Thyristor
Thyristor is a type of
4 layer semiconductor and can be modeled as three diodes connected in series
back to back or 2 transistor with feedback position
it supports hysteresis
phenomena and hysteresis here in thyristor is that the device doesn’t return to its original state even the cause of the action has been
removed and it back again under condition
thyristor can handle
high voltage reach some kilovolts therefore it’s suitable in high power application
types of thyristor :
1. Shockley diode
2. DIAC
3. Silicon Controlled rectifier SCR
4. TRIAC
Shockely diode
Like PN junction diode
in the main concept of that it conducts with forward bias and cut off with
reverse but unlike PN junction diode it can handle high power .
And here the models of
thyristor .
thyristor can be
modeled as 3 PN junction diode connected back to back as following .
And here all
conducting condition :
Condition 1:
Forward blocking
state (OFF) and here j1 and j3
are forward but the voltage between anode and cathode not enough to breakdown
j2 to become conducting and breakdown here doesn’t mean that the junction will Permanently damage, because it fabricated
to withstand excessive reverse voltage and conduct current like Zener diode in
reverse state
Condition 2:
Small reverse
leakage current of J1 or J3 only
and this stat can be considered OFF state
Condition 3:
Forward conducting
state (ON) and here J1 and J3
are forward and voltage between anode and cathode are enough to breakdown J2
and thyristor is fully ON and conduct current.
Then we can say that
shockely diode conduct only if the voltage of anode is higher than cathode with
a value called break over voltage VBO
then VAK ≥ VBO
Once conducts it stay on as long as the current
is above the threshold called holding current IH and we can explain that by BJT equivalent
circuit , once we apply VAK greater
than VBO one of the BJT will switch on causing base current on the
other BJT then the other BJT works causing base current to the first BJT , we
here in positive feedback stat , this loop will continue as long as the current
is greater than holding current , and that explain the hysteresis phenomena ,
once current is below IH one of the BJT will turn off causing base
current in the other BJT to stop then both BJT will stop and diode now OFF
And here the
characteristic curve that explain what we have discussed.
The DIAC
It has the same
characteristic of shockely diode but with one only difference
Shockely diode can
allow current in one direction making it unidirectional device, then two
shockely diode can connect in parallel facing different direction to make a device
can allow current in two direction that device called DIAC
DIAC can now pass
current in the two direction but first the voltage must be higher than VBO
, so the input wave to the output
wave will be shown in the following figure
Also, the characteristic curve shown on the
following figure
It’s the same as the characteristic curve of the
shockely diode but in the two direction
