[ARC5] Hash on Chinese DC-DC converters

[Leslie Smith]

  Hello Ben/All,
  Some time ago an associate bought a function generator.
  It was "clever" - I could program it, vary the duty cycle in a square
  wave and so on.
  It cost close to $2000.

  My associate had a very simple task for this function generator - he
  wanted a sine wave at a precisely settable frequency.
  I connected my (fairly accurate Fluke)  frequency counter to the
  output and measured the frequency.  
  "Spot on" according Mr. Fluke.

  Then I set it to beat against our local ABC radio station on 1512kHz.
  Strange - nothing heard.  I re-checked the frequency using my
  dependable Fluke.   There it was - a frequency of 1510 or 1513 - or
  whatever I asked for.
  Listened for the beat note again, etc.  Nada ...  etc, etc, etc ....

  It turned out that the feedback loop for the PLL was under-damped - so
  that the signal "rushed up and down" on either side of the programmed
  frequency.
  Over a period of 1/2 to one second Mr. Fluke counted the correct
  number of Hz - but the "function" was FM, not a steady CW signal.
  
  
  My guess is that your DC-DC converter has a under-damped feedback loop
  controlling the output voltage..
  My guess is that you can see two super-imposed signals.
  The high frequency "hash" comes from the switching transistors.
  The 170Hz sine wave (I guess) comes from a lower frequency feed-back
  loop controlling the DC output.

  That's my guess, but I guess that your guess would be the same as my
  guess.


  73 de Les Smith
  vk2bcu at operamail.com


On Sun, Oct 18, 2015, at 08:33, Ben Hall wrote:
> Afternoon all,
> 
> I bought two of the Chinese DC-DC converters, so I decided to hook up 
> and test the second one to see if it had the same "sine wave" like
> output.
> 
> For the record, these are the ones I bought:  <http://r.ebay.com/MfTEUQ>
> 
> This first photo is a shot of the output of the converter set at 300 VDC 
> and about 10 mA load.  Scope settings - 2 volts per division Y, 2 msec 
> per division X.  Here the "sine wave" is of approximately 170 Hz, and 
> you can see the higher frequency trash riding on top of it:
> 
> <http://www.kd5byb.net/DCDConverter/ChineseDCDC_01.jpg>
> 
> The second photo shows a zoomed in section of the "sine wave" so you can 
> see the switching pulses.
> 
> <http://www.kd5byb.net/DCDConverter/ChineseDCDC_02.jpg>
> 
> In this third photo, I was trying to show that the switching pulses ride 
> on top of the sine wave:
> 
> <http://www.kd5byb.net/DCDConverter/ChineseDCDC_03.jpg>
> 
> IMHO, something is goofy with these converters.  The 170 Hz wave in this 
> converter and the ~300 Hz wave in the other converter suggests to me 
> that something is goofy in the voltage regulator control loop.
> 
> I'm going to take one apart to see if I can guess at the schematic. 
> Yes, the one chip is marked in Chinese characters so that might be a 
> bust, but I suspect I can at least figure out the pin functions.
> 
> thanks much and 73,
> ben, kd5byb
> 
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

-- 
http://www.fastmail.com - Email service worth paying for. Try it for free