Switching Characteristics of Power Diodes

Voltage and current waveforms for a power diode driven by currents with a specified diF/dt and a specified diR/dt.
  1. Finite times are required for fully turn-on (t1+t2) and turn-off (t3+t4+t5).
  2. This is due to the excess carriers, space charges, and the bulk semiconductor material.
    Two Types of Reverse Recovery Characteristics
    trr = reverse recovery time, measured as the time between the initial zero crossing of the diode current to the time when this current reaches 25% of the peak reverse current. IRR = maximum reverse current ta = time between zero crossing and the maximum reverse current and it is due to the charge stored in the depletion region of the junction tb = time between maximum reverse current IRR and 25% of the of the maximum reverse current IRR and is due to charge stored in the bulk semiconductor material Voltage and current waveforms for a power diode driven by currents with a specified diF/dt and a specified diR/dt. Two Types of Reverse Recovery Characteristics The reverse recovery time is measured from the initial zero crossing from forward conduction to reverse blocking condition of the diode current to 25% of the maximum reverse current IRR. Its magnitude depends on:Reverse Recovery Characteristics- Abrupt recovery
    1. junction temperature
    2. rate of fall of forward current
    3. forward current prior to commutation
    From the graph
    \begin{array}{l}
{t_{rr}} = {t_a} + {t_b}\\
{I_{RR}} = {t_a}\frac{{di}}{{dt}}\\
\frac{{{t_b}}}{{{t_a}}} = {\rm{softness factor (SF)}}
\end{array} 
    Reverse Recovery Charge
    This is the amount of charge carriers that flow across the diode in the reverse direction due to changeover from forward conduction to reverse blocking condition. Its value is determined from the area enclosed by the path of the reverse recovery current (Recall Δ Q = (Δ I)(Δ t)). That is
    {Q_{RR}} = \frac{1}{2}{I_{RR}}{t_a} + \frac{1}{2}{I_{RR}}{t_b} = \frac{1}{2}{I_{RR}}{t_{rr}}
    or {I_{RR}} = \frac{{2{Q_{RR}}}}{{{t_{rr}}}}  or {I_{RR}} = \frac{{2{Q_{RR}}}}{{{t_{rr}}}} = {t_a}\frac{{di}}{{dt}}
    From above equations we get
    {t_a}{t_{rr}} = \frac{{2{Q_{RR}}}}{{di/dt}}
    {\rm{so }}{t_{rr}} \approx \sqrt {\frac{{2{Q_{RR}}}}{{di/dt}}}
    {\rm{Since }}{t_{rr}} \approx {t_a}
    {\rm{so }}{I_{RR}} = \frac{{2{Q_{RR}}}}{{{t_{rr}}}} \approx \sqrt {2{Q_{RR}}\frac{{di}}{{dt}}}
    Ideally diodes should not have a reverse recovery time, and it is possible to construct such a diode. However, the manufacturing cost of such a diode would be quite high for such a feature which in most cases has minor consequences
    previous POWER SEMICONDUCTOR DIODES
    next Types of Power Diodes

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