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,

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 T_JMAX of 125°C will not be exceeded. For this application, the motor power equals 1.8 kW max, the ac mains supply equals 230 V_RMS and, therefore:

Max I_TRIACRMS = P / V = 1800 W / 230 V_RMS = 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 I_GATE of 50 mA is well matched to the typical discrete gate trigger circuit.

From the datasheet, V_O = 1.175 V and R_S = 0.0316 Ω.

Using Eq. 1, P = V_O × 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, R_THJ-A = R_THJ-MB + R_THMB-HS + R_THHS-A.

From the datasheet, R_THJ-MB = 1.5°C/W.

From the table, for the TO-220 package screw mounted without insulator and without heatsink compound, R_THMB-HS = 1.4°C/W.

R_THHS-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, R_THJ-A = 1.5°C/W + 1.4°C/W + 0 = 2.9°C/W.

Using Eq. 6, T_JMAX = T_A + P × R_THJ-A

= 70°C + 10.22 W × 2.9°C/W

= 100°C.

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

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