Power Electronics - Silicon controlled rectifiers

Power Electronics
SILICON CONTROLLED RECTIFIERS
DEPT. OF ELECTRICAL ENGINEERING, COLLEGE OF ENGINEERING TRIVANDRUM 1
2018 Dr. Francis M. Fernandez

Silicon Controlled Rectifier (SCR)
2
DEPT. OF ELECTRICAL ENGINEERING, COLLEGE OF ENGINEERING TRIVANDRUM

SCR operating modes
1. Forward blocking mode
◦ When a positive voltage is applied to the anode of
the SCR with respect to the cathode, and with zero
gate current, the junction J2 will be reverse biased
and therefore the device will not conduct. This is the
forward blocking mode of SCR.
2. Reverse blocking mode
◦ When the cathode of SCR is made positive with
respect to the anode, the junctions J1 and J3 will be
reverse biased and therefore the device will not
conduct. This is the reverse blocking mode of SCR.
3
J1
J2
J3

SCR operating modes
3. Forward Conduction Mode
 An SCR will go into forward conduction mode in two ways:
a) without gate current, the anode voltage is increased beyond a
certain level called breakover voltage VBO
b) A gate current is applied so that the SCR starts conducting
 Once the SCR starts conducting, no gate current is needed to
maintain the anode current.
4

Latching and Holding current
Latching current
 The minimum anode current required to maintain the ON condition even
after removal of the gate current is the latching current.
 Typ value: 25 mA
Holding current
 The minimum anode current below which the SCR will go to forward
blocking state is the holding current.
 Typ value: 10 mA
Latching current > Holding current
5
Vg
VS
RL

SCR Characteristics
IG3 > IG2 > IG1 > IG0

Problem
7
Vg
24V
Rh
For the given circuit, a gate pulse is applied with Rh = 1200 ohms
a) Will the SCR turn on?
b) If the answer is “no” what is the condition to effect a turn on?
c) After turn on how can the SCR be turned off?
Assumptions:
Latching current = 24 mA
Holding current = 10 mA
Forward on state voltage = 0V

Solution
This current is less than the latching current; so the SCR will not turn on
8
Vg
24V
Rh
Current throgh the SCR when it is conducting =
24 V
1200 Ω
= 20 mA
Maximum value of load resistance =
24 V
24 mA
= 1000 Ω
To ensure proper turn on, the load resistance (Rh) should be 1000 ohms or less
To turn off the SCR, the current should be brought down to below 10 mA by
reducing the supply voltage of increasing the load resistance (Rh)

SCR Turn On Methods
1. Forward Voltage Triggering
2. Gate Triggering
3. Radiation Triggering (Light Triggering)
◦ Light falling on the junction created electron-hole pairs and leads to current
flow.
◦ This principle if used in the following devices:
◦ Light activated SCR (LASCR)
◦ Light activate silicon controlled switch(LASCS)
4. Thermal Triggering (Temperature Triggering)
◦ Width of depletion layer decreases with rise in junction temperature. If the
anode is at near break-over voltage and the if the temperature rises, the
device may start conducting.
◦ This is an undesirable triggering
5. dv/dt Triggering
9

SCR Turn On Methods
5. dv/dt Triggering
◦ The reverse biased junction of the SCR may have a capacitance across it.
When a sudden voltage is applied, the device may turn on due to the
capacitance charging current
10
    V
dt
d
C
C
dt
d
V
V
C
dt
d
Q
dt
d
i J
J
J
J
J 



• This is an undesirable triggering
• Typical limit for dv/dt is 10-20 V/μs
A
G
K
P
P
N
N

Snubber Circuit
 A Snubber Circuit may be used to eliminate the dv/dt turn on problem
 A suitable RC network forms the snubber circuit
DS
R1
CS
R2
R1
CS
VS VT
VS VT

