1. INTRODUCTION
POWER ELECTRONICS

2. Power Electronic Devices
Diode
Thyristor (also called as Silicon Controlled Rectifier, SCR)
Gate Turn Off Thyristor (G.T.O)
TRIAC
Bipolar Junction Transistor (BJT)
Metal Oxide Field Effect Transistor (MOSFET)
Insulated Gate Bipolar Junction Transistor (IGBT)

3. Power Electronic Devices Properties
Types and Characteristics
Ideal Switch
Losses
Ratings
Device Comparison
Worked Examples

4. Power semiconductor devices (Power switches)
Power switches operates in two states:
Fully on. i.e. switch closed.
Conducting state
Fully off , i.e. switch opened.
Blocking state
Power switch never operates
in linear mode.
Switch ON(closed)
Switch Off(opened)
Power Semiconductor Devices are categorised into three groups:
Uncontrolled: Diode :
Semi-controlled: Thyristor (SCR).
Fully controlled: Power transistors: e.g. BJT, MOSFET, IGBT, GTO, IGCT

5. Diode PN Junction Diode Conducts during forward bias (ON)
About 0.7v drop when it conducts, carries maximum current
Available with various current ratings(up to 1000A!)
Doesn’t conduct during reverse bias(OFF), minor leakage current
Entire voltage will drop across diode (up to 5000V!)

6. Forward and reverse biasing

7. Diode construction and VI curve

8. Diode Characteristics

9. Diode in different package

10. Thyristor / Silicon Controlled Rectifier(SCR)
Diode with control terminal for conduction
PNPN Structure having Gate terminal at P
What is the role of Gate terminal?

11. SCR Characteristics

12. SCR Characteristics

13. HOLDING CURRENT LATCHING CURRENT
If the anode current is reduced below the critical holding current value, the thyristor cannot maintain the current through it and reverts to its off state usually is associated with turn off the device.
LATCHING CURRENT
After the SCR has switched on, there is a minimum current required to sustain conduction. This current is called the latching current. associated with turn on and is usually greater than holding current.

14. Factors Causing Turn ON of SCR
Forward voltage VAK > VBO. This should be avoided since it may permanently damage the device.
Rise in device temperature can cause unwanted turn ON
and hence should be avoided by cooling the device.
By injecting positive gate current IG until IA = IL where IL is the latching value. This is the preferred method of turning ON the device.
Forward dvAK / dt > rated value causes undesirable turn ON and should prevented by connecting a snubber circuit across the SCR.
Light radiation of specific wavelength incident on junctions of SCR turns ON the device. LASCR’s are turned ON by this method.

15. To turn ON SCR Apply forward voltage to anode/cathode
Apply current to gate (+ve gate/cathode)
resultant anode/cathode current to exceed latching current(!) of device, SCR remains ON
Gate current can be removed
stays ON provided anode to cathode forward current is more than holding current(!) of device)

16. To turn OFF SCR
Anode/cathode current reduced to less than holding current
by forced commutation (dc system)
by natural commutation (ac system)
Blocks in reverse direction up to breakdown voltage.
5kV, 4kA

17. Gate Characteristics of SCR

18. Thyristor Gating Requirement
To turn on a thyristor positively in the shortest time, it is required to have a gate current with a fast rise time up to the maximum permitted value
This rise time is achieved by pulse techniques
It is also referred as firing pulses

19. Gate Turn-Off Thyristor
A gate-turn-off thyristor (GTO) like an SCR can be turned ON by applying a positive gate signal.
It can be turned OFF by a negative gate signal.

20. TRIAC (AC Switch)

21. TRIAC-Modes of Operation

22. Bipolar Junction Transistors (BJT)E
Base
Control terminal
Collector/Emitter
Power terminals B

23. Characteristics of Ideal Switch
ON – conducting with low voltage drop (saturation Region)
OFF – blocking current against applied voltage (Cut-off Region)
For BJT
ON – apply base current
OFF – remove base current
1kV, 400A, Switching frequency – 5kHz

24. Characteristics of BJT

25. Safe Operating Area Peak current Boundary Power Boundary
Avalanche voltage breakdown boundary

26. Avalanche breakdown
A transistor has a maximum collector-to-emitter voltage Vce(sat) that it can withstand, above which avalanche breakdown will occur. Avalanche breakdown need not necessarily result in permanent damage
Secondary Breakdown
It is a destructive phenomenon that results from the current flow to a small portion of the base, producing localized hot spots. If the energy in these hot spots is sufficient the excessive localized heating may damage the transistor permanently.
Peak Current
It is the maximum permissible collector current
Maximum Power or Temperature
Maximum permissible collector power or permissible junction temperature

27. Device Losses Forward conduction Switching Loss
forward voltage drop x conduction current
Leakage current during blocking state
Losses in gate or base circuit path
Switching Loss
energy dissipated during turn-on and turn-off
significant at high frequency

28. Tutorial from text book-EX 1.2 Pg 33
During turn-on and turn-off of a power transistor supplying a resistive load the voltage and current variations are as shown (idealised linear changes with time).
Show that, for each, the switching energy loss is given by (VI/6) T, where V is the off-state voltage, I the on-state current, and T is the switching time.
Determine the mean power loss due to switching at a frequency of 10kHz.
If the transistor is operating with a duty cycle of 0.5, that is equal off and on times, determine the conduction loss if the collector-emitter voltage is 2.1 V at 200 A. Hence determine the total device loss.

