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

Discussion in 'Electronic Design' started by Tom (at tomsweb.net), Jun 9, 2005.

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  1. 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
     
  2. John Smith

    John Smith Guest

    Run the dynamo through a bridge rectifier, perhaps with a 100 uf or so
    cap across the + and - of the output of the bridge...

    You can construct the bridge out of 4 single diodes, 1 amp should be
    more than enough, for instance a 1N4001 or better...

    John
     
  3. Ian Stirling

    Ian Stirling Guest

    Random thoughts:

    The most efficient way would be a AC-DC converter that presented an
    impedance optimised for frequency, to extract maximum power.
    Constant impedance is probably the easiest to do though.
    It should convert the AC waveform into pulsating DC current which is
    fed to the batteries. (as you mention, resistive drops are bad)

    I'd start with something like a standard boost converter - with the
    wrinkle that the power switching device is two complementary FETs back
    to back, so it has no problem switching AC.

    This switches the voltage up to the 6V range, where it's rectified by
    a shocketty or active bridge into the batteries.
    To get the input impedance right, you'd want the control circuitry to
    mimic a resistor, so the feedback loop wouldn't be to keep it at a certain
    output voltage, but a comparison of the current with the absolute value
    of the voltage...

    Anyway, this is probably a bit hard.

    I suspect you'd get a fair portion of the way there with a correctly picked
    maxim/... switcher designed to run from low startup voltages.
    4 biggish shockety diodes (sized for minimum Vf) feeding the switcher chip
    directly, with a small decoupling capacitor.

    The switcher is set to output 6V (or whatever), with a small resistor to the
    batteries.
    Ideally, you'd probably want to feed some fraction of the input supply voltage
    in to the regulator, so it tries to raise the output voltage a little when
    the input rises.

    That's my first thoughts, though I am really a bit tired.
    Anyone else contribute?
    voltage rises.
     
  4. GPG

    GPG Guest

    Have a look at linear's LTC3780,
     
  5. John Smith

    John Smith Guest

    .... once you exceed the forward voltage of the diodes (silicon--about
    ..62 per volt or 1.4 in the bridge circuit) the voltage out of the
    diode/cap circuit with be about 1.4 times the ac in p-p voltage. At 5V
    ac in, this will be ~7V dc out (unloaded.)
    You can spend a lot of time trying for more "efficiency"--I think if you
    try the "occams razor" approach you will be just as well off...

    Warmest regards,
    John
     
  6. 1. I suggest MOSFET for the rectification part, not diodes. You will have
    to check diode specs, but I suspect this alone will give the biggest payoff.
    There are MOSFETs which are well on at 3V gate drive. Perhaps schotkeys in
    parallel will get it started, then the MOSFETs can take over when there is
    DC available.

    2. You could use the configuration used in PFC (Power Factor Correction)
    circuits. This IC probably won't be suitable, but if you grab the LT1249
    datasheet from lt.com (Linear) you will see the idea. The circuit uses a
    single inductor and a mosfet switch. A small cap across the output of the
    rectifier makes the raw DC look like a low impedance source at the 100KHz
    switching frequency. You don't have to do the whole PFC thing, in accurate
    detail.

    3. Personally, I would just rectify into a capacitor, to make it simple.
    The peaky current does reduce efficiency, but numbers I have calculated for
    mains rectifiers tells me that it is not too bad - certainly not the
    disaster you might think intuitively. Using a smaller capacitor to extend
    the conduction angle has so little effect on efficiency, that it is better
    to use a big enough cap to get the ripple down.

    4. You might want a "pause" button which stops charging on hills !

    Roger Lascelles
     
  7. On Thu, 9 Jun 2005 13:05:18 +1000, in sci.electronics.design "Roger

    snip
    going up or going down?




    martin
     
  8. Have a look at linear's LTC3780,

    That looks like a useful chip for the switching part, even if normally
    intended for higher powers (although its minimum Vin of 4V may be a bit
    high for my application). Unfortunately Farnell etc. don't seem to
    carry it. I tried ordering samples from the Linear website... too bad
    LTSpice does not seem to have a model for the LTC3780.

    Thanks for the suggestion.

    greetings,
    Tom
     
  9. John,
    Unfortunately I have no room to "keep it simple" else I would. My
    available input power is 1.5W and I need at least 0.2A x 5.5V = 1.1W
    into my load, so there is not much left for inefficiencies.

    The input voltage under load (which is what matters) drops to 2.5VRMS,
    or ca. 3.5Vpp. If I would use ordinary silicon diodes I would lose
    twice 0.65V leaving me with 1.6Vpp, or a measly 45% efficiency just in
    the rectifier.
    Schottkys are better, but the MOSFET circuit also sounds interesting.

    greetings,
    Tom
     
  10. 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
     
  11. Ian,
    That's an idea, the output does not really need to be "proper" DC. Will
    NiMH batteries charge happily on some sort of pulsating voltage?
    Yes, and a simple boost converter is probably not sufficient, because
    at higher cycling speeds buck operation is needed.
    Looks the most feasible way. So you suggest a SMALL capacitor, to
    maintain a large conduction angle? Someone else suggested this hardly
    improves efficiency vs. a big cap.

