Three phase power supplies can get away with smaller L and C, higher
ripple frequency too. They are great for high power RF transmitters.
But most hams and a lot of small broadcasters don't have three phase AC
service.
Single phase high voltage power suplies can benefit from having enough L
to sustain constant current in the inductor instead of discontinuous
pulses charging the capacitor. By adding enough load resistance (lower
bleeder R) lower value inductors can be made to work. So the trade off
is power consumption/dissipation in this resistor. Selecting a good
inductor ('choke') also requires consideration of the DC resistance, as
this can cause poor regulation as well.
L/C filters suffer from a low frequency resonance, which causes poor
transient response during turn on/off (overshoot) and poor regulation
during keying or low freq audio modulation.This is covered in the
earlier (1964) Pappenfuss/Collins Radio SSB book, and maybe even in the
Terman textbooks. But L/C filters can get by with rectifiers that have
lower peak to average current ratings, such as mercury vapor tubes.
Tuned choke filters work well, if carefully designed. Components need to
be rated for the high 120 Hz voltage that will be across the 'trap' and
the inductor needs to be designed to be the desired L at some DC
current. This is not a swinging choke as was already mentioned here. But
it does need to 'swing' enough that it will go into resonance with the
tuning-capacitor at the desired load, typically when the power supply is
only loaded by the bleeder resistor. Then, when current goes up with RF
power, the resonant choke goes out of resonance and becomes just a choke
input filter.
It is true that the higher frequency ripple components will come through
the filter with less attenuation, unless a second section of L and C are
added. This is because the value of L required for a resonant choke is
much smaller than needed for straight L/C filter.
I spent months developing one of the last commercial tuned choke power
supplies, in 1982 or 83 for Broadcast Electronics Inc. for their single
phase FM transmitters up to 3.7 kW. Tube FM amplifiers are easy, being
class C, with uniform current during operation. But during grid tuning,
and if the driver is switched off, the plate current drops to nothing.
In the 'olde' days the driver was typically a 4CX250B stage, and it was
hard-connected to the final. Around 1980, FM drivers starting to switch
to solid state, 50 ohm stages that had a coaxial interconnect. It became
possible to disconnect the driver and test it into a load. With a
conventional L/C filter, the power supply typically soared to a much
higher voltage. Designers didn't want to compromise the design with very
expensive 20 H chokes or low value bleeders that dissipated hundreds of
watts. Oil/paper capacitors would need to be rated at an even higher
voltage compared to normal operating voltage, like 1.4X. Enter the tuned
choke.... It worked very well in this application, the HV supply would
not soar anymore when unloaded, and the L/C components were relatively
small. I kept detailed notes of the experiments at BE, and later wrote
up my experiences here, can be found in the amps@contesting.com archives
in several places. It never had dangerous explosions or disasters,
because we chose components that could handle the voltage in the tuned
trap. Picking off the shelf L or C and trying to force it to work in
this topology is a compromise that can end in failure. I started this
way, and quickly realized that I would need a custom choke as well as a
custom capacitor. Probably a thousand or more of these rigs were sold to
broadcasters, and the only failure mode I ever heard was when they quit
regulating, starting to soar again when unloaded. This appeared to be
from capacitance changing, long term drift or an internal change.I never
found out what caused that, but replacement capacitor usually fixed it.
Or maybe it was wandering of the power line frequency at night? One more
thing, the choke had a tap on it for 60 and for 50 Hz operation, as the
transmitters were sold internationally.
SSB amplifiers, by nature, have idling current in the PA, even without
drive, so the choke size can be selected along with bleeder to keep the
voltage from soaring. Tuned chokes may be unnecessary.
Nowadays, with silicon rectifiers that are able to handle huge inrush
currents and charging peak currents, the trend is to go to larger
capacitors alone, adding the complication of inrush limiting, stored
energy precautions and needing to have transformer iron designed for
C-input configuration. These filters have eliminated the leak-through
rippled from powerline harmonics, and since the power supply voltage is
set by peak charging voltage on the capacitor, will give higher voltage
and pretty decent load/noload regulation without needing a big bleeder
to accommodate the lightly loaded condition.
73
John
K5PRO
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