Power Electronics

Enhance Triac Reliability Through Thermal Design

Appliance applications illustrate how to perform the necessary thermal calculations using datasheet information supplied by the semiconductor vendor.

Triacs 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.

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)

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 for a SCR, here's how to calculate ITRIACRMS and ITRIACAVG:

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.

Calculating TJMAX

TJMAX 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), 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 RTHJ-MB is the junction-to-mounting base thermal resistance (°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.

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-HS, since 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.

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 motor 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.

From the datasheet, VO = 1.175 V and RS = 0.0316 Ω.

Using Eq. 1, P = VO × ITRIACAVG + RS × ITRIACRMS2 = 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 conditions. It can be regarded as an infinite heatsink with a temperature of 70°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

= 70°C + 10.22 W × 2.9°C/W

= 100°C.

This is below TJMAX of 125°C and, therefore, acceptable.

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. 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.

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.

From the datasheet, VO = 1.264 V and RS = 0.0378 Ω.

Using Eq. 1, P = VO × ITRIACAVG + RS × ITRIACRMS2 = 1.264 V × 1.26 A + 0.0378 Ω × (1.4 A)2 = 1.67 W.

Using Eq. 6, TJMAX = TA + P × RTHJ-A.

TJMAX = 125°C, TA = 40°C and P = 1.67 W.

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.

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.

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. It uses the SOT186A all-plastic package.

From the datasheet, VO = 1.216 V and RS = 0.0416 Ω.

Using Eq. 1, P = VO × ITRIACAVG + RS × ITRIACRMS2 = 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.

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 achievable with a small degree of airflow through the switch module.

Table. NXP triac packages and their thermal resistance specifications.
Package Type Thermal Resistance Specification Value (°C/W)
SOT54
(TO-92)
RTHJ-LEAD
RTHJ-A (free air)
60
150
SOT78
(TO-220)
RTHJ-MB
RTHMB-HS (clip, with grease, no insulator)
RTHMB-HS (screw, with grease, no insulator)
RTHMB-HS (clip, no grease, no insulator)
RTHMB-HS (screw, no grease, no insulator)
RTHMB-HS (clip, with grease, 0.1-mm mica insulator)
RTHMB-HS (clip, with grease, 0.25-mm alumina insulator)
RTHMB-HS (screw, with grease, 0.05-mm mica insulator)
RTHMB-HS (screw, no grease, 0.05-mm mica insulator)
RTHJ-A (free air)
See datasheet
0.30
0.5
1.4
1.4
2.2
0.8
1.6
4.5
60
SOT82 RTHJ-MB
RTHMB-HS (clip, with grease, no insulator)
RTHMB-HS (clip, no grease, no insulator)
RTHMB-HS (clip, with grease, 0.1-mm mica insulator)
RTHMB-HS (clip, no grease, 0.1-mm mica insulator)
RTHJ-A (free air)
See datasheet
0.4
2.0
2.0
5.0
100
SOT186A
(plastic TO-220)
RTHJ-HS (with grease)
RTHJ-HS (no grease)
RTHJ-A (free air)
See datasheet
See data sheet
55
SOT223 RTHJ-SP
RTHJ-A (free air, minimum pad area, FR4 pc board)
See datasheet
150 typical
SOT404
(D2PAK)
RTHJ-MB
RTHJ-A (free air, minimum pad area, FR4 pc board)
See datasheet
55 typical
SOT428
(DPAK)
RTHJ-MB
RTHJ-A (free air, minimum pad area, FR4 pc board)
See datasheet
75 typical
Hide comments

Comments

  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.
Publish