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Old 21st December 2011, 08:28 AM   #1
sekhar is offline sekhar  India
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Location: West bengal/siliguri
Default Need a few suggestion .... Before I start Building.....

Hi guys ,,,

I am planning to make a Lab power supply .... with the following specs...


1. 90-270V Input.
2. 12-24V variable O/p @ 33A.
3. Efficiency > 0.85
4. PF > 0.95

I have made the following assumption on how to achieve the following specs.
a.1&4 will be solved by a Active Pfc front end.
b. The 2&3 I am nt very sure hw to do it . Have a few ideas I will use a half bridge converter with a feed back and regulate the o/p voltage . and to achieve 85% efficency with a two stage converter will be difficult. As because in my last project of 450W smps with active pfc front end I was able to achieve only 82% at low line and 84% at high line...

Guys i need u help me out .... Which will be the simplest topology for the DC_DC stage with the highest efficiency.... And I also need to vary the o/p voltage by almost 50 percent.....


regards
sekhar
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Old 22nd December 2011, 02:55 AM   #2
sekhar is offline sekhar  India
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come on some one ................
i need to vary the o/p voltage of the converter by almost 50% ......any ideas which topology to use .............. can it be done in a half bridge............?????????/

regards
sekhar
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Old 22nd December 2011, 12:45 PM   #3
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CCM Boost PFC followed by Full-Bridge.

.......Its 800W output , so you will be near 900W at input.

Do you think a single FET/Single diode PFC can manage 900W input?..........neither do i.

So you are probably going to have to do a dUAL PHASE ccm PFC.....ie two interleaved pfc stages.

Choose your max duty cyle depending on you r transient response.

You may need fan.

Calculate your switching loss......remember its the length of the miller plateau.

Its probably too much for pri current sense res......so you need current sense trafo.

I think you may find , at your power level that it is NOT POSSIBLE to do it without a fan.......so i feel convinced that you will need a fan.
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Old 22nd December 2011, 01:44 PM   #4
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Location: hobart tasmania
I think deal with your design requirements of 90- 270vac with a very large variac, what a beast it would be.

Then the 12- 24 variable and 33 amps is immediately suggesting to me that you may have to compromise and be happy with 3 settings and use three individual 500va or larger transformers each with full wave bridges heat sinked, install circuit breakers and capacitance equal to 1000uf for each amp of current.

The use of a variac would of course change the secondary voltage of each transformer, which invites some design effort to match what it is doing to your requirement of 12-24Vdc. What about V- don't you need a negative voltage ?

You could then use 3x LM338 for about 5amps out for adjustment of voltages between these ranges, Hopefully just meeting your required needs. V+ raw at maximum current and regulated variable taps at less current.

Suggest you study actual requirements and ask WHY you need 33 amps.

Cheers / Chris

Last edited by Chris Daly; 22nd December 2011 at 01:52 PM.
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Old 26th December 2011, 11:06 AM   #5
sekhar is offline sekhar  India
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ok i will start drawing out the the circuits...............


regards
sekhar
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Old 29th December 2011, 01:15 PM   #6
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Here is the Computation of EI core Transformer Philippines Base

try this one...
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Old 11th January 2012, 02:53 AM   #7
sekhar is offline sekhar  India
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Join Date: Jul 2009
Location: West bengal/siliguri
So,
I have made a few choices and started working on them .
1. It will be a single stage CCM boost PFC.
2. It will be a half bridge DC - DC converter.


regards
sekhar
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Old 11th January 2012, 02:58 AM   #8
sekhar is offline sekhar  India
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As it will be half bridge converter and as i need a variable o/p voltage . I will need to put a huge o/p inductor ..... Can any one tell me how to calculate o/p inductor.....



regards
sekhar
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Old 11th January 2012, 05:33 AM   #9
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Full bridge or phase shifted full bridge for you surely?

Inductor, just keep its rms current low enough to avoid bad losses, and avoid saturation and hysteresis over losses
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Old 11th January 2012, 11:37 AM   #10
dtproff is offline dtproff  United States
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Location: Plano, TX, USA
Going to have to agree with ee2am on this... 800W is definitely CCM PFC and full bridge DC/DC. Phase shifter would probably be best though harder to design for most.

Tony
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