dm00096037

AN4363
Application note
How to select the Triac, ACS, or ACST that fits your application
Introduction
This document gives basic guidelines to select the AC switch device according to the
targeted application requirements. These guidelines will allow the appropriate Triac, ACS or
ACST to be selected, for most of the applications. Some very specific cases could require a
higher level of expertise to ensure a reliable and efficient operation.
September 2015
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Contents
AN4363
Contents
1
2
3
4
Current rating selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
Load current and operating temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2
Turn-off limitation for specific applications . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3
Inrush current and stalled rotor operation . . . . . . . . . . . . . . . . . . . . . . . . . 7
Voltage rating selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1
Peak line voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2
Motor control applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3
Over-voltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Gate current and triggering quadrants . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1
Triac triggering quadrants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2
Operating quadrants or application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Package selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1
Surface mount device or through-hole packages . . . . . . . . . . . . . . . . . . . 16
4.2
Pin-out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
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Current rating selection
1
Current rating selection
1.1
Load current and operating temperature
The maximum current an AC switch (we call AC switch Triac, ACS or ACST in this
document) can handle is limited only by the maximum allowed operating temperature,
except for specific applications where the turn-off capability of the AC switch can also
become a limiting factor (refer to Section 1.2: Turn-off limitation for specific applications).
But, for most applications, where the current waveform is sinusoidal, the current of the AC
switch will depend only on the operating temperature and the device power losses.
For an AC switch, as the switching times (around 100 ns) are very low compared to the
switching period (50 Hz or 60 Hz for most cases), the device power losses are considered
as only linked to the conduction losses.
Then, the junction temperature will be (refer to STMicroelectronics Application note AN533
for further information):
Equation 1
Tj = Ta + Pd ⋅ R th( j − a )
or:
Equation 2
Tj = Tc + Pd ⋅ R th( j − c )
With respectively:
Tj: junction temperature
Ta: ambient temperature
Tc: case temperature
Pd: conduction losses
For a sinusoidal current, Pd is given by:
Equation 3
Pd =
2⋅ 2
2
⋅ Vt 0 ⋅ IRMS + R d ⋅ IRMS
π
Where Vt0 and Rd are respectively the threshold voltage and the dynamic resistance of the
AC switch ON-state voltage drop.
For each device, the datasheet gives the maximum allowed current calculated so that the
associated power losses (with Equation 2) will keep the junction temperature below the
maximum operating temperature (with Equation 1). This maximum temperature is usually
125 °C or 150 °C for high temperature devices.
Figure 1 and Figure 2 give the example of such datasheet curves for respectively an
ACS108-6SA and a T1635T-8FP. As the ACS108-6SA package is a TO92, there is no way
to add a heatsink to the device to evacuate its power losses. That is the reason why the
curve is given versus the operating ambient temperature.
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For example, for a 0.4 A rms current, this device has to work with a maximum ambient
temperature lower than approximately 72 °C. In other words, up to 72°C, the ACS108-6SA
can control a maximum rms current of 0.4 A.
Figure 1. Maximum rms current versus ambient temperature for an ACS108-6SA
IT(RMS) (A)
0.9
TO -92
0.8
SOT-223
0.7
TO-92
0.6
0.5
RMS load 0.4
current
0.3
0.2
Single layer Printed
circuit board FR4
0.1
Ta °C
Natural convection
0.0
0
25
50
75
100
125
Tamb
For the T1635T-8FP, as the device package is a TO220FPAB, and as this device can handle
up to 16 A for a 150 °C junction temperature, a heatsink is used in most of the cases. The
curve is then given versus the operating case temperature as shown in Figure 2.
This figure shows that for instance, for a 12 A rms current, the package case temperature
has to be lower than 107 °C. So the used heatsink has to ensure of evacuating the power
conduction losses (Pd = 13 W for 12 A rms current, as shown in Figure 3) while keeping the
case temperature below 107 °C.
Figure 2. Maximum rms current versus ambient temperature for a T1635T-8FP
18
IT(RMS)(A)
TO -220FPAB
16
14
RMS load 12
current
10
8
6
4
2
TC(°C)
0
0
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50
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100 Tc
125
150
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Current rating selection
Figure 3. Maximum power dissipation versus rms on-state current for a T1635T-8FP
20
P(W)
18
16
Pd
14
12
10
8
6
180°
4
IT(RMS)(A)
2
0
0
1.2
2
4
6
8
10
12
RMS load
current
14
16
Turn-off limitation for specific applications
One particularity of an AC switch is that it may remain ON when the gate current is
removed, even after that the current has reached zero. This kind of behavior could only
occur if the Triac operates with conditions beyond its datasheet guaranteed operation, and
particularly when the rate of load current decrease, or the rate of reapplied voltage after
turn-off are higher than the specified (dI/dt)c and (dV/dt)c rates.
