thyristors and other devicesrev1
TRANSCRIPT
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THYRISTORS & OTHER
DEVICES
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Thyristors are a class of semiconductor devices characterized by 4-
layers of alternating p- and n-material. Four-layer devices acts aeither open or closed switches, for this reason, they are most
frequently used in control applications
Some thyristors and their symbols are:
Introduction
Shockley diode
silicon-controlled rectifier
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Introduction continue
They stay on once they are triggered, and will go off only
if current is too low or when triggered off.
Applications: lamp dimmers, motor speed controls,
ignition systems, charging circuits, etc.
The 4-layer diode (or Shockley diode) is a type of
thyristor thatacts something like an ordinary diode but
conducts in the forward direction only after a certain
anode to cathode voltage called the forward-
breakover voltage is reached..
It does not conduct when it is reverse-biased.
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Advantages and disadvantages
Advantages Disadvantages
Thyristors have low on-state
conduction losses
Higher power handling capabilitythan transistor
High blocking voltage rating
Worse switching performances than
transistors.
Thyristor heating due to currentflowing through them represents
additional power losses in the
distribution system
The thyristor system may also have a
higher equipment cost
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The Basic Four Layer DeviceThe 4-layer diode (or Shockley diode) is a type of thyristor that acts
something like an ordinary diode but conducts in the forward direction onlyafter a certain anode to cathode voltage called the forward-breakover voltage is
reached.
The 4-layer diodehas two leads, labeled the anode (A) and the cathode
(K). The symbol reminds you that it acts like a diode. It does not conduct
when it is reverse-biased.
Equivalent circuit
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The Basic Four Layer Device
It will not conduct when
reverse biased and will not
conduct when forward biased
until the forward breakover
voltage (VBRF)is reached. Itwill continue to conduct as long
as the holding current(IH) is
maintained.
Forward-
conduction
region
Forward-blocking
region
Normal diode
characteristics
When FB, current is
exponential function
of voltage
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Equivalent Circuits(4 Layers diode)
The concept of 4-layer devices is
usually shown as an equivalent
circuit of a pnp and an npn
transistor. Ideally, these devices
would not conduct, but whenforward biased, if there is sufficient
leakage current in the upper pnp
device, it can acts as base current
to the lower npndevice causing itto conduct and bringing both
transistors into saturation.
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Silicon-Controlled Rectifier (SCR)
SCR is another four-layerpnpn device.
Has 3 terminals: anode, cathode, and gate.
In off state, it has a very high resistance.
In on state, there is a small on (forward)
resistance.
Applications: motor controls, time-delay
circuits, heater controls, phase controls, etc.
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The Silicon-Controlled Rectifier
The silicon-controlled rectifier (SCR) is a four layer device with threeterminals, the anode, cathode, and gate.
Equivalent Circuit
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Silicon-Controlled Rectifier
(SCR)The SCR had its roots in the 4-layer diode. By adding a gate connection, the
SCR could be triggered into conduction. This improvement made a much more
useful device than the 4-layer diode.
The SCR can be turned on by exceeding the forward breakover voltage or by
gate current. Notice that the gate current controls the amount of forwardbreakover voltage required for turning it on.
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Once an SCR is switched on, it
remains latched on, even when
the gate signal is removed.
Holding current (IH) is the
minimum required current fromanode to cathode
Reverse breakdown voltage is the
maximum reverse bias voltage for
the SCR
Operation
To switch on an SCR:
Forward bias the
anode-cathode
terminals (VF)AND
Apply sufficient gate
voltage (Vgate) and
gate current (IGT)
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The Silicon-Controlled Rectifier
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Another applications
Another application for
SCRs is in crowbar circuits
The purpose of a crowbar
circuit is to shut down apower supply in case of
over-voltage.
Once triggered, the SCR
latches on.
The SCR can handle a large
current, which causes the
fuse (or circuit breaker) to
open.
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SCSSilicon-Controlled Switch
An SCS is like an SCR, except that it has
two gates: a cathode gate and an anodegate.Operation
Either gate can fire the SCS
A positive pulse or voltage on the
cathode gate A negative pulse or voltage on the anode
gate
Either gate can switch off the SCS
A negative pulse or voltage on the
Cathode gate A positive pulse or voltage on the anode
gate
Note: The anode gate requires higher voltages than the cathode
gate.
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The Diac and Triac
The diacand triacunlike the SCR will conduct in both directions making it
ideal for ac applications. The diac turns on when the breakover voltage is
reached in either direction.
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The Diac and Triac
The triac is basically a diac with a gate terminal. The triac canbe turned by a pulse at the gate.
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The Diac and Triac
In this basic triac phase
control circuit R1
controls the trigger
point at which the triac
turns on for each half of
the cycle. The off timeis called the delay
angle and the on time
is called the conduction
angle.
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Unijunction Transistor (UJT)
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The Unijunction Transistor
(UJT)The UJT has one pn junction and is used mainly as a triggering devicein thyristor circuits and can also be used in oscillator circuits. Thesymbol is similar to a JFET. Note the angle of the emtter.The other
terminals are called base 1 and base 2. The characteristics are quite
different than any other transistor.
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The Unijunction Transistor
(UJT)The resistive equivalent circuit of a UJT shown makes it easier tounderstand its operation. The emitter current controls the value of rB1inversely. The total resistance or interbase resistance (rBB) equals the sum
of rB1and rB2. The standoff ratio() is the ratio rB1/rBB.
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The Programmable UnijunctionTransistor (PUT)
Although it has the same name as a UJT the programmable unijunction
transistor
sstructure is not the same. It is actually more similar to an
SCR. The anode to gate voltage is used to turn it off and on.
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The Programmable UnijunctionTransistor (PUT)
The PUTcan be programmedto turn on at a certain voltage by an
external voltage divider. This yields a curve similar to a UJT therefore it
can used in oscillator circuits like the UJT.