chapter 15 thyristors. this latch is stable in either of two states. latch on state off state q1q1...
TRANSCRIPT
Chapter 15
Thyristors
This latch is stable in either of two states.
Latch On state Off state
Q1
Q2
Q1
Q2
p
p
n
n
{Q1
The four-layer diode
{Q2
Four-layer diode breakover
I
VIH
VK VB
The silicon controlled rectifier (SCR)
Q1
Q2
Anode
Gate
Cathode
p
p
n
n
Anode
Gate
Cathode
Anode
Gate
Cathode
SCRs
• The most widely used thyristors• Use gate triggering for turn-on instead of
breakover triggering• Data sheets list the gate trigger voltage
(VGT) and the gate trigger current (IGT)
• Turn-off requires reducing the current to less than the holding current (IH)
Load
+VAA
A B C D
A B C D
+VAA
An SCR does not turn off at the end of the trigger pulse.
Powersupply
Protectedload
SCR crowbar to protect a load from overvoltage
The overvoltage that triggers the crowbar: VT = VZ + VGT
VT
VZ
VGT
Powersupply
Protectedload
Adding gain to a crowbar circuit
R2
R1
R4
R3
R1 is the trigger adjust
Powersupply
Protectedload
Adding an IC amplifier to a crowbar circuit
10 kW
10 kW 10 kW
IC
VZ
A
The circuit triggers when VA exceeds VZ.
VA
Load
120 V
SCR phase control
qconduction
vline
vload
qfire
R1
R2C
Load
R1
R2C
RC snubber
A snubber network can be used to limit the rate of voltage rise across the SCR.
Bidirectional thyristors can conduct in either direction.
Four-layer diodesin reverse parallel
Diac
I
SCRs in reverse parallel
Triac
IGate
The triac is a popular bidirectional thyristor.
Load
120 V
Triac phase control
vline
vload
R1
R2C
Protectedload
Triac crowbar
R2
R1
R3
R2 is the trigger adjust
120 V
Fuse
Triac
Diac
Sensitivityadjust
RL RL
The photo-SCR istriggered by light.
Maximumsensitivity
Opengate
A B C D
A B C D
RL
RG
The gate-controlled switch is turned off with a reverse-biased trigger.
The silicon-controlled switch is a low-current device.
Cathodegate
AnodeAnodegate
Cathode
Either gate can be used to trigger or open this device.
Unijunction transistor (UJT)
n
p V
vin
E
B2
B1
RE
When vin reaches the standoff voltage, the resistance between the emitter and B1 drops dramatically.
E
B2
B1