Boost Converter Mosfet Problem

Discussion in 'The Projects Forum' started by jdavids1, Mar 26, 2010.

  1. jdavids1

    Thread Starter New Member

    Mar 26, 2010
    For school we are designing a boost converter with PI control.

    The control part and switching works fine up to the mosfet, when testing the mosfet with a purely resistive load (approximately 200ohms VDD-Drain) the mosfet output followed exactly our gate signal (Approx 40Khz). However once i connect the actual boost circuit (150mH inductor, 470uF cap, 25Ohm load, fast recovery diode). The mosfet switching then stops following the gating signal, instead switching at a rate closer to 150Hz giving us a very large ripple, taxing our supply greatly, etc.

    Does anyone have any ideas as to why this could be happening? Our gating signal is still fine even when this is happening. However it could be seen as being a bit more noisy.

    We are using an optocoupler between the switching and the gate driver, however the optocoupler common reference point is the other circuits common point.

    Any insight would be appreciated.

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    You need to post the schematic for commentary.
    Sounds like a problem, though. If both the IRLED and phototransistor share the same ground, no isolation is being provided. Plus, if the driving signal ground is not referenced to the load ground, then the gate signal is floating.
  3. SgtWookie


    Jul 17, 2007
    Optocouplers can be very slow.

    When you post your schematic, be certain that the part numbers of all semiconductors are included, especially the MOSFET and the optocoupler.

    Please note that .png format images are preferred for uploading to this forum. They are compact, require no extra software to view, and are not "lossy" like .jpg images are.

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