MOSFET and Protection Components
Here is the MOSFET circuit including protection and EMC related components.
Firstly that innocuous looking 10k resistor connected to the gate of the IRFZ44N MOSFET. This component makes a huge difference to the amount of RFI (Radio Frequency Interference) that the charge controller generates. Together with the FETs own gate capacitance, it slightly increases the rise and fall times as the FET switches from on to off and back again. This damps out the ringing at these switching transitions which is responsible for RFI generation. It’s highly effective; without it, an AM radio picks up the 122Hz switching tone easily, and at some distance. With the resistor, it’s very hard to pick up any signal at all, even with the radio pressed against the controller and/or its cables.
EMC regulations also require that the controller is not susceptible to interference from other sources. These would include motor vehicle engines, transmissions from police radios and the like, and noise spikes from lightning strikes. The P6KE30CE bidirectional transient suppression diode will absorb any spikes over 30 volts with an instantaneuos power of up to 600 watts. The bidirectional nature of this component works well with the reverse polarity protection that the controller offers.
Finally the TSR845 Schottky diode. This is purely for protection of the charge controller against shorting the solar panel connections while the controller is connected to a battery. Without this diode, the controller would be rapidly destroyed under such conditions. This component is a necessary inconvenience; it would be better if it weren’t needed at all. I’ve selected an advanced component with very low forward voltage and high current handling to minimise its impact.
These two diodes aren’t cheap, but for the protection and EMC compatibility they offer, are a necessary investment.