An unusual switching capacitive shunt regulator from Supertex can cut mains stand-by power to less that 20mW.
It replaces the traditional Zener diode with a mosfet switch in a capacitively-coupled shunt regulator, of the type frequently used to supply microcontrollers from the mains.
"The SR10 is for applications where a conventional switch-mode power supply would be too expensive and where an inefficient capacitor-zener solution wouldn't meet standby power requirements," claimed Supertex v-p of marketing Hernan DeGuzman.
The mosfet is simply switched off when the output drops below a threshold, allowing the mains series capacitor to supply current to the output capacitor (see diagram below), then back on when the output rises above the required output.
There is one subtlety - to prevent high current transients that would reduce efficiency, mosfet switch-on is delayed until close to the mains zero-crossing.
Overall, with a full output load the circuit behaves exactly like the traditional Zener circuit as there is almost no current flowing through the regulator. All dissipation is in the load.
At low load, where the Zener would be dissipating maximum power, the shunt mosfet is on almost all of the time, and so dissipates almost no power.
Mains current is almost exactly 90 degrees out of phase with voltage, so the load is purely reactive and no real power is consumed.
Internal feedback taps allow the output voltage to be set to 6, 12 or 24V, or external resistors can vary it from 6 to 28V.
Output current is inherently limited by the value of the mains series coupling capacitor, which has to be safety-rated.
The chip comes in an eight lead SOIC package, and samples are available now.
Above, the Supertex capcitive switching shunt regulator, with the traditional solution below.
