Fighting with efficiency, Boost Regulator, Eureka!

[georges80]

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"?

-> Linear Tech. Even the venerable LT1618 runs at 1.4MHz. Linear Tech has faster parts as well, with synchronous rectification as well.

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.

-> Yep - the tradeoff of the high freq benefit of being able to use tiny inductors & caps is the time the FETs spend in their linear state.

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.

-> I looked at your inductor - it is large - hence has room for all that wire.

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.

-> True - it all depends on the host torch. But then with D cells the ultimate in efficiency (other than feeling good /ubbthreads/images/graemlins/wink.gif) isn't that critical. Also in that form factor running multiple cells and down converting becomes feasible.

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...

-> I totally agree, lots of tradeoffs, not least of which is cost/size/performance. Like I wrote before, the TI part looks interesting, if you try it at higher drive levels in the 750mA-1A range it would be interesting to see the efficiency numbers (just to verify TI's claim of 90+%). Also, getting an opamp feedback path from a sense resistor will be interesting - mainly to see if the TI switcher's FB control loop can run stable with an active component in that path.

Please keep us informed with your ongoing evaluation of the TI part /ubbthreads/images/graemlins/thumbsup.gif

george.

 

[Jarhead]

Ah, but the LT1618
-doesn't have a 4.5A switch inside it
-plus it uses the shottky diode (eats up your efficiency)
-and with the high switching frequency (eats up efficiency)

The LT1618 datasheet shows around 76% efficiency at load levels of 300mA....would imagine it gets alot worse fast, with heavier loads...

The internal "switch" Vce(sat) of 400mV * 2.0A = .8 W * DF = X amount up in heat alone at full load, within the chip itself, just from it's switching element.

Toss in a schottky diode (lets use MBRM120), lets say an average of 350mA flows through it to the load. 0.35 A * 0.34Vf = 0.119 W up in heat in the schottky diode. Or, if you had a 1 amp load, (0.38Vf * 1A) you'd loose 0.38 W in just the schottky diode.

True, it does have the high frequency advantage so you can use smaller inductors/capacitors. Although, I have gotten
the TI part to work with sub 1 uH parts without issues, and since output ripple isn't a big issue...maybe I will fiddle with making a mini version.

BTW, I do have some self induced losses in my setup with the parts I have sitting around the house, but at 0.8A output, I'm seeing 94.9% efficiency.

 

[dat2zip]

Wow,

I like what I see. I agree with the issues with the LT part and for that matter almost any regulator has issues of one form or another. There is no magic 100% efficient converter and there is always going to be losses.

I'd be interested in working on this design to make a miniature version. I think the only stickler would be getting a small inductor for the 4.5A requirements. Other than that I think this part has a lot of potential.

Tame this into a CC design and you would have a nice converter.

Jarhead,

Can you do a couple of data points for us. Maybe, from 2V to 3V every 1/10V or so. I think the efficiency will stay high if the datasheets are correct.

Wayne

 

[Jarhead]

LTC3402 looks interesting, 3MHz, 2A switch 0.18 ohms, efficiency dies quicky at greater than 0.4 A loads.

 

[Burnt_Retinas]

Jarhead,

Good info. you have provided. The Texas looks good for just what I'd like - driving a 5W LED from a single Li-Ion cell. There are many other chips out there, but most don't have this or don't do that. The biggest pain is there are no decent current constant current solutions. I've just gone through the pain of a 2A+ buck using an opamp for current sense, it works, but it wastes board space. FYI the project will be published when I have the chance. There doesn't appear to be a perfect solution and all require some effort.

My next was to be a 1A+ boost Li-Ion to 6 odd volts at 1A, looks like you're there already!

BTW, yes the LTC3402 looks good, but due to the limited switch current and input and output voltage limitation of 6V then it'd only be good for driving a 1W/3W LED @ 1A from one or two 1.5V cells, or a single 123. It has it's place I guess, and maybe this is your need, but the Texas chip will do a 5W LED at 1A from a single Li-ion. Now that would nice.

Chris

 

[georges80]

[ QUOTE ]
Burnt_Retinas said:
Jarhead,

Good info. you have provided. The Texas looks good for just what I'd like - driving a 5W LED from a single Li-Ion cell.
...
, but the Texas chip will do a 5W LED at 1A from a single Li-ion. Now that would nice.

Chris

[/ QUOTE ]

The TI datasheet specifically says max output is 5.5V... i.e. NO 5W drive - unless there's a trick you can do...

george.

