|Diodes> Thermal Analysis - Appendix B|
On A PC Board - Thermal issues for a diode mounted on a PC board.
In Oil - Thermal issues for a diode operating in oil.
Encapsulated Diodes - Thermal issues for an encapsulated diode.Surface Mount - Thermal issues for a surface mount diode.
Thermal Analysis Table - Of materials commonly used in potted rectifier assemblies.
Typical Diode Configurations - Axial Lead Diode, PCB Mounted
Typical Diode Configurations - Axial Lead Diode, with Copper Heat Sink
Typical Diode Configurations - Surface Mount
|Diode Thermal Analysis
Controlling junction temperature is key to reliable design in any semiconductor package, but high voltage diodes present unique problems that must be addressed. In high voltage diodes, heat is generated by:
Each of these factors change differently with temperature and must be considered carefully over the intended operating temperature range. The following examples depict the relative change in heat sources in a typical example:
Diode Losses vs. Temperature
Diode = 1N6515
PIV = 3000V
VF = 4.0V @ 0.5A
Operating Frequency = 50kHz
Voltage Rise Time = 100ns
Average Rectified Current = 0.5A per diode
Reverse Voltage = 2000V Peak
In high voltage, high frequency diode applications, reverse recovery losses can significantly contribute to the power dissipated in the diodes. Reverse recovery losses occur during the transition from forward current to reverse voltage. When reverse voltage is applied to a diode, it will conduct in the reverse direction for a short time (the reverse recovery time). While the diode is conducting in the reverse direction, the power dissipated is equal to the reverse recovery current multiplied by the reverse voltage.
Theoretically, reverse recovery power losses can be calculated by integrating reverse recovery current times reverse voltage over the time region in which reverse recovery time is a factor and then multiplying the result by the operating frequency. It is not practical to integrate the waveforms, though. An estimate of reverse recovery losses can be found by multiplying reverse recovery time by reverse voltage, multiplying that result by the measured reverse recovery time, and then multiplying by operating frequency. For Figure 2:
PTrr = 0.5 x 0.6A x 250V x 200ns x 40kHz = 0.6 watts
The factor of 0.5 was used because the reverse recovery current waveform is triangular. A peak recovery current of 0.6A was used, along with an average reverse voltage during recovery of 250 V. A recovery time of 200ns was used in the calculation.
The reverse recovery time of a diode is dependent on its junction temperature. The reverse recovery time of a 70ns diode will increase by approximately two and a half times from 25°C to 100°C, so that its reverse recovery losses will also increase by at least two and a half times at 100°C. It is important, when evaluating reverse recovery losses, to take measurements at the maximum operating temperature of the circuit. If reverse recovery losses are too high, the diodes can go into a thermal runaway condition and can fail catastrophically.
| | | | | | |
|Last revised: 03 Aug 2012|