Referring to Figure 2-6, consider the scenario where SSR-6 is kept off and SSR-1 is turned on and off. When SSR-1 is switching, the voltage across the blocking diode D2 changes (the voltage change depends on the instantaneous AC voltage). The voltage change dv/dt across the SSR MOSFETs couples to the blocking diode of SSR6 (which is off) and then through the gate-driver power supply (10 V) low impedance path, couples to the gate-drive transistor circuit (Q2). This can cause momentary base current and resulting collector current in Q2, resulting a gate pick up at the MOSFET gate of SSR6 causing the SSR-6 to conduct. One way to reduce cross coupling is by controlling dv/dt during FET switching, by reducing switching turn on and turn off time by adjusting the gate resistor. The reference design uses decoupling capacitor at the base-emitter of transistor (Q2) to alleviate the unintentional turn on of the gate path transistor Q2, allowing faster MOSFET switching slew rate. The capacitor can be used at the gate to source the of MOSFET to further enhance the noise immunity. shows the simulation results confirming that the gate voltage pick up is negligibly small. The transistor remains off ensuring that there is negligible cross-coupling.

Figure 2-8 Simulation Results Illustrating Negligible Cross-Coupling

The performance comparison of the single isolation multichannel SSR topology against other independent isolation SSR topologies and electromechanical relays are listed in Table 2-1.