Design a Li-Po battery charger with small solar panel and charger controller with embedded MPPT

Question

I'm designing a small solar charger with these characteristics:

SOLAR PANEL

• Voc = 5,65 V
• Vmp = 4,25 V (Voc -25% approx.)
• Isc = 250 mA

IC CHARGER CONTROLLER

STMicroelectronics SPV1040 High efficiency solar battery charger with embedded MPPT

BATTERY

• Type Li-Po (Model LP804367, FULLWAT)
• Nominal Voltage 3.7 V
• Nominal Capacity 2400 mAh
• Standard Charge Current 480 mA
• Max Charge Current 4800 mA
• Charge cut-off Voltage 4.20+/-0.05 V
• Discharge cut-off Voltage 2.75 V

I used the ST Electronics eDesignSuite tool and THIS is the schematic of the design calculated with this tool

QUESTIONS

When I have entered the characteristics of the solar panel in the eDesignSuite tool, I have introduced as open circuit voltage the value Voc = 5.2 V which is the maximum input voltage allowed by the IC SPV1040T. However, the Voc value of the solar panel used is 5.65 V, so I thought of using a zener diode in parallel with the capacitor Cin to not exceed this the maximum voltage allowed at the IC input (5.2 V).

1. Is this correct or is there a better solution than using a zener diode to clamp the input voltage?
2. Could the zener diode decrease to the efficiency of the MPPT algorithm?
3. If using a diode zener is a feasible solution, could this zener diode be used? (1SMA5918BT3G)

Answer 1

Sorry, that chip is largely unsuitable for your needs. It is a boost converter i.e. generates an output voltage that is higher than the input supply voltage. Given that your SP ranges between 4.25 volts and 5.65 volts AND you only require a maximum output voltage of 4.2 volts, you should be looking for a buck (rather than boost) controller.

The block diagram below shows the transistors that prove to me it's a boost converter: -