Mumbai University > EXTC > Sem 3 > Analog Electronics 1

Marks: 5 M

Year: Nov 2013

1. Switching characteristics of diode

Fig1 Switching characteristics of diode

Where

$t_F$ = forward bias time

$t_S$ = Storage time

$t_T$ = transition interval

$t_{RR}$ = reverse recovery time

2. Reverse recovery time:

• From the Fig1, reverse recovery time is addition of storage time and transition interval.

• When the diode is in forward bias and immediately switched to reverse condition, the diode will still conduct current for certain amount of time. The time period for which the diode conduct electricity after switching the voltage is called “reverse recovery time”

• The reason for reverse recovery time is:

  1. In conduction state, we have electrons in p-type material and holes in n-type material as minority carrier.
  2. When we reverse the voltage we expect diode to switch from conducting to nonconducting state immediately but due to minority carrier the reverse current flows through the diode and stays at measureable level for storage time $(t_S)$ required for minority carrier to return to their majority carrier state in the opposite material (n-type material for electron and p-type material for holes).
  3. After this time period transition interval $(t_T)$ required for current to get back to level associated with nonconduction state.

• Reverse recovery time depends on junction temperature, rate of fall of forward current.

• Reverse recovery time should be small for high speed switching application and can be reduced by shortening the length of P region in PN junction diode or by introducing impurities for example Gold.