Re: [Amps] Resonant choke filter

[R.Measures <r@somis.org>]

n
On Aug 4, 2004, at 1:00 AM, peter.chadwick@Zarlink.Com wrote:

> 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?

Probably because we Hams were looking at the  outputs of such supplies 
with an ordinary meter instead of a DC oscilloscope, and because we 
believed that Everything in The Handbook was the gospel.  .
-    The first time I became aware of the problem with unresonant-L 
filters was when I was talking with an OT (old timer), 'Mac', W6SDM 
about converting a surplus, regulated, 24VDC, C-filtered, supply to a 
regulated 14VDC LC-filtered supply.  The idea looked like it worked 
with a VOM since the C-filter delivered c. 1.25 x the transformer's 
V-rms to the regulator, while the new LC-filter would deliver c. 0.8 x 
the V-rms to the regulator.  Was this clever or what?  .  .   I told 
Mac what I had done..  He said that for constant current loads, this 
idea would work, but for varying loads, such as with SSB, there would 
be a problem which could Not be seen with a meter.  He recommended 
looking at the raw DCV with my Tektronix 513D oscilloscope and then 
changing the load current.  I did the test.  The V dipped c. 30% when I 
tripled the load-I, and surged about the same percentage higher when I 
stepped down the load-I.  The surge was no problem for the pass- 
regulator since it had plenty of Vce headroom, but the pass-regulator 
would loose regulation during the dip in raw DCV.  In other words, the 
filter choke was doing exactly what it was supposed to do, maintain 
constant-I when the load-R changed.  It does this by varying the 
output-V.  The moral to the story is that basic electrical theory 
(EE-101, EE-102, and EE-103) is the gospel, not the first book that's 
handy.
> Anyway, back to the point.
>
> ...
>
> 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.

How could a choke whose L varies with load-I stay tuned to one 
frequency with a parallel fixed-C?

cheers, Peter

> 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
>
>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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