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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:

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

  2. #12
    Not the proper way, I confused

  3. #13

    Question

    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; 01-18-2019 at 06:56 PM.

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


  5. #15
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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

  6. #16
    Quote Originally Posted by Silvio View Post
    @ 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. This is not the problem because I tried without feedback too.
    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
    I dont understand everything you said.

    1) What sense voltage ? i suppose you mean to take the feedback before the inductor.
    2) You mean to check the RMS voltage at the transformer primary ?
    3) I think i dont need snubbers for the diodes because I saw this in only one schematic.
    How to use a copper sheet ?
    I have copper tape, how to sandwitch layers ?
    4) Is it necessary to sandwitch the secondary between the primary ?

    Do you know that I didn't used wire strands and maybe thats the problem.
    I used only single wire 0.5mm just for the test. I suppose it has higher inductance and didn't allowed more current to flow at 50khz

    Thanks Silvio, keep the good work and see this amazing schematic I invented -> 400khz switching load because I dont have a load to test my supplys (I only have 4.7ohm 15W resistor and 1x 55W halogen bulb):

    Will control it with STM32F103 pcb from aliexpress, with OLED display and rotary encoder. Im little lazy to write the code but I'll skip the bad.

    Last edited by expressel; 01-20-2019 at 04:55 PM.

  7. #17
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    Hi Expressel using only one strand of 0.5mm and making two turns will not couple enough. You should have known better that you should have used 13 strands according to the Excellent software. The current density with one strand is too high and a lot of voltage drop will occur.
    I realized that with only 2 turns on the primary it would be far better to wind the secondary first in the bottom with wire strands and winding the primary on top with COPPER SHEET cut to to the width of the bobbin. This will give you the best practical winding way with good coupling. Keep in mind that a good manufactured transformer with good coupling and minimum leakage inductance is the key to a successful SMPS.

    Regarding the feedback yes I meant after the output diode before the inductor.

    To gather everything up you should start with a decent wound transformer first then we see if you have further problems later. Just remember that current has to pass through suitable cross section and this will not let you get away with regardless what is the switching frequency. As a rule of thumb you need a current density of 4A/mm for continuous duty while around 6A/mm for 50% duty and 8A/mm for 50% duty helped with forced cooling.

    For loads you should visit the rubbish dump or bring in site and look for old heaters. You can acquire some heating elements that you can load with. Old microwave ovens with grills in them will have a nice 1Kw heater usually with 2 120v filaments in series.

    Good luck Silvio

  8. #18
    I found the problem Silvio, It was the capacitor which converts from DC to AC.
    Today I tried with 150uF electrolytic and now it draws 4.4 amperes.
    However, this capacitor gets hot and I must use full bridge instead.. ( I dont want to use push pull )
    You can use 1uF only for 220V smps-es.
    Regarding the feedback, it must be after the inductor, not before. It works far better this way.


    Thank you for the assistance.

    The waveform now is more clearer, look:

    Last edited by expressel; 01-21-2019 at 08:24 PM.

  9. #19
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    Well that looks better, As for myself I would have used a flyback for a such a converter. It would have been much simpler yet effective.

  10. #20
    Flyback, yes, but the idea was to test the high side bootstrap drive.
    And also I can't use EI33 if I use flyback. See:



    Look what mosfet drive circuit I achieved, 1Mhz freq:


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