Power switch provides soft start

John Haase, Colorado State University, Fort Collins, CO -- EDN, 3/30/2000

In the circuit in Figure 1, series-connected MOSFETs turn on the line voltage near the zero-crossing point and off when the 555-timer delay lapses. That delay ranges from 1 to 7 msec. The MOSFETs' body diodes and the 1N4005 diodes form a full-wave bridge rectifier that provides a floating 12V-dc level via the 10-kW, 3W resistor. The bridge simultaneously delivers the crossing signal to 2N2906 common-base comparator. When the emitter current approaches zero, the collector voltage falls to 4V and triggers the delay timer, initiating gate drive to the switches. Positive-going pin 3 of the 555 also removes the trigger threshold by coupling to the diode OR gate. Because the circuit generates the 12V operating voltage only during off- time, the limit for maximum delay is approximately 7 msec. You trim the delay by selecting a resistance value from Pin 5 to raise (pins 5 to 8) or lower (pins 5 to 1) the upper comparator threshold at Pin 6. Thus, load power is from 0 to 90% of maximum (600W). You must use a heat sink for the power MOSFETs. (DI #2506).