The Circuit Designer's Companion - diagramas.diagram...

General product design 317plenty of cooling time, then a larger thermal capacity will reduce the maximumtemperatures T h and T j reached during a heat pulse. You can analyse this if necessarywith the equivalent circuit of Figure 9.12. Strictly, the other heat transfer componentsalso have an associated thermal capacity which could be included in the analysis ifnecessary.Transient thermal characteristics of the power deviceIn applications where the power dissipated in the device consists of continuous low dutycycle periodic pulses, faster than the heatsink thermal time constant, the instantaneous orpeak junction temperature may be the limiting condition rather than the averagetemperature. In this case you need to consult curves for transient thermal resistance.These curves are normally provided by power semiconductor manufacturers in the formof a correction factor that multiplies R θj-c to allow for the duty cycle of the powerdissipation. Figure 9.13 shows a family of such curves for the IRF640. Because theperiod for most pulsed applications is much shorter than the heatsink’s thermal timeconstant, the values of R θh-a and R θc-h can be multiplied directly by the duty cycle. Thenthe junction temperature can now be calculated fromT j = P Dmax · [K·R θj-c + δ·(R θc-h + R θh-a )] + T Awhere δ is the duty cycle and K is derived from curves as in Figure 9.13 for a particularvalue of δ. P Dmax is still the maximum power dissipated during the conduction period,not the power averaged over the whole cycle. At frequencies greater than a few kHz, andduty cycles more than 20%, cycle-by-cycle temperature fluctuations are small enoughthat the peak junction temperature is determined by the average power dissipation, so thatK tends towards δ.Figure 9.13 Transient thermal impedance curves for the IRF640Source: International RectifierSome applications, notably RF amplifiers or switches driving highly inductive loads,may create severe current crowding conditions on the semiconductor die which invalidatemethods based on thermal resistance or transient thermal impedance. Safe operatingareas and di/dt limits must be observed in these cases.9.5.2 HeatsinksAs the previous section implied, the purpose of a heatsink is to provide a low thermalresistance path between the heat source and the ambient. Strictly speaking, it is theambient environment which is the heat sink; what we conventionally refer to as a heatsink