Fighting with efficiency, Boost Regulator, Eureka!

[Jarhead]

Well, I been fighting with efficiency on various different boost regulators over the past six months.

I tried lots of circuits, and got between 70 and 87%
efficiency, which left me with the biggest loss factor,
the shottky diode.

Finally ended up on a dual MOSFET setup, one MOSFET replaces the schottky diode, to drop the losses.

Got the efficiency of it to day up to 90.4%. It should
have been alot better. Tried all sorts of things to get it up, changing inductor using values and more. I tried higher
current inductors (lower copper losses from DC resistance), to no avail.

Then it occured to me that unlike the previous circuits, this one ran at a rather high frequency (yes, high frequency = higher losses).

Looked up the Sumida CDRH127, couldn't find it's frequency
rating, but they test it at 100kHz....humm

Dug up another inductor I had that was rated for 1MHz.
Bingo! Gained 6.79 % efficiency right off the bat.
Woohoo!

97.1946% efficiency boosting 3V to 3.6V at 350mA. Runs
down to 1.7V, which with two Alkaline batteries, runs them
down to 0.85V each, which at that point, ain't much juice
left in them anyhow. BTW, it will supply 1A with no problem...


Happy Happy, Joy Joy!

I think I've found my circuit. Now I gotta pick a flashlight to put it in.

 

[Jarhead]

BTW, in case some of you tech geeks out there bring it up, I have less than 20mV ripple on the input and less than 30mV ripple on the output.

 

[xbrite]

Dear Jarhead,

May I have the schematic diagram??

Thanks, YS Lee

 

[CM]

Sounds like a synchronous boost regulator which uses a FET instead of a Schottky which is one of the biggest loss contributors. Several manufacturers make controllers for synchronous boost circuits. Are you using one of these or is this a home brewed controller?

CM

 

[Steelwolf]

More info! Schematics! 97% efficiency! Holy Cow! I may never direct drive again!

Seriously, any chance of a look at the schematics?

 

[Jarhead]

Yes it is a synchronous regulator, rather common these days.
At these efficiency levels, even small wires/traces can start to show up. Even the ESR of capacitors can start causing losses. Again, the choice of inductor is critical to getting the high efficiency.

The chip part number is TPS61030PWPR. My home brew hasn't reached this level, but I'll give it more effort as I have time. It is made by TI. It is a bit larger than some chips. There is a choice of TSSOP-16 or the RSA package

http://focus.ti.com/docs/prod/folders/print/tps61030.html

For lower current levels, such as you'd use with 10 Nichia LEDs, also check out the TPS61025DRCR.

 

[CM]

Jarhead,

How did you feedback current? Did you use an op amp to multiply voltage across a small sense resistor? What size sense resistor did you use? Efficiency is somewhat affected by the size of the sense resistor.

CM

 

[CM]

For those wanting to use this with 2x123's to power a 5W Luxeon, the chip has a maximum input voltage of 5.5V /ubbthreads/images/graemlins/frown.gif This severely handicaps it's utility. Many chips I've looked at seemed great until you read all the specs and there always seems to be one major downfall. Now I remember looking at this a while back when TI introduced it and the 5.5V limit was what turned me off of it. However, for Li-Ion's (which the chip was originally targeted at) it is a nice solution.

 

[AilSnail]

how does it do when the voltage sag is included, say the input is about 2.4v for instance?

can it do 5watters as well?
If it can use a li-ion to power a 5w (at 1A?!!), then the max input of 5.5v may not be a problem since two cr123's rapidly decreases their voltage below this when such a load is applied. please, more test data!!

finally, how small could you make the circuit without compromising its efficiency?

 

[georges80]

[ QUOTE ]
AilSnail said:
how does it do when the voltage sag is included, say the input is about 2.4v for instance?

can it do 5watters as well?
If it can use a li-ion to power a 5w (at 1A?!!), then the max input of 5.5v may not be a problem since two cr123's rapidly decreases their voltage below this when such a load is applied. please, more test data!!

finally, how small could you make the circuit without compromising its efficiency?

[/ QUOTE ]

Read the data sheet on the link. Max out 5.5V so too low for 5W. Also, it only switches at 700khz - so rather large inductance required - which means physically bigger inductors with thicker wires to handle the current requirements. Also 96% is only at medium current - at a full 1A the efficiency drops - the efficiency will get worse as the delta between input & output voltage increases. All typical of boost switchers in general.

It's certainly not the 'magic bullet' switcher - but then none of them are...

george.

 

[Burnt_Retinas]

Jarhead,

What was the inductor you finally used?

Even if not going synchronous for PWB size reasons then at least a more efficient/better inductor will help.

Chris

 

[star882]

"Finally ended up on a dual MOSFET setup, one MOSFET replaces the schottky diode, to drop the losses."
Sounds a lot like the CPU voltage regulators.
However, they still have a schottky diode in parallel with the low side MOSFET (probably to keep switching spikes from damaging the MOSFETs).

 

[Jarhead]

700KHz is alot faster than alot of switchers, 6.8uH seems to work well, but in my application, I use a slightly lower value, which has a lower DC resistance of about 0.0023 ohms.