    Perhaps I should consider the possibility of having TWO switchers:
    - a low-voltage 1..5.5V or so Maxim/... step-up switcher for the low
    voltage range
    - a 5.5-36V or so step-down switcher for the higher voltage range

    if both have a shutdown, I could use a comparator (or 2) to select one
    of them depending on the current input voltage.

    That approach would probably not be able to switch fast enough to work
    good with pulsating AC, and a big filter cap might be in order (and
    hysteresis on the comparator).

    Does that sound at all feasible?

    Also an interesting idea, so the demanded battery current depends on
    available voltage. But then the battery state, current demanded by the
    load etc. also becomes important.

    The frequency of the AC also has information about the available energy
    of course (how fast I cycle), but that's getting a bit complicated
    then...

    greetings,
    Tom
     
  12. Fred Bartoli

    Fred Bartoli Guest


    Your pb can be view as a PM motor that you want to brake.

    A full MOSFETs bridge will do:



    .-----------------+--------------.
    | | |
    ||-+ +-|| ---
    Q2 ||<- ->|| Q3 -
    -||-+ +-||- |
    | _ | ---
    | ___ / \ | -
    +----UUU--( ~ )---+ |
    | \_/ | ---
    | | -
    ||-+ +-|| |
    Q1 ||<- ->|| Q4 ---
    -||-+ +-||- -
    | | |
    '-----------------+--------------'

    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

    By having on one half cycle the lower mosfet of one branch permanently on
    (say Q4) and the other branch switching, you'll boost the dynamo voltage to
    the NIMH voltage (Q1 will play the role of the boost the switch, Q2 will be
    the synchronous rectifier).

    This will grant you for the highest efficiency, but driving the upper
    mosfets may not be very practical.

    A slightly lower efficiency, but much easier, solution would be to change
    the upper mosfets for schottkies :


    .-----------------+--------------.
    | | |
    | | ---
    D1 - - D2 -
    ^ ^ |
    | _ | ---
    | ___ / \ | -
    +----UUU--( ~ )---+ |
    | \_/ | ---
    | | -
    ||-+ +-|| |
    Q1 ||<- ->|| Q2 ---
    -||-+ +-||- -
    | | |
    '-----------------+--------------'


    The 0.5V schottky drop, weighted against the 4.8V NIMH voltage would only
    cost you a 10% efficiency penalty.
    The driving circuit will be easy to power from the NIMH.
     
  13. Fred,
    Looks like a very elegant solution!

    Could you maybe show me the control waveforms applied to the gates of
    the transistors in both cases, w.r.t. the incoming sine waves?

    I guess in such a setup I would anyway need a small micro to send out
    the correct pulses, in phase with the incoming AC. So perhaps doing the
    full bridge configuration will not need much extra hardware vs. the
    schottky (apart from maybe a gate driver if the micro's outputs are to
    feeble to properly switch the MOSFETs with a decent risetime).

    greetings,
    Tom
     
  14. Spajky

    Spajky Guest

    rectify the current with (graetz) bridge constructed with 4 schottky
    diodes (+followed by maybe a el.capacitor of few hundreds uF in
    paralell) & than thru a resistor of min. 1W/100ohm or more in series
    to batteries ... IMHO cheap & effective solution ....
     
  15. John Smith

    John Smith Guest

    Absolutely true, Schottkys ARE better.... one real improvement would be
    if a method could be introduced to increase generator (dynamo) output at
    low bike speed...
    I have often wondered about replacing the cheap dynamo of higher output
    with a voltage reg circuit... but seldom ride at night these days and
    haven't been able to find the motivation--but certainly, surplus, a guy
    could get a hold of a better generator--somewhere...

    Warmest regards,
    John
     
  16. Jim Thompson

    Jim Thompson Guest

    [snip]

    I was just struck with a thought... isn't the "pedal rate" relatively
    constant, as opposed to the wheel rate which depends on gear?

    Maybe some gearing directly from the chain to run a dynamo?

    Or magnetize the links alternating and pass through a coil ;-)

    ...Jim Thompson
     
  17. John Smith

    John Smith Guest

    That is an excellent take on it too, directly off the chain, and speed
    regulation would get magnitudes better...
    Magnetized links would increase driver load/fatigue, unless ran on
    non-magnetic sprockets and sufficient distance from magnetic
    metals--there would also be the great danger of the chain attracting
    bits and pieces of magnetic metals and causing increased wear and rider
    injury...

    Warmest regards,
    John
     
  18. Ian Stirling

    Ian Stirling Guest

    They are perfectly happy with pulsating current to charge them.
    Missed that bit, I'll respond again when I look up the high range voltage.
    I dunno, it was a thought.
    The idea was to basically just use the capacitor to provide negligable
    impedance at the switching frequency, not actually smooth the voltage.
    The converter starts up each time the voltage hits 1.2V or so.
    I'll respond more fully a bit later, after I've had time to read the
    whole thread.
     
  19. BobG

    BobG Guest

    Make the generator pully larger diam to slow it down and REDUCE the
    output voltage. Use the mosfet sybchronous rectifier discussed
    previously to charge a 5V supercap.. essentially a D cell. Now get a
    COTS DC to DC converter to go from supercap to battery charger or lamps
    or whatever.
     
  20. Jim Yanik

    Jim Yanik Guest

    Then you don't get any power generation when coasting.
     
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