DC-DC Boost Converter Design

[SJP0tato]

Hi guys,

I'm looking to put together a DC-DC boost converter to amplify the output voltage of ~9-12v of a regular automotive battery to an output of 18v, max 1.5A (typical current load would be ~400mA, but I think 1.5A would be good for safety). I'm hoping for the best efficiency possible, as I will be replacing a much less efficient conversion method being used currently (using a series of off the shelf converters which convert 12DC to 120AC back to 18DC).

I've done some searching online, but the choice of ICs is bewildering to say the least. Some of the schematics I've seen I'm not 100% clear on either. I have seen some that make use of a transistor, but I'm not sure if this would be the best approach for my application.

I do have some experience with circuits (having built a full-wave rectifier), but this seems less straightforward to me.

Any pointers or suggestions on design would be greatly appreciated.

 

[QuixoticOne]

http://www.linear.com/pc/categ...ewCategory.jsp&y=0&x=0

http://www.linear.com/pc/produ...0,C1,C1003,C1042,P1266
http://www.linear.com/pc/produ...0,C1,C1003,C1042,P1430
http://www.linear.com/pc/produ...0,C1,C1003,C1042,P1630
http://www.linear.com/pc/produ...0,C1,C1003,C1042,P1620
http://www.linear.com/pc/produ...0,C1,C1003,C1042,P1620
http://www.linear.com/pc/produ...0,C1,C1003,C1042,P7810
http://www.linear.com/pc/produ...,C1,C1003,C1042,P10091

Several of those are rated in the 89% to 94% efficiency range in similar operating conditions to yours. Several of them can be used with very simple external circuitry.
They have a free switchercad utility to help you select your components and chip and topology and depending on your needs.

Maxim-IC, ON Semiconductor, Texas Instruments, ST, ON semiconductor, National Semiconductor,
et. al. all have other chips that may be similar in function / capability / efficiency / simplicity.

I believe Maxim has an online e-store where you can order small quantities of their parts for some of their parts anyway. Digikey or Mouser or Newark distributors probably have switcher chips from various manufacturers and all the accessory components you need.

I don't think there's a huge difference in efficiency between the available units once you exceed around 90%, so unless you're really selecting for the top 1% of efficiency just decide based on availability / simplicity from the many good 90%+ efficiency designs. Chances are the data sheet will tell you exactly how to hook it up in a very easy to follow manner to get the results you need, and of course there are the free software tools from several of the vendors like national/linear tech et. al. too.

 

[Colt45]

I like the old motorola MC34063. Mostly cause I'm used to them and they're dirt cheap.. like 75cents for a single.

http://www.onsemi.com/pub_link...lateral/MC34063A-D.PDF
http://www.onsemi.com/pub_link/Collateral/AN920-D.PDF

 

[PottedMeat]

Originally posted by: Colt45
I like the old motorola MC34063. Mostly cause I'm used to them and they're dirt cheap.. like 75cents for a single.

http://www.onsemi.com/pub_link...lateral/MC34063A-D.PDF
http://www.onsemi.com/pub_link/Collateral/AN920-D.PDF

Those are real convenient / cheap / easy to use too. Quick for making Buck/Boost/Inverting topologies. I've used them in constant current power LED drivers. Though you probably will need an external power transistor for 1.5A peak output current.

 

[bobsmith1492]

Originally posted by: QuixoticOne
http://www.linear.com/pc/categ...ewCategory.jsp&y=0&x=0

http://www.linear.com/pc/produ...0,C1,C1003,C1042,P1266
http://www.linear.com/pc/produ...0,C1,C1003,C1042,P1430
http://www.linear.com/pc/produ...0,C1,C1003,C1042,P1630
http://www.linear.com/pc/produ...0,C1,C1003,C1042,P1620
http://www.linear.com/pc/produ...0,C1,C1003,C1042,P1620
http://www.linear.com/pc/produ...0,C1,C1003,C1042,P7810
http://www.linear.com/pc/produ...,C1,C1003,C1042,P10091

Several of those are rated in the 89% to 94% efficiency range in similar operating conditions to yours. Several of them can be used with very simple external circuitry.
They have a free switchercad utility to help you select your components and chip and topology and depending on your needs.

Maxim-IC, ON Semiconductor, Texas Instruments, ST, ON semiconductor, National Semiconductor,
et. al. all have other chips that may be similar in function / capability / efficiency / simplicity.

I believe Maxim has an online e-store where you can order small quantities of their parts for some of their parts anyway. Digikey or Mouser or Newark distributors probably have switcher chips from various manufacturers and all the accessory components you need.

I don't think there's a huge difference in efficiency between the available units once you exceed around 90%, so unless you're really selecting for the top 1% of efficiency just decide based on availability / simplicity from the many good 90%+ efficiency designs. Chances are the data sheet will tell you exactly how to hook it up in a very easy to follow manner to get the results you need, and of course there are the free software tools from several of the vendors like national/linear tech et. al. too.

