I had similar experience with LM2596s step down power converter from Aliexpress. Similar units are sold in many places so the chances are it is quite common with these units. The whole module cost less than euro a piece and one LM2596s IC from Mouser or Digikey cost ~6 euros. Even if there were huuuuge volume, it still doesn’t seem plausible they could sell these modules so cheap. So I did some measurements and indeed it used fake LM2596s ic. Conversion frequency was ~28kHz, when it should have been 150. I measured ripple voltage to be 134mV. For the sake of testing, I ordered some original LM2596s ic’s from Mouser and changed to one of the units and re-measured. Frequency was advertized 150kHz, ripple dropped to 72mV. However the fake ic was better in efficiency. I supplied 24V to the module and had 5V out with 61mA load. With fake chip current draw was 0.0232A and original 0.0262A, so the fake was actually bit better in efficiency. However I was bit surprised by the quite big ripple voltage with the original ic. Maybe it’s due to bad quality output capacitor? Out of curiosity ordered some Panasonic low esr caps to see whether that makes any differences.
Makes me wonder why the efficiency was so poor, when these were advertized up to 92% efficiency. Maybe it’s due to so low load?