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 vf 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) (IRP ) (t rr).