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7/30/2019 3rd thyristor
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3rd slide 1
Thyristors
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3rd slide 2
A thyristor is a four-layersemiconductor device of pnpn-structure with three pn-junction.
When the anode voltage is madepositive with respect to thecathode, the junctions J1 and J3are forward biased.
The junction J2 is reverse biasedand only a small leakage currentflows from anode to cathode.
The thyristor is then said to be inthe forward blocking or off-statecondition and the leakagecurrent is known as off-statecurrent, ID.
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3rd slide 3
If the anode to cathode voltage, VAK, isincreased to a sufficiently large value, the
reverse biased junction J2 will break. Thisis knows as avalanche breakdownandthe corresponding voltage is calledforward b reakdown vol tage, V
BO.
Since the other junctions J1 and J3 arealready forward biased, there will be free
movement of carriers across al threejunction, resulting in a large forward anodecurrent. The device will the be in a
conduction state or on state.
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3rd slide 4
In the on-state , the anode current is limited by an
external impedance or a resistance, RL. The
anode current must be more than a value knowsas latch ing cu rrentIL, in order to maintain the
required amount of carrier flow across the
junction, otherwise the device will revert to the
blocking condition as the anode to cathodevoltage is reduced.
Latching current, IL
is the minimum anode current
required to maintain the thyristor in the on state
immediately after a thyristor has been turned on
and the gate signal has been removed.
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3rd slide 5
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The holding current is in the order of mili-
amperes and is less than the latching
current IL. That is IL > IH.
Holding current IH is minimum current to
maintain the thyristor in the on-state. Theholding current is less than the latching
current.
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3rd slide 8
A thyristor can be turned on by increasing theforward voltage VAK beyond VBO, but such aturn-on could be destructive.
In practice, the forward voltage is maintainedbelow VBO and the thyristor is turned on byapplying a positive voltage between its gate and
cathode.
Once a thyristor is turned on by a gating signal andits anode current is greater than the holding
current, the device continues to conduct due topositive feedback, even if the gating signal isremoved. A thyristor is a latching device
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3rd slide 9
Thyristor Turn-On
A thyristor is turned on by increasing the anode
current. This can be accomplished in one of
the following ways.
- Themals
- Lights
- High voltage
- dv/dt- Gate current
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3rd slide 10
Thermal
Temperature then number of electron-hole
pairs this would increase the leakagecurrent.
LightsIf light is allowed to strike the junction of
thyristor, the electron-hole pairs will
increase and thyristor may be turned on.The light-activated thyristors are turned onby allowing the light to strike the siliconwafers.
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3rd slide 11
High-voltage
If the forward anode to cathode voltage VAK is increasedbeyond the forward breakdown voltage VBO , highenough leakage currents will flow, causing regenerativeturn-on. This type of turn-on is destructive and should beavoided.
dv/dtIf the rate or rise of the anode to cathode voltage is high,
(for example, when there is a voltage spike), thecharging current of the capacitive junctions may be highenough to turn on the thyristor. A high value of charging
current may cause damage to the thyristor and must beavoided. Hence, thyristors must be protected againsthigh dv/dt and must be operated within themanufacturer's dv/dt specifications.
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3rd slide 12
Gate Current
The injection of gate current into a forward
biased thyristor would turn-on the device.As the gate current is increased, the
forward voltage required to turn-on the
device decreases.
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3rd slide 13
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3rd slide 14
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Turn-on Time, tonThe turn-on time ton is defined as the time interval
between 10% of steady-state gate current and90% of steady-state thyristor on-state current.
Delay Time, tdThe delay time t
dis defined as the time interval
between 10% of gate current and 10% ofthyristor on-state current.
Rise Time, trThe rise time, tr,,, is defined as the time required
for the anode current to rise from 10% of the on-state current to 90% of the on-state current.
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3rd slide 16
Gate Control Circuit DesignConsideration must be given to the following points when
designing gate control circuits.
- The gate signal should be removed after the thyristor hasbeen turned on. A continuous gate signal will increasethe power loss in the gate junction.
- No gate signal should be applied when the thyristor isreversed biased. If a gate signal is applied under theseconditions, the thyristor may fail due to an increasedleakage current.
- The width of the gate pulse must be greater than the timerequired for the anode current to rise to the holdingcurrent. In practice, the gate pulse width is made wider
than the turn-on time of the thyristor.
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3rd slide 17
di/dt Protection
A minimum time is required for the thyristor tospread the current conduction uniformlythroughout the junctions. If this time is notallotted and the rate of rise of anode current isvery high compared to the spreading velocity atturn-on, then this could lead to localised "hot-spot" heating and the device may fail as a resultof excessive heating.
Protection against di/dt is necessary and anexample is shown in figure below. The circuitanalysis is as follows:
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3rd slide 18
For an inductive load, when thyristor T1
is turned
off, free-wheeling diode Dm conducts load
current. If thyristor T1 is fired when diode Dm is
still conducting, di/dt can be very high. In order
to reduce the high di/dt a series inductor Ls isadded to the circuit as shown. The forward di/dt
is given as
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3rd slide 19
dv/dt Protection
As seen earlier, a high dv/dt may cause
damage to a thyristor. In order to protect athyristor from high dv/dt, the circuits shown
below could be used.
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3rd slide 20
If the switch S1 in figure (a) isclosed at time t = 0, a stepvoltage will be applied
across thyristor T1 anddv/dt may be high enoughto turn on the thyristor.
dv/dt can be limited byconnecting capacitor CSacross the thyristor asshown in figure (b). Since
then
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3rd slide 21
and the rate of rise of voltage is limited by thevalue of the capacitor used. In order tolimit the capacitor discharge current whenthe thyristor is turned on, a resistor RS isinserted in series with the capacitor asshown in figure (c).
This resistor capacitor arrangement is knownas a snubber circuit. For figure (c), whenswitch S1 is closed at time t = 0, thevoltage across the capacitor is given by
and this charging capacitor voltage is seen bythe thyristor anode to cathode terminals asVAK. This is depicted by the waveform offigure (d). The rate of rise of voltageacross the thyristor can be represented by
where 0.632VS is one time constant.
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3rd slide 22
The value of the snubber time constantRSCS can be found for a known dv/dt.
And for a known discharge current ITD ,the value of resistor RS can be foundusing
It is sometimes necessary to use oneresistor for dv/dt and another for
limiting the discharge current of thesnubber capacitor. This arrangement isshown in figure (e). In this circuit, R1and CS are used for dv/dt protection,while R1 + R2 is used for limiting thecapacitor discharge current.
The load can also be placed in series withthe snubber components as shown infigure (f).