# Enhance Triac Reliability Through Thermal Design

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"Enhance Triac Reliability Through Thermal Design"

```					     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 data-
sheet information supplied by the semiconduc-
tor vendor.

T
riacs are used to control ac mains loads in home                           ��
appliances, and commercial and industrial equip-
ment. 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             Fig. 1. With half-wave conduction of an SCR, the average and RMS load
illustrated by several triac (and one silicon-controlled recti-          currents are a function of IPK and the half-cycle time.
ﬁer; SCR) application examples. These include a vacuum
cleaner, refrigerator compressor, washing machine and
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power tool designs.
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Calculating Triac Power                                                                           ���������
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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I TRIACRMS
I TRIAC AVG = 2 × 2 ×                ,               (Eq. 2)
π
where P is the triac power (W), VO is the triac knee voltage
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(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 data-                                ���
sheets on the ITRIAC / VTRIAC curve. If the values are not avail-
able, 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
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Eq. 2. (This assumes full-wave conduction and sinusoidal
load current, which will give worst-case power dissipation.)             Fig. 2. The tangent method is used to calculate VO and RS when these
The value for ITRIACRMS is measured in the application.                  parameters are not given in the datasheet. For worst-case conditions
If half-wave conduction is necessary, as shown in Fig. 1             and a hot triac, the maximum VTRIAC curve at TJ should always be used.
MAX

Power Electronics Technology September 2006                         36                                                          www.powerelectronics.com
for a SCR, here’s how to calculate ITRIACRMS and ITRIACAVG:               (°C/W), RTHMB-HS is the mounting base-to-heatsink thermal
resistance (°C/W) and RTHHS-A is the heatsink-to-ambient
I TRIAC AVG = 2 × IPK × T / π × 2T = IPK / π.          (Eq. 3)        thermal resistance (°C/W).
RTHJ-MB is ﬁxed and governed by the device as it is inﬂu-
I TRIACRMS = (IPK 2 × T) /(2 × 2T) = IPK 2 / 4.
)/(                             (Eq. 4)        enced by die size (refer to the relevant datasheet for the exact
value). RTHMB-HS is controlled by the equipment manufacturer
I TRIACRMS = IPK / 2.                                  (Eq. 5)        because it is governed by the mounting method (for example,
with or without thermal grease, screw or clip-mounted, in-
Calculating VO and RS                                                     sulating pad material). RTHHS-A is governed by the application
If values for VO and RS are not given in the datasheet,               and is under the sole control of the equipment manufacturer.
you will have to generate the data yourself. These can be                 Fig. 3 illustrates these thermal resistance components.
derived from the device’s datasheet, as
shown in Fig. 2. First, make an enlarged

I nsulated G ate B ipolar Transistors
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                                             10 to 125 kHz Hard Switching
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.

Calculating TJMAX
TJ MAX is influenced by ambient
temperature, triac power dissipation
and the thermal resistance between
only the steady-state condition will be
considered. In the short-term transient
condition, transient thermal impedance                Power MOS 7® IGBTs (600, 900, 1200V)
(ZTH) applies. This will always be lower                   PT Technology
than the steady-state thermal resistance                   Ultralow Gate Resistance and Charge
(RTH). The transient condition is more                     Ultralow Switching Losses
Low Cost Alternative to MOSFETs
complicated to analyze and beyond the
Excellent Noise Immunity
scope of this article.                                     Combi with High Speed Diode Available
TJ = TA + P  RTHJ-A,         (Eq. 6)
where TJ is the junction temperature               Field Stop IGBTs (600 & 1200V)
(°C), TA is the ambient temperature                        Trench Technology
(°C), P is the triac power (W) and RTHJ-A
C),                                                      Short Circuit Rated
is the junction-to-ambient thermal                         Very Low Conduction Losses
resistance (°C/W).                                         Easy Paralleling
Combi with High Speed Diode Available
Analysis of RTHJ-A                                    Thunderbolt® IGBTs (600 & 1200V)
Thermal resistance is similar to                        NPT Technology
electrical resistance, in that the total                   Short Circuit Rated
resistance can be broken down into                         Moderate to High Frequency
several smaller resistances in series. For                 Easy Paralleling
the most popular package (TO-220),                         Combi with High Speed Diode Available
RTHJ-A is composed of the following
resistances:                                             ADVANCED POWER TECHNOLOGY IS NOW
RTHJ-A = RTHJ-MB + RTHMB-HS + RTHHS-A                                                                                             TM

www.microsemi.com
(Eq. 7)
Phone: (541)382-8028
where R THJ-MB is the junction-to-
mounting base thermal resistance

www.powerelectronics.com                                             37                                Power Electronics Technology September 2006
TRIAC RELIABILITY
Note that there are some important caveats in the way              tor power equals 1.8 kW max, the ac mains supply equals
the thermal resistance is speciﬁed because it depends on              230 VRMS and, therefore:
the package type and the practicality of isolating a metallic            Max ITRIACRMS = P / V = 1800 W / 230 VRMS = 7.83 A.
