Over Current Protection Using an Existing Crowbar Circuit
Consider a 5 V, 2 A and a 12 V, 3 A flyback power supply. One of the critical specifications for this power supply is over power protection (OPP) on the 5 V output when the 12 V output is at no load or very light load. Both outputs specify a ± 5% regulation requirement.
Of the usual solutions, a sense resistor would hurt cross regulation while a fuse is expensive. There is already a crowbar circuit for over voltage protection (OVP). This circuit addresses the OPP and regulation requirements at the same time and uses part of the crowbar circuit to achieve this.
Referring to Figure 1, R1 and VR1 form an active pre-load on 12 V output, to address the 12 V regulation during light load on 12 V. During the overload condition on the 5 V output, the voltage across 5 V output drops. The pre-load sinks a large amount of current. The resulting drop across R1 can be used to detect this large current. Q1 turns on and triggers the OPP circuit.