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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
New Information:-
Thanks Hen, I'm getting some very interesting results, I changed some values to match what I have lying around, and then realised that a lot of what you believed to be lower frequency harmonics are actually due to the reactances in the second circuit 'interfering' with the reactances in the first circuit.
I still needs to run some more simulations, but now to the interesting bits, firstly, even touching the probe on the final capacitor affects the reactance, and de-stabilises the circuit for nearly a second.
Secondly, with the values shown, this is the highest voltage I've achieved so far, I still need to tweak it some more, but think I can improve on the 25 volts shown here, with the right reactance in the first stage of the filter.
Thirdly, voltage at the output is now stable to within 50uV using a combined total of around 12mH inductance.
This is between ten and twenty times more stable than with just a capacitor in the second leg.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
If you are modelling real parts you absolutely must include the coil DC resistances and capacitor ESRs. Numerical models of LC circuits can get a bit crazy without those resistances providing a bit of damping. Simulation is only as accurate as your component models - the more realistic your models,the more reliable the simulation results are.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
2Spoons wrote ...
If you are modelling real parts you absolutely must include the coil DC resistances and capacitor ESRs. Numerical models of LC circuits can get a bit crazy without those resistances providing a bit of damping. Simulation is only as accurate as your component models - the more realistic your models,the more reliable the simulation results are.
I've been working towards getting it more accurate all night.
I now appear to be getting better, and more consistent results without the notch filter, just using capacitors for the vertical sections of the filter. using two 10mH chokes and 44 4,700uF caps arranged as below, I'm getting around 20uV ripple, although I'm still tweaking things. DC resistance of the chokes is around 139mOhms, the secondary ofn the transformer is 600mOhms, and I can only guess at the ESR of the capacitors, but it's not a lot, especially the banks of ten and 32.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
Be aware also that transient sims of resonant systems can be sensitive to the size of the timestep, and also to the numerical integration method. I use Simetrix for simulation, and usually set the maximum timestep to something quite small. I also choose 'Gear' integration over 'trapezoidal' as it seems to gives more consistent results. Not sure what options you have in LTSpice - I hate it so I don't use it.
Even if you guess 1mOhm for your capacitor ESR, its still better than zero. I don't like zeroes: zeroes tend to produce infinities, and neither are things you want in a numerical simulation.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
2Spoons wrote ...
Be aware also that transient sims of resonant systems can be sensitive to the size of the timestep, and also to the numerical integration method. I use Simetrix for simulation, and usually set the maximum timestep to something quite small. I also choose 'Gear' integration over 'trapezoidal' as it seems to gives more consistent results. Not sure what options you have in LTSpice - I hate it so I don't use it.
Even if you guess 1mOhm for your capacitor ESR, its still better than zero. I don't like zeroes: zeroes tend to produce infinities, and neither are things you want in a numerical simulation.
I think I've chosen 10mOhm for a single 4,700uF quality stud capacitor (electro), but where I have a bank of 30 or more, Ive chosen 1mOhm, etc. I think the minimum I now have in parallel is three, at a combined ESR of 5mOhm, Changing it to ten doesn't seem to make a difference.
I've moved the notch filter, and now I'm starting to get some really impressive results, the 't' scale is 1uV, and it it's to be believed, the voltage at the out put is stable to low nano volts.
It fluctuates for a bit arter the measurement starts, then stabilises after a second and a half. This is the highest resolution LTSpice will go to on the 'y' scale.
EDIT: Here's the same thing with a wider range on the 'y' scale.
50uV on the 'y' scale here, as opposed to 1uV on the 'y' scale in the pic above.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
New information, explanation of 'observed phenomena'...Hope I don't upset Mads
I think the reason why this configuration appears to be orders better than others is as follows, first, as you'd expect, whith the notch filter at the begining, it ought to behave better, as long as there is a 'reasonable' sine wave to 'exclude'.... But...
You have an inductive element (transformer, albeit rectified) driving an LC filter, or to be more precise, initially a notch filter, a 'reactive device'.
I believe the value of the first capacotor to be critical, not just as large as reasonably justified, but.....
It's effectivelt a 'power factor correction' device, to drive, in particular, the 'reactive' notch filter.
This stuff really is a 'black art' there are fo many factors that require balancing.
While I'm aware of PFC and it's basic principles, that is pretty much the extent of mu knowledge on the subject.
Would anyone care to add.expand on the above?
EDIT: It's all about matching 'phase angles' and whatnot when driving a reactive load from a reactive source
EDIT 2: If this does turn out to be a 'new type of filter', it should be named the '4HV voltage filter'
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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Filters aren't defined by their implementation, they're defined by the location of their poles and zeroes, which results in some defining characteristic e.g. Butterworth has a flat passband, Bessel is 'constant delay', Chebychev has a fast initial rolloff.
What is your filter's defining characteristic that makes it special?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
2Spoons wrote ...
Filters aren't defined by their implementation, they're defined by the location of their poles and zeroes, which results in some defining characteristic e.g. Butterworth has a flat passband, Bessel is 'constant delay', Chebychev has a fast initial rolloff.
What is your filter's defining characteristic that makes it special?
That's a complex question, It's effectively two filters in series, with the first capacitor serving two functions, firstly, I believe it serves a power factor correction purpose between the inductive source and inductive load (notch filter), and secondly, as the first element in a Butterworth style LC filter
Butterworth filters tend to have flat passbands, this one will have a flat passband as frequency drops, but at some point, the amplitude of the signal passed will increase further as frequency tends to zero.
The notch filter part, on the other hand, will tend to pass more and more of the signal as the frequency either increases or decreases from 100Hz, albeit distorted
At low frequencies the combined filter will tend to pass most of the signal, but with some distortion, the distortion decreasing with decreased frequency. As frequency rises to 100Hz less and less of the signal will be passed, however above 100Hz, as frequency rises, the notch filter part will tend to pass more of the signal, whilst the Butterwoth type element will tend to stop more of the signal, with there being a significant notch in the passband of the combined filter at 100 Hz. This is similar to a type 2 Chebychev filter in that it has ripple in the stopband, which decreases to a minimum, then starts to increase, before decreasing again.
I guess though, that it's predominantly a Chebychev type 2, with notches in the stop band.
While I can't test all of these scenarios experimentally until I've wound some more chokes (0.4mH, 10mOhm and another 10mH, 150mOhm), that's what both the theory and experimental observation suggest.
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Power supply simulator.
