Virtually all power supplies use silicon rectifiers as the primary ac-to-dc converting device. Rectifier parameters generally are expressed in terms of reverse-voltage ratings and mean-forward-current ratings in a ½-wave rectifier circuit operating from a 60 Hz supply and feeding a purely resistive load.
The three primary reverse-voltage ratings are:
• Peak transient reverse voltage (Vrm)—the maximum value of any nonrecurrent surge voltage. This value must never be exceeded, even for a microsecond.
• Maximum repetitive reverse voltage [Vrm(rep)]—the maximum value of reverse voltage that can be applied recurrently (in every cycle of 60 Hz power). This includes oscillatory voltages that may appear on the sinusoidal supply.
• Working peak reverse voltage [Vrm(wkg)]—the crest value of the sinusoidal voltage of the ac supply at its maximum limit. Rectifier manufacturers generally recommend a value that has a significant safety margin, relative to the peak transient reverse voltage (Vrm), to allow for transient overvoltages on the supply lines.
There are three forward-current ratings of similar importance in the application of silicon rectifiers:
• Nonrecurrent surge current [Ifm(surge)]—the maximum device transient current that must not be exceeded at any time. Ifm(surge) is sometimes given as a single value, but often is presented in the form of a graph of permissible surge-current values vs. time. Because silicon diodes have a relatively small thermal mass, the
potential for short-term current overloads must be given careful consideration.
• Repetitive peak forward current [Ifm(rep)]—the maximum value of forward current reached in each cycle of the 60 Hz waveform. This value does not include random peaks caused by transient disturbances.
• Average forward current [Ifm(av)]—the upper limit for average load current through the device. This limit is always well below the repetitive peak forward current rating to ensure an adequate margin of safety.
Rectifier manufacturers generally supply curves of the instantaneous forward voltage vs. instantaneous forward current at one or more specific operating temperatures. These curves establish the forward-mode upper operating parameters of the device.
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