17/02/2018

RESEARCH: Behavior of QX5252F (and probably CL0116)

Intro

The QX5252F (and it's brother CL0116) are a joule-thief type LED driver that can also use a solar cell to charge a 1.2V rechargeable battery (use YX8018 if you want 2.4V). Here I share my findings to try and figure out how this IC works.


Solar Cell Characterization

First off here is the IV & PV curve of the (shoddy) solar cell I made up. The test was done on a hot summer day with clear skies, so results are rough and don't use an exact 1000W/m² lamp.
As you can see peak power (~390mW) occurs at ~1.7V (~230mA).


QX5252F Tests

Circuit

I used the exact same circuit as shown in the datasheet which you can see here:

L = 100uH

Initially I tried setting the inductor (L) to 100uH, interestingly this limited the battery current to ~40mA. This might be relevant to table on pg3 of datasheet, though this table shows how you can set LED current by using different inductor values.

L = 20uH

I then lowered the inductor to 20uH, this time current was not limited and the battery got a much better charge. Also the battery I used had a capacity of 1200mWhr and the QX5252F managed to charge the battery to 925mWhr (77%) for the day.

SBAT to VBAT Diode Drop

From further tests I concluded a few of things:
  1. The battery is charged directly by the solar-cell via a Schottky diode, hence the voltage drop varies with current. What this means is that at a low charging current you have a higher efficiency and at a high charging current you see a lower efficiency; for example with above data the peak efficiency (98.1%) occurred at a current of 0.01mA, while the lowest efficiency (83.8%) occurred at 136.44mA, also the overall efficiency for the day was 86.9% which is pretty close to the datasheet value of 90%
  2. The QX5252F does not have maximum power point tracking (MPPT). Interestingly enough the peak power (230mW) for the 20uH test occurs at Vsolar-cell ~= 1.7V which if you look at the PV curve (different light conditions) is also the peak power voltage. I think this is more to do with me getting lucky with the solar-cell arrangement, as when I used the same solar-cell on a YX8018 while trying to charge a 2.4V battery the circuit would peak at 10mA before steadily dropping to 1mA (see graph below, terrible charging efficiency).
  3. Strangely the inductor value seems to set a charging current limit for the battery, I am not sure how this works as I thought charging the battery occurred via the schottky diode. Also the oscilloscope did not show any switching DCDC converter behavior when charging the battery (light hitting solar-cell). 
  4. When the battery is discharging the operational frequency of the QX5252F is ~133kHz. This is when the joule thief part of the IC springs into action.

Conclusion

The QX5252F is a pretty nifty IC which makes building a simple solar harvesting circuit very easy. A few small downsides is that:
  • You are limited to a single 1.2V battery, though you might get away with using a YX8018 and a higher Voc solar cell
  • You have to choose solar-cell that has a Voc of at least 2.4V (2x1.2V) for it to work properly
  • As you would expect it does not have MPPT, not a biggie at this price point
Also the inductor sets the peak battery charging current (not expected) as well as the peak LED current (expected). I might have had my data logging circuit wrong, so will have to redo this step in the future

4 comments:

  1. I've been experimenting with the YX8018 from a solar light. A very similar IC. I have replaced the battery with a 270 Farad supercapacitor with very good results. The solar cell is a 2v 120 ma cell and charges up the cap fairly well. Replacing the battery will eliminate the short life of the battery charging and discharging as the capacitor has unlimited cycles. It seems the circuit will run at a fairly low voltage on the cap and plan to chart the discharge. Will let you know of my results.

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    1. Hey there. Sorry for the late reply, my spam filter was a bit too strong...

      Oh cool, thanks for sharing that!

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  2. My solar circuit with the 5252 when switched on won't charge the battery, when switched off I receive the solar panel voltage at the battery terminals. Other than that the circuit operates as it should. So if the battery is fresh it will work until it wears out. Not sure why Have changed out 5252IC already? Thanks, Harold.

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    1. Hey there. Sorry for the late reply, my spam filter was a bit too strong...

      Hmm, I guess the first question is what sort of circuit are you trying to power? Like is it a single LED or do you have something more fancy connected?

      If it's just an LED then maybe your 5252 (or some other component) is faulty? In that case try making a new charging circuit with fresh components, as in don't reuse anything

      If it's something fancy connected then maybe it's playing up with the 5252 light sense function? You could try add a schottky diode between the 5252 output and your secondary circuit

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