Smart-home lighting reference design; no-neutral wireless wall switches
The design, DER-622, describing a wall switch compatible with wiring conditions most commonly found in residential retrofit installations, where a “smart” switch may be installed in place of a simple electrical switch. In a typical domestic installation, the wall switch will break the live line, and only the live wire will be “dropped” from the ceiling wiring; the neutral is run only to the light fixture itself. This solution “bleeds” a small amount of current (microAmp level) through the light in its off-state, sufficient to power the smart switch and to store enough energy to latch a relay when the light is turned on.
Typically, PI observes, 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 (LE) wall switch consuming less than 500 µA in standby mode. The design is based on Power Integrations’ LinkSwitch-TN2 offline switcher ICs, which have quiescent consumption of less than 75 µA. The ICs’ ultra-low current consumption and high light-load efficiency ensure compatibility with energy-efficient LED bulbs rated down to 3W and are ideal 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 ±3%. The ICs enhance system reliability by incorporating numerous safety features including 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 utilized in a non-isolated flyback topology and employs half-wave AC input rectification to reduce solution cost. The power supply provides two outputs – a 12V rail to drive a relay and a 3.8V rail to power a Bluetooth LE controller.
Key applications include wireless lighting control, occupancy and vacancy sensors, motion detectors, wall dimmers and shading controls. DER-622 can be downloaded at https://www.power.com/der-622
Power Integrations; www.power.com
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