2 Parameter Considerations for Replacing a Photorelay With a High Voltage Analog Switch

When making the transition from photo relays to analog switches, making sure compatibility and functionality needs to be taken into consideration from a specification stand point. The important step in this process is examining the key photorelay data sheet parameters Isolation Voltage (V_iso) and Switch Voltage (often denoted as V_switch or OFF-state output terminal voltage). V_iso describes the max voltage between the LED input and the MOSFET output of the photorelay before the dielectric breaks down. Conversely, the V_switch is the max voltage that Photorelay output can handle across the switch. While the naming conventions can change between data sheets, Figure 2-1 shows the relationships between the specifications and the location of the voltage differential used to define the specification.

Figure 2-1 High Voltage Analog Multiplexer Replacement for Photo Relay

When looking at an analog switch to replace the photorelay, the key parameter to compare to from the switch data sheet is the Analog I/O voltage. If the analog I/O voltage rating is limited to at most the V_switch rating of the photomos, in most instances the analog switch or multiplexer can be a viable replacement for the photomos relay. Note that the supply of the switch still needs to encompass the maximum switch voltage (for example, a 200V analog i/o voltage can require a 200V supply). The exception can be in instances where the photorelay input ground (LED side) is not shared with the output (MOSFET side) ground. In these instances, the voltage seen on the analog switch can have the potential to exceed the maximum conditions and can cause damage. Additionally, a shifted GPIO input can cause inconsistencies with the V_IH/V_IL control levels on the switch.