Short Notes: Power Semiconductor Devices | Short Notes for Electrical Engineering - Electrical Engineering (EE) PDF Download

 Page 1


 
 
 
 
 
Properties of ideal switch 
1. Conduction state , 
ON ON
V 0, I ? ? ? ? ? ? 
2. Blocking state , 
OFF OFF
V 0, V ? ? ? ? ? ? 
3. Ideal switch can change its state instantaneously 
ON OFF
T 0 , T 0 ?? 
4. No power loss while switching. 
5. Stable under all operating conditions. 
Classification of switches 
1. Uncontrolled switch (Passive switch) 
Switching state cannot be controlled by any control signal E.g. Diode 
2. Semi-controlled switch 
Only one switching state can be controlled by an external control signal. E.g. SCR 
3. Fully controlled switch 
If both switching states can be controlled by switchable control signal.  E.g. BJT, MOSFET. 
Other Classification 
1. Unipolar switch 
The switch can block only one polarity of voltage when it is in OFF state. 
2. Bipolar switch 
This switch can block both polarity of voltage when it is in blocking state. 
3. Unidirectional switch 
This switch can carry current in only one direction when it is in conduction state. 
4. Bidirectional switch 
This switch can carry current in both the directions when it is in conduction state. 
Page 2


 
 
 
 
 
Properties of ideal switch 
1. Conduction state , 
ON ON
V 0, I ? ? ? ? ? ? 
2. Blocking state , 
OFF OFF
V 0, V ? ? ? ? ? ? 
3. Ideal switch can change its state instantaneously 
ON OFF
T 0 , T 0 ?? 
4. No power loss while switching. 
5. Stable under all operating conditions. 
Classification of switches 
1. Uncontrolled switch (Passive switch) 
Switching state cannot be controlled by any control signal E.g. Diode 
2. Semi-controlled switch 
Only one switching state can be controlled by an external control signal. E.g. SCR 
3. Fully controlled switch 
If both switching states can be controlled by switchable control signal.  E.g. BJT, MOSFET. 
Other Classification 
1. Unipolar switch 
The switch can block only one polarity of voltage when it is in OFF state. 
2. Bipolar switch 
This switch can block both polarity of voltage when it is in blocking state. 
3. Unidirectional switch 
This switch can carry current in only one direction when it is in conduction state. 
4. Bidirectional switch 
This switch can carry current in both the directions when it is in conduction state. 
Page 3


 
 
 
 
 
Properties of ideal switch 
1. Conduction state , 
ON ON
V 0, I ? ? ? ? ? ? 
2. Blocking state , 
OFF OFF
V 0, V ? ? ? ? ? ? 
3. Ideal switch can change its state instantaneously 
ON OFF
T 0 , T 0 ?? 
4. No power loss while switching. 
5. Stable under all operating conditions. 
Classification of switches 
1. Uncontrolled switch (Passive switch) 
Switching state cannot be controlled by any control signal E.g. Diode 
2. Semi-controlled switch 
Only one switching state can be controlled by an external control signal. E.g. SCR 
3. Fully controlled switch 
If both switching states can be controlled by switchable control signal.  E.g. BJT, MOSFET. 
Other Classification 
1. Unipolar switch 
The switch can block only one polarity of voltage when it is in OFF state. 
2. Bipolar switch 
This switch can block both polarity of voltage when it is in blocking state. 
3. Unidirectional switch 
This switch can carry current in only one direction when it is in conduction state. 
4. Bidirectional switch 
This switch can carry current in both the directions when it is in conduction state. 
 
 
 
 
 
Power loss in a switch 
 
1) The average power has in a switch is given by  
               P
T
o
1
vidt
T
?
?
 
 Where v = instantaneous voltage 
  i = instantaneous current 
2) If the device is modeled as a resistance, as in case of a MOSFET 
              
22
rms ON rms ON
P I R V R ?? 
3) If the device is modeled as a voltage source. 
                       
avg
P V I ?
 
