Output inductance of this coupled-inductor/two-phase "buck" converter? - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: September 17, 2020, 02:56:29 pm
content: Hi all,We're testing an AC/DC SMPS with a programmable AC source (Chroma 61503). One test we need to pass is current harmonics with a distorted voltage waveform. (This is a DO-160 aerospace requirement.) In general, we only use the Chroma source with perfect sine wave output, ~0%THD. In this case, the THD is bumped up to 8%, which puts some "piece-wise" discontinuities in the voltage waveform. At these discontinuities and/or at points of high dv/dt, the current waveform shows a low frequency ringing/oscillation. About 5KHz. Capacitance of our EMI filter is about 1uF total. To oscillate at 5KHz, there needs to be about 1mH...somewhere. After checking a multitude of points with a scope, my latest theory is that the "output inductance" of our AC source may be oscillating with the capacitance of the EMI filter.Interestingly, I goofed up and destroyed the output board of our Chroma source...so now I have it out and available for reverse engineering. (The board has been replaced.)The source uses a topology that looks like the below attachment. My best description is a "coupled-inductor/two-phase buck converter without output capacitor". To get a bipolar waveform, it appears to use two identical converters, one for positive half of the wave, one for negative half of the wave. (At least that's what I think.)Each winding measures about 200-220uH @ 100KHz. My only theory is that somehow the "effective inductance" adds up to ~800-900uH, that is the inductance "facing" the EMI filter of the DUT. But, I cannot envision how that math would work out. Does anyone have an idea what the effective output inductance would look like?Thanks,Tim

author: jbb
author_rank: Super Contributor
timestamp: September 17, 2020, 06:48:41 pm
content: The Chroma supply likely has some closed loop control, which will cause the output impedance within the loop bandwidth to be quite different to just the output filter parameters.On that circuit diagram... it’s hard to say. Can you provide some more info?Are the 350V DC busses connected together internally or separated?How many phases is the output?Are you working out the rest of the output circuit from the MOSFETs to the Chroma terminals?What’s the switching frequency?

author: TimNJ
author_rank: Super Contributor
timestamp: September 17, 2020, 10:09:05 pm
content: Quote from: jbb on September 17, 2020, 06:48:41 pmThanks a lot for your inputs.The Chroma clearly has closed loop control for its output voltage, as it has twp voltage divider strings, one for each output. There 350V buses are isolated. The topology of the entire unit is something like this:AC-> Interleaved PFC -> Full-bridge converter with two isolated secondaries -> 2x350V bus -> 2x "coupled-inductor" buck (this board) -> Output board with a few more toroids & control relays. I suppose the last board is where the two waveform halves are "pieced" together? I don't even know what that would look like. TiN did a teardown here for some visuals:This is a single-phase AC output. I am not sure of the switching frequency. The ripple on the output voltage is quite low, so I can't really see much. And, the board that I have outside of the unit is dead.So helpful on my part, I know!But, as you noted, the inductors in the Chroma aren't just passively sitting there. The current through them is being controlled such that the current can't just ring freely (I think). I think the Chroma source is probably not the issue. If we run the Chroma into the EMI filter, and the filter directly into a resistive load (instead of to the PFC section of the converter), the current waveform is just about as ugly as it is with the PFC (and rest of the converter enabled). So, our issue seems pretty isolated to the EMI filter. However, on further review, the current does not truly "ring"..as it's usually only one "cycle", or a half-cycle. I think the current distortion may more simply be related to the X-caps reacting to discontinuities in the voltage waveform by I = C*(dv/dt). Whether or not Chroma's source impedance has a significant impact on this, I am not sure.Attached is an image of the purposefully distorted voltage waveform (magenta) and current waveform (blue), with the DUT's EMI filter powering a resistive load (after diode bridge).Thanks for your help!

author: Phoenix
author_rank: Frequent Contributor
timestamp: September 17, 2020, 10:26:12 pm
content: The total series inductance is actually 4 times the individual winding, because the series current path sees both windings and inductance is N^2.Each of those is a normal hbridge. The coupled Inductor is a way of getting inductance on both phase legs with only 1 core.I doubt the closed loop control is doing much at 5khz.

