Gating, Latching and Holding of SCRs and Triacs

Gating, Latching and Holding of SCRs and Triacs
Teccor® brand Thyristors
Gating, Latching, and Holding of SCRs and Triacs
Introduction
Gating, latching, and holding currents of Thyristors
are some of the most important parameters. These
parameters and their interrelationship determine whether
the SCRs and Triacs will function properly in various circuit
applications.
Triacs (bilateral devices) can be gated on with a gate
signal of either polarity with respect to the MT1 terminal;
however, different polarities have different requirements of
IGT and VGT. Figure AN1002.2 illustrates current flow through
the Triac chip in various gating modes.
This application note describes how the SCR and Triac
parameters are related. This knowledge helps users select
best operating modes for various circuit applications.
MT1(-)
Gate(+)
IGT
N
Gating of SCRs and Triacs
Three general methods are available to switch Thyristors to
on-state condition:
N
P
QUADRANT I
N
P
IT
t "QQMZJOHQSPQFSHBUFTJHOBM
N
MT2(+)
t &YDFFEJOH5IZSJTUPSTUBUJDEWEUDIBSBDUFSJTUJDT
This application note examines only the application of
proper gate signal. Gate signal must exceed the IGT and VGT
requirements of the Thyristor being used. IGT (gate trigger
current) is the minimum gate current required to switch a
Thyristor from the off state to the on state. VGT (gate trigger
voltage) is the voltage required to produce the gate trigger
current.
MT1(-)
Gate(-)
IGT
t &YDFFEJOHWPMUBHFCSFBLPWFSQPJOU
N
N
QUADRANT II
P
N
P
N
MT2(+)
SCRs (unilateral devices) require a positive gate signal with
respect to the cathode polarity. Figure AN1002.1 shows
the current flow in a cross-sectional view of the SCR chip.
MT1(+)
Gate(-)
IGT
Gate
(+) I
Cathode
(-)
GT
N
QUADRANT III
N
(+) I
T
Anode
Figure AN1002.1
©2008 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to http://www.littelfuse.com for current information.
N
MT2(-)
IT
MT1(+)
N
N
QUADRANT IV
N
MT2(-)
Figure AN1002.2
P
N
P
SCR Current Flow
In order for the SCR to latch on, the anode-to-cathode
current (IT) must exceed the latching current (IL)
requirement. Once latched on, the SCR remains on until it
is turned off when anode-to-cathode current drops below
holding current (IH) requirement.
P
Gate(+)
IGT
P
P
N
N
P
N
IT
Triac Current Flow (Four Operating Modes)
Gating, Latching, and Holding of SCRs and Triacs
AN1002
AN1002
Teccor® brand Thyristors
AN1002
Special consideration should be given to gating circuit
design when Quadrants I and IV are used in actual
application, because Quadrant IV has the lowest gate
sensitivity of all four operating quadrants.
IGT
GT(TC = 25°C)
2.0
Ratio of I
Triacs can be gated on in one of four basic gating modes as
shown in Figure AN1002.3. The most common quadrants
for gating on Triacs are Quadrants I and III, where the gate
supply is synchronized with the main terminal supply (gate
positive -- MT2 positive, gate negative -- MT2 negative).
Optimum Triac gate sensitivity is achieved when operating
in Quadrants I and III due to the inherent Thyristor chip
construction. If Quadrants I and III cannot be used, the
next best operating modes are Quadrants II and III where
the gate supply has a negative polarity with an AC main
terminal supply. Typically, Quadrant II is approximately
equal in gate sensitivity to Quadrant I; however, latching
current sensitivity in Quadrant II is lowest. Therefore, it is
difficult for Triacs to latch on in Quadrant II when the main
terminal current supply is very low in value.
1.5
1.0
.5
0
-40
-15
+25
+65
+100
Case Temperature (TC) – °C
Figure AN1002.4
Typical DC Gate Trigger Current versus Case
Temperature
For applications where low temperatures are expected,
gate current supply should be increased to at least two
to eight times the gate trigger current requirements at
25 ºC. The actual factor varies by Thyristor type and the
environmental temperature.
Example of a 10 A Triac:
If IGT(I) = 10 mA at 25 ºC, then
IGT(I) = 20 mA at -40 ºC
ALL POLARITIES ARE REFERENCED TO MT1
MT2
(-)
MT2 POSITIVE
(Positive Half Cycle)
+
IGT
GATE
(+)
IGT
(-)
MT1
IGT
GATE
(+)
MT1
REF
REF
QII QI
QIII QIV
MT2
MT2
IGT
GATE
MT1
REF
*OBQQMJDBUJPOTXIFSFIJHIEJEUIJHITVSHFBOEGBTU
turn-on are expected, gate drive current should be steep
rising (1 μs rise time) and at least twice rated IGT or higher
with minimum 3 μs pulse duration. However, if gate drive
current magnitude is very high, then duration may have
to be limited to keep from overstressing (exceeding the
power dissipation limit of) gate junction.
