Silicon Controlled Rectifier (SCR) - Notes | Study GATE Notes & Videos for Electrical Engineering - Electrical Engineering (EE)

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Power Electronics
SILICON CONTROLLED RECTIFIERS
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Power Electronics
SILICON CONTROLLED RECTIFIERS
Silicon Controlled Rectifier (SCR)
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Power Electronics
SILICON CONTROLLED RECTIFIERS
Silicon Controlled Rectifier (SCR) 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 J
2
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 J
1
and J
3
will be
reverse biased and therefore the device will not
conduct. This is the reverse blocking mode of SCR.
J1
J2
J3
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Power Electronics
SILICON CONTROLLED RECTIFIERS
Silicon Controlled Rectifier (SCR) 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 J
2
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 J
1
and J
3
will be
reverse biased and therefore the device will not
conduct. This is the reverse blocking mode of SCR.
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 V
BO
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.
Page 5


Power Electronics
SILICON CONTROLLED RECTIFIERS
Silicon Controlled Rectifier (SCR) 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 J
2
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 J
1
and J
3
will be
reverse biased and therefore the device will not
conduct. This is the reverse blocking mode of SCR.
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 V
BO
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.
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
V
g
V
S
R
L