I use this step down converter with input 30V DC and output set to exactly 5V. At the output I connected an ESP8266 (WEMOS D1 mini, has 3.3v regulator on its board) and an MP3 player module (DY-SF5W.) One thing is, I experience sparking on the input side of the step down converter module whenever applying power to it. But more concerning, I destroyed my load at the output (the MP3 player module DY-SF5W.)
I set the output to exactly 5V and connected the ESP8266 and the mp3 module to the output of the step down module. After powering up the step down converter 2 or three times (30V DC at the input), the mp3 module was destroyed. The microcontroller on the mp3 board smoked and you could see it glowing. The voltage was definitely set to 5V, and the ESP8266 was still working ok. I then connected another MP3 player board/same model, which first worked ok, but again after 2-3 times powering up the input of step down converter, again the uC of MP3 player board smoked exactly as the first one. The ESP8266 again survived.
I then monitored the output of the step down converter closely with oscilloscope, all looked fine, but after setting time base to 25ns, I could see that at the moment when connecting the input voltage to the step down regulator, its output VCC spikes to plus 18V and down to minus 10 volts, but only 25-50 ns in length, and then back to 5V. The question is, is it possible that this very short Vcc change is dangerous to certain parts, and why I do not find more results when searching google for damages due to that? Even with a 4700uF cap at the output of the step down, I sometimes get those spikes. Another step down converter module of same type shows the same spikes. How can I securely get rid of them?