Electronic – How to achieve the max efficiency from a Buck converter

buckpower supplyswitch-mode-power-supplyswitching-regulatorvoltage-regulator

I have a question about switching step-down regulators.
(As I stated in my previous questions, please consider the fact that that I'm not very expert, so feel free to reply/talk as if I were a student.)

Let's take a practical example of a switching step-down regulator, based on this IC.
(I've seen that is largely used and common in various circuits):

We need to feed a device which needs 12V with a power consumption of 200mA.
Ok: We'll take a buck converter circuit, and as Vin we'll provide, for example, a voltage of 30V from a batteries pack with a total capacity of 2000mAh, then we will set the Vout of the buck converter to 12V. But If we want to make use of a less number of batteries we can also go with a Vin of 20 or less volt: I have read that for the lm2596 IC, the Vin, should be at least greater of 1,5V than the Vout.

I was thinking: If I reduce 30V (from a batteries pack) to 12V, the difference of 18V could be reason of an higher power consumption from the batteries? Am I right? Eg I know that linear regulators (differently from switching regulators) have a bad efficiency because some of the power will be lost as heat. But what about switching regulators? Some days ago, by a search on Google, I've read of a person which had the needs to get 5V using a Buck converter: someone told him that would be better get the 5V from a Vin of 18V instead of using a Vin of 12V.

So, taking again in consideration my example: when using a switching regulator, is better to start from an higher Vin, for obtaining a same Vout? Why?

I'd also like to see some charts of the switching regulators.

Best Answer

TI's got a tool, named WEBENCH which can make a lot of charts and calcualtions for you. Here is its output with your parameters in pdf.

Let me highlight the one about the efficiency. The simulations shows that this IC has a better efficiency when Vin is 20V, but this difference is not that much.

enter image description here

It is not just the Vin that matters, if you change the supplied current from 200mA to 3A a different efficiency chart will be shown. In this case the Vin = 30V is the better choice.

enter image description here

Usually, there are similar charts in the datasheets if tools like this are not available.

If you only need 200mA, you should choose converter which is capable of, let's say 300mA maximum current rather than 3A, efficiency is better near the max current. Another converter, which can drive max 300mA, LMR14203's efficiency chart:

enter image description here

It is again the worst at 30V, but it is around 88% while with the LM2596 it is 79% which is a significant difference. On 20V it is above 90% which is pretty good.