Lesson 6.1 · Module 6: From Schematic to Real PCB Hardware

How to Read a Datasheet: Thermal Resistance Explained

How to read a datasheet to work out whether a part will overheat. Heat flows out of a chip a lot like current flows through resistors.

Think of it like this: In this drawing, “voltage” is temperature and “current” is heat flow. Thermal resistance is how hard it is for heat to escape.

The live, interactive simulation for this lesson opens here when JavaScript is on.

Key ideas

Every datasheet lists maximum ratings; the junction temperature (T_J) inside the chip is the one that kills it.
T_J = room temperature + power × thermal resistance. More copper on the board means lower thermal resistance.
Hot environments leave less headroom: a part fine at 25 °C can fail in a 60 °C box.

Try this in the simulation

Increase the drain current and watch T_J climb on the datasheet overlay.
Add copper area and see the temperature fall.
Raise the ambient temperature to 60 °C and see how much current you can still use.

How do you read a datasheet and work out if a part will overheat?

A datasheet lists the limits of a part, and the junction temperature inside the chip is the one that decides its life. Heat flows out of a chip a lot like current through resistors: the temperature rise equals power times thermal resistance, so T_J = T_ambient + P × R_θJA. A part that is fine at 25 °C can fail inside a hot box, and more copper under the package lowers the thermal resistance.

What you’ll learn

Where to find the maximum ratings and thermal resistance in a datasheet.
How to calculate the junction temperature from power and thermal resistance.
How board copper area and ambient temperature change the result.
How to find the largest safe current for a part.

The formula, letter by letter

\begin{aligned}\htmlClass{mv mv-t_j}{\htmlStyle{font-size:0.80em}{\htmlData{sym=T_J}{T_{\mathrm{J}}}}} &= \htmlClass{mv mv-t_a}{\htmlStyle{font-size:1.00em}{\htmlData{sym=T_A}{T_{\mathrm{A}}}}} + \htmlData{sym=P}{P}\cdot \htmlData{sym=R_thetaJA}{R_{\theta JA}} = \htmlData{funit=°C,v=25}{\text{25.0 °C}} + \htmlData{funit=mW,v=0.1550205153064038}{\text{155 mW}}\times 250\,\htmlData{funit=K/W}{\text{K/W}} = \htmlData{funit=°C,v=63.75507627340702}{\text{63.8 °C}}\\ \htmlData{sym=P}{P} &= \htmlData{sym=I_D}{I_{\mathrm{D}}}^2\,\htmlData{sym=R_DS(on)}{R_{\mathrm{DS(on)}}}(\htmlData{sym=T_J}{T_{\mathrm{J}}}) = \htmlData{funit=A,v=2}{\text{2.00 A}}^2 \times \htmlData{funit=mΩ,v=0.03875512882660095}{\text{38.8 mΩ}}\end{aligned}

Shown with this lesson’s starting values. In the simulation the numbers change as you move the sliders.

$T_{\mathrm{J}}$: Junction temperature (°C · degree Celsius) : How hot the silicon inside the package is.
$T_{\mathrm{A}}$: Ambient temperature (°C · degree Celsius) : The temperature of the air around the part.
$P$: Power (W · watt) : How fast energy is used or turned into heat.
$R_{\theta JA}$: Thermal resistance, junction to air (K/W · kelvin per watt) : How many degrees hotter the chip gets for every watt it turns into heat. A rise of 1 K is the same as 1 °C, so US datasheets write the same number in °C/W.
$I_{\mathrm{D}}$: Drain current (A · ampere) : The current through a MOSFET from drain to source.
$R_{\mathrm{DS(on)}}$: On-resistance (Ω · ohm) : A MOSFET’s resistance when fully switched on. Smaller means less heat.

Common questions

What is thermal resistance?

How many degrees the part gets hotter for every watt it wastes, written in K/W (kelvin per watt). A lower number means the heat escapes more easily.

What is junction temperature?

The temperature of the silicon die inside the package. Datasheets give a maximum, often 150 °C, and staying well below it keeps the part reliable.

What does R_θJA mean?

Thermal resistance from the junction to the surrounding air. It depends on the package and on the board, so datasheets quote it for a stated amount of copper.