thermal reference point. For example, for plastic packages               The triac is ﬁxed to an air-cooled heatsink, without
without a metal mounting base, the expression RTHJ-MB +               thermal grease. Bleed air is allowed to ﬂow through the
RTHMB-HS is replaced by a single parameter of RTHJ-HS , since
J-H
heatsink at all times, even if the main airﬂow is blocked. The
the heatsink is the nearest metallic reference point. Also,           heatsink is double insulated. Absolute maximum heatsink
for low-power plastic packages where a heatsink would not             temperature is 70°C.
be used, only RTHJ-LEAD is speciﬁed, because the leads are the           A 12-A Hi-Com triac is recommended to cope with the
nearest metallic reference point. Most of the heat would be           inductive load and high inrush current. We will take as our
conducted through the leads to the pc board, with a little            example the BTA212-600B. Its IGATE of 50 mA is well matched
radiated directly from the package to ambient. Finally, for           to the typical discrete gate trigger circuit.
some surface-mount packages without a mounting base but                                                    I TRIACRMSS                8
7.83
Using Eq. 2, I TRIAC AVG = 2 × 2 ×          RM
= 2× 2 ×        = 7.05 A.
0
with a solder point instead, RTHJ-MB is replaced by RTHJ-SP.                                                   π                     π
The table lists the NXP triac packages and the means                  From the datasheet, VO = 1.175 V and RS = 0.0316 Ω.
of specifying their thermal resistance. It shows thermal                 Using Eq. 1, P = VO  I TRIACAVG + R S  I TRIACRMS2 =
resistance values where they are ﬁxed by the package type             1.175 V 7.05 A + 0.0316 Ω  (7.83 A)2 = 10.22 W.
or mounting method. If a thermal resistance is inﬂuenced                 Using Eq. 7, RTHJ-A = RTHJ-MB + RTHMB-HS + RTHHS-A.
by the triac die, the speciﬁcation becomes speciﬁc to that               From the datasheet, RTHJ-MB = 1.5°C/W.
particular device, so it will be given in the datasheet.                 From the table, for the TO-220 package screw mounted
without insulator and without heatsink compound,
Vacuum Cleaner Example                                                 RTHMB-HS = 1.4°C/W.
A triac is used in a discrete phase-control circuit to                RTHHS-A can be regarded as zero, since the maximum heat-
control the speed of a vacuum-cleaner motor. Conﬁrm by                sink temperature is ﬁxed at 70°C under worst-case airﬂow
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                                                         www.powerelectronics.com
TRIAC RELIABILITY
conditions. It can be regarded as an inﬁnite heatsink with a                  An 8-A Hi-Com triac is recommended to cope with the
C.
temperature of 70°C. Therefore, RTHJ-A = 1.5°C/W + 1.4°C/W                 inductive load and startup current. A suitable triac is the
+ 0 = 2.9°C/W.                                                             BTA208S-600E, which uses the DPAK package. Its IGATE of
Using Eq. 6, TJMAX = TA + P  RTHJ-A
MA
10 mA is well matched to the drive capability of the micro-
= 70°C + 10.22 W  2.9°C/W                                              controller.
= 100°C.                                                                                                     I TRIACRMSS              1.4
Using Eq. 2, I TRIAC AVG = 2 × 2 ×          RM
= 2× 2 ×       = 1.26 A.
2
This is below TJMAX of 125°C and, therefore, acceptable.                                                         π                     π
MA
From the datasheet, VO = 1.264 V and RS = 0.0378 Ω.
Refrigerator Compressor Example                                               Using Eq. 1, P = V O  I TRIACAVG + R S  I TRIACRMS2 =
A triac is used in an electronic thermostat that controls              1.264 V  1.26 A + 0.0378 Ω  (1.4 A)2 = 1.67 W.
the on-off switching of a refrigerator compressor. The triac                  Using Eq. 6, TJMAX = TA + P  RTHJ-A.
MA
gate is triggered from a microcontroller with 20-mA current                   TJMAX = 125°C, TA = 40°C and P = 1.67 W.
MA
sink capability. What maximum heatsink thermal resistance                     Rearranging the equation gives:
is allowed to keep the triac’s junction temperature within its                RTHJ-A = (TJ – TA) / P = (125°C – 40°C) / 1.67 W = 51°C/W.
TJMAX of 125°C? Steady-state motor current equals 1.4 ARMS.
MA
Using Eq. 7, RTHJ-A = RTHJ-MB + RTHMB-HS + RTHH-SA.
Maximum inrush current equals 17 APK in the ﬁrst half cycle.                  From the datasheet, RTHJ-MB = 2°C/W. We need to ﬁnd
Mains supply equals 230 VRMS. A surface-mounted triac is                   RTHMB-A.
required for direct soldering to the controller pc board.                     Rearranging the equation gives:
Maximum ambient temperature is 40°C.                                          RTHMB-A = RTHJ-A – RTHJ-MB = 51°C/W – 2°C/W = 49°C/W.
