Chinese eBay Switching Converter IC's

[jackbob]

After reading a textbook about power electronics, I wanted to construct some basic buck/boost circuits to study their behavior and better understand these circuits. I could completely design a converter from scratch but my knowledge on feedback systems and control loops for switching converters isn't quite there yet. To somewhat dodge designing the feedback system I bought a bunch of switching regulators such as the LM2596 on eBay. I built the basic circuit with a large output capacitance and inductance to get as close to an ideal converter as possible. The converter worked alright but would often go into a meta-stable mode and could not really deliver the 3A output like the datasheet claims, in fact not really close at all unless the input voltage was very close to the output voltage. Upon further investigation, I found some fishy stuff.

I have attached a picture of the "National Semiconductor" regulator I used in the project. For the price I got them for, no way they are legit. No surprises there. I was however surprised when I probed the PWM output of the converter in a stable mode and found the switching frequency to be 50KHz, one third that of the datasheets 150KHz and suspiciously close to the LM2576's switching frequency of 52KHz. I have attached a capture of this waveform as well.

As for my meta-stability issues, when the input voltage was even mildly high like 10V or greater, the converter would almost never perform as expected. Sure it would maintain the proper output voltage but could rarely deliver more than 1A of output current. The coils would whine at an audible frequency and it was difficult to capture on the oscilloscope due to the constantly changing pulse width causing jitter issues. I did a single shot capture of the waveform and you can see the modulation of the PWM duty cycle, this modulation occurs at an audible frequency. My first thought was that using a large inductor size of 150mH and output capacitance of 2200uF was overkill and causing the converter to go unstable. But from my recent study of buck power supplies, including a larger inductor and capacitor should stabilize the output voltage and current as well as cause the switch current to be closer to the output current with a larger inductor size. I went by the datasheet and lowered the inductance to the recommended value of 47uH and still no dice.

After several hours of double checking the datasheet and playing around with component values, I then checked the switching frequency and found that large discrepancy, my main suspicion then turned to the converter IC. I thought I would share this experience and see what others think of buying silicon off eBay. I'm scared to see as I own a lot of eBay silicon.

[John B]

Ive experienced the same. The LM2596 chips on ebay are probably all the same fakes. I used them for some simple, non crucial things like reading lights, but they certainly aren't LM2596 converters and so the specs in that datasheet are useless.

[thinkfat]

I ended up rolling my own designs (LM2676, LMR14030) after finding that the ebay'd converters never ever match the design specs. There's one particularly nice video on Youtube by one Kerry Wong, who tried characterizing one of the many LM2596 based converter modules and found it very unstable, both electrically and thermally.

https://youtu.be/R32zDhGIGyw