Low Cost Output Disable with Soft Start Kills Current Spike

In order to meet stringent standby power consumption specifications, some multiple output power supplies disconnect outputs when a standby signal is active. Typically this is achieved by switching off series-pass bipolar junction transistors (BJTs) or MOSFETs.  For lower current outputs, BJTs are a viable alternative to MOSFETs, with the advantage of lower cost, as long as the additional voltage drop of the transistor is accounted for in the design of the supply’s transformer.

Figure 1 shows a simple BJT series pass switch for a 12 V, 100 mA output, which has a significant capacitance, CLOAD.  Transistor Q1 is the series pass element, and Q2 turns it on and off according to the state of the standby signal.  Resistor R1 is sized so that the base current of Q1 is enough to guarantee it operates in saturation at minimum Beta and maximum output current.  PI recommends adding an extra capacitor, Cnew,  to moderate the transient current at turn-on. Without Cnew, Q1 turns on very rapidly into the capacitive load and thus conducts a large spike of current.  This requires Q1 to be oversized to accommodate this transient spike, adding to its cost.

By acting as additional “miller capacitance” for Q1, Cnew eliminates the current spike.  The extra capacitance limits the dv/dt of the Q1 collector.  A lower dv/dt results in a lower charging current into Cload.  The capacitance of Cnew was chosen so that the desired output dv/dt of Q1, multiplied by the value of Cnew, would equal the current into R1.


Figure 1 – Simple soft-start circuit disables supply output during standby while eliminating the turn-on current spike, so that a small transistor (Q1) can be used, which keeps cost low.