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Thread: Sg3525 low power psu for dac

  1. #11
    .... Silvio's Avatar
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    I was just wondering why you did use the sg3525 for a non regulated power supply for low power. You could have readily used the IR2153. This would have made your circuit easier and having the oscillator and mosfet drive in one package. This chip could handle medium power fets such as IRF840 or IRF740 and a power between 100 and 400 watts can be achieved quite easily.

    Regarding the gate resistor, well this calculation allows one to find the correct value for the gate resistor.
    This is a combination according to the mosfet driver drive and source current capabilities. The gate resistor is chosen so that the maximum current of the mosfet driver are not exceeded. Hence the mosfet is charged and discharged quickly enough according to the frequency of operation.

    Lets consider an IR2153 chip which have a drive and source current of around 200mA and an IR2110 which has a drive and source current of 2A. As one can see the both chips have different current capabilities and the selection of the gate resistor for both chips will be different.
    I admit the IR2110 is designed to handle larger mosfets with higher gate charge requirements. The explanation is made so that one can calculate the gate resistor according to the driver used.

    It is also important that one must observe the data sheets for both driver and mosfets used. These tend to vary between different manufacturers.

    I hope that helps

    Regards Silvio
    Last edited by Silvio; 02-24-2018 at 11:59 AM.

  2. #12
    Silvio, because I have many sg3525 chip in stock bought at very low price. The idea is to start with this small psu, unregulated, and to continue to learn about smps with some sort of regulation and with bigger psu. One chip to learn is enough for me. Maybe it is not the easiest, but for future use I think it's a good choice. I'm not feel good for the moment with alls the different specifications of this sort of chip but I will look the IR2153 for my knowledge.
    For the moment the psu works very good on alls the outputs. I can use twice the current I need on outputs without any heat for a total of 3.5A at primary side. Idle current is 0.15A. With my dsp/dac plugged, I draw a total of 1.7A on primary side.
    I will try to mount a second psu with just the needed components for the SG to experiment with gate resistor/freq/snubbers/transformer. Maybe I can correct the waveform for a more neat square. The issue seems to be an over-damped form (correct terms ?). I know snubbers can correct overshoot, but here I need the opposite. Right ?

  3. #13
    .... Silvio's Avatar
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    I don't know where are you measuring that wave form is it from the gates of the mosfets or is it from the output of the SG chip?
    It seems that you have a slow rise time and also a slow turn off time. The curves on the wave form clearly shows that. What seems to be the under shoot is the slow turn off and the dead time.

  4. #14

    Thanks for your answer Silvio.
    I have done some tests and play with CT/RT/RD to obtain a 1ÁS dead time and the cleanest square signal I can with just the SG and driver. Ct is 1nF, RT is 15K, RD is 470R. The freq is around 47.5Khz. If I go lower for RD with 250R, dead time is 860ns. With the old CT and RT value charge and discharge time are longer.
    I have done some screenshots of the results, hope they are goods for comprehension:
    SG3525 output alone, the 2 outputs are identical :

    Close view of start and end of the waveform :

    SG3525 + driver TC427 :

    Close view of start and end of the waveform :

    A view of the 2 waveforms with some markers showing 944ns dead time :

    I can see some ringing in the outputs. What can be the cause ? Routing of the board ?

  5. #15
    .... Silvio's Avatar
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    Hello Pichou,

    The output from the mosfet driver is most important as this is driving your fets. From what I can see I understand that you have a very nice square wave form coming out to the gates.

    Regarding the ringing well this may be caused:-

    long runs from driver to gate or from SG to driver.

    long leads from driver to mosfet these need to be as short and as thick as possible

    High current traces near signal traces thus coupling with them

    lastly transformer primary kicks back when switched off from the mosfet. This is only suppressed by a proper snubber across the winding.

    usually this ringing is observed only when smps is loaded. The more load the more it rings

    One other thing that might be causing ringing is the output inductor. This I have experienced myself and may be the case that you have to make another snubber just after the secondary to suppress any feedback to the trafo

    Here is link with some helpful tips for building car audio power supplies

    Hope that helps
    Last edited by Silvio; 03-12-2018 at 09:33 PM.

  6. #16
    Your comments are every time useful and welcome ! Thanks for your time !