 
Silicon Controlled Rectifier 
 
 
 
 
 
 
 
? In forward blocking mode, 
13
J , J are forward biased and 
2
J is reverse biased. 
? In forward conduction mode, 
2
J breakdown, 
13
J , J are forward biased. 
? In reverse blocking mode, 
13
J , J are reverse biased & 
2
J is forward biased. 
Latching Current 
This is the minimum value of anode current above which SCR turns ON. This is related to 
minimum gate pulse width requirement for SCR. 
Holding current 
Minimum value of anode current below which SCR turns OFF. 
 
Page 4


 
 
 
 
 
Properties of ideal switch 
1. Conduction state , 
ON ON
V 0, I ? ? ? ? ? ? 
2. Blocking state , 
OFF OFF
V 0, V ? ? ? ? ? ? 
3. Ideal switch can change its state instantaneously 
ON OFF
T 0 , T 0 ?? 
4. No power loss while switching. 
5. Stable under all operating conditions. 
Classification of switches 
1. Uncontrolled switch (Passive switch) 
Switching state cannot be controlled by any control signal E.g. Diode 
2. Semi-controlled switch 
Only one switching state can be controlled by an external control signal. E.g. SCR 
3. Fully controlled switch 
If both switching states can be controlled by switchable control signal.  E.g. BJT, MOSFET. 
Other Classification 
1. Unipolar switch 
The switch can block only one polarity of voltage when it is in OFF state. 
2. Bipolar switch 
This switch can block both polarity of voltage when it is in blocking state. 
3. Unidirectional switch 
This switch can carry current in only one direction when it is in conduction state. 
4. Bidirectional switch 
This switch can carry current in both the directions when it is in conduction state. 
 
 
 
 
 
Power loss in a switch 
 
1) The average power has in a switch is given by  
               P
T
o
1
vidt
T
?
?
 
 Where v = instantaneous voltage 
  i = instantaneous current 
2) If the device is modeled as a resistance, as in case of a MOSFET 
              
22
rms ON rms ON
P I R V R ?? 
3) If the device is modeled as a voltage source. 
                       
avg
P V I ?
 
 
Silicon Controlled Rectifier 
 
 
 
 
 
 
 
? In forward blocking mode, 
13
J , J are forward biased and 
2
J is reverse biased. 
? In forward conduction mode, 
2
J breakdown, 
13
J , J are forward biased. 
? In reverse blocking mode, 
13
J , J are reverse biased & 
2
J is forward biased. 
Latching Current 
This is the minimum value of anode current above which SCR turns ON. This is related to 
minimum gate pulse width requirement for SCR. 
Holding current 
Minimum value of anode current below which SCR turns OFF. 
 
 
 
 
 
Slope of characteristics = 
??
??
??
di
dt
 
If 
a rr
tt ? 
Area under the curve = 
R
Q 
  
R RM rr
1
Q I t
2
? 
  
? ? RM rr
di
It
dt
? 
  
? ?
2
R rr
1
di
Qt
dt
2
? 
Device & Circuit Turn-off time 
? Device turn off time, 
q rr gr
t t t ?? 
rr
t = reverse recovery time 
gr
t = gate recovery time 
? Circuit turn-off time ? ?
c
t is the time period for which communication circuit applies reverse 
voltage across SCR after anode current becomes zero. 
? For successful communication, ?
cq
tt 
Turn-ON methods of SCR 
1) Forward voltage triggering 
If 
AK BO
VV ? , then 
2
J breakdown & SCR conducts. This can damage the SCR. 
 
2) 
dV
dt
 Triggering 
cj
dv
IC
dt
? , if 
dv
dt
 is high, charging current increase and SCR conducts when 
c latching
II ? . 
 
3) Light Triggering 
If light is incident on 
2
J , charge carriers are generated and 
2
J starts conducting. 
 
4) Thermal Triggering 
When temperature is increased then charge carriers are generated & SCR conducts. 
 
