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# Fail-Safe Protection Circuit

IP.com Disclosure Number: IPCOM000036631D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 2 page(s) / 38K

IBM

## Related People

Ohsawa, T: AUTHOR

## Abstract

An objective of this circuit is to guarantee a protection function when a conventional protection circuit does not work because of some trouble. This circuit can be applied to a primary controlled switching power supply with a secondary voltage sense system.

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Fail-Safe Protection Circuit

An objective of this circuit is to guarantee a protection function when a conventional protection circuit does not work because of some trouble. This circuit can be applied to a primary controlled switching power supply with a secondary voltage sense system.

Recently in a switching power supply, both a protection circuit and an output voltage sense circuit use the same internal voltage in order to reduce cost and device components. However, if an internal voltage is shorted to ground, the power supply will provide undesirable output because it gets out of control and a conventional protection circuit does not work. Therefore, it will cause critical damage to the power supply or load circuits.

To solve this problem, a fail-safe protection circuit, as shown in the figure, has been developed. It comprises a zenor diode and three diodes added to the conventional protection circuit. The key point of this circuit is to optimize the zener voltage VZ in order to get a protection signal which flows like the dotted line shown in the figure when an internal voltage is shorted to ground. Where an output voltage represented as V1, the internal voltage is V2, the zener diode DZ's voltage is VZ, diode D1 and D2's forward drop voltages are VD, and a photo coupler PC1's forward drop voltage is VP, the zener voltage VZ should meet the following equations:
(1) V1 >= V2+2+VD+VP

V1-(V2+2*VD=VP) < VZ < V1-(2*VD+VP)
(2) V1 < V2+2*VD+VP

0 < VZ < V1-(2*VD+VP)...