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[AMPS] resonant choke input

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Subject: [AMPS] resonant choke input
From: (John Lyles)
Date: Fri, 6 Feb 1998 10:37:16 -0700
The Broadcast Electronics FM-3.5A and B 3500 watt FM transmitter (88-108
MHZ, 4CX3500A-based design) uses a resonant choke input filter in the
single phase power supply for the plate voltage. I believe we also may have
used it in the FM-1.5A which was an 8877-based transmitter for 500 to 1500
watts. I designed it in 1983, after reading about the idea in Frederick
Terman's Radio Engineering, page 418, First Edition, 1932. It was also in
the Radio Engineers Handbook, also by Terman, page  608. I believe it was
also mentioned in SSB Principles and Circuits (1964, Bruene, Schoenike, and
Pappenfus of Collins Radio).

Harris used it in the single phase PS for the HFL1000 linear amplifier,
Collins used in (as someone already said), The Galaxy 300 had it, a
Heathkit (Marauder?) had it, probably a lot of other rigs had it too.

Harris used 7 Hy choke and 0.25 uF 5 KV cap, followed with a 6 uF smoothing
The Galaxy had a 4 Hy choke with 0.1 uF 2.5 KV cap, followed with a string
of three series-wired 200 uF
 'lytics for the smoothing section.
The BE FM3.5A used about a 5.3 Hy choke specially designed by SNC
(Wisconsin company, same guy designed the one for the Heathkit 22 years
prior to that) to resonate with 0.45 uF of capacitance across it. It used a
pair of 0.97 uF 2.5 KV polypro capacitors. Then it had a 4 uF oil cap for
smoothing, followed by a small 3.5 Hy choke and another 4 uF smoothing
section. This was a 2 section filter.

Some points that i remember are:

The choke was placed in the negative lead from the rectifiers, to eliminate
the steady DC voltage from coil to frame that would have been present had
it been in the HV lead. It was only required to stand off the momentary
transient voltage where the negative lead might spike to high voltage if
there was a plate supply + output arc to ground with low impedance. But the
AC rating across the choke and capacitor was HIGH. I had scope photos of
the loaded and unloaded AC voltage on the resonating caps, and it was
surprisingly high.

The 3500 watt rig had a plate DC of about 4500 Volts max. I put a tap on
the choke to make it resonant for 120 Hz for America, and at 100 Hz
(additional turns) for folks overseas with 50 Hz mains. If you left the tap
at 100 Hz, and operated it on 60 Hz line, everything looked fine unless you
removed the RF drive the final, virtually unloading the power supply (class
C amplifiers for FM). Then the voltage would soar to > 6KVDC, typical of a
choke input power supply without the critical load on it. The critical load
value was quite impossible to obtain with bleeders unless one wanted to
dump a tremendous amount of DC power into them, or one wanted a gigantic
choke on the input to the filter, to maintain contant current all the time!
We were concerned with cost, weight, size, and stored energy when we made
the decision to resonate the choke. The inherent regulation was the
technical advantage of this thing, allowing me to rate all the HV
components closer to operating point, even the meter scales could be used
in the upper third where they should be, instead of having to deal with the
unsightly soaring of HV if one pulled the drive on the PA. This same
property, of course, is advantageous for SSB amplifiers, having good HV
regulation over voice peaks.

Resonant choke supplies have an additional Pole that can be excited when
the power is switched on and off. This will cause a damped oscillation,
that must be accounted for in the component ratings. A significant
advantage was that inrush current was greatly reduced over what a Cap input
supply would have, helping reduce contactor, ciruit breakers and rectifier
sizes.  It was more like a true choke input, but a smaller choke was used.

The capacitors had to be rated to withstand the high AC voltage across
them, and oil units were not suitable at this voltage. Several KV of AC was
across them, something that capacitors must be rated for. I found that the
voltage doubler capacitors that come from microwave oven power supplies ARE
suitable for this service, having polypropylene dielectric instead of
polyester film and paper in oil. Hence the use of the 0.97 uF units, which
cost only $10-20 each. I series wired them to get more voltage rating. I
bought a quantity of these (they were inexpensive and readily available)
and did a statistical measurement of the capacitance. Using the average
value, I sent the proper capacitors to the choke manufacturer, where they
used these to resonate their choke, taking into account the DC current
affects as well. Remember, chokes will swing in inductance (remember
swinging choke inputs?) as the DC current varies. This meant that the
resonant choke input filter should be resonated for the unloaded condition
where the DC current was small, then it could move out of resonance as the
power supply is fully loaded, where the voltage was not going to soar
anyhow. What this did was gave a well regulated HV supply, with passive
components. The second section of 3.5 Hy choke and 4 uF capacitor was there
to keep the ripple filtering adequate for the non-resonant condition that
was typical at full power, and also to knock down the higher harmonics of
line ripple which will go right through the resonant trap. Such as 240 Hz,
360 Hz, 480 Hz, etc. Stuff that can be audible. This is one of the big
mistakes that can be made, assuming that the resonant filter choke will do
all of the ripple reduction by itself. It's only good if additional
filtering is added, this same point is made by Bruene et all, and Terman.

Because it was a commerical transmitter, we had to make
calculations/measurements of the tempco, the Q, the choke/capacitor
tolerences in the filter, to make sure that it could still work in some
countries where the line frequency regulation was not quite as good as in
the USA. I remember spending days working out these details before we had a
working product. It was also kind of scary to test these things in the lab,
as the capacitors can burst in exciting ways. I would not recommend oil
filled paper/Mylar capacitors in any resonant input choke filter circuit
for HV.

John Lyles

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