Solid State Relay (SSR) Circuit using MOSFETs

SSR or Solid state relays are high power electrical switches that work without involving mechanical contacts, instead they use solid state semiconductors like MOSFETs for switching an electrical load.

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

So far we have learned the theoretical design of an SSR, now let's move ahead and see how a practical solid state relay module could be built, for switching a desired high power AC load, without any external input DC.

The above SSR circuit is configured exactly in the same way as discussed in the earlier basic design. However, here we find two additional diodes D1, and D2, along with the MOSFET internal body diodes D3, D4.

The diodes D1, D2 are introduced for a specific purpose such that it forms a bridge rectifier in conjunction with the D3, D4 MOSFET body diodes.

The tiny on OFF switch could be used for turning the SSR ON/OFF. This switch could be a reed switch or any low current switch.

For high speed switching you can replace the switch with a opto-coupler as shown below.

In essence the circuit now fulfills 3 requirements.

1. It powers the AC load through the MOSFET/Diode SSR configuration.
2. The bridge rectifier formed by D1---D4 simultaneously converts the load AC input into rectified and filtered DC, and this DC is used for biasing the gates of the MOSFETs. This allows the MOSFETs to get appropriately turned ON through the load AC itself, without depending on any external DC.
3. The rectified DC is further terminated as an auxiliary DC output which could be used for powering any suitable external load.

Circuit Problem

A closer look at the above design suggests that, this SSR design might have problems implementing the intended function efficiently. This is because, the moment the switching DC arrives at the gate of the MOSFET, it will begin turning ON, causing a bypassing of the current through the drain/source, depleting the gate/source voltage.

Let's consider the MOSFET T1. As soon as the rectified DC begins reaching the gate of T1, it will begin turning ON right from around 4 V onward, causing a bypassing effect of the the supply via its drain/source terminals. During this moment, the DC will struggle to rise across the zener diode and begin dropping toward zero.

This will in turn cause the MOSFET to turn OFF, and the continuous stale-mate kind of struggle or a tug of war will occur between the MOSFET drain/source and the MOSFET gate/source, preventing the SSR from functioning correctly.

The Solution

The solution to the above issue could be accomplished using the following example circuit concept.

The objective here is, to make sure that the MOSFETs do not conduct until an optimum 15 V is developed across the zener diode, or across the gate/source of the MOSFETs

The op amp ensures that its output fires only once the DC line crosses the 15 V zener diode reference threshold, which allows the MOSFET gates to get an optimal 15 V DC for the conduction.

The red line associated with pin3 of IC 741 can be toggled through an opto coupler for the required switching from an external source.

How it Works: As we can see, the inverting input of the op amp is tied with the 15V zener, which forms a reference level for the op amp pin2. Pin3 which is the non-inverting input of the op amp is connected with the positive line. This configuration ensures that the output pin6 of the op amp produces a 15V supply only once its pin3 voltage reaches above 15 V mark The action ensures that the MOSFETs conduct only through a valid 15 V optimal gate voltage, enabling a proper working of the SSR.

Isolated Switching

The main feature of any SSR is to enable the user an isolated switching of the device through an external signal.

The above op amp based design could be facilitated with this feature as demonstrated in the following concept:

How the Diodes Work Like Bridge Rectifier

During the positive half cycles, the current moves through D1, 100k, zener, D3 and back to the AC source.

During the other half cycle, the current moves through D2, 100k, zener, D4 and back to the AC source.

Reference: SSR