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This diagram is from a PWM solar charge controller datasheet,

http://www.epsolarpv.com/en/uploads/news/201310/1382338419195066.pdf

I have 2 questions about it:

1-Why they used two MOSFET in series rather than one?

2-Can we simply increase the max. current of any PWM solar charger by replacing its MOSFETs by another with higher current or it need to more modifications?

enter image description here

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2 Answers 2

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  1. I've seen this MOSFET configuration in a surge stopper circuit with an ideal diode (e.g. LTC4364). One MOSFET clamps the voltage if it goes above a certain level, while the other prevents reverse current with a smaller voltage drop (and thus less power dissipation) than a diode.

  2. It's not just the MOSFETs that determine the rating of a power converter such as this. Protection and control circuitry, the current sensor, and internal wiring are designed and rated up to a specific power level. Simply replacing the MOSFETs with beefier ones will not achieve a higher-current PWM controller, and could easily compromise safety. Just buy/acquire a different one.

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  • \$\begingroup\$ But if the MOSFET inside solar controller prevent reverse current why all solar panels used a blocking diode? can we remove the diode from the panel for decreasing the losses? \$\endgroup\$
    – M.A.K
    Commented May 22, 2017 at 20:54
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    \$\begingroup\$ The blocking diode included on PV panels is there because PV manufacturers and charge controller manufacturers are typically two different entities, and relying on the other to provide this protection is bad practice. Having two protection elements -- one on the panel and one in the controller -- also provides a safety net in case one element fails. You may be able to remove the diode, but I think you're playing with fire. \$\endgroup\$
    – calcium3000
    Commented May 22, 2017 at 22:32
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    \$\begingroup\$ @mousa The blocking series diode on a solar panel would not prevent reverse polarity on the controller. Just draw it on a piece of paper and you will see why. The series diode on a solar panel is placed to block leakage current while 2 or more panels are connected in parallel and one of them is shaded. Without the diode a shaded panel will pass some ammount of current from other non-shaded panels this way reducing the total generated power of the system. \$\endgroup\$
    – Todor Simeonov
    Commented May 24, 2017 at 14:29
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  1. If you look closer you will see that these 2 MOSFETs are in series but they are mirrored. In this schematic they are connected drain to drain. The first one serves as a reverse polarity protection for the photovoltaic input and the second makes the so called PWM. These type of cheap controllers operate MOSFET switch with PWM having no inductive coil.

  2. It depend how its current protection is connected. Doubling transistors you will achive bether thermal management but it depends on the exact schematic of the controller will you double the charging current or not. Also you must be aware that placing more and more parallel transistor require proper gate driver with higher driving current and from a certain point after it may lower the efficiency, rather than make it higher.

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