 

[Jarhead]

I can think of a couple of tricks offhand, but then the efficiency goes to hay.

 

[INRETECH]

Look at Cuk'

 

[georges80]

[ QUOTE ]
INRETECH said:
Look at Cuk'


[/ QUOTE ]
Sure, and Sepic style etc - but as Jarhead says - the efficiency gots to pot...

Jarhead - remember to tell us if you have success with an opamp feedback scheme for current regulation...

george.

 

[Burnt_Retinas]

DOH

Crap, neither will do!

 

[Jarhead]

Okay, I hooked up a standard off the shelf Cooper Electronics UP4B-4R7 inductor with a DC resistance
of 0.0093 ohms to the TPS61030PWPR boost chip.

Input...Input..Watts.......Output..Output..Watts.....Efficiency
Voltage.Current............Voltage.Current
2.955...0.92...2.7186......3.466...0.768...2.661888..97.91%
2.541...1.16...2.94756.....3.466...0.808...2.800528..95.01%
2.262...1.34...3.03108.....3.466...0.808...2.800528..92.39%
2.031...1.544..3.135864....3.467...0.812...2.815204..89.77%
1.861...1.76...3.27536.....3.471...0.832...2.887872..88.17%
1.728...2.004..3.462912....3.479...0.852...2.964108..85.60%

All measurements with the same meter, and using current sense resistors.

 

[Jarhead]

BTW, the load was an LED, thats why slight changes in output voltage caused larger variations in current.

 

[Jarhead]

Current sense resistors used were Dale-Vishay WSL2512 Power Metal Strip® Resistors, low-inductance, SMT, 1% devices.

 

[georges80]

Jarhead, good info - can you run the tests with 1A approx into the load - presumably you can crank the output voltage up a 'tad' to push Luxeon up the Vf curve a little more.

That should push it into the current realm that will 'prove' the TI's metal vs the opposition /ubbthreads/images/graemlins/grin.gif

thanks,
george.

 

[Jarhead]

Here is the part with ~0.5A load:

Vin.....Iin.....Watts.....Vout..Iout...Watts.....Efficiency
2.980 0.588 1.752240 3.328 0.508 1.690624 96.48%
2.801 0.624 1.747824 3.326 0.504 1.676304 95.91%
2.704 0.648 1.752192 3.326 0.504 1.676304 95.67%
2.598 0.668 1.735464 3.324 0.500 1.662000 95.77%
2.501 0.696 1.740696 3.323 0.500 1.661500 95.45%
2.400 0.724 1.737600 3.321 0.496 1.647216 94.80%
2.300 0.752 1.729600 3.320 0.492 1.633440 94.44%
2.208 0.789 1.722240 3.318 0.488 1.619184 94.02%
2.104 0.816 1.716864 3.316 0.484 1.604944 93.48%
2.003 0.869 1.722580 3.315 0.480 1.591200 92.37%
1.899 0.904 1.716696 3.313 0.480 1.590240 92.63%
1.799 0.969 1.727040 3.313 0.480 1.590240 92.08%
1.702 1.024 1.742848 3.312 0.480 1.589760 91.22%
1.616 1.096 1.771136 3.312 0.480 1.589760 89.76%

 

[Jarhead]

Here is the part with ~0.23A load, note that the data bounces around and is less linear, I assume this is the low power, energy saving burst mode kicking in:

Vin.....Iin.....Watts.....Vout..Iout...Watts.....Efficiency
3.001 0.264 0.792264 3.149 0.244 0.768356 96.98%
2.898 0.260 0.753480 3.140 0.236 0.741040 98.35%
2.810 0.276 0.775560 3.144 0.240 0.754560 97.29%
2.699 0.280 0.755720 3.140 0.240 0.753600 99.72%
2.600 0.292 0.759200 3.139 0.236 0.740804 97.58%
2.506 0.304 0.761824 3.138 0.236 0.740568 97.21%
2.402 0.316 0.759032 3.137 0.236 0.740332 97.54%
2.309 0.332 0.766588 3.136 0.232 0.727552 94.91%
2.210 0.340 0.751400 3.134 0.232 0.727088 96.76%
2.109 0.356 0.750804 3.133 0.228 0.714324 95.14%
2.009 0.372 0.747348 3.132 0.228 0.714096 95.55%
1.905 0.392 0.746760 3.130 0.228 0.713640 95.56%
1.806 0.412 0.744072 3.129 0.224 0.700896 94.20%
1.711 0.436 0.745996 3.128 0.224 0.700672 93.92%
1.618 0.460 0.744280 3.126 0.220 0.687720 92.40%

