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efficient low-voltage ac/dc conversion for bicycle battery charger

  • Thread starter Tom (at tomsweb.net)
  • Start date
R

Roger Lascelles

Tom (at tomsweb.net) said:
Roger,


OK, exactly how would I drive the gates of these MOSFETs? Do they need
a square wave in phase (and/or opposite phase) with the incoming power
frequency, which is variable? If so, do you know a convenient way to
generate that?

Is there no problem with MOSFETs having a parasitic reverse diode which
would inhibit their use as a rectifier? Or do lower-power models not
exhibit this parasitic diode?


I had a quick look at this datasheet and must admit I don't understand
much of it or how it applies to my situation - can you maybe clarify a
bit?


Will it not be a problem that the source may not be able to provide
these high peak currents and go to its knees? If so, would a (bipolar)
capacitor on the input to handle the surge currents help?


Well, uphill the power available will just be too low, and the battery
will not charge (I may need another diode there if the switching
circuit's reverse leakage when off is too high and would discharge the
battery).

Downhill, I would clamp the input voltage to below the max. input of
the switcher chip with a big Zener diode or so.

To have a rough idea of the battery status I plan to add a simple
pushbutton-activated voltmeter that I can press when bicycle is
standing still, i.e. charger circuit is switched off (not very
accurate, I know, but should give enough information).

greetings,
Tom

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
 
J

John Smith

.... gesus, how did I overlook that lol!!!

But, no problem, just close your eyes so you don't get scared... <grin>

John
 
J

John Smith

Roger said:

"Personally, I would put rechargable in my bike light and charge up at
home."

.... or a solar panel on the bike and park it in the sun in the day...

John
 
A

Adrian Tuddenham

Tom (at tomsweb.net) said:
Everyone,

I want to construct a device that takes the AC power generated by my
bicycle dynamo and uses it to charge 4 AA NiMH cells in series (the
batteries may at the same time power a load (GPS) through a low-drop
regulator).

Here are the specs:

input power (dynamo measurements):
* biking speed 10 km/h: 5.6VRMS unloaded, drops to 2.5VRMS at 0.5 amp
load, available power 1.25W
* biking speed 20 km/h: 9.9VRMS unloaded, drops to ca. 4VRMS at 0.5 amp
load, available power 2W
* did not measure the AC frequency but I expect it to be in the range
10..200 Hz, and pretty much sinusoidal

How about:

1) Winding a 2W autotransformer (designed to work down to 10 c/s) which
steps the voltge up to allow efficient full-wave rectification, followed
by some sort of switched-mode stepdown system?

2) Resonating the inductance of the dynamo coil with a capacitor so as
to get higher voltage at lower frequencies?

3) Rewinding the dynamo with more turns?
 
K

Kryten

1) Winding a 2W autotransformer (designed to work down to 10 c/s) which
steps the voltge up to allow efficient full-wave rectification, followed
by some sort of switched-mode step-down system?

Could one use a circuit that used a comparator to switch alternate halves of
a MOSFET bridge (like a diode bridge, only with FETs instead)?

Just an idea to avoid the voltage drops.
 
M

mike

Tom said:
Everyone,

I want to construct a device that takes the AC power generated by my
bicycle dynamo and uses it to charge 4 AA NiMH cells in series (the
batteries may at the same time power a load (GPS) through a low-drop
regulator).

Here are the specs:

input power (dynamo measurements):
* biking speed 10 km/h: 5.6VRMS unloaded, drops to 2.5VRMS at 0.5 amp
load, available power 1.25W
* biking speed 20 km/h: 9.9VRMS unloaded, drops to ca. 4VRMS at 0.5 amp
load, available power 2W
* did not measure the AC frequency but I expect it to be in the range
10..200 Hz, and pretty much sinusoidal

output voltage (for charging batteries): 5.6 V (1.4V/cell)
output current: whatever the source can give and the batteries (+ load,
if any) will take (expected ca 0.3 amp)

Since I have limited power available and need nearly all of it, I am
looking for the most efficient solution (would like 85-90% efficiency).

