Simultaneous Buck-Boost or Flyback Regulator – Help Selecting Chip

powerpower supplyswitch-mode-power-supplyswitching-regulator

I have been trying to find a switching DC power supply that outputs a constant voltage of 12VDC, given a variable input between 4V and 25V. It's to my understanding I'd need a buck-boost converter, but many of the buck-boost converters I find on Digikey seem to show only a single application circuit for buck, and one for boost for the same chip (ie: no circuit that can do both simultaneously).

I am currently using the LM2577, and interestingly enough, it can accept a 12.5V input from a wall wart, as well as a 9V battery, and still has a constant 12V output. I don't know if for this type of chip though, if it makes sense for the input voltage to be greater than the output voltage, because it is a boost/flyback regulator.

So the first part of my question is: can someone recommend a good simple through-hole DC-DC converter that can take an input on the range [4V, 25V] DC and output a constant 12V DC output?

Second, I'm a bit confused on the terminology on Digikey, and Google has only gotten me so far. Could someone point my in the direction of a description of the various topologies? I get what a buck, boost, and buck-boost chip is, but the multiple combinations of boost/buck/forward-converter/flyback/inverting is somewhat daunting. Does flyback imply it is a buck-boost, even if the title is "Step down (buck), flyback", etc? Some such examples are:

  • Step down (buck)
  • Step down, flyback
  • Step down (buck), step up (boost), flyback
  • Step down (buck), step up (boost), flyback, inverting
  • Step up (boost), Cuk, Sepic
  • Switched capacitor (charge pump), flyback

Best Answer

A traditional buck-boost converter (and the Cuk converter) invert the input polarity, so you can get -12V for your 4-25V input voltage range. What's confusing is that there are also buck-boost converters that usually use a 4 switch topology to either buck or boost depending on Vin, and do not invert the polarity. (As does a SEPIC converter.)

A flyback can also use the transformer turns ratio to give outputs above and below the input voltage.

A charge pump's operation depends on the configuration of the switches and the capacitors and the control scheme.

You don't say how much current you need, but something like this part from LTC could work