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EETE MAY 2014

POWER design True active MPPT will seek out the optimum operating point under all conditions. This results in less overall system cost because the smallest panel or smallest battery can be used, reducing the need to over design the system. True MPPT will find the best peak power point and reject false local maximum common in partially shaded panels (note: partial shading power patterns are determined by the number and arrangement of bypass diodes inside the panel). A simple IC solution An IC charging solution that solves the problems outlined above needs to possess many, if not all, of the following attributes: Minimal software and firmware development time Flexible buck/boost topology Active MPPT algorithm Simple, autonomous operation (no μP needed) Termination algorithms for various battery chemistries In-situ charging - to power a load while the battery is being charged Wide input voltage range to accommodate various power sources Wide output voltage range to address multiple battery stacks High output/charging current Small, low profile solution footprints Advanced packaging for improved thermal performance and space efficiency Cost-effective solution Typical convoluted competing solar battery charging systems consist of a DC-DC switching battery charger, a microprocessor plus several ICs and discrete components in an attempt to replicate maximum power point control / tracking functionality. An alternative solution could be a solar module; however these are costly, not simple to design in (require software, firmware, etc.) and tend to lock on to false solar panel maxima and therefore do not operate as efficiently as possible. Fortunately, a simpler solution is at hand, thanks to Linear Technology’s LT8490 buck-boost solar powered Fig. 5: Global MPPT sweep from the LT8490 (Yellow – Panel Voltage, Red – Panel Current, Green – Control Signal from LT8490) Fig. 6: Local dithering by the LT8490, between global sweeps (Yellow – Panel Voltage, Red – Panel Current, Green – Control Signal from LT8490) battery charging controller. An efficient solar-powered solution Linear Technology has developed a simple, innovative high voltage buck-boost charging controller IC specifically for solar applications, one which requires neither software nor firmware development, thus greatly reducing time-to-market. The LT8490, shown in Figure 4, is a synchronous buck-boost battery charging controller for lead acid and Lithium batteries, featuring automatic maximum power point tracking and temperature compensation. The device operates from input voltages above, below or equal to the regulated battery float voltage. The LT8490’s full-featured battery charger offers many selectable constant-current constant-voltage (CC-CV) charging profiles, making it ideal for charging a variety of Lithium or lead acid chemistry types, including sealed lead acid, gel cells and flooded cells. All charge termination algorithms are provided on chip, eliminating the need for software or firmware development, thus reducing design cycle time. The LT8490 operates over a wide 6V to 80V input voltage range and can produce a 1.3V to 80V battery float voltage output using a single inductor with 4-switch synchronous rectification. The device is capable of charging currents as high as 10A depending on the choice of external FETs. The LT8490’s MPPT circuit enables a sweep of the full operating range of a solar panel, finding the true maximum power point, even in the presence of local maxima points caused by partial shading of the panel. Once the true maximum power point is found, the LT8490 will operate at that point while using a dithering technique to quickly track changes in the local maximum power point. With this methodology, the LT8490 fully utilizes the power generated by a solar panel even in non-ideal operating environments. A global MPPT sweep is shown in Figure 5. The yellow trace shows the panel output voltage. The LT8490 commands the panel voltage to go to the open circuit level then subsequently commands the panel to ramp down linearly to the minimum level. The red trace shows the panel current as the panel voltage changes. The current is measured by the LT8490 and the power is calculated inside the IC. Once the sweep is completed, the panel voltage is returned to the point at which maximum power was measured. The dithering technique is used to track smaller changes in the maximum power point between global sweeps. This is shown in Figure 6. About midway through the scope shot, a change to the power point is applied to the panel to simulate a cloud moving in the sky thus changing the amount of sunlight striking the panel. The LT8490 continually moves the panel voltage a small amount above and then below the current MPPT point to check if a better operating point exists. If it finds one, it properly tracks to the new point and repeats the process. In this way, the LT8490 is able to track changes without having to do a global sweep too often. The LT8490 performs automatic temperature compensation of the battery charge voltage by sensing an external thermistor on the battery. The STATUS and FAULT pins can be used to drive LED indicator lamps. Charging current limits can be adjusted by changing as few as 1 or 2 resistors, and a charging time scale can be selected with the appropriate resistor divider. Other features of the device include: input and charge current limit pins, a 3.3V regulated LDO output, status pins and a synchronizable fixed switching frequency from 100kHz to 400kHz. The LT8490 is available in a 0.75mm high 64-pin 7x11mm QFN package and is guaranteed for operation from -40°C to +125°C. 28 Electronic Engineering Times Europe May 2014 www.electronics-eetimes.com


EETE MAY 2014
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