Series LC across output to notch out 100Hz on psu - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: September 14, 2020, 11:29:59 pm
content: Say we have conventional capacitor input filter supplied by a 50Hz transformer and bridge rectifier. Depending on the size of the capacitor and the current drain, there will be some 100Hz ripple in the DC. Now lets put a series LC that is tuned to 100Hz directlyacrossthe output. This would present a very low impedance to 100Hz. Given the physical size of the L and C needed, would doing this be more effective than a conventional LC filter using the same components?

author: SilverSolder
author_rank: Super Contributor
timestamp: September 15, 2020, 12:56:57 am
content: Might draw a lot of current, unless the source impedance of the supply was high (i.e. not a high power supply)?

author: Circlotron
author_rank: Super Contributor
timestamp: September 15, 2020, 01:06:28 am
content: Well, we want it to draw current on the ripple peaks and give the current back on the ripple troughs. Maybe if the main DC filter cap wasn't big enough the resonant current might actually make the DC ripple worse?

author: SilverSolder
author_rank: Super Contributor
timestamp: September 15, 2020, 01:10:30 am
content: I'm not sure a series LC would "give the current back", it might just look like a low impedance short at that frequency?This sounds like something that would be worth playing with in LTSpice or other circuit simulator, to get a feel for how it would behave?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 15, 2020, 01:27:45 am
content: The source impedance is approximately, or somewhat less than, the impedance of the input filter cap. (Consider the rectifier conduction angle.) Which also factors into the effective capacitance used to calculate the trap. Also note amplitude depends on load, so the ripple current goes up with load, as usual.Note that putting a trap across the input filter cap doesn't do very much, as it makes a capacitor divider with the added series capacitor. It's more effective if there is a series resistance or inductance to filter against. Or use a series parallel-resonant trap, to make a band-stop filter, or low-pass Elliptic or whatever.It works fine, but as you can see it depends somewhat on line and load condition, so can't be made too precise (= notch can't be too deep), and only works on the 2nd harmonic, and you need additional stages to deal with other harmonics. And it does nothing for imbalance (fundamental), same thing again.It has been used back in the days of choke filtered supplies, and you can read more about it inRDH4among others.Checked actually to be sure; RDH4 discusses twin-T notch filtering, but not resonant filters!Probably if you consider the cost of all these components, for a given attenuation from supply to output (say for an audio amplifier), it's cheaper overall to use a somewhat larger bulk cap, no filter chokes, and concentrate on improving PSRR. For sure, the inductors most likely will be iron-cored, from which you have very limited selection these days, most likely needing custom parts. So the cost spirals rapidly upwards as you go into this sort of thing.And this was already true back in the day, where triodes might've needed additional filtering because of several reasons, but pentodes in push-pull largely didn't, thanks to their more constant-current characteristic (and therefore high PSRR, even under large signal conditions -- up until clipping, which is to say, a massive increase in IMD, causing ripple to be audible along with the (heavily distorting) signal).The principle applies nicely to RF of course, where parts selection is cheap and plentiful; a typical example is low-pass filtering of DAC output, with additional rejection at the sample frequency and harmonics. (Note that the notches cannot reject sidebands around the harmonics, so this can't be a slam-dunk cure for aliasing. It's a modest optimization to push just a little more BW-SNR out of a system.)Tim

author: TheMG
author_rank: Frequent Contributor
timestamp: September 15, 2020, 01:33:50 am
content: I don't think it would work all that well. You would need comparatively very large L and C for the low impedance path to 100Hz created by the series combination to be much lower relative to the impedance of the load and of the rest of the power supply itself, for it to make much of a difference. There's probably a very good reason I have never ever seen a power supply with such design. There would also still be some ripple, as a full-wave rectified waveform doesn't just have 100Hz frequency components, there are other harmonics in there too.One design technique that was commonly used was a C-L-C filter (parallel C on output of bridge rectifier, followed by series L, followed by another parallel C).And if you needed even less ripple than a C-L-C circuit could give you, you'd follow that up with a linear regulator. No sense in making the power supply unnecessarily large and expensive, when a simple linear regulator can virtually eliminate any ripple. Yeah, not great as far as efficiency, but that's why SMPS were invented.

author: Circlotron
author_rank: Super Contributor
timestamp: September 15, 2020, 01:39:03 am
content: Disclaimer - it's just a thought experiment during lockdown. Not actually going to make one. Just thinking about fundamental concepts. Seems to me like it’s almost the electrical equivalent of a torsional vibration damper on an engine crankshaft. Much higher Q though.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 15, 2020, 02:42:42 am
content: Hi,Here is an LTspice model to play with:I have designed traps for 100 Hz 200 Hz and 300 Hz.I have also implemented the same capacitance used in a conventional manner.The model is attached.Regards,Jay_Diddy_Btraps.asc(3.83 kB - downloaded 104 times.)

author: SilverSolder
author_rank: Super Contributor
timestamp: September 15, 2020, 03:29:00 am
content: Very interesting, the output looks cleaner than the "simple" filter, but not really clean... and it does come at the cost of more RMS current through the capacitors compared with the "simple" filter arrangement.

author: Circlotron
author_rank: Super Contributor
timestamp: September 15, 2020, 03:54:49 am
content: Quote from: Jay_Diddy_B on September 15, 2020, 02:42:42 amInteresting in the second cct there's no big cap directly across the bridge output.I'm using Simetrix. Not sure if it'll import LT Spice. We'll see.

author: SilverSolder
author_rank: Super Contributor
timestamp: September 15, 2020, 04:10:30 am
content: The second circuit has much larger current spikes in the diodes.By far the least ripple comes from the more traditional low pass filter, with almost the same components:

author: Circlotron
author_rank: Super Contributor
timestamp: September 15, 2020, 04:18:23 am
content: Also, for the 100Hz trap I used 100uF and 25.33 mH. How would I determine the optimum LC ratio? And I know this is perhaps a dead end...

author: Circlotron
author_rank: Super Contributor
timestamp: September 15, 2020, 04:51:20 am
content: Even with just a single LC stage the ripple is fairly sinusoidal. That being the case, it would be interesting to put a 100Hz trap across the output and compare it with the same LC as a second filter stage.Edit -> The trap components when shifted as above resonate and make the ripple so much worse...

author: Circlotron
author_rank: Super Contributor
timestamp: September 15, 2020, 05:22:37 am
content: Here's a thought... What if you had 3 single phase choke-input rectifiers each fed from a different phase, and seeing the ripple is mostly sinusoidal, feed all three DC outputs into a zig zag transformer and take the output from the star point. If the ripples are equal, and to the extent they are perfectly sinusoidal, they should complement each other and balance out to perfect DC. Whether the DC fluxes in the zig zag transformer would cancel each other out remains to be seen.