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RX5 4th October 2006 04:03 PM

core/s on SMPS need some info
 
hello all,


can someone tell me in a nutshell WHAT the difference is in using E-E center-GAPPED(2.5mm) ferrite core -versus- no gap E-E core??

I have a real big core here..

wish to drive core at about 90KHz....TL494 in complementary output...

best Regards,
Raff

luka 4th October 2006 07:10 PM

The flux in core will be larger if there is no gap.Gap cores can store much more energy and are also used as choke.

RX5 4th October 2006 10:02 PM

Hi,

so meaning having a GAP in such a core is a good thing for SMPS??

I have done my previous SMPS with NO GAPs.... I dont have idea how it would be if core HAD gaps.. :rolleyes:

sawreyrw 4th October 2006 10:02 PM

Ungapped cores will easily saturate (bad) if there is any volt second imbalance (or DC current) in them. That's why almost all inductors (not transformers) use a gap. Push pull transformers sometime use a gap for the reason mentioned above. As an aside, volt second imbalance can be avoided by using current mode control in push pull circuits.

RX5 4th October 2006 10:07 PM

Quote:

Originally posted by sawreyrw
Ungapped cores will easily saturate (bad) if there is any volt second imbalance (or DC current) in them. That's why almost all inductors (not transformers) use a gap. Push pull transformers sometime use a gap for the reason mentioned above. As an aside, volt second imbalance can be avoided by using current mode control in push pull circuits.


ok... so in push-pull operations using gapped, how about turns? i mean do I have to add more (as compared to ungapped)??

more energy (for MOSFETs) when driving gapped cores?? therefore MORE heatsinks??



;)

sawreyrw 4th October 2006 10:31 PM

A gapped transformer doesn't need more turns, but the magnetizing current will be larger. The flux swing will be the same regardless of the gap. The magnetizing currents will cause slightly higher I^2*R losses, but this is not normally a big deal. Likewise, the current through the switches will be larger. You do need to check the power losses associated with this.

The leakage inductance will be about the same whether or not there is a gap, so the losses associated with them should be the same.

RX5 4th October 2006 10:54 PM

ah ic... so THESE slightly higher I^2*R losses , would be a bit heating of winding, am I correcT? :)

and I think with such slight? heating would mean Higher temp with Driver MOSFETs?

so, i mean WHAT would I normaly expect to get if I changed my working SMPS with a GAPPED core?

same turns ratio and everything, except of course the GAP...

Eva 5th October 2006 02:32 PM

Gapping reduces permeability (and thus magnetizing inductance) and increases saturation current. It also increases the amount of magnetic flux lines leaking the transformer (more EMI). Push-pull transformers should not be gapped, only output inductors and flyback transformers require gapping.

djQUAN 5th October 2006 02:36 PM

I'm assuming you're using this for another car SMPS? :D

then a EE core without gap is the way to go.

RX5 5th October 2006 07:16 PM

Eva,

thanks for the tip... ill keep that in mind... :)



djQUAN,

uhmm yah... got this big E-E core ..gapped.... but since its not good to use GApped in push-pull, then I off again to using U-U core(from flyback,unGApped).. :)


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