EMI reduction in a boost converter - Page 1
Forum section: electronics / projects

author: svofski
author_rank: Regular Contributor
timestamp: December 25, 2012, 11:27:41 pm
content: Hello everybody!I'm new on the forum although I'm an avid follower of EEVblog for a long time. Thumbs up for the excellent videos Dave!I figured this forum might be a great place to ask a question that's bugging me. In the process of trying to figure it out I burned the micro so while I'm waiting for the new one to come I have time to think about things carefully. First the background — it's a bargraph spectrum visualizer with IN-13 linear nixie tubes. The main micro is LPC1769, I use its builtin ADC to sample data at 28000 sps, pass it through 20 narrow biquad filters and convert the peak values into 20 channels of software generated PWM signals that get filtered and used to drive the transistors that regulate the nixie current. It actually works and looks fantastic. The problem however is the occasional surges at ADC that I believe are related to EMI noise. I know that LPC17xx ADC has quirks but the noise that I'm getting is real and can be seen on oscilloscope so I think I should tone it down in one way or another. It comes from the boost converter that I have on the same board.At this point it's probably better if I show the circuits.This is the analog input. I realized that I don't really want separate stereo channels so only the middle tap is used. AMONO is directly connected to an ADC input pin.This is the boost converter. It is driven by hardware PWM from the LPC and a GPIO interrupt from the comparator (HVFEEDBACK) turns it on/off when necessary. The frequency is 60kHz. It seems to be working well for the choke that I've got: 100uH/2.2A. It is set to 145V on +HV.And this is the relevant part of the board layout. The real board is nothing fancy, homemade double-sided.I marked the relevant parts with white text. Also the white trace shows the signal path from the input filters to the ADC input. It is not very clear from this picture but the analog ground goes below all the way to the top side of the LPC chip. It is connected to the main ground near the LPC. There is also a botched in C-L-C filter on the +12V line stuffed between the power jack and the audio jacks, it manages to reject the better part of noise through the +12V line.Now since I can't run the circuit at the moment I can't take any noise measurements and even if it worked, I don't really know how to do EMI measurements right: I just make a loop on the probe and I see huge spikes. What I know for sure is that there is a lot of EMI generated when the boost converter is on. The question is of course, is there something I can do at this point to reduce the switching noise at least a little bit?Just in case you're wondering wtf are the IN-13s, this is what it looked like with 11 bargraphs before I decided to make LPC1769-loaded xmas fireworks. The noise surges can't be seen here, being random they chose not to creep in at the time.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 25, 2012, 11:51:26 pm
content: Hi,I would start by adding a resistor in 10 - 22 Ohm range between the MOSFET driver and the MOSFET gate. This will take the edge of the gate drive. It will increase the dissipation in the MOSFET slightly but it will reduce the noise.You can also put a ceramic capacitor 0.1uF in parallel with C19.Regards,Jay_Diddy_B

author: svofski
author_rank: Regular Contributor
timestamp: December 26, 2012, 12:11:35 am
content: Quote from: Jay_Diddy_B on December 25, 2012, 11:51:26 pm

author: nctnico
author_rank: Super Contributor
timestamp: December 26, 2012, 01:24:00 am
content: A resistor and capacitor in series across the MOSFET will also help to dampen the ringing. You can use ltspice to simulate what values are a good starting point.

author: svofski
author_rank: Regular Contributor
timestamp: December 26, 2012, 10:17:08 am
content: Thanks for the advice guys! Sorry for that previous reply, not sure what happened. I'm trying to create a simulation in LTSpice that would look similar to what I'm seeing so I could start making small changes. So far LTSpice simulation looks too clean to be realistic.

