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Thermal Considerations

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:

  • Forward Voltage
  • Reverse Leakage Current
  • Reverse Recovery Losses

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
TRR= 70ns
PIV = 3000V
VF = 4.0V @ 0.5A

Circuit Conditions
Operating Frequency = 50kHz
Voltage Rise Time = 100ns
Average Rectified Current = 0.5A per diode
Reverse Voltage = 2000V Peak

Example


In the previous example, the junction temperature would exceed +150°C if the package thermal impedance exceeds 6.25 °C/watt. TOTAL heat source consideration must be made. The deceptive difference in recovery losses in high voltage applications is due primarily to the high voltage bias applied while the diode is recovering from forward bias to a blocking mode. The problem presented can be solved by:

a) decreasing forward voltage losses
b) decreasing the reverse recovery losses
c) improving the thermal impedance
d) operating over a reduced temperature range

Both forward voltage and reverse recovery losses are dependent on the diode used in the circuit, as well as the circuit characteristics. In many cases, there are trade-offs to any change made in diode characteristics. For instance, decreasing the reverse recovery time in a diode will generally cause its forward voltage to increase. However, reducing a diodes reverse blocking voltage in order to facilitate a reduction in its forward voltage may increase the risk of exceeding the voltage rating on the part.

Once a diode has been selected for an application, it is necessary to optimize the thermal impedance of the diode package. Rectifier thermal impedance is the resistance against heat energy movement, from the rectifier junction to a heat sink or heat dissipation reservoir. The thermal path for the rectifier will vary depending on the parts packaging configuration. The remainder of Appendix B will list some typical rectifier packaging schemes to address these issues.



Reverse Recovery Power Loss Measurement

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.

Unfortunately, it is not possible to determine the reverse recovery losses for a diode in a circuit without actually testing the circuit. While the reverse recovery time rating of a diode gives a relative indication of its speed, the rating is based on controlled laboratory conditions. In an actual circuit, the conditions which affect reverse recovery time, such as forward operating current, dv/dt of the voltage waveform, reverse voltage, and temperature, can vary considerably.

The best way to evaluate reverse recovery losses is to monitor diode current and reverse voltage waveforms while the diode is operating in the circuit. Figure 1 shows waveforms for a simulated circuit. The voltage waveform shows a peak reverse voltage of 2400V and a nominal reverse voltage of 2000V. The current waveform shows a peak forward current of 600mA and a peak reverse recovery current of 600mA. The operating frequency is 40kHz.

FIGURE 1 - Simulated Current Waveforms

Voltage Waveform

1000 V/div

Current Waveform

500mA/div

5 us / div


FIGURE 2 - Are of waveform circled in Figure 1 - expanded in time to show reverse recovery current in detail

Voltage Waveform

1000 V/div

Current Waveform

500mA/div

400ns / div

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.

Factors that influence reverse recovery losses include the diode recovery time, operating frequency, dv/dt of the voltage waveform, and operating temperature. The faster the recovery time of the diode, the lower the reverse recovery losses will be. Higher operating frequencies and a faster dv/dt will cause higher reverse recovery losses.

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.



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Last revised: 03 Aug 2012