Being based on entirely different switching principles, the direct comparison between semiconductor relays and electromechanical relays turns out to be delicate.
Nevertheless, we attempt to elaborate the advantages of the respective relay types in the following:
When MOSFET semiconductor relays were introduced to the market about 40 years ago, there was speculation that electromechanics would be replaced by semiconductor technology. However, it has been shown that both technologies have their right to exist, depending on their field of application. Whenever miniaturization, energy-saving aspects, optimized processing, zero-failure-rates or additional functions such as short-circuit protection are required, modern semiconductor technology has an advantage. If the focus is on low costs and a simple and robust design, electromechanical solutions have a clear advantage.
Tasks of electromechanical and semiconductor relays
The galvanic separation of logic or control circuits and load circuits is the task of a relay. Furthermore, it links different signal levels at different potentials without interference. Important for this is: to provide a low power consumption of the logic circuit and an interference-free switch in the load circuit with a long life and reliability - independent of switching cycles. An essential difference between electromechanical relays and MOSFET semiconductor relays is the way the load is switched at the output. In electromechanical relays, the switching function is performed by movable metallic contacts, which is considered to be an additional galvanic isolation at the output. The coupling between input and output, which is also electrically isolated, is effected by the magnetic field of the coil at the input. The differences to the MOSFET semiconductor relay are described in more detail below.
Special features of MOSFET relays
MOSFET relays are a special type of solid state relays. Even at an operating current of only a few milliamperes (min. 0.3 mA), the GaAs light-emitting diode in the input circuit of the MOSFET relay emits light in the infrared range. An optically coupled solar cell array, which is separated from the input circuit by a semi-transparent insulator, converts the light into an electrical voltage. This electrically non-conductive connection enables galvanic isolation between the input and output circuits. The generated photovoltage supplies a trigger stage, which in turn drives the gates of two bidirectionally, antiserially connected DMOSFETs (Double Diffused MOSFETs). These power transistors are located directly in the output circuit of the component. Above a certain threshold value of the photovoltage, the integrated trigger circuit reacts and switches the output virtually digitally on and off to enable a defined switching behavior.
Advantages and disadvantages of both technologies or the individual differences can be evaluated more precisely when considering the basic technical parameters such as drive power, signal transmission, RF characteristics, switching times/switching bounce, service life and contact resistance as well as electrical isolation.
Drive power: Modern MOSFET relays can be driven with currents as low as 0.3 mA. The voltage drop of the input LED is typically 1.25 V. This corresponds to a minimum power consumption of about 0.4 mW (example AQY232S). In contrast, the coil power consumption of highly sensitive electromechanical relays is at best 50 mW (TXS relay). However, bistable types are also available here that do not require any holding power at all when switched.
Signal transmission: Signals to be transmitted are usually understood to be small currents and voltages such as those generated by thermocouples, microphones or similar sensors/transducers. A distortion of the signal is critical here. With electromechanical relays, the thermoelectric voltage distorts the signal path. A thermoelectric EMF arises when different conductor materials at the connection points are at different temperature levels. Since the current at the contact points flows through different spring and contact materials, the thermoelectric EMF occurs mostly with monostable relays. The reason for this is that heat is generated after switching on the drive coil and temperature differences build up along the current path through the contact spring set. The standard values for relays with gold contacts are 0.1 μV per Kelvin. Some types such as the SX relay from Panasonic Electric Works are optimized for this application and have a total thermoelectric voltage of 3 μV at nominal operation and thus maximum heating. The thermoelectric voltages of electromechanical relays contrasts with the offset voltage of the MOSFET relays. The offset voltage is generated by free charge carriers in the semiconductor. It is a measure of how the current-voltage characteristic is shifted from the ideal point. The offset voltage is largely independent of temperature and can therefore be considered as a constant in the circuit. Typical values for offset voltages in MOSFET relays can be found at 1 μV.
RF characteristics: To achieve sufficient crosstalk attenuation at high frequencies, relays must have low capacitance at the open contact. For electromechanical signal relays, the values are usually about 1 pF. This results in excellent RF properties. Even at a frequency of 100 MHz, the crosstalk attenuation is still 40 dB. Special high-frequency relays, such as the RJ relay, are even designed for frequencies up to 8 GHz. MOSFET relays can now achieve almost as good RF characteristics as electromechanical relays. The output capacitance of MOSFET relays is also 1 pF, depending on the type, so that MOSFET relays can also be used for frequencies in the MHz range. For applications in the GHz range, however, pin diodes or special RF relays must still be used.
Switching times/switch bounce: From this point of view, MOSFET relays are far superior to electromechanical relays. The typical switch-on time of MOSFET relays is 0.2 ms and depends on the LED current and the ambient temperature. The switch-off time is about one tenth of the switch-on time and is largely independent of the control conditions. For electromechanical relays the switching times are in the range of milliseconds and the contacts bounce when switched on.
Lifetime: With MOSFET relays, the lifetime is mainly determined by the operating time of the LED and is therefore practically unlimited. With uninterrupted operation of a MOSFET relay has a life expectancy of more than 12 years. In contrast, the service life of electromechanical relays depends on the mechanical construction (mechanical life) as well as on the load (electrical life) and is indicated by the number of switching operations. While the mechanical lifetime of modern electromechanical relays can amount to several million switching operations, the electrical lifetime is strongly load-dependent.
Contact resistance: An advantage of MOSFET relays is that the contact resistance is load-independent and remains constant over its service life. However, the value of the contact resistance is higher than with electromechanical relays and can be several ohms, depending on the switching voltage. It is also strongly dependent on the ambient temperature. In the best case, the value for switching voltages up to 30 V is about 30 mΩ, depending on the type. The contact resistance of electromechanical Relays, on the other hand, always move in the mΩ range and can change significantly over the life of the component. Especially on the contacts of open relays, thin oxide layers can form during long storage periods, which increase the contact resistance. As a rule, a few switching cycles under load destroy these layers, and the contact resistance drops back to the data sheet value.
Galvanic isolation: Galvanic isolation is understood to be separation by means of isolation. A distinction must be made between galvanic separation between the control and load side and galvanic separation on the load side. A blocked semiconductor does not guarantee galvanic isolation on the load side. However, optocouplers can be used to achieve at least the separation between the control and load side. Electromechanical relays offer a clear advantage here, as they feature a galvanic isolation on both the control and load sides. Especially in safety applications this can be a decisive criterion.
Conclusion: As the points listed above make clear, both MOSFET and electromechanical relays have their advantages and disadvantages. Depending on the required properties, either the semiconductor or the electromechanical relay is more suitable for the individual application. In the medium term, the market share of MOSFET relays will certainly grow significantly, especially in signaling technology. In addition to continuously falling prices and ever newer, more compact designs, the technical advantages of semiconductor technology are the main guarantee for success. However, the proven electromechanical signal relays will continue to have their place and offer an ideal supplement due to unique selling points such as galvanic isolation at the output.