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Old 12th November 2005, 06:13 PM   #1
Danko is offline Danko  Hungary
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Join Date: Apr 2004
Location: Hungary
Question Switched mode laboratory powersupply

Hi Everyboby!


I have a "cute" 750VA toroidal transformer, this is a 50Hz type. very very heavy

So, I was thinking about, what if I build a laboratory PSU with this trafo, and the electronics is a simple DC/DC converter. A step-down regulator.
The trafo hax 4x23V secundaries, and I plan to use 2-2 series connected, to get about 65V DC.
The specifications would be:
Output voltage: 0-50V, or 0-55V
Output current: 7-8Ampers.
And here comes the "trick". Becouse, I plan to use a step-down converter, I can achieve more current, when the output voltage is lower than 65V DC. For example, when the output voltage is 12V, then the output current can be 20A, or even more, 30A.


Has anyone made a switched mode lab. powersupply?
How can I reach that the output waveform is totally flat? no spikes, no high frequency components....
I could put a simple emitter-follower after the dc/dc regulator, and on the emitter-follower is dropping only a few volts, jut enough, to attenuate the noise.

Any thoughts?
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Old 12th November 2005, 07:03 PM   #2
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One common solution is to have a linear regulator with a switching preregulator. That way you can keep the voltage drop over the pass transistor(s) in the linear regulator low and independent of the output voltage. However, with the currents you talk about, you will still have to burn away quite many Watts in the linear regulator.
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Old 12th November 2005, 08:32 PM   #3
Danko is offline Danko  Hungary
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Join Date: Apr 2004
Location: Hungary
Hello!

Yes, at high currents the linear "part" will dissipate much, but not so much, than a simple emitter-follower, from 65V->12V @ 10A

But now I know, that it's common, to use a switching regulator, and after that a linear regulator, which drops only a few Volts.


Thanks, Christer!
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Old 12th November 2005, 09:20 PM   #4
Danko is offline Danko  Hungary
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Join Date: Apr 2004
Location: Hungary
Oh, sorry, 10A can't be made from that trafo, with a linear regulator
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