Is it possible to get a high efficiency buck-boost converter by switching between the two?

Many buck-boost converters work by boosting to an intermediate voltage, then bucking to the desired voltage. What if instead, it had a separate buck and boost section in parallel, and switched between the two based on input voltage and desired output voltage.

For example, if the converter is set to 15 volts, and it's being supplied with 12 volts, the buck side would be disabled and the boost side would be boosting the 12 volts to 15 volts. Likewise, if you set it for 5 volts, the boost side would be disabled, and the buck side would drop the voltage to 5 volts.

Could this lead to high efficiency buck-boost converters?

• Sure thing! Have you looked at a “buck-boost” with just one inductor and considered the pros and cons over an “buck+boost” with two inductors? – winny Feb 3 '18 at 7:36
• Such a solution would be a challenge to design around the Vin = Vout transition region, as winny mentioned, single inductor BB operates as a buck converter, boost converter or buck-boost converter depending on the Vin vs Vout ratio. – Dean Franks Feb 3 '18 at 8:17

Consider the most efficient type of buck controller; a synchronous type: - Then, in order to turn it off when the parallel boost section becomes active you need a MOSFET on the output like this: - Then consider that you need a couple of MOSFETs (or a MOSFET and diode) for the boost section with it's own inductor and it should become clear that there is a simpler solution namely the buck-boost bridge controller.

Take the drawing above and add another MOSFET (shown in red): - Now compare that drawing with this: - And this one to understand how it works: - Explanation here and below is a real chip that does this: - So, in conclusion, if you begin the process of designing a parallel buck-boost, then if you are shrewd enough you'd notice that the best option is make a H bridge controller because it saves on a MOSFET (or diode) and an inductor and is at least as efficient as what you propose.