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Use thermal analysis to predict an IC's transient behavior and avoid overheating

• Therefore:
  
  

  电容器 电感器

  P = mckA(TB - TA)

  (Eq. 4)

  
  

  Equation 4 can be rewritten as:

  (Eq. 5)

  
  

  Where:
  θBA is thermal resistance - body to ambient.
  TB is the temperature inside the package.
  TA is the ambient temperature outside.
  
  Thus:

  (Eq. 6)

  
  

  Defining the chip as a thermal system

  A clear definition of the system is very important because the thermal result depends on that definition. In the cross section of a chip mounted on a PCB (Figure 1), we see at least three different materials in the path from die to environment: the die itself, the mold epoxy, and the package. Thermal models are based on one of two patterns of heat flow, depending on the location of the dominant heat source: flow from an external source to the die (when the external source is the dominant heat generator), and flow from the die to the environment (when the die is the dominant heat generator). We will discuss each of these patterns of heat flow in turn.
  
  
  Figure 1. A cross section of a chip mounted on a PCB shows the layers of material between the die and the environment.
  
  

  Heat flow from an external source to the chip

  Consider the system of Figure 2, which shows a uniform body gaining energy (heat) from a power source and losing energy to the environment.
  
  
  Figure 2. This thermal model illustrates the flow of heat from an outside power source to the chip (BODY 1) and then back out to the environment.
  
  Heat reaches the internal die through the package and the mold compound. Therefore, this system also models thermal transie大功率电感贴片电感器nts in the chip for heat sources outside the package. The package normally has a much higher thermal resistance than the die itself because the die has lots of metal on it. The die, therefore, tracks the package temperature with almost no lag, thus causing the chip to behave as a single body. We can define this one-body system immediately by using Equation 3. Solving for TB, we have:

  (Eq. 7)

  
  

  Where ko is the constant of integration, which is solved according to the initial conditions. In general, this eq贴片电感uation is useful for defining the thermal transient of a chip when the heat source is outside the chip.
  
  We can illustrate this model with an example. To determine the thermal transient for a chip whose initial temperature 工字电感器is Ti, substitute t = 0 and TB = Ti in Equation 7:

  (Eq. 8)

  
  

  Therefore:

  (Eq. 9)

  
  

  Considering the special case for which Ti = TA:

  (Eq. 10)

  
  

  Using Equation 6, we can rewrite Equations 9 and 10:

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  (Eq. 11)