LC filtering of larger currents - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 12:02:11 pm
content: I'm working on a design to control 25A of PWM power delivery. This no doubt will kick up one hell of a racket from an RFI point of view so I'd like to introduce some power filtering involving capacitors and inductors. If I look up calculators they are aimed at the radio boys with aerials etc. An impedance is generally demanded and I expect I want my impedance to be as near "0" as possible? Are there better or worse configurations ? a simple LC filter is probably going to be less effective than a pi or T filter ?I am also putting some resistance in series with my mosfets so that I can control slew rate but obviously that has to be balanced with power dissipation.

author: danadak
author_rank: Super Contributor
timestamp: October 28, 2017, 12:27:37 pm
content: Not a magnetics expert but at those current levels I assume oneof the main challenges is keeping the cores (toroids, bobbins) outof saturation which negates effects of L.https://electronics.stackexchange.com/questions/71132/what-is-saturation-in-an-inductorhttps://www.quora.com/How-saturation-affects-inductanceRegards, Dana.

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 12:34:35 pm
content: Yes so I am using a 32A rms inductor with a saturation current of 50A. I expect the peak current will be limited by the last capacitor before the load that will be the most responsive "tank".

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 12:39:11 pm
content: This is why I ask, those are not small inductors and the 1000's of uF I could use will take up space as well.

author: nctnico
author_rank: Super Contributor
timestamp: October 28, 2017, 12:51:31 pm
content: With an LC filter you'll always need impedance matching otherwise the filter isn't going to work and you will get high voltage peaking.IMHO a solution which works better is a differential mode choke which cancels the HF getting out of the circuit. This will need some tuning.

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 01:08:11 pm
content: Well tuning wise I would imagine the filters cut off needs to be below the PWM frequency (50-1000Hz) in order to ensure the filter does not get "exited" but this is the problem, to get that low requires significant space for large costly parts.Ultimately I'm only trying to avoid sudden current draws down the supply line that will turn into radiated RFI, I'll probably never make it look like DC but removing harmonics is the first aim which perhaps does not require such a high grade of filtering.If by differential choke you mean two opposite wound coils I'm not sure how they help as here I'm simply trying to slow down the "gulping" of current down the supply line and opposite wound coils will let that straight through. My recommendation would be that the supply line be twisted for that very reason so that in the "inductive divider" most of the inductance falls on my filter circuit and not on the aerial that makes up the supply cable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2017, 02:27:06 pm
content: How about that, so am IFour 16 AWG (1.6mm) wires, wired in pairs for 20A capacity. About 9uH (and disturbingly low leakage inductance between the two windings). Peak capacity in my application is about 800 VA, though this would probably do 200W continuous in a converter.Throwing in series resistance sounds kind of careless... do you have a schematic, layout?Tim

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 02:45:32 pm
content: All I have so far is the attached schematic which is not complete as it lacks the filtering on the power I/O connector. The actual driving will be done from a separate board the connection for which is not shown yet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2017, 05:33:56 pm
content: Well, the labels and symbols are sure helpful... I can't tell a single thing what's going on there.Something about a bunch of transformers wired in parallel??Tim

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 05:39:55 pm
content: Hi Tim, sorry CS is the most stupid program on earth when it comes to outputing somerthing and has black bxed most stuff, out1 is the PWM intput to a mosfet driver (U1) from the control board (it's out1 to that board hence the mixup) the 4 "transformers" are mosfets, the lower legs the sources and the uppers the drain, the diodes are back emf diodes and the motor (load) will be across those. U2 is a current sense amplifier to measure the current draw through R1.Basically it's just low side load switching with back emf diodes and current monitoring.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2017, 07:09:13 pm
content: Ah.If you're paralleling MOSFETs and diodes, consider paralleling the current sense resistor as well. Better strays all around!I do see gate resistors. Is this what you meant by "in series with transistors"? Drain-source is the main current path, so saying "in series" makes me think of that, which would be weird, hence the question.How far will the motor be? Can this be, say, mounted on the back of the motor with no wires, or a nearby mounting frame, and grounded (at least RF grounded, through a capacitor) to it?Can always add an LC filter to the power line, but diff mode filtering is the easy part: common mode filtering needs big fat chokes so it's best to avoid it at the source.Tim

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 07:27:23 pm
content: The circuit would need to be quite close to the motor, I can't really see how I can avoid noise output from that with filtering. Where I wanted to try and filter was on the power input to try and stop noise going up what could be a longer potential aerial.I'm not sure what you mean about paralleling my current sense resistor(s) I need a single point of measurement to measure the current, if I have 4 resistors I'll need 4 lots of measurement circuitry.