Try the SwitcherCAD program; punch your input-output specs and it will give you a few devices that will work. Pick one and it spits out a schematic for you. The version on their website doesn't have the switch selector guide anymore so here's an old version for you. It looks like your best bet would be the LT1270, $10 from Digi Key.

There are a couple of layout things to watch; read the datasheet and you should be ok. Basically put the input cap and output cap right by the chip - the shorter the leads are the better. Same goes for the diode and inductor. Otherwise it just spits out lots of noise which may or may not be an issue (could interfere with nearby AM radios...)

 

[SJP0tato]

Thanks for the help everyone, I think I've narrowed down my selection to either the MC34063, or the LT1270. (bobsmith1492 I'm not able to download your SwitcherCAD.exe to try out the LT1270)

There's one last thing I'm still unsure of: All of the ICs listed have datasheets that give an example setup for a step-up converter for an arbitrary input/output combination, but I'm not sure how to go about figuring out the components necessary for my application.

The OnSemi datasheet specifies some equations, but they don't look like they cover all the components shown on the example given for the step-up converter.

Are there more detailed design/data sheets elsewhere that I'm not able to find, or is this "common sense" knowledge that I need to read up more to be able to figure out?

Thanks again, the suggestions thus far have been very helpful!

 

[bobsmith1492]

Originally posted by: SJP0tato
Thanks for the help everyone, I think I've narrowed down my selection to either the MC34063, or the LT1270. (bobsmith1492 I'm not able to download your SwitcherCAD.exe to try out the LT1270)

There's one last thing I'm still unsure of: All of the ICs listed have datasheets that give an example setup for a step-up converter for an arbitrary input/output combination, but I'm not sure how to go about figuring out the components necessary for my application.

The OnSemi datasheet specifies some equations, but they don't look like they cover all the components shown on the example given for the step-up converter.

Are there more detailed design/data sheets elsewhere that I'm not able to find, or is this "common sense" knowledge that I need to read up more to be able to figure out?

Thanks again, the suggestions thus far have been very helpful!

Hm, not sure why you can't download it... here's a screenshot, anyway.

 

[SJP0tato]

Originally posted by: bobsmith1492
Hm, not sure why you can't download it... here's a screenshot, anyway.

I can download the .exe, but trying to install results in a corruption error. The diagram you provided works great though. Was that obtained from the simulator? Is that typically how design is completed for more involved circuits?

Copying that works okay, but I'd still really like to know how the values could be calculated (for my own knowledge).

 

[Modelworks]

I think this is what you are looking for:
http://www.poyntsource.com/New/18vConverter.htm

 

[bobsmith1492]

Originally posted by: SJP0tato

Originally posted by: bobsmith1492
Hm, not sure why you can't download it... here's a screenshot, anyway.

I can download the .exe, but trying to install results in a corruption error. The diagram you provided works great though. Was that obtained from the simulator? Is that typically how design is completed for more involved circuits?

Copying that works okay, but I'd still really like to know how the values could be calculated (for my own knowledge).

Huh, that's weird... oh well.

Yeah, that simulator makes the circuit for you. Typically for a simple boost switcher you'd go to Digi Key, search for "regulator," look for "boost," and so on (browse the parametric tables for chips that meet the specs you need) then look at a few. The datasheet on each of these chips describe how to design a proper circuit. They're basic enough to design that this simulator program can just spit them out.

Or, go to Linear.com or TI.com and look through their parametric tables which are more detailed.

The main things to watch out for are:
- inductor and diode current: make sure your parts are overrated since the pulse current is higher than the average; get as low of a resistance as you can afford on the inductor to increase efficiency (if it matters)

- output capacitor: you need very low ESR (equivalent series resistance) to keep noise and ripple down. Ceramic capacitors are perfect; you can put one in parallel with a larger value tantalum or electrolytic.

- input capacitor: again, use a low ESR (ceramic) capacitor

- diode: make sure it's a schottky (fast, low forward voltage)

- layout: keep the lines from the input capacitor to the voltage input as short as possible; then keep the output diode, inductor, and ceramic/low ESR capacitor as close as possible also. Datasheets for switching regulators usually have notes or examples for layout.

I used that simulator to design a pair of switchers that I ran off a solar panel (or my laptop power supply) while on a bike trip. They provided 5V to charge my camera and cell phone and 3.3V to charge AA batteries.

 

[SJP0tato]

Thanks for all the help guys. Ironically the link posted by Modelworks is the exact application I'm looking to use it for (well, for my father's telescope anyways).

Everything looks pretty straightforward now, I finally found the datasheet that outlines the equations used to pick the components for the boost circuit, and was able to figure out how & why the values are what they are.

Thanks again, I'll try to post up the results in a week or two once the components are all finished up.

 

[mindless1]

I can appreciate doing a project for the learning experience but if you just want the result, consider using a car laptop power brick since there are several meeting your requirements. It'll tend to cost a lot less too if you can find one as surplus somewhere online.