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Old 19th November 2008, 10:59 PM   #1
diyAudio Member
 
Join Date: Sep 2005
Default newbie can multiple EE cores be paralled for better current handling

Hi Im a newcomer in smps design I mainly do pc repair and tv and audio/video repair and Im venturing into hobby electronics.

So I have about twenty or more burnt atx psu rangin from 300watss to 550watts and I want to use their EE cores to make a power supply for an amp.

Now I have started to do a little research and Im inclining for half bridge or full bridge topology, I found a lot of info around the net on what to consider how to wind a transformer and everything is ok as long as you buy the core specific for your need yet I wish to use these core that I have at hand and thats where I get stuck.

I have a couple of things stuck in my head and if someone could help me sort them out it would be greatly apreciated.
  • 1. can these atx EE cores be paralled somehow to increase current handling.

    2. if not what would be the best voltage/current these cores can handle

I dont specify spec. because I dont have a a specific need Im just researching and learning to build smps and want to see how I can use these cores maybe after I have working psu I would design an amp for it.

Once again thank you very much for your time,please excuse my english its not my native tongue.

Have a nice day
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Old 21st November 2008, 03:18 AM   #2
diyAudio Member
 
Join Date: Jul 2003
Location: Dona paula, Goa
I have a simple logic

An iron core E&Is are anyhow paralleled in a transformer in a conventional psu.
U should try it but all the cores should be of the same types.
In other words, now u will have a transformer like the EI but with the ferrites.

This approach is not done, so what. Nothing wrong in trying.

Only difficulty will be if u design a smpsu fresh, then u will not know which is creating problems(Elex or trafo) in case they occur, as both are not proven (to be working).

This logic may be totally wrong. It's DIY for economy and learning.

Gajanan Phadte
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Old 23rd November 2008, 10:48 AM   #3
dtproff is offline dtproff  United States
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Join Date: Oct 2008
Location: Plano, TX, USA
I'll chime in a bit here.

If the transformers are from the same model number (meaning they are probably wound the same. You can parallel the transformers.

The thing to remmember is that SMPS supplies are very finicky about their transformer. For example a single switch forward will be designed differently that a two switch forward which is different from a half-bridge which is different from a flyback or LLC... You get the picture?

Also when you parallel the primaries the current will double so the current sensing scheme has to change. Also the increased current may change your loop parameters in the feedback and it may go unstable.

I hope this helps a little.

Tony
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Old 30th November 2008, 03:40 PM   #4
diyAudio Member
 
Join Date: Sep 2005
Hi thanks for the replies, I will only use the cores, that is I want to wind the coils myself thats why I ask what would be the best voltage/current combination they can handle, I will wind both cores exactly the same and used in parallel and do the math for current sensing scheme and loop parameters as if it was a single core or should I design separete PSU and and tie outputs in series parallel config to achieve desired output.

the second option has more parts count and would cost more in the other hand I have most parts needed from the surplus atx psu boards
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Old 2nd December 2008, 12:10 AM   #5
TechGuy is offline TechGuy  United States
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Join Date: Apr 2006
Location: Connecticut
Doubling the core size will roughly permit you to run twice the current. However there are some issues. For instance to handle the additional current you must increase winding cross section to prevent copper losses causing thermal runaway. You can do this by using a thicker gauge wire, or parallel two wires with the same gauge (perferred). But, using additional copper may exceed the winding area of the transformer (you won't be able to wind all of the turns necessary). Using multiple thiner gauge wire is perfered because its much easier to wind (less gaps and wasted winding space), and you will get better coupling between the primary and secondary, resulting in reduced inductance leakage and noise.

Generally stacking multiple EE core sets allows you push through the same current load, but at half of the switching speed (to reduce switching loses). Doubling the core size virtually doubles the amount of current you can push through for any given half cycle before the core reaches saturation.

Imagine a box with a pendulum in it. The size of the box limits the swing distance of the pendulum. Now image that you doubled the size of the box, you can swing twice as far. As doubling the box size would be simialar to doubling the core size. Each swing represents one full switching cycle of the transformer.

Swinging the pendulum into the box wall would the same reaching core saturation. You need to avoid saturating the transformer because input current inrush will spike, either destroying your switching transistors (most common result), or blowing the fuse.

The problem is that in order to make use of the extra swing distance when you double the box size, you also have to increase the length of the pendulum (decreasing the switching frequency) or increasing the mass of pendulum (increasing the gauge of the winding for more current flow).

Hopefully my pendulum analogy makes it easier to understand and not end up confusing you more.
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