5) Gate Triggering 
By applying gate pulse in SCR, 
BO
V is lowered and SCR can easily conduct. 
Page 5


 
 
 
 
 
Properties of ideal switch 
1. Conduction state , 
ON ON
V 0, I ? ? ? ? ? ? 
2. Blocking state , 
OFF OFF
V 0, V ? ? ? ? ? ? 
3. Ideal switch can change its state instantaneously 
ON OFF
T 0 , T 0 ?? 
4. No power loss while switching. 
5. Stable under all operating conditions. 
Classification of switches 
1. Uncontrolled switch (Passive switch) 
Switching state cannot be controlled by any control signal E.g. Diode 
2. Semi-controlled switch 
Only one switching state can be controlled by an external control signal. E.g. SCR 
3. Fully controlled switch 
If both switching states can be controlled by switchable control signal.  E.g. BJT, MOSFET. 
Other Classification 
1. Unipolar switch 
The switch can block only one polarity of voltage when it is in OFF state. 
2. Bipolar switch 
This switch can block both polarity of voltage when it is in blocking state. 
3. Unidirectional switch 
This switch can carry current in only one direction when it is in conduction state. 
4. Bidirectional switch 
This switch can carry current in both the directions when it is in conduction state. 
 
 
 
 
 
Power loss in a switch 
 
1) The average power has in a switch is given by  
               P
T
o
1
vidt
T
?
?
 
 Where v = instantaneous voltage 
  i = instantaneous current 
2) If the device is modeled as a resistance, as in case of a MOSFET 
              
22
rms ON rms ON
P I R V R ?? 
3) If the device is modeled as a voltage source. 
                       
avg
P V I ?
 
 
Silicon Controlled Rectifier 
 
 
 
 
 
 
 
? In forward blocking mode, 
13
J , J are forward biased and 
2
J is reverse biased. 
? In forward conduction mode, 
2
J breakdown, 
13
J , J are forward biased. 
? In reverse blocking mode, 
13
J , J are reverse biased & 
2
J is forward biased. 
Latching Current 
This is the minimum value of anode current above which SCR turns ON. This is related to 
minimum gate pulse width requirement for SCR. 
Holding current 
Minimum value of anode current below which SCR turns OFF. 
 
 
 
 
 
Slope of characteristics = 
??
??
??
di
dt
 
If 
a rr
tt ? 
Area under the curve = 
R
Q 
  
R RM rr
1
Q I t
2
? 
  
? ? RM rr
di
It
dt
? 
  
? ?
2
R rr
1
di
Qt
dt
2
? 
Device & Circuit Turn-off time 
? Device turn off time, 
q rr gr
t t t ?? 
rr
t = reverse recovery time 
gr
t = gate recovery time 
? Circuit turn-off time ? ?
c
t is the time period for which communication circuit applies reverse 
voltage across SCR after anode current becomes zero. 
? For successful communication, ?
cq
tt 
Turn-ON methods of SCR 
1) Forward voltage triggering 
If 
AK BO
VV ? , then 
2
J breakdown & SCR conducts. This can damage the SCR. 
 
2) 
dV
dt
 Triggering 
cj
dv
IC
dt
? , if 
dv
dt
 is high, charging current increase and SCR conducts when 
c latching
II ? . 
 
3) Light Triggering 
If light is incident on 
2
J , charge carriers are generated and 
2
J starts conducting. 
 
4) Thermal Triggering 
When temperature is increased then charge carriers are generated & SCR conducts. 
 
5) Gate Triggering 
By applying gate pulse in SCR, 
BO
V is lowered and SCR can easily conduct. 
 
 
 
 
Static V-I characteristics of SCR 
 
 
 
 
 
 
Communication of thyristor 
Communication is defined as process of turning OFF the thyristor. 
Types of Commutations: 
1. Natural or line communication 
In this case nature of supply supports the commutation. 
E.g. Rectifier, AC voltage controllers, Step-down cyclo-converters. 
 
2. Forced Commutation 
 
1) Class A commutation 
 
? Circuit should be under-damped. 
? 
2
4L
R
C
? for damped oscillations. 
? Ringing frequency, ? ? ?
2
2 r
1R
LC 4L
 
? Thyristor conducts for a period of = 
?
?
r