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Measuring DC/DC Isolation Capacitance

Discussion in 'General Electronics Chat' started by Gabriel Sá Pinto, Nov 21, 2017.

  1. Gabriel Sá Pinto

    Gabriel Sá Pinto Member

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    Hi there, I'm working on an EMC Control Plan for a product in my company and I'm trying to simplify all the testing processes as we're doing them in house.
    So there's one EMC that states the following:
    "Secondary power supplies generated by the equipment’s internal DC/DC converter shall respect an isolation of:
    Resistance > 1 M Ohm (see note 1)
    • Capacitance < 5 nF (see note 2 & note 3)
    between the following interfaces:
    • Prim. power hot line/pin and Sec. power hot lines/pins
    • Prim. power return line/pin and Sec. power return line/pin
    • Prim. power hot line/pin and Sec. power return line/pin
    • Prim. power return line/pin and Sec. power hot line/pin
    Note 1) - The resistance measurement shall be done using an Ohm meter. The measurement shall be done for both polarities of the applied ohmmeter. During these measurements, the unit shall be disconnected from power supplies and powered OFF.
    Note 2) - The capacitance measurement is global and shall be done with the test frequency <10 kHz (usually 1 kHz) and according to the procedure described hereafter. The verification might be needed before the DC/DC converter is implemented in the unit’s chassis.
    Note 3) Isolation procedure to measure the DC/DC isolation:
    STEP 1:
    - Connect all primary power lines together and then to the ground
    - Connect all secondary power lines together and then to a capacitor meter with respect to the gnd (frequency < 10kHz) then Cisolation + Csecondary_to_chassis is obtained.
    STEP 2:
    - Disconnect primary lines bundle from ground and inject 1Vpp voltage on this primary power lines bundle with respect to the gnd (keep preferably the same frequency)
    - Perform a voltage measurement on the secondary power lines bundle with respect to the gnd. The output/input voltage ratio corresponds to the bridge attenuation Cisolation / (Cisolation + Csecondary_to_chassis)
    - Multiply the measured capacitor value by the previous voltage ratio and Cisolation is obtained."

    Basically the measurement of the capacitance is made in several steps and through calculations, is there any way the measurement of the isolation capacity between the primary and secondary power lines (and secondary or primary to chassis for that matter) can be done directly with a suited precision capacitance meter?
    I'm having trouble finding suitable information online for this case.

    Thanks in advance for your help.
    Gabriel
     
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    A DC to DC power supply can be complex when measuring capacitance. (C+L+C+L?????)
    I use a standard capacitance meter.
    Just to be certain I measure all four measurements. (the first time follow the instructions) Them after that I short the two input pins together and short the all the output pins together. Then measure input to output. This will give you the highest reading. If that reading is under 5nF then all four will be under 5nF.
    From experiance; the 4 readings will be about the same. And very close to my way of reading all 4 at the same time.
     
  3. Gabriel Sá Pinto

    Gabriel Sá Pinto Member

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    So if I understood you correctly, you don't need to make those steps where you inject 1Vpp in the primary lines and calculate the C isolation through the output/input voltage ratio? You can just measure all four capacitances directly and then measure input to output to see if it checks out, all with a regular capacitance meter?

    Thank you for your answer
     
  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Yes, That is what I do.
     
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  6. Gabriel Sá Pinto

    Gabriel Sá Pinto Member

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    Thank you once again
     

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