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Help needed with power supply design

Other Parts Discussed in Thread: TPS43060, PMP7966, LM5122

Hello all,

  I have been tasked to design a circuit that would have the following specs:

Input: 6V - 28V 

Output: 48V, 1.35A

Operating temp: 0C to 85C

Not being a power engineer, I turned to WBench for help. I got a design using TPS43060 that required 30C ambient temperature and also a custom inductor of 46.3uH and 3mOhm DCR.

Does anyone have any ideas how to go about this? 

Thank you for your time and help.

-- A

  • Hi Amer,

    The low input voltage will be your main limitation so I recommend designing to this. WIll the circuit sit at 6V for an extended period of time or is this some type of short transient (milliseconds time scale)?

    If the 6V minimum is DC condition (seconds time scale), to get up to 85 deg C ambient, you will likely need to parallel the low-side FET to split the power dissipation. WEBENCH shows 78 deg C temp rise when only one FET is used.

    As for the inductor you will want to look at some of the larger inductor families. For example the Coilcraft SER2915H-153 is 15uH with 11.5A RMS current. Also there is the Wurth 7443632200 which has 22uH and 12.5 RMS current rating you could use. These values should work for you since the peak current at minimum VIN can be the more important.

    Best Regards,

  • Hello Anthony,

      Unfortunately the input power may be fixed at any point within the input range and may be used for hours at a time and not for seconds as you state.

      What I also don't understand is how does the ambient temperature play in the design.  My operating temperature spec range is between 0C and 85C so plugged in the max value for the ambient temp.

    Could this be solved with a couple of synchronous controllers in parallel? Something similar to this:

    Thank you 

    -- A

  • One thing you have to remember.  Webench calculates junction temp based on JEDEC standard coupon (usually).  Your PCB may have better performance thermally.  I just did a case were the thermal performance of an EVM was 2x better than the Webench prediction.

  • Designing for 6V input is the best idea then. To adjust for the new ambient temperature a good first estimation is to use the value estimated in WEBENCH to get a change in temperature. Then add this change in temperature to you max temperature. For example, with the WEBENCH design I generated the FET temp would be 78+85 = 163 deg C. In reality it may get a few degrees hotter as Rdson increases with temperature.

    Also in this case the TJA is a good starting point for the FET. The low-side FET in a boost typically doesn't get a lot of copper around it to help with it's thermal dissipation. The pad is connected to the switching node that should be kept as small as possible.

    The LM5122 is another good solution for your application and would likely be higher performance. Using it to put two power stages in parallel will allow you to use 2 inductors of half the size which gives more inductor options. Also it will put parallel low-side FETs to split the power dissipation into two. I think the LM5122 has a 2 phase EVM you could order to prototype a 2 phase design.