Hi! I did the “getting to blinky” KiCAD tutorial and got overconfident and decided I’d like to create my first PCB. I’d like to build a switching power supply that will let me use a 3.5V lipoly battery to power a 12V 120mm computer fan.
I looked around quite a bit at different datasheets and selected the LT1308B (I had a link to the datasheet but the forum forced me to remove it)
Page 20 of the datasheet has pretty much exactly the circuit I’d like to build, so I’m pretty much ripping it off, but adding a couple things to the example application circuit:
- A switch to let me shut down the boost converter (using the SHDN pin)
- A low-battery indicator LED
Here is the schematic I came up with:
http://imgh.us/fanboost.svg
I’ll probably need some help with the component selection. I’d like to pick all SMD parts. The datasheet has some suggested components for outputting 12v @ 300mA, but I would like to output 12v @ 400mA.
I just spent some time messing around in LTspice and I seem to have created a simulation of my power supply. As far as I can tell this looks pretty good:
click here to see the image - I can’t post images because I’m a “new user”
The current and voltage are stabilizing at 12V and 400mA. I put a 30 ohm load on there, which makes it draw the 400mA.
It looks like it’s drawing between 1 and 1.9 amps from the battery, which doesn’t seem too bad. That means (3.75V)(((1.9A + 1 A) / 2)) = 5.4375 watts drawn, and (12V)(.4A) = 4.8 watts output. So that’s ((4.8W)/(5.4375))(100) =~ 88% efficiency… right? Is that good?
Is there something I could be doing to optimize this?
This is pretty much my first real circuit I’ve tried to build other than messing around with Arduino or building kits, so be gentle 
Kevin,
Your calculation for efficiency seems legitimate, although another way to do that is to take instantaneous current and voltage, multiply them together and then average the instantaneous power waveform. This is more accurate because your battery voltage might actually start rippling due to the AC current you are drawing from it, causing extra losses.
As for 88% efficiency being good, that is a very tough question to answer. It depends on your design goals. Do you want the cheapest power supply no matter the trade off for efficiency? Do you want to have the most efficient power supply possible because you are running off of a small battery?
Modern switching power supplies can be upwards of 99% efficient. I work as power electronics engineer and we have a product that does maximum power point tracking on solar panels and is greater than 99% efficient. However your hands are pretty much tied when using this chip, because it seems to be fixed frequency. As a rule of thumb for switching power supplies, as your frequency goes up your switching losses go up and your efficiency goes down. I would say 88% isn’t horrible by any standards.
I guess it depends on how long you want to run the fan off of the battery. Calculate the life of one charge using the amp-hours of the battery and your DC current drawn from the battery and if you aren’t happy make a new design. This is a good first project though!
Ldoiron17
Hey Kevin, congrats on the first project. It can be scary at first, but hopefully very rewarding as well. Boost coverters are fun too (assuming they work!).
I decided to retag this post and create a new category, to “projects”. Thanks for posting!
@ChrisGammell, Thanks for creating the tag, and thanks for creating the forum! Your Getting to Blinky videos gave me enough confidence to give this a shot
@Ldoiron17, Thanks for the insight! That’s very interesting about the switching frequency having an effect on the efficiency. I guess the advantage of a higher frequency is that you can have a smaller inductor? Maybe your output smoothing capacitor can also be smaller since with a higher frequency, the output isn’t going to wobble back and forth with as much amplitude.
I think 88% efficiency will be good enough for a first try. If this project works, maybe I can try to improve the efficiency with a second project! It looks like with a 1200mAh battery (I have a couple of these around) I could get around 50 minutes of runtime. If I had a 100% efficient supply, I could get a full hour. Maybe I could even integrate a LiPoly charging IC
Anyway, the next step is to pick out some parts. I’ll post back when I come up with a parts list.
