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MODULE CODE ELEC0022-A6U
MODULE TITLE POWER ELECTRONICS
Original Exam Duration 3 hours
Answer ALL questions
SECTION A
(a) The reverse breakdown voltage (VBR) sets the performance limit for all semiconductor power electronic devices.
State the four main parameters which affect VBR in a Si p-n junction diode.
[ 4 marks]
Name TWO techniques for reducing air breakdown.
[ 2 marks]
The IGBT is a device with MOSFET gate control and bipolar transistor power handling (i) Sketch the equivalent circuit of an IGBT
(ii) What desirable characteristics and properties from MOSFETs and bipolar transistors does an IGBT combine?
A 3-phase fully controlled converter is shown in Fig. 1.1.
Fig. 1.1 A 3-phase fully controlled bridge converter
Show that the mean load voltage (Vmean) of the 3-phase fully controlled bridge converter in terms of the peak source voltage (Vmax) and firing angle (?) is:
(ii) If the phase voltage of the system is 415 V, evaluate the firing angle if the mean load voltage (Vmean) is 300 V.
[1 mark]
The converter in (i) is designed to supply energy to a 380V 3-phase AC system through a 15kW DC link and another converter operating in inverter mode with interphase reactor. The DC transmission line has a resistance RTL of 0.2 Ω. Neglect the source inductance in the two systems. Answer (iii) to (v) based on the above information.
(iii) Draw the interconnection diagram of the system.
(iv) Evaluate the current in the DC link.
v) Evaluate the firing advance angle (? ) of the 380V AC system.
[2 marks]
ANSWER(Purchase full paper to get all the solution)
1a)i parameter that affect reverse breaks down voltage.
1aii) Techniques for reducing air breakdown.
1bi)
1bii) The IGBT transistor take the high input impedance and high switching speed of a MOSFET with the low saturation voltage of a bipolar transistor and combines them together to produce another type of transistor switching device that is capable of handling large collector-emitter current with virtually zero gate current drive. It also combine the insulated gate of MOSFET with the output performance characteristic of a conventional bipolar transistor.
1ci)
Output voltage waveform can be written as
1cii)
1ciii)
1civ)
1cv)
Finding advance angle (β)
Since L is neglected.
SECTION B
(a) For an npn bipolar transistor with a gain β0 = 50,
(i) What is the ratio between the sustaining voltage, VCEO(sus), and the open emitter collector-base breakdown voltage, VCBO ?
(ii) If the doping concentration of its collector is Nd = 1 x 1020 atoms·m-3, and the critical field strength of the collector-base junction is Ecrit = 1.86x107 V/m, what is the value of
VCBO ?
(iii) What is the value of VCEO(sus)?
(iv) Sketch the doping profile of a typical npn bipolar transistor
(i) What is the difference between a power MOSFET and a bipolar transistor in terms of breakdown in the safe operating area?
(ii) What are the typical values of the parameters VDS_off, ID_on and foperating of a power MOSFET?
(iii) A power MOSFET is with a gate-source parasitic capacitor CGS = 800 pF, a gate-drain parasitic capacitor CGD = 300 pF, and a threshold voltage VTH = 3.5 V. The power MOSFET is initially off and its source is connected to ground. Now apply a 10V step voltage to the gate from the voltage source via a gate resistor Rg = 10 ohm. How long does it take for the gate-source voltage VGS to reach VTH ?
(a) Draw the two-transistor model of a silicon-controlled rectifier (SCR)
After the SCR has been latched the forward voltage drop across the device can be reduced slightly.
Approximately how much will the latching current drop in this process?
(ii) If the current drops much below the above value cut off will occur. Briefly explain the process which causes this to happen.
(iii) After cut-off, if the forward voltage remains applied at a high enough level, the device may turn on again. Explain the reason this may occur
(iv) Which grade is a SCR with a recovery time of 500 µs in?
marks]
A power MOSFET is in the on state. The current pasisng through the MOSFET is 20A, and the on resistance Ron = 0.2 ohm at a junction temperature of 125 ?C.
What is the power dissipation of the power MOSFET in this state?
(ii) The power MOSFET is in a case that is mounted on a heat sink. The ambient
temperature is 25 ?C. What is the junction-ambient thermal resistance between Rθj-a ?
(iii) If the junction-case thermal resistance Rθj-c = 0.8 K/W, and the case-sink thermal resistance Rθc-s = 0.1 K/W, ignore the case-ambient thermal resistance Rθc-a, what is the maximum sink-ambient thermal resistance Rθs-a in order to keep the junction temperature not exceeding 125 ?C?
Fig. 4.1 shows a three-phase full-control converter.
(a) (i) Why is it a full-control converter?
What is the required firing angle condition to operate the converter in the inverter mode?
What is the relation between firing angle (?) and firing advance angle (?)
[3 marks]
The firing advance angle (?) of the converter shown in Fig. 4.1 is 15º. Based on the first phase voltage (v1) provided in Fig. A4.1 on page 6, draw, in the same figure,
the second phase voltage (v2),
the third phase voltage (v3),
the gate current of thyristor T1 (ig1),
the gate current of thyristor T2 (ig2),
the gate current of thyristor T3 (ig3),
the current of thyristor T1 (iT1),
the current of thyristor T2 (iT2),
the current of thyristor T3 (iT3), and
the load voltage (vL)
the voltage of thyristor T1 (vT1)
[10 marks]
Derive the mean output voltage (Vmean) of the converter at the load when operating in the inverter mode. Define your parameters in the equation clearly.
If the power supply is heavily inductive,
explain what the potential problem is, and (ii) suggest a precaution for the problem.
[4 marks]
Fig. 4.1 A three-phase full-control converter
Candidate No.: ___________________________
Fig. A4.1
5. (a) Explain the four modes of operation of a DC chopper on a motorised vehicle in terms of the
VL/IL diagram.
[4 marks] (b) Fig. 5.1 shows a chopper circuit, explain how it can operate as
class A chopper
class B chopper
Consider the DC chopper in Fig. 5.1 operates as a class A DC chopper. Derive
the thyristor on-time (t1),
the thyristor off-time (t2), and
the RMS value of the ripple current of the output.
in terms of the output ripple current (?I), load inductance (L) and the source voltage (Vs) and output voltage (Ea). State all your assumptions.
Fig. 5.1 A DC chopper
A class A DC chopper is operating at a frequency of 4 kHz from a 72 V DC source to supply
a load of resistance 6 ? and inductance 30 mH. If the mean load voltage is 48 V. Evaluate,
the mark-space radio of the output waveform,
the maximum load current, and (iii) the minimum load current.
6 marks]
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Last updated: Jun 29, 2021 10:44 PM
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