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Thread: TL494 schematic by 0xff

  1. #11
    Quote Originally Posted by expressel View Post
    kanon, you are right that the totem pole must be connected to the source of the high side fet but the first transistor's emitter must be connected to ground because its controlled by the TL494.

    Like this:

    Attachment 6843Attachment 6843
    Oops, I mistaked! you are right kanon!
    This is the proper way:

    Last edited by expressel; 01-12-2019 at 08:38 AM.

  2. #12
    Not the proper way, I confused

  3. #13


    I was waiting for my new scope to arrive until my father told me that he didn't ordered it yet

    Today I didn't had a work and decided to test the bootstrap high side circuit with my old crappy scope at low voltage - 15V.

    I decided to make a notebook adapter for car.

    I soldered this schematic and calculated the transformer with ExcellentIT (2 primary turns, 10 turns secondary, I wound 11 turns secondary).

    I expected to have 5A consumation with a 4.7 ohm load when I saw that its only 1.2A and the output voltage drops from 19V to 6.2V
    Why this happened ? I thinked a lot and didn't figured it. Ideas are welcome.

    I used 4x 1uF capacitors in parallel to convert from DC to AC. This is 0.8 ohm reactance at 50khz.

    I didn't used snubber because I forgot.

    Here's the schematic and waveforms:

    This is the high side waveform at the gate:

    This is the waveform at the transformer:

    Last edited by expressel; Yesterday at 06:56 PM.

  4. #14
    Here is also a screenshot from the ExcellentIT:

  5. #15
    .... Silvio's Avatar
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    @ Expressel

    Your calculations seem ok to me. I think you should try a few things
    1) try to take the sense voltage before the output inductor.
    2) check the input voltage at load across the transformer primary. See that its not dropping too much here.
    3) The wave shape at the transformer primary is not so nice. This could be due to the output inductor. You may have to use a snubber across each output diode to suppress it.
    The wave form can be disturbed due to incorrect coupling within the transformer (too much leakage inductance) I know winding 13 strands is not an easy task, try using a copper sheet instead. If you get hold of some copper tape although its thin (you can always sandwich a few layers on top of each other) is another alternative to copper sheet for the primary windings.
    4) Try to cover the whole width of the bobbin with the windings for good coupling
    Lastly I know sandwich the secondary between the primary is not an easy task if you are using wire strands side by side. However you can bundle your primary wire in 4 bundles of 3 wires each making a total of 12 strands. here you will have 4 bundles side by side to handle during winding. Check that they will fit in one layer.

    I hope this will help you solve the problem

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