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Oscillations in the gate signal with IXFN 150N65X2 transistors

Discussion in 'Power Electronics' started by Bernat Feixas, Sep 20, 2019.

  1. Bernat Feixas

    Bernat Feixas

    4
    0
    Sep 20, 2019
    We are testing a 300kW converter made with 12 modules of 25kW in parallel.

    Each module has a full bridge of 4 IXFN150N65X2 transistors. The drivers are made with HCPL3120 opto-drivers

    We have dangerous oscillations in gate, at frequencies between 40MHz and 50MHz, which increase with voltage and power. We cannot eliminate them with toroid ferrites, and the only thing that reduces its amplitude is placing a RC of 4.7ohms and 10nF between the driver and the Mosfet, which excessively increases the consumption of the driver.

    In what other way we could improve it?
     

    Attached Files:

  2. WHONOES

    WHONOES

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    May 20, 2017
    Please explain what and how regarding the toroid ferrites used.
     
  3. Alec_t

    Alec_t

    2,703
    717
    Jul 7, 2015
    There is a bunch of components between the HCPL3120 and the FET gate. I haven't tried to work out what purpose it all serves, but it inevitably introduces parasitic capacitance and trace inductance which could cause ringing or pick up EMI.
     
  4. Bernat Feixas

    Bernat Feixas

    4
    0
    Sep 20, 2019
    The ferrites were placed in series to the gate resistors of 4.7 ohms, but without visible improvements.
     
  5. Bernat Feixas

    Bernat Feixas

    4
    0
    Sep 20, 2019
    Those bunch of componets can be divided into two groups:

    1) The resistor of 15 ohms, the antiparallel diode, and the resistor of 4.7 ohm are for a turn-ON smoother than the turn-OFF.

    2) The rest of the components are part of a MOSFET desaturation detector, and the sending of an alarm to the control through an optocoupler in case of saturation.

    We suspect that the problem is not a lack of inductance but excess cable to the gate. We have a circuit with a spacer and from that they leave the cables that go to the 4 transistors.

    The cable could be the error.
     
  6. Alec_t

    Alec_t

    2,703
    717
    Jul 7, 2015
    Very likely.
     
  7. WHONOES

    WHONOES

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    May 20, 2017
    You don't want any additional inductance in the gate leads (the inductor will ring of its own accord
    ) or for that matter, resistance. You should try adding ferrite beads to the gate leads as close to the gat pin as possible. These will present a high impedance to any unwanted oscillation on the gate leads and may well cure your problem. as Alec_t intimated, long leads are a disaster in your circumstance and are to be avoided at all costs.
     
  8. Bernat Feixas

    Bernat Feixas

    4
    0
    Sep 20, 2019
    Could it be a good idea to replace the cable drivers from the PCB drivers to the base PCB with a coaxial, RG174 or similar type?
     
  9. WHONOES

    WHONOES

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    153
    May 20, 2017
    No. Coax will ring like hell unless it is properly terminated which is not practical in your circumstance.
     
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