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current sensors for high frequency converters

Discussion in 'Electronic Design' started by aravind, Mar 22, 2007.

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  1. aravind

    aravind Guest

    hi all,
    I am working on a project which involves, sensing the current
    through the switch (connected in series with the flyback transformer)
    of a flyback converter (max current is abt 15 amp).Operating frequency
    is 50KHz. I have tried to use LEM Hall Effect sensor LA 55-P, but the
    sensor is not able to sense the currents at this frequency faithfully,
    though its bandwidth is about 200KHz (at -1 dB), can anyone please
    suggest anyother sensor, or anyother way of sensing the current
    faithfully.
     
  2. Guest

    You need a current transformer. Do you want to design one or measure
    current once?

    If all you want to do is measure, take a look at something like a
    Tektronix P6042.
    It is a clip-on DC-50MHz current probe. I'm sure there are modern
    instruments like this.

    If you are in-circuit, you can add a low value resistor and sense the
    current through that. Bandwidth of a resistor is quite high ;)
     
  3. scada

    scada Guest

    try www.ohiosemitronics.com
     
  4. Tom Bruhns

    Tom Bruhns Guest


    It's common to use a current transformer, though you should be careful
    about saturation of the core, especially since the switch current must
    have a large DC component in your flyback design. Of course, that
    won't give you the DC value. It's also common to put in a resistor
    across which you can measure the voltage drop to determine the
    current. If you can put it in the source/emitter lead of the switch,
    it will probably be easier to measure the drop across it, since it
    will be referred (presumably) to ground. You are operating at a low
    enough frequency that you should be able to find an appropriate
    resistor with low enough inductance. Assuming the current is 15A for
    50% of the time, the power dissipation in an 0.01 ohm resistor would
    be 1.125 watts. You'd want the inductance of such a low-value
    resistor to be less than about 5 nanohenries, preferably quite a bit
    less, to make it useful out to 200kHz and beyond. One form such
    resistors take is a wide strap of metal with three terminals at each
    end, allowing a Kelvin connection to measure the drop across the
    resistance.

    You can find current transformers (really just small ferrite toroid
    cores with a secondary already wound on them) designed specifically
    for monitoring currents in switching power supplies. They have a
    "flat" frequency response out to several MHz, usually. They add a
    small amount of inductance and resistance to the lead around which
    they are placed, but it's not a lot if you load the secondary properly
    with a low resistance.

    Cheers,
    Tom
     
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