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Thread: TL494 halbridge like ATX problem

  1. #31
    Thanks for very informative reply!

    Regarding voltage drop:
    I observed very strange waveform on switching transistors, when converter is loaded about 50W.
    convertermeasurement.png
    This is how I connected scope, on the other transistor it's the same.
    1595832255.jpgconvertermeasurement2.jpg
    On the first screenshot when lighter load (few watts) the voltage on the collector is just fine, going from 325V to ground - very small voltage drop on transistors. But on the second screenshot, with higher load 50W there's smaller voltage swing - higher voltage drop on transistors? Is that becaouse of too small base current? Could this create voltage drop on the output and heat up transistors?
    I used a base drive transformer from old atx, maybe I should rewind it? Do You have some Idea for numbers of turns?

    And lastly, I changed number of turns of secondary to 2x10 and using 4x0,6 wire to fit it in window.

    Thanks, Fifi

  2. #32
    Yes i can also see that the pulses are not symmetrical.
    This could be the highside i guess. Try to change the capacitor feeding the gate to correct. Try a different value and see for any changes. The task is to pulse the gate hard at all times. Try a higher aux supply like 15 or 17v feeding oscillator circuit and BDT primary. If gate is not drven hard enough then transistors enter linear mode and get hot.

  3. #33
    Hello after long time!

    First: Silvio - BIG Thanks for help!

    I didn't wasted it. I realised, that BJT's are tricky. They need proporcional base drive, and I was unable to get it working in a needed range. I switched to mosfet with GDT. I will upload working schematics, pictures within two weeks. But there is one small problem: Resistors that are with capacitors ( 10ohm, 1nF ) across double diode at the output heat up and turn black (they're 1/4W as in ATX). I don't think that this is problem with the converter, because oscilloscope shows some spikes there, but they are super short. I used this values from ATX supply, but my supply is 3x voltage. Can I safly increase resistance to 33/47 ohm? What those resistors really are snubbering? - there is snubber on the primary side.

  4. #34
    Quote Originally Posted by fifi_22 View Post
    Hello after long time!

    First: Silvio - BIG Thanks for help!

    I didn't wasted it. I realised, that BJT's are tricky. They need proporcional base drive, and I was unable to get it working in a needed range. I switched to mosfet with GDT. I will upload working schematics, pictures within two weeks. But there is one small problem: Resistors that are with capacitors ( 10ohm, 1nF ) across double diode at the output heat up and turn black (they're 1/4W as in ATX). I don't think that this is problem with the converter, because oscilloscope shows some spikes there, but they are super short. I used this values from ATX supply, but my supply is 3x voltage. Can I safly increase resistance to 33/47 ohm? What those resistors really are snubbering? - there is snubber on the primary side.
    I do not really suggest that you remove the output snubber, it is quite reasonable that your resistor tend to burn. For the output voltage being 3 times higher I suggest that you enlarge the wattage of the resistor to 0.5 watt and lower the capacitance. That way you make the output snubber softer. The output snubber is needed because your smps is regulated. Some spikes from the output inductor may reflect back at the primary without it. Do not forget that the initial voltage was not more than 20 to 30v now your voltage is rather high so a softer load with the snubber is needed. I forgot your peak voltage and operating frequency on the smps but if you tell me I can suggest some values for you to try out. You can start with 470pf and 10 ohms for a start and see how it works.

    good luck Silvio

  5. #35

    Wink Posting Schematic and transformer details.

    @Silvio. I just changed resistors to 2W ones, and they are running ok.

    smps.jpg smps_test.jpg

    So, this supply works. BUT, not I'm not yet 100% proud of it.

    Yesterday I let it running on the full load for half of the day (my dummy load had bad time), it survived. As I wrote before: It regulates nicely, it has full power, it doesn't heat up too much. Efficiency is defenetly >80%, I will measure it, when I will be able to go to school and use their meters (I dont have true RMS to measure 230V power). Unfortunetly, this is not lab power supply. Why? it produces audible noise (feedback) without load and light load. It has some voltage overshoot at powerup, when load is connected/disconnected also voltage slightly overshoots and so on. I do not recommend to built it, unless you want some foolproof, high power PSU and know what you are doing. DO NOT power anythink delicate using this supply (eg. arduino). I'm sure feedback circuitry can be re-developed to work better, but it's not only bad thing: Ouput capacitance and choke - thay also disqualifies this supply from lab use. Too much capacitance create HUGE current peaks which easly kill the contacts in switches/relays etc. Output inductor creates voltage spikes, when output shorted, and disconnected.

    To solve all of these issues I want to add linear postregulator. As Silvio sugessted - SMPS will folow output voltage + ~4V to provide room for regulation. This is in progress.
    If anyone has schematics of simple, but well working linear psu, please share with me - this will speed up design process. (0-35V 0-6A yes, I want to regulate current from <1mA).

    Im not posting PCB image, due it had some errors (and additional circuitry, which don't want to work), but if anyone is interested - I can solve them and post it without errors.

    SMPS35V6A.zip

  6. #36
    Hi Fifi, I am posting you a circuit of what I did in my dual psu with a post linear regulator. The linear regulator is very simple to construct and can go up to around 6 or more amps depending on the output transistor capability. I used a TIP2955 for myself which I found that it can go to 5 or 6 amps quite easily with a good heatsink. I am sorry I do not have the full proper schematics as I did these along my experimenting but I am sure you can understand what I did.
    First I made the linear regulator which works well and has current limit. I got this circuit from the datasheet of the 317T, after I added a small transistor and a zener and a few resistors to feed the feedback pin 1 of the TL494 of the modified ATX computer smps. There will be the need to create a negative voltage (9-15v) to feed the added opamp (LM301) to create the current limit. The current limit starts at about 100mA and this is sensed via the 0.2 ohm resistor in the output line. if you want to go below this limit then you can make a switchable resistor of higher value so that you can sense lower currents. For example x10 so it will be 2 ohms instead for low current liming. This is my idea for you. I have already posted to you the schematic for the TL494 earlier in this tread and there is also the compensation circuitry. As for the output inductor the value should be around 80 to 100uH followed by a smaller inductor of about 6uH to filter the high frequency ripple.
    There is also description in the pdf file so that you can understand. My bench smps is a dual version which has the master and slave supply. I can use indipendently or in series. I am also giving you the link to see it on youtube.

    Link https://www.youtube.com/watch?v=uXzr...lvioDeLeonardo

    The description in the files are not present due to these being uploads of a year or more ago. I cannot upload new files as there is something wrong with the website.
    Attached Files Attached Files

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