Package Type                         Thermal Resistance Speciﬁcation                                      Value (°C/W)
SOT54                                RTH                                                                  60
(TO-92)                              RTH         (free air)                                               150
J-A

SOT78                                RTH                                                                  See datasheet
J-MB
(TO-220)                             RTH          (clip, with grease, no insulator)                       0.30
MB-HS
RTH          (screw, with grease, no insulator)                      0.5
MB-HS
RTH          (clip, no grease, no insulator)                         1.4
MB-HS
RTH          (screw, no grease, no insulator)                        1.4
MB-HS
RTH          (clip, with grease, 0.1-mm mica insulator)              2.2
MB-HS
RTH          (clip, with grease, 0.25-mm alumina insulator)          0.8
MB-HS
RTH          (screw, with grease, 0.05-mm mica insulator)            1.6
MB-HS
RTH          (screw, no grease, 0.05-mm mica insulator)              4.5
MB-HS
RTH         (free air)                                               60
J-A

SOT82                                RTH                                                                  See datasheet
J-MB
RTH          (clip, with grease, no insulator)                       0.4
MB-HS
RTH          (clip, no grease, no insulator)                         2.0
MB-HS
RTH          (clip, with grease, 0.1-mm mica insulator)              2.0
MB-HS
RTH          (clip, no grease, 0.1-mm mica insulator)                5.0
MB-HS
RTH         (free air)                                               100
J-A

SOT186A                              RTH        (with grease)                                             See datasheet
J-HS
(plastic TO-220)                     RTH        (no grease)                                               See data sheet
J-HS
RTH        (free air)                                                55
J-A

SOT223                               RTH                                                                  See datasheet
J-SP
RTH        (free air, minimum pad area, FR4 pc board)                150 typical
J-A

SOT404                               RTH                                                                  See datasheet
MB
J-MB
(D2PAK)                              RTH     (free air, minimum pad area, FR4 pc board)                   55 typical
J-A

SOT428                               RTH                                                                  See datasheet
MB
J-MB
(DPAK)                               RTH     (free air, minimum pad area, FR4 pc board)                   75 typical
J-A

Table. NXP triac packages and their thermal resistance speciﬁcations.

www.powerelectronics.com                                              39                                   Power Electronics Technology September 2006
TRIAC RELIABILITY
This is effectively the heatsink thermal resistance, since                   It uses the SOT186A all-plastic package.
the pc board is the heatsink in this case. As an approximate                                                          I TRIACRMSS              1.3
Using Eq. 2, I TRIAC AVG = 2 × 2 ×          RM
= 2× 2 ×       = 1.17 A.
1
guide, this thermal resistance can be obtained with a cop-                                                                π                     π
per pad area of 500 mm2 (refer to the NXP application note                          From the datasheet, VO = 1.216 V and RS = 0.0416 Ω.
“Surface Mounted Triacs and Thyristors,” document order                             Using Eq. 1, P = VO  I TRIACAVG + R S  I TRIACRMS2 =
number 9397 750 02622).                                                          1.216 V  1.17 A + 0.0416 Ω  (1.3 A)2 = 1.49 W.
Please note that the actual thermal resistance will be re-                      Using Eq. 6, TJ = TA + P  RTHJ-A.
duced by other, nondissipating components in close proxim-                          We already know that TA = 40°C and P = 1.49 W.
ity to the triac, while it will be increased by any components                      From the datasheet, RTHJ-A for the SOT186A package in
that dissipate power. It is essential to measure the prototype                   free air is 55°C/W.
to discover the true thermal performance.                                           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
Vertical-Axis Washing Machine Example                                            operated without heatsinks.
The washing machine uses a reversing induction motor
that’s controlled by two triacs. Will the triacs’ TJMAX of 125°C                 Power Tool Example
be exceeded if they are operated without a heatsink?                                         A heavy-duty electric drill uses a universal (brush) mo-
Full load motor power equals 300 W. The ac mains supply                               tor whose speed is controlled by a half-wave phase-control
equals 230 VRMS. Therefore:                                                              circuit. Calculate the maximum power dissipation in the
Max ITRIACRMS = P / V = 300 W / 230 VRMS = 1.3 A.                                     SCR and calculate the heatsink thermal resistance required
An isolated triac package is required, and the maximum                                to maintain the junction temperature below TJMAX.
ambient temperature is 40°C. Calculations are as follows:                                    Peak motor current during normal running = 5 A.
This application requires 1000-V triacs to withstand the                              A surface-mounted triac is required for mounting within the
high ac mains voltage that the motor imposes across them.                                trigger switch. Maximum ambient temperature is 50°C.
A three-quadrant design is mandatory for maximum im-                                         The SCR is air-cooled by the motor cooling fan. The
munity to spurious triggering. The BTA208X-1000C is rec-                                 BTH151S-650R is chosen for its high repetitive surge
ommended. It is an 8-A Hi-Com triac with IGATE of 35 mA.                                 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 achiev-
Visit www.bussco.com/busbar
to download our design guide or brochure. Call toll free: 866-481-5137      able with a small degree of airﬂow through
the switch module.                        PETech

Power Electronics Technology September 2006                                 40                                                         www.powerelectronics.com

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