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Old 7th April 2009, 10:39 AM   #1
DavidJE is offline DavidJE  United Kingdom
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Default Clean DC supply?

Hi,

I want to make a circuit that can take a simple 10-15V DC input and give me a multiple clean regulated 12V DC outputs. I want the output's to be isolated from the input and from each other to stop interference. The output's would only need to supply about 2-4 amps each maximum. That's the idea I have in my head anyway. I'm an electronics novice so I don't know how much of this is possible.

This is to supply power to several pieces of Hi-Fi equipment that require 12V DC supplies.

Could anyone point me in the direction of a suitable design for this?

May thanks,

David.
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Old 7th April 2009, 11:57 AM   #2
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Google '12V buck boost'.

Thats what you want, a bunch of 12 volt buck/boost regulators. They'll be a switching regulators, so I don't know about 'clean DC' but it'll be regulated.

Or you could use a single boost regulator to say 18V and a bunch of linears to regulate back down. A 7812 will give you 12.6V if you connect the ground pin via a diode, then you can connect that to the bases of a bunch of (NPN) pass transistors all with their collectors connected to 18V and you'll get 12V at the emitters.

w
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Old 7th April 2009, 12:00 PM   #3
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Since you want multiple isolated voltages, you must use a transformer-coupled converter. The maximum output power will determine the topology. At low power levels one might consider flyback, at higher power levels push-pull. Personally I would recommend using push-pull since the input voltage is very low. At low input voltage and high output power the input current will be large though.

But if you have never designed a switching converter or even studied related issues, I would say forget it. Or give the design task to someone who knows how to do it.

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Old 7th April 2009, 02:12 PM   #4
DavidJE is offline DavidJE  United Kingdom
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Thanks for the replies,

So far I've mainly looked into the buck boost regulators. These seem to fit the requirement for varying input voltage and constant output voltage. I've seen these called DC/DC converters which I've heard of before. Is a buck boost regulator the same as a DC/DC converter?

For example, I found THIS slightly expensive dual output DC/DC converter which can handle 9-36V DC input with a fixed 12V DC output at 6A. Would this do the job?

I'm not sure if this provides isolation between it's outputs though.
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Old 7th April 2009, 02:33 PM   #5
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by DavidJE
...Is a buck boost regulator the same as a DC/DC converter?...
A buck-boost converter is a type of DC-DC converter. There are various topologies that have different properties, with the main feature of the buck-boost topology being that it can both step up and step down the voltage.

Like Bootstrapper said, if you want isolation then you need a transformer. The simplest transformer-based topology is the flyback converter. However, as Bootstrapper also said, a DC-DC converter is not a good project for a novice.
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Old 7th April 2009, 05:54 PM   #6
marce is offline marce  United Kingdom
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You could possibly look at computer PSU's or commercial DC-DC convertors initialy, though the 10-15V input may be a problem. For the output power you require up to 48W, and multiple supplys, a custom built circuit is gonna be rather complex. To keep the transformers size down to a reasonable level will require a higher switching frequency, more noise. The capacitors will have to be good quality low ESR, not cheep. And the layout problems!!! I have worked on a similar spec commercial product and it took us 6 iterations of circuitry, layout, thermal anaysis etc to get it right. Dont forget a big mosfet for switching those sort of currents, that adds heat, and switching losses. Thats why most commercial SMPS at these currents come in an metal case , EMC shielding and somewhere to bolt active components to get rid of the heat.
If you realy did want to go the DIY route, a well used topology for your input output voltage is an initial boost section maybe to 20V, with reservoir capacitors, then a secondry flyback to give you your isolated 12V. The bigger the initial boost section determines the amount of power you are storing and could vary depending on how many 12V outputs you want. This can often be the cheeper option in component cost as the reservoir caps can be relatively cheap, as apposed to a single stage buck/boost design which would be more demanding.
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Old 8th April 2009, 10:35 AM   #7
DavidJE is offline DavidJE  United Kingdom
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Thanks for the replies everyone, there's some really interesting info here.

For clarification, the DC input into this PSU is from a 12V battery, hence the possibly fluctuation input voltage. I mainly need one regulated 12V DC output from this but I stated multiple outputs for future proofing. If this is going to seriously complicate things then I'll probably drop that requirement.

Also, the amount of current drawn from this PSU will probably be much less than 4A, especially if I only have one output. If I have one output it will probably not be above 1A. With two outputs probably no more than 2A.

If I do decide to have multiple outputs, I'm not sure if I need them to be isolated from each other. Would isolating them from each other provide any benefit to the sound quality of the connected devices?

Also, to provide isolation between the two outputs, could I just use two separate buck-boost converters? The DC/DC converter I linked to earlier states that it's output is isolated from it's input, so I would have thought that connecting two to the same input would provide isolation between the two outputs. Please excuse my ignorance if this is entirely wrong.

Many thanks,

David.
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Old 8th April 2009, 11:40 AM   #8
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Mascot 8660 8A 12V-12V DC converter

They have other power ratings too.

w
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Old 9th April 2009, 06:38 PM   #9
marce is offline marce  United Kingdom
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Hi,
you might find this link informative.
http://www.national.com/AU/design/courses/253/
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