Bidirectional super capacitor charger
The LT8705 can also be configured as a bidirectional supercapacitor charger as shown in the Figure 4
simplified schematic. This circuit operates from a from a PCI backplane with a 12V nominal input voltage. When the input voltage is present, power passes directly to the system load DC/DC converters and to the LT8705 supercap charger circuit through a blocking diode. A bank of six each supercaps in series are charged with a 1 amp charge current to 15V. When the 12V PCI backplane input is removed, the LT8705 operates in the opposite direction with the supercap energy being supplied to the 12V loads. Up to 6A of current can be supplied from during this mode.
In this way, a single bi-directional supply replaces two traditional pulse width modulated supplies producing a substantial cost and parts savings. No additional power routing/management is needed, which reduces the parts count and power losses which would be incurred by the additional circuitry.
Figure 4: Simplified LT8705 bidirectional super capacitor backup circuit
Click on images to enlarge
The LT8705 activates a fault sequence under certain operating conditions. If any of these conditions occur, such as an over current or over voltage condition, the internal switching and clock output are disabled. At the same time, a timeout sequence commences where the soft start function needs to be reinitialize. If the fault persist, like during an over current condition, the soft start function will not be allowed to restart the converter. After the fault condition has been removed and a predefined timeout period has ended, the converter will restart at a rate dependent upon the capacitor value assigned to the soft start pin on the LT8705. The timeout period relieves the part and other downstream power components from electrical and thermal stress.
It is evident that utility companies around the world will be raising their rates over the next several years. Therefore, the use of solar panels to generate clean energy can reduce and possibly eliminate consumer dependency on utility company power. This will inevitably lead to many solar installations to be deployed. Nevertheless, how solar panels are configured, their relative size and number of panels result in a wide variation of voltage and power availability. This wide variation can cause the need for multiple power stages to achieve the desired output voltage and maximum power levels. Fortunately, the LT8705 with its wide input voltage range and four regulation loops can reduce the number of power stages required offering an efficient simplified solution. Thereby simplifying the design for the system architect. And finally, a side benefit of clean energy generators will be a greener environment for all of us.
(1) Patriot Solar and Roofing, May 2009 provided information on Required Infrastructure Investment, the Compound Pricing Effect and Environmental Regulations
About the author
Bruce Haug is senior product marketing engineer at Linear Technology Corporation
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