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41 minutes ago, Tony Brooks said:

Too much for my brain SirN. I just know it happens and why. I also know confuses the hell out of peeps when making a stab at diagnosing charging/ battery faults from voltage alone. I am far from sure that an MPPT controller or even a PWM one can be considered as pure DC. Pulsed DC possibly but that will give rise to induction.

What you are thinking of applies more to wound components like alternators, motors and transformers. The simple explanation is that devices like solar controllers are really only current sources. When the current meets an obstacle such as resistance or the voltage of a battery, the flow of the current creates a voltage. Although we like to think of such things as having a regulated voltage, and whilst this is true at first glance, that regulation is derived from controlling the current, not the voltage (the voltage, as I said, being a “byproduct” of stuffing current around a circuit). So in the case mentioned, the amount of energy produced by the panels is simply insufficient to stuff enough current around the circuit to get the voltage up to the regulated value of 28.8v or whatever. The “current tap” on the controller is wide open but the flow only manages to create a paltry 15.5v or whatever it was. Inductance doesn’t come into it.

Edited by nicknorman
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1 hour ago, nicknorman said:

What you are thinking of applies more to wound components like alternators, motors and transformers. The simple explanation is that devices like solar controllers are really only current sources. When the current meets an obstacle such as resistance or the voltage of a battery, the flow of the current creates a voltage. Although we like to think of such things as having a regulated voltage, and whilst this is true at first glance, that regulation is derived from controlling the current, not the voltage (the voltage, as I said, being a “byproduct” of stuffing current around a circuit). So in the case mentioned, the amount of energy produced by the panels is simply insufficient to stuff enough current around the circuit to get the voltage up to the regulated value of 28.8v or whatever. The “current tap” on the controller is wide open but the flow only manages to create a paltry 15.5v or whatever it was. Inductance doesn’t come into it.

So what I need to know is how MPPT controller can play with current and voltage when dealing with many amps. Showing my ignorance but I would suspect there is a high frequency transformer and rectifier circuit inside. The frequency may be all on one side of zero so not AC  but still capable of induction.

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1 hour ago, Tony Brooks said:

So what I need to know is how MPPT controller can play with current and voltage when dealing with many amps. Showing my ignorance but I would suspect there is a high frequency transformer and rectifier circuit inside. The frequency may be all on one side of zero so not AC  but still capable of induction.

It is correct that within an MPPT controller there is a switch mode buck converter, which of course has an inductor. There probably is no transformer, and the “rectifier” will be a single diode which doesn’t rectify because there is no ac, it just acts as a freewheel diode during the switching process. But to use that inductor as part of an explanation as to why the controller couldn’t achieve its target voltage of 28v or whatever, would be like explaining that the maximum speed a car can reach is determined by tyre friction.

Edited by nicknorman
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37 minutes ago, nicknorman said:

It is correct that within an MPPT controller there is a switch mode buck converter, which of course has an inductor. There probably is no transformer, and the “rectifier” will be a single diode which doesn’t rectify because there is no ac, it just acts as a freewheel diode during the switching process. But to use that inductor as part of an explanation as to why the controller couldn’t achieve its target voltage of 28v or whatever, would be like explaining that the maximum speed a car can reach is determined by tyre friction.

Thanks. Next question - what components are in a buck converter that can handle tens of amps and how does it work?

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4 minutes ago, Tony Brooks said:

Thanks. Next question - what components are in a buck converter that can handle tens of amps and how does it work?

See diagram. The higher voltage supply is on the left, the load on the right. When the switch (which is of course a MOSFET) closes, current starts to build up through the inductor and the current, passing through the load, creates a voltage. When the desired voltage (less than the input voltage) is reached the switch opens. The property of inductance is such that it tries to maintain current, so current continues to flow through the inductor now being supplied via the diode, only slowly reducing. The current and hence voltage decreases below a threshold voltage and the switch is turned on again. The cycle repeats at high frequency, 10s or 100s of KHz. The duty cycle (on time vs off time) is adjusted by the control system to maintain the desired output voltage as the load varies. Since the switch is either on or off, and since (in an ideal world) the inductor has no significant resistance, there is very little power dissipated and as the output voltage reduces compared to the input voltage, so the output current increase compared to the input current. In other words, power output is roughly the same as power input.

 

If the load becomes too great for the supply, the switch will remain permanently closed in an attempt to get the output voltage up. With the switch permanently closed, only dc is now flowing through the inductor and so any theories about inductance limiting the supply are false.

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