Specifications related to forward bias operation of power diodes
Maximum RMS Forward current (IFRMS):
Due to predominantly resistive nature of the forward voltage drop across a forward biased power diode, RMS value of the forward current determines the conduction power loss. The specification gives the maximum allowable RMS value of the forward current of a given wave shape (usually a half cycle sine wave of power frequency) and at a specified case temperature. However, this specification can be used as a guideline for almost all wave shapes of the forward current.
Maximum Average Forward Current (IFAVM):
This specification gives the maximum average value of power frequency half cycle sine wave current allowed to flow through the diode in the forward direction. Average current rating of a diode decreases with reduction in conduction angle due to increase in current “form factor”. Both IFRMS and IFAVM ratings are given at a specified case temperature. If the case temperature increases beyond this limit these ratings has to be reduced correspondingly. “Derating curves” provide by the manufacturers give the relationship between IFAVM (IFRMS) with allowable case temperature.
Fig: Derating curves for the forward current, Average forward power loss vs. average forward current, Peak Repetitive surge current VS time curve, Non-repetitive surge current and surge current integral vs. current pulse width characteristics of a power
Average Forward Power loss (PAVF):
Almost all power loss in a diode occurs during forward conduction state. Average forward power loss over a full cycle is specified by the manufacturers as a function of the average forward current (IAVF) for different conduction angles.
Surge and Fault Current:
In some rectifier applications a diode may be required to conduct forward currents far in excess of its RMS or average forward current rating for some duration (several cycles of the power frequency). This is called the repetitive surge forward current of a diode. A diode is expected to operate normally after the surge duration is over.
On the other hand, fault current arising due to some abnormality in the power circuit may have a higher peak valve but exists for shorter duration (usually less than an half cycle of the power frequency). A diode circuit is expected to be disconnected from the power line following a fault. Therefore, a fault current is a non-repetitive surge current. Power diodes are capable of withstanding both types of surge currents and this capability is expressed in terms of two surge current ratings.
Peak Repetitive surge current rating (IFRM):
This is the peak valve of the repetitive surge current that can be allowed to flow through the diode for a specific duration and for specified conditions before and after the surge. The surge current waveform is assumed to be half sinusoidal of power frequency with current pulses separated by “OFF” periods of equal duration. The diode should be capable of withstanding maximum repetitive peak reverse voltage (VRRM) and Maximum allowable average forward current (IFAVM) following the surge.
Peak Non-Repetitive surge current (IFRM):
This specification is similar to the previous one except that the current pulse duration is assumed to be within one half cycle of the power frequency. This specification is given as a function of the current pulse duration.
Maximum surge current Integral (∫i2dt):
This is a surge current related specification and gives a measure of the heat energy generated inside the device during a non-repetitive surge. It is useful for selecting the protective fuse to be connected in series with the diode. This specification is also given as a function of the current pulse duration.