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General Board Current Monitoring on Scope

Discussion in 'Electronic Design' started by VA, Jul 3, 2007.

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  1. VA

    VA Guest

    Hi all,

    I am a power electronics engineer and work for a company that
    manufacturers multphase synchronous buck converter control ICs (VRM)
    for core power applications (among other things).

    For an analysis that I am doing I am trying to capture on the scope
    multiple phases inductor currents to see controller's current sharing
    scheme. The controller uses a DCR current sense network and the IC is
    voltage mode control. I use a standard Textronic 4 channel
    oscilloscope (TDS 3032).

    http://www.valuetronics.com/Details.aspx?ProdID=4238&Model=Tektronix_TDS3032
    I have tried the following ways

    1. Using a current probe and a current loop near the inductor. I want
    to avoid such a measurement since the loop adds inductances to my
    current path and will interfere with DCR time constants and my current
    sense network

    2. Using a differential probe and measure the voltage across across
    the Capacitor (Cb) of the DCR sensing network. The signal levels are
    in the order of 30-50mV at full load. The offsets in the probe cause
    erroneous readings. If there is a good differential probes that any of
    you recommend, it would be helpful.

    3. Floating Ground measurements: Float the scopes ground and measure
    the differentia voltage across Cb by removing the ground connections
    on a standard scope probe. One end of Cb is close to Vout and hence is
    a quiet point which I can use as a reference. This measurement
    technique gives me accurate results but I cannot observe more than one
    phase current at a time as there is a circulating current path through
    the VRM and the scope that messes up my scope capture.

    If anybody has used any circuit, instrument or have ideas which you
    think can help me....do let me know. Looking for ideas....thanks in
    advance

    With this...I complete my third part of my triology of some of my
    interesting problems....great to find a discussion forum like this.
    VA
    ps:
     
  2. Paul Mathews

    Paul Mathews Guest

    I think you mean TDS3034, since TDS3032 is 2 channel, but it hardly
    matters.
    I generally use TCP202 current probe, having done the circuit layout
    with probing in mind. If not, I put some effort into modifying the
    circuit so that as little inductance as possible is added to the
    switching loop. (By the way, in cases where you're measuring the
    current in a sizable inductance, it may not matter much that you've
    added a few nH in series.) Note that Tek current probes have one jaw
    smaller than the other: put the smaller jaw through a smaller loop.
    If you end up observing voltage drops, HV differential probes such as
    P5205 can be quite useful, even at relatively low voltages. You can
    also use an isolation transformer to power your circuit, rather than
    lifting the scope ground. Note that the combination of a variable
    transformer and an isolation transformer, while very handy for mains
    power supply bench work, presents a rather different source impedance
    to your circuit compared to the mains. If you're then able to ground
    one end of a sense resistor, ordinary single-ended probes will do.
    However, most scopes, including TDS3034, will not have sufficient CMRR
    to prevent displaying lots of spikes that the control circuitry
    doesn't necessarily see. Check this by observing what you get with the
    probe tip grounded. Of course, it's absolutely essential to minimize
    the loop area at the scope probe end. You can also get improved CMRR
    by passing the scope probe coax through a large ferrite toroid or by
    clamping ferrite clamps over it.
    Paul Mathews
     
  3. Fred Bartoli

    Fred Bartoli Guest

    VA a écrit :
    If the needed voltage compliance isn't too high (under 10V) then there's
    a superb diff probe amplifier for you : AD8129-8130. And it'll provide
    way more BW than you'll ever dream.
    A small PCB will the usual surrounding protection stuff for all the
    channels you need and voilà.
     
  4. VA

    VA Guest

    Dear Mr. Mathews,
    Thanks for your ideas.....I will try them out.

    Just a note. Our applications typically use inductors of about
    100-200nH.....and I have seen that even 10nH can screw up the already
    delicate current sharing between multiple phases especially while
    trying to meet Intel's tough VRM requirements. I will give you
    feedback as and when I try it. Thanks

    Krishnan
     
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