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Boost converter switch node spiking

  1. Oct 14, 2013 #1
    Greetings.

    I'm making a high power boost converter, supposed to deliver constant current at 15-50V from about 12V input. 100KHz frequency.
    xb-sch2.png
    There are problems, however.

    There was a lot of ringing at the switch node - meeting point of the diode, inductor and MOSFET.
    5.8Mhz clear decaying sine wave, got rid of the most of it with an RC snubber - 5Ω, 10nF.

    There is still, however, a big peak at the moment the switch closes.

    15.5V 7.75A out, 12V 10A in (20V vertical)
    bc-10a.jpg

    21V 10.5A out, 20A in.
    The spike is already at nearly 60V.
    bc-20a.jpg

    At this rate the spike will exceed the 100V rating of the transistor well before the 100A maximum input current i was aiming for.
    There is a TVS diode across it, but i doubt it would help much to prevent the explosion.

    Is there any way to get rid of this spike?
    I suspect the devil is in the layout, but i'm not good enough to determine just what it is.

    Here is what it looks like, input on the right, output to the left:
    http://orbides.1gb.ru/img/bc-side.jpg
    http://orbides.1gb.ru/img/bc-top.jpg

    I tried moving the wires around, with no noticeable effect, and there is not much i can do to shorten the wires.
    The MOSFET package is claimed to be "Low internal inductance", so that parasitic inductance must come from somewhere else, right?
     
  2. jcsd
  3. Oct 14, 2013 #2

    meBigGuy

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    The diode needs to be a schottky diode. It is creating the spike when the switch opens. At that point the inductor slews to whatever voltage it needs to maintain its current. Since a normal switching diode has stored charge, it takes time for the diode to turn on. The other problem with non schottky diodes is that they appear as a momentary short when they are reverse biased (huge current spike through diode when the switch turns on)
     
  4. Oct 15, 2013 #3
    Does not seem to help much.
    Placing another diode in parallel with the existing one reduces the spike ever so slightly, but actually replacing the existing one with a schottky (40CPQ060 to be exact) makes the spike worse.

    There is a slight reduction of the spike if i screw the existing diode directly onto the MOSFET without the wire, but it's only about 5% or so.

    Also, the spike happens when the switch closes, not when it opens.

    40ns reverse recovery time on this one, you can't do much better than that.
     
  5. Oct 15, 2013 #4

    meBigGuy

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    I'm confused then with respect to what your scope trace is. I assumed it was the voltage at the switch-inductor-diode junction. When the switch is closed that voltage is 0, then it jumps to the output voltage when the switch is opened. It then collapses (with ringing you suppressed with the snubber) to zero when the inductor current dies out. When it jumps to the output voltage it overshoots because the diode doesn't turn on right away (or some oither problem).

    One way to determine whether wiring is affecting this is to move the ground trace of the scope. If you get radically different spikes grounded at the output vs grounded at the input the there is a layout problem. (or measure voltage from input ground to output ground)

    Another very real possibility is that the inductance of the output capacitor is too high. Is it a low esr capacitor (very important)? Try a 100uF low esr capacitor and a 2.2uF ceramic in parallel with the 6800uF output capacitor. What spiking do you see on the output capacitor itself? If it shows no spiking then that is not the issue.
     
  6. Oct 15, 2013 #5
    The traces above are measured across the mosfet, with probe and ground connected directly to it's bolts.
    Assuming switch=mosfet, the voltage is at the output level when it's closed, and drops to zero when it conducts.
    As the switch closes there is a spike.

    The ringing was that spike being a decaying sine wave.
    The current should not die out - it runs in continuous mode at any interesting load.


    Moving the ground around do not produce any drastic changes.

    Across the output capacitor there is a bit of bursts of ringing, +-1V or so, coinciding with opens and closes, it does seem to reduce if i add various small capacitors to it.
     
  7. Oct 15, 2013 #6

    meBigGuy

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    We are confusing electrical terms. When the mosfet conducts == it is on == the switch is closed. When the mosfet turns off == the switch is open == mosfet doesn't conduct. That is pretty much the standard terminology.

    So, the spike is definitely caused by the inductor trying to maintain its current when the mosfet stops conducting. If there is no comparable spike on the output capacitor, then the voltage is across the diode and is caused by the diode not turning on fast enough. You can prove this by slowing down the turn-off (non-conduction) of the mosfet. But be careful. If you slow it too much it will fry since the currents are so high.

    You never mentioned the timebase of the waveforms.
     
  8. Oct 15, 2013 #7

    meBigGuy

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    Maybe you need a faster diode, although I'd expect the schottky you tried to be fine. Try the Vishay TMBS series. They are optimized for high frequency.

    If the lead inductance of the diode is the issue you will again see different voltages at the diode leads (input and output). But, I expect it is just too much too fast for the diodes you have chosen (although I really don't see a problem with your choices).

    You could try a capacitor across the diode just to see what it does to the spike.

    I haven't worked with switching currents as large as you are dealing with.
     
