Switching Characteristics of PN Junction Diode

Switching Characteristics of Diode

• When the diode is switched from forward biased to reverse biased or viceversa, it takes finite time to reach the final steady state. The behaviour of diode during this time is called switching characteristics of diode.

• To study the switching characteristics of diode consider simple diode circuit and an input waveform as shown in Fig. 2.15.1.

The following events will take place due to the nature of the applied voltage.

• Event 1: Till time t₁, the forward voltage applied is V_F and diode is forward biased. The value of R is large enough such that drop across forward biased diode is very small compared to drop across R. The forward current is then I_F ≈ V_F/R, neglecting forward resistance of diode.

• Event 2: At time t₁, the applied voltage is suddenly reversed and reverse voltage of ‒V_R is applied to the circuit. Ideally diode also must become OFF from ON state instantly. But this does not happen instantly. The number of minority carriers take time to reduce from P_n ‒P_no to zero at the junction as shown in Fig. 2.15.2 (b). Due to this, at tı current just reverses and remains at that reversed value ‒I_R till the minority carrier concentration reduces to zero. This current is given by ‒I_R = ‒V_R / R. This continues to flow till time t₂.

Key Point: During time t₁ to t₂, the minority charge carriers remain stored and decrease slowly to zero. Hence this time is called storage time denoted as t_s.

• Event 3 : From t₂ onwards, the diode voltage starts to reverse and the diode current starts decreasing as shown in Fig. 2.15.2 (c). At t=t₃; the diode state completely gets reversed and attains steady state in reverse biased condition.

Key Point: The time from t₂ to t₃ i.e. time required by the diode current to reduce to its reverse saturation value is called the transition interval or transition time denoted as t_t.

• The total time required by the diode which is the sum of storage time and transition time, to recover completely from the change of state is called reverse recovery time of the diode and denoted as t_rr. This is an important consideration in high speed switching applications.

Key Point: For quick switching from ON to OFF state, the reverse recovery time should be as small as practicable.

 t_rr = t_s + t_t                     .. (2.15.1)

• The reverse recovery time depends on the RC time constant where C is a transition capacitance of a diode.

Key Point: To have fast switching from ON to OFF of a diode, the transition capacitance should be as small as possible.

• Thus the transition capacitance plays an important role in the switching circuits using diodes.

• Most commercially available switching diodes have the reverse recovery time ranging from a few nanoseconds to one microsecond. However, diodes are specially manufactured having reverse recovery time as small as only a few picoseconds.

• The total switching time t_rr puts the limit on the maximum operating frequency of the diode. Hence t_rr is an important datasheet specification. To minimize the effect of the reverse current, the time period of the operating frequency must be atleast ten times t_rr.

T = 10 t_rr

 f_max =  1 / T = 1 / 10t_rr

• where f_max is the maximum operating frequency.

Review Questions

1. Explain the switching characteristics of diode with the help of simple diode circuit.

2. Write short notes on diode switching characteristics.