This power supply is based on the LM2596. The LM2596 series of regulators are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator, capable of driving a 3A load with excellent line and load regulation. These devices are available in fixed output voltages of 3.3V, 5V, 12V, and an adjustable output version.
There are many of these sold on the internet, typically the board looks like this.
LM2596 DC 3 40V to DC 1.5 35V Buck Converter Step Down Transformer Module
The schematic for this is similar to the one below
Lm2596 PSU
Links
LM2596 datasheet
1 Pcs DC-DC LM2596 Step Down Adjustable Converter Power Supply Module
LM2596 DC-DC Adjustable Step-Down Power Converter Module+LED Voltmeter+USB Port