Saturday, February 17, 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 (2.4V will now work, for this use YX8018) rechargeable battery. 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 charging current to ~40mA. This might be relevant to table on pg3 of datasheet, but this is do LED current not battery current ?_?

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. I don't think the QX5252F does any 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 choosing an appropriate 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 (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 an inline diode. Also looking at the oscilloscope wave-forms I did not see any switching DC-DC converter behavior when charging the battery (light hitting solar-cell). 
  4. When the battery is discharging the operational frequency of the QX5252F is ~133kHz.

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, you have to choose solar-cell that has a Voc of at least 2.4V (2x1.2V) for it to work properly, and there is no MPPT. 

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