Comparing Solid-State and Mechanical Relays for Fault Insertion

    Pickering Interfaces offers both mechanical relay and solid-state switch solutions for Fault Insertion applications. If you have a choice of which solution to use, this information might help chose the switch technology.

    Mechanical Relays.

    For medium and low-current applications mechanical relays can be a good solution for fault insertion switches. The relays are generally lower cost than solid-state solutions and often provide the lowest path resistance. Leakage currents are very low and in may cases they will withstand higher open circuit voltages than solid-state relays with few compromises on the path resistance. But they do have some compromises:

    • Relays are relatively slow to operate, so reproducing intermittent short term faults can be a problem. 
    • They have a limited life, and that life is shortened if they are required to hot switch signals. 
    • Hot switch powers are limited, particularly where DC switching at high voltage is required 
    • For very high current applications the switches become large, making them less suitable for integration into PXI modules. 
    • Some relays do not tolerate high inrush currents, for example when switching power supply connections into capacitive loads. 

    High-current relays can have a minimum switch capacity, so can have issues when they are used for low-current applications, overcoming this problem adds complexity to the switch design

    Solid-State Solutions.

    Solid-state switches for fault insertion are implemented using two MOSFET's and an isolated gate control to form an AC switch. The switches are fast to operate, so they can easily reproduce faults that are intermittent. They can also have very high hot switch power capability without having any impact on their service life. For high current applications they tend to be smaller than mechanical relays. Solid-state switches are also remarkably tolerant of high inrush currents without switch life degradations, our designs will tolerate inrush current often greater than five times the continuous rating with no effect. Again though, the switches are not perfect:

    • They can have leakage currents, for Pickering Interfaces products the leakage current is less than 1uA at maximum voltage and normal operating conditions. 
    • As the voltage rating of the switch rises, so does the resistance of the FET's chosen to implement the switch, so a 10A 40V switch has much lower resistance than a 10A 200V switch. So chose the voltage rating with care, it has a direct impact on the path resistance and therefore the switch thermal losses under load. 
    • They have a high switch input capacitance, which limits BW

    Summary

    In general, mechanical relay solutions often offer better solutions for low-current fault insertion applications, solid-state solutions generally offer better solutions for high-current applications. In both cases, other considerations such as speed or path resistance may drive the decision to a different switch type. 

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