ES3E00
THE UNIVERSITY OF WARWICK
Paper Details
Paper Code: ES3E0
Paper Title: Power Electronics
Exam Period: Summer 2023
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Exam Rubric
Time Allowed: 2 hours
Exam Type: Written – Standard
Answer all three questions
Approved Calculators: Only calculators that conform to the list of models approved by the School of
Engineering may be used in this examination.
Instructions: Candidates should answer all four questions. Read carefully the instructions on the
answer book and make sure that the particulars required are entered on each answer book.
Additional Stationery: Answer book, Engineering Data book 8th edition.
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1.
Constant Symbol Value Units
Charge of an electron q 1.6×10-19 C
Critical Electric Field of Si ΕCrit,Si 2.5×105 V/cm
Permittivity of Si εSi 1.054×10-12 F/cm
Table 1: Constants for use in question 1.
a) Sketch the cross-section of a practically realisable PiN diode including appropriate labels
of the terminals and the different semiconductor regions.
(5 marks)
b) Draw a graph detailing the approximate charge distribution throughout the drift region in
the on-state. Show what effect carrier lifetime will have on the charge distribution and
explain its impact on conduction and switching losses.
(7 marks)
c) Using the constants in Table 1, calculate the drift region width of a 1200 V silicon PiN
diode, using a drift region doping of 1×1014 cm-3
.
(10 marks)
Question 1 continued overleaf
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Question 1 continued
Figure 1: A reproduction of the reverse recovery waveform of a Si PiN diode.
d) A 1200 V, 50 A silicon PiN diode is used in a rectification application.
i) The switching characteristics of the PiN diode have been reproduced in Figure 1. It
is to be turned off from a nominal forward current with a di/dt of 40 A/μs. The
datasheet states that the recovered charge QRR is 0.5 μC at that di/dt and 25 °C. Find
the maximum reverse recovery current IRR and tRR. Assume that all the charge is
recovered by t2, i.e. t3-t2 is negligible.
(7 marks)
ii) Explain why reverse recovery occurs in a PiN diode. What alternative device might
you use in place of a 1200 V Si PiN diode that will minimise reverse recovery
effects What alternative problems might this cause
(7 marks)
Total 36 Marks
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CONTINUED
IF
-IRR
t t1 t2 t3
IA
0
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2.
a)
i) Sketch the cross-section of a power MOSFET including appropriate labels of the
terminals and the different semiconductor and insulator regions. Indicate on this
diagram (or on a second diagram) the origin of all the stray capacitances, resistances
and other parasitic effects.
(10 marks)
ii) Compile a list of fabrication instructions (known as a process flow), which would
allow your MOSFET design in part a)i) to
be manufactured in a clean room. Start
with a wafer of silicon of one doping type.
(7 marks)
iii) Explain why two p-type regions are required beneath the gate and source of a
MOSFET.
(3 marks)
b) Explain what is meant by the ‘unipolar limit’ of a material, referring to the drift region
width of a device. Sketch a graph representing the unipolar limits of silicon and silicon
carbide. Indicate the location on this graph of the unipolar and bipolar switching devices
covered in the lectures, namely silicon MOSFETs, IGBTs, BJTs and Thyristors, and
silicon carbide MOSFETs.
(8 marks)
Question 2 continued overleaf
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Question 2 continued
c) Table 2 lists some of the fundamental properties of silicon and silicon carbide. Explain
how these properties allow a silicon carbide MOSFET to operate at higher temperature,
increased efficiency and higher frequency than a silicon device at an equivalent
blocking voltage.
(8 marks)
Semiconductor Property Silicon Silicon Carbide
Bandgap, EG (eV) 1.1 3.26
Critical Electric Field, Emax (MV cm 1
) 0.25 2.2
Thermal Conductivity (W cm 1 K 1
) 1.5 3.8
Table 2: Fundamental properties of silicon and silicon carbide.
Total 36 Marks
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CONTINUED
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3.
a) A full bridge converter/inverter is used as part of a DC power supply as shown in Figure
B.1. The output voltage is required to be 60 V. The input voltage Vdc is obtained by
rectification of the mains and can vary from 130 to 380 V.
Vdc
Va Vb Vc
0V
D4
D1
D2
D3
T1
T2
T3
T4
D D
D D
0 V
Vdc
Supplied by a AC/ DC
rectifier
D21 D22
D23 D24
+
–
Transformer
Full bridge converter/inverter
Figure B.1
i) Explain why the standard Unipolar and Bipolar mode switching control strategies
may not be directly applied for controlling this full bridge converter/inverter.
ii) If the isolating transformer has the turn ratio N2:N1 of 1:2, with the switching
control strategy shown in Figure B.2, determine the range of the duty ratio
required.
Figure B.2
(12 Marks)
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Question 3 continued overleaf
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Question 3 continued
b) If the DC link voltage of the inverter is from a single-phase AC/DC diode
converter with a phase voltage of 230 V (rms), explain why a step-up
transformer is required to achieve the required DC line voltage. Write down your
explanation in the following space.
(6 Marks)
c) A flyback converter for 5 V to 100 V conversion is shown in Figure B.3. The converter
supplies power to a purely resistive load at R=250 .
i) Find the current and voltage rating of the switching device SW.
ii) Find the current and voltage rating of the diode D.
Figure B.3
(10 Marks)
Total 28 Marks
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END
5:100
5V 100 V R L C
SW
D