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MOSFETs for relay market.

Solid-state relays have been available for over 20 years. Typically, they have been based on silicon-controlled -rectifier (SCR) technology. Advantages include high surge, high voltage, low cost, and relatively low forward-voltage drop. But disadvantages include limited control, high noise generation during switching, thermal latch, and slow switching speeds.

In some applications many peripheral passive components must be added to the SCR output relay to offset SCR problems. For example, in light dimming, large inductors serve to slow down the waveform to the lamp.

The fast transition that occurs when "phasing" an SCR generates radio-frequency interference (RFI), and this interferes with other types of electronic equipment. Resistors, capacitors, and inductors are typically used to alter the waveform in an attempt to reduce RFI.

Existing SCR relays, when used in random mode for phase control, alter the phase angle by delaying the on time with respect to the sine wave's zero-cross starting reference. For resistive loads such as heater elements, this type of switching degrades element life.

In some cases, lamp loads produce a "singing" noise. Most lamps have lower resistance when cold, and the elements are designed for normal sine-wave heating. Rapid transitions from cold to hot will stress the elements and generate significant noise.

Finally, zero-cross SCR solid-state relays do not turn on or off precisely at zero. Turn-off occurs automatically with each cycle of AC voltage, but larger SCR devices involve increased holding current, and the zero off-point shifts accordingly. Worse, as loads become more inductive, the differential is greater.

Thus, there is ample reason to try something new, and MOSFET and IGBT units are now viable alternatives. They reduce RFI, provide alterable switching slopes, suffer less off-state leakage, and have higher immunity to false triggering. But, with comparable on-state rated current, there is a higher forward voltage drop than with SCR devices. This means that more heat is generated for equal current passed. However, by taking advantage of on-off switching in the efficient operating region of the actual load to be switched, it is possible to generate less heat on an average basis.

A MOSFET or IGBT will generally cost more as the output device in a solid-state relay than SCRS. However, on an overall system basis, the advantages achieved can offset the higher cost. An example would be phase control applications for light dimmers. Use of these devices can eliminate the need for inductors. This is accomplished by phase controlling the off side of the sine wave and altering the slope. In addition, noise is significantly reduced, making it easier for compliance with FCC noise requirements. Life expectancy of the filament is also increased.

For more information, contact Gentron Corp, Scottsdale, AZ. Phone 602-443-1288.
COPYRIGHT 1991 Nelson Publishing
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Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:silicon-controlled-rectifier relays
Publication:Tooling & Production
Date:Feb 1, 1991
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