author: TimNJ
author_rank: Super Contributor
timestamp: September 17, 2020, 11:09:17 pm
content: Quote from: Phoenix on September 17, 2020, 10:26:12 pmThanks for the good information. So, the coupled inductor seems to be mainly for...space saving purpose? In this case, the equivalent series inductance is around 900uH, which is close to the inductance I estimated based on "ring" frequency of the current. Interestingly, the Chroma has the ability to set the output impedance in the form (L + R). You can set it to "0mH + 0R"...Can it compensate for itself somehow? Maybe this setting isn't what I think it is. I tried a variety of programmable inductance values, and did not see any appreciable change in the current waveform.

author: Phoenix
author_rank: Frequent Contributor
timestamp: September 17, 2020, 11:39:49 pm
content: Quote from: TimNJ on September 17, 2020, 11:09:17 pmThere will be a bandwidth limitation of this programmable impedance. It should work at 50Hz/60Hz and some low frequency harmonics, but I doubt it will contribute anything at kHz+. It will give you a voltage droop as you draw more current. It would also be useful if you're looking at mains frequency reactive power stuff and maybe for power sharing when paralleling modules.If the controller is using output voltage feedback (likely) it is already "compensating" for the voltage drop of the filter reactance and appearing like a voltage source to your load - within its control bandwidth.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 17, 2020, 11:55:29 pm
content: Hi,Look at this model and the results:I have attached the LTspice model.I had to make some guesstimates.Jay_Diddy_Bchroma with load.asc(1.63 kB - downloaded 87 times.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2020, 12:02:12 am
content: Space saving, probably yeah. The downside is, if the H-bridge is slightly mismatched, the common mode error goes straight through (it's a diff mode choke). It should really be designed as two normal-mode inverters that happen to be complementary. That is, half bridge, single phase filter with respect to ground.If the two channels are wired in parallel for higher output current, they have to track current independently, i.e. as a basic matter of control, some mismatch is necessary (you can't simply wire two inverters in parallel, with the same gate drives, they won't share current evenly), and this this is that much more critical.If they're two independent channels, say going to a Scott-T transformer to synthesize proper 3-phase, that gives the degree of freedom necessary to avoid this. Note, the primary windings still need to be isolated from each other; an autoformer configuration can't be used.Or maybe there's a third board for total three independent phases, and they do need to track in some way, I don't know.Anyway, between the output filter (looks to be a single axial film (per channel) on the board, waaait, that's a double decker isn't it?) and whatever connects afterward, yeah there'll be some manner of control loop (which can't be any faster than the LC shown here), which can modify the natural behavior of those networks; it might be a different resonant frequency (or multiple resonances, since you say there's EMI filtering after, so more than just 2 poles), it might be some manner of damping, it might be unstable (but obviously, it's not tunedthatbadly).I would guess that the control circuit has, either a rather complicated analog control, or perhaps/hopefully more likely a digital control with an MCU. I don't have a good feel on how old this design is... much of it looks like it could be 90s or even 80s tech, with all the thru-hole parts. Ah, flipping through the video above I see it's a TMS320 DSP, and I see some date codes that look to be current-ish, 2009 probably?In any case, it can only control itself within the control bandwidth, with an asymptotic cutoff set by loop stability and filtering. It might be programmable to "1H" source inductance, but clearly it's not going to produce many kV when you charge that up and let it sproing away; obviously, it'll saturate before then, but it also won't saturate instantly, it has to charge up the filter network, and that can only be done so fast. Which in turn means you can only get "1H" behavior up to some limiting frequency (and again, amplitude as well, but let's assume small-signal conditions so we can ignore nonlinearity and dynamic range). Or if you set "100mH" behavior, it can probably do that to about 10x the frequency, and so on. And likewise for R and C characteristics, where the ratio is with respect to the characteristic impedance of the output filter (and to frequency, in the case of capacitance and inductance).So, probably they would only guarantee that the RLC output impedance is within such-and-such a range, at or below some nominal output frequency (perhaps 60Hz), and that's that: a square operating region of guaranteed behavior.But as you can see, the possible operating range isn't restricted to a square area, if it's allowed to vary with frequency then we can define a sloped, trapezoidal or whatever region (or really parallelepiped, in however many dimensions (V, I, Z, F) we're talking about now... sorry, it gets messy doing multi-dimensional controls after all), where you can set increasingly extreme values (very high or very low R/L/C) as frequency drops.So the problem comes in here, when setting an arbitrary waveform: the impedance-bandwidth, as it were, is constant, but the frequencies of the wave's harmonics obviously aren't. So you can only reliably set a desired output impedance, up to some limiting harmonic of that waveform.Not that the settable impedance characteristic was your primary concern, but it kind of is, in the reverse direction: whatever the case, it seems your load impedance, and desired bandwidth, are simply not well matched to the combined filter network and control loop, and hence you see waveform distortion (in the time-domain pulse distortion sense; this is really just an LTI frequency response thing, so I want to highlight the special use of "distortion" here).The solution is to:- Reduce waveform bandwidth, i.e., simply don't expect so much bandwidth from a system that's just not tuned for it anyway;- Match the load, so the filter is well damped (to whatever useful meaning that has -- you fundamentally cannot reproduce a waveform with excess bandwidth, so it's up to you to decide how to best fit it);- And continue to ensure the load is linear and resistive; which, is probably a hard sell for a SMPS load...That last point may simply mean that you need a much better (faster?) source to perform this test. Alternately, perhaps this is accommodated by the standard after all, and a careful reading will yield better understanding. (The mains network itself isn't infinite bandwidth, after all; a typical model is, say, (50Ω || 200uH) + 50mΩ or thereabouts.)Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 18, 2020, 12:16:19 am
content: Hi,Further to my last post, you can use the .FOUR commands to measure the harmonics and the THD on the voltage and current waveforms. The results are in the SPICE error log:With the example I gave there is 9.7% THD on the voltage waveform and 17.4% on the current waveform.Regards,Jay_Diddy_B

