EE-321 N

Lecture-2Diodes (Contd) &

Thyristors

Ex. 2.2 MHR

The reverse recovery time of a diode is trr = 3 µsand the rate of fall of the diode current isdi/dt = 30 A/µs. Determine(a) the storage charge QRR, and(b) the peak reverse current IRR

5-Sep-12 2

Solution

(a)

(b)

( )

2

26

2 1/ 2

0.5 30 3 10 135 μC

RRrr RR rr

Q dit Q tdi dt dt

−

= ⇒ =

= × × × =

6 62 2 135 10 30 10

90 A

RR RRdiI Qdt

−= = × × × ×

=

5-Sep-12 3

See Prob. 2.1 MHR

45-Sep-12

Power Diode Types

55-Sep-12

1. General Purpose Diode

• Also called line frequency or mains diode• ON state voltage very low (below 1 V)• Large trr (about 25 µs)• Very high current (up to 5 kA) and voltage

(5 kV) ratings• Used in line-frequency (50/60 Hz) applications

such as rectifiers for electroplating, welding,traction

65-Sep-12

2. Fast Recovery Diode

• Very low trr (<1 µs)• Power levels at several hundred volts and

several hundred amps• Normally used in high frequency circuits like

choppers, inverters, induction heating

75-Sep-12

3. Schottky Diode

• Very low forward voltage drop (typical 0.3 V)• Limited blocking voltage (50-100 V)• Used in low voltage, high current application

such as switched mode power supplies.

85-Sep-12

See SiC Diodes in MHR

5-Sep-12 9

Today’s Task

Find out• A list of power electronic device manufacturers• Any magazine (Non IEEE) pertaining to PE• Relevant analysis softwares (other than

MATLAB)• Read out an article on significance of Power

Electronics

105-Sep-12

Thyristors(SCRs)

5-Sep-12 11

Thyristors

• Four layer, three terminal, minority carrierbased semi-controlled device

• Available in a wide range of ratings rangingfrom few A to several kA and few tens of voltsto several kV

• One typical rating is 4500 V, 4000 A, 10 kHz fora Gate Turn Off Thyristor (GTO)

• Thyristor family consists of a large number ofdevices (to be discussed later)

5-Sep-12 12

Thyristors

• Needs to be forward biased + a positivecurrent pulse supplied through the gateterminal to turn ON and conduct

• Once it latches into ON condition, the gateterminal loses its control and the devicecontinues to conduct just like a diode

• Gate does not play any role in turning OFF• That is why it is a semi-controlled device

5-Sep-12 13

Device Structure

5-Sep-12 14

Physical Layout

15

Thyratron Tube Thyristor

Device Symbol

A(+)

G, iG

K(−)

A(+)

K(−)

G, iG5-Sep-12

Operation & Characteristics

165-Sep-12

GiG

Ideal Characteristics

175-Sep-12

Forward Conduction

SCR Operation

Three modes:1. Reverse Blocking Mode2. Forward Blocking Mode3. Forward Conduction Mode

185-Sep-12

SCR Junctions

5-Sep-12 19

Forward Blocking Mode

• Anode is +ve w. r. t. cathode• Junctions J1 & J3 are forward biased while J2 is

reverse biased• Only a small leakage current flows from anode

to cathode• SCR does not conduct and this is known as the

forward blocking mode• Junction J2 breaks down at a sufficiently high

value of VAK called breakover voltage VBO

205-Sep-12

Forward Conduction Mode

• At a sufficiently high value of VAK called breakovervoltage VBO junction J2 suffers avalanchebreakdown

• Since J1 and J3 are already forward biased, therewill be free movement of carriers across all threejunctions, resulting in a large forward anodecurrent.

• The device will be in a conducting state or ON-state.

• The voltage drop would be due to the ohmic dropin four layers and typically 1 V

215-Sep-12

Contd...

• In the ON-state, the current is limited by theexternal impedance.

• The current must be greater than the latchingcurrent IL in order for the device to conduct;otherwise, the device will go into the blockingmode as the anode-cathode voltage isreduced.

22

Contd...

• If the current is reduced below a value whichis known as the holding current IH thethyristor will go into the blocking state.

• The holding current is in the range ofmilliamperes.

• Devices ceases to conduct if the current fallsbelow holding current level

23

2 to 3 times of L HI I=