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Constant current boost converter problems

  1. Apr 20, 2014 #1
    Hi.

    I want to make a constant current driver for a Xenon arc lamp.
    It takes 12V as input, should give out 8A at 15 - 20V, and maintain 50V idle voltage.

    I used a TL494 to control it, single-ended output driving a boost converter.
    100KHz frequency.
    Schematics attached below.

    It basically works, but there are problems.
    The thing surges and whines in audible range, the input is whole 20A at 12V with output at 8A at 15V, and the current is often a few A above the setting.

    The symptoms are here sometimes, and not here the other times - depending on the load or voltage, or just nothing specific at all.

    What am i doing wrong?

    Tried different inductors, from 10uH to 500uH, capacitors on the current feedback input, chokes - the whine changes tone, but never goes away.
     

    Attached Files:

  2. jcsd
  3. Apr 20, 2014 #2
    The intermittant sounds like a intermittant resistance point due to possibly a poor connection, forming an unplanned RC circuit. I've seen circuit boards do this and usually solved the problem by re-flowing the connections. I don't see any problems on the shematics but if you can identify which component is whining, might help in isolating the signal generation
     
  4. Apr 20, 2014 #3

    meBigGuy

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    It depends on the nature of the irregularity.

    For example, one issue I have seen is what I call "clumping". It is cause by poor layout and is characterized by groups (bursts) of switching pulses during which the current goes way up. I've actually fixed it by re-orienting the inductor (it was talking to pcb loops), but generally it takes more. It varies with load, etc as you described.

    As for the sound, what oscillator frequency is it actually running at? The chart says about 20K. What frequency are you hearing? "Clumping" can cause lower frequency variable audible effects. Not sure what could make audible sounds other than the inductor.

    Have you layed out switching regulators before? I was surprised there was no layout note in the app note or data sheet. Getting the grounds right so there are current loops affecting the feedback signals can be tricky. Google TL494 pcb layout and see what you might want to change. Sometimes you can add some braid to hack the grounds.

    Try a cap from pin 15 to ground.

    Try disconnecting the current feedback and drawing some current at 50V. How stable is it then?
     
  5. Apr 21, 2014 #4
    Clumping sounds about right.

    If i put a 50Ω load at full voltage it would whine with a clean high sound, and the gate would get a regular missed pulse (first attach).

    If i get it into constant current at 2A, then it starts making a much lower tone with some surging, but similar picture of bursts of pulses at semi-regular intervals (second attachment).

    Third attach is the controller board layout.
    The power part is basically a fet and a diode on a radiator with capacitors on both sides and shortest possible wires - i'll take pictures this evening.
    The controller board was powered by a separate battery, so big ground loops are unlikely.

    High, but audible whines.
    The oscillator runs at 100KHz as it supposed to.

    Yes, with varied amount of success or lack of it.
    It's a hobby of mine, so i don't usually get to know all the problems.
     

    Attached Files:

  6. Apr 21, 2014 #5

    meBigGuy

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    I'm not an expert in tracking ground loops in designs. You have them, for sure.

    You have to think about where the currents flow when things switch. That flow causes voltages that, if they get into the feedback path, cause problems such as you are seeing.

    There are all sorts of examples of good layouts (google switching regulator pcb design, etc), but what you are doing isn't even close.
    Here are two
    http://www.edn.com/design/component...leties-in-switching-regulator-and-LDO-designs
    http://cds.linear.com/docs/en/application-note/an136f.pdf

    I would say 50% or more of the design effort in a switching regulator is proper PCB design, with the rest being calculations and component selection.

    You could play with LTspice and add trace resistances and inductances to see what happens.
    http://www.linear.com/designtools/software/
     
    Last edited: Apr 21, 2014
  7. Apr 21, 2014 #6

    Baluncore

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    You have built a voltage booster with a capacitance across the output.
    An arc lamp needs inductance to stabilise the arc current.
    As it is current spikes will probably damage your output capacitor driving the non-linear arc.
    Insert another inductor, but after the capacitor, in series with the discharge lamp.
     
  8. Apr 21, 2014 #7

    jim hardy

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    I'm with Baluncore .

    Discharge lamps are non-linear.
    If I understand , in slide rule days we'd have called this a "Relaxation Oscillator".
     
  9. Apr 21, 2014 #8
    Overview of the build attached.
    I don't see any obvious current loops to interfere with the sensing.

