R
Roger Lascelles
Tom (at tomsweb.net) said:
Ouch
So many questions - serves me right for opening my mouth.---- SWITCHIN CONVERTER ------
PFC correction is a way to prevent peaky charge current, and makes the input
current look like a sinewave. It does that by running a switched mode
converter at a much higher frequrency than the supply frequency and varying
the switching pulse width and hence adjusting supply current throughout the
low frequency cycle. The magic of the PFC circuit is that you still end up
with a capacitor charged up to a DC voltage, just as you do with a simple
rectifier. You were concerned about inefficiency due to peaky current, and
the only way to work around that is either a choke input filter (you don't
want the weight of the choke) or a PFC type circuit, or perhaps some kind of
resonant filter system (you don't have a fixed frequency). So PFC
techniques are of interest to you.
Let me go straight to the point. A small, modern microprocessor will do it
all for you, turning on and off the MOSFET in your your PFC type switching
regulator, and varying current as the instantaneous voltage varies. For all
I know, it could probably turn on and off the MOSFETs which form your
rectifier. As well, it can handle stuff like cutting the pedal effort at
low speed and battery charge management. The micro will have an A/D
converter and onboard voltage reference.
The magic of the micro controlling a switchmode is that you can tune power
drawn from the source - ie you can move up and down the current vs voltage
characteristic of the generator. Big generators run at high efficiency,
with generator losses much less that load power. However, you may want to
try loading your small generator down to the maximum output power, where
generator losses equal output power, the impedance *matched* condition where
you increase the load until the output voltage drops to half. Or anywhere in
between.
For example, at 20Km/h, your alternator drops 9.9 - 4 volts at 0.5 amp - ie
it is a source resistance of 11.8 ohms. With a 11.8 ohm load, you will get
equal power delivered to generator and load. Max power in load at 20km/h
will be half of ( 9.9V ^2 ) / ( 2 * 11.8 ) = 2.1 watts, which is close to
your calculated 2 Watts !! So PFC lets you get the current just right for
maximum power !! Of course, *you* have to pedal harder.
About your buck vs boost issue. Once you turn your inductor into a
transformer with two windings, you can manage any different voltages on
primary and secondary. Probably use a flyback converter. So you combine
that with PFC and a single switching circuit with the one transformer does
it all.
For me, all this is too much trouble for a one-off. Enough R&D for a
commercial product. I would check out the simple rectifier approach. As
discussed next.
---- SIMPLE RECTIFIER ------
Batteries do not like charge-discharge from cycle to cycle. While you can
get away with putting the juice in as pulses, you must not put in on one
part of the cycle and take out on the other part of the cycle, thus using
the battery as a kind of capacitor. You end up wearing out the battery
without even running it flat ! Some big wet NiCD types handle this - in
relephone exchanges, but not any small types that I know of - maybe others
can suggest.
About those current peaks. The source won't "go to its knees" trying to
provide the peaks- if by that you mean "give up". Rather, the conduction
angle will lengthen. It may seem so messy, but that is how electronic
devices got their power from the mains, until about 15 years ago, when PFC
came along. Inside your radio, the transformer copper resistance pretty
well determines how much current flows on peaks, and in conjunction with the
capacitor, how wide are the peaks. Much simpler than PFC, and it may be as
far as you need to go.
If you want to check out the possibilities with a simple rectifier, I would
suggest using SPICE to model possible circuits. I would model the
alternator as an AC source in series with 5.9 ohms resistance. You can
calculate efficiency, losses etc. This would be my first job, because if it
is acceptable, I avoid a big R&D job on the fancy version.
Personally, I would put rechargable in my bike light and charge up at home.
Roger
