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Thread: Sg3525 low power psu for dac

  1. #1

    Sg3525 low power psu for dac

    Hi Alls !
    I'm freshly registered here, but I read you since many years.

    Now is my day to take the plunge and to show you my smps project.
    I don't have engineer knowledge, only school memories.
    Since the time I read the forum trying to understand the smps rules, I have now finished the schematics of a low power switchmode psu for preamp use.

    The PSU is based on the SG3525 chip and don't use regulation since I will use some regulators at the outputs.
    Push-pull design with TN23/3C90 core transformer with 3 secondary (2 for analog +/GND/- and another for digital +/GND) 2x5 PRIM / 2x9+1x5 turns SEC
    I use a mosfet driver TC427 with 2x IRFS7440 MOSFET.
    I'm planning to use at maximum SMD components for compact design and because I prefer soldering SMD than through hole (my eyes are still good for the moment )
    For sure some parts cannot be put in SMD (snubbers essentially).

    In attache the schematic of the entire PSU.
    Many "sub-block" are inspired from well-know others PSU like BCAE1 or Elliot Sound or valveaudio. I'm not able to design entirely this circuit. So maybe I have done some beginners mitakes.

    I will have some questions if someone can look the schematic.
    Since the secondary winding ( 8- 9 for digital ) is isolated from the analog windings (4 - 5 - 6 -7), I have 2 dif GND, one digital and one analog. It is good for the DAC.
    With this configuration does I need to use 2 common mode inductor (one for each output) or does I need to wind all of the secondary in the same inductor ? If I do thos does I need to connect absolutely the 2 gnd ?
    I prefer to use 2 different inductor since for this power, I can probably buy them ready to use.
    Since the PSU is unregulated, does I really need the inductors ? I need the cleanest possible psu, and in my head, running unregulated is better for noise.

    Any comments or suggestion will be appreciated,

    Thanks in advance to those who take time to read the post,

    Cheers,PSU Pichou v3.pdf

  2. #2
    Sorry, missing the parts numbers on the schematics.
    I'm playing with Excellent IT, and I think the TN23 core I planned to use is not good. A TN26 will be better.
    Print screen in attache.

    IT core psu v3.jpg

    Attachment 6459
    Attached Files Attached Files
    Last edited by Pichou; 11-22-2017 at 06:16 PM.

  3. #3
    diysmps Member* res_smps's Avatar
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    i think you have to reverse diode direction like this
    pichou.jpg

  4. #4
    diysmps Senior Member
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    hi ,pichou nice project . please post photos of your smps while testing it.

  5. #5
    Hi,
    Thanks for your answer

    Ouuuupsss 4 diodes in my nose

    I will start routing the board first and I will show you the result.

  6. #6
    .... Silvio's Avatar
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    @Pichou
    Through my experience so far common mode inductors on the output tend to be more noisy than single bead inductors. I do not know what load you have on the output but as the smps is unregulated then noise is not so prominent.
    You have enough capacitance on the output to filter out. I also suggest that you make a smaller capacitor such as 1uf film cap instead of the 100uf in the input of the regulators this will filter better any residual noise. You can also try ring inductors but separately wound and not in common mode fashion.

    Your input and output snubbers have to be trimmed on the actual setup as ringing if any has to be suppressed on the actual ringing frequency. This tend to vary with different transformers and winding technique.

    Use tight coupling and symmetry in your windings to eliminate leakage inductance as much as possible.

    Regards Silvio

  7. #7
    Silvio
    Thanks for your answer and advices.
    Load on the output will be approx 0.5A on analog symmetric output and something like 1A on digital.

    Ok for the capacitors and the inductors. To be sure about the inductors you advise me to use, you talk about one single ring inductor for each transformer output ? Or a common mode for analog and a separate single for digital ?
    I have an old amp at home to make some tests (Esoteric 7056, the amp side is burnt put the psu is ok). If i remove the regulation to go 50% duty and the secondary common mode inductor I experiment some ripple. If I leave the inductor in place but still without the regulation, ripple is better.
    Something like this can be used for digital side (rated at twice the current needed) ? https://www.mouser.fr/ProductDetail/...zfYJDYvjIr8%3d

    The snubbers in the schematics are "just to put a value". I will try use this jig to found the correct value when the transformers will work : http://www.diyaudio.com/forums/power...-test-jig.html
    Good idea ?
    Anyway, I will leave a place free in the board and populate this only when I will found the good value.

    Another question, is, what about the manner I have plug the IN-/IN+/COMP/VREF to run unregulated, do you think it is correct ?

    Cheers !

  8. #8
    .... Silvio's Avatar
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    @Pichou
    About the output inductors I was talking about the dual rail output. I was experimenting with smps of higher current than that you are using (8-10A) when I encountered this problem with noise using the common mode inductor wound on the same core for the 2 rails. I got rid of it when I used separate inductor for each rail.

    Usually they use a common inductor for several outputs ( Like the ones used in ATX power supplies) These all have a single rail though and not a double output. They do it like this usually for economy and also to get better regulation across all outputs these using the same ferrite ring with separate windings for each rail.
    The inductor helps in both ways as it chokes a little the output and also help with better smoothing. It also causes some voltage drop so some headroom in the output for regulation is needed. It will also help out for surges in the output and limits the surge current.


    Regarding snubbers well you can also check the blog post I posted called Calculation of snubber components and driver gate resistors.

    About the IN+ and IN- I guess you are talking of the sg3525 chip. All I can see is that R15 is too low and this should be around 470K the duty cycle will suffer if it is too low. I also recommend that you make a small preset of around 500R instead of the discharge resistor R9 and then you can set the dead time to around 1-1.5uS. You can put a fixed resistor instead when the correct value is found.

    I hope that helps Silvio

  9. #9
    That helps

    Ok for the inductors !
    That I can do is to put some dual footprint on the pcb to run common mode or separate inductors (with some headroom between them, I think is necessary) depending results...

    I will look your entry blog about snubbers !

    Thanks for the correction about the sg3525. I will put 500R for R9 in the schematics and move the value after some tests. I think I need first to have a good square wave signal before moving deadtime (would be difficult to measure DT if the signal have some spikes or other distorded form), so first I will found the good snubbers.

    Now, i'm in the routing !

  10. #10
    Hi !
    The PSU seems to work properly !
    I have play with the snubbers, but i think I don't need them, at the primary and secondary side, there is no overshoot or something strange, it's even the opposite. In attache a view of the mosfet command. Smooth square signal and little undershoot at the end of the waveform.

    Silvio, I have try to use your method to found the correct value for the gate resistor, I found 16R. I still use the smallest resistors I have for the moment, 22R....I don't understand the second part of your how to.

    NewFile2.jpg

    NewFile3.jpg

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