Since I use voltage feedback, I just tweak it for the specific luxeon. Yes, the luxeon's forward voltage will change, and thus the current, as it's temperature changes, but the better you heatsink it, the less of this effect you will see.

With voltage feedback, I've ran it down to 2.0V on batteries, and haven't seen any change in the power delivered to the LED, I'll work on it more, with more details, once I fix another bench supply that gave up the ghost.

I'll probably rig up a current feedback setup when I get a chance, and see how it does.

 

[Jarhead]

No Star882.

A cpu switcher is a buck, not a boost regulator, and the circuit works a bit differently, and the inductor is moved from the power input to the power output.

During the first cycle the power flows through the top fet, through the inductor, to the load on the first cycle. Once their is enough energy stored in the inductor, the top fet is turned off, then the bottom fet shorts the input side of the inductor to ground, so the current flows out the inductor, through the load, though the fet, and back into the inductor. Once the energy drops, the bottom fet is turned off, and the power is again applied to the inductor, "filling it up". And repeat.

Also, the shottky performs a little different function. Losses incurred during a brief period just before or just after the switching transition, during which the freewheeling MOSFET conducts the current with zero gate voltage. This forces the current to flow in the internal body diode and has a significant impact on the efficiency because of a much higher voltage drop across the device during this period. Putting a shottky diode across the lower FET lowers these losses during this period.

 

[Jarhead]

I am using some HC7-1R5 made by cooper electronics, in a series/parallel combination.

I was using a 10uH inductor, think it was a CDRH127 style.

Also tried some UP4B cooper inductors.

I'm waiting for a HC2-2R2 and a HC2-6R0 from them.

 

[Jarhead]

Also, Panasonic makes a decent part also, a ETQPAF4R8HF, which starts out at 6.8uH and drops to about 4.8 at like 10A or so, and it has a DC resistance of 0.0029.

The actual numbers might be slightly different, doing all this from memory.

There is a CEP125-6R0 that doesn't seem too bad either, from Sumida, with a DC resistance of 0.008(max) to 0.0066(typ.).

 

[Jarhead]

The Zetex ZXSC300, ZXSC310, ZXSC400 and such "regulators" used in many of the boost circuits talked about around here only run at 200KHz, which requires an even bigger inductor than the TI chip does, and for the same ripple as the Zetex part, it requires much lower capacitor values. So, both the inductors and capacitor physical sizes can be lowered.

Plus, both the fets are internal to the TI part...

 

[georges80]

[ QUOTE ]
Jarhead said:
700KHz is alot faster than alot of switchers, 6.8uH seems to work well, but in my application, I use a slightly lower value, which has a lower DC resistance of about 0.0023 ohms.

Since I use voltage feedback, I just tweak it for the specific luxeon. Yes, the luxeon's forward voltage will change, and thus the current, as it's temperature changes, but the better you heatsink it, the less of this effect you will see.



[/ QUOTE ]

6.8uH (or a bit lower as you say) and 0.0023ohms - is that a misprint or a large inductor with THICK wires?

700KHz is still low if you want tiny inductors that can handle decent currents. A lot of the modern switchers are up in the 1.5 - 3MHz range. The inductance you need is basically inversely proportional to frequency and for the tiny switchers that CPF folk love to use - you will be forced into the 2-3uH range to carry 1A to the load with a small footprint inductor like the Coilcraft DO1608 family.

The panasonic part you quoted is larger than a sandwich ;-)

The TI is probably nice for the 350mA range - you'll need to verify that an opamp current sense feedback loop can is stable, i.e. not upset the feedback control loop.

Keep us updated on your progress and I'd LOVE to see your efficiency numbers and runtimes with a Luxeon III running at 750mA or greater /ubbthreads/images/graemlins/grin.gif

Yeah, it's xmas day, but the kids are napping after a morning of present induced hyper activity - so now I can relax catching up on stuff /ubbthreads/images/graemlins/smile.gif

george.

 

[LightScene]

It sounds like your boost regulator would be ideal to drive a 3 watt Luxeon in a 2C cell M@g with either alkalines or rechargeables.

 

[Jarhead]

..."A lot of the modern switchers are up in the 1.5 - 3MHz range. The inductance you need is basically inversely proportional to frequency and for the tiny switchers that CPF folk love to use"....

Okay, besides the Zetex ZXSC300, which alot of folks seem to use here, which ones are they using that run in the "1.5-3MHz range"?

Higher switching frequencies also equates to higher loses on the MOSFET gates, because you need to drive that gate capacitance (and other associated capacitances) around. The faster you drive it, the more energy you waste there.

Power loss due to gate charge = Switching frequency * Total Gate Charge (Qgd_total) * Maximum Gate Voltage

..."6.8uH (or a bit lower as you say) and 0.0023ohms - is that a misprint or a large inductor with THICK wires?"...

No misprint, wound with litz wire, many strands of fine magnet wire (the strands are probably about 42 guage, looking at it), designed to reduce eddy current losses.

Used a 4 wire kelvin bridge (basically have to at those low values).

Yeah, that Panasonic part is as big as a sandwich, but it is no problem for D cell mag-lite mod.

A designing a switching power supply is quite a dance to get the optimium efficiency in the desired form factor at the power levels needed, then add in the input voltage range, etc. and it can become quite an endeavor...