# Which circuit to maximize power at regulated 3V3 DC from low power 6VAC/20mA source?

The power output of provides a guaranteed 6VAC/20mA/50kHz minimum output into a resistive load with fluctations possible up to 12VAC. Other loads are not specified. The output impedance is not specified, but I think we could model the source as 12VAC/50kHz/300$$\{\Omega}\$$ output impedance.

I want to power a DC circuit from it that may have "instantaneous" peak consumptions higher than 20mA.

I wonder how to maximize DC power @3V3. Without losses, 0.12W max would be available. A big enough capacitor would help cope with peak currents.

One method I think of is to add an up transformer to multiply the source voltage by 4 or 5 times to stay within safe limits at all times. So 6V/300$$\{\Omega}\$$ source would become a 30VAC/50kHz/1k5$$\{\Omega}\$$ (max 60V) source that could be used by a switched mode rectifier.

EDIT: I found an interesting search term: "Synchronous rectification", used in secondaries of SMPS.
I found chips that do not need (direct) control concerning the primary side.

• Diodes/APR347 operates from 4.5V up.
• OnSemi/NCP43080 specified for up to 1MHz.
• Diodes/APR34330 incorporates the mosfet.
• Active bridge controller: NXP/TEA2208T
• TI/UCC24612 - sensing up to 230V.

Most of those seem to do only "half-wave" rectification.

EDIT2: I search for the term "Resonant rectification" while thinking of the principle used in a class D RF power amplifier with an LC-tank. This gave some other interesting results - I am not sure it's fundamentally different from "Synchronous rectification".

Resources:

• ST/SRK2001 - Dual gate driver for use with a transformer with center tap.
• onsemi/NCP4390 - Dual gate, requires primary current sensor.
• Patent US5701243A

Other thoughts?

• hard to answer without a list of wich you looked at datasheets and rejects explaining for each one why you rejected it exaclty. Are you sure you detailed all you specifications/needs in the post ? or it is only the main onces ?
– user201301
Commented Nov 26, 2021 at 14:34
• I did not do a research on components that I could use. I've been thinking about the design principle since a while, but this is the best I can think of. In the end this is to use that power to do IoT transmissions. The power actually available would determine the possibilities. Using WiFi for IoT type transmissions seems a possibility: youtu.be/d80X2GtTNBk?t=2000 . Commented Nov 26, 2021 at 15:44
• "I want to power a DC circuit from it that may have "instantaneous" peak consumptions higher than 20mA." Large output capacitor. Commented Nov 26, 2021 at 17:56
• "I wonder how to maximize DC power @3V3. Without losses, 0.12W max would be available" Is your AC source a sinusodial waveform or something else, like square wave? Commented Nov 26, 2021 at 17:56
• The output is sinuoidal, but can have "50% distortion". Sure, the capacitor has to be big enough. Commented Nov 26, 2021 at 20:43