At least 60 percent of today’s home appliances worldwide contain electronic devices, and almost 100 percent of ‘wet’ appliances can say the same. This move from electromechanical to digital control was first accomplished with off-the-shelf electronic devices, with the systems’ architecture built around a microcontroller, discrete transistors, and high-voltage triacs.

This soft revolution in home appliances is motivated in part by the growing need to reduce energy and water waste, as well as to increase consumer convenience.

Performance and cost efficiencies have always been the challenges for home-appliance manufacturers, and the globalization of the marketplace and its standards have added to that challenge. This has changed the face of AC power controls.

The need for differentiation within the home-appliance market pushes manufacturers to improve system electrical performance and to offer new appliance functions that focus on electrical safety and power savings in normal or standby modes.

Performance and safety improvements

Enlarge this picture Fig. 1. New planar AC switch removes all external switch protection.

When designing a control system, designers aim for higher electrical immunity and increased robustness. The IEC61000-4 series standards relate to the electromagnetic compatibility requirements linked to the AC line, such as high-voltage surges, electrical fast transient bursts, and electrostatic discharges. The standard defines the levels and criteria of power-control board immunity, and requires increasing gradually this immunity and the robustness of the home-appliance systems.

In refrigeration designs, for example, better food preservation and higher compressor efficiency are achieved by using a digital control. A 3 DegC cabinet temperature hysteresis and a power-consumption decrease of 20 percent can be expected.

A safety task in a clothes washer could collect and analyze electrical and washing parameters to avoid the risk of flooding or a lack of water. Orders to stop the heating elements, open the water valves, or power the drain pump can be achieved with the help of power-control circuitries.

Improvements in AC switches

Enlarge this picture Fig. 2. Protected AC switch: NeoS example.

In the first electronic boards, the triac fulfilled the needs for compactness. Five times smaller than a relay in the sub-Amperes range, it offered EMI-free switching operation, a fast response time, a multi-million switching cycles reliability, and a lower consumption drive.

To further simplify power designs, the triac was improved. The snubber circuit placed in parallel of the standard triac was removed, and designers only had to be concerned about the commutation parameter (dI/dt)_C, which is selected according to the load turn-off current.

Nevertheless, the technology of triacs is only robust within the limits of its rated blocking voltage (V_DRM / V_RRM). Beyond this limit, an over-voltage may create irreversible degradations of the switch as an uncontrolled over-voltage triggering provokes hot points in the junction area. The triac must be protected with an external suppressor.

The most critical constraint is the voltage surge as described in the IEC61000-4-5 standard. A 2 kV 1.2/50 μs voltage level is usually required. Two main protection methods are possible to resist this 40 Joules surge:

• Clamping: an external voltage suppressor like a varistor absorbs the energy of the surge.
• Crowbar: the triac is safely turned on, and the surge energy is dissipated into the load impedance.

Developed around a bi-face full-planar technology, new protected triacs feature outstanding built-in over-voltage robustness that enhances system reliability.

When its terminal voltage exceeds its avalanche voltage, the switch is safely self-triggered in crowbar mode. The voltage drops down to a few Volts, and the over-voltage stress is turned into a current flowing through the switch. The planar AC switch then recovers its blocking capability on the end of the line cycle. This behavior is consistent with the IEC60730 standard.

Along with integration targets for reliability and design ease, a further evolution has been realized with the ACS switch. This new switch integrates a gate level shifter, enabling a MCU logic-level drive with higher electrical transient immunity.

For instance, a 0.8 A switch guarantees 500 V/μs immunity — 10 times more than an equivalent 1 A triac with same gate sensitivity (I_GT = 10mA).

This simplifies the design by removing the need for any noise suppressor, and the entire control can meet IEC61000-4-4 standards.

While executing a water valve on/off control, a 0.8 A AC switch safely withstands turn-off operation, absorbing the inductive energy of the load by clamping. Guaranteed by design, the switch energy capability is confirmed by severe tests on highly inductive loads, 28 H.

Previously impossible with the triac structure, the unique gate structure of the ACS switch allows the backside of the die to be electrically stable. Arrays of AC switches can be created in a single frame package, dedicated to the centralized actuator drive of an appliance such as a dishwasher.

Energy savings in refrigerators

Enlarge this picture Fig. 3. Switch failure detector operation prevents catastrophic effects on the appliance.

Energy savings in refrigerators is generated in several ways, including:

• Electronic control improves compressor efficiency by removing the causes of starter leakages and providing better temperature control.
• The starter, a positive temperature coefficient resistor (PTC), continuously absorbs 2.5 W because of its leakage current. These losses can be removed if a solid-state AC switch turns off the PTC after start-up.
• Smoother temperature control reduces the input average power by 20 percent and increases the on/off repetitive rate of the compressor by 50 percent.
• A 10-year compressor lifetime corresponds to 270,000 on/off cycles, which justifies the use of solid-state technology.
• With their 2kV over-voltage robustness and their 200V/μs transient immunity, the new planar triacs or ACS provide the required off-state reliability (half of the lifetime).

In addition, at a similar system cost to the electromechanical solutions, the solid-state technological breakthrough allows refrigerators or freezers to fulfill European Class A+ consumption labels, resulting in better food preservation as well as spark-free and EMI-free operation.

Switches and control

Enlarge this picture Fig. 4. AC switch over-temperature protection. With 25 percent over load (IRMS = 0.95A, TAMB = 25 DegC) a 0.8 A AC switch and its load are protected.

With the use of system-in-package and power-planar technologies, the micro-module combination of an AC switch and a power controller can be envisioned. Because the ACS switch die backside has a stable voltage, it can be placed next to a power IC to implement new functions such as failure detection or overload over-thermal protection.

The safety of home appliances is reinforced through UL and IEC standards. The appliance control monitors the operating state of the AC switches and detects failure modes. Close to the switch, a detection circuit is required to sense AC operation. Some catastrophic load and appliance failure can be eliminated, such as the DC operation of high inductive load due to a triac diode mode failure, and the over-heating of resistive load or the water over-flow due to a switch short circuit.

The innovative concept of smart AC switches, such as the STMicroelectronics NeoS project, answers this challenge by detecting switch failure in a cost-effective manner. An AC detector allows all switch states to be monitored and compared with drive orders, preventing catastrophic failure of the appliance.

Furthermore, the capability of implementing thermal protection of the switch opens new ways to prevent overload effects. Knowing the thermal state of the AC switch allows a shutdown protection to be designed. Thus, if the electrical actuator is misconnected in final assembly or is gradually damaged under special stress, the switch is able to detect this failure. An alarm signal provides information to the appliance control, limiting the maintenance action to changing the electrical actuator rather than the entire electronic board.

This kind of protection is justified with critical electrical loads such as those in cooling fans, drain pumps, or heating elements, which are exposed to aggressive environments. Already, switch-failure detection and overload monitoring features are available in a cost-effective manner, paving the way for the development of remote maintenance services for home automation applications.

Conclusion

Appliance performance, power and resource savings, and user safety and convenience, have all been improved by the development of robust and full planar AC power switches and AC power control circuits. These devices contribute to increased electromagnetic compatibility within the power drive. And their voltage over-voltage robustness along with fast transient immunity allows international EMC standards to be not only met, but exceeded.

They also upgrade the reliability and the quality of appliance control while mastering the cost of the materials. Because they ease the design of power controls, design resources can be refocused to more user-oriented and system-differentiating functions.

A new generation of smart ACS is breaking the “discrete” barrier and is integrating the power controller, which improves electrical-load safety and provides time-saving diagnostic features.


For more information email: Thierry.Castagnet@st.com