Discussion in 'Power Electronics' started by f123, Oct 26, 2018.

1. ### f123

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Oct 26, 2018
Hello everyone!

Does anyone know of a book or a paper that teaches how to design mppt charge controller?

Is there topologies to follow?

Thank u

2. ### Harald KappModeratorModerator

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Nov 17, 2011
Welcome to EP.

When you Google "mppt charge controller design" you'll find reference designs by several manufacturers. You should be able to learn a lot from these reference designs.

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Generally speaking you have a buck or boost topology switch mode converter controlled such that you deliver the maximum power to the load.

In the case of charging a battery, the output voltage will be held relatively constant, so the power is proportional to the current.

Changing the duty cycle will change the current.

The task of the controller is to tune the duty cycle for maximum current (therefore maximum power).

One method is to increment the duty cycle in one direction as long as the current continues to rise. If it falls, the direction is reversed and the procedure continued.

The general procedure is the same no matter what the load, but the measurement of power may differ.

4. ### BobK

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Jan 5, 2010
How does it change the current with a constant voltage?

Are you saying the average current over the duty cycle?

Bob

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
If you are charging a battery the voltage across the battery stays relatively constant regardless of current.

Altering the duty cycle until you achieve the maximum current will put you at the point where the most power is being delivered to the load. This will be the point at which you are getting the maximum power out of the solar panel, wind turbine, or other similar device.

Assume a solar panel with an open circuit voltage of 24V and a 12V battery connected by a DC to DC buck converter where we have direct control of the duty cycle.

Obviously at 0% duty, no power is delivered. At 100% duty the panel is effectively connected straight across the battery, and we will assume that it delivers very close to the short circuit current.

As we reduce the duty cycle from 100%, the panel voltage rises, and the current reduces, but by a smaller amount. The buck converter is still producing the same voltage output, but the current into the battery rises (because the input power is higher). Reducing the duty cycle continues to increase the power to the battery until you reach a point where it doesn't. The controller needs to maintain the duty cycle at the point where maximum power is delivered.

If the level of illumination changes, then the power available will reduce and the duty cycle at which maximum power is delivered will also change.

The MPPT controller is always hunting for the optimum duty cycle to ensure that it extracts the most power from the source.

6. ### BobK

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Jan 5, 2010
Hmm. So the battery must have non-linear behavior when charging. I guess that makes sense, but I never thought about it. I always assumed the current was controlled by the voltage input and the internal resistance of the battery. Any idea how one would model this in LTSPICE?

The other thing I have never understood about an MPPT controller is, when the duty cycle is less than 100%, you are not drawing current from the solar panel all the time. Does that not reduce the total energy output? Or does the solar panel act a bit like a charging capacitor and build up a reserve when no current is being drawn for short periods?

Bob

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
A zener diode is one option.

A long cable run with a large capacitor at the end will act a bit like an LC filter, tending to smooth out those pulses. The lower the ripple the panel sees, the better.

If the DC-DC converter runs in continuous mode, you see something more like a ramping of current up and down rather than pulses.

8. ### f123

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Oct 26, 2018
Thank you very much steve!

9. ### f123

9
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Oct 26, 2018
if i change the duty cycle, then the output voltage will also change(from the buck equation). so if we connect this output voltage to the battery, it might damage the battery since the output voltage maybe much higher than the max voltage of the battery. right? or unless we have feedback in the converter

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
No it won't. Your battery is not a resistor.

And most loads are not resistors either. If you have a buck (or boost) DC-DC converter, the mark/space ratio is determined not only by the voltage that is required, but the current which is drawn. Typically the voltage is regulated by the SMPS and the current is allowed to do what it wants.

In a MPPT regulator the voltage is typically fixed by the load and the current is maximised.

What you are doing is energising an inductor at a certain rate (determined by the duty cycle of the regulator) and dumping that charge into the load. The voltage is determined by the battery voltage, and the current is determined by the amount of energy in the inductor.

The rate at which you want to put energy into the inductor should equate to the point at which the maximum power is delivered from the panel. The voltage and the current will vary.

Because the battery voltage is nearly fixed, the current into the battery is a proxy for the power.

11. ### BobK

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Jan 5, 2010
But in the buck converter topology the source is completely disconnected during the off part of the duty cycle. The smoothing of the output occurs because current continues to flow in the inductor. So, the panel is not producing any current during the off part of the cycle.

Are you referring to a capacitor between the panel and the switch? I guess that would do it. The on pulse partially discharges the cap and the panel replenishes its charge during the off cycle.

Bob

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Yes

The second sentence of my post (that you didn't quote) would only be useful for the panels if the DC-DC converter was a simple boost converter (as you point out).

13. ### BobK

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Jan 5, 2010
Agreed, boost topology has the power source connected all the time.

Bob