[AMPS] Bridge vs. Doubler

[Phil T. (VE3OZZ)]

At 06:53 AM 5/7/99 +0100, you wrote:

>One thought, largely in ignorance: is the 'less iron' bit really true?
>If the peak currents in a doubler system are higher, don't you need
>higher magnetic flux to support that? If that is the case, then should
>you have more iron to handle the flux?
>

For a wishy washy answer : yes and no.  Yes, less iron, because the the
smaller winding size means a smaller "window" area is required in the core
which means a smaller core (less iron).  This is usually what is done.
However, the cross sectional area of the core should still be the same as a
FWB transformer for equal VA ratings.  VA capacity is directly related to
core area, which is a function of the selected flux density.  On this point
alone, you could easily come to the conclusion that no reduction in iron is
possible because the VA ratings have to be the same (FWB vs FWD).  However,
the smaller window area is what really allows the size reduction.  Further,
nobody sizes the core or windings to fully accomodate the peak currents
(such as the ones you've noted).  To do so would result in transformer with
a VA rating several times that of the load VA, and thus several times
larger and heavier than a FWB transformer.  Fortunately, because
transformer impedance limits the peak currents to some extent, and because
the conduction angle of the current pulses is small,  and because typical
amateur duty cycle is only a fraction of 100%,  it is quite practical to
use a smaller transformer.  If the FWD supply was actually for some sort of
continuous use, 100% duty cycle application, your comments above would be
absolutely correct, and you would end up with a very large transformer -
even larger than a FWB transformer for the same application.  So your
thinking is correct.  Don't forget though that physical size, weight and
profit margin are also transformer design criteria that don't show-up in
SPICE, or any of the technical design formulas for transformers.  These 3
criteria are often set first, the technical calculations proceed, then they
look to see if what falls out of the bottom will get the job done.  Its
totally backwards to the approach you and I take. (On an individual
hobbyist basis,  we design first, then look to see how big the thing will
be and how much it will cost).

Hope this helps

Phil


>Steve
>
>


--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm