I am designing a flyback converter circuit using the LT8302 flyback converter IC. Now, the efficiency of this chip is approx 85% as shown from the datasheet and the screenshot -


The value of 85% is an approx. Ofcourse, it depends on the load current and such. Now, this IC has an internal MOSFET for the switching controlling. The Rds of the MOSFET is 80mOhms. Now, when it is said that the efficiency is 85%, I understand it means approx 85% of the input power is able to be transferred to the output.

But, does this also include the power dissipated in the MOSFET as well ? And the power transferred ,does this mean the power transferred to the primary of the transformer or to the secondary ?

Because, the flyback transformer can add/introduce its own inefficiencies, right ?


The curve shown in the graph is meant to be a typical performance chart, and it represents the overall input-to-output efficiency for the specific circuit shown. It includes all of the losses, including the MOSFET inside the chip, the external diode, and other miscellaneous losses.

It's just meant to give you a general idea of what kind of efficiency you can expect from a circuit built using the chip. They generally come up with a circuit that shows the chip in its best-case scenario, so you can expect that any other circuit you build can only do as well or worse, but never better.


The eighty-five percent is going to be just for the IC. It will conserver ~85% of the input energy and push it into the flyback primary. The other 15% probably goes to heat and random fields inside the device. A manufacturer would never be able to rate the efficiency of their device plus a third-party flyback, because the flyback efficiency id entirely dependant on design which varies by manufacturer and also by model.

  • \$\begingroup\$ This is basically incorrect. It's virtually impossible to isolate and measure the chip losses alone. \$\endgroup\$ – Dave Tweed Mar 21 '16 at 12:57
  • \$\begingroup\$ They could use a flyback with a known efficiency, and do a bit of arithmetic to figure out that of the chip. \$\endgroup\$ – KilowattLaser Mar 21 '16 at 18:40

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