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Old 22nd December 2008, 03:23 PM   #21
dtproff is offline dtproff  United States
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I'll agree with Conrad...

Please don't be discouraged.

Eva is a wonderful Engineer and I've read many of her posts and they are always insightful. She is correct that a well layed out PCB will be necessary to squeeze all the performance out of the circuit that you appear to want. You should also keep in mind that only 10% of the design is getting the correct volts and amps. The other 90% of the design is gettign the fail-safe modes of operation, EMI, and margin testing of the stresses on the components corrected. It's the old saw of 10% inspiration and 90% persperation.

I assumed when I saw your equations at first that you were using McLymans book. Maybe you were or not... Just be careful, sometimes listed equations are plain wrong and at other times they have been simplified with fudge factors that can come back to bite you later.

If you need help... Ask and there is probably someone here that has already screwed it up that way before and can help get you out of the weeds.

I will caution you that in general a 1KW PSU is for the very advanced PSU engineer. Be careful, we would hate for you to have to type with only one hand.

Take care and the best of luck.

Happy holidays.

Tony
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Old 24th December 2008, 07:42 AM   #22
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Winding your own transformers works, but it's hard to get each winding 100% perfect or even. Toroids are not the easiest, and as mentioned here, it's hard to tell exactly what core is used for the toroid.

Something that has worked really well for me is to use the transformers from old computer SMPS power supplies. Those cores are already wound for you, and they handle anywhere between 40-100khz with ease. Some are CT on all windings, some not. I've had great success with push-pull using the 12V winding on the transformers as in input for the 12V, and taking the output from the 120V winding. You can also input on the 5V winding and use the 12V winding as a step-up output as well. These cores are great and don't saturate much, and hardly require much dead-time. I've run mine at max pulse width from TL494/594 with no problems.

Another tip that may help, although not completely necessary, is to make 2 snubbers from 0.1uf + 50ohm 10W power resistors, one for each 12V input half of the winding. The snubbers help with inductance spikes that can occur, and help to keep the mosfets cooler.

Also since you are wanting to build an offline Mains SMPS in the future, a computer SMPS transformer would work here too - since that's what they are designed for.
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Old 24th December 2008, 12:21 PM   #23
Eva is offline Eva  Spain
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Specific RC snubber values have to be found for each circuit. There are posts describing methods to do so and with links to related documents. The formula F=1/RC usually gives a good approach, where F is the frequency of the ringing to be damped.

The values given are wrong and result in very high losses in the resisors because 100nF is really high. When the transformer and capacitor are optimum, a 1W resistor is usually enough. I use 0.25W resisotrs in the RC snubbers of my car amplifier.

The problem with E core transformers in 12V push-pull applications is asymmetry between windings that may lead to saturation under heavy load (when I*R voltage drop becomes quite different for each winding).

Have you found the posts that explain how to wind a push-pull toroid properly?
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Old 25th December 2008, 04:07 AM   #24
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Quote:
Originally posted by Eva


The values given are wrong and result in very high losses in the resisors because 100nF is really high. When the transformer and capacitor are optimum, a 1W resistor is usually enough. I use 0.25W resisotrs in the RC snubbers of my car amplifier.

The problem with E core transformers in 12V push-pull applications is asymmetry between windings that may lead to saturation under heavy load (when I*R voltage drop becomes quite different for each winding).

Have you found the posts that explain how to wind a push-pull toroid properly?
I used 10W resistors so they are oversized. The 100nf would explain why the resistors get hot. I was mainly making the point to use snubbers, and I'm pretty sure that my values are not perfect, but I'm sure any ringing is damped out. I saw Rod Elliott's SMPS using 100nf and 2W 56 ohm resistors, so I did similar to that, but with what parts were available. Thanks for the info about the cap, and that will keep snubbers cooler.


I'm aware that E-cores may not be perfect, but the ones I have salvaged and used measure pretty evenly. I'm more than sure a better toroid can be wound, but I mentioned the Computer PSU transformer as an easy solution for a beginner that may not have their transformer quite right. It worked for me as a beginner before I wound my own toroids.
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Old 25th December 2008, 06:13 AM   #25
Eva is offline Eva  Spain
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10W resistors are usually wirewound and very inductive, which may make the snubber ineffective (or may make it better if you tune snubber RLC self resonance to match the circuit resonance to be damped).
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Old 25th December 2008, 09:24 AM   #26
aandy is offline aandy  United Kingdom
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i got some big toridal cores in f44 3c90 Very good results as well got >1kw out of the one i have here. 60mm od 35mm id and 25mm high.
i think the thing to do is to find power ferrite and not the common -2 and filter types that need the gap and have low flux
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Old 8th January 2009, 01:36 AM   #27
B.VDBOS is offline B.VDBOS  New Zealand
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