# How to apply passive PFC on low side of transformer in power supply?

I have a low power power supply, built with a 60Hz 120v/12v 0.3A transformer. It will have an ordinary diode bridge and capacitor filter. What I want to achieve is reduction of current/magnetic field spikes caused by power factor effects.

I read about PFC, but can only see examples with parts on "mains" side of transformer. I dont want to mess with mains, so I decided to put PFC after secondary winding.

What parts, configuration and values are needed to achieve good low voltage PFC for very low noise power supply?

The reason for placing the PFC circuit on the primary is that it's the transformer itself which is inductive!
But there's absolutely no need to do PFC on a 3.6W power supply. Just forget about the whole thing.

• But isn't it a capacitance of fiter caps after bridges which causes the need in PFC ? I expect the transformer's inductance to be very very low. – user924 Jun 29 '11 at 16:19
• PFC mainly corrects the unpredictable, HF current draw from the DC/DC converter by making that power stage appear resistive to the mains. – Adam Lawrence Jun 29 '11 at 20:31
• @Rocket Surgeon: what capacitor value do you want to use??? A 1000$\mu$F won't be noticeable at the primary! – Federico Russo Jun 30 '11 at 6:34
• Yes, 1000μF. My whole approach with low noise power supply is to get to 1ppm noise/drift/oscillation/hum as close as possible in range of 0..+10V. – user924 Jun 30 '11 at 14:42
• @Rocket Surgeon: don't forget to post here when you're done. Which means: I don't think 1ppm is feasable. – Federico Russo Jul 1 '11 at 14:41

You likely will not be able to achieve good power factor correction at that power level even if the PFC implementation was on the primary side. To get good PFC, the front-end boost stage should be running in continuous mode to get the PF high, which almost never happens at such light loads. I'd forget about PFC for this application.

You're not crazy to do it on the secondary side; as long as your transformer only draws on load reactance (somewhat subject to temperature, spot-on 60Hz AC, what you do with VOMs and shunt resistors if anything,) you can simply draw current evenly enough to match your rather particular 1ppm spec and somehow raise a flag on whatever needs changing if a capacitor goes out of spec in the lifetime of your supply. Moreover you can skip the isolation transformer and pick an RC, LC or LRC conductance mode filter to feed your baby charger/supply (which does sufficient AC/DC isolation in its own components, in exchange for attention to voltage tolerances, leakage, etc.)

Your spec of 1ppm is kind of nuts; it sounds like a solar or other energy harvesting (The Hills Have Ls?) supply is needlessly being put into a cheap inverter that can only tolerate these expensive (in parts or engineering, or both) DC supplies, when it's trivial to make adequate inverter or a DC (even 24V) tap. Not to say it can't be awesome the way you initially called it, shorting a cap on the primary's not going to help reactance noise and guarantees the power loss which premises PFC equipment could have mitigated. Spec of <1ppm on the noise, drift and wvdc is different; and do you need that to stay true close at 0v, where you suddenly need two or more extra decades (after 6) of hardness to RFI? Are Fairchild and ADI design models, which are almost certainly overkill but permit you keen instrumentation, truly inadequate so far?