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Top258 Design Frequency, Cable Cross Sections and Transformer Heating

Posted by: Abdullah on Tue, 06/01/2021


In the TOP258YN chip (132kHz) design that we have designed in PI expert, it indicates the cross-section of the cable used in the primary winding of the transformer as 24 AWG.
However, as can be seen in the table with cable properties in the appendix, the skin effect occurs in 24AWG 68Khz. For this reason, our circuit overheats under 24V 4A operating conditions. The most heated element is observed as transformer. When we reduce the switching frequency, we are asked to increase the Lp. In summary, is it normal to use a 24AWG section cable in 132khz design? What could be the reason for the heating of our transformer and circuit? We also use a good heatsink for TOP258 and output diodes. At this point, we ask for your help. Required documents are attached( I hope I was able to explain.

Best regards,

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Submitted by PI-csbabu on Fri, 06/04/2021

Hi Abdullah,

        The wire table properties are based on sinusoidal current. In the current flyback design unit is working in continuous current mode of operation at full load, so the primary current has ac ripple plus dc bias current. The tool optimizes the design and suggests the wire AWG as a starting point. Based on primary RMS current, 2X 24AWG is good for your design. Are you using single 24AWG or two wires in parallel? Based on design, you have some window area, so I suggest to experiment with higher AWG and more strands in parallel.

     If you would like to design with 66kHz switching frequency, need to change primary turns to avoid core saturation and Lmag to deliver full power. Please use PIxeprt tool to optimize design with 66 kHz switching frequency.