An AC switch can be compared to two Thyristors mounted back-to-back and coupled with a
single control area. To trigger the two Thyristors, the control area overlaps the two
conduction areas.
During the conduction time, a certain quantity of charge is injected into the structure. This
charge disappears by recombination during the current decrease and by extraction after the
turn-off with the reverse recovery current (refer to Figure 4.a). Nonetheless, if an excess of
charge remains, particularly in the neighboring regions of the gate, this can induce the
triggering of the other conduction area when the mains voltage is reapplied across the Triac
(refer to Figure Figure 4.b).
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Figure 4. Triac turn-off (inductive load), suitable turn-off (a) or missed turn-off (b)
a)
b)
dV/dtOFF
VT (100 V/div)
VMains (100 V/div)
VT (50 V/div)
Recovery current
IT (10 mA/div)
IT (10 mA/div)
dI/dtOFF
To avoid such unwanted retriggering, ensure that the rate of decrease of the load current
(dI/dtOFF) and the rate of increase of the reapplied voltage (dV/dtOFF) are both lower than
the device specified minimum (dI/dt)c and (dV/dt)c parameters.
Note for Snubberless, ACS, and ACST devices, only the (dI/dt)OFF has to be checked as
turn-off is guaranteed whatever the (dV/dt)OFF is.
Checking the (dI/dt)OFF of an application can be done easily as long as the current is
sinusoidal. This parameter is the derivative of the current waveform. It is given by the
following equations, according to the mains frequency (f) and the load peak or RMS currents
(IPEAK, IRMS):
Equation 4
dI
= IPEAK ⋅ 2π ⋅ f
dt OFF
Then, for 50 Hz, and with appropriate units, Equation 4 gives:
dI
= 0.44 · IRMS(A)
dt OFF(A / ms)
For 60 Hz, equation 2 gives:
dI
= 0.53 · IRMS(A)
dt OFF ( A / ms )
Knowing the peak current gives then the (dI/dt)OFF rate and the right AC switch device has
to be selected so that it features a higher (dI/dt)c than this calculated rate.
Usually, a Triac features a (dI/dt)c parameter which is 0.44 or 0.5 times higher than its
IT(RMS). So using such a Triac for a load rms current lower than the guaranteed IT(RMS) will
ensure a proper operation as the (dI/dt)OFF will then be automatically lower than the device
(dI/dt)c as explained by Equation 4.
Particular care has to be taken with sensitive devices (with Igt current lower than 10 mA) as
the (dI/dt)c could be lower than half the IT(RMS).
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The only applications where (dI/dt)OFF rate has to be carefully checked (and measured) in
the application are:
1.3
•
Universal motors: Due to the brush commutations, the rate of decrease of the current is
higher than the one calculated from the rms current from the equations.Typically the
rate can be three times higher.
•
Loads (such as universal motor or valve) connected in a diode bridge (refer to
STMicroelectronics Application note AN439): In this case, the rate of decrease of the
Triac current is limited only by the mains inductance. This is the reason why an inductor
has to be put in series with the Triac to reduce this rate of decrease.
Inrush current and stalled rotor operation
AC switches thanks to their N-P-N-P layer silicon structure feature the best overcurrent
capability among semiconductor switches. For example, the maximum peak current an AC
switch can withstand during a line cycle (10 ms for a 50 Hz frequency) is usually 6 or 10
times higher than its nominal rms current (IT(RMS)).
For a 1 ms pulse length, the ratio can even reach 20 to 40.
In all AC switch datasheet, the overcurrent capability is given by both the ITSM parameter
and two curves giving the variation of this parameter according to the pulse duration. One
ITSM curve is given for pulse durations longer than a line cycle, and another curve is given
for pulse durations shorter than 10 ms.
These two curves allow users to know if the selected device fits with their application
requirements, particularly if overcurrent due to inrush currents or stalled rotor operation can
occur.
These curves demonstrate that is not necessarily mandatory to over-rate an AC switch
current rating for a motor control application.
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2
Voltage rating selection
2.1
Peak line voltage
An AC switch is used in series with the load and the line voltage in most applications. This
means that at the OFF state, the Triac has to withstand at least SQRT(2) times the peak line
voltage.
