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Thread: Sg3525 low power psu for dac

  1. #21
    .... Silvio's Avatar
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    Hi Pichou

    I still do not know your input voltage but I am assuming it is 12v (correct me if I am wrong)

    It will be better to put 2 snubbers across each primary winding (Your topology is push pull from what I can see)

    You can also fit an 18v zener across drain source so that any voltage generated during switch off will be shorted out with the zener

    Try lower capacitance with your snubber like 10nf and work out again the resistor value

    Through experience I found that when the pwm is working in full duty not much noise is generated and usually the output inductor is not needed, this can generate a spike back to the primary winding. This is why the secondary snubber is needed

    Having spikes in your wave forms can disturb the operation of the pwm signal to the gates

    You need to use better symmetry in your trafo try to interleave the windings together if possible Primary and secondary side by side and wound together to the best possible way due to different turns in each winding.

  2. #22
    .... Silvio's Avatar
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    one last comment on the winding of the transformer

    The symmetry in the primary windings need to be equal as possible. Make this winding in bifilar method so that the outcome will be as close as possible. Any difference between the two windings tend to generate a different wave shape or spike.

  3. #23
    Hi Silvio,

    Yes, it's a car psu. During tests I use have 12.35v.
    Yes it's pushpull with a snubber on each prim side.
    I will try with lower cap. I've read some articles and see a bell curve for the snubber action. So, bigger is not better.

    For the 18v zener, i assume I need to put it in the opposite direction of the internal diode of the mosfet ?

    OK, I will try to make a new transformer.
    Probably with 0.5x3 on primary and 2x0.4 and 1x0.6 for secondary.

  4. #24
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    The zener should be placed the same direction of the mosfet diode and also parallel with the snubber so that only when there is over voltage will it conduct.

    Regarding your snubber well this should be enough to dump excessive ringing and not dumping it all together otherwise you will be loading your fets for nothing.

  5. #25
    Hi Silvo,
    Tried 2 news transformers following your instructions with the same visible step on primary.
    Here is a photo of one transformer with only the primary and the 7 turns secondary. There is one winding of the 7 turns alone in the middle of the primary entry.
    Primary is made with 3x0.5 mm wire and are winding parallel on the core with the other primary inside it (to made a 6 wire parallel profile = sec1 wire1/sec2 wire 1/sec1 wire 2/sec 2 wire 2 and so..).
    Better is to twist each 3 stands first and twist the 2 primary together ?

    For the moment impossible to measure the snubbers. With no load, I can see something like ringing, but with little load this first part of the waveform goes down to make the "step".

    No load :
    NewFile3.jpg

    Little load (0.3A@15v) :
    NewFile5.jpg

    Little more load :
    NewFile4.jpg

    Photo of the first transformer :
    Same behaviour
    IMG_20180410_202632.jpg

    Photo of the second transformer :
    Same behaviour
    IMG_20180412_195255.jpgIMG_20180412_195328.jpg

  6. #26
    .... Silvio's Avatar
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    Hi Pichou,

    A few tips to help you out solve the problem

    from what I can see the rise time is not fast enough and this may be the cause of the step you are seeing. Try lowering a bit more the gate resistor like 10 ohms and see the outcome

    Please check also what is coming out of the SG chip as it could be the signal is not coming out as it should and thus disturbing the driver output stage. The duty cycle should be around 47-48%

    The trafo winding looks good and I do not think is the problem now.

    check again your dead time and see that it is around 1us, I can see you have a lot of ringing try to play a bit with the dead time and widen it a bit like 1,5 or 2uS and see the outcome

    Push pull windings tend to ring a bit but that can be suppressed quite considerably with a good tailored snubber,

    bridge out your output inductors and check again without them.

    You can measure one side at a time to see more clearly

    If I am not mistaken this problem came out since you mounted the smps on the pcb. If so check your tracks and see that you do not have a mistake somewhere especially around the sg chip. decouple the supply rail to the sg with a 100nf ceramic cap as closely as possible to the chip or directly with its pins

    good luck

  7. #27
    Hi Silvio !

    Gate resistor is already 11R (2x22r). Duty cycle is ok, dead time 1Ás from 3Ás without resolving my problem.
    Decoupling is 100/10/0.1 on each power device.

    All the problems disappear when I remove the inductor on output ! You are right with them ! Over....engineering from my head...thinking making better with the self...not in all case I understand...

    Returning to step "charge psu and calculate snubber" !

    Another time, thanks for you help Silvio !

  8. #28
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    You're welcome

  9. #29
    Hi,

    Trying to define the snubber at primary side, but I don't understand something on your how to Silvio.
    First, I measure the ringing freq with the psu loaded on each output (total of 2.6A on primary at 12.35v).
    I found something like 14.7Mhz, it's very high....but the ringing is not a proper sine wave, there is some "non linear" behaviour.

    NewFile1.jpg

    When I put a 10nf cap on the snubber, freq is reduced to 3.48 mhz.
    I don't have the same behaviour your are talking on your how to (the freq is shifted up).
    NewFile2.jpg

  10. #30
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    Hi Pichou, well I guess your capacitor is too large you may try something smaller like 1nF or 470pf and see the outcome. It also seems that you already have a high ringing frequency.
    Regarding behavior of the ringing wave that is quite normal that it starts diminishing and lowering in frequency at the end.

    The practice I used was read in a book and if I am not mistaken it was called power supply cookbook by Marty Brown. I tried it myself and it worked. The only difference between us here is that I was working on an off line half bridge smps and my voltage at the primary was around 160v. Well yours now is only 12v so that may be the different behavior.

    Do not give up on this you can experiment with different value caps and different resistors. The ringing may not be all dumped but reducing the amplitude to 1/3 of what it was originally will be acceptable. A lot of dumping may cause unnecessary load on the switching transistors and this will bring more losses to the smps. It will also may affect the rise and fall times. Try to find a good compromise.
    I would like to point out that snubber resistor wattage may be 2 and even 3 watts at times.

    Regards Silvio

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