K resistor 11. The diode 3 to the +5 volt output is selected in preference to the corresponding diode to the +25 volt output when the specific application calls for a steadier load on the +5 volt output.

The output of U1A is connected to the junction of 100K resistor 13 and 390 ohm resistor 15. The other terminal of resistor 15 is connected to the negative terminal of comparator U1B and through the 0.22 Vf capacitor 17 to ground. 10K resistor 19 is connected to the positive terminal of U1B and from that point through 10K resistor 21 to ground. The output of U1B is connected through 430K resistor 23 to the plus terminal of U1B and the minus terminal of U1A. The +5 volt source is connected to resistors 11, 13, 19 and 25. Resistor 25 is 10K, and its other terminal is connected to the output lead 27 of U1B. The signal on lead 27 is the PDI signal.

The voltage on lead 5 is a negative value that is proportional to the AC line voltage when diode 3 is back-biased. Resistor 7 and resistor 11 level shift and scale this voltage for input to U1A. Capacitor 9 provides high frequency noise filtering. The threshold for both U1A and U1B is set by resistor 19 and resistor
21. Hysteresis is provided by resistor 23.

When rated line voltage is received by the power supply 1, the signal on the positive terminal of U1A forces the output of U1A low. The voltage at the negative terminal of U1A is dictated by the ratio of resistors 19 and 21 di...