# Diode Switching Characteristics (Reverse Recovery Characteristics)

#### Diode Switching Characteristics (Reverse Recovery Characteristics):

We have already discussed about the Power diode basics and Power diode Structure in our previous posts. In this post let us see the dynamic characteristics of the power diode....

• Unlike in majority-carrier devices like Schottky diodes, forward current in power diodes, is due to the flow of electrons as well as holes.
• This current results in an accumulation of electrons in the p-region and of holes in the n-region.
• The charges (called “stored charge”) which had been carrying forward current must be “swept out” from the junction region before the reverse non-conducting state can be established.
• When forward voltage is applied to a power diode, a short turn-on time elapses before the charges (electrons & holes) at the PN junction reach equilibrium to carry full forward current.
• This time is in terms of nanoseconds.
• Similarly when reverse voltage is applied to turn off the power diode, a short time elapses before reaching full blocking mode.
• This time is in terms of microseconds.

So in diode, turn-on characteristics does not play important role in device selection process. In this post let us see the switching characteristics (turn-off characteristics) of the power diode in detail.

• When the diode is conducting (in forward conduction mode), to turn it off, reverse voltage is applied.
• As shown in the above figure, the diode current momentarily becomes negative before becoming zero .
Ie, when the reverse voltage is applied, the forward diode current (IF) decreases linearly(due to leakage inductance) to zero, the diode continues to conduct in the reverse direction because of the presence of stored charges in the two layers.
• The reverse current flows for a time called reverse recovery time trr.
• It is usually less than 1 microsecond.
• The trr is defined as the time between the instant forward diode current(IF) becomes zero and the instant reverse recovery current(IRRM) decays to 25% of its peak reverse value IRRM.

Trr consists of two segments t2 & t3
trr = t2+t3
t2 = time between zero crossing of forward current and peak reverse current IRRM.
During this time charged stored in the depletion layer is removed.

t3 = time between the instant IRRM to the instant where 0.25 IRRM is reached.
During this time, charge from the two semiconductor layers is removed.

The reverse recovery charge QRR must be removed during tRR.
As a thumb rule, the lower tRR the faster the diode can be switched.
The ratio of t2/t3 is called as softness factor.

When
S factor >1 then diode has large oscillatory over voltages.

S-factor = 1 then diode is called soft recovery diode.

S-factor <1  then diode is called as fast recovery diode.

• The Reverse Recovery Time (tRR) and Peak Inverse Current (IRRM) depends on forward current(IF).
• High di/dt at turn-off causes a reverse voltage (Vrrm) that is higher than the applied voltage (VR).
• Peak reverse current (IRM), Stored charge (QRR), S-factor, PIV (usually mentioned in datasheets) important diode parameters to select the power diode for the particular application.
• Based on reverse recovery characteristics, the power diodes are classified as
(i) Slow-Recovery Diodes,
(ii) Fast-Recovery Diodes, and
(iii) Schottky Diodes.
• The Slow-recovery diodes are used in 50/60-Hz power rectification applications.
• Fast-recovery types are used in feedback/ freewheeling of converters and snubber circuits.
• Schottky diodes are used in high-frequency converters as they provide very fast switching....