Glenn McNeil wrote:
>
>A few questions about voltage doublers. I'm building a single tube 4CX250R for
>70cm using a voltage doubler
>supply. This is my first attempt at FWD supplies.
>
>I have a transformer with about 750vac secondary, 12ohms dc resistance. I'm
>planning on using IN5408 diodes and
>a string of electro's.
>
In one word, DON'T. The secondary resistance of the transformer -
combined with the effects of primary resistance and diode on-resistance,
which you also need to factor in - will ruin the voltage regulation. You
will get disappointingly low voltage at zero-signal anode current, and
it will droop severely down from there. The result will be quite hard
limiting of RF output on speech peaks. Been there, done exactly that,
and sorely wished I hadn't built a complete amp around that useless
transformer!
It was only many years later that I really thought about the standard
FWD design charts, which were there in the ARRL Handbook all the time...
and they could have told me exactly what was going to happen.
The best practical advice is "don't ever use FWD unless you KNOW the
transformer was specifically designed for FWD." Otherwise it is not
going to have a very low secondary and primary resistance, because they
aren't needed for bridge or biphase operation.
>(1) I can either use 5 x 190 uf 400vdc caps in each side of the stack, second
>hand, or 5 x 1500 uf 250vdc caps
>which are new but will give a much greater overall capacity. Is there such a
>thing as too much capacity in the stack,
>and will this make the supply too "stiff" if things go bang
>
You got it. Ideally you want just enough capacitance for adequate ripple
rejection, and no more. Any excess is literally storing up trouble if
things go bang. Unfortunately FWD requires more capacitance than bridge
or biphase in order to get the voltage regulation (although even
unlimited capacitance won't rescue a transformer that has too much
resistance).
>(2) Is modern practice to still put C and R across each individual diode in
>the
>rectifier stack.
>
This has been discussed here in times past, so I hope this is a fair
summary:
There are some valid reasons for using R and/or C, but much of what you
see in published circuits is a holdover from the dark ages where PIV was
very expensive and reverse breakdown was by an internal arc that was
instantly fatal.
Modern rectifier diodes have avalanche reverse breakdown characteristics
and shouldn't need R or C (especially if they are matched in reverse
breakdown characteristics). Editions of the ARRL Handbook since 1995
explain this.
A modern concern is that the effects of R and C on the diode turn-off
and recovery characteristics have some EMC implications. If anyone has a
good SPICE model of the 1N5408, for example, I'd very much like to do
some more work on this.
Some commercial designers use both R and C, some R only, some C only.
Others use lots of diodes to get a very high PIV, with no R or C. To
some extent these decisions seem to be guided by the availability and
costs of components. For example, manufacturers can buy well-matched
(and that's important) high voltage ceramic capacitors very cheaply;
hams can't, so it may be more cost-effective for us to use the "many
diodes" approach.
>(3) Is it good practice to place a diode across each cap in the stack, cathode
>towards the + side, for reverse current
>protection.
>
AFAIK, the only time that reverse current could occur in a capacitor is
when a charged stack is being completely discharged through an external
load (this is not what happens at routine switch-off, when each cap
discharges through its own voltage-equalising resistor). Even in a 'big
bang', a single cap could only reverse polarity when all the others were
almost completely discharged too, so there's not much risk of severe
reverse current. And finally, reversal could only occur if one or more
caps is severely mismatched from the others in the stack. Unless there
is some other scenario I haven't thought of, those diodes don't seem to
be necessary.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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