Faster alternative to solid state relay

Question

I am currently designing a circuit that includes a solid state relay, the CPC1125 . I chose it because it is one of the faster normally closed solid state relays I found. I want it to be optical because I want to isolate the two parts of the circuits which are linked by the relay. The problem is that all the relays I found are slow, switching times are on the order of a few 100 us, which is weird to me because they use transistors and optical elements which should all be faster than that (?). I think I am missing some key word to find the right alternative, is there any?

For completeness, here is a schematic of the circuit. the relay is used to short the integrating part of an integrator circuit.

The LED and the op-amp are different grounds.

If the op-amp and diode are common-ground, surely isolation is not required, and an analog switch or JFET would suffice? — Tim Williams, Mar 13 '24 at 20:41
Why not just use fast dio isolation (Analog Devices?) to switch a fast mux or analog switch? — Chris Knudsen, Mar 13 '24 at 20:45
I will look into that, knudsen, thanks. I am a beginner so I just don't know about these devices. — DarkBulle, Mar 13 '24 at 20:46
You can use a simple latch circuit that drive a led of optocoupler? It can be fast as 1us or so. Btw, you probably need a two opto against each other for both current directions in integrator. — Michal Podmanický, Mar 13 '24 at 21:03
@DarkBulle Opto-isolated circuits have only one reason for existing - to separate you from high voltages. As a beginner, if you're working on anything with high voltages (including mains) then the only advice ever is *DON'T*. And if you aren't, then you almost certainly aren't using the right thing here. It's called an "X-Y problem", where you ask a detailed question and get a good detailed answer, but it isn't what you actually need. A better question to ask is "I need to do this, with this switching speed - what device should I use?" — Graham, Mar 14 '24 at 06:43

Marcus Müller · Answer 1 · 2024-03-13T21:52:37.907

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You probably just want a simple optcoupler, not a full solid-state relay; that would solve the issue, and you can comfortable get below 1µs latency, assuming sufficient drive strength on the input.

Depending on voltage requirements, you could choose to drive a JFET or similar bidirectional switch with that optocoupler, coming closer to the original idea of an SSR. But depending on the type of optocoupler, that might be unnecessary.

edited Mar 13 '24 at 21:52

answered Mar 13 '24 at 21:12

Marcus Müller

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The optos with exposed base terminals can switch on to off faster. https://electronics.stackexchange.com/questions/48462/optocoupler-with-phototransistor-base-lead – hacktastical Mar 13 '24 at 22:00
Thanks, I will look into this, why would people use solid states relays then ? – DarkBulle Mar 13 '24 at 23:20
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@DarkBulle for cases where they actually need the features of SSRs; higher current, higher voltages… – Marcus Müller Mar 13 '24 at 23:36

score 6 · Accepted Answer · answered Mar 14 '24 at 00:19

This kind of SSR uses a photovoltaic cell internally, which can only produce microamperes so it takes some time to charge the internal MOSFET gates. They are fast enough for many applications and have the advantage of not requiring any power supply on the switched side.

For faster action you probably have to separate the power and the isolation and switch functions.

If you power an analog switch V+/V- from the op-amp supplies and use an analog switch with level shifting you may only need to add an isolator. A 6N137 will switch in 10 or 15ns. The type of analog switch you would need depends on the supply voltages. +/-15V supplies will require a more expensive part than if they are +/-5 or +5/0.

score 5 · Answer 3 · answered Mar 14 '24 at 00:14

Opto MOSFET solid state relays are made with a LED that shines on a photovoltaic cell that drives the gate of a MOSFET. The current available to drive the gate is tiny, so they don't switch fast.

I doubt the digital and analog grounds in your circuit are floating, they're probably tied together somewhere. If this is the case then you don't need 1500V isolation, and you can use a much faster analog switch.

Here's an example: DG468. Switching time is 50-130ns typical.

V+ and V- are the analog supply, and voltage on both sides of the switch must be between these two supplies.

The switch is closed by applying more than 2.4V between IN and GND so it is compatible with 3V3 logic levels.

IN and GND pins must be between V+ and V-.

The point is, the "GND" pin isn't the analog ground, it's the zero volts reference for the digital control signal. As long as the digital control signals (IN and GND) are between the positive and negative analog supplies, it will work.

Faster alternative to solid state relay

3 Answers3