At 11:14 AM 2000-02-03 -0500, Phil T. (VA3UX) wrote:
>
>At 07:50 AM 2/3/2000 -0700, you wrote:
>>>
>>>That's the half cycle (8.3 ms), non-repetitive rating Peter. Max
>>>continuous rating is 3 amps at some max operating temp.
>>
>>As I understand it, a rating of say 8.3mS indicates the period of the
>>allowable repetitive peak.
>>
>
>No. 8.3ms is the time of one-half cycle of a complete cycle @ 60Hz. One
>full cycle is 16.67ms. The surge rating of 200 amps (in this case) is only
>valid for this half cycle. In other words, the diode is only "guaranteed"
>to take it once.
The nonrepetitive peak surge current rating, I(FSM), is defined in the
Motorola "Silicon Rectifier Manual" as:
"A final temperature limit is the peak surge junction temperature, T(JSM).
If the surge is only allowed a few times (few being defined by JEDEC as
less than 100) during a rectifier's life, then no harm results providing
the surge is limited. The surge temperature is the basis for the I(FSM)
rating."
>The diode is designed (supposedly) to take the 3 amp rated current
>REPetitively, for succeeding cycles at infinitum. Hence the "Irep" rating
>of 3 amps.
Motorola does not use an "Irep" specification, nor can I find any such
mention in the ITT Reference Data for Radio Engineers. In the Motorola
"Silicon Rectifier Manual, the average forward current rating, Io, which is
3.0 amperes for the 1N5408, is defined as:
"It is customary, as well as a JEDEC requirement, to rate rectifiers in
terms of average current delivered to a resistive load at a specified case
temperature in a 60 Hz half-wave circuit. The rated current is defined as Io."
Due to Motorola's obvious concern with specifying rectifiers according to
JEDEC requirements, I would think that were there such a specification as
Irep, they would have one. There is no mention of Irep. There IS a mention
of rms current per rectifier in the ITT handbook, which is defined as 0.707
times the dc average output current for inductive loads or 0.785 for
resistive loads. The rms current for a capacitor-input filter is the same
as for an inductor-input filter.
I'm rapidly acquiring the impression that some of us might benefit from a
reading of the design explanations and procedures for rectifier-filter
circuits in the various data and reference books; very little of what has
been said today has been correct. Power supply design, in comparison to
high-power linear amplifier design, may not be "sexy" or pretty; but as my
Elmer stated long ago: "The most important aspect of any electronic
equipment is the power supply design.". Without a well-designed and
reliable power supply, your amplifier might as well be behind glass on a
shelf.
73, Steve K0XP
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