author: ptricks
author_rank: Frequent Contributor
timestamp: December 26, 2012, 12:41:39 pm
content: The trace you have highlighted , I would move away from the U7 part, put a ground between the trace and the part U7, with a PWM signal being that close to a power rail it can easily inject noise. Think of any traces connected to a PWM as being a small radio antenna broadcasting to parts around the trace as it crosses the board.Something to read that might give you some more clues on how to solve :http://cds.linear.com/docs/Application%20Note/an136f.pdf

author: penfold
author_rank: Frequent Contributor
timestamp: December 26, 2012, 01:18:45 pm
content: From the diagram it looks like you've got a common ground plane across the board being shared by all the circuits. Try breaking it into zones and join them together where the signal paths demand it. EMI problems at 60kHz will be more likely due to switching transients causing voltage differences in the ground path, and much less due to actual 'radio' interference.Treating either problem will still do plenty of good to the circuit, keeping loops as small as possible etc

author: svofski
author_rank: Regular Contributor
timestamp: December 26, 2012, 04:25:16 pm
content: I didn't make my pictures entirely clear. Here's hoping to fix some of that:I have 3 large ground areas. AGND, which is turquoise tinted, boost area polygon (magenta) is separated and connected in its top-left corner, general GND which occupies the better part of the board.Part U7 is a 78L05 that powers the input opamps, there is no PWM around that region and the white trace is surrounded by grounded "shield" traces. I outlined the traces that carry most of the PWM signals. I don't think that they radiate too much.The truth is, it is rather hard to measure what actually happens there because the scope ground clamp picks up much more than the actual circuit. Perhaps I would have to try and make a local ground probe from a staple.

author: ptricks
author_rank: Frequent Contributor
timestamp: December 26, 2012, 08:41:36 pm
content: Here is an article from TI on how to find and solve EMI power issues.http://www.ti.com/lit/an/snoa382/snoa382.pdf

author: Niklas
author_rank: Frequent Contributor
timestamp: December 26, 2012, 10:59:41 pm
content: You should also minimize the high current loop as much as possible. Some quick ideas:- Move C19 between Q1 and P2, negative pin towards IC3.- Let D2 and C25 change places. D2 horizontal with the cathode towards the logo. C25 with its negative pin towards IC3.- Add more GND vias to Q1- Add ceramic capacitors (X7R or X5R, not Y5V) across C25 and C19. Something like 100 nF-1 uFThen you can add some small chokes/coils before and after the linear voltage regulator and also move the buffer capacitor a bit closer to the regulator's input. The idea goes like this: 12V-choke-capacitor-regulator input. Regulator output-choke-capacitor. Murata's EMIFIL- or NFM31-series are also good LCL-type filters. Combinations of L-C-L or C-L-C can also do good. I have used Murata's BLM-series, SMDs in 0603, 0805 or 1206, with impedances of 1k to 2k, in combination with ceramic capacitors of approx 100 nF and X7R with good results.

author: penfold
author_rank: Frequent Contributor
timestamp: December 26, 2012, 11:20:34 pm
content: I re-read your original question, just need to clarify if the glitches are in fact regular or more sporadic

author: svofski
author_rank: Regular Contributor
timestamp: December 26, 2012, 11:47:06 pm
content: Quote from: penfold on December 26, 2012, 11:20:34 pmI do not know the answer to this question— The spikes that I can observe on the scope are regular. The most visible ones are characteristic of fast switching circuits, very sharp and very fast. Their rate also corresponds to the switching frequency.— The glitches that I see as the result of processing in the microcontroller are sporadic.— The relationship between the boost converter and scope-observable EMI is 100% confirmed— The relationship between EMI and the output spikes is not confirmed.Tomorrow I should be getting the replacement chip so I can continue with the experiments.It's worth noting that the boost converter operates at fixed freq/fixed duty cycle. I suspect that the major EMI bursts happen at the beginning or end of pulse bursts, even though the software makes sure that the PWM cycle is complete and the output is at 0 before terminating the burst. When the circuit is operating again I'll try to catch those moments.@Niklas: the board is already done so I can only introduce minimal changes like adding parts across others, cut traces etc. Swapping parts sounds problematic even though I agree that I did not think well about the "whip" formed by D2. I like the idea of adding a smaller film cap across C25 (tank) and adding ground vias to Q1, these are #1 things to do on my list now. BTW, I checked with the local shop has and it appears that in the >250V range the capacitors are generally film (PTFE or polypropylene or something) — do you think it would matter here?@ptricks: thanks for the paper, interesting read.

