Power Diode under Reverse Bias conditions
Reverse Bias condition:
- The applied reverse voltage is supported by the depletion layer formed at the p n- metallurgical junction. Overall neutrality of the space change region dictates that the number of ionized atoms in the p region should be same as that in the n- region.
- However, since NdD << NaA, the space charge region almost exclusively extends into the n- drift region. Now the physical width of the drift region (WD) can be either larger or smaller than the depletion layer width at the break down voltage.
- Consequently two types of diodes exist, (i) non punch through type, (ii) punch through type.
- In “non-punch through” diodes the depletion layer boundary doesn’t reach the end of the drift layer. On the other hand in “punch through” diodes the depletion layer spans the entire drift region and is in contact with the n cathode.
- In non-punch through type diodes the electric field strength is maximum at the p n- junction and decrease to zero at the end of the depletion region. Whereas, in the punch through construction the field strength is more uniform.
Specifications of a power Diode:
Fig: (a) Reverse bias i-v characteristics of a power Diode; Reverse Voltage ratings of a power diode; (b) Supply voltage wave form; (c) Reverse i-v characteristics
DC Blocking Voltage (VRDC):
Maximum direct voltage that can be applied in the reverse direction (i.e cathode positive with respect to anode) across the device for indefinite period of time. It is useful for selecting free-wheeling diodes in DC-DC Choppers and DC-AC voltage source inverter circuits.
RMS Reverse Voltage (VRMS):
It is the RMS value of the power frequency (50/60 HZ) since wave voltage that can be directly applied across the device. Useful for selecting diodes for controlled / uncontrolled power frequency line commutated AC to DC rectifiers. It is given by the manufacturer under the assumption that the supply voltage may rise by 10% at the most. This rating is different for resistive and capacitive loads.
Peak Repetitive Reverse Voltage (VRRM):
This is the maximum permissible value of the instantiations reverse voltage appearing periodically across the device. The time period between two consecutive appearances is assumed to be equal to half the power cycle (i.e. 10ms for 50 HZ supply). This type of period reverse voltage may appear due to “commutation” in a converter.
Peak Non-Repetitive Reverse Voltage (VRSM):
It is the maximum allowable value of the instantaneous reverse voltage across the device that must not recur. Such transient reverse voltage can be generated by power line switching (i.e. circuit Breaker opening / closing) or lightning surges.