    On the screenshots, there is only the primary side mounted on the board. So ringing is certainly due to my board layout. I think is due to the poor Rigol quality probes too. If I move them vertically to the pcb, ringing is....different...

    I will continue to experiment first by calculate and placing gate resistors and mosfets and play with that, make new screenshots and wind a new transformer. When the transformer will be ok, I will continue with new screenshots and by placing rectification and see what happens with a load, add the caps and output inductor etc etc. Step by step approach to learn.

    I have a question, does I need to load each output of the transformer to define the primary snubber ? If I understand, this snubber correct only the winding of the primary ? But can pass some strange effects from the secondary like a mirror.
    I can put snubbers on secondary side, I have let the place. I can put a snubbers for each secondary output. But for the symmetric analog output, does the best way is to put the snubbers across the 2 legs of the secondary or between each leg and middle point ?

    Thanks for the bcae1 link ! I know it and more I read it more I understand the "rules" for smps.

  7. #17
    Some new screenshot of the gate signal direct with the probe on the pin of the mosfet.
    I think I have a good (the best) result without gate resistor. A little better slope if I put a 10K in pulldown. I have done some test still without gate resistor but with different value of the pull-down, and if I decrease the value I have very little dead time added and no visible enhancement on the slope of the curve. Does it's a problem to run without gate or pull-down resistor ?

    Gate without anything :

    Gate with 10K pull-down :

    Gate with 3K pull-down :

    Gate with 1K pull-down :

  8. #18
    .... Silvio's Avatar
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    Hi Pichou,

    It is advisable that you use a gate resistor. This will limit the gate current as your diver might not last long without it.
    Your driver can deliver around 1.5 amps.

    You can take a look at the blog post regarding this subject.If you are using the same fets described in the schematic you posted earlier then use the specified one (22R) You can use a reverse diode to enhance switch off of the fet (1n4148 diode will do) see diagram I posted

    The pull down resistor is there to prevent false switching of the fet. The driver actually switches off the fet, 10K ohms is enough in this case.

    You can take some scope shots across the primary windings. You will notice that the rise and fall times will be much better there.

    .I would like to see some pics of your project this will help me with better suggestions


    regards Silvio

  9. #19
    .... Silvio's Avatar
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    Regarding the snubber you will try first without it and see how things perform.

    If you will be needing to load the smps it does not really matter which winding you load on the secondary it still has the same effect as long as your loads are resistive. (Car bulbs in series will do) I don't know your output voltage/s.

    Lastly use symmetry in your windings and tight coupling especially with the main secondary winding which will carry most of the load.
    A good transformer is the key for a clean waveform.

  10. #20
    Here is a photo of the psu. Don't know if you can see it in large dimensions...
    Transfo is a PRIM 6+6T made with 0.7x1 / SEC1 11+11T with 0.5x1 / SEC2 7T with 0.6x1
    RD 470R / CT 1nf / RT 15k / F=48khz
    R gate 11R + pull down 10K + Diode like your schematic.
    Fall and rise time are better !
    Snubber for PRIM is 100nF / 100R. Can't get lower for the resistor or higher for the cap because resistors heat quite a lot...Calculated value is near then 12R....impossible to put that value without huge current consumption and heat...Have to work on this more I think I don't understand or I made mistake somewhere....
    No snubber for secondary for the moment.
    Iddle current is 0.54A
    Secondary voltage with no load at the regulators inputs are : 13.87v for bridge secondary, and +/-22.2v for dual rail output.

    With no load here is some screenshots, strange spike, not seen before....
    Gate with no load :
    Transformer primary with no load. What cause this "step" ?
    Now some screenshots with load on the output (after the regulators).
    1.35A at 7.8V on Digi output, 0.55Ax2 at 12v on analog output, total current drained on primary is 2.52A.
    Gate is ok I think, but at the primary of the transformer the "step" is clearly visible. Does it's a problem to have this waveform ?
    Because I found the same strange form on secondary....for sure...
    Here is a view of the bridge secondary : same "step"
    And a view just after the diodes. This problem is visible even after the diode.NewFile11.jpg
    The same problem is visible on the analog dual rail output.
    I know I have some parasitics on the gate trace, but all my problems seems to show a bad transformer construction I think...

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