author: penfold
author_rank: Frequent Contributor
timestamp: December 27, 2012, 10:46:23 am
content: Its certainly an interesting problem, since what ever is the culprit is sufficient to occasionally kill the micro, its most likely not the noise from a comparatively baby boost converter unless there's something properly messed up.Any chance of a few scope traces of voltages around the boost converter? MOSFET drain voltage and output voltage, would nice if at all possible, a bit of a sketch would clear things up.

author: Niklas
author_rank: Frequent Contributor
timestamp: December 27, 2012, 10:58:34 am
content: Film caps can work if the dielectric material is fast. Look for metallized polypropylene or, with those voltages, rather an X1-, X2-, Y1- or Y2-rated capacitor.A choke in series with the 12 Volts can also be added on the actual board by cutting and patching a trace. In that case select a filtering choke, those with a rated impedance in ohms and not rated in Henrys. Usually those chokes are made from compressed powder instead of copper wire.

author: Neilm
author_rank: Super Contributor
timestamp: December 27, 2012, 12:02:09 pm
content: Quote from: svofski on December 26, 2012, 10:17:08 amLTSpice will simulate the information you have given it. Unless you have used the "Real World" components in the libraries, you have given it perfect components. Try going through and adding some of the parasitic effects in real components and you will probably find that you can get a simulation that is close. I have managed to get a simulation of a fly-back system so close as to see exactly the same signals on the scope and the simulator. I had to take several measurements from components to achieve this.Neil

author: svofski
author_rank: Regular Contributor
timestamp: December 27, 2012, 01:07:57 pm
content: Quote from: penfold on December 27, 2012, 10:46:23 amOh no, the noise did not kill the micro. I killed the micro with my crooked hands. I guess I gave it some +12 across 3V3 bus or something similar.The shop failed to deliver the replacement today. I sincerely hope they burn in hell but the day after tomorrow, after I will have received the micro.

author: penfold
author_rank: Frequent Contributor
timestamp: December 27, 2012, 09:39:07 pm
content: Haha, I know the feeling entirely.If the glitch isn't doing any damage, any possibility of detecting the glitch in software and ignoring it? Not exactly an elegant solution, but given the other option is stabbing in the dark it at least gets something working before fiddling around with circuit designs

author: svofski
author_rank: Regular Contributor
timestamp: December 27, 2012, 09:46:21 pm
content: @penfold: Actually I could. I even tried some very basic filtering but since I do not really know the enemy well the results were worse than expected.Meanwhile I've been reading up on LPC17xx ADC glitches and I have found this interesting page:http://mbed.org/users/chris/notebook/Getting-best-ADC-performance/Apparently one of the software/hardware things I should do is to try and make unused ADC pins digital outputs. This alone must reduce the glitch rate significantly.

author: svofski
author_rank: Regular Contributor
timestamp: December 28, 2012, 08:40:06 pm
content: Just a quick update. I took measurements with a minimal load: only one bargraph with only the "pilot" light on.Original ringing at D1: 40VppThe charge burst at Ctank looks like:And those tiny spikes are actually this (~18Vpp):I bought a 0.1µF film capacitor, EPCOS no idea what type exactly — there wasn't much choice. Looks like a little blue cube. Here's the ringing at the Ctank with added 0.1µF:The ringing at D1 also appears to have reduced a little bit, 32Vpp:Now adding 12? resistor at the gate of Q1, this is what the front at D1 looks like now, 19Vpp, that's 2-fold!And the spikes at Ctank now look just like this: 6VppEven if this is not going to solve all of my problems, it definitely makes the circuit much more well-mannered. Many thanks to all who participated!