29. MOSFET
Drain
Structure
Gate
Source
Symbol

30. MOSFET is a voltage control device
MOSFETs are of two types
Depletion MOSFETs
Enhancement MOSFETs
500V, 200A and very high switching frequency up to 1MHz
It has high input impedance.(No loading of source,less current drawn and low input power dissipation)
Internal(dynamic) resistance between drain and source during on state Rds(on) limits power handling capacity of MOSFET

31. N-channel Depletion MOSFET
N-channel Enhancement MOSFET

32. MOSFET Characteristics
Apply gate voltage to switch ON
Remove gate voltage to switch OFF
Match demand current with gate voltage. ,

33. Advantages Disadvantages No Power switching losses
No secondary breakdown
Much higher switching speed
Better reliability and thermal stability
Higher peak current handling capability
Disadvantages
The only disadvantage is higher conduction drop –typically 4.5V

34. Insulated Gate Bipolar Transistor(IGBT)
IGBT is a voltage controlled device.
It has high input impedance like a MOSFET and low on-state conduction losses like a BJT.

35. With collector and gate voltage positive with respect to emitter the device is in forward blocking mode.
When gate to emitter voltage becomes greater than the threshold voltage of IGBT, a n-channel is formed in the P-region. Now device is in forward conducting state
In this state p+ substrate injects holes into the epitaxial n- layer.
Ratings
3.3kV, 1.2kA and typical switching frequency is 20-50kHz

36. Device Comparisons Ideally Thyristor
unlimited voltage and current ratings
instant turn-on and turn-off times
zero leakage current
zero conduction and switching loss
zero gate or base power requirement
ability to withstand current overloads and voltage transient
easy to protect against spurious turn-on and fault conditions
low cost and ease of assembly
Thyristor
highest ratings, robust, low conduction loss, slow, difficult to turn off in dc systems.
Used in electricity supply systems - WHY ?

37. Ratings Ratings normally expressed as Voltage Current Temperature Time
forward, reverse, repetitive, non-repetitive, rate of rise
Current
forward, leakage, rms or mean (waveshape), rate of rise.
Temperature
losses, cooling strategies
Time
current and voltage rates, switching speed
Example
Transistor Safe Operating Area (SOA)

38. Five Comparison Between MOSFET and BJT
Voltage and Current Ratings The voltage and current ratings of MOSFET family is lesser than the BJT. It is 500V & 200A for MOSFET and 1kV & 400A for BJT. Speed of Operation Typical frequency of MOSFET is greater than 100kHz and for BJT it is less than 100kHz. Thus MOSFET is used for high frequency applications as dc-dc converter and switch mode power supplies

39. Conduction Loss, Switching Loss The conduction loss of BJT is lower than that of MOSFET, but the switching loss MOSFET is lower than BJT, thus making it good for high frequency application. The conduction loss of BJT is less due to low on state voltage drop (1-2V) Robustness Both the devices can operate with junction temperature upto 125 o c. Both the devices cannot withstand reverse voltage without additional protection and the MOSFET can be damaged by static charge handling. Cost The BJT tends to be cheaper for similar ratings. However, the cost of drive electronics is cheaper for MOSFET

40. Switches Comparison Thyristor GTO BJT MOSFET IGBT Availability
Early 60s
Mid 80s
Late 70s
Early 80s
Late 80s
Voltage Ratings
5kV
1kV
500V
3.3kV
Current Ratings
4kA
5kA
400A
200A
1.2kA
Switching Frequencies
low
2kHz
5kHz
1MHz
100kHz
On-state voltage drop 2V
2-3V
1-2V
I*Rds(on)
4.5V
Drive Circuit
Simple
Very difficult
Difficult
Very simple
Comments
Cannot turn-off using gate signals
King in very high power
Phasing out in new products
Good performance in high frequency
Best overall performance

41. Power Electronics Devices: Summary

42. Power Electronics Devices: Summary
Figure 4Power Devices, Symbol, Characteristics Courtesy:‘Power Electronics’; M.H Rashid, Prentice Hall,2003

43. Types of Power Electronics Converters
AC to DC: RECTIFIER
AC input
DC output
AC to AC:
CYCLO-
CONVERTER
DC input
AC output
DC to AC: INVERTER
DC input
DC output
DC to DC: CHOPPER

44. Applications Static applications
involves non-rotating or moving mechanical components.
Examples:
DC Power supply, Un-interruptible power supply, Power generation and transmission (HVDC), Electroplating, Welding, Heating, Cooling, Electronic ballast
Drive applications
contains moving or rotating components such as motors.
Electric trains, Electric vehicles, Air-conditioning System, Pumps, Compressor, Conveyer Belt (Factory automation).

45. Applications
Figure 5 Applications of power devices Courtesy:‘Power Electronics’; M.H Rashid, Prentice Hall,2003

46. Applications-HVDC TRANSMISSION

47. Applications-AC DC DRIVES

48. Applications-STANDBY INVERTER

49. Applications-RENEWABLE ENERGY

50. Applications-ELECTRIC VEHICLE

51. Applications-VARIOUS OTHER APPLICATIONS