For single phase voltages, the power grid with the highest rms level that can be
encountered is 277 V. This leads to a 390 V peak voltage applied across the AC switch. If
we even assume a 15% increase of the nominal grid voltage, the applied voltage will be still
lower than 448 V. This is the reason why 600 V devices fit with most single-phase
applications (except those listed in Section 2.2: Motor control applications).
For a 3-phase power grid, the line-to-line voltage is SQRT(3) times higher than the line-toneutral voltage. Table 1 gives the standard values according to the different classic power
grids.
Table 1. Nominal voltages for single and 3-phase grids
Single-Phase voltage VLN (V)
Phase-to-Phase voltage UKL (V)
220
380
230
400
240
415
277
480
Then, the maximum voltage applied across the switch will then depend on load
configuration. As the switch is in series with the load, then the maximum applied voltage will
be:
•
For delta configuration (see Figure 6): VT max = UPeak
•
For star configuration (see Figure 5): VT max = UPeak 3 (worst case for floating load
2
neutral)
Figure 5. Star connection
IY
U12
V1
V2
V3
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Figure 6. Delta connection
IΔ
U12
J 13
V1
V2
V3
The highest voltage across the Triac is then: UPeak (peak line-to-line voltage). This voltage,
according to Table 1 values, is very close or above 600 V. For such applications, AC
switches able to withstand at least 800 V are then required. For motor control, due to the
back electro-motive force (BEMF) that can be added to the voltage held by the AC switch at
OFF state, 1000 V or 1200 V devices are preferred.
2.2
Motor control applications
A BEMF can be applied by a motor when the series AC switch is OFF. A motor can still be
running due to inertia or if a mechanical torque is applied.
A BEMF will occur across a motor if it is turning and if an inductive field is applied. This is the
reason why, such a BEMF does not occur with universal motors (as the exciting field applied
by the rotor is switched off when the stator is turned off, as both windings are in series) and
asynchronous induction motors (as the rotor field is generated by the stator field). A lowlevel BEMF could occur in case of permanent induction due to motor iron saturation but
such a BEMF can usually be neglected.
A BEMF will actually appear at stator winding turn-off usually when permanent magnets are
used. This is the case of permanent magnet synchronous motors (PMSM). Typically the
BEMF could be between 30 to 50% of the line voltage. So a 50% margin on the AC switch
voltage capability, versus the peak line voltage has to be taken into account.
There are also specific topologies where a capacitor can be used to control a motor. These
two topologies are:
•
Induction motors, with split-phase capacitors (see Figure 8): such motors are used for
drum motor control (mainly in vertical axis washing machines), rolling shutters, garagedoor openers, compressors, etc. The Triac voltage typically reaches 550-650 V at
device turn-off for 230-240 V applications (see STMicroelectronics Application note
AN2991).
•
Motors used in a diode-bridge in series with an AC capacitor (used as a ballast to stepdown the Line voltage, see Figure 7): such a topology is used to operate a low-voltage
electronic load like a BLDC motor, directly from the mains. At turn-off, the voltage
across the Triac reaches 2 times the line peak voltage due to the nearly perfect 180°
phase-shift between line and capacitor voltages. This means 746 V can be applied for
a 264 V line voltage (240 V + 10%).
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Figure 7. BLDC low-voltage motor ON/OFF control with a Triac
DC
M
AC
VLINE
Figure 8. Split-phase induction motor ON/OFF control with two Triacs (doubledirection)
M
VLINE
2.3
C
R
L
Over-voltage protection
As an AC switch is mostly used in series with a load and the line voltage, any surge or
transient voltage applied to the grid will be seen by the AC switch if it is in the OFF state.
ACS and ACST are a new AC switch family that is guaranteed to turn on safely by
breakover as long as the applied current stress is within the guaranteed limits. So no
overvoltage protection device is required in parallel to the device to protect it either from
voltage surges coming from the mains or from an overvoltage generated at turn-off on
inductive load (overvoltage could occur due to the holding current, refer to
STMicroelectronics Application note AN1172).
Any other device has to be protected from a voltage exceeding their maximum voltage
capability (VDRM,VRRM or VDSM,VRSM parameters according to the technology). Two
solutions are used to implement an overvoltage protection. The most common is to add a
metal-oxide varistor (MOV) in parallel to the AC switch terminals (for example, A1 and A2 for
a Triac).
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It has to be noted that any MOV presents a series resistance that makes the voltage
clamped by this device increase with the current created by the surge.
