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    Switching power supply much lower output than LTSPICE model

    I've been through several iterations of a specialized switching power supply. It underperforms its LTSPICE model by a factor of about 2, and I don't understand why. Here's the schematic:

    Click image for larger version. 

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    These have a selector electromagnet with an inductance of about 5.5H and resistance of 220 ohms. To pull this in takes about 120V at 60mA for about 2ms to overcome the inductance, after which about 12V is enough to sustain.

    So the circuit charges up a 2uf capacitor to 120V while the data signal is 0, and when the data signal goes from 0 to 1, dumps the cap into the output. The cap must be charged in less than 22ms (that's one bit time at 45 baud). The whole thing runs from a USB port, so only 500mA at 4.8V is available.

    The design is standard; there's a transformer (Coilcraft FA2469-AL) connected to ground through a MOSFET, which is being driven by a 50% duty cycle square wave at about 100KHz. On the output side, there's a diode, so the voltage in the caps ratchets up, much like a photoflash power supply.

    All this works, but it takes about 40-50ms to charge the cap to 120V, instead of the 20ms or so that LTSpice predicts. I need more output, and can't figure out how to get it, or why the sim is so far off.

    Things I've tried:

    - On the real PC board, shorting out all that spike filtering and resistance on the high side of the transformer primary. (That's there to protect the USB port of the computer driving this.) That increases the output by about 20-30%.

    - Increasing the MOSFET gate drive. No effect.

    - In simulation, changing the switching frequency over a factor of 2 above and below 100KHz doesn't do much.

    This is my first switcher, so I don't really know what I'm doing. But clearly I'm doing something wrong.

    The project is in KiCAD on Github, here: https://github.com/John-Nagle/ttyloopdriver

    LTSpice model is here: https://github.com/John-Nagle/ttyloo.../ttydriver.asc
    Last edited by andre_teprom; 22nd June 2017 at 13:39. Reason: upload image from external file sharing server

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    Re: Switching power supply much lower output than LTSPICE model

    Please post the zipped *.asc file at Edaboard, or rename it to *.txt.

    You should probe the MOSFET gate and drain waveforms and compare with the simulation results.



    •   Alt13th June 2017, 07:52

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    Re: Switching power supply much lower output than LTSPICE model

    Quote Originally Posted by FvM View Post
    Please post the zipped *.asc file at Edaboard, or rename it to *.txt.
    Just save this link to get the LTSpice model:

    https://raw.githubusercontent.com/Jo.../ttydriver.asc

    No external model files required other than the ones that come with LTSpice, so that should run.
    Not too sure about the transformer, which is just two LTSpice coupled inductors. In reality, it's a Coilcraft FA2469. (Data sheet)

    The MOSFET has an LTSpice model from LTSpice, so that should be reasonably close.

    You should probe the MOSFET gate and drain waveforms and compare with the simulation results.
    I'll do that tomorrow. I've been looking mostly at the capacitor charge waveforms.



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    Re: Switching power supply much lower output than LTSPICE model

    I'm not sure if the github files can be expected to be still accessible after years, if not posting the files on github would violate forum rules about attached files. I imported the file and got unprintable characters for µ needing manual editing, the file seems to be saved as html and is mapping some characters. Another reason to post the *.asc file at Edaboard.

    According to my understanding of the datasheet, the Coilcraft transformer shouldn't saturate at 1 or 1.5 A, unfortunately there's no ∫Vdt or L versus I specification.



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    Re: Switching power supply much lower output than LTSPICE model

    LTspice file attached: ttydriver.txt.

    You think the inductor is saturating? That's a possibility; it's a tiny surface-mount inductor.

    Notes:
    - Was "ttydriver.asc", but I can't upload that suffix per board restrictions.
    - It has µ characters on lines such as "SYMATTR Value 470µf". That's a standard UTF-8 text file and it's how LTspice specifies values.



