Enhance Triac Reliability
Through Thermal Design
By Nick Ham, Principal Applications Engineer, Bipolar Product
Line, NXP Semiconductors, Hazel Grove, United Kingdom
Appliance applications illustrate how to perform
the necessary thermal calculations using datasheet information supplied by the semiconductor vendor.
T
riacs are used to control ac mains loads in home
appliances, and commercial and industrial equipment. In the majority of applications, the triac
will dissipate sufficient power to make thermal
considerations necessary. The size of heatsinks
must be calculated, and the maximum junction temperature
must be predicted. These thermal design procedures must be
followed to ensure long-term reliability of the application.
The thermal design requires several stages of calculation
involving power, thermal resistance and temperature rise, as
illustrated by several triac (and one silicon-controlled rectifier; SCR) application examples. These include a vacuum
cleaner, refrigerator compressor, washing machine and
power tool designs.
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Fig. 1. With half-wave conduction of an SCR, the average and RMS load
currents are a function of IPK and the half-cycle time.
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Calculating Triac Power
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Triac power dissipation is influenced by the load current.
Full sine-wave current (full-wave conduction) is assumed, as
it presents the worst-case condition of maximum triac power
dissipation. It also makes for the easiest calculations.
P = VO  ITRIACAVG + RS  ITRIACRMS2
(Eq. 1)
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(Eq. 2)
where P is the triac power (W), VO is the triac knee voltage
(V), ITRIACAVG is the average load current (A), RS is the triac
slope resistance (Ω) and ITRIACRMS is the root-mean-square
(RMS) load current (A).
VO and RS are given in the NXP Semiconductors datasheets on the ITRIAC / VTRIAC curve. If the values are not available, they can be obtained from the ITRIAC / VTRIAC curve as
described under the heading “Calculating VO and RS.” ITRIACAVG
is calculated from the application’s RMS load current using
Eq. 2. (This assumes full-wave conduction and sinusoidal
load current, which will give worst-case power dissipation.)
The value for ITRIACRMS is measured in the application.
If half-wave conduction is necessary, as shown in Fig. 1
π
Power Electronics Technology September 2006
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I TRIAC AVG = 2 × 2 ×
I TRIACRMS
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Fig. 2. The tangent method is used to calculate VO and RS when these
parameters are not given in the datasheet. For worst-case conditions
and a hot triac, the maximum VTRIAC curve at TJ should always be used.
MAX
36
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for a SCR, here’s how to calculate ITRIACRMS and ITRIACAVG:
I TRIAC AVG = 2 × IPK × T / π × 2T = IPK / π.
(Eq. 3)
I TRIACRMS = (IPK 2 × T))/(
/(2 × 2T) = IPK 2 / 4.
(Eq. 4)
I TRIACRMS = IPK / 2.
(Eq. 5)
(°C/W), RTHMB-HS is the mounting base-to-heatsink thermal
resistance (°C/W) and RTHHS-A is the heatsink-to-ambient
thermal resistance (°C/W).
RTHJ-MB is fixed and governed by the device as it is influenced by die size (refer to the relevant datasheet for the exact
value). RTHMB-HS is controlled by the equipment manufacturer
because it is governed by the mounting method (for example,
with or without thermal grease, screw or clip-mounted, insulating pad material). RTHHS-A is governed by the application
and is under the sole control of the equipment manufacturer.
Fig. 3 illustrates these thermal resistance components.
Calculating VO and RS
If values for VO and RS are not given in the datasheet,
you will have to generate the data yourself. These can be
derived from the device’s datasheet, as
shown in Fig. 2. First, make an enlarged
photocopy of the ITRIAC / VTRIAC curve to
increase accuracy. Second, in the graph
of ITRIAC versus the maximum VTRIAC for
TJMAX , draw a tangent through the point
on the curve corresponding to the rated
current of the triac. Third, the point
where the tangent crosses the VTRIAC
axis gives VO. In the fourth and final
step, the slope of the tangent VTRIAC /
ITRIAC gives RS.
