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TL494 Latching up

maurice shelton1
Prodigy 20 points
Other Parts Discussed in Thread: TL494

 I have inherited an old power supply design using a TL494 driving the primary of a transformer in push-pull mode.  Recently we have had several board that when VCC power is applied the TL494 output transistors just won't start switching.  The Osc at pin 5 always starts when VCC is applied and all the control voltage seem correct.  But the output may not start switching for a few mSec to several minutes.  If power is turned off and then back on quickly everything looks normal but if power is left of for a few minutes the part is likely to be latched up again.    

  All the failed units have the same date code '32C3NRK'.  Has this been reported as a questionable lot?  This design did not use the soft-start, DTC is tied to ground and they are pushing a 4.4v level pulse signal into the feedback pin 3 to stop switching while some sensitive signals are measured.  I don't like that approach, but I also don't see why that should be causing the problem.  

Any suggestions?

Thanks 

Maurice

  • 0
    TI__Guru**** 191445 points

    That is a 30 year old device.  It may be difficult to find anyone familiar with it.  You could go thru your distribution channel to get one or two of the failed devices submitted for failure analysis.  I think that would be my first step.  Can you tell me about the history of the design?  Can you provide your schematic and some waveforms that illustrate your problem?  I'll see if I can locate someone familiar with that device.

  • 0
    TI__Guru**** 164976 points

    Maurice,

    If pin 3, feedback, is within the range pin 5, CT, saw tooth waveform then the output should switch. Also verify that Vref is 5V. Is pin 13, output ctrl,  tied to ground or Vref? If the signals are good and the outputs do not switch, then this is  failure mode I have not seen before.

    30 years in production and many more to come.

  • 0 in reply to Ron Michallick
    Prodigy 20 points

    Thanks for the reply Ron,

      The level on the feedback pin 3 is low (0v), CT pin 5 is a 0 to 3v saw tooth 66KHz signal.  VRef is a good solid 5v. OC pin 13 is tied to vRef for push-pull.  But once VCC is applied, the switching start is very intermittent.  Some times it start right away and some times it doesn't.  I will try to attach a file showing the waveforms.  If you can see the pdf file, C3 (blue) shows the 10v applied to VCC pin 12, the green and red traces show the switching starting 150mS later.  Sometimes the switching start right with VCC, but sometimes it doesn't start at all.

      This is happening with about 20% of are current test units.  The other 80% always starts reliably.

    Any help or suggestion you could give would be most welcomed,

    Maurice 

    6837.VCC to Pretrig 1.pdf

  • 0 in reply to maurice shelton1
    TI__Guru**** 164976 points

    Maurice,

    With Low FB voltage with a good oscillator and Vref, output pulses should be immediate and reliable. I suggest returning some failing devices to your distributor.