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DC-DC 120W power supply

I need to design the power supply for an automotive application, to run from car or truck supplies, i.e. 12V and 24V DC nominal supply under various conditions, essentially 11V - 30V DC input.
Output DC voltage needs to be controllable 20V - 34V DC.
Maximum input current 11A DC approx.
Maximum output current 3.6A DC. (full power not required at the lower output voltages)
Maximum output power 120W.
I have designed many switchmode powers supplies into products but mostly at lower power levels, using simple inductors in buck or boost mode, for which the webench tool has been very useful.
Having considered various topologies for the power supply circuit, I think that the main alternatives are forward converter, interleaved forward converter or push-pull transformer configurations. Possible driver ICs range from the simple LM5030 and 3525 to such as LM5045 but my main problem is how to specify the transformer and the optimum oscillation frequency.
Help would be much appreciated.

5 Replies

  • Paul,

    Thank you for your interest in TI parts. We do have a controller that can meet your needs, and it is the LM25118 buck-boost controller (

    There was no mention of a need for isolation, so I have assumed you don't need it. However, since your stated input voltage can be less than or greater than your output voltage, you will need a topology that can meet such requirements, like this buck-boost. This controller will operate as a boost when Vin<Vout, and it will transition to a Buck when Vin>Vout.

    Please visit the link above to access the datasheet for the device, and to start designing with the WebBench tool in which you can enter your design requirements. This tool will help you determine some parameters like an optimum Fsw.

    If isolation is required, then there are a number of other topologies available, at the expense of an additional isolating transformer. The specifications of the isolating transformer will depend on the chosen topology.

    At 120W, you would be near the upper limit for a flyback. While a flyback would still work, the current stresses would start to be prohibitive. 120W would best be addressed with a forward converter if indeed you do need isolation.

    Thank you.


  • In reply to Martin Cardella:


    Thanks for your advice.

    Having studied the data sheet and the webench results, I feel that the 120W requirement would be stretching this topology too near to its limits although the device would be useful for future applications.

    I am looking at the possibility of a LM5030-based push-pull design using a planar transformer to achieve 90% efficiency and to have satisfactory EMC performance. To meet the low end input voltage requirement, I would run the device Vcc from a 8V regulator (also used elsewhere in the circuit) as otherwise it would not start up reliably. Is this a reasonable solution or is there a more suitable device? As for the transformer, Payton may be able to provide a suitable part. Are there any other transformer manufacturers that you would suggest?



  • In reply to Paul Dixey:


    The LM5030 is a good choice if you want to use push-pull and a transformer. It will give you the freedom to extend the power range if required.

    Since we don't design or provide magnetics, I can't steer you to any specific vendors.

    Thank you.


  • In reply to Martin Cardella:

    Hello Martin,

    I am trying to use the LM25118 controller for a school project. Following the datasheet of the product I was able to get most of the values for my components. However, It is not clear to me how to get the input and output capacitors, as well as the components for the error amplifier configuration. I also wanted to simulate the design in SPICE (this is the software i am most familiar with) but I could not find a model for it.

    my specs for my project:

    Vin = 11V - 16 V

    Vout = 12V

    Iout = 8A

    Also, I just used WeBench for the first time to simulate, and I does the job but I would like to know where all the values came from.Thanks you.


  • In reply to Luis riveros:

    Hi Luis,

    Thank you for your interest in TI parts. While I can't offer any experience with that particular device, I can give you some general information on designing general devices within a non-isolated Buck switcher.

    I have attached a design example paper for a device I am familiar with, the TPS40422. This paper has not yet been released so use at your own risk. This device is not equivalent to the LM25118, but designing input caps, output inductors and output caps is pretty much the same process.

    Also attached are two very helpful papers which should help with loop compensation design.

    I hope this helps.


    6837.DRAFT TPS40422 Design Example.pdf

    2311.SEM300 1984 Appendix A Error Amp and Compensation Network Design.pdf

    0066.SEM300 1984 Closing the FB loop.pdf

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