I nsulated G ate B ipolar Transistors
10 to 125 kHz Hard Switching
Calculating TJMAX
TJ MAX is influenced by ambient
temperature, triac power dissipation
and the thermal resistance between
junction and ambient. For this article,
only the steady-state condition will be
considered. In the short-term transient
condition, transient thermal impedance
(ZTH) applies. This will always be lower
than the steady-state thermal resistance
(RTH). The transient condition is more
complicated to analyze and beyond the
scope of this article.
TJ = TA + P  RTHJ-A,
(Eq. 6)
where TJ is the junction temperature
(°C), TA is the ambient temperature
(°C),
C), P is the triac power (W) and RTHJ-A
is the junction-to-ambient thermal
resistance (°C/W).
Analysis of RTHJ-A
Thermal resistance is similar to
electrical resistance, in that the total
resistance can be broken down into
several smaller resistances in series. For
the most popular package (TO-220),
RTHJ-A is composed of the following
resistances:
RTHJ-A = RTHJ-MB + RTHMB-HS + RTHHS-A
(Eq. 7)
where R THJ-MB is the junction-tomounting base thermal resistance
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Power Electronics Technology September 2006
TRIAC RELIABILITY
Note that there are some important caveats in the way
the thermal resistance is specified because it depends on
the package type and the practicality of isolating a metallic
thermal reference point. For example, for plastic packages
without a metal mounting base, the expression RTHJ-MB +
RTHMB-HS is replaced by a single parameter of RTHJ-H
, since
J-HS
the heatsink is the nearest metallic reference point. Also,
for low-power plastic packages where a heatsink would not
be used, only RTHJ-LEAD is specified, because the leads are the
nearest metallic reference point. Most of the heat would be
conducted through the leads to the pc board, with a little
radiated directly from the package to ambient. Finally, for
some surface-mount packages without a mounting base but
with a solder point instead, RTHJ-MB is replaced by RTHJ-SP.
The table lists the NXP triac packages and the means
of specifying their thermal resistance. It shows thermal
resistance values where they are fixed by the package type
or mounting method. If a thermal resistance is influenced
by the triac die, the specification becomes specific to that
particular device, so it will be given in the datasheet.
tor power equals 1.8 kW max, the ac mains supply equals
230 VRMS and, therefore:
Max ITRIACRMS = P / V = 1800 W / 230 VRMS = 7.83 A.
The triac is fixed to an air-cooled heatsink, without
thermal grease. Bleed air is allowed to flow through the
heatsink at all times, even if the main airflow is blocked. The
heatsink is double insulated. Absolute maximum heatsink
temperature is 70°C.
A 12-A Hi-Com triac is recommended to cope with the
inductive load and high inrush current. We will take as our
example the BTA212-600B. Its IGATE of 50 mA is well matched
to the typical discrete gate trigger circuit.
Using Eq. 2, I TRIAC AVG = 2 × 2 ×
I TRIACRMS
RMS
π
= 2× 2 ×
7.883
= 7.005 A.
π
From the datasheet, VO = 1.175 V and RS = 0.0316 Ω.
Using Eq. 1, P = VO  I TRIACAVG + R S  I TRIACRMS2 =
1.175 V 7.05 A + 0.0316 Ω  (7.83 A)2 = 10.22 W.
Using Eq. 7, RTHJ-A = RTHJ-MB + RTHMB-HS + RTHHS-A.
From the datasheet, RTHJ-MB = 1.5°C/W.
From the table, for the TO-220 package screw mounted
without insulator and without heatsink compound,
RTHMB-HS = 1.4°C/W.
RTHHS-A can be regarded as zero, since the maximum heatsink temperature is fixed at 70°C under worst-case airflow
Vacuum Cleaner Example
A triac is used in a discrete phase-control circuit to
control the speed of a vacuum-cleaner motor. Confirm by
calculating for worst-case conditions that the triac’s TJMAX
of 125°C will not be exceeded. For this application, the mo-
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Fig. 3. There are three major thermal resistance components in the
complete thermal path for a TO-220 package mounted to a heatsink.
Power Electronics Technology September 2006
38
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TRIAC RELIABILITY
conditions. It can be regarded as an infinite heatsink with a
temperature of 70°C.
C. Therefore, RTHJ-A = 1.5°C/W + 1.4°C/W
+ 0 = 2.9°C/W.
