# Buck Vs. Buck-Boost for Portable Variable SMPSU

I'm looking to design a variable output power supply that can do 0-30V and up to 10A. I am going to build a custom battery pack for it out of 18650 cells so I have control over the capacity and voltage. The idea was to build a battery pack and then using an adjustable Buck converter or Buck-Boost converter set the output voltage. I've never designed a power supply before but I have a good understanding of how the two topologies work, but I have one question that I can't find the answer too and that is which one would be more efficient?

Since the system is going to run on battery power in the field, extending battery life is my goal so is one better then the other or are they pretty much the same? Would I be better off building a pack with 30+ VDC and stepping it down or building a pack of the same physical size with more capacity at a lower voltage, say 12 VDC, and stepping it up when needed?

• Buck will have better efficiency than buck-boost. You could achieve 80-95% with buck against 75-85% with buck-boost. Numbers depend on many things but you can use them for a simple comparison between these two topologies. Keep in mind that 10A is a big goal for a first design. – Todor Simeonov Oct 4 '17 at 16:57
• @TodorSimeonov Does that still hold true if say I have to run something small like a microcontroller which takes 5v or 3.3V down from a 30~V source? Yeah I know 10A is shooting high. I might drop that to 2 for a first go. – Wired365 Oct 4 '17 at 17:00
• Buck will always have better efficiency than buck-boost compared at the same input and ouput voltages. Of course at light loads (couple of mA) both will have poor efficiency, because the control circuit will draw more current than the output. With the ouput voltage going close to zero both topologies will have poor efficiency, but still buck will be better. – Todor Simeonov Oct 4 '17 at 17:11
• On the other hand if you plan to use this source mainly with 5V or 3.3V output, you could consider lower battery voltage to reduce losses. If you want to get 30V@10A with boost from a 7.2V battery you should source more than 40A from the battery! For a buck-boost it would even be 80A pulses and voltage stress is doubled also. A more efficient approach would be to make couple of power sources - one for light loads - like 0 to 5V@1A and one for the heavy stuff. And both of them based on buck ;) – Todor Simeonov Oct 4 '17 at 17:20
• @TodorSimeonov Ooooh that's a really good point. I didn't think of the huge current draw that could occur trying to get to the upper voltages. Multiple sources sounds like a good idea for what I intend to use them for, thanks! – Wired365 Oct 4 '17 at 17:22