 

[Jarhead]

Here is the part with ~0.36A load (e.g. slightly over driven 1W Luxeon)

Vin....Iin.....Watts......Vout..Iout...Watts......Efficiency
3.000 0.416 1.248000 3.247 0.376 1.220872 97.83%
2.919 0.428 1.249332 3.247 0.376 1.220872 97.72%
2.808 0.444 1.246752 3.246 0.372 1.207512 96.85%
2.717 0.456 1.238952 3.244 0.372 1.206768 97.40%
2.613 0.472 1.233336 3.242 0.368 1.193056 96.73%
2.502 0.488 1.220976 3.241 0.364 1.179724 96.62%
2.416 0.500 1.208000 3.241 0.360 1.166760 96.59%
2.308 0.528 1.218624 3.240 0.364 1.179360 96.78%
2.219 0.552 1.224888 3.239 0.364 1.178996 96.25%
2.100 0.588 1.234800 3.238 0.360 1.165680 94.40%
1.899 0.656 1.245744 3.236 0.360 1.164960 93.52%
1.813 0.688 1.247344 3.234 0.356 1.151304 92.30%
1.697 0.732 1.242204 3.233 0.356 1.150948 92.65%
1.594 0.780 1.243320 3.230 0.352 1.136960 91.45%

 

[Jarhead]

Disregard, I calculated some stuff wrong.....

Here is the part with ~1A load (Luxeon in parallel with the resistor to handle the power out.)

Disregard, I calculated some stuff wrong.....

 

[georges80]

[ QUOTE ]
Jarhead said:
Here is the part with ~1A load (Luxeon in parallel with the resistor to handle the power out.)

Vin.....Iin.....Watts.....Vout..Iout...Watts.....Efficiency
3.012 1.256 3.783072 3.437 1.052 3.615724 95.58%
2.903 1.240 3.599720 3.411 1.000 3.411000 94.76%
2.805 1.232 3.455760 3.391 0.964 3.268924 94.59%
2.704 1.228 3.320512 3.375 0.928 3.132000 94.32%
2.611 1.224 3.195864 3.358 0.896 3.008768 94.15%
2.513 1.220 3.065860 3.340 0.860 2.872400 93.69%
2.416 1.216 2.937856 3.323 0.828 2.751444 93.65%
2.298 1.216 2.794368 3.302 0.784 2.588768 92.64%
2.204 1.216 2.680064 3.284 0.756 2.482704 92.64%
2.123 1.216 2.581568 3.268 0.728 2.379104 92.16%
2.025 1.212 2.454300 3.248 0.696 2.260608 92.11%
1.904 1.184 2.254336 3.210 0.640 2.054400 91.13%
1.809 1.152 2.083968 3.176 0.596 1.892896 90.83%
1.711 1.164 1.991604 3.155 0.568 1.792040 89.98%
1.613 1.184 1.909792 3.134 0.544 1.704896 89.27%

[/ QUOTE ]

Jarhead, it would be nice to have the lower range of voltages - down to 2.2V with ~1A, this would give an indication of the efficiency when run from nimhs. If you have a pot to set the output voltage on the FB pin - can you crank it up a few mV? The true indication of how good the switch elements in the TI part are is high output current with low input voltage.

Actually, why is the Vout dropping so much as you lower the input voltage? I though you had said the switcher was configured with a resistive divider on the FB pin - if so the output voltage should be pretty steady /ubbthreads/images/graemlins/confused.gif

thanks,
george.

 

[Jarhead]

Re-Ran the 1A test, and calc'd the resistors wrong.
This time I used a little tiny inductor, as you'd asked in the PM, a Cooper Electric 2.2uH UP1B-2R2 DCR 0.0363, I loose a good chunk of efficiency (appx. 2% from the inductor alone).

Vin.....Iin....Watts......Vout..Iout..Watts......Efficiency
3.012 1.300 3.915600 3.497 1.052 3.678844 93.95%
2.898 1.364 3.952872 3.495 1.052 3.676740 93.01%
2.698 1.484 4.003832 3.493 1.052 3.674636 91.78%
2.615 1.540 4.027100 3.492 1.052 3.673584 91.22%
2.511 1.620 4.067820 3.490 1.052 3.671480 90.26%
2.409 1.704 4.104936 3.489 1.052 3.670428 89.41%
2.306 1.836 4.233816 3.488 1.048 3.655424 86.34%
2.197 1.916 4.209452 3.486 1.048 3.653328 86.79%