SLUSA75B July 2010 – January 2020 BQ24650
A typical solar panel comprises of a lot of cells in a series connection, and each cell is a forward-biased p-n junction. So, the open-circuit voltage (VOC) of a solar cell has a temperature coefficient that is similar to a common p-n diode, or about –2 mV/°C. A crystalline solar panel specification always provides both open-circuit voltage VOC and peak power point voltage VMP. The difference between VOC and VMP can be approximated as fixed and temperature-independent, so the temperature coefficient for the peak power point is similar to that of VOC. Normally, panel manufacturers specify the 25°C values for VOC and VMP, and the temperature coefficient for VOC, as shown in Figure 17.
The BQ24650 employs a feedback network to the MPPSET pin to program the input regulation voltage. Because the temperature characteristic for a typical solar panel VMP voltage is almost linear, a simple solution for tracking this characteristic can be implemented by using an LM234 3-terminal current source, which can create an easily programmable, linear temperature dependent current to compensate the negative temperature coefficient of the solar panel output voltage.
In the circuit shown in Figure 18, for the LM234 temperature sensor,
The current node equation is Equation 26:
To have a zero temperature coefficient on VREG,
For example, given a common 18-cell solar panel that has the following specified characteristics:
Open-circuit voltage (VOC) = 10.3 V
Maximum power voltage (VMP) = 9V
Open-circuit voltage temperature coefficient (VOC) = –38 mV/°C
Applying the following parameters into the equations of R3 and R4:
The resistor values are RSET = 1 kΩ, R3 = 167.4 kΩ, and R4=10.6 kΩ. Selecting standard 1% accuracy resistors and RSET = 1 kΩ, R3 = 169 kΩ, and R4=10.7 kΩ.