  9. Oct 15, 2013 #8
    Ah, sorry about the confusion - english is not my primary language.
    Traces are at 2us, 20V per division.

    I never seen forward turn on time mentioned in diode datasheets.

    I tried 40CPQ060 schottky and 10sq045 schottky barrier - both reduce the spike ever so slightly if placed in parallel to the big one, and increase the spike if placed instead of it.
    So, either these are just as bad or there is something more going on there.

    One thing i can think of is using synchronous rectification, but i have no clue how to find the right IC for the job.
     
  10. Oct 15, 2013 #9
    Now, that did something.

    Things started happening at 100nF, film cap across the diode.

    Before:
    FET, 2us, 10V.
    bc-fet-before.jpg
    Output, 2us, 1V.
    bc-out-pre.jpg

    After:
    FET, 2us, 10V.
    bc-fet-after.jpg
    Output, 2us, 1V.
    bc-out-after.jpg

    Not all the way, making different oscillations, and heating the capacitor noticeably, but feels like progress.
    Is there any theory behind what capacitor to choose, and how much heat to expect on it?

    And where can that new ringing be coming from?
     
  11. Oct 15, 2013 #10
    One more thing - it's the ESR/ESL of the capacitor that matter, not it's value.
    Same 100nF, but as a quality X2 capacitor:
    bc-fet-after-x2.jpg

    Looks almost perfect, spike-wise.
    However, i don't like that low frequency ringing that is now all over the place.
     
  12. Oct 15, 2013 #11

    eq1

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    The problem is the turn on time of the diode is too slow. When the FET turns off the inductor must still conduct. But there is a finite amount of time for the diode to go from non conducting to conducting. So the inductor just keeps pushing charge into Cj of the diode until it can be drained out. Because Cj is small and I is large there is a lot of V. (You can confirm this experimentally by varying the Idc in the L and observing the peak change. If you have a load R vary it.) By putting a cap in parallel with the diode you increase the C but Q stays basically the same so there is less V. (Note: you have to be able to access the C. If there is excessive ESR or ESL the C cannot be accessed by the high frequency I induced by the switch.)

    In general this is not a preferred way to damp the peak compared to a switch node RC snubber because I think you'll notice the efficiency of the conversion just went way down.

    The snubber should've worked. It's purpose is also to provide an escape path for the inductor current while the diode is turning on (some people think of it as an AC short, or damping the LC, its all just different sides of the same coin in my opinion).

    Try using this technique to pick the snubber RC with the Schottky. I've used it plenty of times and it always works for me.
    http://www.ti.com/lit/an/slva255/slva255.pdf
     
    Last edited: Oct 15, 2013
  13. Oct 15, 2013 #12

    eq1

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    I just looked at the images of the converter you constructed. It's actually kind of beautiful in my opinion. But you'll never going get the parasitics down low enough in that setup for these switch times. You're definitely going to need the snubber.
     
  14. Oct 15, 2013 #13
    That's exactly the article i used to pick the snubber.
    It worked, turning a raging ringing into a little spike.
    But that little is still a lot.

    Hardly way down - the capacitor barely heats at all, and the efficiency remain around 85-90% all the way into KW range.

    What troubles me is what this capacitor adds.
    The spike is not actually removed completely by it - it is reduced, but on top of that there is a new wave in there, and a spike that follows it.

    Can't quite describe it, so here is a video.
    Input going from 10A to 75A, then back down (AFAIK the switching current ripples +-5A from input one, based on F and L used).
    20V/2us per division, across the MOSFET
    https://www.youtube.com/watch?v=mHgYDOsXigA

    It's still getting uncomfortably close to the breakdown voltage, and now it's not a mere spike, but a big wide interval.
     
  15. Oct 15, 2013 #14

    meBigGuy

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    I have a problem with the schottky diodes behaving worse. Makes me think it is inductive effects of your wiring.

    Adding a capacitor across the diode is not really an acceptable solution. I just suggested it to see if it, by providing a path for the inductor current, verified that the problem is the diode turn on. But it has other side effects that mask the data.

    With the capacitor in the circuit you now have a series resonance which is causing a sine wave superimposed on the current ramp. As that beats with the controller it sometime switches when current is low, and sometimes when current is high, therefore sometimes no spike, sometimes huge spike.

    I suggest the following:
    1. With the probe ground in 1 place (at the input or mosfet ground), probe *all* points in the circuit and see if any spiking is caused by ground or lead inductances. The reason I suggest this again is that a schottky is supposed to have no reverse recovery time. If that is the case, then wire inductances in your free-air layout must be causing the spiking. The diode has a specified inductance. How does that jive with the spike you are seeing. Probe both sides of the diode as close as possible to the package. If the spike is less on the input side, then it is the lead inductance. You are looking for any changes across a wire. Again, probe both sides of all wires.

    2. Slow down the turn-off time of the mosfet. This will increase the power in the mosfet, but give the current a place to go while the diode turns on. It will reduce the spike.

