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

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

Have a look at linear's LTC3780,

... 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...

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.

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.

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

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

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

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

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

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...

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

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...

snip

going up or going down?

martin

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.

"Tom (at tomsweb.net)" a écrit dans le message de news: snipped-for-privacy@g44g2000cwa.googlegroups.com...

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

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.

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

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 ....

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.

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

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

John