Characteristics of power semiconductor devices - rectifiers
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Voltage and Current Rating -- Silicon-rectifier ratings are generally expressed in terms of reverse-voltage ratings and of mean-forward-current ratings in a half-wave circuit operating from a 60-hertz sinusoidal supply and into a ...
Voltage and Current Rating -- Silicon-rectifier ratings are generally expressed in terms of reverse-voltage ratings and of mean-forward-current ratings in a half-wave circuit operating from a 60-hertz sinusoidal supply and into a purely resistive load.
There are three reverse-voltage ratings of importance:
Peak transient reverse voltage VRM
Maximum repetitive reverse voltage Vrm(rep)
working peak reverse voltage Vrm(wkg)
Peak transient reverse voltage Vna is the rated maximum value of any nonrecurrent surge voltage, and this value must not be exceeded under any circumstances, even for a micorsecond. Maximum repetitive reverse voltage (Vrm(rep)) is the maximum value of the reverse voltage that may be applied recurrently, e.g. in every cycle, and will include any circuit oscillatory voltage that may appea on the sinusoidal supply voltage. Working peak rteverse voltage (Vrm(wkg)) is the crest value of the sinusoidal voltage of the supply at its maximum limit.
The manufacturer generally recommends a Vrm(wkg) that has an appreciable safety margin in relation to the Vrm to allow for the commonly experienced transient over-volatges on power mains.
Three forward-current ratings are similarly of importance:
Nonrecurrent surge currewnt Ifm(surge)
Repetitive peak forward current Ifm(rep)
Average forward current If(av)
Silicon diodes have comparatively small thermal mass, and care must be taken to ensure that short-term overload currents are limited. The nonrecurrent surge current is sometimes given as a single value that must not be exceeded at any time, but it is more generally given in the form of a graph of permissible surge current versus time. It is important to observe whethger the surge-current scale is marked in peak, rms, or average value in order that the data be correctly interpreted. The repetitive peak forward current is the peak value of the forward currnt reached in every cycle and excludes random peaks due to transients. Its relation to the average forward current depends on the circuit used and the load that is applied. For example, the repetitive peak is about three times the average for a half-wave or bridge circuit working into a resistive load; it may be many times greater when the same circuitsd works into capacities loads.
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