8. Providing MPPT for each panel
An improved method of solar system design is to use micro-inverters. A micro-inverter is a 250W inverter that is connected to each panel. MPPT is performed by the micro-inverter at the panel level. Figure 11
shows a system consisting of thirty micro-inverters, one per panel. The micro-inverters AC outputs are connected together and properly phased with the AC line.
Micro-inverters are fairly complex electronic products. A picture of one from STMicroelectronics for evaluation purposes, with its components clearly visible, is shown in Figure 12.
A simpler method of implementing a photovoltaic power system with MPPT is to use an active power optimizer such as the SPV1020. An overall system diagram is shown in Figure 13
In this case one active power optimizer is connected to each panel. It boosts the panel output voltage and performs the MPPT function while doing so. Panel output voltage must be at least 6.5VDC. The SPV1020 output voltage can be as high as 40VDC. A typical value is 35VDC as shown in Figure 13
. The active power optimizer utilizes the Perturb and Observe algorithm until it finds the maximum power point on the power vs. voltage curve as shown in Figure 7
. This optimizer measures the input power to determine the panel’s Vmp. There are other types of MPPT converters available but they assume Vmp is a fixed percentage of Voc. This could be case under one specific operating condition and thermisitors are required to approximate the change in Vmp with temperature. The SPV1020 makes none of these assumptions. It measures input voltage and input current to determine the actual input power in setting the maximum power transfer operating point. Figure 12
shows the simple external connections. The panel is connected through the boost inductor at the Lx input and the load is connected to Vout. No other source of power is required. The resistor divider connected at the panel output senses the chip input voltage for MPPT purposes. Input power is determined by sensing the current through the main MOSFET switch and multiplying its value by the input voltage. The resistor divider connected at Vout sets the value of the output voltage.
The SPV1020 is an interleaved four channel converter. Figure 14
shows one of the four switching channels. Of particular interest is its 320W the power handling capability. This is accomplished in the small PowerSSO-36 package by splitting the power handling components into the four channels.
The 4-phase interleaved topology as shown in Figure 15
There are four interleaved switching sections interleaved every 90 degrees. This diagram shows a single panel and single load, and how the four switching section are connected. Each section has its own inductor. The switch and diode pictured are both MOSFETs with low Rds on. At the default switching frequency of 100kHz, each section operates at 25kHz. The SPV1020 also integrates four zero crossing blocks, one for each branch. Their role is turn off the related synchronous rectifier to prevent reverse current flow from output to input.
In order to guarantee a correct power-up sequence, the converter initially operates in burst mode. When the input voltage is greater than 6.5 V, the converter sequentially activates each of the four phases. Initially, only phase 1 starts to work in burst mode, charging the inductor only for one cycle over 15 cycles. Then the duty cycle is progressively increased until phase 1 is switched on at every cycle at the default switching frequency of 100 kHz. After phase 1 has reached its steady-state condition, the other phases are progressively switched on in the following sequence: phase 3, phase 2 and, lastly, phase 4. If lower power than 320W is required, it may be possible to use only two of the four phases, eliminating the cost and space requirements of two inductors.
A major advantage of interleaved architecture is low ripple. Assuming a resistive load, the output voltage ripple is proportional to the output current ripple. In interleaved-4 phase architecture, total output current is the sum of the four currents flowing in each inductor. Since each phase carries one fourth the total current, for a given value of inductance, the peak to peak ripple would be one fourth the value of a single phase architecture system.