This current is limited by the load if the MOV is well placed in parallel with the Triac and not
at the line input (see Figure 9a). This is the preferred position. But if the load power is high,
this means the load impedance is low and the surge current, applied to the MOV, can be
very high, leading to a high voltage applied across the AC switch.
One solution is then to use a Transil connected across Triac A2 and Gate terminals as
shown in Figure 9b. This allows a crowbar protection to be implemented. This solution
allows to more accurately control the maximum voltage applied to the Triac but it will lead to
a spurious triggering of the device in case a surge is applied. More information on this
solution can be found in STMicroelectronics Application note AN1966.
Figure 9. Triac overvoltage protection by MOV (a) or Transil (b)
a)
b)
Load
Load
Transil
VLINE
MOV
VLINE
RGK
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AN4363
3
Gate current and triggering quadrants
3.1
Triac triggering quadrants
To switch-on an SCR, Triac, ACS or ACST, a gate current must be applied on its gate pin
(G). The gate current flows between gate (G) and cathode (K) for SCR, or between gate and
terminal A1 for Triac, or between gate and terminal COM for ACST and ACS.
For Triac and ACST, the gate current could be positive or negative. Figure 10 illustrates the
simplified schematic silicon structure of a Triac or an ACST and its associated equivalent
diagram. As shown on this figure, a Triac or an ACST could be switched on either by a
positive or a negative gate current thanks to the two diodes embedded back-to-back
between G and A1. These 2 diodes are implemented by the P1-N1 and P1-N2 junctions.
Figure 10. Triac simplified silicon structure
A1 (or COM)
G
A1 (or COM)
IVT
N2
G
N2
P1
N1
N
I+
N+
IT
A2 (or OUT)
P
A2 (or OUT)
The silicon structure of an ACS is different from a Triac or an ACST. Here the gate is the
emitter of a NPN bipolar transistor. So there is only one PN junction. The gate current can
then only be sunk from the gate, and not sourced to it (refer to STMicroelectronics
Application note AN3168)
Figure 11 gives the symbol of a Triac, ACS or ACST. The symbol of the ACS illustrates the
embedded gate driver that allows these devices to reach unbeatable immunity –sensitivity
trade-offs. The crowbar symbol, for ACS and ACST, illustrates that these devices are
overvoltage protected and can safely turn on by breakover.
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Gate current and triggering quadrants
Figure 11. AC switches symbols
Triac
ACST
ACS
A2
Out
Out
G
G
A1
G
Com
Com
Four triggering quadrants can be defined according to the polarity of the gate current and
the polarity of the voltage applied across the device, as shown in Figure 12.
Figure 12. Triac triggering quadrants
VT
+ 2 nd
A 2 (or OUT)
++
1 st
IG
3 rd
4 th
- -
- +
VT
G
IG
A 1 (or COM)
For an SCR, only a positive gate current can switch-on the device. Thus, the triggering
quadrants are not considered for SCR devices.
The usable triggering quadrants depend on the family and the class of the device used. The
table 2 sums-up the triggering quadrants for STMicroelectronics devices.
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Table 2. Usable triggering quadrants according to device family and class
Triggering quadrants
Family
Class
Triac
Q1
Q2
Q3
Q4
Standard
Yes
Yes
Yes
Yes
Snubberless and logic level
Yes
Yes
Yes
NA
Snubberless high temperature
Yes
Yes
Yes
NA
ACS
NA
Yes
Yes
NA
ACST
Yes
Yes
Yes
NA
ACS / ACST
3.2
Operating quadrants or application
The choice of the triggering quadrants will depend only on the control circuit. Or vice-versa,
the control circuit choice will depend on the operating quadrants of the selected AC switch.
The most common control circuit consists of controlling the AC switch gate directly by a lowvoltage circuit which can be a microcontroller unit (MCU), ASIC, logic circuit, discrete lowvoltage transistor or an opto-coupler.
Then there are two options in the way the drive reference of the AC switch (cathode K, for
an SCR, terminal A1 for a Triac, terminal COM for an ACST or an ACS) is connected to the
reference of the control circuit:
•
Solution 1, called the “positive supply”, where the control circuit ground (VSS) is
connected to K or A1, as shown on Figure 13a.
•
Solution 2, called the “negative supply”, where the control circuit voltage supply (VDD)
is connected to A1 or COM (see Figure 13b or refer to STMicroelectronics Application
note AN3168 for more information on this solution).
