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Thread: TL494 Lab PSU

  1. #1

    TL494 Lab PSU

    I've started building a lab PSU using the TL494. The schematic is attached below, it's missing a lot of things but I'm still testing stuff on the breadboard.
    The problem I have is with the gate drive transformer. When I connect it, it pulls a lot of current through Q1, which quickly heats up. It's made of a green toroid with trifilar windings, 16 turns each (minimum calculated is 13 turns). Inductance is around 945uH. Frequency is 70KHz.
    Is it something stupid that I've done or is it the core that's not suitable?

    IMG_20180317_005414.jpg
    Attached Files Attached Files

  2. #2
    .... Silvio's Avatar
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    Hello Bogdan,

    It could be few things that are making Q1 heat up, it is either there is not enough gate current to it and the fet is running in linear mode. The inductance of the gate trafo in not enough, you could try a series resistor to lessen the current through it and lastly Q1 cannot handle the current through it. I suggest also try to put a small heat sink to it and see how it performs.

    I did not check what Q1 is and its parameters. Usually it needs 10v and over at the gate so that it can switch fully on quickly without entering the linear region.

    Check your wave forms with the oscilloscope to see the rise time etc. Check also the wave form at the primary of the gate trafo to see what actually is arriving there, you need a good square wave so that it can produce a good wave form at the secondary windings.

    I hope that helps

    Regards Silvio
    Last edited by Silvio; 03-17-2018 at 12:28 AM.

  3. #3
    I've used the FQP50N06 transistor (60V/50A). What I've noticed is that in single ended mode the TL494 gives a 90% duty cycle square wave with the DTC pin grounded (image 1 shows the signal at the gate of the mosfet).
    So I've added a trimpot and fixed the duty cycle to around 50% (image 2). That didn't change much, maybe the current was slightly lower but still too high.
    Then I put the transistor on a heatsink and used a small GDT from an old power supply. This one doesn't pull that much current, but the heatsink still gets considerably hot after a short while. Image 3 shows the primary and secondary waveforms (the ringing is mainly from the wires). Anything higher than 50% just pulls the supply into protection.
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  4. #4
    .... Silvio's Avatar
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    Hi Bogdan, Try to take a shot between drain and source to see the wave form there. I still cannot believe that mosfet is getting hot. That should run cool without a heat sink. I think it is still the fact that its not driven hard enough. Try to make amplifier with a couple of transistors feeding the gate of the fet. The TL494 is not so hefty when it comes to drive current

    When you are talking of current you did not mention how much. I believe that it should run in the region of 100-300 mA without load.

    You can also try to put a small resistor in series and measure across it to see the current peaks with the scope. This will give you a better idea of what is going on. It is also the norm to put a series resistor with the gate trafo primary to limit current. Try some values and see what are the results.

    Lastly if all looks well there might be an issue with switching losses and core material.

    I guess the trafo from the old psu was meant to work at around 20Khz

    Try lowering or increasing the frequency slowly and see how it performs.

    I hope this helps.

  5. #5
    I made a simple buffer and connected one of the windings of the toroid to it. I can't believe how well it works. Only around 20mA with not load on the secondary.
    Primary waveform:



    Secondary waveform:



    Circuit:



    On the other circuit, I tried messing with the frequency and duty cycle and burned the transistor (short circuit between drain and source). So something was definitely wrong with that one.

  6. #6
    .... Silvio's Avatar
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    I am glad you found a solution to your problem. It is now a question of drive power from that buffer stage. I do not know what power are you seeking from the lab psu. Well that I leave to your judgement. You can take some ideas for old computer power supplies and you can judge well from some comparisons.

    Good luck

  7. #7
    I'm looking at 10A limit, or even 5A will do. I'm more interested in higher voltages, 50-60V.
    Don't know about the driving power. But maybe bigger transistors such as BD139/BD140 would work if needed.

  8. #8
    .... Silvio's Avatar
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    Hi Bogdan,
    Well at that voltage I would rather go for 7 amps at 60v at the most (420w) this is what a medium power fet can go. Try to be modest at first as the more power the more problems will arise. You will be needing an ETD39 more or less or something equivalent for that power.

    I would divide the voltage if I where you thus having 30-0-30 on the output. It will be useful when testing audio equipment. It will also be a bit difficult to go down to low voltages with high power at a high input voltage as the fets tend to switch in the linear mode down there. Another option is to switch between with more tappings on the traffo so at low voltages you can still maintain a good output and the pulse width will not be to narrow. I am talking through experience here as this happened to me.

    Well with the buffer stages I think the BD transistors will be a better choice however this depends on the fets used.
    Regarding fets try your tests with low cost fets such as IRF740 these are quite capable for the load wanted if not held at high loads for long periods. These fets are easily driven with around 200mA. Use a separate supply to drive the pcb as at low voltages an auxiliary feed from the main trafo will not supply adequate voltage. Try to buy genuine fets as the ones on ebay are mostly fakes and cannot withstand even half the load. Take a look at the video on Youtube by me there is also a blog post here about the subject.

    Good luck and happy experimenting

    regards Silvio

  9. #9
    Thank you for the really good ideas! I will consider the 30-0-30 configuration as I will definitely need it for power amps. That's about the only power-hungry application I can think of.
    I do have the IRF740 and the IRF840. For 2-switch forward I think the 740 will do fine. I'll obviously get genuine parts once I'm confident with the design!

    For center tapped windings the main issue that I can think of is current sensing. I will have to investigate how that could be implemented.
    Last edited by bogdan2011; 03-18-2018 at 07:39 PM.

  10. #10
    .... Silvio's Avatar
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    Hi Bogdan, There are a few options for you to choose, Make a stationary switching psu at the back with center tapped windings, the two separate outputs can be adjusted with a couple of buck converters these have adjustable voltage and current. There are quite a few to choose from on ebay as its not worth building them as they are rather cheap being around 3 pounds sterling.

    The other option is to build two linear regulators with current limit. If you want to take an idea you can take a look on the link and I will explain what I did for myself

    https://www.youtube.com/watch?v=uXzrmLrwPcg&t=7s

    stand by aux supply

    https://www.ebay.co.uk/itm/3-3V-5V-9...rxMPrQbkvQ96gQ

    Buck convertor

    https://www.ebay.co.uk/itm/DC-DC-CC-...oAAOSww9xZAbCF

    Regards Silvio

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