    The lamp refuses to ignite without considerable output capacitance.

    Even so, the "clumping" also happen with a resistive load, and even in constant voltage mode.
    So whatever is wrong here is likely wrong regardless of the arc lamp.

    I think the series igniter acts as such an inductor.
    For resistive load it's presence or absence makes no difference.
     

    Attached Files:

  10. Apr 21, 2014 #9
    Hm, moving big output capacitor negative end to after the sense resistor makes the whine go away.
    It still gives out 8A instead of 5A it's set to, but the input is now 15A instead of 21A.
     
  11. Apr 21, 2014 #10
    Here is what i see across the current sense resistor (with 10nF capacitor at the input), with the arc lamp and current set to 5A (0.5V on the amp input).

    I think it sort of averages to 0.5A, but it does not look good at all, and DC meter shows 8-9A.
    So, no whine does not equal no problem...
     

    Attached Files:

  12. Apr 22, 2014 #11
    Ok, i'm making progress even though i'm still moving ahead blindly.

    I've rearranged the components as shown below.
    There is a 30uF capacitor before the sense resistor and a 100uF one after it, the sense resistor have a 10nF capacitor across it.
    This makes it much more stable, but still far from perfection.

    The driver is set for 5A, and it gives about 5A, give or take one.
    The thing is very sensitive to anything, and seem to settle in one of a few stable states - a bit more current, a bit less current, a bit more whine, a bit less whine.
    An extra pulse from the igniter, moving wires around, or the lamp heating up - many things can make it jump from one state to another.

    Here is a video of it, you can hear the whines and noises it makes if you turn the volume up:


    For reference, there is a high pitch whine at the start, which turns into white noise-ish sound when i move the lamp around, then turns into a series of lower tones when i fire the igniter one more time, jumping an amp up and down for some time, then settle at half an amp above the setting with a very high whine.

    Just in case it's some recognizable issue.

    All in all, i'm at loss what else to try.
     
    Last edited by a moderator: Sep 25, 2014
  13. Apr 22, 2014 #12

    meBigGuy

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    It is ground loops and isolation, bypassing, etc. Did you read through the two example layout application notes I posted?

    Do you get the same issues when you use a resistive load?

    Once it works cleanly for resistive loads you may need to compensate for the lamp load. Others would be more helpful for that.

    I think you are essentially trying to add components to cover up design issues. Sometimes you can get close enough to call it working, sometimes not.
     
  14. Apr 22, 2014 #13
    Yeah, i'm going through them slowly.
    Quite a bit of it is irrelevant, but many things from the second one are.

    For example, i found out that i can reliably switch between modes of wrongness by moving the current feedback wires a tiny bit relative to each other.
    This is so wrong...

    The big question is, however, how to do it right?

    Pretty much.
    It's a bit more stable with a resistor, but same kind of issues remain.
     
  15. Apr 22, 2014 #14

    Baluncore

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    I once tried to stabilise magnetron current by using a vacuum tube as a constant current supply. No matter what I did I could not outrun the hard switching characteristics of the magnetron. In the end I reduced supply capacitance and used an RF inductor.

    You have two problems here. You have a voltage regulator topology with a capacitor, not an inductor close to the load. You have no-low pass spike filter between your current sense resistor and the regulator.

    Once started the arc must continue and be stable, that is the job of a series inductor, probably with an RF ferrite core, not LF iron powder. The current feedback must be low pass filtered to remove spikes faster than the regulator switching rate. That is confusing the slow cycling regulator. It explains why moving your output capacitor across the current sense resistor calms things down a little.

    What type and part number is the xenon lamp ?
    How does it start ?
     
  16. Apr 22, 2014 #15

    AlephZero

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    Looking at your photos, it might be worth reviewing the "design rules" for the layout of the circuits and havign a general tidy up. With wires carrying 8A or more at 100 kHz, you don't need much "random" coupling to create measurable feedback currents and voltages. If everything is solidly "nailed down", at least you might have just problem to debug instead of random variations.

    But sometimes you can solve this type of problem by NOT "nailing things down". We once had an instability problem in a machine generating about 50MW of power, where we calculated the amount of power that needed to get to the wrong place and trigger the problem was only about 5W, i.e. one millionth of the total power available. We fixed that one by reprogramming the electronic control system to continually "waggle the gas pedal" by a tiny amount, which had no practical effect on the machine except that whatever conditions caused the feedback loop never existed for long enough to let the problem develop. That was a lot quicker and cheaper than trying to find where the 5W was coming from, and stop it.
     
