The main parameters of power diodes

The main parameters of power diodes

1. Forward average current IF (AV)

The forward average current IF (AV) of the power diode refers to the average value of the maximum power frequency sine half-wave current that the power diode is allowed to flow under the specified case temperature (referred to as shell temperature, expressed by Tc) and heat dissipation conditions during long-term operation. And this current is called the rated current of the power diode.

From the definition of the forward average current IF (AV) of the power diode, it can be seen that IF (AV) is measured by the tube under the specified brightness temperature limit. That is, it is defined according to the principle of “heating effect of current is within the allowable range”, and the current parameter related to heat is the effective value of the current. Therefore, the heating effect caused by the maximum sinusoidal half-wave current allowed by the power diode should be equal to the heating effect caused by the power diode when it is used. That is, the principle that the effective values of the two waveform currents are equal is used to select the current rating of the power diode, and a certain margin should be left.

The power frequency sine half-wave current waveform is shown in Figure 1. Set the current peak value as Im, and the corresponding sine half-wave average current is

The main parameters of power diodes
(1-1)

The effective value of the sine half-wave current is

The main parameters of power diodes
(1-2)

It can be seen that the relationship between the average value of the sine half-wave waveform and the effective value is 1:1.57, and the maximum current allowed by the power diode is 1.57IF (AV).

The main parameters of power diodes
Figure 1 – Power frequency sine half-wave current waveform

If it is known that the effective value of a certain power diode that needs to flow a certain waveform current in the circuit is ID, and the margin of 1.5 to 2 times is considered, then the rated current of the power diode is selected as

The main parameters of power diodes
(1-3)

It should be noted that when used in high-frequency occasions, the heat of the power diode is not only the on-state loss caused by the forward current, but also its switching consumption can often not be ignored; when a power diode with a large reverse leakage current is used, the heating effect caused by its off-state loss is not small. These should be considered when choosing the forward current rating of the power diode.

2. Forward pressure drop UF

Forward voltage drop UF refers to the corresponding forward voltage drop when a specified steady-state forward current flows through a power diode at a specified temperature. Sometimes, the parameter table also gives the maximum instantaneous forward voltage drop of the power diode when a large transient forward current flows at a specified temperature.

3. Repetitive peak reverse voltage URRM

Repetitive peak reverse voltage refers to the highest peak reverse voltage that can be repeatedly applied to the power diode. It is usually 2/3 of the avalanche breakdown voltage UB. When used, this parameter is often selected according to 2 times the maximum reverse peak voltage that the power diode in the circuit may withstand.

4. Maximum operating junction temperature TJM

The junction temperature refers to the average temperature of the PN junction of the die, which is represented by TJ. The maximum operating junction temperature refers to the highest average temperature that the PN junction can withstand without damaging it, and is expressed by TJM. TJM is usually between 125 and 175°C.

5. Reverse recovery time trr

Reverse recovery time refers to the time from when the forward current of the power diode crosses zero to when the reverse current drops to 10% of its peak value, as shown in Figure 2b.

6. Surge current IFSM

Surge current refers to the maximum continuous overcurrent of one or several power frequency cycles that a power diode can withstand.

Switching characteristics of power diodes
Figure 2 – Dynamic characteristics of power diodes