In message <19990309.153130.-7749.163.km1h@juno.com>, km1h@juno.com
writes
>
>Maybe I missed something during the recent discussion.
>
>In the common voltage doubler PS as used in many amps, is not each
>capacitor subjected to 50/60 Hz rather than 100/120Hz as in a FWB ?
>If that is true, what effect does that have on all the cap ratings which
>are specified at 120Hz?
>
>Ive tried calling CDE for an answer but the product mgrs must all be out
>playing golf down in SC.
>
>73 Carl KM1H
>I seem to remember Tom Rauch commenting about extra capacitor stress in a
>FW doubler compared to a FWB. Also remember a rather heated discussion
>amongst the engrs at National that resulted in them ordering special HD
>rated caps for the NCL-2000.
>
>Oh well.
>
>73 Carl KM1H
Carl, it appears that you're absolutely correct about this. The caps in
this circuit do charge at 60 Hz, but the overall ripple current is still
120 Hz.
I got curious about the comment on extra cap stress in the FWD circuit so
I E-mailed Tom (W8JI) and got a pretty good explanation (see below).
Further, I spoke to Keith Billings who is author of McGraw-Hill's
Switchmode Power Supply Handbook (Keith lives not far from here and was
employed by Hammond for many years). Keith concurs 100% with Tom's
explanation of capacitor operation in a FWD compared to a FWB.
Keith went on a little further about the benefits to transformer
requirements that arise from using the FWD circuit. He also mentioned that
electrolytics have improved so much during the past 10years that 2 or 3
amps ripple current is "peanuts" for modern capacitors (provided they're
properly spec'd to begin with). AT power-line frequency, the frequency of
the ripple (whether 60 or 120 Hz) is a non-issue.
Thought you'd want to know.
Phil
----------------------------------------------------------------------------
--------------------------------
To: <amps@contesting.com>
Date: Fri, 12 Mar 1999 12:24:55 -0500
From: Tom Rauch <w8ji@contesting.com>
Subject: Re: Caps in full wave doublers
To: "Phil T. (VE3OZZ)" <phil@vaxxine.com>
Reply-to: W8JI@contesting.com
X-Mailer: Pegasus Mail for Win32 (v3.01d)
Priority: normal
Hi Phil,
> Are the caps (series string of e-lytics) in a full wave voltage doubler
> under any more stress than the same string of caps would be if they were
> in a conventional full wave bridge circuit ?
Yes, they are.
A doubler requires twice as much capacitance for the same ripple
percentage into the same load resistance. The cascade voltage
doubler has 60 Hz ripple, the full wave (conventional) doubler 120
Hz ripple.
RMS current per each rectifier element is twice as high. RMS
current in the transformer secondary is twice as high. RMS current
in each capacitor is twice as high.
Ripple factor (rms ripple/ average voltage output) is nearly twice as
high compared to non-doubler in fullwave systems.
A doubler is two half-wave rectifiers in series. In a full wave doubler,
they 1/2 wave rectifiers are fed 180 degree out-of-phase voltages. In
a cascade multiplier, they are fed in-phase. If you treat the analysis
as two (or more) groups of 1/2 wave rectifiers in series you can use
conventional formulas. Just be sure to consider the phase of each
series system.
When you are all done with this, you'll see there is very little
difference in cost for the same performance if everything is scaled
correctly. The transformers are cheaper to wind, that's about all.
73, Tom W8JI
w8ji@contesting.com
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