Switch intelligently controls current

-March 30, 2000

The circuit in Figure 1 can intelligently control ac or dc current when connected in series with a load. The circuit "steals" its power by turning off the load at a low duty cycle. The switch uses the MOSFETs' parasitic body diode to its advantage. While the MOSFETs are off, the body diodes, along with D1 and D2, serve as two legs of a diode bridge. Current flows through the load and the bridge, charging C1 to the peak ac or dc voltage. The relatively small control-block supply current continues to flow through the load when the load is turned off. The circuit has low insertion loss because of the MOSFET's bidirectional nature. The control block connects power to the load by turning MOSFETs Q1 and Q2 on. On alternating cycles, either Q1 or Q2 becomes reverse-biased, but current does not flow through the body diode because the MOSFET can conduct in either direction. The insertion loss is equivalent to the loss in two times the MOSFET's on-resistance.

While the load is turned on, the control block draws current from C1. The circuit must periodically recharge C1 by briefly turning off the load . You can allow the duty cycle to go as high as 99.99% with a high-current load and a micropower control circuit. The maximum duty cycle is approximately ILOAD/(ICONTROL+ILOAD). For example, with a 1A load and a control-circuit current of 1 mA, the maximum duty cycle is 99.9%. By choosing MOSFETs and diodes with higher current ratings, you can adapt the circuit to control high-power loads. Many applications are possible-lamp dimmers and thermostats, for example. The controller can optionally synchronize to the ac zero-crossing point as shown. (DI # 2505)

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