-
+
IGT
MT2
Latching Current of SCRs and Triacs
IGT
GATE
MT2 NEGATIVE
(Negative Half Cycle)
MT1
REF
NOTE: Alternistors will not operate in Q IV
Figure AN1002.3
Definition of Operating Quadrants in Triacs
The following table shows the relationships between
different gating modes in current required to gate on Triacs.
In the illustrations in Figure AN1002.5, the Thyristor does
not stay on after gate drive is removed due to insufficient
available principal current (which is lower than the latching
current requirement).
IGT (in given Quadrant)
Typical Ratio of -----------------------------------------at 25OC
IGT(Quadrant 1)
Type
Operating Mode
Quadrant I
Quadrant II Quadrant III Quadrant IV
4 A Triac
1
1.6
2.5
2.7
10 A Triac
1
1.5
1.4
3.1
Example of 4 A Triac:
If
Latching current (IL) is the minimum principal current
required to maintain the Thyristor in the on state
immediately after the switching from off state to on state
has occurred and the triggering signal has been removed.
Latching current can best be understood by relating to the
“pick-up” or “pull-in” level of a mechanical relay. Figure
AN1002.5 and Figure AN1002.6 illustrate typical Thyristor
latching phenomenon.
Gate Pulse
(Gate Drive to Thyristor)
Time
Latching
Current
Requirement
Principal
Current
Through
Thyristor
IGT(I) = 10 mA, then
IGT(II) = 16 mA
IGT(III) = 25 mA
IGT(IV) = 27 mA
Gate trigger current is temperature-dependent as shown
in Figure AN1002.4. Thyristors become less sensitive with
decreasing temperature and more sensitive with increasing
temperature.
Gating, Latching, and Holding of SCRs and Triacs
Zero
Crossing Point
Time
Figure AN1002.5
Latching Characteristic of Thyristor (Device
Not Latched)
In the illustration in Figure AN1002.6 the device stays on
for the remainder of the half cycle until the principal current
falls below the holding current level. Figure AN1002.5
shows the characteristics of the same device if gate
drive is removed or shortened before latching current
requirement has been met.
©2008 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to http://www.littelfuse.com for current information.
Teccor® brand Thyristors
Gate
Drive
to Thyristor
Holding current modes of the Thyristor are strictly related
to the voltage polarity across the main terminals. The
following table illustrates how the positive and negative
holding current modes of Triacs relate to each other.
Gate Pulse
Time
Typical Triac Holding Current Ratio
Principal
Current
Through
Thyristor
Latching
Current
Point
Holding Current Point
Zero Crossing Point
Time
Figure AN1002.6
Latching and Holding Characteristics of
Thyristor
Similar to gating, latching current requirements for
Triacs are different for each operating mode (quadrant).
Definitions of latching modes (quadrants) are the same
as gating modes. Therefore, definitions shown in Figure
AN1002.2 and Figure AN1002.3 can be used to describe
latching modes (quadrants) as well. The following table
shows how different latching modes (quadrants) relate
to each other. As previously stated, Quadrant II has the
lowest latching current sensitivity of all four operating
quadrants.
I (in given Quadrant)
L
Typical Ratio of -----------------------------------------at 25OC
IL(Quadrant 1)
Operating Mode
Quadrant I Quadrant II Quadrant III Quadrant IV
4 A Triac
1
4
1.2
1.1
10 A Triac
1
4
1.1
1
Example of a 4 Amp Triac:
If
IH(+)
IH(–)
4 A Triac
1
1.1
10 A Triac
1
1.3
Example of a 10 A Triac:
IH(+) = 10 mA, then
If
IH(-) = 13 mA
Holding current is also temperature-dependent like gating
and latching shown in Figure AN1002.7. The initial onstate current is 200 mA to ensure that the Thyristor is
fully latched on prior to holding current measurement.
Again, applications with low temperature requirements
should have sufficient principal (anode) current available to
maintain the Thyristor in the on-state condition.
Both minimum and maximum holding current
specifications may be important, depending on application.
Maximum holding current must be considered if the
Thyristor is to stay in conduction at low principal (anode)
current; the minimum holding current must be considered
if the device is expected to turn off at a low principal
(anode) current.
2.0
IL(I) = 10 mA, then
INITIAL ON-STATE CURRENT = 200 mA dc
IL(IV) = 11 mA
Latching current has even somewhat greater temperature
dependence compared to the DC gate trigger current.
Applications with low temperature requirements should
have sufficient principal current (anode current) available to
ensure Thyristor latch-on.
