Switching characteristics of Power diodes
Importance of behavior of the diode current and voltage during these switching periods:
- Severe over voltage / over current may be caused by a diode switching at different points in the circuit using the diode.
- Voltage and current exist simultaneously during switching operation of a diode. Therefore, every switching of the diode is associated with some energy loss. At high switching frequency this may contribute significantly to the overall power loss in the diode.
Observed Turn ON behavior of a power Diode & Turn OFF behavior of a Power Diode:
Silent features of Turn-On operation characteristic are:
Fig: Forward current and voltage waveforms of a Power diode during Turn-On operation
- Diodes are often used in circuits with di/dt limiting inductors.
- The rate of rise of the forward current through the diode during Turn ON has significant effect on the forward voltage drop characteristics.
- The forward diode voltage during turn ON may transiently reach a significantly higher value Vfr compared to the steady slate voltage drop at the steady current IF.
- In some power converter circuits (e.g voltage source inverter) where a freewheeling diode is used across an asymmetrical blocking power switch (i.e GTO) this transient over voltage may be high enough to destroy the main power switch. Vfr (called forward recovery voltage) is given as a function of the forward di/dt in the manufacturer’s data sheet.
- Typical values lie within the range of 10-30V. Forward recovery time (tfr) is typically within 10 us.
Salient features of Reverse Recovery characteristic are:
Fig: Reverse Recovery characteristics of a power diode
- The diode current does not stop at zero; instead it grows in the negative direction to Irr called “peak reverse recovery current” which can be comparable to IF. In many power electronic circuits (e.g. choppers, inverters) this reverse current flows through the main power switch in addition to the load current. Therefore, this reverse recovery current has to be accounted for while selecting the main switch.
- Voltage drop across the diode does not change appreciably from its steady state value till the diode current reaches reverse recovery level. In many power electric circuits (choppers, inverters) this may create an effective short circuit across the supply, current being limited only by the stray wiring inductance. Also in high frequency switching circuits (e.g, SMPS) if the time period t4 is comparable to switching cycle qualitative modification to the circuit behavior is possible.
- Towards the end of the reverse recovery period if the reverse current falls too sharply, (low value of S), stray circuit inductance may cause dangerous over voltage (Vrr) across the device. It may be required to protect the diode using an RC snubber.