Design of Snubber Circuit









S
t
S
T e
V
V 
1 S
T
S V
V
t
At 632
.
0
, 

S
S
S
S
C
R
V
V
dt
dv
1
632
.
0
632
.
0



1 , is the maximum discharge current of SCR
S
TD
TD
V
Also R where I
I

R1
CS
1
0.632 S
S
V
dv
dt R C


Problem
A SCR is to operate in a circuit where the supply voltage is 200 VDC. The
dv/dt should be limited to 100 V/ µs. Series R and C are connected across
the SCR for limiting dv/dt. The maximum discharge current from C into
the SCR, if and when it is turned ON is to be limited to 100 A. Using an
approximate expression, obtain the values of R and C.
R1
CS

Solution



 2
100
200
1
TD
S
I
V
R
S
S
C
R
V
dt
dv
1
632
.
0

S
C




2
.
2
200
632
.
0
10
100
6
F
CS 
575
.
0
100
2
.
2
10
200
632
.
0 6






Select 2.2 ohms
Select 0.68 μF
R1
CS

di/dt Rating
 Critical rate of rise of current is the maximum rate of rise of anode
current in the ON state that the device can safely withstand.
 If the rate of rise if faster than the spreading velocity of carriers across
the junction, hot spots may develop and damage the device.
 Specified for the highest value of junction temperature.
 Typical di/dt ratings are in the range of 50-800 A/μs
 Protection provided with a series inductor.
s
s
L
V
dt
di


Two Transistor Analogy
Eber Moll equation: = +
For TR1: = 1 +
For TR2: = 2 +
− − = 0 = +
= + + +
For gating signal = +
= ( + ) + + +

= ( + ) + + +
=
+ +
1 − ( + )
( + ) is called the loop gain
− − = + +
Two Transistor Analogy Contd.
(1 − ( + )) = + +
When ( + ) = 1, IA becomes very high.
Here, IA is limited by the external resistance (Load)

Parallel operation of SCRs
Forward
Current
Voltage Drop
T1 T2
T1
T2
I1
I2
I = I1 + I2
 Parallel operation is needed when the load current is more than
device rating
 Should be done carefully if the VI characteristics are different
L

Proper Current Sharing
Current sharing using series resistance
R2
R1
R1 + RT1 = R2 + RT2
T1
T2
The SCRs should turn on simultaneously
◦ SCRs are mounted symmetrically on the heat
sink to reduce difference of inductance of
conducting paths
◦ Series resistance connected in gate circuit

Proper Current Sharing
L2
L1
T1
T2
Current Sharing using magnetically
coupled series reactance

Series Operation of SCRs
 SCR with higher resistance will
have larger voltage drop
across it
 Results in unequal distribution
of voltages
 String efficiency reduces
T1
T2
SCRs
of
Number
SCR
one
of
rating
Voltage
string
the
of
rating
voltage
Actual
Efficiency
String



Voltage Equalisation
T1
T2
R2
C
R2
C
R1
R1
• Static Equalisation
• A uniform voltage distribution in
steady state can be achieved by
connecting a suitable resistance
(same value) across each SCR
  
1
max min
1
Where
number of devices
maximum permissible breakdown voltage
string voltage
device blocking current
s D s
s b b
s
D
s
b
n E E
R
n I I
n
E
E
I
 

 




R
E
P
dissipated
Power R
2
, 

Voltage Equalisation
T1
T2
R2
C
R2
C
R1
R1
• Dynamic Equalisation
• A simple resistor used for static
voltage equalization cannot maintain
equal voltage distribution under
transient conditions
• Parallel connected RC network does
the dynamic compensation
• Also acts as snubber circuit
 
type
same
the
of
SCRs
of
charge
recovery
reverse
between
difference
maximum
voltage
breakdown
e
permissibl
maximum
devices
of
number
Where
1
max
max









Q
E
n
E
E
n
Q
n
C
D
s
s
D
s
s

Structure of SCR

TRIAC
Ig = 0
Ig
1
Ig
2
Ig
3
Ig =
0
Ig
1
Ig
2
Ig
3
IT
VT
Symbol
Characteristics
Internal Structure
 Conducts in both half cycles of AC supply voltage
 Has two main terminals and a gate

Triac Circuit & Waveforms
SCR TRIAC

Power Electronics - Silicon controlled rectifiers

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