I want to make a wireless sensor node that will run at about 2.5v. I am trying to decide between running 3 alkaline batteries in series with a buck converter or in parallel with a boost converter. I guess if both converters have the same efficiency, let's say 88%, they would run just as long as each other?
The current required for this project should be <200mA
I make this sort of thing for a living. Presumably your sensor will spend most of its time asleep, only waking up to take a reading or send data periodically. In that case a linear regulator might be a better option.
For example, a TPS78225 would give you 2.5V @ 150mA. Keep in mind spikes in current demand can be met by caps and inductors. This device has a 0.5uA quiescent current, so if you device spends most of its life asleep you should be able to get down to the single digit uA range.
Rather than using 3 cells consider using either 4 (2x2) or perhaps a different chemistry (lithium will give you 3.6V per cell). Obviously you need to minimise the voltage drop for the LDO.
This method will likely be much more efficient than any switching system if your idle current is very low. You could even do a hybrid solution where you use an LDO for the microcontroller and switching regulator that is powered up only when needed.
In general, and all other things being more-or-less equal, I2R losses are going to be the chief cost of efficiency in a power converter. Therefore, it would be a goal to keep currents as low as possible.
This would imply that the overall system efficiency will be better with a buck converter than with a boost converter. The average currents in the batteries, coil and the switching element will all be lower.
With a booster you might still be able to get it to work down close to 0.5 volts. If the 3 in series all dropped to about 0 9 volts you'd probably be bust. So it's a booster for my choice but choose your chip carefully. Also, you need to consider that all three batteries need to be of equal charge or there'll be other losses. Not so in series.
Extra info on battery ESR that should be taken into account- the effective series resistance of a "typical" alkaline battery is about 0.3 ohms (see Energizer PDF file). For a series configuration, the ESR will be about 0.9 ohms and for 3 equally "charged" parallel batteries, the ESR is 0.1 ohms.
"I guess if both converters have the same efficiency, let's say 88%, they would run just as long as each other?"
Correct, but efficiency varies with load, so they would have to have the same efficiency profile over your operating range. Also, some controllers/converters have light load efficiency modes that can make a big difference for battery life if your sensor doesn't draw much current or if it has a sleep or energy saving mode.
