switching wall wart with power line sawtooth output ripple?

Discussion in 'Power Electronics' started by michael8, Nov 23, 2015.

  1. michael8

    Thread Starter New Member

    Jan 11, 2015
    18
    2
    Anyone have an idea what's inside the following switching wall wart which
    was supplied for a SB5101 cable modem which takes "12VDC" and at glance
    runs an internal on board switcher to 3.3 VDC.

    Leader Electronics Inc.
    I.T.E. Power Supply
    P/N: 503913-004
    model no: MT20-21120-A00F
    input: 110-127V ~ 60Hz 0.25A
    output: 12.0V = 750mA
    outside of plug -, inside of plug +
    efficiency level: IV LEI-4 Made in China JL Rev 1

    The output seems ok on a DMM (11.7 VDC) but it consists of a sawtooth of
    about .4 V peak-to-peak with a period of about 8 mS (powerline is 60 Hz)
    and a 50 ohm load. (1 V pp with 100 ohm load!)

    It's clearly a switching supply as it's very light weight... Why/How the
    sawtooth at the full wave power line frequency?
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    filter cap dried out.
     
  3. michael8

    Thread Starter New Member

    Jan 11, 2015
    18
    2
    It's clearly some sort of switching power supply so I'd expect the output ripple/sawtooth to be much higher than 120 Hz.

    What's inside that it manages to get the power line sawtooth on the output?
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the line voltage is rectified and filtered to provide the switching voltage for the smps inside. a small electrolytic with a fairly high voltage rating.one I have open on the banch here has two 10 mfd 400 vdc caps for filters inside.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    If the rectifier/reservoir cap has dried up the SMPSU will get full wave half cycles (rough DC) - the regulation circuit will try to put the design voltage on its output, but there are limits.

    Well worth inspecting all the electrolytics for bulged/split tops or any other signs of distress.

    All the secondary side caps have to be very low ESR, the reservoir cap tends to be less critical - but it does have to handle ripple current at the SMPSU frequency as well as 2x mains frequency.
     
  6. michael8

    Thread Starter New Member

    Jan 11, 2015
    18
    2
    Here are some pictures of the insides of the mt20-21120-A00F. There are
    4 magnetics two of which cross the isolation barrier and no optoisolator.
    There is a 33 uF 250 V capacitor on the line side and a 1000 uF 16 V
    capacitor on the load side. Visually both capacitors appear fine.
    mt20-01-label.gif mt20-02-top.jpg mt20-03-bottom.jpg
    With a 50 ohm load on the output and powered via an isolation transformer
    I see about a 5 Vpp sawtooth along with 167 V of DC on the 33 uF
    capacitor. There is also about 400 mV pp at the nominal 12 VDC load.

    The power into the load is (12**2)/50 -> 2.88 W
    2.88 W / 167 V (on line side) is -> 17 mA

    So the sawtooth on the 33 uF capacitor should be:

    17e-3*(1/120.)/33e-6 -> 4.3 Vpp which is close to what I see

    Why doesn't the switcher remove the 120 Hz sawtooth as it has to be
    switching faster than 120 Hz (it has small magnetics)?
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Its a switchmode psu, the sawtooth is on the Primary side, at 120hz thats mains ripple due to the charging and discharging of the capacitor.

    Whats the problem with it?
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
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    33uF isn't much for a reservoir, and since its 120V mains the ripple current will be higher than European units.

    Having said that - I've seen as little as 47uF in a VGA monitor.

    That capacitor could well be on its way out.
     
  9. michael8

    Thread Starter New Member

    Jan 11, 2015
    18
    2
    I think the question is why the 12V output has 120 Hz sawtooth ripple of about 400mV (w/ 50 ohm load). Is this normal for this power supply?

    The design of this wall wart is clearly "different" as there's some magnetics instead of an optoisolator for feedback across the isolation boundary. It's almost possible that the 120 Hz sawtooth on the output is normal for this supply since it's supplying power to a cable modem which has it's own switcher running off the 12 V from this supply.

    Perhaps the sawtooth on the output shouldn't be this high an amplitude. A failing output 1000 uF capacitor would cause this...

    PS: The 33 uF line side capacitor measurements tell me that it's fine, the sawtooth there matches the calculations.
     
  10. michael8

    Thread Starter New Member

    Jan 11, 2015
    18
    2
    I've replaced the output capacitor and still see 0.4 Vpp sawtooth at the power line full wave ripple frequency on the output.

    This likely works since the cable modem contains another switcher to get down to 3.3VDC, but I'm not happy with it.

    I think this supply is just a AC to HV DC and then DC "transformer". It doesn't really strictly regulate the output -- the output sawtooth
    ripple is directly from the HV capacitor sawtooth just divided by the 120 Vin/12Vout ratio...

    There's only 1 transistor Q1 (13003 HV power NPN) and Q2 a diode looking device. I'm a bit uncertain what Q2 is, my best guess is a diac.
    I supplied a variable DC voltage via a 10K resistor to it in-circuit. At around 29 VDC the voltage across it dropped to 26 V.

    Here's two more photos, the first is a trace of the bottom circuit board traces/holes flipped to be a view from the top. I've hand
    drawn the components on it so it's close to a schematic.

    The second photo is a view of the top of the power supply with the goop removed and the HV capacitor unfolded so the components below
    are visible.

    I'm going to abandon this power supply (demoted to parts) and use a different one I've repaired to power the cable modem. It has a much cleaner
    output than the mt20 now that I've replaced it's bulging output capacitors and shorted output overvoltage zener. I can then get my lab power supply back from powering the cable modem.
    mt20-04-top-circuit.jpg mt20-05-top-degoop.jpg
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    400mV ripple would be pretty nasty if it was feeding anything with an audio output. But if there's more regulators in the final equipment, the manufacturer probably decided it doesn't have to be perfect.

    On one occasion I found an Olivetti laptop power brick with the regulation circuit entirely confined to the primary side. Since the laptop had a secondary local regulator for every rail, regulation was unimportant.

    The output voltage dropped fairly rapidly under load to such an extent, I was able to use it directly as a nickel battery charger.

    Never bothered checking the ripple - but the capacitors were smaller than I'd have expected, and looked as if they'd led a hard life.
     
  12. michael8

    Thread Starter New Member

    Jan 11, 2015
    18
    2
    The repaired switching power supply (not the original) has been
    running my SB5101 cable modem for about a year now without problems.

    It's interesting that the problems with the SB5101 were caused by the
    wall wart power supply it came with failing rather than something in
    the SB5101 itself. I wonder how many cable modems (and other gear)
    has been replaced or thrown out due to just a bad power supply.
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    If you think about it - the rectifier reservoir cap has to handle charging pulses at twice mains frequency, and also current draw pulses at the SMPSU frequency.

    In practice; the secondary side filter/reservoirs fail much more often, but the mains reservoirs do fail sometimes - I had to replace the one in my DVD player about a month ago. It was still working, but the eject button gave erratic results.
     
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