Switching characteristic is the dynamic characteristic which shows the variation of the diode current or voltage with respect to the time. A power diode takes finite time to change its state from ON (conduction-state) to OFF (blocking-state) and vice-versa. This time taken by the diode is an important parameter of a diode. Switching characteristics gives the following information about the diode:
- shows the time taken by the diode to change its state
- also shows the variation in the magnitude of voltage and current during the switching period.
The switching properties and values of a diode are given on the specification sheet of the diode. Both the switching time of a diode and the waveforms of the voltage and current of the diode are affected by the intrinsic properties of the diode and also by the circuits in which they are used.
NOTE: The switching characteristics of a power diode is different from the signal level diode.
Switching Characteristics is divided into two regions
- Turn-on Transient
- Turn-off Transient (i.e. the Reverse Recovery Characteristics)
Turn On Transient
It is the transient condition obtained when the Diode enters the Forward biased state (i.e. the On-state) from the Reversed Biased state (i.e. the Off-state). Turn-on transient (or turn-on characteristics) of a diode is marked in the above switching characteristics as well as shown separately in the below figure.
- Turn-on transient spans over time periods t1 and t2 and two processes occur during these periods
- During t1: space-charge stored in the depletion region due to the reverse biasing is removed because the diode is now forward biased.
- During t2: the forward biased diode causes the injection of the excess carriers into the drift region actually double injection takes place.
- During the Reversed Biased condition, the stored charge exists in the depletion region hence the region acts like a capacitor. Hence, even if the Diode is abruptly forward biased, the voltage across the diode increases smoothly because of the capacitance of the Space-charge region and current starts to increase see the waveform during t1.
- Current also increases smoothly due to the inductance of the material and similarly the voltage across the depletion region increases smoothly due to the capacitance of the Depletion Region (There is no Conductivity Modulation until the depletion region decreases to its normal open circuit value. Therefore, the resistance of the Drift Region is large till then).
where, v(t) = voltage across the diode,
vΩ = voltage due to the resistance R of the Drift Region,
vL = voltage due to the inductance L of the material
- v(t) becomes maximum when current increases to the maximum value IF at t= t1. After t1, the Conductivity Modulation starts hence the resistance of the drift region decreases so VΩ (= R.iF) also decreases and VL = 0 since iF attains a constant value IF. Therefore, Vdiode decreases from peak value to steady state value Von and at t = t2, Conductivity Modulation is completed.
- Forward turn ON time i.e. ton = t1 + t2.
Turn OFF Transient / Reverse Recovery Characteristics
It is the transient condition obtained when the Diode enters the Reverse biased state (i.e. the Off-state) from the Forward Biased state (i.e. the On-state). It is exactly the reverse process of Turn ON transient. Turn-off characteristic is also known as Reverse Recovery Characteristics.
Here, mainly the excess charges stored due to the Double Injection in the Drift Region are removed first so the depletion region could be formed and the diode can be reversed biased (i.e. Off-state).
This is the important process of the turn-off transient which is known as the reverse recovery (to be discussed in detail in the next topic i.e. ‘Reverse Recovery Characteristics’).