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    Re: Switching power supply much lower output than LTSPICE model

    At a glance I can't see anything wrong with the simulation. Whatever the problem(s) is, they will fit into two broad categories. Either the switcher is not pulling enough power from the input, or the switcher is less efficient than expected, possibly due to leakage current on the output. Can you measure what the input current/voltage is while the output is charging? Can you show some waveforms of the winding voltages?

    You'll notice that on startup the current draw exceeds 0.5A, which might be causing your current limiting switch to trip. And that's not counting the quiescent current of the other circuitry (especially the 12V isolated supply). What happens if you remove some of the extra circuitry and just test the relevant parts that are in the simulation?
    Last edited by mtwieg; 13th June 2017 at 23:37.



    •   Alt13th June 2017, 23:14

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    Re: Switching power supply much lower output than LTSPICE model

    Have you determined that the USB supply is not the current
    limiter? 120V*60mA/5V=1.44A which seems more stout than
    I recall standard (or older, at least) USB being capable of.
    Heck, I've had external USB HDDs that needed two USB plugs
    to get adequate power.

    Seems like the cap charging should only take 10mA (CdV/dt)
    which might only demand 250mA (@100% eff). But let the
    efficiency slide and you might be on the wrong side of 500mA
    source current.



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    Re: Switching power supply much lower output than LTSPICE model

    Quote Originally Posted by mtwieg View Post
    Can you measure what the input current/voltage is while the output is charging? Can you show some waveforms of the winding voltages?
    Will make voltage measurements at the transformer windings and post pictures of the scope traces, to compare with the sim. I can get a value for primary side current by measuring the voltage across R18.

    You'll notice that on startup the current draw exceeds 0.5A, which might be causing your current limiting switch to trip. And that's not counting the quiescent current of the other circuitry (especially the 12V isolated supply). What happens if you remove some of the extra circuitry and just test the relevant parts that are in the simulation?
    The current limiting device does go into current limit as the filter caps charge. But once the system has started, the current draw should be below 400mA.

    The AP2553W6 acts as a linear current limiter under small or brief overloads until it overheats. Then it turns off until it cools. This is intended to allow big filter caps to charge at startup. It's a physically small part and can't dissipate much power, so it can't act as a linear regulator for long. I've shorted it as a test; after about two seconds the output voltage drops to zero, as it performs its short circuit protection function. If it was going into current limit during operation, the whole circuit would shut down and power cycle every few seconds. It's really obvious if that happens, which it did in an earlier rev.



    •   Alt14th June 2017, 06:32

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    Re: Switching power supply much lower output than LTSPICE model

    OK, as requested, measured waveforms from an oscilloscope vs. simulated waveforms from LTSpice.

    First, cap charging. This is measured between test points W3 and W5.

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    This is what's supposed to happen. The cap charges to 120V in 17ms; then the 120V Zener stops further charging.


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    What actually happens.

    In the 22ms bit time, the voltage only reaches 65-70V.

    Click image for larger version. 

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    With enough time, the cap charges fully, and the Zener clamps it.

    But it takes more than two bit times to reach full charge.

    (Next, the transformer primary waveforms.)

    - - - Updated - - -

    Now here's what the transformer primary is doing. As above, here's the sim plot.

    Click image for larger version. 

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    The two waveforms are the two ends of the transformer primary, relative to ground.
    Looks reasonable. What matters is the difference between those two. (Can LTSpice graph that?) Very stable. What's happening on the secondary side isn't affecting the primary much.

    Here's the real world:

    Click image for larger version. 

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    There's no ground ref here, so this is symmetrical.

    10x probe, across the primary coil. Voltage goes from +40 to -40. The sim goes up to about +45, but only goes down to about -5. Unclear why. Note how the voltage increases over time. That has to come from effects from what's happening on the secondary side, where the cap is charging up. So this is where the model diverges badly from reality.
    Last edited by andre_teprom; 22nd June 2017 at 13:55. Reason: upload image from external file sharing server



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    Re: Switching power supply much lower output than LTSPICE model

    And, finally, what's going on with the secondary.


