Parameters defining the turn off characteristics of a Power diode
Diode overvoltage protection circuit:
Fig: Diode overvoltage protection circuit; (a) “Snappy recovery characteristics; (b) Capacitive snubber circuit; (c) snubber characteristics.
- The snappiness factor S depends mainly on the construction of the diode (e.g. drift region width, doping lever, carrier life time etc.). Other parameters are interrelated and also depend on S. Manufacturers usually specify these parameters as functions of diF/dt for different values of IF. Both Irr and Qrr increases with IF and diF/dt while trr increases with IF and decreases with diF/dt.
- Diode reverse recovery time can be reduce by increasing the rate of decrease of the forward current (i.e, by reducing stray circuit inductance) and by using “snappy” recovery (S<<1) diode.
The problems with this approach are:
- Increase of diF/dt also increases the magnitude of Irr
- Large recovery current coupled with ”snappy” recovery may give rise to current and voltage oscillation in the diode due to the resonant circuit formed by the stray circuit inductance and the diode depletion layer capacitance.
- Large reverse recovery current may lead to reverse voltage peak (Vrr) in excess of VRSM and destroy the device. Snubber circuits increase the overall power loss in the circuit since the energy stored in the snubber capacitor is dissipated in the snubber resistance during turning ON of the diode.
- In high frequency circuits other types of fast recovery diodes (Inverter grade) are preferred. Fast recovery diodes offer significant reduction in both Irr and trr (10% - 20% of a rectifier grade diode of comparable rating). This improvement in turn OFF performance, however, comes at the expense of the steady state performance.
- The forward voltage drop in a diode is directly proportion to the width of the drift region and inversely proportional to the carrier life time in the drift region. both Irr and trr increases with increase in carrier life time and drift region width. Therefore if Irr and trr are reduced by reducing the carrier life time, forward voltage drop increases.
- If the drift region width is reduced the reverse break down voltage of the diode reduces. The performance of a fast recovery diode is therefore, a compromise between the steady state performance and the switching performance. In high voltage high frequency circuits switching loss is the dominant component of the overall power loss.
- Some increase in the forward voltage drop in the diode (and hence conduction power lass) can be tolerated since the Turn OFF loss associated with reverse recovery is greatly reduced.