ELEC-E8421
Components of Power Electronics
Thyristors
Thyristors – Turn on and the di/dt rating
• At turn on the gate current goes to cathode only
at the small region near the gate.
• The initial turn on area is quite small and
increases only about 0,02…0,1 mm/μs
• Thyristor may be destroyed if the anode current
increases more than its di/dt rating.
• Typical di/dt ratings: 50 A …1000 A / µs
Current paths in the
beginning of the turn-on
Recommendations:
• Gate pulse magnitude ≈ 1…2 A
• Increase rate of gate current ≈ 1…2 A/μs
Methods used to improve thyristor di/dt rating
Gate
Cathode surface
Interdigitated gate
• The area initially conducting at turn-on can
be increased by extending the gate in the
cathode area
• With a dense gate pattern also the time
required to fully turn on the thyristor is
minimized
• The drawback is naturally high gate current
Methods used to improve thyristor di/dt rating
• Amplifying gate consists of small pilot thyristor that feeds the current to the
interdigitated main gate. Thus a large thyristor can be turned on with small
gate current pulse.
Equivalent circuit
Structure
Thyristor's du/dt rating and recovery
• The reverse recovery is similar to the diode recovery
• When entering to forward blocking state the trapped charges near
gate may turn on the thyristor
• The du/dt rating defines how fast
charge
the voltage may rise without
depletion
carrier
turning on the thyristor
region
cloud
• The du/dt rating depends on
temperature, hold-off time tH and
voltage during hold-off time
• The hold-off time has to be
longer than the recovery time tQ
of the thyristor
Methods to improve thyristor du/dt
gate
emitter shorts
cathode
cathode
emitter short
gate
anode
• Emitter shorts allow the trapped charges to
flow from gate to cathode without crossing
p-n junction
• Drawback is slower turn on
• Typical du/dt ratings: 100 V…1000 V/µs
Different thyristor types
•
•
•
•
50/60 Hz thyristors: tQ ≈ 150…400 µs
Triac
Integrated Gate Commutated Thyristors (IGCT)
Light triggered thyristors
•
•
•
•
•
Fast thyristors: tQ ≈ 4…70 µs
Asymmetric fast thyristors
Reverse conducting fast thyristors
Gate Assisted Turn Off (GATO) thyristors
Gate Turn Off (GTO) thyristors
NOT USED ANYMORE
IN NEW PRODUCTS
Asymmetric thyristor
• n+ buffer layer at the anode side of middle nregion increases forward blocking voltage but
at the same time decreases reverse blocking
voltage to about 30…50 V
• Asymmetric fast thyristors were used in 80's in
applications where thyristors had anti-parallel
diodes and thus never had high reverse
voltages (for example in force commutated
frequency converters)
Circuit symbols
Reverse conducting thyristors
• Used at the end of 80's in some applications to replace asymmetric thyristor
and its antiparallel diode.
Diode section
Circuit symbol
Internal structure
Light triggered thyristor (LTT)
•
•
•
•
Used in high voltage DC transmission (HVDC) applications.
Fired by laser light pulse via fiber-optic cable
Multiple pilot thyristors are used to amplify the gate current
Often over voltage protection functions are also integrated (BOD = Break
Over Diode)
Gate turn-off thyristor (GTO)
• GTO can be turned off by a negative gate current pulse that draws holes out of the gatecathode junction. Typically required gate current is 20 % of anode current.
• Cathode is divided in small islands that allow gate to efficiently draw holes out at turn-off
• High voltage up to 4500 V, maximum turn off current up to 5 kA.
• GTO replaced fast thyristors at the 80's but was itself replaced by IGBTs and IGCTs in 90's
GTO wafer from cathode side
GTO turn-off
Anode –
cathode
voltage
• Most critical moment is
when anode current is
falling: the anode cathode
voltage should not exceed
about 600 V level
• Voltage rise limitation
requires quite big snubber
capacitor
Anode –
cathode
current
Gate –
cathode
current
Current distribution
in cathode island
Gate
current
Integrated Gate Commutated Thyristor (IGCT)
• The GTO turn off problem and snubbers
can be avoided if gate current is very fast
increased to be greater than anode
current. Then cathode current is zero and
does not cause local hot spots
• Requires high but short gate current pulse
• Special flange contact to gate is needed as
low inductance is essential for fast gate
current increase
• Usually sold with integrated gate driver
• Replaced GTOs at the end of 90s
Advantages
•
Low conduction losses
•
Snubber circuit is not needed
•
Connection in series is possible
Disadvantages
•
Short circuit current may exceed turn off
capability
•
High switching losses limit pulse
frequency to about 300 Hz
Triac
• Equivalent to two thyristors but has only one
gate. Thus very simple to use.
• Negative gate pulse turns on the triac
regardless of the polarity of the voltage U21
between main terminals
• Also positive gate pulse turns the triac on, but
this is not always recommended if U21 < 0
• Used in dimmers, vacuum cleaners, hand
drills, etc for stepless control of the device
Lamp
dimmer
Triak with inductive load – problems with turn-off
Triak circuit diagram with inductive load
Voltage and current curves
• For older triacs maximum allowed du/dt is only 5…10 V/μs when di/dt is nominal
• Many new triacs survive even without snubber, but snubber is recommended due to EMC
Voltage and current ratings of thyristors (2015)
100000
a) 50/60 Hz
thyristors with
light control
b) Normal 50/60 Hz
thyristors
c) GTO
d) IGCT
e) Triacs (Current
value is RMS)
UDRM / V
b)
a)
10000
e)
c)
1000
d)
100
100
1000
10000
ITAV / A
Electrical properties listed in a thyristor data sheet
• Most of the symbols are the same than with diodes
• However, subscript T is used instead of F, for example rated average
current of thyristor is ITAV
• For thyristors some special parameters are given in addition to di/dt, du/dt,
tQ and forward blocking ratings, for example:
– Latching current ILAT (anode current needed to keep thyristor conducting after turn-on)
– Holding current IH (anode current below which the thyristor starts itself turn off)
Both latching and holding currents are high for GTOs and IGCTs, several tens of amperes.
These require continuous gate current in order to keep all cathode islands conducting
Current – voltage curve of the gate
• Manufacturers give diagrams
that show the spread in the
characteristica
• As the turn on is more difficult
in low temperatures the
diagram shows areas where
the turn on may not be
succesfull.
Gate circuit
characteristica