    Click image for larger version. 

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    Voltage across the secondary.

    Huge excursions on the downward side. Strange effects as the cap charges up.

    Here's the real world.

    Click image for larger version. 

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    Voltage across secondary

    The key point here is that the voltage isn't rising fast enough. It's only at 60v on the high side after 22ms in the real world, instead of hitting 120v in 17 ms in sim.

    So that's what I've measured. Comments? Thanks.
    Last edited by andre_teprom; 22nd June 2017 at 13:57. Reason: upload image from external file sharing server



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    Re: Switching power supply much lower output than LTSPICE model

    You have not answered the question about whether the
    xfmr is saturating. That's a big one.

    Have you any primary current waveforms? Might not be
    that hard to put a sense resistor in the S leg of the dual
    FET package.

    If that C4 of 1000pF is supposed to be some sort of
    snubber, maybe look at that - the topology looks like
    flyback but C-only snubbers are not the done thing as
    far as my limited reading suggests - RC or RCD are less
    lossy. See if you can find any discussion of why that
    cap is there shunting the FET (raises Coss, limits slew
    rate, returns energy to ground - what's supposed to
    be the upside?).



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    Re: Switching power supply much lower output than LTSPICE model

    Just discovered that the default LTSpice inductor model cannot saturate. The "Peak Current" parameter doesn't do anything. Wrote to Coilcraft to see if they have a SPICE model of the transformer. They have SPICE models for their two-terminal inductors, but not their transformers.



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    Re: Switching power supply much lower output than LTSPICE model

    Your transformer waveforms aren't really useful... why can't you connect the scope probe to circuit ground? Should be fine with a USB powered device, at least on the primary side. I don't really need to see the high frequency switching waveform, I'm more interested in the DC voltages before and after your input filter (the voltages on test point W1 and across C5).

    Also agree that C4 is probably hurting you. I would try removing it, or changing it to an optimized RC snubber.

    I doubt that the transformer is saturating, so long as you are applying near the expected pulse width.



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    Re: Switching power supply much lower output than LTSPICE model

    10x probe, across the primary coil. Voltage goes from +40 to -40.
    Just not possible because the MOSFET is clamping negative voltages. Suggests that the measurement results are somehow crappy. Or you have a bad circuit layout with huge parasitic inductances. In the latter case I won't be surprised about erratic behavior.

    Instead of showing waveforms with ms time base, it makes more sense to look at a few oscillation periods in the middle of charge action, see rise and fall times and other relevant details.



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    Re: Switching power supply much lower output than LTSPICE model

    I did a rough calc and got 1.3A for 5uS on time, 5V, 19uH.
    That's close-ish to the 1.5A primary rating on the datasheet
    but I did not see what the basis was, for that.

    Now, if the core is not fully resetting during the "off" time
    (and I see nothing explicitly making that so) maybe there
    is flux walk / staircasing that leads to eventual saturation.
    But flyback converters are kinda outside my scope. Just
    a thing to consider.

    Given that this is an "open source hardware" design (or
    gives that whiff), have you looked into whether others
    have a similar experience and maybe solutions? Just
    because it's on Teh InterWebz doesn't -necessarily-
    mean it's true and completely correct. Or so I've heard.
    Point being if it's so accessible and been tried, maybe
    there's a pool of knowledge more connected to it, with
    more relevant direct experience than our speculations
    are. Maybe you find somebody tested it to the point
    where a load-line is shown. Or improvements identified.

    The core question seems to be why the result does not
    match the simulation. But a more useful one might be,
    does your result match other independent builds' bench
    results. That would be the tie-breaker of choice.



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    Re: Switching power supply much lower output than LTSPICE model

    As requested, transformer primary current scope traces.

    Click image for larger version. 

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    Current through L1, simulated.

