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Thread: TL494 halbridge like ATX problem

  1. #21
    Thanks for very informative replies!

    As I wrote before, I have some quesions regarding this kind of psu's. The component names are from the schematic in attachment.

    1. What is purpose of using capacitor C2 in series with resistor R6 in the error opamp feedback? Maybe some "speed up" for feedback? How to choose this components - I have seen values from 10nF to 100nF and from 8.2k to 47k in different ATX schematics.

    2. How do we calculate (estimate) the value of output inductor? Currently I'm using 93uH measured, wound on Yellow core from ATX, 4 wires of 0.4mm diameter each. Should this be a litz wire or can be solid inductor? (I heard that litz is needed only when direction of current changes, so in transformers, not chokes). Also, when set to low voltage, and shorting output, there are some voltage spikes - are they coming from the inductor? Maybe it is too big? How to check if this inductor doesn't saturate at high currents?

    3. How do we calculate (estimate) output capacitance? Is that just to be in resonance with output inductor on working freuency? I think more caps=better, but when working in CC mode and shorting output, there is still huge charge in these capacitors - can kill connected device.

    4. Currently I'm using 2sc5027, and transistors are switching correctly (checked with oscilloscope), but they get warm (I know that's normal) But when using lower voltage drop transistors (eg. MJE13007 - only 1V at 2A) will the power dissipated in them lower?

    5. Is it worth to figth with synchronous rectification in such circuit?

    6. Is it worth to try PFC with such curcuit?

    Big thanks for Your time!

    atx_psp_lab_schema_01_2011.gif

  2. #22
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    Quote Originally Posted by fifi_22 View Post
    Thanks for very informative replies!

    As I wrote before, I have some quesions regarding this kind of psu's. The component names are from the schematic in attachment.

    1. What is purpose of using capacitor C2 in series with resistor R6 in the error opamp feedback? Maybe some "speed up" for feedback? How to choose this components - I have seen values from 10nF to 100nF and from 8.2k to 47k in different ATX schematics.

    2. How do we calculate (estimate) the value of output inductor? Currently I'm using 93uH measured, wound on Yellow core from ATX, 4 wires of 0.4mm diameter each. Should this be a litz wire or can be solid inductor? (I heard that litz is needed only when direction of current changes, so in transformers, not chokes). Also, when set to low voltage, and shorting output, there are some voltage spikes - are they coming from the inductor? Maybe it is too big? How to check if this inductor doesn't saturate at high currents?

    3. How do we calculate (estimate) output capacitance? Is that just to be in resonance with output inductor on working freuency? I think more caps=better, but when working in CC mode and shorting output, there is still huge charge in these capacitors - can kill connected device.

    4. Currently I'm using 2sc5027, and transistors are switching correctly (checked with oscilloscope), but they get warm (I know that's normal) But when using lower voltage drop transistors (eg. MJE13007 - only 1V at 2A) will the power dissipated in them lower?

    5. Is it worth to figth with synchronous rectification in such circuit?

    6. Is it worth to try PFC with such curcuit?

    Big thanks for Your time!

    atx_psp_lab_schema_01_2011.gif
    I will try to answer your questions to the best of my knowledge. I am also a hobbyist just like you.

    1) As I can really see it may be controlling hysteresis in the opamp (The reaction time between each change of state giving a little delay to prevent oscillation) This may vary between each setup according to the frequency of operation in each case.

    2) I do not really know how to calculate it but from my experience is that the lower the minimum current the higher the output choke would be. I have a software called Excellent IT which is also on this site to help out in calculation for smps transformers. IF in the input data of this software you will opt for a minimal current of say 0.2A the output inductor will get really high compared to say an a minimal current of 1A. You will see that this will offer the necessary continuous voltage at the output while the dead time gets bigger at minimum pulse width. I guess with higher inductance there is more stored voltage in the inductor to compensate for this. The impedance will also be larger but this at low current will not offer much voltage drop. The type of copper wire used being stranded or solid. I have always seen in ATX smps that this would be of the solid type in the DC line. If say an inductor is used where the magnetic field is changing (like the one used in LLC smps in the center of the half bridge) I have seen both solid and also stranded ecw in pro amplifiers.
    As you say the stored charge in the capacitors may overcome the CC regulation in the initial surge. This may not happen because often there should always be a minimum current shunt resistor for this purpose and also not to let the chip work in discontinuous mode. This resistor may be put before the ampere meter not to interfere with the actual output current going to the PSU terminals.
    [I]Saturation [/I]A blog exist on this site where a chap with the name Fortytwo made a saturation tester.
    here is the link. https://www.diysmps.com/forums/entry...ation-testerII

    3) I think the output capacitance will depend to what ripple voltage at load is estimated. This will also get calculated according to the output inductance (choke) used. Well to sum this up the higher the output choke the less capacitance needed to maintain the calculated ripple voltage at load. Keep in mind that its not only the capacitance used but also the ESR of these capacitors. Low ESR capacitors need be less in capacitance for the same performance when using normal capacitors at the output.

    4) [I]The power dissipation in a given transistor will depend on the facts that I mention here.[/I] The saturation voltage (BJT) or RDS on (mosfet) The lower these get the less dissipation. The switching frequency has to be taken also in account as the more switching cycles per second the more switching losses. In each case one can check the datasheet and see the graph for a given current versus frequency in operation, with this you can calculate the total dissipation. Do not forget to check the temperature graph also.

    5) Synchronous rectification I would consider when I have higher currents over 15A depending also on the duty cycle of the smps in use. I would prefer to use scottkey diodes that can handle the maximum voltage used in your case.

    6) PFC up to 500w, I do not think it is really needed. You have the output regulated by PWM so if you have enough headroom in the output the output voltage at load will still be maintained. Active PFC will enhance efficiency but remember this is a lab PSU which will be used occasionally. [I]What I learnt and seen[/I]. Feed properly the input capacitor and voltage drop at the halfbridge will not be bad. Use a rule of thumb for 1uF to 3uF per watt of power across the the 320v rail. if you are using 200v capacitors than double the value of capacitance. This is because capacitors in series will halve the capacitance value.

    Regards Silvio
    Last edited by Silvio; 06-25-2020 at 08:48 PM.

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