WEBENCH® Design Center

PCB Thermal Calculator - Help

Notice

The PCB thermal calculator serves as a first pass estimation tool to determine the thermal impact on device performance when changes occur on the printed circuit board (PCB). Use this calculator only for estimates. If the system is complex or thermally sensitive, you should consider a more detailed analysis. For all applications, validate the actual device temperature with measurements in the final system.

The operation, performance, and reliability of TI devices are based on compliance to the Maximum Operating Conditions as specified in the device datasheets.

You assume all responsiblity and liabilty for using the PCB Thermal Calculator, including any problems or failures of your products and applications. TI assumes no liability or responsibilty for any problems or failtures with your products and applications, applications assistance, customer product design, tool performance or infringement of patents or other intellectual property rights of third parties.

Questions, comments, concerns, and feedback can posted here:

Help Sections

  • For a more detailed explanation of the method and supporting data, view the following TI Application Note.

Packaging

This calculator is designed specifically to estimate the junction temperatures of components which use exposed-pad packages. An exposed pad is a device pad or thermal tab that provides a direct thermal path from the device to the PCB.

Several examples of these packages are shown here.

PCB Thermal Calculator - Package Types

This calculator is designed for use with exposed packages for two reasons:

  1. Exposed-pad packages are commonly used for devices which dissipate heat primarily through the PCB.
  2. Exposed-pad packages have a simple conduction heat-flow path, which correlates the modeling and measured data to facilitate a simple and accurate calculation.

Back to Top

Method

The following equation calculates the total thermal resistance for the system:

Tj = Ta + (theta-JC,bottom + theta-CA) * Power Dissipated

Where:

  • Tj = the estimate of the junction temperature of the device
  • Ta = the ambient temperature around the PCB
  • Theta-JC,bottom = the thermal resistance from the junction to the bottom of the device
  • Theta-CA = the thermal resistance from the bottom of the device to the surrounding ambient air through the PCB
  • Power Dissipated = the dissipated power of the device

PCB Thermal Calculator - Theta-CA and Theta-JC bottom

The data used in the thermal calculator is generated from measured laboratory data combined with a careful, thermal-modeling analysis that uses a series of thousands of data points. The result of the correlation is used to create the curves and the data points that appear in the thermal calculator.

The thermal calculator also supports the input of the board temperature as a reference point. In this case, the junction temperature results are estimated from a simple calculation that is based on the psi-JB thermal parameter.

Key assumptions for this calculator:

  • The ambient temperature is the ambient air temperature to which the copper cooling area of the PCB is exposed. This ambient temperature is not necessarily the same temperature as the external ambient temperature nor is it the same as the ambient temperature that is measured at other locations in the system.
  • The copper spreading area is dedicated to the device. If other hot devices are on the copper plane, then the copper spreading area to be used in the Calculator Results Section should be scaled down based on the ratio of the power dissipation of the devices.
  • The calculator uses natural convection, which typically requires at least 6 mm of space above and below the PCB for air currents to develop. For tighter enclosures, you must consider the loss in effectiveness of the convection.
  • For exposed-pad packages, the PCB is often the primary contributor to thermal resistance. The PCB thermal resistance is often much more significant than the device thermal resistance.

Back to Top

Calculator Inputs

Following are brief descriptions of each field:

Enter/Select a Device

This field allows you to select the device to use in the thermal calculator. The auto-complete and drop-down features allow you to type a device name or scroll through the list of available devices. If a device is not in the "select a device" field, the device may not currently be available with an exposed-pad package, or the device has not yet been included in the calculator. To find out about future additions to the device list, post a question in the forum at www.ti.com/e2e-pcbthermalcalc.

Theta-JC (bottom)

This field displays the theta-JC, Bottom value for the selected device; the value is used in the calculation.

Device Power Dissipation

This field allows you to enter the power dissipation of the device. The Power Dissipation value can be calculated based on the usage condition in the real application. This value can be estimated from the device data sheet and/or from an appropriate application note.

NOTE: Dissipated power is NOT equal to the input or output power; dissipated power is the power that is lost or dissipated in the device.

System Reference Temperature

The PCB Thermal Calculator estimates the temperature rise above the System Reference Temperature. This field allows you to select the reference temperature to use in the calculator. Ambient temperature is the more common parameter and gives output curves based on the PCB copper area. Board temperature is a more accurate parameter and gives a single output value of the junction temperature.

Calculate Button

This is enabled when the input fields are completed.

Print Button

Enables you to print the calculated results.

Back to Top

Results Section

When a reference ambient temperature is provided, the results section shows a curve. The key features of the curve are:

  • The x-axis is the total area in mm2 of the copper spreading plane for the device.
  • The y-axis is the temperature in degrees C of the various curves.
  • Four result curves are given. One way to view the results is to slide the mouse over the curves to see the copper area and temperature values.
    • The junction temperature on a minimum thermal PCB
    • The junction temperature on an enhanced thermal PCB.
    • The board temperature on a minimum thermal PCB.
    • The board temperature on an enhanced thermal PCB.
  • The junction temperature refers to the maximum temperature on the device, which should not exceed the operating conditions specified in the datasheet.
  • The board temperature refers to the temperature of the board next to the device. The board temperature can be used as a reference point to validate the PCB temperature rise, as well as to indicate a valid measurement point.
  • The "Minimum Thermal PCB" data refers to a PCB with a single copper spreading plane that is constructed of 1/2 Oz foil.
  • The "Enhanced Thermal PCB" data refers to a PCB with a single copper spreading plane that is constructed of 1 Oz foil. PCBs with thicker foil or additional spreading planes are somewhat better than this data point, but often not dramatically better.
  • The data listed below the graph includes the copper spreading area in mm2 and in2. The four data points are based on the location of the slider, which can be moved to your desired copper spreading area by clicking on it and sliding it to the left or right.

When a reference board temperature is provided, the results section shows two results (no curves). These results are the estimated junction temperature on a minimum thermal PCB and an enhanced thermal PCB.