Linear load circuit to control switching power supplies under minimum load conditions

摘要

An embodiment of the present invention is a switching power supply comprising a transformer (12) with a primary (14) and two secondaries (16,18), a switching transistor (20) in series with the primary, a rectifier (22) and filter (24) on one secondary to provide regulated output power, a feedback control circuit (26) that monitors the secondary (18) and adjusts the duty cycle of an on-off control signal to the switching transistor (20) to maintain a constant output at the terminals (30,32), and a monitor (34) that detects when the power level flowing through the switching regulator has dropped to a predetermined minimum and then linearly turns on a load to the transformer (12). Transistor (20) is typically a power MOSFET. The monitor (34) tracks a summing junction current before any slope compensation is added. Load (36) is a shunt regulator that is activated when the summing junction current falls below 12% of full scale. Alternatively, the predetermined value of 12% is adjustable. The shunt regulator load (36) increases the load on secondary until the summing junction current returns to 12% of full scale.