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BQ25570 max solar panel size

Attila Eredics
Prodigy 195 points
Other Parts Discussed in Thread: BQ25570, BQ25505

Hello,

I have an application which runs on a solar panel to charge a Li-ion battery (of about 2200 mAh). Since the device will possibly be operated in low light conditions (e.g. in the forest) I'd like to "oversize" the solar panel, if possible. This way the relatively large panel area can generate more power even under low light conditions. But on the other hand, under full sun the oversized panel could give more power than necessary (or the power handling capability of BQ25570).

My application circuit with BQ25570:

  • VBAT_OV = 4.2V
  • VIN_DC = 0.5 ... 4 V depending on the selected solar panel
  • boost charger indutor 22uH (Wuerth WE-TPC 744031220) rated current 510 mA(max), saturation current 360 mA(typ).

The datasheet says that the abs. max. power rating of BQ25570 is 510 mW and the input current capability of the chip is about 100 mA. So with a 4V 100 mA panel I'm on the safe side.

If the panel provides more current (Isc (short circuit) > ~100 mA) the BQ25570 regulates the input current to about 100 mA. I measured the following setups:

  • Solar panel: Uoc = 0.5 V, Isc = 850 mA (Pmax ~ 425 mW), measured input under full sun: U = 0.4V, I = 126 mA
  • Solar panel: Uoc = 1.0 V, Isc = 200 mA (Pmax ~ 200 mW), measured input under full sun: U = 0.95V, I = 119 mA
  • Solar panel: Uoc = 2.0 V, Isc = 380 mA (Pmax ~ 760 mW), measured input under full sun: U = 1.9V, I = 115 mA

The above measurements show that the current limiting occurs at about 120 mA.

However the third panel has a power rating which clearly exceeds the abs. max power rating of BQ25570 (760 mW > 510 mW). My questions are:

  • Is it safe to use solar panels with maximum power > 510 mW, since the chip will limit the input current anyways? I mean panels with Uoc <= 4 V.
  • How is that current limiting done? The excess power is converted into heat? If so, which component dissipates the excess energy?
  • Can I increase the power handling capability e.g. with better cooling?

Thank you very much!

  • 0
    TI__Mastermind 20200 points
    The maximum power rating of the panel does not equal to the VOC * ISC. At VOC, the panel does not have any output current. At ISC, the voltage of the panel should be zero. Most of the solar panel the maximum power point occurs around 80% of VOC. Solar cell specification should have the maximum power point (MPP) spec. Looking at your measured data under full sun, V = 1.9V and I = 115mA, this data could possibly be your mpp voltage and current. The resulting power, 218.5 mW, is lower than the abs max spec of the bq25505.
  • 0 in reply to Jing Zou
    Prodigy 195 points

    Dear Jing Zou,

    Thank you for your answer.

    You are right, that my MPP power calculation was wrong, therefore I made additional measurements with the Uoc=2V (nominal) Isc=380mA (nominal) panel. I used a lamp as a light source and varied its distance and angle to obtain different light levels. I measured the currents with a DMM and the average voltage with an oscope. I always waited min 16 sec to wait for the Uoc sampling of BQ25570 and the stabilization of the control loop. Since Isc is proportional to the illumination level, I plotted all data against Isc. The results are below:

    On the bottom graph you see how Uoc and Umpp varies with the light level. The dotted blue line shows 80% of Uoc. Until Isc=115mA Umpp approximately tracks the dotted 0.8*Uoc line as expected.

    On the top graph You see Impp (red line) increases linearly until 115mA, and approximately Impp = 0.98*Isc (based on the mesaured data).

    Pmpp is calculated as Impp*Umpp.

    The 'Pmpp theory' line shows the extrapolated Pmpp without the Bq25570 current limiting action, calculated as: 0.8*Uoc * 0.98*Isc. You see, that this dotted line closely follows the measured Pmpp values until Isc=115mA illumination level.

    So the panel IS ABLE to deliver more than 600 mW of MPP power (dotted purple line), but due to the current limiting action of BQ25570, the input current never exceeds 115mA, Umpp can reach max 2.02V thus total power input is max 232mW (solid purple line).

    So my question remains:

    • is it safe to use this panel, since it could deliver more power than 510mW (abs. max. rating of BQ25570)? But in reality the input power can never exceed 232mW, due to the 115mA current limit.
    • Because this current limiting always occurs, in theory I can use solar panels as huge as I want (even 1m2, with 100W nominal power), as long as the panel mpp voltage does not exceed 4V, since 4V*115mA = 460mW. Am I right?

    Thanks,

    Attila

  • 0 in reply to Attila Eredics
    Prodigy 20 points
    I'm interested too, so the original questions remain:

    - Is it safe to use solar panels with maximum power > 510 mW, since the chip will limit the input current anyways? I mean panels with Uoc <= 4 V.
    - How is that current limiting done? The excess power is converted into heat? If so, which component dissipates the excess energy?
    - Can I increase the power handling capability e.g. with better cooling?

    Could someone please respond?
    Thanks.
    Enrico
  • 0 in reply to Enrico Rosalli
    TI__Guru**** 151155 points
    Hi Enrico,

    The IC measures input current and turns off the boost converter power FET when that current reaches IIN(DC)=200mA typical. The 200mA is fixed and cannot be changed.
  • 0 in reply to Jeff F
    Prodigy 20 points
    Thanks. Everything is clear now