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Old 3rd December 2005, 12:19 AM   #1
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Default 12V SMPS 1kW

Hi guys!
Great forum you have here, already found a lot of usefull things!

But for my actuall problem i found not so much.
I need some more information about dc/dc-converters, especially 12V with power round about 1kW output voltage +/-25V. Some catchwords will be usefull for me.

My idea was to design a push-pull-concept with switching frequency round about 500kHz, RM pot core and active rectifying.What do you think about this, is it practicable?

Thanx
Wolfgang
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Old 3rd December 2005, 08:38 AM   #2
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Pushpull is the only way to go if you want 1000 watts, but this will draw huge amps fron your 12v source.

Iam building a smps myself and i can tell you its no game. I have blown rectifier diodes (on the output of the thing), overvolted the test amp with 200+v, chasing problems causing oscillation and noise for days.

You could salvage the smps from a busted 1kw car amp, yeah that wont be as fun as building it but i knocked out tv reception here with my smps sometimes so building from scratch is not reccomended, especially not in your power ranges.
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Old 3rd December 2005, 09:08 AM   #3
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My question is now, how to dimension the smps. Is it usefull to take the formulas for the ideal converter, like it is the way for lower power?

Or where can i find a mathematic model which is for the reality?
In my opinion i think it will be better to dimension the devices
with a model which is equivalent to the reality, in such power regions. What is you opinion?
Are there any links or do you know any usefull books?

Thnax for your help!
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Old 3rd December 2005, 10:52 AM   #4
sss is offline sss  Israel
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for that power u should use 2 or more transformers
and i think your freq range is too high , up to 100khz sounds more practical .
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Old 3rd December 2005, 10:56 AM   #5
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Default Re: 12V SMPS 1kW

Quote:
Originally posted by Wolfgang

My idea was to design a push-pull-concept with switching frequency round about 500kHz
500khz is too high.below 100khz is recomended.
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Old 3rd December 2005, 02:22 PM   #6
Eva is offline Eva  Spain
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Try to develop your own model. For example : Assume a continuous output current value. Find out rectifier dissipation according to datasheets. Select a suitable model and amount of devices in paralell to suit your heatsinks (both mechanically and thermally).

Build or figure out a transformer prototype, measure the required magnet wire length and find out primary and secondary DC resistance and theoretical wire dissipation. If it's too high then build a new prototype with more magnet wire (the best way to know that is to measure actual temperature increase in a working prototype). Don't forget to consider the effective cross-sectional area reduction in the wires due to skin effect if your switching frequency is well over 30Khz.

Find out primary winding current. Assume a certain crossover time, for example 100nS, and calculate switching losses. Then calculate conduction losses for several switch models and amounts of devices until something that suits your heatsinks is found...

If you know some programming language, you may find useful to write small programs in order to provide some degree of automation thus making trial and error calculations much easier (It may sound a bit weird, but I use the good old GWBASIC for that purpose).

Finally, if you have doubts concerning basic component behaviour and forumulas, try to search that forum and try also google (sometimes component datasheets and application notes include precious information).
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Old 4th December 2005, 12:00 PM   #7
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For example when i want to find out the dissipation for my active rectifier i must use the equivalent circuit diagram.
What is usefull for me, i think i have to decide about two models.
First the Rdson and the capacity loss like Pv=Pon+Pswitch
Second only the capacity loss in the switching moment like Pv=Pswitch
And then i will use the biggeset one, i think that is right...
But i'am afraid of the transformer ...
What is important therefore? I think the transformer is the device with the biggest loss or do i get it wrong?
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Old 4th December 2005, 01:00 PM   #8
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Quote:
Originally posted by Wolfgang
For example when i want to find out the dissipation for my active rectifier i must use the equivalent circuit diagram.
What is usefull for me, i think i have to decide about two models.
First the Rdson and the capacity loss like Pv=Pon+Pswitch
Second only the capacity loss in the switching moment like Pv=Pswitch
And then i will use the biggeset one, i think that is right...
But i'am afraid of the transformer ...
What is important therefore? I think the transformer is the device with the biggest loss or do i get it wrong?
You are definitely going to wind up stacking several E-Cores for this job -- this necessitates making your own bobbin. Not that difficult -- if you can get a copy of the ARRL Handbook at your local ham radio club in Germany (they are plentiful) the technique is well described. The XQ2FOD desinged 1/2 kW off line switcher used 5 Amidon EE-77-625 cores (equivalent TDK EE42/42/15) -- and you will need more!

When you calculate the transformer parameters, you need to know both the minimum and maximum current that the powered device will consume (SMPS don't take kindly to devices that idle with low current consumption). You also have to know the range of input voltages that the switching devices will see -- for a car it will be something like 7 to 15 volts.

Just a rough guess -- primary 3.5 turns, secondary 15 turns for 100kHz SMPS -- something between 660uF and 1,000 uF as the output filtration capacitor -- something like 20uH as the output inductor. Should be good from 50W to 1kW.
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Old 4th December 2005, 01:49 PM   #9
mzzj is offline mzzj  Finland
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Quote:
Originally posted by jackinnj
[B]

You are definitely going to wind up stacking several E-Cores for this job -]
1kW from single EE-core should be no problem at all. EE65 should do more than 3kW at around 100khz and EE55 should fit this job wery well, as suggested by Eva.
Single EE42 would be pushing your luck @1kW , but should be do-able if operated around 200-400khz in ZVS bridge.

ARRL article used very low switching frequency because bipolar transistors.
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Old 4th December 2005, 02:23 PM   #10
Eva is offline Eva  Spain
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I agree with mzzj, furthermore, I've obtained up to 1.8Kw output from two E42/21/20 transformers (a pair of cores in each) at 33Khz with some air cooling. However, this was an off-line application and low voltage windings tend to require more space and plenty of coil former pins or soldering the magnet wires directly to the PCB.

Another alternative is toroid cores, as the ones employed in most car-audio amplifiers, see the picture :

Click the image to open in full size.

This one has 6+6:12+12 turns and produces +-23V output. It was operated around 40Khz, I think, and approx. 400W output may be expected from something of that size. Note that output voltage will always sag under load in unregulated push-pull converters due to leakage inductance, but paralelling several small transformers solves the problem.
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