Simplifying solar-based battery charging: Page 6 of 7

May 06, 2014 //By Steve Knoth and Albert Wu
Simplifying solar-based battery charging
Steve Knoth, Senior Product Marketing Engineer, Power Products Group and Albert Wu, Design Manager, Power Products Group, Linear Technology Corporation explains how a buck-boost solar powered battery charging controller can offer an efficient solar-powered solution.
point is found, the device 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 device 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.

Figure 5: Global MPPT sweep from the LT8490 (Yellow – Panel Voltage, Red – Panel   Current, Green – Control Signal from LT8490)

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 device continuallly 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 device is able to track changes without having to do a global sweep too often.

Figure 6: Local dithering by the LT8490, between Global Sweeps (Yellow – Panel Voltage, Red – Panel Current, Green – Control Signal from LT8490)

The charging controller performs automatic temperature compensation of the battery charge voltage by sensing an external thermistor on the battery. The STATUS and

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