Using Eq. 6, TJMAX
= TA + P  RTHJ-A
MA
= 70°C + 10.22 W  2.9°C/W
= 100°C.
This is below TJMAX
of 125°C and, therefore, acceptable.
MA
An 8-A Hi-Com triac is recommended to cope with the
inductive load and startup current. A suitable triac is the
BTA208S-600E, which uses the DPAK package. Its IGATE of
10 mA is well matched to the drive capability of the microcontroller.
Using Eq. 2, I TRIAC AVG = 2 × 2 ×
I TRIACRMS
RMS
π
= 2× 2 ×
From the datasheet, VO = 1.264 V and RS = 0.0378 Ω.
Using Eq. 1, P = V O  I TRIACAVG + R S  I TRIACRMS2 =
1.264 V  1.26 A + 0.0378 Ω  (1.4 A)2 = 1.67 W.
Using Eq. 6, TJMAX
= TA + P  RTHJ-A.
MA
TJMAX
=
125°C,
T
=
40°C and P = 1.67 W.
A
MA
Rearranging the equation gives:
RTHJ-A = (TJ – TA) / P = (125°C – 40°C) / 1.67 W = 51°C/W.
Using Eq. 7, RTHJ-A = RTHJ-MB + RTHMB-HS + RTHH-SA.
From the datasheet, RTHJ-MB = 2°C/W. We need to find
RTHMB-A.
Rearranging the equation gives:
RTHMB-A = RTHJ-A – RTHJ-MB = 51°C/W – 2°C/W = 49°C/W.
Refrigerator Compressor Example
A triac is used in an electronic thermostat that controls
the on-off switching of a refrigerator compressor. The triac
gate is triggered from a microcontroller with 20-mA current
sink capability. What maximum heatsink thermal resistance
is allowed to keep the triac’s junction temperature within its
TJMAX
of 125°C? Steady-state motor current equals 1.4 ARMS.
MA
Maximum inrush current equals 17 APK in the first half cycle.
Mains supply equals 230 VRMS. A surface-mounted triac is
required for direct soldering to the controller pc board.
Maximum ambient temperature is 40°C.
Package Type
Thermal Resistance Specification
Value (°C/W)
SOT54
(TO-92)
RTH
60
J-LEAD
RTH
(free air)
J-A
SOT78
(TO-220)
See datasheet
J-MB
RTH
(clip, with grease, no insulator)
0.30
RTH
(screw, with grease, no insulator)
0.5
RTH
(clip, no grease, no insulator)
1.4
RTH
(screw, no grease, no insulator)
1.4
RTH
(clip, with grease, 0.1-mm mica insulator)
2.2
RTH
(clip, with grease, 0.25-mm alumina insulator)
0.8
RTH
(screw, with grease, 0.05-mm mica insulator)
1.6
RTH
(screw, no grease, 0.05-mm mica insulator)
4.5
RTH
(free air)
60
MB-HS
MB-HS
MB-HS
MB-HS
MB-HS
MB-HS
MB-HS
J-A
RTH
See datasheet
J-MB
RTH
(clip, with grease, no insulator)
0.4
RTH
(clip, no grease, no insulator)
2.0
RTH
(clip, with grease, 0.1-mm mica insulator)
2.0
RTH
(clip, no grease, 0.1-mm mica insulator)
5.0
RTH
(free air)
100
MB-HS
MB-HS
MB-HS
MB-HS
J-A
SOT186A
(plastic TO-220)
RTH
(with grease)
See datasheet
RTH
(no grease)
See data sheet
RTH
(free air)
55
J-HS
J-HS
J-A
SOT223
150
RTH
MB-HS
SOT82
RTH
J-SP
RTH
J-A
1.4
= 1.226 A.
π
See datasheet
(free air, minimum pad area, FR4 pc board)
150 typical
SOT404
(D2PAK)
RTH
J-MB
MB
RTH
(free air, minimum pad area, FR4 pc board)
See datasheet
SOT428
(DPAK)
RTH
J-MB
MB
RTH
(free air, minimum pad area, FR4 pc board)
See datasheet
75 typical
J-A
J-A
55 typical
Table. NXP triac packages and their thermal resistance specifications.
