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LM5161: LM5161 with wurth-750314597 and primary inductance calculation

Part Number: LM5161


I am thinking to design isolated Power supply using LM5161 and two 750314597 transformers whose primary is connected in parallel to be driven by LM5161.

750314597 specs : L_PRI = 60uH; PRI:SEC ratio :: (1:1.5) ;

According to calculation With Vin(max) =36V, Vin(nom) = 24V, Vo(buck) = 12V, Fsw=350KHz, Isec(max) = 0.5A, Vo(sec)=18V and considering Lpri = 30uH ( paralleling of 2 750314597) ;

Primary side ripple current Ipri-ripple(p-p max) would be 0.739A @ 36V input supply and Ipri-peak  = 1.12A.

Here, Ipri-peak is below LM5161 max current rating of 1.3A but the value of Ipri-ripple(p-p max) seems well above recommended 40% approximation.

I would like to know : what will be the adverse effect over the long period of time of operation, with this parameters of design?

I can minimize the value of Ipri-ripple(p-p max) by driving the 750314597 combo at 650KHz.

In this case, how it will affect the transformer itself and what will be its outcome over the long operation?


  • Hi

    I will not suggest to use 30uH, the calculation is based on no any power loss when transfer the energy from primary to secondary, but the actual Iprimary_dc will be larger  than calculation result when we consider the efficiency

    second, you only mentioned the normal operation voltage is 24V , how about min operation voltage? for flybuck , the desired duty in all working range need to be smaller than 0.5 which make the transformer have enough time to deliver the energy to secondary. Smaller (1-D)*Tsw, worse load regulation. so increase the fsw also will make the load regulation poor.