Figure 13. Triac control with positive (a) or negative (b) power supply
b)
a)
VAC
+VDD
LOAD
T
IG
Control
circuit
+VDD
VAC
VSS
IG
LOAD
Control
circuit
VSS
In Figure 13, the Tb bipolar transistor is optional and the AC switch can be controlled
directly by the logic circuit if its I/O pin can supply a current higher than the AC switch
maximum Igt.
For the “positive supply”, the current is always sourced to the gate (as it circulated from VDD
down to the ground through the gate). According to the line voltage polarity, the AC switch
will then be triggered in quadrants Q1 or Q4. For such circuits, only standard Triacs and
SCR can then be selected.
For the “negative supply”, the current is always sunk from the gate (as it circulated from VDD
through the gate, down to the ground). According to the line voltage polarity, the AC switch
will then be triggered in quadrants Q2 or Q3.
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Gate current and triggering quadrants
For such circuits, all Triacs, ACS and ACST can then be selected. Only SCR cannot be
controlled as they require a positive gate current. STMicroelectronics Application note
AN3168 gives a circuit which can be implemented to trigger an SCR from a negative power
supply.
Two other control circuits make the AC switch operate in quadrants Q1 and Q3. These
circuits are those using a diac or an opto-Triac (refer respectively to Figure 14a and
Figure 14b). For these two circuits, the gate current polarity is the same as the voltage
polarity.
Such circuits can trigger any kind of Triac and ACST. SCR could be triggered at least for
positive line cycles. But ACS devices, which require to be triggered in quadrants Q2 and Q3
can not operate with such circuits.
Figure 14. Triac control circuits operating in quadrants Q1 and Q3
LOAD
Lf
LOAD
P
Cf
Diac
C
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4
Package selection
4.1
Surface mount device or through-hole packages
Through-hole packages are convenient for limiting the area used on the printed circuit board
(PCB).
They are also the only solution when the device has to be connected to an external heatsink
or to the appliance chassis.
Surface mount devices (SMD) are more often used in applications where devices are
automatically mounted on the PCB. A heatsink, to limit the temperature of the device, can
be implemented on the PCB by connecting a copper area on the device tab. Such solutions
can be efficient for low load current (typically lower than 2 A).
4.2
Pin-out
The device pin-out can change between two technologies. This is mainly the case between
ACS and Triac technologies. Two devices are then not pin-to-pin compatible.
Figure 15 gives an example of a PCB layout to allow a Z01xx Triac or an ACS108 device to
be used on a same PCB. The selected device, at production step, is then soldered to the
appropriate foot-print. A 180° rotation of the TO92 is enough for assembling compliance.
Figure 15. Triac and ACS pin-out compatibility (in TO92)
ACS pinout
Z01 pinout
Z01
Z01
A1
COM
G
G
A2
OUT
ACS
ACS
Figure 16 gives another example of PCB layout to allow production use of either an ACS108
or a Z01 Triac. These two devices do not offer the same pin-out when they are placed in a
SOT-223 package. One measure could consist of using the Z01 in a SMBflat-3L package.
As this package is around half the size of a SOT-223, it could use the SOT-223 footprint
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Package selection
while keeping a long enough creepage distance. And this is achieved with a very high
density board.
Figure 16. Triac and ACS™ layout compatibility (in SOT223 and SMB-Flat)
Footprint for
ACS108-6SN
COM
Footprint for
Z0109MUF
A1
SOT-223
A2
SMBFIat3L
G
OUT COM
G
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Conclusion
5
AN4363
Conclusion
We have seen in this paper the fundamentals of selecting an AC switch device:
•
Current rating, which is mainly linked to the operating temperature and also to the
device turn-off capability
•
Voltage rating, according to the load to control and the mains voltage;
•
Triggering quadrants and the Igt which depend mainly on the application control circuit
Further information can be found in STMicroelectronics Application notes already listed in
this document (AN439, AN533, AN1172, AN1966, AN3168) and in the following
STMicroelectronics Application notes, which offer useful information for designers not fully
familiar with these devices:
18/20
•
AN2703: Definition of Triac, ACST, ACS, and SCR datasheet parameters
•
AN302 and AN303: Details on the latching and holding current parameters
•
AN392: Triac control with a microcontroller
•
AN437: Designing a snubber circuit
•
AN4030: Impacts of a capacitor connected between device gate and drive reference
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6
Revision history
Revision history
Table 3. Document revision history
Date
Revision
Changes
23-Jan-2014
1
Initial release.
24-Sep-2015
2
Updated two formulas in Equation 4.
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