1. Institute of Technology
Bahir Dar University
Institute of Technology
School of Computing and Electrical Engineering
Eeng-5181 : Power Electronics and Electric Drives
Nov, 2017

2. 1. Introduction to power electronics & electrical Drive
Power Electronics is the art of converting electrical energy from one form to another in an efficient, clean, compact, and robust manner for convenient utilization.
Exmple : A passenger lift in a modern building equipped with a Variable-Voltage-Variable-Speed induction-machine drive offers a comfortable ride and stops exactly at the floor level.

3. Interdisciplinary Nature of Power Electronics

4. Power Electronics involves on the study of:
Power semiconductor devices - their physics, characteristics, drive requirements and their protection for optimum utilization of their capacities,
Power converter topologies involving them,
Control strategies of the converters,
Digital, analogue and microelectronics involved,

5. Rotating and static electrical devices,
Capacitive and magnetic energy storage elements,
Rotating and static electrical devices,
Quality of waveforms generated,
Electro Magnetic and Radio Frequency Interference,
Thermal Management

6. Application of Power Electronics

7. Power Electronics verses Linear Electronics
In Power Electronics all devices are operated in the switching mode either 'FULLY-ON' or 'FULLY-OFF' states.
The linear amplifier concentrates on fidelity in
signal amplification, requiring semiconductor devices to operate strictly in the linear (active) zone.
Power semiconductor devices are operating at ‘CUT OFF’ or ‘SATURATION’ region.

8. Typical Bipolar transistor based (a) linear (common emitter) (voltage) amplifier stage and (b) switching (power) amplifier

9. Operating zones for operating a Bipolar Junction Transistor as a linear and a switching amplifier

10. Review of power semiconductor devices & their characteristics
power semiconductor devices includes on the study of:
power diode,
Thyristor (SCR, Triac, GTO, MCT)
BJT,
MOSFET,
IJBT.

11. Power diodes play an important role in power electronics circuits
They are mainly used as:
Uncontrolled rectifiers ,
Electro plating (coat a metal object by electrolytic deposition with another metal),
Battery charging,
Welding,
Power supply (dc or ac),
Variable frequency drives,
Feed back and freewheeling functions.

12. Construction and Characteristics Power Diodes
Power diodes are usually constructed using silicon because silicon diodes can operate at higher current and at higher junction temperatures than germanium diodes.
Conventional diode is two layer of p-n junction device.
But, a typical Power diode has P-I-N structure (additional or drift layer) in between p-n layer.
This is to improve high power holding capacity with minimum power loss.

13. Construction of Power Diodes

14. voltage-current characteristics of Power Diodes
Ideal condition:
Forward conduction: Resistanceless
Reverse blocking: Lossless
Switching ON/OFF time: Instantaneous
a. Ideal
b. Practical

15. Types of Power diode
Line-frequency diodes:- These are PIN diodes with general-purpose rectifier type applications
High speed /or fast recovery/:-used for high frequency switching of power convertors.
Schottky diode:-have low on state voltage and very small recovery time, typically nanoseconds

16. Thyristor:
The thyristor is the general name given to a family of power semi conductor device(SCR, Triac, GTO, MCT)
Silicon controlled rectifier (SCR):
SCR is basically a four-layer three-junction PNPN device.
It has three terminals: anode(A), cathode(K), and gate(G).

17. (a)
(b)
(c)

18. voltage-current characteristics of SCR

19. Latching Current:-This is the minimum anode current required to maintain the thyristor in the on-state immediately thyristor has been turned on and the get signal has been removed.
Holding current:- This is the minimum anode current required to maintain the thyristor in the on-state.

20. Turn-ON Method of Thyristor
Forward voltage triggering
Thermal triggering
Radiation triggering
dv/dt- triggering
Get trigering
Turn-OFF Method of Thyristor
Natural commutation
Forced commutation

21. Example1: The latching current of an SCR circuit below is mA, the duration of the firing pulse is 50µs. Will the SCR get fired?
The answer will be ‘NO’

22. Example2: If the latching current of an SCR circuit below is mA, obtain the minimum width of the gating pulse required to properly turn on the SCR.

23. TRIAC: The Triac is a member of the thyristor family.
But unlike a thyristor which conducts only in one direction a triac can conduct in both directions.
Thus a triac is similar to two back to back (anti parallel) connected thyristosr but with only three terminals
The triac turns off only when the current through the main terminals become zero

24. It is extensively used in:
residential lamp dimmers,
heater control and
for speed control of small single phase series and induction motors

25. (a) anti parallel connected thyristosr
(a) Electrical symbol
Exit

26. Steady state V-I characteristics of Triac

27. GTO (Gate Turn-Off Thyrstor)
Gate Turn-Off Thyrstor(GTO) as the name indicates, is basically a thyristor type device that can be turned on by small positive gate current pulse.
But in addition, has the capability of being turned off by a negative gate current pulse.
Has slow switching speeds
Used at very high power levels

28. A GTO : (a) Electrical Symbol,
(b) practical v-I characteristics,
(c) idealized v-I characteristics

29. The power MOSFET
Unlike a diode, turn on and turn off of a transistor are controllable.
Types of transistors used in power electronics circuits include MOSFET, BJT, IGBT.
MOSFET is a Voltage controlled device
Its construction includs a parasitic (body) diode.
It is a three terminal device where the voltage on the gate terminal controls the flow of current between the output terminals, Source and Drain.

30. Circuit symbol of MOSFET

31. V-I characteristic of MOSFET

32. IGBT
The IGBT has the high input impedance and high-speed characteristics of a MOSFET with low saturation voltage of a bipolar transistor.
The IGBT is turned on by applying a positive voltage between the gate and emitter
And turned off by making the gate signal zero or slightly negative.
It has a much lower voltage drop than a MOSFET of similar ratings.

33. The IGBT cell has a parasitic p-n-p-n thyristor structure embedded into it
The constituent p-n-p &, n-p-n transistor and the driver MOSFET are shown in its structure
Important resistances in the current flow path are also indicated.

34. Parasitic thyristor in an IGBT cell.
Schematic structure
Exact equivalent circuit
Approximate equivalent circuit
Circuit symbol
EXIT

35. THANK YOU!