Two types of circuit modules equipped with a boost circuit and MPPT (maximum power point tracking) function. The one seen in the lower left is a module used for the photovoltaic cell for mobile phones (above). The other on the lower right has a photovoltaic cell loaded on its back.
Two types of circuit modules equipped with a boost circuit and MPPT (maximum power point tracking) function. The one seen in the lower left is a module used for the photovoltaic cell for mobile phones (above). The other on the lower right has a photovoltaic cell loaded on its back.
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NTT Energy and Environment Systems Laboratories developed a circuit module that enables to charge a lead storage battery with a single photovoltaic cell.

The module was exhibited at NTT R&D Forum 2009, which took place from Feb 19 to 20, 2009, at NTT's Musashino R&D Center.

This circuit module consists of a boost circuit with a DC-DC converter and an IC equipped with the "MPPT (maximum power point tracking)" function. The boost circuit can increase the typical voltage of a single photovoltaic cell, about 0.5V, up to 15V. The MPPT is a function to maximize the output from a photovoltaic cell in accordance with the change in sunshine conditions.

"The use of the MPPT has increased the amount of electricity generated per day by 1.5 times," according to the Laboratories.

However, the efficiency of the boost circuit is somewhat low at approximately 70 to 80%, though it varies depending on the inductor to be used.

"If the circuit module is used for solar batteries for mobile phones, for example, its low efficiency is a minor disadvantage compared with the fact that it enables charging by a single cell," NTT said.

With existing photovoltaic cells, several to several tens of small cells have to be connected in series to obtain enough voltage because the voltage generated from a single cell was too low to be useful. The drawback of this approach is that the output of the total system sharply decreases if the output from any one of the cells connected in series drops due to failure or being shaded by the hand.

Meanwhile, when only one cell is used, the output does not significantly drop even if the cell is partially shaded.

NTT Energy and Environment Systems Laboratories has developed this type of circuit module for some time, but all of the circuits, including the MPPT, were discrete parts. In addition, in the previous circuit modules, the MPPT circuit's withstand voltage was several volts at the most, limiting the types of secondary batteries that can be charged.

For the new circuit module, the BiCMOS technology was adopted so as to realize the MPPT circuit as a 4 x 4mm square-shaped IC, and the withstand voltage of the IC was enhanced to 15V or higher to charge lead batteries. The IC and the module will be distributed by NTT Electronics Corp. Sample shipment has already started, and the company is "ready for mass production," according to the Laboratories.