I'm designing a 48V to 12V, 300W buck converter. I am considering using the LM5148 controller and NTMFSC004N08MC switches. I'm confused about how synchronous DC-DC controllers can list efficiency curves without knowing which FETs the user is using (let alone caps/inductor).

eg. LM5148 efficiency curve:

enter image description here

I can't fully attribute any system losses to the controller other than the IC operating losses, which are minimal, without knowing the switch/passive parameters. I understand that the controller's dead-time and switching frequency affect most switch losses (gate charge losses, output capacitance losses, dead-time losses, etc.), but those losses are still dependent on the switch parameters as well.

Perhaps I'm misunderstanding this graph as full system efficiency with some average part selection, when it is instead efficiency of the controller IC itself?

Note - I did some simple modeling of the controller + switches in MATLAB. I varied the input voltage (40-56V) and power output (100-300W) and calculated all switch losses/full system efficiency at 100kHz (without passive/IC operating losses). This resulted in ~97-99.5% efficiency across the surface. This doesn't exactly line up with the efficiency curves (although the curve listed is 5V out and stops at 10A, plus I chose an expensive FET).


1 Answer 1


The curves are typically measured on the manufacturer's demo board, or on a reference design that the manufacturer has done showing how to use the controller.

You're exactly right that it may not be representative of what you might get with different FETs and a different inductor. (Or even other passive components, or switching frequency selection if available.)

The goal of the graph is just to show what kind of typical efficiency is possible for the given part.

  • \$\begingroup\$ One demo board linked on p.42 of the datasheet, but the switching freq in Fig 7.1 suggests the test circuit may be from p.46. \$\endgroup\$
    – user16324
    Commented Jul 14, 2023 at 19:42

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