Take-Home Exam A

EE 5522
Fall, 2015
Take-Home Final Exam A (Due 12-16-2015 at 4PM) [You may alternatively
choose Exam B]
*** TWO Students may work together to prepare this examination ***
Problem Statement:
Create an engineering design document that provides the preliminary component
and functional design for a portable USB re-charging station. It must have the
following capabilities:
Energy storage will be accomplished using a super-capacitor, namely a
Nichion 4000 farad, 2.5 V cap.
Your design will start with a full cap. No need to design a chargin circuit for
The device will supply a constant 5 volt output at up to USB 3.0 current
The device will have a basic human-machine interface, such as indicator
lights, to display “state-of-charge” in the capacitors and indicate connection
to a USB load.
A programmable device, such as arduino, will be used to measure output
voltage and control the duty cycle that maintains output voltage. You are
NOT required to write any code for the programmable device.
User safety must be considered (Contact potentials, over-current protection,
The document MUST contain the following components:
1. A full electric schematic of the storage capacitors DC/DC converter that
maintains 5.0 volts (even though the capacitor voltage drops as its charge
depletes), over-current protection, circuitry for indicator lights, etc.
2. Design calculations on sizing the filter inductor for the DC/DC converter to
maintain CCM operation, etc. Also, make sure to calculate the time necessary
to put a full charge on the capacitors for your design. It looks like the Nichion
cap has an internal resistance of 2.2 milli-ohms. Assume you can have I2R
heating of 2 watts during capacitor discharge.
3. Discussion about design choices, such as MOSFET vs. IGBT or Si Diode vs.
Schottky. Remember, you’re potentially only working with 5.0 volts DC,
unless your design creates additional potentials.
4. Discuss/show how your programmable device receives its power. If it is
powered from the energy stored in the super-capacitors, demonstrate how
the system will power-up reliably at the start of a new charging cycle and
EE 5522
Fall, 2015
how it will shut -down at the end of a discharge cycle. If the device is
powered by separate energy storage, such as a back-up battery, design the
circuit that will re-charge it and show with calculations that it will last
sufficiently long to operate the device.
5. At a minimum, the device should clearly indicate a state of full charge and a
state of impending full discharge (you could use LEDs).
6. The device should protect against overcurrent discharges (fuse, re-settable
circuit breaker, etc).
7. The device should protect against overvoltage conditions on the supercapacitor and the USB output. At least state how your programmable device
would react to such an overvoltage. Good designs might include some sort of
physical back-up to isolate the capacitors (or USB output) in the event of an
un-resolvable overvoltage condition.
8. The device must have two operating states: Dormant and Discharging.
Design steps should be taken to ensure that the Dormant state is a very low
power condition for the super-capacitors.
9. A Bill of Materials (BOM) listing the components and retail prices, summed to
a total material cost.