Figure 5-10 Normalized effective transient thermal impedance vs. square wave pulse duration.

Typically most power applications are characterized by use of a series of electrical pulses rather than a single pulse. When a square wave with a duty factor DDTon/T, as shown in Figure 5-9, is applied to a single transistor, the resulting junction temperature will also depend on the pulse duration ton. The shorter the pulse, the smaller the junction temperature will rise. The variation in thermal resistance with pulse duration is shown graphically in Figure 5-9. The thermal resistance is normalized so that

This family of curves allows to calculate the junction temperature if power dissipation in the chip PDM, duty cycle D,and pulse duration ton are known, by using the expression

Figure 5-11 Thermal transient response dependence on single pulse duration.