How does MPPT work? Explained for the beginner.

[smaBonham]

Firstly, I've been following the YouTube videos for a while and love the wealth of information they and sites like this provide. Thank you for all your hard work.

I'm a relative newbie, though I do have a basic understanding of electronics and I have a MPPT question that I just can't  fathom out the answer to. I would be extremely grateful to whomever can help me understand and better still, can do it using straight forward terms.

Basically, how do MPPT circuits up the amperage to match the output wattage with the input wattage?

From what I can understand so far, MPPT uses a buck converter. A MOSFET opens and closes, governed by a driving PWM frequency, that in turn charges an inductor and a capacitor. A second MOSFET opens and closes in equal and opposite timing to the first MOSFET, a synchronous rectifier, creating a circuit for the inductor and capacitor to drain and drive a load when the main input is off when the first MOSFET is open.

Sorta got my head round that.

But the hard part, it seems the MOSFETs act like a PWM stepping down the voltage  and the inductor keeps the current relatively constant, which is all well and good but what raises the amperage on the output of MPPT?

I'm not a maths person so please, no hyper complex equations.

For very rough example:

If a solar panel output 1amp @ 20volt we would have 20watts
The buck converter takes this 20watt input and steps the voltage down via opening and closing to say 10volts but the inductor keeps the amps @ 1, we would have 10watts at the output.

So how does MPPT get that 10volts to 2amps to equal 20watts again?

My challenge and hope is to make an MPPT arduino charge controller but I'm  clearly not getting one of the systems fundamental principals and can find nowhere that explains every step of the circuit and system in beginner terms.

Is there a solar Jedi out there? Help me Obi Wan Kenobi, you're my only hope.

Cheers and many thanks in advance.

[SeanB]

MPPT is very simple. You start off at the middle and set the PWM to that, then measure the input voltage and current. Then every cycle through the loop you measure the voltage and current, and multiply them to get power. Compare to the previous values. If the voltage is higher and the power is less then increase the PWM value by 1 and store the values. If the voltage is lower and the power is less then decrease by one. In the end you end up with a PWM value set so that every time the controller runs through the loop it is either going up or down by 1 step, and keeps the power at the value where the maximum power is being transferred irrespective of the input voltage and current.

There will be another loop that is used to control the output voltage as the battery charges, and this one will override the PWM loop when the battery voltage is fully charged to reduce the power to a low level to float charge the battery.

Very simple, the thing to remember is that it will always change the PWM value and tends to drive towards a value where it will oscillate gently around a point of maximum power drawn from the panel, until the battery is charged when it will drop to a low level and then power back every few seconds to keep the battery at the float voltage. These loops do not have to be fast, most are generally run through about every 10 seconds, the microcontroller is going to be idle most of the time as the values generally are very slow changing.

[smaBonham]

Thank you for the reply.

I'm not sure I entirely understand but I think your reply refers to the MPPT algorithm used to control the PWM frequency to the first MOSFET in the buck converter circuitry.

I'm not yet that far ahead in my understanding, but thank you all the same.

I'm actually after information on the circuit itself and how it's components work as the DC to DC converter before the battery charging systems.

http://upload.wikimedia.org/wikipedia/commons/6/63/Buck_chronogram.png

This diagram is the best example I've found as to how a buck converter steps down the voltage while working with an inductor to keep the current relatively constant. My confusion is to how this circuit can raise amperage as it lowers voltage to maintain the input wattage on the output.

Is MPPT more than just a cleverly controlled buck converter? If so, what is it?

Many thanks.

[SeanB]

A buck converter steps up current by having a capacitor on input and output. It stores energy in the inductor as the switch is closed, drawing a current out of the capacitor on the input that is going to ramp up to a defined current larger than input and a little above the output current. This is flowing in the load as well. When the switch opens the inductor reduces the energy stored in it to zero by carrying on supplying current to the load. When the current reaches zero or a low level the switch is closed again. The current drawn from the input is time averaged by the input capacitor, so a lower current ( average) on the input supplies a higher current continuously on the output.

MPPT can be either buck, boost or a buck-boost converter, depending on the input voltage range from the solar panel and the open circuit voltage relative to the battery. Typically the buck-boost converter is used, as it will provide an output over a wide range of input voltages, even if the voltage from the panel is above the battery voltage. This allows a lower cost of cable to the panel, and still works as the sun sets to provide usable power from the last light. Simplest is a boost converter with panels whose open circuit voltage is below the battery, as it always has to step up.

[smaBonham]

I think what I really need is a component by component beginners guide to how MPPT charge controllers are built and work. Sigh.

[bricofoy]

Well, I think the previous reply from SeanB explain exactly what you want. Let's try to clarify this.

Based on this topic : http://mjlorton.com/forum/index.php?topic=68.0 wich is the simplest mppt system I could find. We should have a look at the schematic :


the relevant part here is only the basic part of the buck converter : C2, Q2, Q3, L1, C3

as you said, you already understood how the buck converter works. Q2 closes, C3 and L1 are charging. Q2 opens, Q3 closes, L1 discharge into C3. Then Q3 opens, Q2 closes, L1 is charging, etc etc.
C2 is here to average the current flowing from the solar panel to L1.
The PWM ratio that controls Q2/Q3 is controlling this average solar panel current.

As we have seen, C3 is charged by the converter.  But C3 is also directly connected to the output of the system, and viewed from the output, it acts as a battery, that provide current to the load.

If the PWM ratio is set to get 1A and 20V at solar input, we get 20W input. So we charge 20W in the C3 capacitor.
If at the output the load is imposing 10V, the current drawn from C3 will be 2A. So you also get 20W at the output.

What you need to understand here is when you charge a battery, it's the battery that determines the voltage at the output. The current is just the result of the available power, drawn at the voltage the battery "wants".

So what allow the current to be different between the input and the output of the buck converter is because C3 acts as a storage. So you can consider C3 alone (regardless of the converter) as a power source that provides fixed power to the load.
So the current at output is the current the load ask when powered with the amount of power available.

the current at the input is the current that charges L1, not C3.
the current at the output is the current that discharges C3, not L1.


Well, I'm not sure it will really be clearer now... at least I tried