Power diodes | Power electronics

Power diodes

Introduction of power diode

• Power diode is a two layer, two terminal, p-n semiconductor diode.
• The two terminal of diode are called anode and cathode.

Characteristic of power diodes

Circuit diagram for characteristic curve of power diode

• When diode is positive with respect to cathode, diode is said to be forward biased.
• With increase of V_s from zero value, initially diode current I is zero.
• From V_s = 0 to cut-in voltage (threshold voltage or truth-on voltage), the forward diode current is very small.
• After cut in voltage, the forward diode current increases rapidly and the diode is said to conduct.
• For Si diode, the value of cut-in voltage is 0.7V
• When diode conduct, there is a forward voltage drop of the order of 0.8 to 1V.

Characteristic curve of power diode

• When cathode is positive with respect to anode, the diode is said to be reverse biased.
• In reverse bias condition, a small reverse current, known as leakage current flow in the circuit and this current is of the order of µA or mA.
• The value of leakage current increases slowly with the increase in reverse voltage up to breakdown or avalanche voltage.
• At breakdown voltage, the diode is turned on in the reverse direction.
• The reverse avalanche breakdown of a diode is avoided by operating the diode below specified peak repetitive reverse voltage V_RRM.
• PIV = peak inverse voltage of a diode.
• The PIV of a diode is the maximum reverse voltage to which a diode may be subjected during its working. It is same as V_RRM.

Diode reverse recovery characteristics

• After the forward diode current decays to zero, the diode continuous to conduct in the reverse direction, due to the presence of stored charges in the two layers.
• The reverse current flows for a time called reverse recovery time t_rr.

Reverse recovery characteristic of power diode

Reverse recovery time (t_rr)

• Reverse recovery time is defined as the time between the instant forward diode current becomes zero and the instant reverse recovery current decays to 25% of its reverse peak value I_RM.
• t_rr = t_a + t_b
• t_a : The time between zero crossing of forward current and peak reverse current I_RM. During this time the charge stored in depletion region is removed.
• t_b : It is measured from the instant of I_RM to the instant where 0.25 (or 25%) I_RM is reached. During this time the charge from the semiconductor layers is removed.
• During t_rr, the stored or reverse recovery charge Q_R must be removed.

Softness factor or S factor

• The ratio of  t_b and t_a is known as softness factor or S-factor.
• The S factor is a measure of the voltage transient that occur during the time, diode recovers.
• Usually the value of softness factor is found to be unity, which indicates low oscillatory reverse recovery process.
• If softness factor is small, then diode has large oscillatory over voltages.
• If S-factor is 1, the diode is called soft-recovery diode.
• If S-factor is less than 1, the diode is called or fast recovery diode. or snappy recovery diode.
• If v_f is forward voltage drop across diode, and i_f is forward current
• v_f x i_f is power loss in diode
• Fig (c) shows that major power loss in a diode occurs during t_b
• From fig (a), I_RM = t_a (di/dt)
• Here I_RM = Peak inverse current, and di/dt = Rate of change of reverse current
• If we assume that reverse recovery characteristic of fig. (a) is triangular, then
• Q_R = ½ I_RM x t_rr ⇒ I_RM = 2 (Q_R / t_rr)
• ∵  I_RM = t_a (di/dt)  and  t_a = t_rr 

• Thus t_rr (reverse recovery time) and I_RM (peak inverse current) depends on Q_R (storage charge) and di/dt (rate of change of current)
• Since Q_R depends on i_f (forward diode current)
• So t_rr and I_RM also depends on i_f.
• So a power electronics engineer must know the value of I_RM, Q_R, S-factor and PIV to design the circuit in which power diodes are used.

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