Patent ID: 7382595

Claim:
A method for overcurrent protection of a solid state high frequency PWM switching power supply connected to an AC power source, the power supply having a maximum current capacity rating and being operated with a duty cycle corresponding to a dimming level and output voltage, the duty cycle, dimming level and output voltage being variable in a range between minimum and maximum values, said method comprising: filtering the output of a series solid state switch; supplying the filtered output current to a load circuit; sensing the filtered output current supplied to the load circuit; providing the sensed output current to a microprocessor based programmable controller; performing a family of overcurrent tests in the controller during each cycle of the AC power source, the family of tests including a plurality of different tests satisfied by the presence of different output currents for different time durations; establishing an overcurrent condition in response to satisfaction of any of the family of tests; switching the series solid state switch to a nonconductive condition in response to the overcurrent condition; detecting zero crossings of the AC power source, and performing the family of overcurrent tests in the controller following each zero crossing of the AC power source; a first test of the family of tests including determining if the output voltage is in a predetermined portion of the range between minimum and maximum values and determining if the current exceeds an overcurrent threshold value for a predetermined first duration of time, the overcurrent threshold value being smaller than the maximum current capacity rating, and a second test of the family of tests being carried out only when the output voltage is in the lower third of the output voltage range.