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Old 27th October 2005, 02:51 PM   #1
corrieb is offline corrieb  South Africa
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Default SMPS transformer overwinding

Hi All,

Can anyone perhaps give me a close estimate for how many volts to over wind to compensate for a particular amps/wattage required?

I have wound a transformer with the following specs.

It is regulated at 35v but over wind to 59v. When loading the smps I can pull 290watts continuous without dropping below 35v and I am pulling 8.66 Amps. Now I wonder are there perhaps a standard of volts to over wind per amp required?

thanks
corrieb
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Old 27th October 2005, 03:02 PM   #2
Did it Himself
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You overwind a little (10% or less) to account for losses. That is all. If you overwind to 59V for a 35V supply you have huge regulation problems and your core is simply being grossly overloaded.

Accounting for losses, the transformer will supply the correct voltage at the specified current, and if the drawn current is less the voltage will rise a bit inline with the regulation factor of the transformer.
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Old 27th October 2005, 03:13 PM   #3
Eva is offline Eva  Spain
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It's not so easy to determine and some basic data is required:

- Input voltage range. It would be usually 220V to 350V DC for conventional 230V mains, accounting for some ripple and extreme voltage drop in areas where mains power quality is very bad.

- Maximum effective duty cycle, including the effects of leakage inductance, that effectively shortens each switching cycle by delaying the rise of transformer current until it reaches the value of output inductor current. It would be usually 85% or more at full load, but note that the value decreases as output current increases, so a worst case approach should be taken.

- Voltage drops. This should include switches, primary and secondary windings, diodes and output inductor, and it may be analysed for primary side and secondary side independently. Let's assume 4V for primary side and 2V for secondary side. Note that this also increases as output current increases, so a worst-case approach should be also taken.

With these data, minimum transformer turn-ratio may be calculated:

(Vin-Vdrop_pri) * ratio * max_d_c = (Vout+Vdrop_sec)

(220-4) * X * .85 = 35 + 2

X = (35 + 2) / (216 * .85) = 0.2

So 0.2 secondary turns are required for each primary turn, a 5:1 ratio if we look at it in the other way.

This is only an example, you should calculate it more thoughtfully according to the numbers of your prototype, altough the result will probably be similar.
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Old 28th October 2005, 10:26 AM   #4
corrieb is offline corrieb  South Africa
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Richie00boy,


What I can't place is the fact that when I remove the feedback regulation and let it run at max duty cycle which is 42.5% in a push-pull circuit running at 40Khz I get the 59v ouput. But when I start to load it, the volts start to drop. When I pull my full current which is 8 Amps the voltage is around 35v.

Now correct my if I am wrong please. I am using the TL494CN controller as my pwm. If I wind a transformer to deliver say 35v, the pwm will start up to almost max duty cycle. Now when I load it, the smps will try to deliver the current requested by adjusting the duty cycle. Now if the pwm is already almost at the max how is it going to compensate for the current required? For this reason I thought that you need to over wind to compensate for the current amount required when regulating the output.

thanks for you feedback and your help is much appreciated!
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Old 28th October 2005, 11:48 AM   #5
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Usually the transformer is wound so that at full load current the voltage is correct. Then under any lighter loading the voltage will rise in accordance with the regulation of the transformer (this is all ignoring duty cycle and loop regulation for the moment).

So if your input voltage is 12V and your wanted full load output is 24V you wind a 1:2 ratio with maybe an extra 10% turns on the secondary to account for losses. If you find that under full load the voltage drops below what you wanted, maybe your primary has too high losses or your core doesn't have high enough flux density to cope.

If your duty cycle is not 50%, then you may need to adjust the turns ratio. If you are using 42.5%, try increasing the secondary ratio by 7.5%.
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Old 28th October 2005, 12:13 PM   #6
corrieb is offline corrieb  South Africa
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Richie00boy,

I understand what you are saying.

My core that I am using is a toroid from MagInc and has an Ae of 1.16cm^2 ; OD 49.1mm; ID 33.8mm; H 15.9mm and I am using 6+6 turns on the primary and 28+28 on the secondary for a flux density of 1450 Gausses P-P. Currently the primary is wound first and the secondary on top of it. The problem that I have in taking the flux density up is the fewer turns, if I use for instance 3+3 turns on the primary wound in a bifilar way, there is no way that I will get around the core. Will it affect the core if I only wind the primary in the one side for instance?

If you want I can give you the formulas that I used to generate the results of the transformer for me. I derived them from the Switchmode powersupply handbook from Keith Billings.

How do you recommend I wind the transformer?

thanks,
corrieb
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Old 28th October 2005, 01:08 PM   #7
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If you only have a few windings to do - be they pri or sec - they should be evenly distributed around the core not just at one section.

The primary turns is a balance between losses (lots of turns) and saturation (too few turns).
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Old 28th October 2005, 02:30 PM   #8
Eva is offline Eva  Spain
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corrieb :

Sorry, I misunderstood you, I thought you were working in an offline application. Well, you have to find out where is coming so much voltage drop.

Do you have an oscilloscope at hand?
Are you using a fully regulated push-pull with inductors connected just between the output diodes and the output capacitors?
More details would be appreciated.

Concerning your transformer, with 6+6 turns my calculatios predict approx. 100mT peak flux. This is quite low for most materials, so you may consider going for 4+4 primary turns and 160mT, this will also have a great impact on voltage drop, since leakage inductance will be halved. Note that the flux density that should be considered is not peak to peak, but just the peak value in one direction, and up to 200mT may be ok for most materials.

Furthermore, if you are using a fully regulated topology with output inductors, you can go for 3 turns and 200mT peak since maximum duty cycle and maximum input voltage won't happen at the same time (except during load transients, but this is not a concern from the point of view of core losses).

The best way to place the windings is to spread all them evenyly around the toroid. Primaries should be bifilar and as symmetrical as possible, and same applies for the secondaries.

Should 3 turns be too hard to wind, you can reduce the switching frequency to 30Khz (60Khz clock) and use 4 turns instead.
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Old 28th October 2005, 02:50 PM   #9
corrieb is offline corrieb  South Africa
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Eva,

Thanks for the reply.

- I do have a scope.
- For the full regulated push-pull, yes I am using it just as you said.
- I will go a draw a diagram in electronic format this weekend and post it, currently it is paper.
- Thanks for the tips on the flux density on the core it self, it has always been a concern to me.
- For the winding of the core, which winding should go first on the core the primary or the secondary?


Regards,
corrieb
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Old 28th October 2005, 03:23 PM   #10
Eva is offline Eva  Spain
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With an oscilloscope you can do a lot of things to know from where the voltage drop comes the first one to check is what voltage is actually being applied to the primaries...

Concerning windings, they should be ideally placed at the same level, so wind the dual primaries thight and with all wires evenly spaced over the core, and then wind the secondaries over them trying to fill the gaps (take a look at toroids from other power supplies, some chinese amplifiers feature really nicely wound toroids, altough I don't fully understand how they manage to do that).

Also, with nearly a 5:1 turns ratio you may consider using two 2.5:1 transformers and connecting their secondaries in series.

Concerning the calculations from my first post, since you are using the same topology, the formula still applies.
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