ZAMBIA UNIVERSITY COLLEGE OF TECHNOLOGY
SCHOOL OF ENGINEERING
BEng Electrical and Electronics Engineering
EEE4040 – Power Electronics
Instructions: Answer ALL questions.
Draw neat and well labelled DIAGRAMS to support your answer.
Copied work will NOT be marked.
Task: Assignment 1
Due Date: 4th March, 2023
Marks: 50
QUESTION ONE [10 Marks]
The following information was obtained from the data sheet of a GN2470 IGBT.
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Turn-on delay time tdon = 8ns
Rise time tr = 400ns
Turn-off delay time tdoff = 20ns
Full time tf = 7000ns
Collector-to-emitter on-state voltage drop VCE(on) = 4.5V
The device was tested at a voltage of 400V and current of 2A with a switch duty ratio
of 0.5 to obtain its total power loss.
a) Sketch a suitable circuit for such a test
b) Sketch the waveforms for VGE, VCE, ic and PT (the instantaneous power dissipated in the
switch)
c) From the waveforms derive expression for the total power dissipated in the switch.
d) Calculate the switching losses and the on-state losses.
e) Calculate the total power dissipated during the test.
QUESTION TWO [20 Marks]
a) What is power electronics
b) Discuss the four main types of converters used in power electronics
c) Two 1.2kV thyristors are series connected and 2kV is connected across them
(Fig. 2.1). The thyristor off-state resistance, RT1 = 60kΩ of the first thyristor
and RT2 = 80kΩ of the second thyristor. Determine;
i.
ii.
iii.
Figure 2.1
the voltage drop (VT1) across thyristor T1 and voltage drop (VT2) across
thyristor T2
Voltage equalizing resistors (R) of 6.8kΩ are now connected across each
thyristor. Calculate the voltage across each thyristor.
Determine the power dissipation in the equalizing resistors.
d) Consider Figure 2.2 showing the turn-off switching characteristics of a power
diode in Amps /µs.
Figure 1.2
i.
Show that the reverse recovery current, Irr is given by the expression;
 di 
2Qrr  r 
 dt 
I rr 
1 S
ii.
Determine the;
a.
b.
c.
d.
Rate of fall of diode current
Softness factor
Peak reverse recovery current
Reverse recovery charge
QUESTION THREE [20 Marks]
a) Describe using diagrams the following power switching devices in terms of construction;
operation and voltage-current (v-i) characteristics:
i. BJT
ii. SCR
iii. MOSFET
b) A resistive load is to be supplied at 3 kV and 1100A. The thyristors available are rated at 800V,
200A. Calculate the number of series and parallel connected thyristors required, allowing one
extra series and parallel thyristor as a safety factor.
c) Determine suitable values for the series static and dynamic equalizing components, when the
maximum leakage current is 6 mA, the maximum difference in recovery charge is 20µC, and
the maximum rate of change of voltage is 200V/µs
THE END