  17. Apr 22, 2014 #16

    jim hardy

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    Baluncore asked two very good questions..

    here are two articles on xenon lamps, maybe they'll help

    first one
    http://www.excelitas.com/Downloads/Cermax_Eng_Guide.pdf
    describes trigger, boost and DC phases around pages 20-26
    I found section 4.2.3 interesting, and looking at your current waveform I'd wager that your trouble lies in boost to DC transition.

    second one
    http://ww1.microchip.com/downloads/en/AppNotes/01372A.pdf
    has an interesting description of lamp startup surrounding figure 3 on page 2

    now I've never messed with these lamps
    but your scheme looks to me like it might need a few more parts.
     
  18. Apr 23, 2014 #17

    meBigGuy

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    Regardless of the xenon lamp issues, you need to first fix the instability issues with resistive loads. The other instability issues due to load will generally take a different form and should be attacked later (unless you want to do LTspice simulations with a realistic model of the lamp now).

    There are many different examples and application notes addressing proper switching converter layout. They all have large low inductance ground areas that properly channel the input and load currents. I agree the 2nd article had more pertinent information. There are better ones out there. You read enough and suddenly you see the current paths and what you need to do. No shortcuts.

    I've seen articles on "clumping" or "bunching" also, but I can't find them at the moment. There was one solution that fed back a switching transient into the reference, or something like that (a coworker told me about it). I'll keep looking.

    Try adding a snubber also, just for grins.

    The only time I had to troubleshoot a problem such as yours it was a circuit someone else designed. I added additional bypassing, some ground straps, and reversed the inductor and was lucky enough to get it to work "good enough".
     
  19. Apr 23, 2014 #18

    meBigGuy

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    From an ON semi paper:
    There are two rules of thumb for PCB layouts:
    “short and fat”
    for all power carrying traces and
    “one point grounding”
    for the three different types of grounds within
    a switching power supply. Traces that are short and fat
    minimize the inductive and resistive aspects of the trace,
    thus reducing noise within the circuits and RFI. One point
    grounding keeps the noise sources separated from the
    sensitive control circuits. The three types of grounds are
    the input power return ground, the output power return
    ground and the low-level control ground.
     
  20. Apr 23, 2014 #19

    jim hardy

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    second that.

    Your current feedback voltage looks as if the lamp fires every 60 microseconds
    current reaches a 20 amp peak then decays to near zero on ~25-30 μsec time constant

    and the 494 doesn't pulse again until current goes below 5 amps

    control theory says feedback needs to be faster than the process
    but yours is instantaneous

    if you can risk blowing up a lamp
    or try it with a 2 ohm resistor load

    insert a little bit of RC delay in series with pin 16
    perhaps 100 microsecond time constant for starters
    so that the poor little 494 is responding to time average of its last several cycles, not the instantaneous current

    things should get either much better or much worse.

    but you'll learn something. And if you post another photo, so will I.



    old jim
     
  21. Apr 23, 2014 #20
    Helps quite a lot. :)

    The first one in particular, since it gave me a few ideas - i haven't realized that you might get away with a buck converter if you use a diode-isolated boost for initial ignition, instead of a 64V battery or something impractical like that.
    That might be something to try if all else fails, since a buck sounds like something much more stable in a constant current role.

    Also, nice detailed description on how the lamp starts - no wonder the output capacitors were necessary, without them there is nothing to provide the current between ignition and the driver catching up with it.

    The effect is completely opposite - the current regulation pretty much goes out of the window.
    No difference between 100Ω or 1KΩ or 10KΩ, with 10nF capacitor.

    Tried with 3Ω and 5Ω loads - 5Ω goes up to voltage limit (and 8A), 3Ω goes over the input limit, essentially unregulated.
    Both work almost fine without the RC filter.

    The scope shows a flat line on the cc input, with progressively less noise as the R increases.
    Without it, there is noisy pulsing.

    It's a soviet 150W short arc xenon lamp, ДКсШ-150, runs at 7.5A.
    Starts with an in-series igniter - an HV capacitor discharges into a primary of the ring transformer with the main wire wound around as secondary.

    Not sure what to make of that...
     
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