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SSR vs mosfet with drivers, why is SSR so slow?

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JohanHoltby:
Hi!
I have been looking in to solid state relays and noticed that they are very slow compared to optoisolated  gate drivers for mosfets.
Why is this? I would like to have a "SSR" switching at 1mhz. This is a safety switch which should be turned of when an electric level is outside the allowed rates.

BravoV:

--- Quote from: JohanHoltby on August 30, 2019, 03:16:11 pm ---I would like to have a "SSR" switching at 1mhz.

--- End quote ---

1 mhz ? or 1 Mhz ?

T3sl4co1l:
You really wouldn't want that, actually -- at 1MHz, you'll get all sorts of nasty peak voltages and currents from the unavoidable reactances (inductance and capacitance) present in the surrounding wiring.  And coming so frequently that the thing will destroy itself.  If it survives the first cycle, even.  You really need extra circuitry around the switching device to limit that, and that circuitry tends to make it special-purpose.  So that's basically why we still have bare transistors around, for the most part.

Note there are basically two kinds of SSRs: AC-only (thyristor based), and AC/DC (MOSFET(s) + photovoltaic driver).  Thyristors (namely TRIAC or SCRs) turn on reasonably fast (fractional microsecond, which can be tricky concerning peak currents through capacitors), but stay on until current drops to zero, so can only be used on AC.  The PV driver delivers very little current (microamperes), so turns on the MOSFETs very slowly (several milliseconds).  The switching loss can therefore be pretty high, but that just means you don't want to turn it on and off very frequently (which is usually fine for such applications).

The MOS SSRs are also fairly useless at delivering fault current, so, won't clear a fuse before blowing up themselves.  Thyristors can survive such rough treatment, just barely, if the fuse blows quickly.

Tim

Zero999:
An isolated MOSFET driver requires power on the secondary side permanently, whether it's on or off. A photovoltaically coupled MOSFET gets its power from an LED, on the primary side. As mentioned above, the current generated by the photocell is tiny, so the MOSFET takes a long time to switch.

A solid state relay should never be trusted in a safety critical application. It can easily fail short circuit, causing electrocution or a motor to run causing a serious injury, when it should be off.

David Hess:
Internally SSRs use a light source and photovoltaic cell to create the voltage to turn on the MOSFETs.  Unlike an optocoupler, the voltage is sufficient to fully enhance the gate so 5 to 15 volts.  The current available in low so it takes time to charge the gate capacitance of a large device and turn it on.  For the same reason, the gate to source shunt resistance is high and takes time to remove charge to shut it off.

So switching times are typically in the 100s of microseconds to milliseconds.

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