Two key test conditions on latching current specifications
are gate drive and available principal (anode) current
durations. Shortening the gate drive duration can result in
higher latching current values.
Holding Current of SCRs and Triacs
Holding current (IH) is the minimum principal current
required to maintain the Thyristor in the on state. Holding
current can best be understood by relating it to the “dropout” or “must release” level of a mechanical relay. Figure
AN1002.6 shows the sequences of gate, latching, and
holding currents. Holding current will always be less than
latching. However, the more sensitive the device, the
closer the holding current value approaches its latching
current value.
Holding current is independent of gating and latching, but
the device must be fully latched on before a holding current
limit can be determined.
©2008 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to http://www.littelfuse.com for current information.
1.5
1.0
Ratio of
IL(III) = 12 mA
IH
IL(II) = 40 mA
IH (TC = 25 °C)
Type
Operating Mode
Type
.5
0
-40
-15
+25
+65
+100
Case Temperature (TC) – °C
Figure AN1002.7
Typical DC Holding Current vs Case
Temperatures
Example of a 10 A Triac:
If
IH(+) = 10 mA at 25 ºC, then
IH(+) ≈ 7.5 mA at 65 ºC
Relationship of Gating, Latching, and Holding Currents
Although gating, latching, and holding currents are
independent of each other in some ways, the parameter
values are related. If gating is very sensitive, latching and
holding will also be very sensitive and vice versa. One way
to obtain a sensitive gate and not-so-sensitive latchingholding characteristic is to have an “amplified gate” as
shown in Figure AN1002.8.
Gating, Latching, and Holding of SCRs and Triacs
AN1002
AN1002
Teccor® brand Thyristors
AN1002
The following table and Figure AN1002.9 show the
relationship of gating, latching, and holding of a 4 A device.
A
*
A
Sensitive
SCR
G
Typical 4 A Triac Gating, Latching,
and Holding Relationship
Power
SCR
K
Parameter
K
G
*
Quadrant I
Quadrant II Quadrant III Quadrant IV
IGT (mA)
10
17
18
27
IL (mA)
12
48
12
13
IH (mA)
10
10
12
12
MT2
MT2
Sensitive
Triac
G
Quadrants or Operating Mode
Power
Triac
MT1
MT1
G
*
Resistor is provided for limiting gate
current (IGTM) peaks to power device.
Figure AN1002.8
“Amplified Gate” Thyristor Circuit
(mA)
20
QUADRANT II
IH(+)
QUADRANT I
IGT (Solid Line)
IL (Dotted Line)
10
(mA)
50
40
30
20
10
0
10
20
30
40
10
20
QUADRANT III
Figure AN1002.9
QUADRANT IV
IH(–)
Typical Gating, Latching, and Holding Relationships of 4 A Triac at 25 ºC
The relationships of gating, latching, and holding for
several device types are shown in the following table. For
convenience all ratios are referenced to Quadrant I gating.
Typical Ratio of Gating, Latching, and Holding Current at 25 OC
Ratio
IGT (II)
-----------IGT(I)
IGT (III)
-----------IGT(I)
IGT (IV)
-----------IGT(I)
IL (I)
-----------IGT(I)
IL (II)
-----------IGT(I)
IL (III)
-----------IGT(I)
IL (IV)
-----------IGT(I)
IH (+)
-----------IGT(I)
IH (–)
-----------IGT(I)
4A Triac
1.6
2.5
2.7
1.2
4.8
1.2
1.3
1.0
1.2
10A Triac
1.5
1.4
3.1
1.6
4.0
1.8
2.0
1.1
1.6
15A Alternistor
1.5
1.8
–
2.4
7.0
2.1
–
2.2
1.9
Devices
1A Sensitive SCR
–
–
–
25
–
–
–
25
–
6A SCR
–
–
–
3.2
–
–
–
2.6
–
Gating, Latching, and Holding of SCRs and Triacs
©2008 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to http://www.littelfuse.com for current information.
Teccor® brand Thyristors
AN1002
If
AN1002
Examples of a 10 A Triac:
IGT(I) = 10 mA, then
IGT(II) = 15 mA
IGT(III) = 14 mA
IGT(IV) = 31 mA
If
IL(I) = 16 mA, then
IL(II) = 40 mA
IL(III) = 18 mA
IL(IV) = 20 mA
If
IH(+) = 11 mA at 25 ºC, then
IH(+) = 16 mA
Summary
Gating, latching, and holding current characteristics of
Thyristors are quite important yet predictable (once a single
parameter value is known). Their interrelationships (ratios)
can also be used to help designers in both initial circuit
application design as well as device selection.
©2008 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to http://www.littelfuse.com for current information.
Gating, Latching, and Holding of SCRs and Triacs
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