In AC power-switching applications, closing the load switch during the zero-crossing of the AC line voltage minimizes the inrush current during turn-on. Figure 7-1 shows the implementation of a solid-state relay based on the TPSI3050-Q1 device. The TPSI3050-Q1 is an isolated switch driver with an integrated 10V gate supply. The two external, back-to-back, n-type field effect transistor (NMOS) power switches are turned on when the EN pin of the TPSI3050-Q1 is driven high. These power switches are turned off when the EN pin is low. In this application, the AMC23C10-Q1 detects the zero crossing of the AC line voltage and gates the EN signal to the TPSI3050-Q1. The high-side of the AMC23C10-Q1 is powered from the integrated 10V gate drive supply of the TPSI3050-Q1. A separate power supply is not required for the high-side.

The AC line voltage is clamped by R5 and two small-signal, antiparallel diodes to limit the voltage at the input of the AMC23C10-Q1. Close to the zero crossing of the AC line voltage, neither diode conducts and the voltage at the INP pin equals the AC line voltage. As illustrated in Figure 7-2, OUT1 (an open-drain output) toggles from high-to-low during the rising zero-crossing event and from low-to-high during the falling zero-crossing event. OUT2 (a push-pull output) toggles with the opposite polarity. Use OUT2 in a discrete logic block or with a microcontroller to gate the EN signal of the TPSI3050-Q1.