I am designing a discontinous conduction mode flyback converter, 100kHz using UC3844 IC with the spec. 220V,50Hz to 5V 5A. The issue is that after testing it for an hour, the transformer starts heating up (tried with different cores.)
Ambient temperature: 27degC
1st try: EE20/10/6 core, 0.5mm gap, Al=100nH/turns2 (EE20/10/6 Datasheet)
Turns ratio: 21 to keep Vds(max) low
Primary: 1.7mH, 30 SWG, 130 turns, 1 strand
Secondary: 5-6uH, 6 turns, 30 SWG, 7 strands (I know, the diameter that I used is very low but could fit only this in the winding area and the strands have to be increased, for testing purpose designed this)
Auxiliary: 52uH, 14 turns,30SWG, 1 strand.
With this configuration, the transformer was heating up to 80deg C in just 30 min.
From the calculations for the core loss, it was coming to 170mW, but copper losses were huge
Primary: Irms=0.4A, Resistance_dc=(total length)Res. per length, Rp=(7.64130)0.221 mOhm=873mOhm, P=0.40.40.873*1.5=0.2W.
Secondary: Irms=9A, R_dc=(7.646/5)0.221=8mOhm, P=99*.008*1.5=0.972W(This is the prob)
Seeing this with the thermal resistivity of 50K/W, the temperature would rise to almost 80 degC. So, it confirms the problem.
I could not fit more diameter winding on this bobbin, I had to increase the core size.
2nd try: EE25/13/7 0.2mm gap Al=290nH/turns2 EE25/13/7
- Same inductance parameters.
- Primary: 77turns, 30 SWG, 1 strand
- Secondary: 22SWG, 5 strand, 4 turns.
But even then it is still rising to 65-70degC.
Currently working at 78% efficiency.
- What is happening in this case?
- Are my previous calculations correct?
- Is it because AC resistance has increased due to skin effect since operating at 100 Khz because of 22SWG wire, or something else?
EE25/13/7 core: 0.4mm gap, 187nH/turns2 [earlier(340nh/turns2)]
- Primary: 30SWG, 1 strand, 95 turns, 1.6mH
- Secondary: 22SWG, 5 strand, 5 turns, 5.2mH
- Auxiliary: 35 SWG, 1 strand, 16turns.
Although the temp decreased but still hovering around 65degC.