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AN-1059 Datasheet(PDF) 2 Page - International Rectifier |
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AN-1059 Datasheet(HTML) 2 Page - International Rectifier |
2 / 11 page Introduction Encapsulated power semiconductors in packages such as the TO-220 or D-Pak are fairly easy to model thermally, using only one thermal parameter. The assumption is that most of the power generated in the silicon chip travels in one direction. The assumption is reasonable because the silicon chip is soldered (or epoxy-attached) to a lead frame that provides the main cooling path to the environment. Heat flow in the opposite direction is limited because the die is insulated with a layer of encapsulation ‘plastic’. The dissipation of heat from the lead-frame into the environment is often enhanced by fitting a heat sink. DirectFET ® is fundamentally different. Its construction encourages heat to disperse from the die in opposite directions, cooling through both the substrate pad connections (source and gate) and the metal can on top of the device. The can-to-ambient thermal interface can be maximized by fitting a heat sink. The design of the can also provides a parallel or shunt thermal path from the can to substrate. Figure 1 shows the direction of heat flow from a DirectFET device and an approximate thermal equivalent circuit for such a device in use. Measuring the thermal resistance of a DirectFET device inevitably produces a composite result based on the temperatures measured at the junction, can or substrate using the total power dissipated by the silicon. While this gives the effective thermal resistance under particular cooling conditions, it does not give a value that applies under other conditions. When determining a value for dual-sided cooling conditions, the most significant factors are the thermal resistance of the heat sink and substrate. To assess these correctly, the power flow through each path must be known. This requires a method of predicting the proportion of power flow through the paths. The temperature of the can and the substrate will change with different levels of can and substrate cooling. Indeed, it is this feature of DirectFET construction – which enables cooling from both sides of the silicon die – that gives the devices their particular benefits. This application note provides an easy method for assessing the proportion of power flow from each of a DirectFET device’s surfaces, so that the appropriate thermal resistance figures are used and the true rating is accurately determined. can heat sink junction substrate heat sink interface material can (drain) source pads gate pad Tambient Rth chassis / heat sink T chassis / heat sink Rth gap filler Tcan Rth can-substrate (can) Rth junction-can Tjunction Rth junction-substrate (source) Tsubstrate Rth substrate Rth heat sink (substrate attachment, if present) Tambient Figure 1a Directions of heat flow (indicated by the red arrows) Figure 1b Approximate thermal equivalent circuit DirectFET ® Technology AN-1059 Thermal Model and Rating Calculator www.irf.com Version 3, September 2010 Page 2 of 11 |
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