My daughter got a toy train which uses AA sized battery cells to operate. It has space to put two AA cells in parallel, so it requires just 1.5 V to work. To avoid buying AA cells, I tried to make it rechargeable with a 18650 lithium-ion cell with protection board having USB port. But…
Attached document demonstrates calculations involved for a SEPIC converter. Calculations are based on “AN-1484 Designing A SEPIC Converter” by Dongbing Zhang from Texas Instruments.