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.
