How do I use the equation and table that are attached to the transient thermal resistance curve in the data sheet?

Using the transient thermal resistance curve, table, and associated equation from a datasheet allows you to estimate the junction temperature rise of a semiconductor device during transient conditions. Here’s how you can interpret and use this information effectively:

  1. Understand the Key Elements
  • Transient Thermal Resistance Curve:
  • Shows how thermal resistance changes over time (i.e., how quickly the device can transfer heat from the junction to the case or ambient).
  • Typically plotted as ZθJC(t) or ZθJA(t) (thermal impedance over time) vs. time.
  • Table:
  • Provides numerical values of thermal impedance for specific time intervals, making it easier to read precise data from the curve.
  • Equation:
  • May represent the curve mathematically for calculation purposes, often using an exponential model to account for thermal capacitance and resistance.
  1. Key Parameters to Identify
  • Power Dissipation (): The amount of power the device is dissipating during operation.
  • Thermal Impedance (): The time-dependent thermal resistance, typically provided for junction-to-case () or junction-to-ambient ().
  • Temperature Rise (): The increase in junction temperature due to power dissipation, calculated as:
  1. How to Use the Transient Thermal Resistance Curve
  2. Determine the Time Duration:
  • Identify the duration of the transient event (e.g., a pulse or switching operation) where the thermal response is critical.
  1. Locate Thermal Impedance:
  • Use the curve or table to find the corresponding value for the specific time duration.
  1. Calculate Temperature Rise:
  • Apply the equation:
  • Add this rise to the ambient or case temperature to estimate the junction temperature:
  1. Practical Steps

Using the Curve:

  • Locate the time duration of interest on the x-axis.
  • Trace upward to intersect the curve.
  • Read the corresponding thermal impedance value on the y-axis.

Using the Table:

  • Find the time duration in the table.
  • Read the corresponding value for that duration.

Using the Equation:

  • Substitute the time duration into the provided mathematical model.
  • Compute to use in the temperature rise formula.
  1. Considerations
  • Steady-State vs. Transient:

For long-duration events, approaches the steady-state thermal resistance ( or ).

  • Pulse Widths and Duty Cycles:

For repetitive pulses, ensure you consider the duty cycle and time-averaged power dissipation.

  • Thermal Interface Material:

Include the thermal resistance of any interface material (e.g., thermal grease or pads) between the junction and ambient in your calculations.

  • Safety Margins:

Ensure the calculated is well below the maximum allowable junction temperature () specified in the datasheet.

Example

  • Given:
  • Power Dissipation () = 50 W
  • Time Duration = 10 ms
  • From Curve/Table:
  • Ambient Temperature () = 25°C
  • Calculate:

This calculation shows the junction temperature under the given transient condition.

By interpreting the transient thermal resistance curve, table, and equation, you can accurately predict how a device handles thermal stresses during dynamic operations and ensure it operates within safe limits.

 

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