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Old 11th August 2006, 03:59 AM   #11
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Hi Crissty,

Ralphs99 and I are on the same page: Increasing your frequency will enable you to reduce the required core size, and the number of turns required for all your windings. For the wire sized you ahve selected, you should be good for about 50kHz (switching) frequency before having to worry about skin effect. All of the major manufacturers of ATX (and older AT) supplies wind their heavy-current secondaries (5V section) either bi-filar or tri-filar. I have rarely seen a quadri-filar winding. So, bumping your switching frequency up to 35kHz should be no problem.

As promised, here is the design procedure Chryssis uses for selecting the input cap. It is somewhat backwards, as it calculates the effifiency, and the I(in), first. Anyway, here it is:


P(out) = 800W, P(in) = P(out) / h, where h is your efficiency coefficient. So, say for h = 0.8 (80%) efficiency, your P(in) = 1kW.

Now, we find the input current:

I = P(in) / V(in - nominal) = 1000W / 360V = 2.778Amps.

Now we calculate the Cap's value:

C = [I x t] / DV,

where t = the charging time of the cap, 10mS for 50Hz, and 8.33mS for 60Hz, and DV is the max. desired ripple voltage.

So, for our equation, we get C = [2.778A x 0.01S] / [20V] = 0.00138888888888888F, or 1,388mF.

Therefore you should use two 680 mF, 400V caps in parallel, or any combination of 400V caps to equal roughly 20% over the rated cap value.

As for your main switching MOSFETs, I would recommend the 500V IRFP460 (I(d) = 20A). A 500V rating would give you the option of later replacing your non-PFC'ed front end with an active PFC. Of course, your caps would now want to be rated at 450V for safety.

Ralphs99 also plays it safe for his creepage distance. While almose all regualtory agencies (UL, CSA, VDE, etc.) require a minimun of 4mm for the creepage distance between the high-voltage and low-voltage sections for all circuits over 60VDC, 6mm is more preferred, again for safety's sake.

Give us your thoughts and feedback, but DO please consifer raising the switching frequency.

For your secondary, I further agree with Ralphs99 in using a center-tap winding. This way, you will be able to use the first two diodes to collect the (+) voltages and the other two diodes to collect the (-) voltages for a bi-polar (+/-) output. Ultra-fast rectifiers are OK, but if you can find some Schottkys with high enough PIV rating, I would recommend them, as they have a lower forward voltage drop.
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Old 3rd December 2006, 01:53 PM   #12
crissty is offline crissty  Greenland
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Attached image of near complete SMPS, just secondary side circuit is not made (i.e. Bridge & filter)

I tested the circuit giving input of 230V (rectified 320V DC) thru a series 200W lamp (it glows as started and goes off immediately i.e. till the cap is charged.

Very little heating on the hetsink, right now i am giving 12V regulated thru seperate transformer & 7812 to the circuit.

I have a 15V tap on trafo, i wanna know what value of startup resistace from +320V to positive voltage for circuit, so that it should be able to startup the circuit and be capable of fully charging the Mosfets properly.

Progress is little slow and steady, now to add output bridge and filter, and checking for fast bridge rectifiers @20A. I have 2-3 rectifier of old PC SMPS but it is only for +ve rectification.

Regards
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Old 3rd December 2006, 02:01 PM   #13
crissty is offline crissty  Greenland
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Attached image of near complete SMPS, just secondary side circuit is not made (i.e. Bridge & filter)

I tested the circuit giving input of 230V (rectified 320V DC) thru a series 200W lamp (it glows as started and goes off immediately i.e. till the cap is charged.

Very little heating on the hetsink, right now i am giving 12V regulated thru seperate transformer & 7812 to the circuit.

I have a 15V tap on trafo, i wanna know what value of startup resistace from +320V to positive voltage for circuit, so that it should be able to startup the circuit and be capable of fully charging the Mosfets properly.

Progress is little slow and steady, now to add output bridge and filter, and checking for fast bridge rectifiers @20A. I have 2-3 rectifier of old PC SMPS but it is only for +ve rectification.

Regards
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File Type: jpg smps completed1.jpg (38.6 KB, 350 views)
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Old 3rd December 2006, 02:02 PM   #14
crissty is offline crissty  Greenland
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Attached image of near complete SMPS, just secondary side circuit is not made (i.e. Bridge & filter)

I tested the circuit giving input of 230V (rectified 320V DC) thru a series 200W lamp (it glows as started and goes off immediately i.e. till the cap is charged.

Very little heating on the hetsink, right now i am giving 12V regulated thru seperate transformer & 7812 to the circuit.

I have a 15V tap on trafo, i wanna know what value of startup resistace from +320V to positive voltage for circuit, so that it should be able to startup the circuit and be capable of fully charging the Mosfets properly.

Progress is little slow but steady, cause of time constaints, now to add output bridge and filter, and checking for fast bridge rectifiers @20A. I have 2-3 rectifier of old PC SMPS but it is only for +ve rectification.

Regards
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File Type: jpg smps completed1.jpg (38.6 KB, 286 views)
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Old 3rd December 2006, 03:06 PM   #15
crissty is offline crissty  Greenland
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OK now, no load current drain from AC 230V is 0.06A (i.e. - 14W), checked this with still in series with 200W bulb.
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Old 3rd December 2006, 03:29 PM   #16
crissty is offline crissty  Greenland
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Output voltage cheked as 33.5V with an LED as Load.
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Old 3rd December 2006, 03:41 PM   #17
crissty is offline crissty  Greenland
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Now used a fuse instead of 200W Bulb , everything OK, the no load current fluctuate from 0.06A to 0.07A and the output voltg\age reading shows 34.5V with Led as load.
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Old 3rd December 2006, 05:05 PM   #18
Eva is offline Eva  Spain
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How about an oscilloscope and an isolation transformer? You can't fully succeed in such a project without these items, particularly you should analyse switching waveforms and suppress RF ringing (you must have a lot as that seems like one of your first projects).
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Old 4th December 2006, 02:12 AM   #19
crissty is offline crissty  Greenland
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i will post the switching waveforms soon, and above procedures are pre startup checks.
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Old 4th December 2006, 03:19 AM   #20
crissty is offline crissty  Greenland
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Waveform on the gate of Mosfet
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