TVS Diode with rectifier diode ?

Discussion in 'The Projects Forum' started by urb-nurd, Mar 16, 2015.

  1. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    i am trying to stop the transients on my transformers secondary from charging my capacitor (C2) up to the peak transient voltage.

    The dc-dc converter has a max dc input voltage of 75V, however C2 jumps from 36V to 80V when im switching my load at 30+ KHz.

    I have purchased a unidirectional TVS diode to put across the capacitor in question.
    But i then realised that when the polarity of the transformer output alternates, the TVS would short out C2 every cycle.

    I was hoping i could put a diode in series with the TVS diode to block the reverse polarity of the supply, if this will work that is.

    Then this got me thinking - what about my zener diodes? are they acting as a short when the lower supply line goes negative, causing C3 to discharge and then subsequently re-charge from C2?

    I have noticed the resistor R1 getting very very hot but i had put this down to the potential difference between my 24V C3 and the 80V of C2 - but if C3 is shorting via the zeners every cycle, then the additional current flows through R1.

    In summary:
    Will i be able to prevent my capacitor charging to the transient voltage level if i use TVS diode with a rectifier diode?

    Would it also be recommended to put a rectifier diode in with the zener diode to block the reverse polarity of the supply?

    Last edited: Mar 16, 2015
  2. MaxHeadRoom


    Jul 18, 2013
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    Does the voltage out of the transformer also jump up?
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    c2 voltage owuld short out every cycle? it is a polarized cap, do you really want it to go reverse?
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    I don't see how that circuit works, when the mosfet M3 turns on it will conduct through the 1n4004, and diodes d1, d2 to put R1 across the supply.
  6. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    I believe so, I have discussed the issue with a professor at my university and thought it to be the interaction of the primary and secondary fields. Usually a change in one field interacts with the other and an equilibrium is reached, but switching at high frequencies can stop this equilibrium between the primary and secondary fields being reached causing a spike in the secondary output voltage (I believe, I certainly do not know this for sure - just my best guess).
    I think this jump in secondary voltage is charging my cap up to the level of the switching transient voltage.

    What i mean is that with a unidirectional TVS diode across C2, it would conduct as a standard forward biased diode when the supply polarity changed.
    Causing the TVS (forward biased) to conduct and short out C2 by connecting the bottom supply terminal (going by the schematic) to the positive terminal of the capacitor - thus the +ve and -ve terminal of the cap would be shorted through the forward biased TVS diode.

    The diode is connected incorrectly in the schematic (not on my circuit though). I didn't spot that!

    With the diode across the fan (inductor) reversed, then the resistor is across C3 and C2 - with the resultant voltage across R equal to VC3 - Vc2.
    This is why my resistor gets very very hot when switching - as the voltage of C2 jumps up to 80ish V so the potential across R is 80-24 = 56V.
  7. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    Thanks for the link - ill have a look at that before the morning.

    The reason i asked in the manner i did was that i had ordered a unidirectional TVS and just made the realization that it would be forward biased when the supply polarity inverts.
  8. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    Original post has been updated to show amended schematic.
    Fan diode is now in the correct polarity and the unidirectional TVS is shown across C2.
  9. urb-nurd

    Thread Starter Member

    Jul 9, 2014
  10. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    Also, it appears the the diode is not required !
    i goofed (as usual) with my comprehension of the voltages.