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Old 26th July 2007, 11:12 AM   #1
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Default Variable output SMPS

Hi,

So far all the bench top variable DC output power supplies i have seen are linear AC-DC power supplies.

Does anyone have any idea about a variable output AC-DC SMPS?

By variable i mean output is 0 Volts DC to X Volts DC. And, maybe working on Universal input range.

It these are available, how these are compensated in design?
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Old 26th July 2007, 01:32 PM   #2
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Some CPU voltage regulators can regulate down to almost 0v but also operate at low voltages.
The main power supply for a digital amp can be something to get ideas from. The newer digital amps modulate the supply voltage to decrease precision needed at low volumes.
Try experimenting with a small buck circuit with a variable voltage reference. You'll probably need two stages if operating from the line.
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Old 26th July 2007, 11:27 PM   #3
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Check these out:

http://www.sorensen.com/products/DCS/DCS_Overview.htm

I see these pretty regularly on eBay and in fact I have a DCS-80-13E I bought for $75 (not working) and got it working by adding new pots and the programming dongle. You may also be able to purchase a service manual for one on eBay too to see their design.

I've seen the schematics and they are pretty straightforward. SG3524 SMPS driver, op-amps for voltage/current monitoring and feedback, LED drivers forming the E/I meters, push/pull switching primaries driving a ferrite transformer, etc.
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Old 27th July 2007, 03:26 AM   #4
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DCPreamp..
Can we see the schematics ?
Thanks.
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Old 27th July 2007, 05:02 PM   #5
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I was only able to view them, not save them. So unfortunately, I don't have anything to post. If, by chance, I come across the service manual again, I'll see if I can scan and post them.

FWIW, their 1KW model uses FETs for primary switching and their 3KW model uses IGBTs.
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Old 28th July 2007, 05:07 PM   #6
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to get output to 0V, i suggest building a bias into the output. similarly to a class D amplifier, where 50% duty ratio is 0V out. this will remove the requirement for a large per-unit dynamic range (eg, D is acceptable over D/(1-D) ) and allow you to use a buck converter*. clearly the negative range should be less then the positive range to allow a wider control range, or the supply should be designed to produce +V or -V with regulation.

you can use a forward or half-bridge converter for this application.
I suggest using the reference on secondary side using an opto-isolator and shunt regulator, as this will allow you to adjust the setpoint with an element on the secondary side.

for multiple outputs you will want to set up a fixed pulse-width isolation stage (half bridge) to act as a DC transformer, then build simple buck stages to provide regulation on a per-output basis.

*edit -- buck _derivied_ converter, like a half-bridge or double-ended forward, both of which would be applicable for a line powered application with isolation requirements.
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Old 3rd August 2007, 11:34 PM   #7
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Not quite sure what regulation down to 0V does for anyone but hey ho. presuming you mean down to 'low voltage' then consider a fixed duty cycle high efficiency front end with one of the forward converter topologies with mag amp post regulation on the output or outputs. (can make the front end resonant and get all the low noise high efficiency benefits there but resonant introduces some subtle but possiby not important limitations so if you are 'new' to th game stick to conventional switched). Mag amps need some skil in power magetics design but you can have independently variable outputs, closely regulated and no oto / galvanic isolation closed loops to worry about. Its the design approach im taking on my class A amp where I can vary the rails as the volume pot is turned up / down. You get class A but minimise the waste power when you have the amp low which is most of the time. The topology I describe also has many other advatages but that would keep me typing all night. Hope that helps. StreeterA
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Old 3rd August 2007, 11:37 PM   #8
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would add that mag amps are of course a form of secondary side buck regulator in this approach but have noise and other advatages over conventional secondary switched buck regulators.
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