Solid-State AC Switches

April 1, 2002
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New technology serves electronically controlled appliances.

As home appliances turn increasingly to electronic controls to conserve energy and improve reliability, the solid-state switch of choice is clearly the triac. Today’s compact, efficient, and rugged, triacs are well suited for controlling loads of up to 5 A. Moreover, they are continuously being improved and, in the near future, will include in one package switching and control functions that allow for power management and isolate users from shock hazards.

Already, reliable static AC switches are emerging which are easy to use and have robust off-state resistance to voltage surges in compliance with the requirements of IEC61000. Moreover, they meet the high-quality standards for mass market applications, offer small size, and connect directly with a microcontroller. Moreover next-generation electronic appliance controllers are about to emerge with built-in controllers or microcontroller interfaces, reducing the size and cost of appliance electronics and, eventually, adding monitoring and diagnostic features.

For example, a planar solid-state fabrication technology called ASD has produced rugged, noise-immune AC switches capable of handling from 1A to 25 A under direct command of a microcontroller. As a replacement for electromechanical relays, switches built using this technique clear the way for building smart AC timers and other controls in one and the same package as the switch itself.

A refrigerator compressor example shows one way that such direct control can improve efficiency by eliminating the positive temperature coefficient resistor used to start the compressor motor (see Fig. 1). By controlling the start and run windings directly, the processor controlled switch recovers the resistor’s 2.5 W of power dissipation, increasing efficiency by 2.5 percent to 7 percent.

Choosing a solid-state AC power switch involves considerations of reliability, compactness, and cost. Specific factors also include power handling capability, galvanic isolation, overall power dissipation, and robustness in the face of overvoltage transients. The fact is that, today, electromechanical relays still dominate where load currents exceed 1.5 A. As switches, relays offer low power dissipation and good galvanic isolation.

But for load currents under 1.5 A, triacs are a popular choice, especially where fast switching times and high reliability are critical. About one-fifth the size of a relay for sub-1-A loads, triacs offer zero-voltage turn on and zero-current turn off to yield EMI-free switching. In addition, switching occurs in about a millisecond and reliability reaches into the multimillions of operations at full current. Finally, the triac can be controlled by applying low-power logic-level control signals to its gate.

Not without their downside, triacs suffer from relatively high power dissipation during conduction and poor galvanic isolation compared to relays. But these shortcomings are being overcome and enhancements made possible with the emergence of new planar silicon-fabrication technologies like STMicroelectronics’ ASD process. Specifically, the prospects for future triac-like switches include increased off-state resistance to line voltage transients and surges thanks to built-in overvoltage protection; feedback on current, voltage, and temperature conditions for power regulation, safety, and maintenance; and the use of optical couplers to achieve relay-like galvanic isolation.

As an example of the increased robustness available through planar technology, consider the characteristics one of a new breed of AC switches, the model ACST6-7S. For one thing, the switch can drive a 230 V compressor induction motor and handle up to 300 VA (see Fig. 2). Carrying ratings of 700 V at 6 A, it can operate with a phase-shift capacitor to switch off a stalled rotor without the aid of a snubber circuit.

At the same time, with a 10-mA maximum gate-triggering current, the ACST6-7S can be driven directly by the 20-mA output current of a microcontroller. Moreover, the device features a junction temperature rating of 125?C and is immune to noise pulses having slew rates of up to 200 V/µs, a fivefold improvement over the noise immunity of triacs, dramatically reducing the size of a noise suppressor.

Voltage protection in the ASD switch operates like a thyristor-based crowbar switch. That is, the switch is triggered when the terminal voltage exceeds its avalanche voltage. At that point, the terminal voltage drops from 1,300 V down to just a few volts. In this way, the voltage stress is translated into a fast surge current. Moreover, because of the switch’s thyristor topology, it conducts every half cycle.

In cold appliances, reliability is an especially important factor related to product life. In its expected 10-year or 90,000-hour lifetime, a compressor will turn off and on about 300,000 times and be turned off for about 60,000 hours. Here, the off-state robustness of the ASD switch makes it a compelling choice for this type of application.

To be sure, the ASD fabrication technology encourages still further innovation in developing AC switches. One example is STMicroelectronic’s ACS family of switches for handling currents of under 2 A and power ratings of less than 200 W. The ACS family integrates the same overvoltage protection and off-state robustness as the ACST device mentioned earlier. In addition, it can clamp and absorb the turn-off energy from inductive loads, like a relay or valve coil, of up to 25 Henries without external protection. Moreover, a key innovation of the ACS switch is the integration of a high-voltage level shifter, which imparts three important advantages.

First, the level shifter inserts a separating block between the power switch and a controller. In this way, not only can the switch, with its 10-mA gate triggering current, be controlled by a processor, but the processor itself is protected from any overvoltages on the gate.

Second, the level shifter increases the switch’s noise immunity tenfold compared to conventional triacs. Thus, for example, the ACS108-5S, which has a 10-mA triggering current, can withstand a static dV/dt of more than 500 V/µs, obviating the need for an external RC circuit.

Third, the level shifter allows the switch’s internal back-to-back thyristor switch to be physically inverted. In this way, the backside becomes the neutral line allowing several switches to be mounted in one package to save space and cut assembly time (see Fig. 3). Alternatively, discrete switches can share the same heat sink without the need for an insulating plate. The result is to reduce the radiator size and simplify mounting.

The option of putting multiple switches in one package clears the way for the addition of a power controller or, to impart galvanic isolation, an optical coupler. Indeed, an example already exists in an electronic fluorescent-tube starter that includes a 1-A, 800-V switch; a 1,400-V ignition device; and a 12-V, 5-mA AC-to-DC power supply. Other ACS switches have been manufactured with built-in optical couplers. And in the future, still other integrated combinations will emerge. Likely products to appear could include a smart plug that embeds a controller for drive and monitoring functions, and even a network interface.

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