author: jbb
author_rank: Super Contributor
timestamp: September 18, 2020, 12:30:48 am
content: There are also limits to control response set by the switching frequency. It’s hard to get control bandwidth much higher than 1/5th (usually 1/10th) of the switching frequency, so control of a 5kHz ring would suggest using a switching frequency of 25kHz or more. That’s quite fast...Ultimately there is a reason that giantlineargrid emulators are a thing. They’re huge and expensive but very fast (I used one with a 1us rise time on 400V!).

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 18, 2020, 12:44:56 am
content: TimNJ,Can you tell by looking at the failed board what these components do?I suspect that they are a damping network after the inductor.Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 18, 2020, 12:53:45 am
content: Hi,This model almost matches the shape of the waveforms that you shared earlier perfectly:VersusThe really circuit isn't like this. There would be too much dissipation in the 100damping resistor.The damping is probably coming from a combination of the voltage feedback loop and some passive damping.I have attached the model.Jay_Diddy_Bchroma with load 2.asc(2.54 kB - downloaded 68 times.)

author: Phoenix
author_rank: Frequent Contributor
timestamp: September 18, 2020, 01:14:44 am
content: Quote from: T3sl4co1l on September 18, 2020, 12:02:12 amIt's a single phase output unit. The Chroma website says it has output voltage modes 0-150V/0-300V so I expect it has some relays to do series-parallel switching of the two H-bridge outputs.

