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Enabling clean power generation

Posted: 25 Feb 2013     Print Version  Bookmark and Share

Keywords:solar panels  power stages  DC/DC controller 

When VIN is significantly higher than VOUT, the part will run in buck (step-down) mode. In this region M3 is always off and M4 is always on unless reverse current is detected while in Burst Mode operation or discontinuous mode. At the start of every cycle, synchronous switch M2 is turned on first and the inductor current is sensed by an internal amplifier. A slope compensation ramp is added to the sensed voltage which is then compared to a reference voltage. After the sensed inductor current falls below the reference, switch M2 is turned off and M1 (synchronous rectifier) is turned on for the remainder of the cycle. Switches M1 and M2 will alternate, behaving like a typical synchronous buck regulator.

As VIN and VOUT get close to each other, the duty cycle decreases until the minimum duty cycle of the converter in buck mode is reached and the part moves into the buck-boost region and all four MOSFETs are switching.

When VOUT is significantly higher than VIN, the part will run in boost (step-up) mode. In this region M1 is always on and switch M2 is always off. At the start of every cycle, switch M3 is turned on first and the inductor current is sensed by an internal amplifier. After the sensed inductor current rises above the reference voltage, switch M3 is turned off and switch M4 is turned on for the remainder of the cycle. Switches M3and M4 will alternate, behaving like a typical synchronous boost regulator.

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.

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 reinitialise. 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. 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, it is touted to simplify the design for the system architect. As a result, 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 Corp.

To download the PDF version of this article, click here.

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