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,

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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 = V_O × I_TRIACAVG + R_S × I_TRIACRMS² (Eq. 1)

where P is the triac power (W), V_O is the triac knee voltage (V), I_TRIACAVG is the average load current (A), R_S is the triac slope resistance (Ω) and I_TRIACRMS is the root-mean-square (RMS) load current (A).

V_O and R_S are given in the NXP Semiconductors datasheets on the I_TRIAC / V_TRIAC curve. If the values are not available, they can be obtained from the I_TRIAC / V_TRIAC curve as described under the heading “Calculating V_O and R_S.” I_TRIACAVG 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 I_TRIACRMS is measured in the application.

If half-wave conduction is necessary, as shown in Fig. 1 for a SCR, here's how to calculate I_TRIACRMS and I_TRIACAVG:

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