[AMPS] Bridge vs. Doubler

[Phil T. (VE3OZZ)]

At 01:29 PM 5/4/99 -0400, you wrote:

>First, I think the G3RZP comment about higher peak currents refutes AG6K's
>comment about wire size. If the currents were equal, Rich would be correct
>- however the transformer currents are not equal for a given output load
>current.  Conservation of energy applies, so if the rectifier circuitry
>raises the output voltage, the input current must increase.  The analogy is
>somewhat equivalent to an impedance matching transformer.
>

If I may, I think Rich is assuming (but not stating) that the transformer
in a FWD would be DESIGNED for such service, and as such, would have
windings with wire having at least 2 ( and maybe up to 4) times the
circular mil area compared to an equivalently rated transformer (KVA and
current rating that is)  for FWB service.  Consequently, the I^2R losses
would be no different, so there's no disadvantage.  Second, the series
resistance of such a transformer is considerably lower (if designed
correctly) than a FWB transformer of the same KVA rating - which is an
advantage toward improved regulation.

>Second, the voltage doubler circuits typical to ham amplifiers are in
>reality two half-wave circuits with the outputs stacked.  The percent
>voltage regulation for a full wave bridge is significantly superior to a
>half-wave circuit.  Two half-wave rectifiers are not going to be superior
>to a full wave bridge if regulation is the criteria.
>

These are full wave rectifiers.  The regulation is determined largely by
the ratio of the series resistance ( ie, total transformer winding
resistance all referred to the secondary winding,  plus diode resistance)
to the load resistance.  The value of C in the filter is the next major
'influencer' of regulation. For equal load resistance, the total amount of
C in a FWD should be twice the amount of C in a FWB supply (quite often,
this isn't what you find).   Knowing this in advance, you could build a FWD
supply that would perform identically to a FWB supply.

Your comments are well founded in a sense because sub-par performance is
probably what you've seen out of most amateur doubler supplies.  In my
opinion, this is due to manufacturers installing a sub-par transformer -
prompted by their overwhelming need to make some form of profit, and to
make everything fit into a desktop box that the market prefers.

>IMHO, either circuit will work fine with the appropriate transformer. 

Yep,  there's the key.

>There may be other factors too - for example, if your filter cap is a
>single oil-filled unit, I don't see how you can use a doubler without
>buying another, identical cap.  On the other hand, the rectifiers in a
>doubler only carry half the current so this might allow some flexibility in
>component sizing.
>

The rms current in each doubler diode is twice the DC load current, and the
peak secondary winding current is at least 2.8 times the DC load current.
This is why a transformer supplying a doubler circuit should be designed
for it.  It isn't enough to just look at the DC load current or the KVA
rating of the transformer when selecting one.  Knowing the wire size of
both windings is also a key piece of information.

Sorry for the diatribe, but there's my 2 cents worth.  Transformers are
more interesting than they look.

Phil T




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