author: Phoenix
author_rank: Frequent Contributor
timestamp: December 30, 2012, 08:50:53 am
content: Howdy, thought I'd put my 2c worth in as my first post!The ring looks very high frequency, that suggests to me it's due to parasitic capacitive coupling in the circuit. This can be reduced with better component layout and some extra components. Better to tackle the root cause of the problem if you can.==Components==Gate drive resistor- 2ohm would be smallest based on current rating of driver chip- bigger will reduce dv/dt and thus ring due to parasitic capacitive coupling- your choice of 12ohm looks fineNon-polar capacitor accross C25 and C19 are required for high frequency current (known as bypass caps)- locate capacitors as physically close as possible to inductor and diodeSubbering is a useful addition if the circuit is operating in discontinuous conduction mode (which it most likely is at small load)- an RC accross the FET will reduce the amplitude and frequency of the diode turn off ring, this will need to be designed correctly to be beneficial however-http://www.onsemi.com/pub_link/Collateral/TND396-D.PDFand more with a bit of google==Boost track layout==Hard to see but I can't make out a good high current/low inductance path for the FET Q1 source to GND- looks like they are connected via a thin track to the gnd pins of IC3 and a small via- bigger/shorter tracks usually mean less parasitic inductance- can you put the via under Q1, perhaps 2 vias?- rotate Q1 90º clockwise so the drain pins are closer to the inductor and diodeThe tracks connecting Q1, the choke and D2 could be a lot bigger, there is heaps of room- perhaps a polygon to join them allThe top side plane fill on an switching converter may not be a great idea, as you want to minimise parasitic capacitive coupling. It will also free up space for larger and shorter tracks.Rotate the whole boost circuitry 90º CCW, so the 12V input as at the top left of the group of components, not the top right. The output will still be at the bottom. This will reduce the length of the 12V supply track, and should allow better placement of C19 and it's new bypass cap.==3.3V layout==The tracks for the input bypass capacitor look very small.==Some questions==Does the circuit operate in discontinuous conduction? I.e. the inductor current reaches zero during a switching period?Can you provide a waveform probing the drain of Q1 to gnd, include a full cycle?What is your boost switching frequency?Curiously, why the AC coupling capacitor C23 and diode clamping for the PWMDRIVE?In brief -> shorter tracks, bigger tracks, bypass capacitors.Cheers

author: svofski
author_rank: Regular Contributor
timestamp: December 30, 2012, 03:13:58 pm
content: Hi Phoenix! I really appreciate your analysis of my circuit! Even though I cannot follow each of your advice it's something I should remember for future projects.You are correct about the only one via near Q1. I'm trying to shove a better connection to the ground there. I cannot put it under Q1 since it would be dangerous to the board (I already desoldered Q1 once and homemade boards tend to only be able to bear this much abuse), but I can put it very close. My vias are not that small though, they are not plated but have a 0.5mm wire passing through them.The top plane fill is actually not connected, do you think that it still adds parasitic effects?The boost switching frequency is ~60kHz.I'm reluctant to make measurements now because the board is away from the workbench presently and it's a messy business to carry all these christmas lights aroundI'm not sure how to be 100% sure that the inductor reaches zero but I think it should, wouldn't it be a very inefficient mode of operation if it didn't? It appears to reach saturation though since it's getting rather hot when it has to work fulltime. I found the frequency/duty cycle by trial and error.Just BTW: Unfortunately, my converter appears to be insufficient to provide enough current to fully light up all 20 tubes (I can see that the output voltage drops down significantly and because of the way IN-13 work it results in curious display artefacts). Fortunately, unless I listen to Slayer through a compressor, all 20 tubes never need to light up fully at once.I use AC coupling in the boost drive to make sure that when the PWM signal stops unexpectedly (for example when I program the microcontroller and press reset, not to mention debugging screwups), the MOSFET stays closed and keeps the magic smoke inside. The diode is to reference the AC signal to the ground.

