Active PFC AC/DC converter high efficiency low voltage

[quux]

Hi all,

So, I've been looking far and wide on the tinterweb and have yet to find a good solution to this problem. Here it is.
I have a Hub Dynamo on my bicycle that I want to hook up to a USB power connector (to power my phone). I want to have high efficiency as my phone has a pretty high current draw.
The hub dynamo is rated at 6V/3W AC. The final output should be 5VDC.

Given the low input voltage it would seem to me that I would want to avoid using a diode rectifier bridge.
Similarly given the need for high efficiency, I would want a SMPS setup (possibly using Active PFC, given that the source is likely to be highly inductive).

Therefore, does anyone have any recommendations on how I could put something like this together?

I think I want to have an active MOSFET rectifier with an active PFC buck-boost PSU.
Although also once saw someone build a boost PSU in parallel with an inverting SEPIC and a pair of MOSFETs switching between the two.

 

[Orson Cart]

A schottky diode bridge followed by a sepic or Cuk converter (or buck-boost) will do the trick.

 

[quux]

Even a Germanium Schottky bridge would drop 0.6V across it, which would drop (I think) about 10% of the power.
Given that the USB specification requires a supply of 5V at 500mA (ie 2.5W) a Schottky bridge would give very little efficiency to work with on the sepic/Cuk to work within.

 

[Orson Cart]

Dear quux, you should try measuring some of the schottkies out there, a lot will give 0.2 volt forward for low currents...

 

[quux]

Okies, if its a 0.2V drop across a bridge then that would be doable.
I do assume you mean across the bridge rather than across a single Schottky diode.