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Will solid-state relays now pose a real challenge?

01 January, 2001

Will solid-state relays now pose a real challenge?

A new design of solid-state relay is said to overcome many of the traditional drawbacks of these devices, and to last ten times longer than earlier versions.

Although solid-state relays (SSRs) have been around for many years, they have yet to make significant inroads into sales of traditional electromechanical relays. The global market for SSRs is worth just £50m, with the UK accounting for about 10% of this.

One reason that SSRs have failed to make a greater impact is that potential users are worried about some aspects of their performance. For example, the relays generate considerable heat, they can cause electromagnetic interference, they can be damaged by over-voltages and short-circuits, they can suffer internal failures as a result of thermal stresses, and they can sometimes even catch fire.

On the other hand, the electronic relay does offer several potential benefits. It is quiet, dustproof, resistant to shock and vibrations, switches quickly, and avoids the problems of contact bounce. It also responds to low-power inputs and offers both synchronous and non-synchronous switching.

Two years ago, Crouzet, one of the leading manufacturers of solid-state relays, embarked on a project to develop a new generation of SSRs which would tackle most of the earlier concerns. Starting with a blank sheet of paper, the designers set themselves the task of developing the most reliable and robust SSR on the market.

The biggest cause of SSR failures is heat. The SCR (silicon-controlled rectifier) switching circuits that control the current flow, generate heat which must be dissipated through a ceramic substrate and baseplate before reaching a heatsink. If the internal heat transfer is inefficient, or if the relay is not matched correctly to its load, the device may overheat and fail.

In the new design, the ceramic substrate is fused with copper to minimise thermal impedance between the SCRs and the baseplate. According to Crouzet, this direct copper bonding (DCB) technology has several attractions.

For example, it allows the use of SCR dies that are 40% thinner than before, reducing the on-state voltage drop. This, in turn, means that there is less power to be dissipated and reduces the size of heatsink required.

The copper also forms a stronger bond with the ceramic than the previous thick-film technology, again cutting the thermal resistance.

Another heat-related problem with conventional electronic relays involves their internal soldered joints. As the relay goes through repeated cycles of heating up and cooling down, these joints can fracture, leading to failure.

In a traditional SSR, load current typically flows into a relay terminal, through a solder junction into the circuit board, through another solder junction into the power lead, and through yet another junction to the SCR. This process is repeated in reverse as the current flows back out of the relay to the load or AC mains. Each junction generates heat and is a potential point of failure.

In the new design, known as the "new generation" GN relay, most of the junctions are eliminated using a lead frame that provides direct connections to the power leads.

Another innovation involves the baseplates. Instead of the traditional stamped baseplates which tend to curve slightly, reducing the area available for heat transfer, the new SSRs use flat, cast aluminium baseplates.

Traditionally, metal oxide varistors have been used to protect SSRs against spikes and over-voltages. However, these devices have a limited life, so Crouzet has adopted a new silicon-based transient protection device known as a "Tranzorb" for the new relays. These fast-acting devices are said to have an almost unlimited operating life.

According to Brian Hornsby, Crouzet`s SSR product manager in the UK, the performance predictions for the new GN relays have been borne out in the laboratory. In one series of thermal tests, designed to stress relays to the point of failure, Crouzet`s old models and relays currently produced by its main rivals failed after about 800 cycles. The new design lasted for an average of 8,000-10,000 cycles before expiring.

There are two versions of the new SSR. One provides zero volt switching for single-phase resistive loads or for motors where the power factor is greater than 0.8. For inductive loads, such as motors, where the power factor is less than 0.8, a random switching version is offered.

Both versions have a diagnostic LED to show when the SSR is on. They are available in current ratings from 10A to 125A at 280V or 660V AC. The inputs accept 5V logic signals or 4-32V regulated DC signals such as PLC outputs.

The new relays are CE marked for compliance with the Low Voltage Directive and Crouzet says that devices are also EMC compatible. Unlike some SSR suppliers, which regard RFI protection as being the responsibility of the user, Crouzet has included suppression components on both the inputs and the outputs of the new relays. It argues that this will save time and money for engineers and designers who use the devices.

According to Horsnby, the new SSRs are priced similarly to rival relays that do not include EMC components.

Crouzet was recently acquired by Schneider Electric.




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