Posted on Dec 22, 2012 5683
A power MOSFET and a quad exclusive-OR oscillator makes an effective solid-state relay. Figure 75-5(a)`s capacitively iso
lated drive circuit provides gate drive to turn on the n-channel device. This consists of a gated oscillator (IC1A and IC1B running at 500 kHz, set by Rl, R2, and CI). The diode bridge (Dl through D4) rectifies the charge transferred through C2 and C3. When you disable the oscillator, R3 discharges the stored gate charge, thereby turning off the MOSFET. R3 needs to allow fast turn-off times without loading the gate"s enhancement voltage. A value of 10-KOhmhm is sufficient to produce a turn-off time of 800 with an 18-mQ SMP60N06-18 MOSFET. The measured turn-on time is 150 . You can reduce the turn-off time to 100 by using a pnp transistor as a diode-steering emitter-follower in the MOSFET gate circuit (Fig. 75-5(b)). Adding a hex buffer to Fig. 75-5A"s circuit increases the drive capability of the complementary outputs.