Lutron Softswitch Technology

Background

Up until the early 1990's, relays needed to be concerned with only incandescent and inductive loads. The worst case inrush for these load types was 10X and the relays were designed accordingly.

Unfortunately, the advent of electronic ballasts changed the 'rules' as the inrush for these loads can be as high as 50X, more typically 25-40X. At these levels of inrush, the relays that were designed for 10X will typically experience a lower life. In some cases, this could be only weeks after initial operation.

The primary cause of relay failure is heat which is caused when the typical relay is closed during turn on. An arc is created as the relay contacts close, 'bouncing' several times before finally closing. As Figure 1 shows, this results in wasted energy that is dissipated in the contacts of the relay as heat. The higher the inrush, the more heat is created. Ultimately, this heat causes the contacts to weld and become inoperative.

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Figure 1

Lutron’s Solution

Knowing that heat was the source of relay impairment, Lutron's approach was aimed at reducing the heat by eliminating the root cause - the arc.

Figure 2A shows three components—a mechanically held relay (which is the primary component of other relays), an electronically held relay, and a solid-state triac. When signalled to close (or turn on), the electronically held relay is closed before the triac is signalled to begin conducting (Figure 2B). Once the electronically held relay is closed, the triac begins conducting, energizing the load. Any inrush at this point is passed through the triac. (Triacs are inherently more capable of handling surges than relays because they have no moving parts and therefore no arc.) Since the electronic relay is already closed, it is not affected by any inrush.

	Hot/Live
			Series Switch Electronically Held Relay

	Parallel Switch Mechanically Held Relay		Series Triac Solid State Device
	Switched Hot/Live

Figure 2A – Soft Switch Circuit

This is the circuit prior to activation. Three components are shown – a mechanically held relay, an electronically held relay, and a solid-state triac.

	Hot/Live
			Series Switch Electronically Held Relay

	Parallel Switch Mechanically Held Relay		Series Triac Solid State Device
	Switched Hot/Live

Figure 2B – Softswitch Circuit

Upon activation, the electronically held relay is closed and then the triac begins conducting. Once the load is energized, inrush is passed through the triac (the closed relay is not affected).

	Hot/Live
			Series Switch Electronically Held Relay

	Parallel Switch Mechanically Held Relay		Series Triac Solid State Device
	Switched Hot/Live

Figure 2C – Softswitch Circuit

Now the mechanically held relay closes as well. Because of the higher resistance than the electronically held relay, no current passes through the mechanically held relay, resulting in no arc.

At this point, the mechanically held relay is closed (Figure 2C). When the contacts for this relay 'bounce', there is no current passing through the contacts because the 'bouncing' contacts have a higher resistance than the already completed circuit passing through the electronically held relay and triac. Accordingly, this relay closes without an arc. Figure 3 reflects this.

	Hot/Live
			Series Switch Electronically Held Relay

	Parallel Switch Mechanically Held Relay		Series Triac Solid State Device
	Switched Hot/Live

Figure 2D – Softswitch Circuit

The electronically held relay now opens and the triac is removed from the circuit. When the circuit is turned off, both relays open again providing an air gap.

Finally, the electronically held relay opens (Figure 2D) and takes the triac (and any heat or inefficiency—most triacs are 2-3% inefficient) out of the circuit. The notch in the waveform in Figure 3 is the triac remaining in the circuit. The electronically held relay opens after 10 line cycles, removing the triac from the circuit.

In turning off the circuit, both relays are opened using the same sequence in reverse, ensuring an air gap off that is consistent with all Lutron products.

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Figure 3

Triac in Circuit (will be removed from circuit after 10 cycles

Summary

With the Softswitch™ design, Lutron has verified relay cycles (on/off) in excess of 100,000 times without adverse effects on relay operation. It is also independent of source - resistive, inductive, or capacitive - where most other relays are rated for resistive only. Figure 4 shows the various levels of typical inrush and Lutron's solution which is capable of handling 50 times inrush.

Another 'new' relay circuit introduced recently is a 'zero cross relay'. These relays are supposed to reduce the inrush by turning on where the power sine wave crosses zero. These relays do reduce the inrush but there is still inefficiency in this operation. Figure 1 on page one is a zero cross relay. Other common relays would be far worse. Zero cross switching is implemented in order to minimize the effects of arcing. With Lutron's Softswitch technology there are no moving parts to protect due to the triac; therefore, zero cross switching is not necessary.

One last comment is that many facilities have not experienced a problem with traditional relay operation. There are several factors that can contribute to this. The two most common are: circuits that are not fully loaded and line impedance (longer wire runs as an example). It is important to note that Lutron had the same perspective four years ago. Because we do operate some circuits as nondim, we began to witness issues with our circuits (and components) that led us to this solution. With this solution in place, we have not experienced any problems.

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Figure 4 – Lutron’s Patented Switching Scheme