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MODULE CODE ELEC0022-A6U

MODULE TITLE POWER ELECTRONICS

Original Exam Duration 3 hours

Answer ALL questions

SECTION A

1. (a) Describe what you think is meant by “Power Electronics” and give two examples of applications that would use power electronic devices.[3 marks]

(b) Describe the operation of a power Zener diode. Give the range of power values that this device can handle, and a typical value for the transient suppression power. What does the latter value mainly depend on?[4 marks]

(c) The required performance of a p-n diode in power applications can be achieved by a number of methods. Name three methods.[3 marks]

(d) Explain the terms uncontrolled, half-controlled and fully controlled relating to converters.[3 marks]

(e) Figure 1.1 shows a three-phase half wave fully controlled converter.

Given that v1 = Vm cos ωt , derive the mean output voltage Vmean of the converter in 

terms of the peak phase voltage Vm and firing angle α.[7 marks]

Answer(Purchase full paper to get all the solution)

1a)

Power electronics is a study that utilizes electronics power devices from converting one form of electric power into another form of electric form of electric power with proper control.

Two example of applications that will use power electronics device.

Ac/Dc converter

Fan regulator

1b)

Power Zener diode operate in reverse breakdown mode at low voltage (peak electric fields in scl sufficient to tunnel electrons from valence to conduction band). The range of power handling 250mW to 75W, but large area diodes (high ) are difficult to fabricate with hard breakdown. Transient suppression power handling up to 500kW, but depends on duty-cycle and heat sinking.

1c)

Method of achieving required performance of p-n diode.

The diode area can be made larger to increase I current.

Power losses can be minimized by high doping levels and careful design of contacts to minimize losses.

Heat can be dissipated by reverse mounting to place junction close to heat sink.

1d)

Uncontrolled converter: give rise to an overlap which affect the load voltage 

Half controlled converter: they are single quadrant converters having one polarity of voltage and current at Dc terminals.

Full controlled converter: these are sometimes used when thyristors are fired at 3rd quadrant and allows negative current pass through the load.

SECTION B

2. (a) Draw a plot of a typical doping profile and the associated electric field, through a power Bipolar Junction Transistor (BJT). What is the effect on the breakdown voltage of the n-region of the device?[5 marks]

(b) The breakdown voltage of the power BJT in reverse bias can be reduced by four methods. Briefly describe two of these.[6 marks]

(c) Sketch a plot of Ic versus Vce for the two methods you described in Question 2(b). Show the circuit diagram of the power BJT for the two methods.[6 marks]

(d) Explain the usual origin of the second breakdown in a power BJT.[3 marks]

3. (a) A Silicon Controlled Rectifier (SCR) remains latched on until two conditions are met. 

(i) State these conditions.

(ii) Draw a schematic of Ia and VAK against time illustrating the turn off sequence when 

forced commutation is used.

How soon can forward current be re-applied without breakdown occurring?[6 marks]

(b) Draw a schematic showing the general device structure of a gate turn off Thyristor (GTO). With reference to your schematic briefly describe how turn on and turn off is achieved in this device.[5 marks]

(c) In a MOSFET, the gate-source voltage Vgs controls the current flow to a large extent. 

(i) Briefly describe the operation of the device as the gate to source voltage is increased 

from 0 to 5 volts 

(ii) What is the approximate maximum sustainable value of Vgs to avoid “punch through” occurring?

(iii) Write down the relationship between the channel resistance and the breakdown voltage of the device if VDS > 100 V.[5 marks]

(d) Draw the equivalent circuit for an Insulated Gate Bipolar Transistor (IGBT). Briefly describe the operation of this device.[4 marks]

4 (a) A step-down converter using a power transistor is shown in Figure 4.1. Draw the equivalent circuit of the converter when the transistor is switched on and off. 

(b) Assume that the inductor current varies linearly during the switch on/off periods, sketch the waveforms of iT, iD, iL and iC which show two complete on-off cycles of operation.[8 marks]

(c) Given the transistor switch-on time (ton) and switch-off time (toff) are

Show that the peak-to-peak ripple current of the inductor L ( dI ) can be expressed as

where k is the duty cycle and f is the switching frequency.

(d) Given the input voltage Vs = 12 V, the required output voltage V0 = 5 V at the load. The switching frequency is 20 kHz. If the peak-to-peak ripple current of the inductor is limitedto 0.5 A, determine the duty cycle k and the filter inductance L.[6 marks]

5 (a) Name two types of snubber circuit.[2 marks]

(b) Figure 5.1 shows a snubber circuit.

(i) Identify and explain the operations of this snubber circuit.[4 marks]

(ii) The thyristor has a maximum dv/dt rating of 7×106 Vs-1and supplies a load resistance of 10 ohms. The maximum step change in voltage is 350 V. Find the value of the capacitor C which could protect the thyristor.

(c) What is the problem with the snubber circuit shown in Figure 5.1?[3 marks]

(d) Suggest a circuit which can rectify the problem in 5(c) above. Explain how your suggestion can solve the problem.[7 marks]

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Last updated: Sep 17, 2021 10:23 PM