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Junction Temperature by Sense Diode

Thermal

Junction temperature from a diode VF reading, and the measured RθJA of your assembly.

Inputs

Typically 600–750 mV at 1 mA
V
Usually −1.6 to −2.0 mV/°C. Measure it rather than assume it.
mV/°C
V
A
1 mA
W
°C

Results

1 passed · 1 to check
Junction temperature TJ
97.2°C
Pass

Checked: TJ ≤ 125 °C; 150 °C is the usual absolute limit for silicon
Measured RθJA

(TJ − TA)/PD at 1 W
72.2°C/W
Resolution
0.56°C/mV
All results (3)
Rise above ambient
+ 72.2°C
Calibration slope
-1.800mV/°C
VF at 0 °C (intercept)
695mV
Your assembly measures 72.2 °C/W at 1 W. Compare it with the datasheet RθJA.
Cut the heating power to measure, and read VF within a few hundred microseconds, before the die cools.

Calibration line: VF = 0.6950 V − 1.800 mV/°C × T. A 1 mV reading error is 0.56 °C.

The sense current dissipates 520 µW in the junction, about 0.04 °C of self-heating at 72.2 °C/W. Keep Imeas at 1 mA or less, and use the same current for calibration and measurement.

slope -1.80 mV/°C

Parameter sweep

Vary one input over a range and see the answer and verdict at each step, as a table and a curve.Pro

Worst-case corners

Put a tolerance on each input and get the worst-case band around the answer.Pro

Circuit

The principle

Any silicon junction is a thermometer. At a fixed small current its forward voltage falls with temperature, roughly linearly, by about 2 mV per degree. Calibrate that line, read VF under load, and you have the junction temperature and the real Rθ of your assembly.

Diode thermometer
VF(T)=VF0+m (T−T0),m≈−1.6  to  −2.0  mV/°CV_F(T) = V_{F0} + m\,(T - T_{0}), \qquad m \approx -1.6 \;\t{to}\; -2.0\;\t{mV/°C}
Typically 600–750 mV at 25 °C and about 1 mA.
Junction temperature from a reading
TJ=T0+VF−VF0mT_J = T_0 + \frac{V_F - V_{F0}}{m}
Two-point calibration
m=VF2−VF1T2−T1VF0=VF1−m T1m = \frac{V_{F2}-V_{F1}}{T_2-T_1} \qquad V_{F0} = V_{F1} - m\,T_1
Extracting thermal resistance
RθJA=TJ−TAPDR_{θJA} = \frac{T_J - T_A}{P_D}
Apply a known power, let it settle, then read VF. This is the Rθ of your assembly, not of a JEDEC test board.
Why the slope exists
VF=nkTqln⁡ ⁣(IFIS(T))V_F = \frac{nkT}{q}\ln\!\left(\frac{I_F}{I_S(T)}\right)
IS rises so fast with temperature that it outweighs the kT/q term, so the slope is negative.
  • VFV_Fforward voltage at the fixed sense current
  • mmtemperature coefficient, negative, in V/°C
  • ImeasI_{meas}sense current, about 1 mA, small enough not to self-heat
  • PDP_Dheating power applied to produce the temperature rise
More detail

Which diode

Any junction you can reach: a substrate diode, an input protection diode, a base-emitter junction or an output clamp. The die is close to isothermal, so any of them reads close to the temperature you want.

Calibration

  • Cover at least the junction temperature range you expect in service. Do not extrapolate.
  • Use a small, known current, about 1 mA: enough to measure cleanly, too little to heat the junction noticeably.
  • Step the chamber in 25 °C steps, let each settle, and record VF. Six points give a solid line and expose a bad reading.
  • Heat the die with a different structure, and read the diode. That keeps the thermometer separate from the heater.

Cautions

Give it time. A board takes 10 to 15 minutes to settle. Read early and you under-report the rise.
Avoid IR drop. The heating current shares your ground. Interrupt the heating power to read, and use four-wire (Kelvin) sensing.
Above about 2 W, spread the heating across several devices rather than one saturated output.

Where the datasheet gives ΨJT, a thermocouple on the package top is usually a faster route to TJ.

Method adapted from Allegro MicroSystems AN295014 Rev. 1, on measuring IC temperature with a sense diode. It reports about −1.8 mV/°C from 25 °C to 175 °C. The explanations are our own.

Engine version ⁨1.18.3⁩