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flyback transformer windings temperature

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franticEB

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Hi,
i'm designing an isolated DCM flyback with this feature:
Vin_min=36Vdc
Vin_nom=48Vdc
Vout_max=63Vdc
Vout=250V
Iout=0.2A

flb.png

At the moment the problems are on transformer windings.

The transformer i've made is RM8 N87 gapped AL 250nH with 10 turns of primary (AWG 21 equivalent litz wire) and 86 turns of secondary (AWG 26).

The secondary winding goes to 90° after 3 minutes since startup.

I did not expect this increase in temperature from the calculations I had done.

Could you help me in understanding this fact?

Thanks
 

How many feet of wire are in the secondary?
Have you measured the primary current when under load?
 

The primary current in the primary under load is 2.6Arms and 6.75Apkpk.
 

According to my hand calculation, the secondary winding losses are almost negligible. But the core is driven into saturation, resulting in huge losses.
 

Are you sure about the secondary wire size?
Do you mean 36 AWG instead of 26 AWG?
With primary 10T of 21 AWG and secondary 86T of 36 AWG, the fill is already 96%.
 

Do you mean 36 AWG instead of 26 AWG?
With primary 10T of 21 AWG and secondary 86T of 36 AWG, the fill is already 96%.
Click to expand...
?? RM8 coil former has 30 mm² area.
 

You can maybe squeeze it in there, but you will have no insulation/space safety margins.
 

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FvM said:
According to my hand calculation, the secondary winding losses are almost negligible. But the core is driven into saturation, resulting in huge losses.
Click to expand...

I'll try to make again the calculation but i dimensioned the transformer for B=0.2T.
I used coil former for power application

coil.JPG
 

I'll try to make again the calculation but i dimensioned the transformer for B=0.2T.
Click to expand...
Would be easier to verify having all related data.
 

What is your Primary Inductance?
What frequency are you runing at?

Low frequency will cause excessive heating on that transformer.
 

franticEB said:
The secondary winding goes to 90° after 3 minutes since startup.

I did not expect this increase in temperature from the calculations I had done.

Could you help me in understanding this fact?
Click to expand...

The first step is to calculate the true rms currents from the expected triangular current waveforms.

DCM flybacks are notorious for having very high current peaks, with short narrow pulses that produce extremely large crest factors.
The average output of 200 mA sounds harmless enough, but the current peaks can be many times that, and the true rms current much higher than expected.

Try to keep your conduction periods as long as possible using all the available time at maximum load, and at minimum input voltage, by not making the air gap any larger than absolutely necessary.

Also be fanatical about skin effect, it will make a much bigger difference where very high crest factors are involved. Reducing the switching frequency may be an option ?
 

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