    3. Go to sync rectifier, as you stated.
     
  16. Oct 16, 2013 #15

    eq1

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    In addition to what mrBigGuy said, maybe try moving the snubber. From the pictures I think it is connected directly across the DS of the FET. This means the snubber and FET are sharing a return current to the input cap in that heavy wire. There can be ~150A in that wire so even a small ESL there will make a big difference. Maybe try removing that snubber cap from the S of the FET and connect it to the input cap ground directly in its own wire. Do your best to keep the snubber wire length as short a possible.

    Also, make sure the snubber has a low ESR and ESL cap in it as well.
     
  17. Oct 16, 2013 #16

    eq1

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    You might also try adding the Cout-byp that the TI app note recommends. Connect it directly to the cathode of the diode and give that cap it's own wire to the ground pin of the input cap as well. Again, do your best to keep that wire short and use a low ESL cap if possible. The 100nF thin film will probably work well.
     
  18. Oct 16, 2013 #17

    meBigGuy

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    Actually, the more I think of it the more I think it is inductive effects at your high currents. I just don't think a schottky forward recovery time can be that large, and using shottky made it worse (which could have been wire related)

    V= Ldi/dt
    To get 50V in 300ns given 50A only requires 300nH. 1 inch of straight wire in free space is about 40 to 50nH. (http://www.consultrsr.com/resources/eis/induct5.htm has a calculator)

    You can decrease inductance by going to wide braided wire, multiple wires, using a ground plane or by shorter wires.

    I really think you should be able to see the spike levels change as you probe different points.

    Do you plan to lay this out? If not, maybe you should build it on a sheet of heavy copper covered pcb material
     
  19. Oct 17, 2013 #18
    A capacitor across the diode appear to only be effective at lower power - higher up it gets overwhelmed by the trapezoidal peak.

    Moving the snubber to input ground makes a bit of a difference, but not really significant. Having two snubbers - one on DS, other D to input ground seems better than either alone.
    High frequency noise on the diode is cleaned up by adding an RC snubber across it - 3300pF, 15Ω. That one also contribute a few percent reduction to the spike.
    Changing all snubber caps to MKP X2 shaves off another percent or two.

    The biggest effect, however, is adding a capacitor from the output end of the diode straight to the input ground - 2uF MKP X2. That reduces the spike at the MOSFET about as well as the capacitor across the diode did, but without additional ringing. The bigger this one is - the better, but above 1-2 uF it no longer change much.

    Bad news about it is - at high power it heats a lot, and the spike turns into a full scale calamity.
    20V/2us per division, across the MOSFET, 10A to 80A input:
    https://www.youtube.com/watch?v=FJG2j0JfTXI

    To me that looks worse than the ringing with a cold capacitor across a diode.

    For the reference, here what it looks without anything at the diode.
    20V/2us per division, across the MOSFET, 10A to 50A input:
    https://www.youtube.com/watch?v=_9-fS9Ljiaw

    So ultimately, all these diode shenanigans are quite pointless at anything above the lower power levels.
    It seems to me that there are two spikes superimposed over each other - one is small, and is filtered out by various ways of putting a capacitor on a diode, and the other is big and growing with power, and is not affected by anything i do.

    I was expecting current layout to be final, but it seems like there is no fixing it like it is.
    Any advice on how to lay it out? I can't quite visualize it on a PCB when even 4mm wires are warming up.

    I do have a 2mm thick sheet of copper, so maybe use it as a ground plane, and mount everything in it's line?
    Kind of like this:
    xb-nlay.png

    Any advice on what sort of IC to look for?
     
  20. Oct 17, 2013 #19

    Baluncore

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    Your input capacitor at 100,000 uF is unnecessarily huge with lots of inductance. I am surprised it is needed. Surely 1000 uF would be more than enough. If your input leads are very long then a Pi input filter should be a better investment as it will offer lower impedance at the switching frequency.

    The 6800 uF output capacitor is much too inductive for that application. It needs to be paralleled with a small fast high current capacitor to catch the switching edge. Big capacitors are not better for spikes, they are far worse. Big capacitors are good for ripple. Use two different capacitors for the best of both worlds.

    Since the transition that causes the problem involves only the 20 uH inductor and the fast diode the best way to snub the spike is to put a capacitor across both those components. That would be between the (+)input and (+)output and would divert the spike from the ground inductance into a much smaller current loop. The capacitor need only handle the spike so an 0.1uF disc ceramic, possibly in parallel with a metallised polypropylene suppression capacitor should eliminate most of that spike.
     
  21. Oct 18, 2013 #20

    Baluncore

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    Re: Metallized Polypropylene Film Capacitors.

    Take a look at the table in this document;
    http://www.cde.com/catalogs/935.pdf
    You will see ripple current specified in the table.

    13A, 48V output suggests an input current from a 12V supply of about 50A flowing through the inductor and diode.
    You will need a few small caps in parallel as your spike catcher and/or output filter.
    They are available from Digikey in USA.
     
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