Buck vs Boost Converter

[rentner]

I am thinking about, what type of DC/DC Converter is more efficient. I was actually about, to build a Buck converter, but than I thought about: Hey, is a boost converter possibly more efficient?

I want the highes efficiency possible for a "beginner" like me.

[8086]

You generally need to decide which to use based on your input/output voltages.

If in < out then you need a boost
If in > out then you need a buck

[rentner]

Well, that was not the question. I will vary the Input voltage based on the fact, which one is more efficient. Or are they equaly efficient, if calculated properly?

[smashedProton]

They are both in the 90 percent efficient range.   However, using a boost is attractive because you only need one cell which can potentially have a higher capacity

[dr.diesel]

If you can properly adjust the source voltage then I suggest neither, as there is always loss. 

You probably need to explain your application in more detail.

[Falcon69]

they make them that do both.

http://www.mouser.com/Power/Power-Management-ICs/DC-DC-Switching-Regulators/_/N-6g7lw?Keyword=buck+boost&FS=True

[c4757p]

they make them that do both.

In a pinch, if your power input is floating, you can use an inverting converter to get whatever negative voltage you want, then just rename the new negative rail "ground" and the existing ground "positive". Efficiency usually isn't as high, though.

[Rufus]

They are both in the 90 percent efficient range.   However, using a boost is attractive because you only need one cell which can potentially have a higher capacity

Buck is inherently more efficient. For a given output current the rms current in the switch and inductor is lower and only a proportion of the output current flows through the diode.

[Rick Law]

If you are not sure what source voltage you got, and what final voltage you need, the best is first boost then buck.  This gives you the most flexible solution at the cost of efficiency (80% range best case).

Use any wall-wart with sufficient power at any voltage within range (say an LM2577, it will take anything from 2 V range to about 30 V.   Now boost that 2-30V to the LM2577’s max -around 27V.  Buck that down with the LM2596 to the desire end voltage by adjusting just the LM2596.  So you end up with a Boost-Buck that will accept any input voltage (2-30V range), and output any voltage (1.25 to around 27V) and merely single adjustment at the LM2596.

I use a cheap board that does the job just that way.

But, the lower the input voltage, the less the efficiency - just the Laws of Physics.  So if your wall-wart is merely 2Volts and you want 20V out, be prepare to see efficiency well below 50%.

Rick