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LM5168: LM5168: Transformer construction support

Part Number: LM5168
Other Parts Discussed in Thread: LM5169


We are designing a multi output power supply using the LM5168F with the following specs :Input : 20 – 60Vdc. Out1: 5V/50mA non-isolated, Out2: 6V/150mA, Out3: 6V/50mA

Using the datasheet and spreadsheet calculators - L = 66uH for selected Fsw = 680KHz. Hence we designed the transformer with higher L of 80uH.

The output are generated but the voltage for Out2 drops for 20Vin and full load. Full load is supported beyond ~45Vin.

Next we changed the transformers to two new types(2 & 3) as below and tested again.
1. L = 80uH, AL = 90 (first version we tested)
2. L = 124uH, AL = 145, same turns as 1,primary turns split 50-50, higher L achieved by using higher AL core
3. L = 90uH, AL = 90, higher turns than 1 & 2, primary turns split 50-50, same core as 1, higher L achieved through higher number of turns

Support for full load is best with 124uH transformer except slight drop for Out2 at 20-24Vdc. Drops are much lower for 90uH transformer also compared to type 1.

Does this indicate that L values below 125uH are not able to transfer adequate energy to the isolated secondaries?
If yes, please suggest the L we should try next - around 150uH?
Also, to achieve this there are two options - more turns or higher AL - which is preferable? 

2nd query: Please advise the recommended voltage rating for Ca and Cb for a max Vin of 60V.

Best regards,

  • Hi Tanmay,

    At 20Vin, it looks like you may be violating the current limits, which is why you may see a slight drop in output, and that is related to the inductance value, as it is possible that the ripple is hitting the current limit. You may try increasing the inductor value, but if performance is adequate at 124uH, then no need to change. I'm not too familiar regarding whether changing the turns or higher Al is preferable, you may ask the inductor vendor. 

    As long as Ca and Cb are above VIN rating, I think it is ok. In the EVM, Ca is rated to 100V, and Cb is rated to 50V. 



  • Hi Richard,

    Thanks for the reply.
    We will have to increase / fine tune the L as the Sec1 o/p drops to ~5.65V and after LDO final o/p is 4.65V which is not adequate for 5V systems.

    The behaviour is the identical with LM5169 also (in same setup) which has double the current capacity, so current limit violations seems unlikely.
    Also, for transformer with an L of 90uH also, which as per my understanding would have a higher current than 124uH, the drops are only slightly worse.
    So if not current limit, can we deduce that total energy stored/ transferred to isolated side is inadequate at Vin min and hence the drop at full load?

    If yes, we will have to increase the L. Is there any method to estimate the increase required or will we have to estimate in % terms?
    Kindly revert at the earliest possible.

    Best regards,

  • Hello Tanmay, 

    Richard is currently out of office, but will respond to this thread as soon as he returns. Thank you for your patience. 


    Oscar Ambriz