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

3.3 Power Dissipation (PD)

The power dissipation is calculated with Equation 2.

Equation 2. GUID-20201105-CA0I-WVSB-KTVP-LXQHZD4MDKWN-low.gif

The efficiency of the converter is represented by η. As previously mentioned, the efficiency is somewhat in the control of the designer. The maximum output power (VOUT × IOUT) is set by the application, but there is some opportunity to select a device with good efficiency. Obviously, the higher the efficiency the lower the power dissipation. Lower input voltages tend to improve the efficiency of a regulator. As an example, if the output of a DC/DC converter is 3.3 V, then using a 5-V rail to power the converter (rather than something higher like 12 V) will help to improve efficiency. Switching frequency also has a big impact on the losses in the converter. Selecting a lower switching frequency will reduce the frequency dependent losses. The trade-off here is that a larger LC filter will be needed, increasing the solution size and possibly cost. Reducing the power loss has other benefits for the system as well. With less heat being generated, the other components will experience a cooler ambient, helping to improve overall reliability. The efficiency used in the equation is found in the data sheet or measured under actual operating conditions. In some cases the published efficiency includes the losses in the inductor. These can be approximately accounted for if the inductor resistance is known, using Equation 3.

Equation 3. GUID-20201105-CA0I-2GVM-NLSF-QXCCWNNKBQH6-low.gif

Simply subtract Equation 3 from Equation 2 to get the approximate power loss in the regulator. Although the total inductor power dissipation consists of both AC and DC losses, for the purposes of this example only the DC power loss is accounted for, as represented by the stated resistance of the inductor. If a more accurate loss model for the inductor is needed, the inductor manufacturer should be consulted.

Usually the efficiency found in most data sheets will be taken at an ambient temperature of 25°C. The efficiency at higher temperatures will be somewhat lower. Many times the regulator manufacturer can provide efficiency data at elevated temperatures to aid the user in making power dissipation calculations. In any case the published curves can be used to estimate the efficiency under the specific user conditions and this number can be reduced by a point or two, to account for higher temperature operation. Table 3-1 gives a guide as to how regulator operating conditions affect efficiency and can be used to aid in estimating efficiency under conditions not given in the data sheet.

Table 3-1 Variation of Efficiency With System Parameters
ConditionEffect on Efficiency
Input VoltageDecreases with increasing voltage
Output VoltageIncreases with increasing voltage
Switching FrequencyDecreases with increasing frequency
Ambient TemperatureDecreases with increasing temperature