A solid state relay (switch) is a term often used to describe a relay where the path is formed by a semiconductor material (silicon, gallium arsenide). Relays made in this way have advantages and disadvantages compared to mechanical switches (reed relays, EMR's):
- Can have much higher hot switch powers
- No wear amount mechanism through frequent use within specification
- Faster operation
- No contact bounce
- No minimum switch capacity issues
- Low thermoelectric effects
- Higher DC path resistance
- Higher capacitance on MOSFET SSR's, which limits BW
- Off state has higher leakage
- Lower standoff voltages (hot switching voltage same as cold switching voltage)
There are many types of solid state relays (switches) available but only a few are generally considered suited to test and measurement applications. Two types are used in Pickering Interfaces switching products, CMOS switches and MOSFET SSR's.
CMOS switches are implemented by a number of semiconductor companies using variations on their standard CMOS processes. Typically these switches have relatively low operating voltage (+/-22V or lower) and are susceptible to problems when power is applied to the signal pins of the switch while their own power supply voltage is absent. Such conditions can cause the switch to operate incorrectly, produce unexpected loads on the UUT or in some circumstances damage the switch. To be suited for inclusion in a test system CMOS switches should be protected, ideally in a way that produces no load on the UUT when the UUT is powered but the switching system (LXI or PXI) is not.
Some CMOS switches include Fault Protection circuits - as is used in the Pickering Interfaces 40-680. In a Fault Protection switch additional series devices are implemented in the switch which automatically open circuit the signal connection if the UUT voltage rises above the switch power supply voltage, ensuring that in these circumstances the switch does not load the UUT. Provided the voltage from the UUT is constrained to within the protection voltage limits (in the case of 40-680 to greater than +/-40V) the switch is protected and does not load the UUT.
In more recent years CMOS has also been applied to RF switches with some success, using advanced CMOS processes RF switches with greater than 6GHz BW have been implemented. These switches tend to have reduced DC current/voltage handling capability compared to mechanical switches and consequently are best implemented in designs where the DC component is removed by AC coupling capacitors to avoid the switches being damaged by the user. The 40-880 series RF 6GHz switches use CMOS technology and are available in a variety of MUX and matrix topologies. Design of these switches requires careful design practices, particularly when optimizing the VSWR of modules which use significant numbers of CMOS switches in series to provide expanded functionality and improved isolation as is the case with 40-880 series. It requires the addition of additional matching circuits and the use of carefully designed circuits on composite board materials that offer low loss and the rigidity required in PXI modules.
40-880 6GHz MUX using CMOS switches
For users though all these issues are hidden, for handling frequencies to 6Ghz CMOS RF switches are ideal when fast switching speeds and long life are required.
MOSFET Solid State Relays (SSR)
MOSFET's can be used to create high current isolated relays. In a typical relay two N type enhancement mode MOSFET's are used with the source and gate connections tied together and their drain contacts used as the signal input/output. An isolated differential voltage is applied between the gate and the source to turn the MOSFET's on, with no voltage applied the MOSFET's turn off. Two FET's are required to allow the switch to operate from either polarity of signal applied. In the ON state both FET's are on, in the OFF state both FET's are off but one of them will have a parasitic body diode present which prevents it from turning off, the second FET will ensure this conduction route is blocked. The use of two MOSFET's in this way ensures the SSR can handle voltages of either polarity across the signal (drain) connections.
SSR Using Two N Channels MOSFET'S whose gate bias is derived from a photocell excited by an LED
The isolated supply can be implemented in a number of ways, the simplest being the use of an LED illuminating a solar cell to create the voltage needed to turn the relay on. Packaged device are available that integrate the LED, solar cell and FET in a single device, such as that used on the Pickering Interfaces 40-682 MUX and the 40-563 BRIC. An advantage with this approach is that the PCB small footprint is small, however the ratings and operating speed are limited by the integrated MOSFET's and the low power available to drive the MOSFET gates.
For higher current and voltage applications though a discrete design is required, such as that used on the 40-191 ad 40-192 Fault Insertion Switch and its derivatives. The 40-191 and 40-192 offers very high hot switch powers - over 1 kW - without any long term degradation or reliability issues. This is a major advantage for solid state relays of this type, especially when switching power supplies with high capacitor inrush currents (the inrush current handling is typical at least three times the continuous rating with no switch degradation) or trying to simulate intermittent faults in fault insertion applications. To drive high current MOSFET's with fast operating speed (which reduces the thermal stress on the switch) these types of solution use isolated DC to DC power supplies to drive the MOSFET's.
40-191 Solid State high capacity fault insertion module
Solid State Relays of this type are very effective, and can be very compact compared to EMR's for high current applications. They are particularly effective when switching DC voltages above 24V. EMR's suffer a rapid decline in lifetime as DC voltage increases and in particular contact breaking causes electrical arc's to form which rapidly erode contact materials and closures result in welding. For this reason as DC voltage rises the handling power of EMR's rapidly declines. This does not occur with MOSFET SSR's, as long as the MOSFET's transition rapidly being the on and off state they tolerate the transition with no effect and do not suffer from arc generation. This also means the contacts do no bounce (another issue that limits EMR life under load). MOSFET SSR's are usually the most compact and effective solution for switching high current signals on DC voltages above 24V.
They do have their compromises though, just like any other switch technology. As the required voltage rating rises, so does the on resistance of the MOSFET's. Larger MOSFET's have lower on resistance so larger devices can be used to compensate, but they also have higher capacitance which in turn restricts the bandwidth. For power applications though the BW is rarely of great concern since the source impedance must be low (to avoid losses) - the ability to stand high inrush currents and high DC voltages is a much more important factor.
The voltage ratings on SSR's needs to be considered whenever highly inductive loads are switched. The sudden interruption of current in an inductive load creates voltage spikes which can create avalanche breakdown events in the MOSFET. This is not a destructive process provided that the energy is within the capability of the MOSFET to handle it, either as a single event or as a repeating event. Similar limitations apply to mechanical relays, but in this case it is the creation of signal arc's that creates a potential problem. Ratings always assume that the load being switched is resistive, for inductive loads if a user has concerns they should consult Pickering Interfaces for information on the MOISFET's used.
Users also sometimes express concern about leakage currents. All MOSFET's have a leakage current specification and Pickering Interfaces apply these numbers to the MOSFET SSR data sheets. A typical number quoted is 1uA at 25C and rated applied voltage. As temperature rises this leakage will also rise, a temperature rise could because of an increased ambient temperature or it could be heating from a previous on state where the MOSFET's were varying a high current. In the context of say a switch that can carry 10A or more 1uA is a very small number - an on/off ratio of some 140dB. The leakage number is also usually very conservative - it is a number that is easily tested for by the semiconductor manufacturers in an industry that needs high yields and fast test to keep costs down. Typical SSR implementations used in Pickering Interfaces products measure with much less leakage than 1uA (by at least two orders of magnitude). For customers with concerns about leakage current Pickering Interfaces are able to offer versions tested to more stringent limits at extra cost.