Reverse Recovery Characteristics

At the end of forward  conduction in diode  reverse current  flows for  a short  time. The  device  doesn't  attain its full blocking  capability  until  the reverse current  cease. The  reverse current flows  in the interval called  reverse  recovery time. During  this time  charge  carriers stored in the diode at the end of forward  conduction are removed. Actually when a  power diode  has been conduction in the  forward direction sufficiently long to  establish  the steady state there  will be a charge  due to  minority  carriers  present. Before  the device  can  block in the reverse direction  this charge  must be extracted.

This  extraction takes  the from  of a transient reverse current and this  together  with the  reverse bias  voltage results in additional power  dissipation  which reduces the rectification efficiency.  Reverse recovery time is measured  instant the current recovers to 25% of its peak  reverse value. low  reverse state forward  current and low reverse bias  voltage increase recovery time. High  rate of all of anode  current reduces recovery time but  increase stored charge. High  junction temperature is  increase both   recovery time and  stored charge.

There  are two  parts of  reverse recovery  time. One  is the time  between  zero  crossing  of forward current  and peak reverse current. During  this time  period, charges stored  in  depletion  region  is removed.  The other part  of t measured form the instant of peak reverse current to the instant where  25% of peak reverse  current  is reached. During this time period charges from the two  semiconductor layer are removed.

The  shaded  area in figure  represents  the stored  charge  or reverse  recovery charge  which  must be  removed  during  the reverse recovery time. The  ration ½ is known  as softness factor. Voltages  transient occurs during  the time  diode  recovers is measured by the factor.

b.   forward  voltage  drop v_f and forward  current  if gives  the power  loss  in a diode.  The total power  loss in  given by  average value of V _f i _f  during  time t2 major power  loss  occurs  in a diode. As shown in  figure peak reverse current  IFP is given by

 I =  RP = t1 di/ dt

Where Do/ dt  is the  rate of  rise of  reverse  current. If  the reverse  recovery  characteristics  is assumed as a triangle shape  then storage charge  Q can be written as .

Q =  ( ½) (I_RP ) (t _rr).