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Over-voltage protection for device with photovoltaic cell source

I have a solar panel rated for 6V open-circuit and up to 150mA. I would like to connect this to an [SPV1040](https://www.st.com/en/power-management/spv1040.html) or similar buck-boost converter with MPPT functionality to be able to charge a 3.6V battery under cloudy conditions.

The problem is that the SPV1040 has a maximum input voltage of 5.5V, while the panel is rated for 6V and I have measured up to 7.5V in sunny conditions. Of course, 6V is an open-circuit specification and the 7.5V measurement was open-circuit as well. I read that the maximum power point is usually around 75% of the open-circuit rating, which would give 4.5V which is below the 5.5V maximum rating of the SPV1040.

Would it still be necessary or sensible to place a zener diode voltage regulator in front of the SPV1040 to make sure the input voltage will not exceed 5.5V? If so, is it necessary to have a resistor in series with the zener diode to limit the current? I can imagine that since the power source is current limited anyway this may not be needed, and I don't want to waste power in the resistor in the common case that the zener diode is not dissipating any power. So, given the 6V open-circuit and a 5.5V zener diode that can handle 150mA, do I need the resistor in this circuit?

![Example circuit: a DC power source, resistor, zener diode, and DC/DC-converter. The resistor and zener diode form a typical voltage regulator for the DC/DC-converter, to limit the ](https://electrical.codidact.com/uploads/icb899vfg0zltq4ujjf6g51bhmrw)

Or am I going about this wrong and is another type of over-voltage protection preferable?

(I have also looked for other buck-boost converters with MPPT functionality and a wider input voltage range, but without success. There is the LTC3119, but it is far more complex (and expensive). Other chips with a higher maximum input voltage typically also have a higher *minimum* input voltage, which is not ideal; or they are only available in packages that I wouldn't be able to solder by hand.)