DIPLOMA IN ELECTRICAL ENGINEERING
LEVEL III
POWER ELECTRONICS
DRAFT 1
PREP BY: Hughes Chifulwa
QUESTION 1
a) A single-phase fully controlled bridge rectifier is supplied at 120V,
determine the average voltage drop for firing angle of 30˚. Assume each
thyristor having a voltage drop of 0.78V. [5 marks]
b) Find the displacement factor for a fully controlled converter corresponding
to a firing angle of λ = 45˚. [5 marks]
c) Determine the power factor of a single-phase fully-converter with highly
inductive load for α = 45˚. [5 marks]
d) Explain the reason why the power factor of semi-converters is better than
full-converters. [5 marks]
QUESTION 2
In details, explain the following SCR protection methods;
a)
b) dv/dt protection
c) Over-current protection
d) Over-voltage protection [5 marks each]
QUESTION 3
a) In a step down Chopper, the supply voltage is 250V, the switching
frequency is 300Hz, K=0.6, R=5Ω, L=5mH and Eb=100V. find;
i.
The average output current of a Chopper[5 marks]
ii.
The maximum current[4 marks]
iii.
The minimum current of steady state output current[3 marks]
b) For a step-up chopper, it is required to deliver a load voltage of 300V from
100V dc source. If the non-condition time of SCR is 100µs, find out the
required Pulse Width. (ton) [8 marks]
QUESTION 4
Consider the step-down converter circuit shown below with a purely capacitive
snubber having Cs=Cs1. Assume Io=25A, an input voltage Vd=200V and a reverserecovery time, trr=0.2µs for the free-wheeling diode;
a) Calculate the reduction in the turn-off losses in the BJT due to the use of
the snubber capacitor. [Assume a switching frequency of 20kHz and a
current fall time tfi of 0.4µs. [10 marks]
b) Calculate the increase in BJT losses during turn-on due to Cs. [Assume that
during the turn-on dic/dt=50A/µs. [10 marks]
QUESTION 5
In a single-phase full-bridge PWM inverter, the input dc voltage varies in a range
of 295V – 325V because of low distortion required in the output Vo, ma<1.0.
a) Determine the highest Vo1 that can be obtained and stamped on its name
plate as its voltage rating. [5 marks]
b) Its nameplate volt-ampere rating is specified as 2000VA, that is,
Vo19max)Io(max) =2000VA, where io is assumed to be sinusoidal. Calculate the
combined switch utilization ratio when the inverter is supplying its rated
volt-amperes. [15 marks]
QUESTION 6
a) The small-signal gate-source capacitance of a MOSFET decreases as VGS is
increased from zero volts. For VGS˃VGS(th), the small signal capacitance is
constant. Qualitatively explain the reason for this behavior. [6 marks]
b) An n-channel MOSFET is to be used in a step-down converter circuit. The dc
voltage Vd=300V, the load current Io=10A,the free-wheeling diode is ideal,
and the MOSFET is driven by a 15-V (base to peak) square wave (50% duty
cycle and zero dc value) in series with 50Ω. The MOSFET characteristics are;
VGS(th)=4V, ID=10A at VGS=7V, Cgs=1000pF, Cgd=150pF and rDS(on)=0.5Ω.
i.
Sketch and dimension VDS(t) and iD(t). [10 marks]
ii.
Estimate the power dissipation at a switching frequency of 20kHz.
[4 marks]
QUESTION 7
a) Explain the difference in operation between the Astable Multivibrator and
Monostable multivibrator. Include circuit diagrams in you explanation.
[10 marks]
b) A 555 timer configured to run in the astable mode is shown below.
Determine;
i.
The frequency of the output[5 marks]
ii.
The duty cycle[5 marks]
QUESTION 8
A 230V, 50Hz supply is connected to a 1-Phase transformer which feeds a diode
bridge rectifier. Primary to secondary turns ratio for transformer is 0.5 and load RL
has a ripple free current Io = 10A. Determine; [8marks]
a) The average value of output voltage[2marks]
b) Input distorting factor[2marks]
c) Input displacement factor[2marks]
d) Input power factor[2marks]
e) Input current harmonic factor (or THD) [2marks]
f) Crest factor[2marks]