Switching characteristics of power diodes

Switching characteristics of power diodes

In the previous article, the characteristics of voltage and current changing with time are used to describe the transition process of power diode between zero bias (zero applied voltage), forward bias and reverse bias. It is called the dynamic characteristics of power diode, which is called switching characteristics for short. This article will introduce the switching characteristics of power diodes in detail.

(1) Turn-on characteristics

Figure 1a shows the time-varying waveforms of the current and voltage waveforms during the turn-on process when the power diode is switched from zero bias to forward bias. It can be seen that in this dynamic process, the forward voltage drop of the power diode will also have an overshoot UFP, and it will take a period of time to approach a certain value of the steady-state voltage drop (2V in Figure 1). This dynamic process time is called the forward recovery time tfr. The phenomenon that the current lags behind the voltage during the turn-on process of the power diode is also called the inductance phenomenon. In addition to internal structural reasons, the appearance of this phenomenon is also related to factors such as lead length and the use of magnetic materials for equipment packaging. Therefore, the greater the power diode current rise rate during turn-on, the higher the peak voltage UFP and the longer the forward recovery time tfr. In addition, when the junction temperature rises, the UFP and tfr values will also increase.

When the power diode is converted from reverse bias to forward bias, in addition to the above time, the adjustment of the barrier capacitance charge also requires more time to complete.

In power electronic circuits, when used as a fast switching device, the effect of the forward recovery time of the power diode should be considered.

Switching characteristics of power diodes
Figure 1 – Dynamic characteristics of power diodes

(2) Turn-off characteristics

Figure 1b shows the waveform of the voltage and current changing with time during the off-state process when the power diode is converted from forward bias to reverse bias. It can be seen from Figure 1b that at tF, the applied voltage of the power diode that was originally in the forward conduction state changes from forward to reverse, and the power diode cannot be turned off immediately, and its forward current IF begins to decrease. The rate of decrease is determined by the magnitude of the reverse voltage and the inductance in the circuit. By T0, although the current of the power diode drops to zero, there are still a large number of minority carriers on both sides of the PN junction at this time, and the power diode has not recovered its blocking ability. Until T1, when the minority carrier stored in the PN junction is exhausted, the reverse current reaches the maximum value IRP, the power diode begins to recover its blocking ability, and the reverse recovery current begins to decrease. The inductance in the external circuit induces a higher electromotive force, which causes the device to withstand a very high reverse voltage URP.

td=t1-t0 is called the delay time, tf=t2-t1 is called the current fall time, and trr=td+tf, it is called the reverse recovery time of the power diode. The ratio of the fall time to the delay time tf / td is called the softness of the recovery characteristic, or the recovery coefficient, expressed by Sr. The larger the Sr, the softer the recovery characteristic. In fact, the reverse current fall time is relatively longer, so the reverse voltage overshoot URP caused by the same external circuit condition is smaller.

The junction capacitance affects the operating frequency of the PN junction, especially in the state of high-speed switching, it may make its unidirectional conductivity worse, or even fail to work, and attention should be paid to it during application.