Need help w/ efficient DC-DC switching converter - diyAudio
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Old 8th November 2007, 09:38 PM   #1
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Default Need help w/ efficient DC-DC switching converter

I know this is a little off topic, but people in this forum seem quite knowledgeable about the construction of DC-DC converters, so please, please don't roast me =)

I need to build a dc-dc converter that takes an input that constantly varies from ~18-24v and outputs 12vdc at ~12 amps continuous. The voltage may go transiently go to 36v or it may drop to 0v at times. Below an input of 14v or so it should shut off and it should also have over current/thermal protection if the current gets too high. The input voltage will be 18-24v most of the time. Precise regulation of the output voltage is not important.

Efficiency of this circuit is the most important to me, followed by low part count, then cost. It also needs to practically design itself since I'm pretty unskilled when it comes to circuit design=)

My current plan is to use a few LM2677's in parallel with a 555 timer as suggested by "n-channel": here:

http://www.diyaudio.com/forums/showt...812#post167812

Does anyone have any better suggestions for IC's? Something more efficient or less complex? How about whole circuits that are ready to go?
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Old 8th November 2007, 10:09 PM   #2
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Hi,

Try searching for something like "Synchronous Buck Controller"

I'm sure you will find something that can do the job

Look for ST, Maxim, TI, IR, Infinion, Onsemi...... +lots more.

Happy hunting

\\\Jens
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Old 9th November 2007, 09:38 AM   #3
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Default synchronous buck

I've tried searching with those terms, but I can never find examples of regulators using multiple ICs or a single ic controlling multiple mosfets. As far as I know no one makes single IC solution that meets my power requirements.
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Old 9th November 2007, 11:17 AM   #4
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Hello

I did a google search: http://www.google.com/search?hl=da&q...%22%2B+40V&lr=

I'm sure you can find something that will work.

\Jens
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Old 10th November 2007, 08:09 AM   #5
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Hi,

This looks like it could be ok in your application:
http://focus.ti.com/lit/ds/symlink/tps40057.pdf

When you do the layout, be sure to seperate the noisy part of the circuit from the feedback part.

That will secure proper circuit operation.

Best of luck

\Jens
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Old 16th November 2007, 02:59 PM   #6
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I agree with Jens; a synchronous rectifier is the way to go if high efficiency is a must.
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Old 16th November 2007, 04:28 PM   #7
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You can may be look for ready-to-go module.

Search for: PS100-24S12, 8.5A

PowerSolve
PowerTrends
Texas
ASTEC
PowerOne
TRACOPOWER

And others, look for "DC/DC convertor module" at Farnell. But at this power, its expensive.

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Old 16th November 2007, 11:13 PM   #8
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Thanks Guys,

stef1777:

These converters and others I've seen like them have a minimum voltage of 36v and 48v is needed to power up. I need a lower minimum like 14v.


JensRasmussen:

The tps40057 looks good, even though it's more complicated than I had hoped.

Any suggestions for an efficient switching mosfet to pair with this chip?
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Old 18th November 2007, 09:22 AM   #9
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Quote:
Originally posted by pulexirritans
Any suggestions for an efficient switching mosfet to pair with this chip?
Hi,

I would go for something you can hand solder so Dpack or D2pack type mosfets is a place to start.

The TOP mosfet (switches from Vin) may be a single mosfet for this converter, but for the bottom mosfet (Active diode) it might be a good idea to parallel two devices to get the conduction losses down.

I would look at 30V-40V N-channel mosfets from:
Onsemi
Vishay
IR
Fairchild
ST

Low Qg and Low Rds is what you want.

What about inductor?
Switching frequency?

Remember that the ripple current in the inductor only is related to L, Vin, Vout and Fswitch.

In and output capacitor(s) (Ceramics are great here)

Some of the things to consider

There is no easy way to convert 100+ W with little loss

\\\Jens
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Old 18th November 2007, 09:51 AM   #10
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Quote:
Originally posted by JensRasmussen


Some of the things to consider

\\\Jens

And one of the most important -> PCB layout.

But the best and simple idea is to use 3 integrated step-down regulator as the LM2679 in parallel. This kind of chip need less external components and some can be shared.

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