Solid State Relay (SSR) Circuit using Two MOSFETs

SSR or Solid state relays are high power electrical switches that work without involving mechanical contacts, instead they use just a couple of solid state semiconductors like MOSFETs for switching an electrical load, smoothly, and with high efficiency.

SSRs can be used for operating high power loads, through a small input trigger voltage with negligible current.

These devices can be used for operating high power AC loads as well as DC loads.

Solid State Relays are highly efficient compared to the electro-mechanical relays due to a few distinct features.

Main Features and Advantages of SSR

The main features and advantages of solid state relays or SSRs are:

• SSRs can be built easily using a minimum number ordinary electronic parts
• They work without any form of clicking sound due to the absence of mechanical contacts.
• Being solid state also means SSRs can switch at much faster speed than the traditional electro-mechanical types.
• SSRs do not depend external supply for switching ON, rather extract the supply from the load itself.
• They work using negligible current and therefore do not drain battery in battery operated systems. This also ensures negligible idle current for the device.

Basic SSR Working Concept using MOSFETs

In one of my earlier posts I explained how a MOSFET based bidirectional switch could be used for operating any desired electrical load, just like a standard mechanical switch , but with exceptional advantages.

The same MOSFET bidirectional switch concept could be applied for making an ideal SSR device.

For a Triac based SSR please refer to this post

Basic SSR Design

In the above shown basic SSR design, we can see a couple of appropriately rated MOSFETs T1 and T2 connected back to back with their source and gate terminals joined in common with each other.

D1 and D2 are internal body diodes across the drain/source of the respective MOSFETs.

An input DC supply can also be seen attached across the common gate/source terminals of the two MOSFETs. This supply is used for triggering the MOSFETs ON or for enabling permanent switch ON for the MOSFETs while the SSR unit is operational.

The AC supply which could be up to grid mains level and the load are connected in series across the two drains of the MOSFETs.

How it Works

The working of the proposed sold state relay can be understood by referring to the following diagram, and the corresponding details:

With the above setup, due to the input gate supply connected, T1 and T2 are both in the switched ON position.

When the load side AC input is switch ON, the left diagram shows how the positive half cycle conducts through the relevant MOSFET/diode pair (T1, D2) and the right side diagram shows how the negative AC cycle conducts through the other complementing MOSFET/diode pair (T2, D1).

In the left diagram we find one of the AC half cycles goes through T1, and D2 (T2 being reverse biased), and finally completes the cycle via the load.

The right side diagram shows how the other half cycle completes the circuit in the opposite direction by conducting through the load, T2, D1 (T1 being reversed biased in this case).

In this way the two MOSFETs T1, T2 along with their respective drain/source body diodes D1, D2, allow both the half cycles of the AC to conduct, powering the AC load perfectly, and accomplishing the SSR role efficiently.

Here's an excerpt from the datasheet of the article.

Video showing the testing of the above SSR circuit

Making a Practical SSR Circuit

Why We Use Two MOSFETs For AC

When we want to switch AC using MOSFETs then we find that a single MOSFET cannot block current in both directions because of its body diode.

So we use two N Channel MOSFETs and we connect their sources together and we keep their drains toward the AC line terminals.

When we do this arrangement then the body diodes oppose each other and the AC can be fully blocked when the gate drive is zero. So the pair works like an AC switch.

Why We Need A Floating 15 V Supply

When we use back to back MOSFETs in AC circuits then the common source point does not stay at ground level.

This point keeps moving up and down with the AC waveform, so now we understand that the gate voltage also must follow this moving point, because the MOSFET only cares about the Vgs difference.

When the source rises, then gate must rise above it, when the source falls, then gate must fall above it.

So we need an isolated 15 V supply that floats along with the MOSFET sources.

When the supply is floating, then it does not care about earth or neutral and it simply rides with the circuit, so the MOSFET always sees a correct gate voltage.

Why The Source Node And The 15 V Negative Can Touch

Many people think that isolation is broken when the isolated supply negative is connected to the MOSFET source node.

But since the isolated supply has no link with mains or earth, it does not matter. It only follows the MOSFETs.

So the 0 V of the 15 V supply becomes the reference for the MOSFET gates and sources.

When we do this connection, everything still stays isolated from the outside world.

How The Gate Is Driven From The 12 V Control Side

We use an opto or a small control section in the 12 V side. When we give 12 V input to this module then the opto transistor pulls the floating 15 V supply positive toward the MOSFET gate through the gate resistor.

When this happens then the MOSFETs get a Vgs of around 12 V and they turn ON.

When we remove the 12 V input then the gate is left without drive and the resistor that we added slowly discharges the gate. When the gate reaches the same level as the source then the MOSFETs turn OFF.

How The Circuit Behaves When AC Passes

When the AC line is connected to the MOSFET drains then common source node becomes a floating midpoint that shifts according to the load and the waveform.

But the gate supply also floats in the same way so the Vgs stays correct.

When we give the 12 V command, then MOSFETs turn ON in both half cycles and the AC flows without restriction.

When the input is removed then the gate falls down through the 1 M resistor and the MOSFETs turn OFF and block both directions due to the anti series body diode configuration.

Why 220k Is Acceptable In This SSR

We use 220k because we do not need ultra fast gate discharge, and because the SSR is not switching at very high speed.

The AC cycle itself is very slow compared to MOSFET switching, so even a 10 ms or 20 ms fall time is perfectly OK. The 220k value also keeps the loading of the 15 V supply to a very small level, but if you want a little faster discharge you can use 100 K also. Both values work fine because the switching is slow.

Final View Of The Completed SSR

So now the whole SSR becomes very simple. Two N Channel MOSFETs are connected source to source, the drains go to AC.

The floating 15 V positive is fed to the gates through individual 22 Ohms.

The floating 15 V negative goes to the common source.

A 220 K resistor is placed between the gates and the sources of each of the MOSFETs.

The isolated 12 V side drives the opto, and the opto connects the 15 V positive to the gate when it receives the command.

Conclusion

So now we get a simple, very easy to understand bidirectional MOSFET SSR which can switch AC efficiently.

The use of the floating 15 V module makes the design work smoothly even when the AC line moves up and down.

The resistors keeps the gate under control.

Source.

Reference: SSR