Flyback Zener causing oscillations

Discussion in 'Analog & Mixed-Signal Design' started by patpin, May 12, 2018.

  1. patpin

    Thread Starter Active Member

    Sep 15, 2012
    I tried to sim a part of an ignition system (car coil).
    I include asc and lib.
    When I put a flyback Zener over the IGBT I see oscillations on secondary, which I don't want.
    Further I do not understand why the current through IGBT drain/collector is so low.
    Can someone look at it? Thanks
    Last edited: May 12, 2018
  2. DickCappels


    Aug 21, 2008
    For those without LTSpice available, such as those using a cell phone or tablet.
  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    The type of ignition system using points and a coil also has a capacitor across the points.
    When the points are closed a cureent builds through the coil primary.
    When the points open that current continues to flow and charges the capacitor. The voltage across the coil primary rises to around 300V and this is transformed to ~10kV across the secondary.

    Your circuit has no capacitor and a zener would limit the peak voltage across the primary.
    Also it seems that the RB238T100 is not a zener but instead a schottky diode with a maximum reverse voltage of 100V.
    patpin likes this.
  4. patpin

    Thread Starter Active Member

    Sep 15, 2012
    Thanks for info on RBT . I read the reverse voltage is max. 600V in real life. On my sim it was 1.8KV.
    Max Reverse voltage 600 or 1.8Kv?
    What would be the most appropriate: Zener on the coil or on IGBT?
    And should I go for low Vf and what zener tension? 20V?
    Should I add a capacitor. (In the original ignition (electronic) there was a .1µF 600V condenser over a power transistor.)