author: TimNJ
author_rank: Super Contributor
timestamp: September 18, 2020, 03:30:40 am
content: To all...I am overwhelmed (in a good way) with all of the helpful responses. Although, I shouldn't be surprised. This isn't the first time you've all come to the rescue.Quote from: T3sl4co1l on September 18, 2020, 12:02:12 amThis design is at least 20 years old. The board that failed is Rev A, 2001-2002 date codes. By the looks of it, the design might be ~mid-90's ish. The MOSFETs are 2SK1517. The earliest datasheet I can find online is from 1999, so could be as recent as that.Quote from: T3sl4co1l on September 18, 2020, 12:02:12 amOriginally, we tried to directly synthesize a distorted waveform according to the standard's requirements. The Chroma's waveform editor allows you to specify the %distortion in each harmonic and the phase w.r.t. the fundamental. But, the point of this test is to check how the DUT behaves with a voltage distorted due to non-linear loading on a shared bus (i.e. neighboring equipment with poor power factor). The waveform straight from the Chroma "complied" with the harmonic definition from the standard, but ultimately was not realistic.So, we used the test setup the standard recommends: a parallel non-linear load (bridge rectifier -> E-cap || R-load) with a series reactor/resistor to drop voltage across, therby distorting the voltage at the input of the DUT. The waveform from the Chroma with this setup is a perfect 400Hz sine wave, no distortion.I think I was confusing in my original explanation. We did indeed use the Chroma as an abritrary waveform generator initially, but then just moved onto the "traditional" test setup with distortion done by actual non-linear load.That's good to hear you say the "distortion" is more of a transient response thing than anything else. I've shifted my understanding of this waveform away from true oscillation and more towards this explanation.Quote from: T3sl4co1l on September 18, 2020, 12:02:12 amIt's interesting that the standard really has no mention of what the AC source ought to look like. I've actually been quite frustrated with the ambiguity of this standard. DO-160 is a voluntary standard that manufacturers may opt to certify to, to prove their equipment is safe to fly. But, ultimately, the installer (Boeing, etc.) needs to decide if the equipment is actually suitable. I don't understand the aerospace industry well, but I don't think the FAA really gets in the middle of this. So, since flight worthiness may be a "case-by-case" thing, maybe they leave some parts ambiguous, for negotiation between the equipment manufacturer and installer...I don't know. But then...why have a standard?Seems like there should be an LISN (or something) in between to make this test repeatable at different locations.Thank you.

author: TimNJ
author_rank: Super Contributor
timestamp: September 18, 2020, 03:54:55 am
content: Quote from: Jay_Diddy_B on September 18, 2020, 12:16:19 amWow...and here I was reading the peak values of the LTPSpice FFT. Thanks for the tip.Quote from: Jay_Diddy_B on September 18, 2020, 12:44:56 amIndeed. The two caps are 2x1.0uF in parallel. Under the aluminum plate bolted to the heatsink are 3 wirewound (cement box) radial power resistors, in series. After the board failed a second time (after my repair attempt which worked for about 1 hour), I checked the snubber to see if perhaps it had failed, causing overstress on the semiconductors. Both networks were fine. Off the top of my head, it was 3x1.5R in series, but I might be wrong.Quote from: Jay_Diddy_B on September 18, 2020, 12:53:45 amWow! Thanks. I really appreciate that. It's so close! Maybe just a touch more series damping in the real case. I will play with the model. One question: What does the 5KHz LP represent? Anything real? Or just part of the model?This was rather instructive to show that the "overshoot" at the abrupt change in dv/dt of the voltage waveform is (as Tim also mentioned) more transient response than oscillation, per say. Though, it seems like that overshoot is a good way tostartoscillation. In the attached image, I set L1 to 1uH (basically 0).Thank you.

author: TimNJ
author_rank: Super Contributor
timestamp: September 18, 2020, 03:58:30 am
content: Quote from: Phoenix on September 17, 2020, 11:39:49 pmAh. Thanks. Yes it behaves very well as a voltage source, maintains close to 0% THDv regardless of load.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 18, 2020, 10:41:23 am
content: Hi TimNJ,Quote from: TimNJ on September 18, 2020, 03:54:55 amYou can specify how many harmonics that you want to measure..FOUR <FREQ>  <data trace>So you can get the higher order harmonics if you need them.Quote from: TimNJ on September 18, 2020, 03:54:55 amThe 5 kHz LP filter is to round the corners on the voltage waveform, by attenuating the higher order harmonics.Without the LPF the waveforms are:Nothing real, except trying to match the voltage waveform.Jay_Diddy_B