EDS and PES TAKE HOME QUIZ 1

EDS and PES

TAKE HOME QUIZ 1, 8 September 2024

Due date: 15 September 2024

Question 1

Consider the circuit given below. If the switch turns ON and OFF repeatedly as also illustrated in the figure below, sketch the inductor current and the voltage waveforms until the inductor reaches steady state (at which its value at the beginning of each switching cycle is the same). Then mathematically express the steady-state condition.

Question 2

Consider the switching circuit given in the figure with an ideal diode. Assume the initial inductor current is zero and the switch is turned on at t=0 and turned offat t=10 μs. Sketch the inductor current waveform. for  t>0. If the diode has a 5 Ω forward resistance, what is iL att=0.5 μs, 10 μs, and 100 μs? At what time does the inductor becomes discharged?

Question 3

In the step down converter shown below, the transistor switch and the freewheeling diode have the thermal  resistances junction to case of 1.2 oC/W and 1.3 oC/W respectively.  The maximum junction temperature of the transistor and the diode is 150 oC. The converter is designed to operate in an ambient temperature of 40  oC. At the maximum switching frequency and the duty cycle, the switching losses of the transistor is measured as 10W and the conduction losses is measured as 60W. The diode’s conduction loss is found 30W. Assume athermal resistance case-heatsink of 0.2 oC/W for both the transistor and the diode.

a)   Draw the thermal equivalent model of the power devices if they are assembled on a common heatsink.

b)   Calculate the required heat sink thermal resistance assuming the junction operates at its thermal limit.

c)   Calculate the temperature of the case and the heatsink.

d)   Comment on the case temperature about the heatsink selection

Question 4

Reference to the step down converter given in the figure below, if the input supply is 12V, and if the converter has to provide 3.3V output voltage at 10A maximum current to a load. Ignore the switching losses in the transistor. Assume that the power components have the following parameters:

MOSFET : RDS(ON) = 10mΩ

Resistance of the inductor : RInd(Cu) = 2 mΩ

Diode forward voltage VD = 0.5V.

a) Calculate the total conduction losses at full load in the MOSFET, in the diode and in the inductor.

Hint: Pconduction = IO2  RDS(ON)  D + IO  VD  (1 - D) + IO2  RInd(Cu)

b)  Calculate the efficiency of the converter.

c)   Compare and comment on the conduction losses of the MOSFET, the diode and the inductor

Question 5

To improve efficiency of the converter, the diode in the previous question (Question 4) is replaced with another identical MOSFET. This new converter topology is called “synchronous buck converter” and two transistors are driven by a complementary signal.

a)   Draw the new circuit.

b)  Sketch the main PWM control signal and the control signals of two transistors.

c)   Calculate the total conduction losses at full load.