The buck converter is arguably the most important part of the board. A buck converter works by manipulating the electric and magnetic fields of an inductor to convert a high voltage and low current into a lower voltage and higher current. This is much more efficient than simply using a series of resistors to drop the voltage. Simulation shows the efficiency of this buck converter to be around 93%.
By controlling the frequency and duty cycle of the drive signal to the MOSFET, the desired voltage at the output can be achieved. The microcontroller drives the MOSFET via the PWM line and uses feedback from the Voltage and Current Sensing Circuitry to achieve highly accurate control of the output voltage.
The IC sitting between the MOSFET and PWM line is a MOSFET driver chip. MOSFET gates have a lot of capacitance, and in order to switch them rapidly on-and-off requires the use of a MOSFET driver IC. Faster switching speeds improves efficiency of the buck converter.
The current MOSFET driver has an upper supply voltage limit of 40 volts. Future designs will look into drivers with higher voltage limits, such as the AUIRS21811S high voltage drivers from International Rectifier. This would allow the board to interface with higher voltage, amorphous silicon solar panels such as this one manufactured by Kaneka. Using a new driver, however, would require a complete redesign and testing of the buck converter circuitry.
While the voltage limit of the board is determined by the MOSFET and MOSFET driver IC, the current limit is set by the inductor and PCB trace widths. The inductor used on the current board is a 100 uH, 7 amp, 2300 series inductor from Bourns. The high inductance of the coil means it can be driven at a lower frequency (25Khz in the current design), which improves efficiency by minimizing switching losses in the MOSFET. The amp rating of the coil determines the maximum current that can be driven through it before it reaches magnetic saturation. A higher amp rating is always better.
Designed for Hacking
Because traces on a printed circuit board (PCB) are thin, they do not work well for carrying large amount of current. The power traces on our board have large through holes at each node for attaching extra wiring. Adding extra copper in this way improves the efficiency and current capacity of the board. Additionally, a large area of the board is dedicated to the buck convert inductor. It is hoped that better coils can be found or made by future users. The large mounting holes and board real estate make it possible to attach many different power coils.
On the topic of hacking, through hole parts and large DIP package ICs have been used whenever possible to allow users to try different hardware configurations and make their own repairs. As more people use this device, improvements will be found. Every effort has been exerted to allow anyone with basic soldering skills to manipulate their own circuit.