9.2.2.6 MPPT Temperature Compensation

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 (V_OC) 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 V_OC and peak power point voltage V_MP. The difference between V_OC and V_MP can be approximated as fixed and temperature-independent, so the temperature coefficient for the peak power point is similar to that of V_OC. Normally, panel manufacturers specify the 25°C values for V_OC and V_MP, and the temperature coefficient for V_OC, 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 V_MP 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,

Thus,

The current node equation is Equation 26:

To have a zero temperature coefficient on V_REG,

For example, given a common 18-cell solar panel that has the following specified characteristics:

Open-circuit voltage (V_OC) = 10.3 V
Maximum power voltage (V_MP) = 9V
Open-circuit voltage temperature coefficient (V_OC) = –38 mV/°C

Applying the following parameters into the equations of R₃ and R₄:

1. Temperature coefficient for V_MP (same as that of V_OC) of –38 mV/°C
2. Peak power voltage of 9 V
3. MPPSET regulation voltage of 1.2 V

And choosing R_SET = 1000 Ω.

The resistor values are R_SET = 1 kΩ, R₃ = 167.4 kΩ, and R₄=10.6 kΩ. Selecting standard 1% accuracy resistors and R_SET = 1 kΩ, R₃ = 169 kΩ, and R₄=10.7 kΩ.