Reference design enables no-neutral wireless wall switches for smart home lighting

A reference design from Power Integrations is for two-wire smart wall switch, compatible with retrofit wiring and LED lights.

Reference design, DER-622 describes a smart wall switch, compatible with wiring conditions most commonly found in residential retrofit installations.Typically, smart wall switches with wireless connectivity, occupancy/vacancy sensing and/or voice control require a neutral return wire to power the unit, which is not always available in retrofit situations.

No-neutral products are available for legacy incandescent bulbs, because the small AC input current that is allowed to leak through the load when the smart switch is in standby mode is insufficient to heat the filament. However, for LED and compact fluorescent designs, high standby-mode current from the smart switch’s internal power supply can lead to unacceptable flicker often known as ghosting, caused by the leakage energy accumulating in the lamp and initiating intermittent start-up and brief light activation.

DER-622 illustrates a Bluetooth Low Energy wall switch consuming less than 500-microA in standby mode. The design is based on Power Integrations’  LinkSwitch-TN2 offline switcher ICs, which have quiescent  consumption of less than 75-microA. The low current consumption and high light-load efficiency ensure compatibility with energy-efficient LED bulbs rated down to 3.0W, says Power Integration, and are suitable for no-neutral wall switch wiring.

LinkSwitch-TN2 devices may be configured to support flyback or buck topologies and deliver highly accurate output, providing voltage regulation of better than ± three per cent. The ICs enhance system reliability by incorporating safety features such as input and output over-voltage protection, over-temperature, and output short-circuit protection along with a rugged 725V power MOSFET.

In DER-622, the LinkSwitch-TN2 power supply IC is used in a non-isolated flyback topology and employs half-wave AC input rectification to save on cost. The power supply provides two outputs, a 12V rail to drive a relay and a 3.8V rail to power a Bluetooth Low Energy controller.

Key applications include wireless lighting control, occupancy and vacancy sensors, motion detectors, wall dimmers and shading controls. DER-622 can be freely-downloaded from the company website.


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