AN-1011
Defining Maximum Operating Junction Temperature
Application Note
AN-1011: Defining Maximum Operating Junction Temperature (TJ)
Applications Department, APD Semiconductor, San Jose CA
Introduction
There are three main factors that determine the maximum operating junction temperature (TJ) of a
diode. They are the following:
(1) Melting Temperature of the Device Materials
(2) Maximum Thermal Runaway Temperature
(3) Reliability Considerations, including High Temperature Operating Life (HTOL) and
High Temperature Reverse Bias (HTRB)
1) Melting Temperature of Device Materials
The melting point of both the material used for the molding compound and the material used for
the solder during the die-attached must be higher than the maximum operating junction
temperature (TJ). The lower of the two melting points will be the upper limit to the maximum
operating junction temperature (TJ).
2) Maximum Thermal Runaway Temperature
Thermal runaway occurs when the reverse power exceeds the diode’s capability to dissipate heat at
the same rate it is generated. The failure mode is defined at the point where the heat generated is
greater than the heat dissipated, thus eventually destroying the barrier of the diode. This point is
defined as the maximum thermal runaway temperature.
The thermal runaway test is performed by applying a reverse voltage equivalent to the maximum
rated blocking voltage, VR, to the diode. For a device in a TO-220 package, a standard vaned
heatsink1 is mounted on the device. The device under test (DUT) is then placed in a temperature
oven as the temperature is increased incrementally. The device will have to withstand the reverse
power at the ambient temperature for a period of 20 minutes and show a stable leakage before the
device can qualify for that junction temperature. This test is continued until the device reaches an
ambient temperature where thermal runaway occurs in the device.
1
Standard heatsink size of 2cm x 3cm x 0.2cm
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AN-1011
Defining Maximum Operating Junction Temperature
Application Note
3) Reliability Considerations
(A) High Temperature Operating Life (HTOL)
HTOL test is used in determining if the device can reliably withstand the forward and reverse
power generated at the rated maximum operating junction temperature for a time period of 1000
hours. A 50% duty cycle pulse is used at the rated output current and at the rated blocking
voltage. A sample waveform of the HTOL test is illustrated in Figure 1. The final forward
voltage (VF) and the final reverse leakage (IR) of the DUT are measured at 25C to determine if it is
still within the maximum specifications.
Figure 1. Diode Operating Life Waveform
(B) High Temperature Reverse Bias (HTRB)
HTRB test consists of applying a DC voltage that is 80% of the maximum rated blocking voltage
at the maximum operating junction temperature for a time period of 1000 hours. The final
forward voltage (VF) and the final reverse leakage (IR) of the DUT are measured at 25C to
determine if it is still within the maximum specifications.
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November 2005, Rev 1.0
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