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Use thermal analysis to predict an IC's transient behavior and avoid overheating
发布时间:2014-12-03 阅读：次 [打印文章] [关闭窗口]
Figure 9. Equation 34, fitted to a couple of diode-voltage measurements, closely tracks all the diode measurements for a chip that is cooling after being heated with a heat gun.
As we can see in Figure 9, Equation 34 closely follows the measured data for kA = -0.0175. To verify that our equations are correct, we try to fit the falling curve on Equation 28 with the value determined for kA. The equation fits very accurately (Figure 10). Thus, we see that Equation 34 for the system discussed in System definition 1 closely matches the experimental data.
Figure 10. The curve-fitted Equation 28 closely tracks diode-voltage measurements for the falling (heating) portion of the curve.System definition 2
Verification of the system 2 Equations 30, 31, and 32 is more difficult. We must generate heat on the die, measure the die temperature using the diode forward voltage, and fit that temperature value to a simulated value for the C1 voltage of the proposed RC network. This task is accomplished by writing a program using MATLAB.
It is important to record the电感器的原理 thermal transient at a time for which the initial temperature of the whole chip is known. In that way we also know the initial capacitor voltages for solving the RC network. Using the same test setup (see Figure 7), we now turn on the current and capture the diode voltage on a scope (Figure 11).
Figure 11. A forward-voltage transient from the MAX16828's internal diode signals that an on-board MOSFET has turned on and is generating heat.
Similar transients are recorded for three different power-dissipation levels, and one curve is fitted to that data. The circuit in Figure 12 is the result of fitting from the first set of data in which the power dissipation is 1.626W. The graph in Figure 13 compares the measured and fitted data. S上海 电感器imilarly, the graph in Figure 14 shows how the RC network fits the second set of readings (power dissipation of 2.02W). The graph in Figure 15 shows how i工字电感器t fits the third set of readings (power dissipation of 1.223W).
Figure 12. With component values as shown, this RC network models the chip's thermal transient when heat is generated on the die.
Figure 13. Measured vs. curve-fitted results for the chip's heating curve when the die is dissipating 1.626W.
Figure 14. Measured vs. curve-fitted results for the chip's heating curve when the die is dissipating 2.02W.
Figure 15. Measured vs. curve-fitted results for the chip's heating curve when the die is dissipating 1.223W.
These experimental results show that the measured results closely match the theoretical model. Such modeling is useful for simulating the transient temperature of an IC, once you have modeled the RC network for that particular chip. The model can also be used for chips of similar size to determine their thermal characteristics during the definition phase. That capability can indicate the operational limitations of the chip. That information, in turn, helps you define the chip's operational modes to prevent overheating.Conclusion
This article has described a way to model the thermal behavior of a chip as an RC network, which can then be simulated easily using a SPICE tool. The following measures can improve the accuracy of this model:
- Take data at both extreme电感生产厂家s of power dissipation and at one level in the middle. Fitting the RC network to all three levels simultaneously makes the model usable for most practical p大功率电感ower-dissipation levels.
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- Take data at both extreme电感生产厂家s of power dissipation and at one level in the middle. Fitting the RC network to all three levels simultaneously makes the model usable for most practical p大功率电感ower-dissipation levels.
