TL494 Duty Cycle

Discussion in 'The Projects Forum' started by dude_500, Apr 30, 2010.

  1. dude_500

    Thread Starter New Member

    Oct 22, 2009
    16
    0
    I have a TL494 oscillator driving a switching power supply and right now the best I can get is 40-45% duty cycle. I know the datasheet says 45 is the recommended maximum duty cycle, but it does mention that absolute max is 50. I've seen people online reference to their TL494 switching supplies and have scope traces of 50% duty cycle, but I can't find any schematic reference to how to force this chip to go to 50% (I could get by with 48-49% probably, but 50 would be best).

    Any tricks on getting this to happen?
     
  2. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    The TL494 is by default a push-pull controller, which is why it has 2 switching transistors.
    It turns on one transistor for some duty cycle, then the other. With 100% duty cycle for a single transistor you still only get 50% because it alternates.
    The two transistors will never be on at the same time, effectively giving you less than 50% duty absolute maximum if you're only looking at one transistor, because there must be some dead time (both transistors off).

    If you want to do single ended (as in you only need to switch one thing or don't need the deadtime) you can parallel the transistors.
    The datasheet actually gives 48% maximum duty cycle, which gives you 96% if you parallel the two transistors.

    Look at the Motorola datasheet:
    http://www.datasheetcatalog.com/datasheets_pdf/T/L/4/9/TL494.shtml

    Figure 17 on page 8 shows how to parallel them.
     
  3. dude_500

    Thread Starter New Member

    Oct 22, 2009
    16
    0
    Thanks much! Shorting them together is behaving exactly as I wanted
     
  4. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    Cool, you're welcome.
     
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