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TIDA-00199: shall I use 2.7k instead of 511 Ohm to make a negative voltage ?

Dave Hunson
Expert 1740 points
Part Number: TIDA-00199

hi every one

good day.

TIDA-00199

I was trying to use 511 Ohm resistor for making negative voltage(-8v) side  (R4,R5,R6,R7),

but sadly my aluminum capacitor got over heat within a few hours .

so I replace them with 2.7k it got better ,I am using 150A IGBT module with input capacitor of 13nf.,

my question is : 2.7k is ok for making negative side for that IGBT?

thanks for attention.

Dave.

  • 0
    TI__Mastermind 25590 points

    Dave,

    No I don't believe 2.7k will give you the same or similar results. R4/5/6/7 will determine the amount of current flowing through the zener diodes D1/3/5/7. Lower resistance generally means better regulation but worse efficiency and a slightly decreased regulated output voltage. But 2.7k may be too much:

    13nF at 100kHz gives ~20mA from +15V and ~10mA from -8V. That means we have ~10mA flowing into the mid-point (connecting the resistor and zener) and through the resistor. ~10mA through 511Ω gives us ~5V. That means the zener must be passing enough current (~6mA) to drop 8V across the resistor.

    Increasing the resistance to 2.7k will increase the 10mA drop to 27V which doesn't work to give us our desired voltages.

    Where is your aluminum capacitor?

    -Sam

  • 0 in reply to Samuel Jaffe
    Expert 1740 points

    thanks dear .

    truth be told the condition was a bit different .

    I had 70khz switching and +-15 volt

    I put the LM7824 to test in 24v it got a bit better but still I have heat in my caps.

    I could not figure out why is that happened !!!

    to customize for my application I slightly changed it. I used 

    3SMBJ5929B and aluminum capacitors are very close to the diode zener(DZ16v) and resistor(511Ohm) so it got heat so fast about 30 min.
     
    let's not forget that when I made a distance between caps and diode zener (DZ16v)and R(511Ohm) 30mm the capacitors temperature was fine (35c~45c).
     
    capacitors are near to IGBT G-E.
     
    peak currents => sink 3A/us ,source 2.5A/us
     
    thanks for attention  
     
    I test it with several caps  with 35V,50 V no change...
    in full load (driving IGBT Gate drive) draw current was 42mA.
  • 0 in reply to Dave Hunson
    TI__Mastermind 25590 points

    Dave,

    I see. The electrolytic capacitor is in place of C10. The only reasonable way for the capacitor to heat up on its own is from high ESR and high AC current. Have you tried using a capacitor with lower ESR or replacing with a ceramic capacitor?

    -Sam

  • 0 in reply to Samuel Jaffe
    Expert 1740 points

    dear Sam

    no I did not replace it,
    because I could not find the 330uf tantalum ,so I used 330uf electrolytic  cap along with 10uf tantalum  cap .but I  paralleled a tantalum cap along with electrolytic but  nothing  is changed !

    Dave.

  • 0 in reply to Dave Hunson
    TI__Mastermind 25590 points

    Dave,

    I do not see a 330uF capacitor on the schematic from TIDA-00199. Can you please attach your schematic?

    -Sam