    Click image for larger version. 

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    Voltage across R18, actual (inverted).

    R18, 2.2Ω, is in series with the high end of the primary of the transformer. So, per I = E/R, 1v on the scope = 0.45A. We only have about 1A in the real world, where we have about 4-5A in simulation. Now we're getting closer to the problem.

    That 4 to 5 amps comes from C3, the 470uF cap which levels out the load. The input side of C3 only draws less than half an amp in sim, and C14, another 470uF cap, levels out the load on the USB power source to keep it under 400mA. So there's enough power to make this go.

    So what's holding back the current into L1? Something is far different in sim and in the real world.

    Could the filter cap have too high an ESR? I use 2 ohms in the sim. It's a Panasonic ECA-1VM471. Data sheet. 35V, no problem there. Most electrolytics in this size have an ESR under 1 ohm. That shouldn't be it.

    The MOSFET model is one that came with LTSpice for that device. The primary inductance and resistance of the transformer are from its Coilcraft data sheet.

    What am I missing?
    Last edited by andre_teprom; 22nd June 2017 at 13:37. Reason: upload image from external file sharing server



    •   Alt18th June 2017, 02:23

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  17. #17
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    Re: Switching power supply much lower output than LTSPICE model

    OK, so LTSpice's peak current once the load is charged is
    close to what a basic V=L*dI/dt (flipped to dI=V/L*dt,
    where we assume current starts from zero so dI is Ipeak,
    and dt is your 5uS on time, and L is the primary inductance
    as specified). So far, so good - LTSpice is reasonable when
    no output current is being taken.

    Now for a loaded transformer the peak current should be
    well higher. The current taken by the load, times the turns
    ratio. At the beginning of LTSpice sim there's a really big
    spike (=?) and then reasonable-ish 4-5A - so figure 3-4A
    primary, which at 18:1 means maybe 200mA delivered
    (peak) out the secondary? Still reasonable-ish relative to
    the advertised / expected charging.

    Now we can see that the top of the primary sags 2 - 0.5V
    for the duration of the cycle. This will reduce the primary
    current ramp peak against what's calculated.

    Maybe you could keep on going with this 'scope work, in
    more of an orderly "matchup" way - follow the input power
    path, node by node, 'scope and LTSpice probes; put them
    side by side (same scale and base if possible - LTSpice
    will let you do this with trace properties) and you ought
    to plainly see where things start to deviate.



  18. #18
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    Re: Switching power supply much lower output than LTSPICE model

    You're latest observations are apparently referring to a different simulation circuit than appended in post #5. There's e.g. no R18 at all. Please clarify.

    Now for a loaded transformer the peak current should be well higher. The current taken by the load, times the turns ratio.
    The original schematic shows a flyback configuration. Respectively there's no direct current transformation taking place.



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    Re: Switching power supply much lower output than LTSPICE model

    You're comparing those two waveforms improperly. The simulated waveform above clearly shows the peak current in the primary, while the measurement of R18 shows the average primary current. The two aren't going to be equal. You should be comparing that scope waveform to the simulated voltage across R18, which looks something like this when I run it:
    Click image for larger version. 

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    So the start of your waveform matches up fairly well, but at 10ms instead of plateauing around 0.6V (0.27A), the input current decreases and starts ringing until stopping completely at 22ms. Try probing other points in the circuit and see if you see any odd behaviors correlating with that.



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    Re: Switching power supply much lower output than LTSPICE model

    Quote Originally Posted by FvM View Post
    You're latest observations are apparently referring to a different simulation circuit than appended in post #5. There's e.g. no R18 at all. Please clarify.
    The circuit at post 5 has an R18 at line 308:

    SYMATTR InstName R18

    Click image for larger version. 

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    Screenshot in LTSpice
    Note R18 near upper left.
    Last edited by andre_teprom; 22nd June 2017 at 13:38. Reason: upload image from external file sharing server



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