Electronic – Step-down Regulator Voltage Drop / Frying

Boost converters are typically less efficient than Buck converters, but not by much. The fundamental reason has to do with the inductor current flowing directly to ground during the on-time, instead of through the load, like it does on Buck converters. There is an EE-Times article that mentions this: Unscrambling the power losses in switching boost converters

Generally, however, since inductor current flows to ground during the on time, only a fraction (off time to period ratio) flows to the output, as illustrated by the pulsing currents in Figure 2 (this is the reason why boost converters are generally less power efficient than buck converters)

Since the efficiency differences are not significant, you are probably better served deciding on additional criteria instead of regulator efficiency alone, including:

• Charger complexity (single cell is simpler).
• Cost
• Size
• Cells in series will be limited by their weakest cell.
• Cells in parallel tend to charge one another, and there is an efficiency hit due to the chemical process.
• Losses due to higher input current with lower voltages (parallel cells).

The LM2576 needs a high current path for the input.

You can't run the wimpy output of a prox sensor into the input and expect it to work.

Connect the LM2576 input directly to your 24V supply.

temporarily ground pin 5 (ON/OFF) and verify that the power supply is actually delivering +5V.

Then disconnect pin 5 from ground and attach it to the bk wire of the NPN version of the sensor.

That should allow the +5V output to be controlled by your sensor.