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 07:30:04 pm
content: what is becoming clear is that I can't simply have a filter with a lower cut off frequency than the PWM frequency, I'll just have to rely on the fact that with some slew rate control and the fact that the motor inductance will resist a sudden change in current some capacitance and inductance will go a long way to curb the fastest of spikes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2017, 08:13:14 pm
content: Quote from: Simon on October 28, 2017, 07:27:23 pmYou can try to filter the piss out of a noise source, or you can prevent the piss from being generated in the first place.Keeping the switching noise bottled up inside the motor's casing is an excellent way to prevent noise in the first place. With no RF voltage drop between the motor and this board, there's nothing (additional) to transmit up the power lead.If you cannot keep the circuit close to the motor, then you can use shielded cable, effectively extending the ground from the motor casing up to the circuit. Again, RF grounding is needed at each end: if not a galvanic connection, then bypass capacitors at least. Do not break the shield.QuoteNo, I said parallel -- it's still a single point, but you get 1/4th the ESL of a single resistor. Better switching.Tim

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 08:22:59 pm
content: Well my worry with splitting the current sense resistors is that should there be any differences I would loose control of my measurement. I want fairly accurate measurement of ongoing current for speed control, I'm not simply trying to protect from over current.I can't put the circuit in the motor as I'm controlling an off the shelf motor. Either way I can't get out of the fact that I am switching large currents at a frequency and that can propagate up the supply loom. I'm thinking of 2uH with 5'000-10'000uF for 3000-2250 Hz cut off with a switching frequency of 50-500 Hz

author: floobydust
author_rank: Super Contributor
timestamp: October 28, 2017, 09:03:30 pm
content: I don't understand why the PWM carrier frequency is so low 50-1,000Hz.It's just the magnetics/capacitors get kinda huge and the acoustic noise was annoying.The only time I've used 100Hz PWM is on brushed DC motors on robots and model train engines, as the pulsating jiggles the armature and thus the motors can run at very low speeds without stiction.

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 09:07:19 pm
content: Well I suppose it will all come down to how hot the mosfets get, they are rated at 20A each and 5mR so I've tried to give myself headroom at 25A, I could increase the frequency if they can cope with it, of course with a higher frequency the inductance of the motor won't discharge so much so the turn it back on current may be lower so the slew rate control less of a problem.

author: dmills
author_rank: Super Contributor
timestamp: October 29, 2017, 01:56:21 pm
content: Don't bother trying to filter the PWM frequency the effective motor inductance will do that for you if the frequency is reasonably high, instead filter to slow the edge rates, for RFI reduction, think uH and at most single digit uF, much more manageable.For sub kW DC machines I would be looking at PWM starting at a few kHz and going up rapidly to maybe 50kHz or so, hundred Hz to single digit Khz is for IGBT traction drives on railways and other similarly butch applications.Regards, Dan.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: October 29, 2017, 03:07:43 pm
content: Quote from: Simon on October 28, 2017, 12:02:11 pmAdding resistance in series with the MOSFETs won't help slew rate it will increase the conduction losses. You can reduce slew rate by increasing gate resistance. This increases switching losses but not steady state conduction losses.Andy

author: Simon
author_rank: Unknown
timestamp: October 29, 2017, 03:11:16 pm
content: Quote from: ahbushnell on October 29, 2017, 03:07:43 pmYes it's gate resistor I am using. There is 1 2mR current sense resistor.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: October 29, 2017, 03:12:47 pm
content: What does the gate resistor have to do with a current sense resistor?

author: Simon
author_rank: Unknown
timestamp: October 29, 2017, 03:14:47 pm
content: Nothing, it's a gate resistor for slew control, the only resistor in series with the main current path.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2017, 03:38:53 pm
content: Quote from: Simon on October 29, 2017, 03:14:47 pmThis is contradictory.Tim

author: Simon
author_rank: Unknown
timestamp: October 29, 2017, 04:06:05 pm
content: Sorry typing on a phone is hard, there is a gate resistor for slew and the only resistor in series with the main current path is the 2mR sense resistor.