SNVA951 November   2020 LM61460-Q1 , LM63615-Q1 , LM63625-Q1 , LM63635-Q1 , LMR33620-Q1 , LMR33630-Q1

 

  1.   Trademarks
  2. 1 Introduction
  3. 2 The Goal of Thermal Management
  4. 3 Junction Temperature Calculation
    1. 3.1 Regulator Junction Temperature (TJ)
    2. 3.2 Ambient Temperature (TA)
    3. 3.3 Power Dissipation (PD)
    4. 3.4 Thermal Resistance (θJA)
      1. 3.4.1 Thermal Metrics
  5. 4 Package Type
  6. 5 PCB Copper Heat Sink
  7. 6 PCB Layout Tips
  8. 7 Estimating and Measuring θJA
    1. 7.1 Simple Guideline
    2. 7.2 Data Sheet Curves
    3. 7.3 Simplified Heat Flow Spreadsheet
    4. 7.4 Online Database
    5. 7.5 Thermal Simulators
  9. 8 Measuring Thermal Performance
    1. 8.1 Thermal Camera
    2. 8.2 Thermocouple
    3. 8.3 Internal Diode
  10. 9 Thermal Design Example
  11. 10Conclusion
  12. 11References

7 Estimating and Measuring θJA

As mentioned the goal of thermal management is to ensure that the device junction temperature stays within the desired limits. The key to estimating the junction temperature is generating a good estimate of θJA. There are several ways to arrive at an estimate depending on what resources are available. The best way to get a good estimate is to use sophisticated thermal modeling programs. This can be time consuming and expensive. The methods outlined below are not as accurate, but are much faster than conducting a complete thermal analysis and give a good idea whether the PCB/package combination will perform as desired. One common consideration with any method of thermal calculation is determining the "effective" copper area acting as the heatsink. On a crowded PCB, the regulator is only one small piece of the entire system, and it is obvious that the entire PCB copper area is not effective in acting as a heatsink for the power supply. The question becomes: "how much of the copper area surrounding the converter/inductor is effective in removing heat". The answer is related to the thermal footprint mentioned above and requires some prior experience and judgment. As a first estimate the effective copper area can be approximated as about 18 times the area of the heat producing components. However, components such as connecters, and so forth, that break the copper plane will tend to block the heat flow and reduce the effective heatsink area. On the other hand, large surface mount components, such as aluminum electrolytic capacitors, may tend to improve the thermal performance by acting as additional heatsink elements. The example given later may help to clarify some of these considerations.