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39
Power Electronics Technology September 2006
TRIAC RELIABILITY
This is effectively the heatsink thermal resistance, since
the pc board is the heatsink in this case. As an approximate
guide, this thermal resistance can be obtained with a copper pad area of 500 mm2 (refer to the NXP application note
“Surface Mounted Triacs and Thyristors,” document order
number 9397 750 02622).
Please note that the actual thermal resistance will be reduced by other, nondissipating components in close proximity to the triac, while it will be increased by any components
that dissipate power. It is essential to measure the prototype
to discover the true thermal performance.
It uses the SOT186A all-plastic package.
Using Eq. 2, I TRIAC AVG = 2 × 2 ×
I TRIACRMS
RMS
π
= 2× 2 ×
1.3
= 1.117 A.
π
From the datasheet, VO = 1.216 V and RS = 0.0416 Ω.
Using Eq. 1, P = VO  I TRIACAVG + R S  I TRIACRMS2 =
1.216 V  1.17 A + 0.0416 Ω  (1.3 A)2 = 1.49 W.
Using Eq. 6, TJ = TA + P  RTHJ-A.
We already know that TA = 40°C and P = 1.49 W.
From the datasheet, RTHJ-A for the SOT186A package in
free air is 55°C/W.
Therefore, TJ = 40°C + 1.49 W  55°C/W = 122°C. This
is below the TJMAX of 125°C. Therefore, the triacs can be
operated without heatsinks.
Vertical-Axis Washing Machine Example
The washing machine uses a reversing induction motor
that’s controlled by two triacs. Will the triacs’ TJMAX of 125°C
be exceeded if they are operated without a heatsink?
Full load motor power equals 300 W. The ac mains supply
equals 230 VRMS. Therefore:
Max ITRIACRMS = P / V = 300 W / 230 VRMS = 1.3 A.
An isolated triac package is required, and the maximum
ambient temperature is 40°C. Calculations are as follows:
This application requires 1000-V triacs to withstand the
high ac mains voltage that the motor imposes across them.
A three-quadrant design is mandatory for maximum immunity to spurious triggering. The BTA208X-1000C is recommended. It is an 8-A Hi-Com triac with IGATE of 35 mA.
Power Tool Example
A heavy-duty electric drill uses a universal (brush) motor whose speed is controlled by a half-wave phase-control
circuit. Calculate the maximum power dissipation in the
SCR and calculate the heatsink thermal resistance required
to maintain the junction temperature below TJMAX.
Peak motor current during normal running = 5 A.
A surface-mounted triac is required for mounting within the
trigger switch. Maximum ambient temperature is 50°C.
The SCR is air-cooled by the motor cooling fan. The
BTH151S-650R is chosen for its high repetitive surge
guarantee for the repetitive overload conditions it will have
to face. It is rated at 12 ARMS and comes in
the SOT428 (DPAK) package.
Using Eq. 3, ITRIACAVG= IPK / π = 5 / π =
1.59 A.
Using Eq. 5, ITRIACRMS= IPK/2 = 5/2 = 2.5 A.
From the datasheet, VO = 1.06 V and
RS = 0.0304 Ω.
Using Eq. 1, P = VO  ITRIACAVG + RS 
ITRIACRMS2 = 1.06 V  1.59 A + 0.0304 Ω 
(2.5 A)2 = 1.88 W.
Using Eq. 6, TJ = TA + P  RTHJ-A.
We already know that TA = 50°C and
P = 1.88 W and, in this case, TJ = TJMAX =
125°C.
Rearranging the equation gives:
RTHJ-A= (TJ – TA) / P = (125°C – 50°C) /
1.88 W = 39.9°C/W.
Using Eq. 7, RTHJ-A = RTHJ-MB + RTHMB-HS
+ RTHHS-A.
From the datasheet, RTHJ-MB = 1.8°C/W.
We need to find RTHMB-A.
Rearranging the equation gives:
RTHMB-A = RTHJ-A – RTHJ-MB = 39.9°C/W –
1.8°C/W = 38.1°C/W.
A maximum heatsink thermal resistance
of 38°C/W will keep TJ at or below 125°C.
This heatsink thermal resistance covers the
steady-state condition and is easily achievVisit www.bussco.com/busbar
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