Is it hard to build a voltage step up converter?

Caleb Chia · 13th March 2010, 10:01 PM

Hi,
I have a nice big 23Ah nicad battery that I would like to build a battery powered guitar amp with, but its only 6 volts.
Is it very difficult to build a step up power supply like what they have in car amps? (does anyone have any schematics of one?)
My other option is to buy a ready made voltage converter but they're quite expensive.
Or could I build a low voltage high current amp that will run on 6v and put a step up transformer on the output to the speaker?
What do you think?
Thanks.

Th3 uN1Qu3 · 14th March 2010, 10:30 AM

The 2nd option isn't an option really. You will need to build a buck converter (if you just need one voltage) or a flyback converter (if you need several voltage outputs). They are not hard to build if you have the appropriate tools, but it's not something that you can just jump into. You should read "Switching Power Supply Design Third Edition" by Abraham Pressman, Keith Billings and Taylor Morey.

I have built only a couple low powered converters myself (12v->250v, 15W) but i'm on my way on designing a 150W supply for my new amplifier. The book "Power Supply Cookbook" by Marty Brown is also very helpful (that's what i had been reading at first), but it sometimes forgets to mention all the units so the formulas can be confusing to figure out. In the one i mentioned above everything is clear.

gootee · 15th March 2010, 01:31 AM

Or go to national.com and let the automated Webench app design it for you.

Linear.com also has lots of chips that can be used to build a boost-mode switching power supply. (I thought "buck" referred to a step-down switching supply.)

Both sites also have tons of already-designed circuits, and application notes that explain most or all of what you would need to know.

Th3 uN1Qu3 · 15th March 2010, 09:38 AM

Yes, my bad, i wanted to say boost instead of buck. The buck is indeed a stepdown converter/regulator.

I neglected the highly integrated solutions because you can't find any of those here. But where you can, there's no reason not to use them.

WhiteD3vil · 16th March 2010, 03:17 AM

This biggest problem you'll face is the magnetics required for a step-up converter: Neither one is going to use a conventional transformer.

Switched-mode circuits like a flyback converter require something that looks like a transformer, but functions more like an inductor with dual windings that transfers energy from one set of windings to the other through the core, with the output quasi-proportional to the ration of the windings. Calculating everything is not straightforward, probably not available off-the-shelf.

Boost converters use a conventional inductor, so its more straightforward to design. The drawback is the efficiency losses that increase with the ratio of output voltage to input voltage. You'll need lots of well-sinked FETs to produce a lot of current.

Caleb Chia · 16th March 2010, 09:36 AM

Thanks for the tips

Quote:

Originally Posted by WhiteD3vil

The drawback is the efficiency losses that increase with the ratio of output voltage to input voltage. You'll need lots of well-sinked FETs to produce a lot of current.

How much efficiency loss would be a ballpark figure? I guess a few per cent of loss could be all right, the battery is quite big.

star882 · 16th March 2010, 02:06 PM

For your application, a push-pull topology is probably the best. I would recommend TI NEXFET MOSFETs, which offer very low internal resistance.

WhiteD3vil · 16th March 2010, 07:55 PM

^Good FETS like those will help, but the losses will be in the magnetics and the windings. Its been a while since I was making switchers, but at the time the magnetics limited the Vout/Vin to about 6 with about 70% efficiency. I'm sure that's improved, but it's more than a few percent.

Caleb Chia · 17th March 2010, 07:46 AM

hmm, 70% doesn't sound too good. How much idle power would these circuits use?

Maybe I'll try a bridge mode amp into parallel 4ohm speakers.

WhiteD3vil · 17th March 2010, 07:04 PM

Idle power would probably be a few watts, unless the amp is class A.

13th March 2010, 10:01 PM	#1
Caleb Chia diyAudio Member Join Date: May 2008	Is it hard to build a voltage step up converter? Hi, I have a nice big 23Ah nicad battery that I would like to build a battery powered guitar amp with, but its only 6 volts. Is it very difficult to build a step up power supply like what they have in car amps? (does anyone have any schematics of one?) My other option is to buy a ready made voltage converter but they're quite expensive. Or could I build a low voltage high current amp that will run on 6v and put a step up transformer on the output to the speaker? What do you think? Thanks.

16th March 2010, 02:06 PM	#7
star882 diyAudio Member Join Date: Mar 2007	For your application, a push-pull topology is probably the best. I would recommend TI NEXFET MOSFETs, which offer very low internal resistance. __________________ "Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!

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14th March 2010, 10:30 AM	#2
Th3 uN1Qu3 diyAudio Member Join Date: Feb 2010 Location: Bucharest	The 2nd option isn't an option really. You will need to build a buck converter (if you just need one voltage) or a flyback converter (if you need several voltage outputs). They are not hard to build if you have the appropriate tools, but it's not something that you can just jump into. You should read "Switching Power Supply Design Third Edition" by Abraham Pressman, Keith Billings and Taylor Morey. I have built only a couple low powered converters myself (12v->250v, 15W) but i'm on my way on designing a 150W supply for my new amplifier. The book "Power Supply Cookbook" by Marty Brown is also very helpful (that's what i had been reading at first), but it sometimes forgets to mention all the units so the formulas can be confusing to figure out. In the one i mentioned above everything is clear.

15th March 2010, 01:31 AM	#3
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	Or go to national.com and let the automated Webench app design it for you. Linear.com also has lots of chips that can be used to build a boost-mode switching power supply. (I thought "buck" referred to a step-down switching supply.) Both sites also have tons of already-designed circuits, and application notes that explain most or all of what you would need to know.

15th March 2010, 09:38 AM	#4
Th3 uN1Qu3 diyAudio Member Join Date: Feb 2010 Location: Bucharest	Yes, my bad, i wanted to say boost instead of buck. The buck is indeed a stepdown converter/regulator. I neglected the highly integrated solutions because you can't find any of those here. But where you can, there's no reason not to use them.

16th March 2010, 03:17 AM	#5
WhiteD3vil diyAudio Member Join Date: Mar 2010 Location: San Jose, CA	This biggest problem you'll face is the magnetics required for a step-up converter: Neither one is going to use a conventional transformer. Switched-mode circuits like a flyback converter require something that looks like a transformer, but functions more like an inductor with dual windings that transfers energy from one set of windings to the other through the core, with the output quasi-proportional to the ration of the windings. Calculating everything is not straightforward, probably not available off-the-shelf. Boost converters use a conventional inductor, so its more straightforward to design. The drawback is the efficiency losses that increase with the ratio of output voltage to input voltage. You'll need lots of well-sinked FETs to produce a lot of current.

16th March 2010, 07:55 PM	#8
WhiteD3vil diyAudio Member Join Date: Mar 2010 Location: San Jose, CA	^Good FETS like those will help, but the losses will be in the magnetics and the windings. Its been a while since I was making switchers, but at the time the magnetics limited the Vout/Vin to about 6 with about 70% efficiency. I'm sure that's improved, but it's more than a few percent.

17th March 2010, 07:46 AM	#9
Caleb Chia diyAudio Member Join Date: May 2008	hmm, 70% doesn't sound too good. How much idle power would these circuits use? Maybe I'll try a bridge mode amp into parallel 4ohm speakers.

17th March 2010, 07:04 PM	#10
WhiteD3vil diyAudio Member Join Date: Mar 2010 Location: San Jose, CA	Idle power would probably be a few watts, unless the amp is class A.

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