author: Phoenix
author_rank: Frequent Contributor
timestamp: December 31, 2012, 01:57:54 am
content: QuoteTo be honest I'm not sure... that's the trouble with parasitic effects. But the thinner tracks that split direction will add to inductance and ring. For example the junction of the inductor, FET and diode. When the FET is on the current is flowing to the right and up to the FET. When the FET is turned off the current now needs to commutate into the diode, which is down and left through some small tracks. The longer this commutation takes (track inductance) the higher the voltage spike you will see on the inductor. If you made the junction a big polygon connecting everything directly the commutation would be quicker giving a smaller voltage spike on the inductor. As you said, not a lot you can do nowQuoteDiscontinuous/continuous shouldn't effect efficiency that much. It's more to do with specifying the peak current that the inductor will see for the operating condition (smaller inductor is cheaper, but you will get higher peak current and more chance of discontinuous).There are two issues with discontinuous operation. The first is that when the diode stops conducting (inductor current reaches zero) there is generally a high frequency ring, which requires snubbering (RC circuit accross the FET). The second problem is that the output voltage is load depedent. I.e. the more load the lower the output voltage will be. If you want to maintain 150V you will need closed loop control. In continuous operation the output voltage is (ideally) constant. The equations that govern the boost converter operation are shown quite well on wikipediahttp://en.wikipedia.org/wiki/Boost_converterQuoteInteresting idea. I assume it works...

author: mazurov
author_rank: Frequent Contributor
timestamp: December 31, 2012, 08:25:21 am
content: Stop guessing and build yourself an EMI sniffer probe ( Linear Technology Application Note 70, Appendix J ->http://cds.linear.com/docs/Application%20Note/an70.pdf. Bruce Carsten, Jim Williams ).Here the probe is sniffing LT1376-based 500KHz buck converter under 1A load. Lower trace shows converter output, AC-coupled, 50mv/div. A trigger is set to switching glitch visible at second division. An upper trace response of the sniffer probe placed close to pin 3 of LT1376 ( a switch node ). Everywhere else it shows straight line pointing at switch node as a source of the glitch.

author: CarlG
author_rank: Regular Contributor
timestamp: December 31, 2012, 01:14:45 pm
content: Many good advices you've got already. Maybe these below are covered above, but anyways:- The MOSFET driver IC3 (MIC4420)musthave a GOOD decap, with its GND near the source of MOSFET D1.- IC3 GND must be close to D1 source- add ferrite beads on +12V going to IC3 and C19 (maybe one's good enough but consider separate beads)- add filter on +HV close to D2, C25 (the low-loss intrinsic capacitance on the +HV node acts as a sort of unintentional decap across C25)- (the previously suggested) snubber acrosss D1 D-S may be better to place across D2 if its capacitance is dominating (lowest) the resonant frequencyNot EMI related, but a small (10-100 pF; experiment) cap across the U6A comparator 10M FB resistor might give you a more stable function. [Does U6 have any HF decap? I see that there's some 3-pin component close to pin 8 but is that a cap?] I assume you figured it's OK using an OP-amp as comparator, although some (like me) could argue against that;)//C

author: svofski
author_rank: Regular Contributor
timestamp: January 02, 2013, 12:49:48 am
content: Quote from: Phoenix on December 31, 2012, 01:57:54 amThis is interesting, I never really heard of different modes before. I guess my mode is discontinuous because I always set up my system to fully discharge the inductor. Do you know some simple reference implementation of a boost converter running in continuous mode? I'm afraid that if I just increase the switching frequency while keeping the same on-time, I'm going to end up with another burned FET very quickly. These SO8 packaged FETs suck at indurance.@CargG, mazurov: thanks for your suggestions!

author: Neilm
author_rank: Super Contributor
timestamp: January 02, 2013, 07:11:39 pm
content: Quote from: svofski on January 02, 2013, 12:49:48 amThere are several low inductance FEts that fit on a SO8 package. For examplehttp://www.vishay.com/docs/65171/sir846dp.pdfOther manufacturers call the package by different names.They do fit on an SO-8 footprint but if going with a lot of current you will need the pad underneath.I have used something similar at frequencies of 1.25MHz.Neil