Swinging choke vs std. choke in PS

[Curly Woods]

OK,

I have read some comments that a swinging choke in a power supply can cause motorboating vs. a standard type power supply choke. Can anyone elaborate why it is so difficult to use a swinging choke in a power supply, or is this nonsense?

Thanks.

 

[Tweeker]

Any choke bearing supply can ring if its of high Q. Motorboating is usually a symptom of inadequate pre and/or driver stage decoupling. Choke input supplies are typically less vulnerable to it than capacitor input supplies as choke input supplies have lower output impedance. But choke filters have resonances that will ring if they are too underdamped.

Fundamentally the way to get a damped filter response is to add more capacitance and/or resistance.

How to calculate LC filter Q:

Resonance Frequency: Fres(low)= 1/ {2pi*[sqrt (L*C)]}

Fres(low) Q= (1/ Rchoke)*[sqrt(L/C)]

If we want to be more thorough, the load resistance across the capacitor also damps the resonance, and can be added to Rchoke via:

Rdamp= L/ (C*Rload)

Rload= VB+/ IDC + Rchoke

Q= [1/ (Rchoke + Rdamp)]*[sqrt(L/C)]

Series regulators provide no damping.

If Q is > .707 there will be a hump in the response of the filter. Q= .5 is critically damped. A very high Q is likely to lead to ringing. Look for it in PSUDII with the voltage across the choke in addition to the output voltage. A stepped load is helpful in looking at this.

Each octave the low frequency resonance is reduced by gives 12dB more filtering.

With less than perfect diodes there is a time when no diodes are conducting during the 0 crossing as the foward voltage drop for the diode to conduct has not been overcome, the inductor tries to maintain current causing a voltage to develop. This is why some recommend 220nFish caps to ground in front and behind the choke as snubbers. Math courtesy Morgan Jones, Valve Amplifiers.


One unrelated drawback of swinging chokes is that is hard to find ones of new production.

What are your idle and max currents?

 

[sherelec]

Michael,

The short answer is that inductors, unless they are air core, are all swinging chokes. The iron or ferrite materials in the core cause the apparent inductance to be max at 0 current and to quickly or slowly show reduced inductance as the magnetization current increases, on out to core saturation.

As far as I know chokes designated as swinging are typically used for line related power applications and historically used for choke input LC filters right after the rectifiers. The design approach is to start with the minimum load current and to use a choke inductance at that current sufficient to keep the choke from "drying out". If the maximum load current is less than the choke max current spec; then there is no danger of "drying out" at intermediate currents as the L does not fall fast enough to become a problem.

Often a design will incorporate some additional load resistance to accomodate the choke requirements (that is raise the minimum load to accomodate the inductance of the choke used). The above would be applicable to any choke, whether designated swinging or not (for a choke input filter).

The change of inductance with current is an analytical problem in designing filter responses as many analyses use a fixed L. Real inductors, unless air core, are not so well behaved.

Back in the cobwebs of my mind I believe that the "Radiotron Designer's Handbook" has most of a chapter devoted to LC and CLC filters after rectification. No SPICE then, so lots of graphs!

I hope this is helpful,
VSR