Lec 5 Thyristors.pptx - THYRISTOR Thyristor symbol three PN...

THYRISTOR

Thyristor symbol & three PN junctions

Background On Thyristors
• The name Thyristor comes from two similar
device names ‘Thyratron’ and ‘Transistor’
• Thyristors are useful due to their ability to
handle large current in power applications and
fast switching
• The most common thyristor is the SCR which
stands for “Silicon Controlled Rectifier”

Thyristor circuit & static characteristics

Thyristor Operating Modes
•
Thyristors have 3 operation modes
Forward blocking mode: Anode is positive w.r.t cathode, but the
anode voltage is less than the break over voltage (VBO) .
•
Only leakage current flows, so thyristor is not conducting .
•
Forward conducting mode: When anode voltage becomes greater
than VBO, thyristor switches from forward blocking to forward
conduction state, a large forward current flows
•
If the gate is triggered, thyristor can be turned ON even when
anode voltage is less than VBO.
• The current must be more than the latching current (IL).
• If the current reduced less than the holding current (IH), thyristor
switches back to forward blocking state.

Thyristor Operating Modes
•
Reverse blocking mode: When cathode is
more positive than anode , small reverse
leakage current flows.
•
However if cathode voltage is increased to
reverse breakdown voltage , Avalanche
breakdown occurs and large current flows

Thyristor Turn-ON Methods
Thyristor turning ON is also known as Triggering.
With anode positive with respect to cathode, a
thyristor can be turned ON by any one of the following
techniques :
• High voltage triggering
• dv/dt triggering
• Thermal triggering
• Light triggering
• Gate triggering

High Voltage Triggering
• When breakover voltage (VBO) across a thyristor is
exceeded than the rated maximum voltage of the device,
thyristor turns ON.
• Breakover voltage triggering is not normally used as a
triggering method, and most circuit designs attempt to
avoid its occurrence.
• When a thyristor is triggered by exceeding VBO, the fall
time of the forward voltage is quite low (about 1/20th of
the time taken when the thyristor is gate-triggered).