My question is simple, why are we still using relays ? Can't we say that MOSFET are the future of relays?
Can't we say that MOSFET are the future of relays?
You will probably never replace some radio frequency relays with MOSFETs because a MOSFET has several pF of drain-source capacitance and at 1 GHz that will continue to look pretty much like a closed contact. At 1 GHz, 100 pF has an impedance of 1.59 ohms - It wouldn't make much of an open-contact if you used a MOSFET.
Even at (say) audio frequencies this capacitance is problematic if you were considering using a MOSFET as a signal switch. At 10 kHz, 100 pF has an impedance of 159 kohms and this could still let through a significant signal into a high impedance amplifier.
To add to Andy's response, relays and MOSFETs do have their own application ranges and are not always interchangeable. For example, MOSFETs have a much, much higher switching frequency than relays — you wouldn't want to drive a stepper motor with relays. On the other hand relays provide insulation between command and output when jelly-bean MOSFETs generally do not. In some circumstances, relays may even take less space than MOSFET equivalent solutions, e.g. when using H-bridges to drive motors in two directions.
Choosing between relays or MOSFETs depends upon the application field and various constraints such as space, consumption, driver stage requirements, power supply, switching voltage, switching current, capacitive/inductive/resistive load, insulation (command/gate and output)... There's no straight formula that drives the decision between both, it's up to the design engineer to opt for which best fits.
Relays have several desirable properties.
- Resistance when on is very low
- Leakage when off is very low
- When off current flow is blocked in both directions (mosfets have a "body diode" and the body is normally connected to the source).
- Input and output sides are isolated.
- They are generally robust against spikes, heat etc.
They also have several undesirable properites.
- Cycle life is relatively short.
- Physical size is large
Mosfets have been improving in recent years but still for applications involving infrequent switching of high currents relays are hard to beat.