transient suppression - TVS specification understanding

I have some questions regarding Transient Voltage Suppressors (TVS).

The specific datasheet I'm referring to is this, from 1SMB22AT3G.

Here is some of its specs:

• \$V = 22V\$


• \$V_c = 35.5V\$


• \$I_{pp} = 16.9A\$


• \$tp = 10us\$ (protection pulse width)


• \$P_p = 5kW\$ (power rating)

My questions are:

• How do I calculate its Junction Temperature at \$T_a = 55°C\$?



• If the datasheet says the device can dissipate 5kW @10us pulse width, how is it that its \$R_{JA}\$ is 226°C/W?

Answer

Calculating the peak junction temperature for a short pulse is difficult. The 226°C/W is for steady state or dissipation that is much longer than the thermal time constant of the device.

The pulse rating curve (Figure 1, above) in the data sheet shows how much power the device can dissipate for a single pulse event. It's probably safe to assume that the pulses along the line cause the junction to heat to somewhere between 120°C and the maximum of 150°C. From that you might be able to estimate a thermal time constant and come up with a general formula for junction temperature for a given pulse.

You can see how that would work in this WikiPedia article: Thermal time constant.