3V->15V 15W switcher topology?

[Joel Kolstad]

I'm contemplating building a step-up converter from about 3V (it's a lithium
cell and realistically it's almost completely discharged at 3V) to something a
little shy of 15V, where I need 15W plus a few more to cover losses. Starting
with such a low voltage obviously requires some pretty hefty currents, so I'm
thinking that a switcher topology of either a half-forward converter (nice,
simple transformer and low parts count -- peak switch current around 12A) or a
push-pull topology (peak switch currents around 6A, but need a center-tapped
transformer, and controller ICs become quite uncommon given the very low input
voltage since most push-pull applications seem to be in the area of much
higher power). Flybacks or a straight boost converter seem highly undesirable
due to peak currents in the ballpark of 27.5A -- ouch! Since this is battery
powered, I care about efficiency, but I do have plenty of room, so I figured
I'd just use a low (couple hundred kHz) switching frequency and big magnetics
to achieve that end.

Does this sound like a decent strategy?

Also... anyone know of off-the-shelf modules that can perform this job? TI
has one that's close --
http://focus.ti.com/docs/prod/folders/print/ptn04050c.html -- but it's only
12W output power. (It is amazingly small!) This is a low-volume application,
so price isn't a huge concern.

Thanks,
---Joel Kolstad

 

[Tim Wescott]

I'm contemplating building a step-up converter from about 3V (it's a lithium
cell and realistically it's almost completely discharged at 3V) to something a
little shy of 15V, where I need 15W plus a few more to cover losses. Starting
with such a low voltage obviously requires some pretty hefty currents, so I'm
thinking that a switcher topology of either a half-forward converter (nice,
simple transformer and low parts count -- peak switch current around 12A) or a
push-pull topology (peak switch currents around 6A, but need a center-tapped
transformer, and controller ICs become quite uncommon given the very low input
voltage since most push-pull applications seem to be in the area of much
higher power). Flybacks or a straight boost converter seem highly undesirable
due to peak currents in the ballpark of 27.5A -- ouch! Since this is battery
powered, I care about efficiency, but I do have plenty of room, so I figured
I'd just use a low (couple hundred kHz) switching frequency and big magnetics
to achieve that end.

Does this sound like a decent strategy?

Also... anyone know of off-the-shelf modules that can perform this job? TI
has one that's close --
http://focus.ti.com/docs/prod/folders/print/ptn04050c.html -- but it's only
12W output power. (It is amazingly small!) This is a low-volume application,
so price isn't a huge concern.

Thanks,
---Joel Kolstad

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

 

[colin]

I'm contemplating building a step-up converter from about 3V (it's a
lithium cell and realistically it's almost completely discharged at 3V) to
something a little shy of 15V, where I need 15W plus a few more to cover
losses. Starting with such a low voltage obviously requires some pretty
hefty currents, so I'm thinking that a switcher topology of either a
half-forward converter (nice, simple transformer and low parts count --
peak switch current around 12A) or a push-pull topology (peak switch
currents around 6A, but need a center-tapped transformer, and controller
ICs become quite uncommon given the very low input voltage since most
push-pull applications seem to be in the area of much higher power).
Flybacks or a straight boost converter seem highly undesirable due to peak
currents in the ballpark of 27.5A -- ouch! Since this is battery powered,
I care about efficiency, but I do have plenty of room, so I figured I'd
just use a low (couple hundred kHz) switching frequency and big magnetics
to achieve that end.

Does this sound like a decent strategy?

Also... anyone know of off-the-shelf modules that can perform this job?
TI has one that's close --
http://focus.ti.com/docs/prod/folders/print/ptn04050c.html -- but it's
only 12W output power. (It is amazingly small!) This is a low-volume
application, so price isn't a huge concern.

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 =^.^=

 

[Nico Coesel]

I'm contemplating building a step-up converter from about 3V (it's a lithium
cell and realistically it's almost completely discharged at 3V) to something a
little shy of 15V, where I need 15W plus a few more to cover losses. Starting
with such a low voltage obviously requires some pretty hefty currents, so I'm
thinking that a switcher topology of either a half-forward converter (nice,
simple transformer and low parts count -- peak switch current around 12A) or a
push-pull topology (peak switch currents around 6A, but need a center-tapped
transformer, and controller ICs become quite uncommon given the very low input
voltage since most push-pull applications seem to be in the area of much
higher power). Flybacks or a straight boost converter seem highly undesirable
due to peak currents in the ballpark of 27.5A -- ouch! Since this is battery
powered, I care about efficiency, but I do have plenty of room, so I figured
I'd just use a low (couple hundred kHz) switching frequency and big magnetics
to achieve that end.

Does this sound like a decent strategy?

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.

 

[Joel Kolstad]

Hi Colin,

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.

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

im not sure what controllers go down to 3v though,

There are a surprising number these days, actually!

---Joel

 

[Joel Kolstad]

Hi Nico,

I'd use a microcontroller to implement the controller in software. In
this way you have a charge and discharge controller almost for free.

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

---Joel

 

[colin]

Hi Colin,



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

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

There are a surprising number these days, actually!

---Joel

yes but Im not so familiar with them.

Colin =^.^=

 

[kell]

I'm contemplating building a step-up converter from about 3V (it's a lithium
cell and realistically it's almost completely discharged at 3V) to something a
little shy of 15V, where I need 15W plus a few more to cover losses. Starting
with such a low voltage obviously requires some pretty hefty currents, so I'm
thinking that a switcher topology of either a half-forward converter (nice,
simple transformer and low parts count -- peak switch current around 12A) or a
push-pull topology (peak switch currents around 6A, but need a center-tapped
transformer, and controller ICs become quite uncommon given the very low input
voltage since most push-pull applications seem to be in the area of much
higher power). Flybacks or a straight boost converter seem highly undesirable
due to peak currents in the ballpark of 27.5A -- ouch! Since this is battery
powered, I care about efficiency, but I do have plenty of room, so I figured
I'd just use a low (couple hundred kHz) switching frequency and big magnetics
to achieve that end.

Does this sound like a decent strategy?

Also... anyone know of off-the-shelf modules that can perform this job? TI
has one that's close -- http://focus.ti.com/docs/prod/folders/print/ptn04050c.html-- but it's only
12W output power. (It is amazingly small!) This is a low-volume application,
so price isn't a huge concern.

Thanks,
---Joel Kolstad

Look at the very bottom of this circuit by Tim Williams, in the area
where you see Q14:
http://webpages.charter.net/dawill/Images/Induction_Heater_Draft1.gif
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.

 

[kell]

Look at the very bottom of this circuit by Tim Williams, in the area
where you see Q14:http://webpages.charter.net/dawill/Images/Induction_Heater_Draft1.gif
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.- Hide quoted text -

- Show quoted text -

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

 

[colin]

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.

this would easily do what you want....

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,P22513,D18284

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 =^.^=

 

[[email protected]]

... I'd just use a low (couple hundred kHz) switching frequency and big magnetics
to achieve that end.

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

Does this sound like a decent strategy?

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 ;-).

 

[Nico Coesel]

Hi Nico,



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

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

 

[colin]

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

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 =^.^=

 

[Joel Kolstad]

Hi Frithof,

"[email protected]"

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 ;-).

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

 

[Joel Kolstad]

Hi Nico,

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.

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).

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.

Yep, agreed.

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

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

---Joel

 

[colin]

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.

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

Colin =^.^=

 

[Nico Coesel]

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.

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.

 

[Nico Coesel]

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).

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.

Yep, agreed.

See my reply to Colin.