3V->15V 15W switcher topology?

I'm not entirely clear on what you mean by "half forward converter", but you should avoid conduction diodes on the 3V side, they'll eat your efficiency for lunch and dinner.

Given that, a controller that will implement any two-transistor solution will probably be suitable for a push-pull with two N-channel FETs, so why not go that way from the start? If you can't find a controller that will work adequately from 3V, think about using something light-duty and cheesy to make the rail for the controller (Jorge should chime in here with how to do it with a '2222 and less than four passives), or ask yourself if you can get by with serious inefficiencies and misbehavior at startup as long as the controller works well enough to get the rail up so that it can do _something_ right.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

I dont know how you arrive at >27amps for a boost, my quick calcs show a lot less than this, more like just over 10 amps, and Low voltage mosfets have amazingly low rdson in small devices.

or consider a 2 phase boost. im not sure what controllers go down to 3v though, but maybe tghe battery has a little more volts at switch on enough to operate a more avialable controller long enough to boot strap itself before the current draw makes it drop ?

Colin =^.^=

I'd use a microcontroller to implement the controller in software. In this way you have a charge and discharge controller almost for free. Lithium is very sensitive to overcharge and overdischarge. I recently saw what happened with a big lithium battery being overcharged: it simply becomes a blow torche.

--
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl

Hi Colin,

I'm using peak current ~= 5.5*Pout/Vin from Marty Brown's book.

There are a surprising number these days, actually!

---Joel

Hi Nico,

Yeah, but my time isn't free. :-) I like the idea though; thanks.

---Joel

I dont know that book myself, that sounds far too generalized, but :- if Power transfered via inductor =(vout-vin)*Iout = 12W voltage accros transformer = 12v, 3v power during on = Power * (12+3)/12 = 15, for max on time but still discontinuos Ipk = 2 * Pon/vin = 10amps

yes but Im not so familiar with them.

Colin =^.^=

but it's only

Look at the very bottom of this circuit by Tim Williams, in the area where you see Q14:

The cap he has across the transformer primary may be a litte fat, you could try something smaller. For low power you might be able to dispense with the secondary of the transformer and just pick pulses off the 20 uH inductor with a diode. To regulate it you can use a TL431 to shunt the transistor base to ground, with the tl431 feedback pin attached to a voltage divider on the output.

a

Starting

a

input

undesirable

it's only

application,

Q14:

P.S. I was suggesting this as a low-power supply to drive a chip.

this would easily do what you want....

a quick sim in ltspice sees peak current of 10-11 amps at 3.3v they have an example sim circuit just need to change the feedback resistor, and the laod resistor.

Colin =^.^=

magnetics

Maybe lower - I find that around 30-50 kHz, with modern ferrites, losses are "invisible". If you got room.

I think you want a push-pull inverter at this low voltage to cut down the peak currents. How about a current fed Royer oscillator? If you really want, you can regulate the secondary with a mag-amp to keep it all discrete ;-).

If you are going to use lithium cells and care about there life expectancy, then there are very little other options than checking the battery voltage droop versus time (dV/dt) to see if the battery is empty. The voltage across the cell may vary a lot with temperature.

If you are going to use a boost converter you'll need a chip with a huge switch capable of handling 30A (16.6% duty cycle, 5A average input current). I think a converter with a transformer is a good option here to keep the primary current down. Wurth has flex transformers with which you can choose the turns ratio by combining 6 individual windings. Never used them in a circuit, but I will if get the chance :-)

--
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl

with 3v input and 15v output that coresponds to an on time of 80% for edge of discontinuos mode. I ran a sim and found 10.5amps peak for 3.3v with continuos mode this is even lower.

Colin =^.^=

Hi Frithof,

That's a little too fancy for me, although I was fascinated some years ago reading Pressman's book and his all-discrete design using mag amps for the controller. Quite impressive, and while it's not super-high on the list, it'd be cool to duplicate someday. (Speaking of which... does anyone have a link to Pressman's obituary? Or even just when he died?)

I've been thinking of using the LT1950 controller IC for this; it's very much designed to go down to 3V using a little built-in boost converter for bootstrapping the gate drive up to 8V. Alas, I probably won't get back to serious design for another week here.

Thanks for your input,

---Joel

Hi Nico,

Well... I'm doing charging with an off-the-shelf TI (formerly Benchmarq) IC. In this application the device is supposed to spend the vast bulk of its time plugged-in, so other than various watchdog ICs that effectively remove the load below about 2.7V, there isn't any discharge monitoring circuitry that I've added. The battery itself comes with the usual built-in protection, though -- it disconnects itself at too high of a charge rate or too low of a voltage (2.5V, it says).

Yep, agreed.

Cool, thanks for the name; I'll definitely check them out.

---Joel

has everyone gone crazy, why only work at 16.6% duty cycle?

Colin =^.^=

You are right. I got the duty cycle the wrong way around, but the currents are still huge. During Toff the current through the rectifier diode will be 5A which leads to considerable losses even with schottky diodes. Synchronous rectification is something to consider to save power.

--
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl

Panasonic has some excellent documents on-line on charging and discharging Lithium cells. But it seems your battery pack has this intelligence build into it.

See my reply to Colin.

--
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl