Patent ID: 6061257
Filing Date: 2000-05-09
Classification: H02M

Abstract:
A method of protecting a DC-DC flyback converter from a short circuit, the DC-DC flyback converter self-oscillating either at a variable frequency or functioning at a fixed frequency in a discontinuous manner, the DC-DC flyback converter including a transformer for storing and transferring energy to a load, and having primary, secondary and auxiliary windings, the primary winding being driven by a power switch, the auxiliary winding synchronizing the turning on of the power switch under a zero current condition in the primary winding when self-oscillating at a variable frequency, a voltage induced on the auxiliary winding from a current flowing in the secondary winding is rectified and filtered to power a control circuit during a steady state of operation, the turning on of the power switch being driven during a start-up or recovery phase by a primary control loop when the supply voltage of the control circuit reaches or is over a preestablished enabling threshold of the control circuit, a secondary control loop comprising an output error voltage being photocoupled to an input of the control circuit to which a compensation capacitor is connected, the method comprising the steps of:comparing the voltage present on the auxiliary winding with a first threshold of a value which is higher than the voltage that is induced on the auxiliary winding under a short circuit condition of the secondary winding, to produce a first comparison signal;comparing the voltage present on the compensation capacitor with a second threshold of a value substantially equal to the value of maximum charge of the compensation capacitor, to produce a second comparison signal;logically combining the first and second comparison signals during a time interval sufficiently delayed from a turn-off instant of the power switch to discriminate an output short circuit condition from a start-up or recovery condition;disabling a turning on of the power switch in response to the output short circuit condition.