EE 5522 UMD Fall, 2016 Name: ______

Practice EXAM : 20 HW Points (Show Work for Partial Credit)

1. (40 Points) The Tesla PowerWall is a $3000 lithium battery pack intended for home use. It has a capacity of 7 KWH (kilo-watt-hours) of stored electric energy. The battery is claimed to be 92% efficient on a round-trip charge-discharge cycle. Assume a grid-tied inverter, 95% efficient, connects a PowerWall to the residential wiring of a typical U.S. home using 900 KWH per 30-day month. The inverter can both charge and discharge the battery at 95% efficiency. Answer the following:

a. How many joules of energy does the PowerWall contain at full charge? (1 watt = 1 joule/second)

b. How many KWH of energy are lost (in the PowerWall and the inverter, together) in a round trip charge- discharge cycle?

c. In a home where no alternate source of energy exists (no wind , solar, etc), an economic benefit still exists for the PowerWall if the utility charges for electricity on a “Time of Day” rate. Assume one-half of all energy use in this home occurs during the “peak” hours from Noon to 8PM. Also assume that the utility charges $0.30 per KWH during the peak hours and $0.10 per KWH during off peak. How much money can be saved in an average day, by charging the PowerWall during off-peak and discharging it on- peak?

d. How many years will it take to pay back the $3000 investment in a PowerWall under the conditions in c. above?

1S.R.Norr EE 5522 UMD Fall, 2016

2. (25 Pts) The International Rectifier IRF540, N-Channel MOSFET has an “ON” resistance of Rds = 0.044 ohm. It is used in a Buck Chopper that converts 24 volts to 12 volts and serves a 4 ohm load in CCM.

a. Calculate the conduction losses in the MOSFET

b. If the conduction loss above is the only loss in the converter, what is its efficiency?

2S.R.Norr EE 5522 UMD Fall, 2016

3. (35 Pts) Congratulations! You’ve been hired by Billy’s Pretty Good Power Converters as a Design Engineer I. Your new boss, Earl, (whom you already consider to be an idiot) has presented you with a schematic of his latest buck chopper design, which he has designated “Earl, Jr. 100”. He says, “The 100 watt Junior has an S5BC diode with 100 volts of reverse breakdown and 6 amps of average current conduction, an IRF540 MOSFET that’s 100 volt and 30 amps and a 100 µH filter inductor that can handle 8.5 amps of average current.” He also states that the converter is intended to use a 48 volt source, produce a constant 12 volt output and operate at a PWM frequency of 200 kHz. Finally, he says, “The brilliance of the Junior is the 100 ohm resistor that I put in parallel with the load terminals to ensure CCM.”

a. Is Earl correct? Will the 100 ohm permanent load ensure CCM? (calculate Rcrit) If not, suggest three options to correct the design, ordered by increasing cost.

b. Is it a 100 watt converter? Calculate the maximum power output for the converter, and identify the component or condition that is the limiting factor?

3S.R.Norr