Enhance Triac Reliability Through Thermal Design

Sep 1, 2006 12:00 PM
By Nick Ham, Principal Applications Engineer, Bipolar Product Line, NXP Semiconductors, Hazel Grove,

Vertical-Axis Washing Machine Example

The washing machine uses a reversing induction motor that's controlled by two triacs. Will the triacs' T_JMAX 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 V_RMS. Therefore:

Max I_TRIACRMS = P / V = 300 W / 230 V_RMS = 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 I_GATE of 35 mA. It uses the SOT186A all-plastic package.

From the datasheet, V_O = 1.216 V and R_S = 0.0416 Ω.

Using Eq. 1, P = V_O × 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, T_J = T_A + P × R_THJ-A.

We already know that T_A = 40°C and P = 1.49 W.

From the datasheet, R_THJ-A for the SOT186A package in free air is 55°C/W.

Therefore, T_J = 40°C + 1.49 W × 55°C/W = 122°C. This is below the T_JMAX 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 T_JMAX.

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 A_RMS and comes in the SOT428 (DPAK) package.

Using Eq. 3, I_TRIACAVG = I_PK / π = 5 / π = 1.59 A.

Using Eq. 5, I_TRIACRMS = I_PK/2 = 5/2 = 2.5 A.

From the datasheet, V_O = 1.06 V and R_S = 0.0304 Ω.

Using Eq. 1, P = V_O × I_TRIACAVG + R_S × I_TRIACRMS² = 1.06 V × 1.59 A + 0.0304 Ω × (2.5 A)² = 1.88 W.

Using Eq. 6, T_J = T_A + P × R_THJ-A.

We already know that T_A = 50°C and P = 1.88 W and, in this case, T_J = T_JMAX = 125°C.

Rearranging the equation gives:

R_THJ-A = (T_J - T_A) / P = (125°C - 50°C) / 1.88 W = 39.9°C/W.

Using Eq. 7, R_THJ-A = R_THJ-MB + R_THMB-HS + R_THHS-A.

From the datasheet, R_THJ-MB = 1.8°C/W. We need to find R_THMB-A.

Rearranging the equation gives:

R_THMB-A = R_THJ-A - R_THJ-MB = 39.9°C/W - 1.8°C/W = 38.1°C/W.

A maximum heatsink thermal resistance of 38°C/W will keep T_J 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