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Old 10th January 2012, 06:17 AM   #1
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Default Calculating power handling of cores?

Hi. I've done a search, and tried to read thru a bunch of literature, but am lost...
I would like to calculate the size of RM core I need for a flyback with a given power output. There must be some formulae I am missing. I know how to calculate the necessary AL value to stay out of saturation, and also to find the minimum wire size, but not how to figure out which core size. I havent found it in the datasheets either, but I am sure it is right in front of me ;(
Any help would be appreciated.
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Old 10th January 2012, 11:03 AM   #2
dtproff is offline dtproff  United States
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Basically, you can run any amount of power through any core if you can get enough turns in it or can run it at a high enough frequency.

However here are a couple rules of thumb, for RM10 you can run a 65W flyback continuously at full power using about 52 turns and 65KHz as your swithcing frequency. Peak power handling capability in a non-aircooled enclosure would be about 90W on a 10% duty cycle (Bmax<370mT for Tmax=100C using PC44 material and a u=2400). You can then scale power and accordingly.

I hope that helps.

Tony
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Old 10th January 2012, 01:19 PM   #3
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Thanks Tony. Thanks for reply.
I am trying to make this very compact and was hoping to use two RM6 in parallel for about 30W rated output at a max duty cycle of a little over 40%. But it seems the core temp rises quickly when I pass 25watts, or 12.5watts per tranny, so that must be RM6s max. At least in this setup.
So now I'm testing RM8, but not sure if they will fit inside the enclosure I am hoping to use.
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Old 10th January 2012, 03:12 PM   #4
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See: www.ti.com/lit/ml/slup127/slup127.pdf Basically, the core size is determined as an area product. The main factors are the maximum flux density and the wire current density.
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Old 10th January 2012, 03:25 PM   #5
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If you PM me with your e-mail address I've got a 20 odd page datasheet that might help you.

It details how the various parameters of a ferrite core are determined and how they can be applied.
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Old 11th January 2012, 04:13 AM   #6
dtproff is offline dtproff  United States
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Quote:
Originally Posted by sawreyrw View Post
See: www.ti.com/lit/ml/slup127/slup127.pdf Basically, the core size is determined as an area product. The main factors are the maximum flux density and the wire current density.
The area product came originally from McLyman's work. It's a decent approach but does not fully utilize the core.

For 30W continuous you will need about 60 Ae. I am not a big fan of paralleling cores on a flyback as I get more leakage than I want. Anyway. Best of luck.

Tony
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Old 11th January 2012, 04:16 AM   #7
dtproff is offline dtproff  United States
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I have done 50 Ae for 30W but it is a very tricky design. The thermals get tricky.
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Old 11th January 2012, 08:27 AM   #8
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Thanks folks.
It looks like RM8s do the trick, two of them in parallel gives me 30watts continous with only a slight temp increase. The parasites seem to be under check, only some ringing on the MOSFET drain, but nothing nasty.
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Old 11th January 2012, 11:05 AM   #9
dtproff is offline dtproff  United States
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Nicely done.

Good lock.

Tony
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