# Confusion about solar cells and MPPT

Pardon the potentially silly question.
I have been reading about photovoltaic (PV) cells and max power point tracking (MPPT). From what I understand, a MPPT module can use a DC-DC converter to change the voltage on the PV's terminals to its corresponding MPP voltage (Vm). My question is, how are the PV cells terminal voltage clamped to Vm? For example, the PV terminal voltage is 20 V and Vm is 17 V. Can we create a circuit like this to achieve MPP?

PV 20V->DC-DC Converter 17V -> Other stuff

This means that the PV terminal voltage is still at 20 V.
Or, do we need to accomplish this?

PV 17V -> Other stuff

Where a DC-DC converter forces the terminal PV voltage to be 17 V. If this is the case, how is this actually done?

I feel like I'm missing something really obvious.

• What they said. One version of almost-MPPT decides an optimum Panel Vout under typical conditions and adjusts load until panel is at this voltage. Comes close tpo truye MPPT. Can be improved by slightly increasing V as I provided rises. – Russell McMahon Nov 30 '15 at 0:53

A DC-DC converter under MPPT control must have a suitable load on it in which to dump all the power the panels can supply, for instance charging a big enough battery bank, or an inverter into the area's mains power supply.

It then simply increases the current it draws from the panels, which drops the panel voltage, until the algorithm decides that it is drawing the maximum power it can. Drawing any more current at this point would reduce the voltage so much that the power drops.

The MPPT controller doesn't have magic in it to predict what the voltage will drop will be. It will dither the current either side of the operating point, and see what the conditions are, and work near the MPPT. That way it can keep up with changing illumination conditions.

Of course the voltage at output is varying, since the battery bank has very low resistance a small incresae of voltage meens big incresae of power. The PV voltage is held on its maximum MPPT point, while the voltage at the batery is set such that energy from PV is transfered to the battery (load). If there is no load at output, then the MPPT controller can't sustain that point.

Firstly, please find my answer for another guy Questions concerning Solar MPPT tracker And forgive me that I do not really want to type these equations, so I just copy my answer.

For your concerns, it is because you do not know the MPPT working principle. Here is the details:

A dc-dc converter is generally used in a PV system as an interface between a PV string and a load, as shown below:

The control parameter d refers to the duty cycle of the converter. Assuming that M(d) is the voltage conversion ratio, the relationship between the input and output voltages (currents) for the dc-dc converter can be written as: $$V_{in} = \frac{V_{out}}{M(d)}, I_{in} = \frac{M(d)}{I_{out}}$$ Divide Vin into Iin, we have: $$R_{in} = \frac{V_{in}}{I_{in}} = \frac{V_{out}/M(d)}{M(d) \cdot I_{out}} = \frac{1}{M(d)} \cdot \frac{V_{out}}{I_{out}} = \frac{R_{out}}{M(d)}$$ In the PV system, we have $$R_{pv} = \frac{R_{load}}{M(d)}$$ where Rpv and Rload refer to the equivalent resistance for PV and load.

Then, the operating condition resulted by a PV string connected with a dc-dc converter is shown as below:

As shown in this figure, when you change d, the operating point will be changed even if Rload is constant. The aforementioned part is how the MPPT works with duty cycle.

For your concerns, the PV output voltage can be changed from 20V to 17V just by changing its duty cycle d.