The temperature is then elevated to a higher value ( 50°C , for example), the DUT is allowed to reach thermal equilibrium, and the current pulse is again delivered. Voltages recorded at this temperature are labeled the VF2 at TJ2 ( 50° C in this example).
These steps can be repeated over a number of values, then plotted as voltage versus junction temperature (Fig. 3). Use at least three temperatures in the analysis to check for any discrepancies in the approximation. You can now calculate the slope (m) of the line as well as the intercept, using Equation 1:
TJ = (m × VF ) + T0
TJ2 - TJ1 = m( VF2 - VF1) (point-slope form of Equation 1)
m = (TJ2 - TJ1)/(VF2 - VF1) (2)
and you can then calculate T0 by extrapolation:
TJ2 - TJ1 = m(VF2 - VF1) (point-slope form of Equation 1)
By setting VF2 to 0, Equation 2 becomes:
TJ2 = TJ1 - m VF1
TJ2 in this case is equal to the intercept, or T0 .
T0 = TJ2 = TJ1 - m VF1.
Real World Example: High-Brightness LED
In this example, a new high-brightness LED die is being developed. The device is designed to carry more current than previous units, and we need to ensure high heat flux to minimize the junction temperature. This will certify adequate device lifetime for some of our more rigorous applications.
A common LED failure occurs when the bond wire connecting the anode or cathode to the LED die is broken. The common cause of breakage stems from temperature cycling of the bond wire, which results from elevated junction temperatures created by inadequate heat removal.
We place the LED die in an oven and follow the above prescribed test plan. We measure the following results:
VF1 at TJ1 (25 °C ) = 1.01 V
VF2 at TJ2 (50 °C ) = 0.78 V
m = (50 - 25 )°C /(0.78 - 1.01)V = - 108.70 °C/V
T0 = TJ1 - mVF1 = 25 °C - (-108.70°C/V) × (1.01V) = 134.79 °C
Therefore, the first-order equation describing the junction temperature versus forward voltage for this device is:
TJ = (-108.70°C/V) × VF ) + 134.79°C
We can now vary other aspects of the evaluation, such as operating current, environmental/case temperature, and packaging, and simply measure the V F to determine the actual junction temperature.
Sources Of Error
The largest source of measurement error comes from the measurement uncertainty of the temperature in the environmental chamber. This measurement is typically made using a thermocouple, which can have errors of ± 2 ° or more. Additional accuracy can be obtained by placing a more accurate thermal-measurement sensor, such as a thermistor or resistor temperature detector (RTD), near the DUT and using a separate digital multimeter to measure the temperature.
Uncertainty in the voltage measurement also adds to the error when calculating the junction temperature. Selecting an instrument with a high degree of accuracy and resolution on voltage measurements is the key to minimizing this error.
Errors in the junction temperature measurement can propagate to other thermal calculations, such as thermal impedance and resistance. Therefore, minimizing these errors is critical to obtaining accurate measurement results.
The data gleaned from this simple technique for measuring a semiconductor junction's temperature can be used to analyze the effects of heat-sinking, environmental, and source conditions on the junction in question.
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