Achieving better home appliance safety with smart AC control design methodologies - Embedded.com

Achieving better home appliance safety with smart AC control design methodologies

At least 60 percent of today's home appliances containelectronic devices, and almost 100 percent of “wet” appliances can saythe same.

This move from electromechanical to digital control was firstaccomplished with off- the-shelf electronic devices – the systemsarchitecture being built around an MCU, discrete transistors andhigh-voltage triacs.

This mini revolution in home appliances is motivated in part by thegrowing need to reduce energy and water wastes, as well as to promoteease-of-use.

Performance and cost efficiencies have always been challenges forhome-appliance manufacturers, along with the globalization of themarketplace and its standards. This has changed the face of AC powercontrols.

When designing a control system, designers aim for higher electricalimmunity and increased robustness. The IEC61000-4 seriesstandards relate to EMC 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 controlboards' immunity, and requires increasing this immunity and therobustness of home appliance systems gradually.

In refrigeration designs, for example, better food preservation andhigher compressor efficiency are achieved by using a digital control; a3 degree Centigrade cabinet temperature hysteresis and a powerconsumption decrease of 20 percent can be expected.

A safety task in a clothes washer could collect and analyzeelectrical and washing parameters to avoid the risk of flooding orshortage of water. Orders to stop heating elements, open water valvesor power-drain pumps can be achieved with the help of power-controlcircuitry.

Figure1: The use of a planar AC switch removes all external switch protection.

Emerging solutions
In the first electronic boards, the triac fulfilled the need forcompactness. Five times smaller than a relay in the sub-amps range, itoffered EMI-free switching operation, afast response time, a multimillion switching-cycle reliability, and alower consumption drive.

To further simplify power designs, the triac was improved. Thesnubber circuit placed in parallel with 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 thelimits of its rated blocking voltage (Vdrm / Vrrm). Beyond this limit,an overvoltage may create irreversible degradations of the switch – anuncontrolled overvoltage triggering provokes hot points in the junctionarea. The triac must be protected with an external suppressor.

Figure2: Switch-failure detector operation (top) prevents catastrophiceffects on the appliance.

The most critical constraint is the voltage surge as described inthe IEC61000-4-5 standard. A 2kV 1.2/50µs voltage level isusually required. Two main protection methods are possible to resistthis 40 Joule (J) surge:

Clamping. Anexternal voltage suppressor like a varistor absorbs the energy of thesurge.

Crowbar. Thetriac is turned on safely, and the surge energy is dissipated into theload impedance.

Triacs to the rescue
Developed around a bi-face full-planar technology, new protected triacs feature outstandingbuilt-inovervoltage robustness, thus enhancing system reliability.

When its terminal voltage exceeds its avalanche voltage, the switchis safely self-triggered in crowbar mode. The voltage quickly dropsdown to a few volts, and the overvoltage stress is turned into acurrent flowing through the switch. The planar AC switch then recoversits blocking capability at the end of the line cycle. This behavior isconsistent with the IEC60730 standard.

Along with integration targets for reliability and design ease, afurther evolution has been realized with the ACS Switch (Figure 1,above ). This new switch integrates a gate level shifter,enabling anMCU logic level drive with higher electrical transient immunity.

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

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

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

Previously impossible with the triac structure, the gate structureof the ACS switch allows the backside of the die to be electricallystable: arrays of AC switches can be created in a single-frame package,dedicated to the centralized actuator drive of an appliance such as adishwasher.

Figure3: Shown is an example of AC switch overtemperature protection.

Energy savings
Electronic control improves compressor efficiency by removing thecauses of starter leakages and providing better temperature control.

The starter – a positivetemperature coefficient (PTC) resistor – continuouslyabsorbs 2.5W because of its leakage current. These losses can beremoved if a solid-state AC switch switches off the PTC after startup.

Smoother temperature control reduces the input average power by 20percent and increases the on/off repetitive rate of the compressor by50 percent. A 10-year compressor lifetime corresponds to 270,000 on/offcycles, which justify the use of solid-state technology.

With their 2kV overvoltage robustness and their 200V/microsecondtransient immunity, the new planar triacs provide the requiredoff-state reliability (half of the lifetime).

At a similar system cost to the electromechanical solutions, thisallows refrigerators or freezersto fulfill Class A+ consumption labels, ensuring better foodpreservation as well as spark- and EMI-free operation.

Closer association
With the use of system-in-package and power planar technologies, themicromodule combination of an AC switch and a power controller can beenvisioned.

Because the ACS switch die backside has a stable voltage, it can beplaced next to a power IC to implement new functions such as failuredetection or overload overthermal protection.

The safety of home appliances is reinforced through UL and IECstandards. The appliance control monitors the operating state of the ACswitches and detects failure modes. Close to the switch, a detectioncircuit is required to sense AC operation. Some catastrophic load andappliance failure can be eliminated.

Furthermore, the capability of implementing thermal protection ofthe switch opens new ways to prevent overload effects. Knowing thethermal state of the AC switch allows a shutdown protection to bedesigned.

Thus, if the electrical actuator is misconnected in final assemblyor is gradually damaged under special stress, the switch is able todetect this failure. An alarm signal provides information to theappliance control, thus limiting the maintenance action to changing theelectrical actuator rather than the entire electronic board.

This kind of protection is justified with critical electrical loadssuch as those in cooling fans, drain pumps or heating elements, whichare exposed to aggressive environments.

Already, switch-failure detection and overload monitoring featurescan be achieved in a costeffective manner, paving the way for thedevelopment of remote maintenance services for home automationapplications.

Thierry Castagnet is Manager,Strategic Marketing for Power Management and Industrial Controls, ASD& IPAD Division, MPA Group at STMicroelectronics. For a PDF versionof this article go to “Homeappliance safety with smart AC control.”

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