Dealing with negative resistance

Discussion in 'General Electronics Chat' started by nsaspook, Feb 26, 2015.

  1. nsaspook

    Thread Starter AAC Fanatic!

    Aug 27, 2009
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    This more of a rant than anything else.

    I'm working on a problem with a circuit that drives a indirectly heated cathode in a ion source. The problem is the switching bias supply (600v 2.6a in current control mode) keeps exploding the output filter caps when the source is unstable when ionizing some gases. The data seems to indicate we are getting arcing (instead of a normal plasma discharge) from the heater plate 20 to 10 that's connected to the arc supply (60v 10a) as the source ages. This condition (while maintaining current control) is feeding back to the supply causing oscillation issues leading to a 'magic smoke' failure that I think is caused by NIR in the power supply.

    http://www.powerguru.org/negative-input-resistance-and-rms-input-currents/

    Typical IHC Ion Source diagram
    [​IMG]
    Does anyone have experience in solving this sort of problem with external output filters. I'm trying a few configurations of common-mode chokes and DC line capacitors before the choke to damp the load variations at the terminal of the bias supply. The vendor (not the supply OEM) has replaced the original internal aluminum electrolytics with low ESR types that I think are only making the problem worse.
    Figure 8:
    http://www.interpoint.com/product_documents/DC_DC_Converters_Output_Noise.pdf


    GEN600-2.6 isolated analog programming
    http://www.mouser.com/ds/2/475/93507100-253078.pdf
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    how about contamination buildup inside? across 24 to 10, causing arcing from the cathode plate? switching cap output filters do not usually explode unless they are exposed to much higher voltages than they are rated for. current out of supply does not usually affect them.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @nsaspook
    You could be right. The Low ESR will allow the cap to last longer at high oscillation + high voltage conditions because it will reduce self heating. The problem with higher ESR caps is that they do not expel their heat well. Kind of a damned if you do, damned if you don't situation.

    One thing you could try is putting a fraction-ohm resistor in series with your low-esr cap. Use the high-wattage resistors with the aluminum case with fins to expel heat quickly.

    http://www.smpstech.com/filter01.htm
     
    Last edited: Feb 26, 2015
  4. nsaspook

    Thread Starter AAC Fanatic!

    Aug 27, 2009
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    The designer of the 'upgrade' has installed an external protection box to clamp the cathode voltage at slightly above 600v along with negative swing voltage protection but it's not working. Contamination buildup is the cause and is normally burned off until we have a 'hard' arc that the supply can handle but before that happens we expect the supply to go into a protect mode, foldback or shutdown with a little shorting so we can replace the source instead of smoking with all the control leds and displays blinking at a high rate.:( The primary control point of the circuit is the resistor (110) Ie beam (74) extraction current. If we need 20mA of current at 60kV extraction the source controller needs to create a set density plasma (a current path for the arc voltage) inside the arc chamber. The arc supply is in voltage control mode so the arc current is set by the heat (~1kW) generated by the bias supply current from the heated filament (30) outside the chamber to the hot cathode block inside the arc chamber. Once the gas heats to the point it starts conducting from thermal energy a normal plasma arc is struck causing arc current to flow, the control loop then adjusts that arc current (by varying the plasma resistance) for the needed beam current from the ionized gas. If we get beam instability the controller tries to correct quickly and those quick changes are causing the bias supply (it's control loop) to internally oscillate causing failure.

    I can't mod the power supply as they are still being repaired under warranty by the vendor. We tried a different brand but the effects are the same (smoking dead supply) so we suspect it's a generic problem with switchers in this application.
    http://www.programmablepower.com/dc-power-supply/XG/downloads/Sorensen_XG_Series_Brochure_2013.pdf
     
  5. nsaspook

    Thread Starter AAC Fanatic!

    Aug 27, 2009
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    Finally got this problem solved with an old fashioned parallel RL filter (20mH on a T107 core/10K ohm 100W) in series to the cathode and a 17mH common-mode filter behind that and the power supply electrically isolated +- outputs. The power supply normally runs in constant current mode but a 'option' in the controller software was enabled to take it into voltage mode to increase filament life by increasing voltage and decreasing current for the same cathode power causing a software Differential Negative Resistance 'Z' curve. That got turned off and the filters damp down changes in current from the plasma NDR behavior to something the switchers can handle. No power supplies smoked in over a month so on to the next headache, life is good. :)
    [​IMG]
    http://www.allaboutcircuits.com/vol_2/chpt_3/4.html
     
    Last edited: Apr 24, 2015
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