I see basically two major options:
1) rectify + filter AC to DC and use a switching converter to the fixed
output voltage
2) use a circuit which switches the AC directly into a DC output
voltage

for 1)
- what rectifier circuit to use? a bridge rectifier with schottky
diodes seems the most straightforward, but still costs two schottky
drops of wasted power, not negligible for such low input voltages;
perhaps some sort of active rectifier circuit with MOSFETs or so is in
order?
- will the typical short conduction angle of such rectifiers, where a
peak current is drawn to recharge the filter capacitor, negatively
impact efficiency? if so, how to avoid this?
- since the desired output voltage of ca. 5.6V can be higher or lower
than the input voltage (input expected to vary between 2VRMS and going
upto 20VRMS at high speeds), some sort of buck-boost regulator seems
needed; I was hoping to be able to use a simple IC switching regulator
but most seem to be either buck or boost, rarely both
- perhaps a voltage doubling rectifier can get the voltage high enough
so a simple buck regulator can be used; will this cost me efficiency?

for 2)
- intuitively I would say this would allow higher efficiencies, but all
switching IC's I looked at take in DC, and I would expect a
configuration like in an off-line switching power supply (with
transformer) to be very inefficient at these low voltages, power levels
and frequencies; any ideas?

Thanks for any suggestions!

greetings,
Tom

PS I am aware I could use a solar panel but that is not convenient to
add, and the dynamo is there anyway

Draw yourself a schematic for a full-wave voltage doubler.
Now replace each of the caps with two cells.
Now replace each of the diodes with a fet or scr
and turn off the active deivice when the voltage or the current or the
battery temp is too high or any of the above.
And yes, if you use fets, you'll have to modulate them to do the
synchronous rectification.
Pic processors are cheap.
mike
mike

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C

colin

Tom (at tomsweb.net) said:
Everyone,

I want to construct a device that takes the AC power generated by my
bicycle dynamo and uses it to charge 4 AA NiMH cells in series (the
batteries may at the same time power a load (GPS) through a low-drop
regulator).

Here are the specs:

input power (dynamo measurements):
* biking speed 10 km/h: 5.6VRMS unloaded, drops to 2.5VRMS at 0.5 amp
load, available power 1.25W
* biking speed 20 km/h: 9.9VRMS unloaded, drops to ca. 4VRMS at 0.5 amp
load, available power 2W
* did not measure the AC frequency but I expect it to be in the range
10..200 Hz, and pretty much sinusoidal

output voltage (for charging batteries): 5.6 V (1.4V/cell)
output current: whatever the source can give and the batteries (+ load,
if any) will take (expected ca 0.3 amp)

Since I have limited power available and need nearly all of it, I am
looking for the most efficient solution (would like 85-90% efficiency).

I see basically two major options:
1) rectify + filter AC to DC and use a switching converter to the fixed
output voltage
2) use a circuit which switches the AC directly into a DC output
voltage

for 1)
- what rectifier circuit to use? a bridge rectifier with schottky
diodes seems the most straightforward, but still costs two schottky
drops of wasted power, not negligible for such low input voltages;
perhaps some sort of active rectifier circuit with MOSFETs or so is in
order?
- will the typical short conduction angle of such rectifiers, where a
peak current is drawn to recharge the filter capacitor, negatively
impact efficiency? if so, how to avoid this?
- since the desired output voltage of ca. 5.6V can be higher or lower
than the input voltage (input expected to vary between 2VRMS and going
upto 20VRMS at high speeds), some sort of buck-boost regulator seems
needed; I was hoping to be able to use a simple IC switching regulator
but most seem to be either buck or boost, rarely both
- perhaps a voltage doubling rectifier can get the voltage high enough
so a simple buck regulator can be used; will this cost me efficiency?

for 2)
- intuitively I would say this would allow higher efficiencies, but all
switching IC's I looked at take in DC, and I would expect a
configuration like in an off-line switching power supply (with
transformer) to be very inefficient at these low voltages, power levels
and frequencies; any ideas?

Thanks for any suggestions!

greetings,
Tom

PS I am aware I could use a solar panel but that is not convenient to
add, and the dynamo is there anyway

hmm, well avoiding mentioning ac/dc, there is a posibility to achieve more
efficiency that will deal with most of your points, and maybe quite simple,
that is to avoid the ineficiencies of rectification by not doing it until
the voltage is steped up a bit wich is probably what you mean by (2)

basicaly the idea would be to switch the step up transformer directly acros
the ac (oops i mentioned it), mosfets have inherent diodes so it will need
two in series (back to back or drain to drain) so when they are both off
they block the curent.

The output of the stepup device could be rectified with lower loss, (some
rectification here is inevitable anyway). it would need full wave
recification wich may be done with 2 windings to avoid more than one diode
drop.

if the midpoint of the two mosfets is considered ground they could be driven
directly by many of the available smps ics. these could provide simple
output regulation by PWM. a split primary would avoid high frequency being
fed to the generator wich would need decoupling to ground.


Colin =^.^=
 
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