[Amps] Resonant choke filter

[peter.chadwick at Zarlink.Com]

Because the critical choke inductance in a choke input filter varies
inversely with the current, the swinging choke can have less air gap i.e.
less reluctance than a non swinging choke. This means that it needs less
wire for any given inductance, and so can be smaller. As the air gap is
reduced, so the flux density in the iron goes up - less reluctance -  so
the number of ampere turns needed to start reducing permeability reduces.

Now look at the numbers. Suppose we have a 2kV supply with a bleeder
current of say 50mA, which is still 100 watts of heat.

Now Terman gives   Lcrit>R/1130 = 40,000/1130 = 35.4 Henries for a 60Hz
single phase full wave rectifier.

At 1 amp, however, we only need  1.77H.

So you can see why a  swinging choke is used.

Interesting question: if the transient performance is so bad, why were
swinging chokes considered OK for Class AB and B audio amplifiers used as
modulators in AM tx's?

Anyway, back to the point.

At 120Hz, the reactance of 35.4H is 26.7K

If we used a 20H choke, the reactance is 15.08K. Now shunt that with 0.08
microfarad. The result is


1[1/(-jXc) + 1/(jXl)] equivalent to 221.2 Henries, so we've made a small
choke look like a much bigger one.

the resonant frequency is 125.8Hz. ( f = 1/ 2pi. rt LC)


Now this is all rather idealised insofar as no account has been taken of
the choke resistance, which will modify the resonant frequency (resonance
being defined as the frequency at which the circuit dealt with as a one
port looks purely resistive) and the effective inductance. But you can see
that adding the 'tuning' capacitor makes the choke look like a much bigger
choke. Now if a swinging choke is used, as the current increases, the choke
inductance drops, but that doesn't matter as long as the circuit still
offers more than critical inductance.

My suspicion is that a swinging choke provides worse transient regulation
because when the load decreases, until the domains within the magnetic
material can start de-aligning, i.e. getting further away from saturation,
the choke critical inductance is too low, so the volts start to swing up.
In any case, you have a parallel tuned circuit with varying current, so
there will be a biggish transient.  I would have thought that a fairly big
output capacitor would cure this from the viewpoint of the load, but the
rectifiers could see a large PIV. I've never used a swinging choke in a
tuned choke system, though. Nevertheless, all chokes with iron cores change
inductance to some extent as the current varies - some more than others.

As Will said, tolerances mean that very careful choice of components is
needed if you are to be certain that the choke is tuned HF: if you tune it
LF, it will look like a lossy capacitor and the whole thing will revert to
a capacitor input filter. If you get the wrong resonant condition, you can
get enormous voltages built up too, so this is not an exercise for the guy
who isn't experienced in working on HV circuits. Tom, W4JI, has a
frightening story about that, which you'll find somehwere in the Amps
archives. I find a Variac very useful at this early stage of the game:
later, when you want to get realistic loads on the PSU, the Variac isn't
such a good idea - they're not renowned for their regulation.

73

Peter G3RZP