Buck-boost converter from LT3791-1

oktoober 22nd, 2013
by jaanus
Finished prototype, some mosfets and big inductor.

Finished prototype, some mosfets and big inductor.

I'm building quite high power (for me) buck-boost converter for a friend. It takes in 12-45 V and has to output regulated 24 V. Total output power has to be more than 300 W. I don't understand switching regulators well enough to build more than 100 W regulators from 555 timer (that is basically how your computer power supply is built). Just too much phase shifting and output oscillations and whatever else to take into account. You know, the things you learn in electronics classes (which there was none in my school).

LT3791-1 in the middle of a debugging session.

LT3791-1 in the middle of a debugging session.

So I took the highest power buck-boost controller I could find - LT3791-1. It satisfied all my specifications so I calculated what parts would I need to make a 300 W board out of it. After ordering stuff and soldering it together it looked awesome. But after some playing around I couldn't get it to boost more than 100W, buck worked well but boost didn't. After lots of debugging and bunch of fried chips I finally figured out that the MOSFET drivers are too weak. My big and high power mosfets didn't even get warm at 300W output, but the chips internal voltage regulator fried all the time.

So, lessons learned from high power:

  • Every watt adds problems to MOSFET driving
  • Don't over specify parts three times

Next prototypes will be smaller and there will be several of them in parallel. Maybe some pictures soon.

Posted in Small projects | Comments (8)

8 Responses to “Buck-boost converter from LT3791-1”

  1. Albert Kurucz Says:

    Very interesting project!
    Have you tried to measure the power loss?
    A chart showing the efficiency as a function of output power would be extremely helpful.

  2. jaanus Says:

    Yes, I have measured. In both modes the power loss was in the order of magnitude of 1 W. Even while giving out more than 150 W. But I don't have better numbers before I buy myself some more good multimeters.

  3. Sleepwalker3 Says:

    Got a circuit diagram?

  4. jaanus Says:

    I have it but can't share. I'm building it as a contract work and I'm not allowed to share the schematics. But it is quite similar to typical application circuit.

  5. Placido Says:

    Good job!!
    I have just a question: How did you solve the problem of weak MOSFET driver in order to reach really high power ?

    Thanks for your help and good luck

  6. MarkR Says:

    Another option to look at is a multi-phase design with current sharing. It is how the high current regulator on motherboards get 100A.
    A side benefit of the multipase, is the ripple currents smooth out because one of the phases is always on.
    Its not the total wattage that causes problems, its the Iout which create the need for large MOSFETs with their large gates. ANother option is to place a gate drive between the controller and the FETS, but that creates it's own issues.

    Finding a multiphase buck-booth is the challenge. It can be hacked with some of the LT parts that accept sync inputs. An op-amp is used to measure current through a sense resistor (usually just a sized trace) and the output of the amp is used to drive the FB of the 2nd switcher.

  7. jaanus Says:

    I solved it with smaller mosfets, cascading regulators and adding heatsinks. Not an pretty solution but working one.

    Actually this same chip can be cascaded to make it a multiphase one, it also has application notes for current sharing.

  8. nelsoro Says:

    I've been looking for the same design for a while. The most important feature being the parallel/load sharing capability. My application requires a 240 watt output module and ability to stack up to four of this things. If Interested drop me a line

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