Yesterday, I built a very simple DIY solar-powered USB charger for my TP-link 10400mAh USB Power Bank. All I needed was a 6V/3.5W solar panel and the TD1410-based 5V buck converter module. I bought both of them on Aliexpress for less than $8.
It was one of the easiest projects I built. All I needed to do was to connect the input of the 5V step-down buck converter to the output of the solar panel using two wires.
From TD1410 datasheet,
The TD1410 is a 380 KHz fixed frequency monolithic step down switch mode regulator with a built in internal Power MOSFET. It achieves 2A continuous output current over a wide input supply range with excellent load and line regulation. The device includes a voltage reference, oscillation circuit, error amplifier, internal PMOS and etc.
For 5V output from TD1410, the input voltage can range from 5.5V to 20V (from the datasheet). I measured the solar panel open-circuit voltage under sunny conditions to be ~ 6.5V. Under the load of 200 mA, the solar panel output voltage was reduced to ~6.0V. The TD1410-based 5V buck converter module that I purchased on Aliexpress already has a diode at its input for polarity protection. So I didn’t need any extra diode for the solar panel output as it directly goes to the input of the buck converter. The nice thing about this buck converter is it has a USB-A female port for output, which is same as found in standard USB chargers. It also comes with a plastic enclosure (see pictures below).
For more detail: A very simple DIY solar-powered USB charger