your opinion on this SMPS

Discussion in 'General Electronics Chat' started by Salvador123, Oct 9, 2015.

  1. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    Hi folks.

    So I recently were talking here about making an smps out of some leftover BJT's , after some failures and problems to drive it , i think i will try my hand at making a smps out of components dedicated for such use.

    Since I have a IR2110 chip already , i google a schematic based on this driver chip and found some good looking schematics , so i will post them here and ask what do you think.
    basically all i need is the oscillator chip SG3525, and a few resistors and diodes.

    also , what would change if I attached the float capacitor not to the middle point of the smoothing capacitors but to the ground ?Because if I understand correctly now that the transformer primary is tied via the capacitor to the middle of the rectifier capacitors the mosfets drive the primary only half the voltage of the rectified mains , If I would attache the capacitor to the ground the mosfets would drive the capacitor to the full rectified voltage correct? the power output would be larger , why is this not done here for example?
    I plan on using IRFP 460 as drivers , If i recall correctly they were 400 volt devices.

    I used a red arrow in the schematic to point the thing I was asking about the capacitor connection.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    The reason the transformer its at half supply is so each fet pulses it at +/_ 155v, if you connect it your way the lower fet wont have a voltage across the coil, if you need more voltage output, put more turns on the secondary side..

    What voltage range output do you need, and what current, your using the correct chip, there are other chips simpler available..


    http://danyk.cz/s_atx_en.html
     
    Last edited: Oct 9, 2015
  3. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    I mean this circuit is simple enough and suits me fine, also i have one of the two chips already at ahnd , to my eye it seems to work I just wanted to know your opinion.

    could i just go without the octocoupler ? or is it needed to keep frequency stability and control voltage levels?
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    the opto is there to isolate the output voltage from the feedback pin on the chip, which would normally be under Live mains voltage,your circuit has a transformer, the output voltage is regulated by pulling the V- sense pin high , the opto turns on when the zeners strike ,lights the opto led and this stops the pwm, then the cycle repeats, other designs use a TL431 precision zener.
     
    Last edited: Oct 9, 2015
  5. Salvador123

    Thread Starter New Member

    Oct 3, 2015
    22
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    i found a nice small transformer , it outputs about 20 volts which after a full wave rectifier and smoothing will be about 28 volts, what do you think , could I use that voltage to supply the sg3525 and ir2110 directly or should I use somekind of zener to limit the voltage to some 15 or 20 volts?

    the sg datasheet says its operation is up to 35 volts, couldn't find the same info about the ir2110

    also while searching various schematics on google i stumbled upon such a self excited half bridge smps , the output being similar to the schematic i posted above but the excitation is as you see.
    what do you think is it worth messing around with this or should i better just go with the IC's?
    the stability with the IC's and safety is going to be better i assume.
     
    Last edited: Oct 9, 2015
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Last edited: Oct 9, 2015
  7. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    i have known the ir2110 datasheet before just couldn't locate the number given for supply voltage.
    yes I agree I too would rather go with the IC's.

    well if it has 25v max then i guess i will just go with some 2w or higher power rated zener at zome 20 volts or about that level.since teh transformers size is very small and good suited for my test board on which i have tried to make many smps before but some of them never finished and some just werent good enough.

    well if i will be succeseful with this one I will use it to power two channels of a 800w or so amplifier boards that i built some years ago and never started to use because i already had many amplifiers both self made and factory ones to use , also I have kinda got tired of the weight and size needed for high power amins transformers just to drive an amplifier board which itself is kinda small and lightweight even with the heatsink.

    i will use about 80-0-80 volts on the secondary.

    i guess i will order the sg3525 soon and some other parts , after all need only a few there and see how that works out for me.
     
  8. Salvador123

    Thread Starter New Member

    Oct 3, 2015
    22
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    also I wanted to know, if I want two separate transformers for two channels, I plan on using two IR2110 each driving two transistors for each transformer , now do I need also a separate SG3525 chip for each IR2110 or can I just run both IR2110 from a single SG3525?
    I think it can be done from a single SG chip by the way, but I would like to hear your opinion.

    thank you.
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The Sg3525 doesn't need an external driver, you can drive the mosfets direct from the Sg, read datasheet.
     
  10. Salvador123

    Thread Starter New Member

    Oct 3, 2015
    22
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    ok but then why in the schematic as well as in many others i see both sg3525 driving a ir2110 which then drives the mosfets?
    Someone lobbying chip manufacturers? :D

    Ok in all seriousness i checked and i already kinda though that the sg can drive mosfets up to a certain level itself.I assume the gate capacitances of the IRFP460 are not as big so the sg could drive them directly.
    but if im making two transformers each one for each channel and each is driven in half bridge so in total of 4 IRFP460 wouldn't that be too much for the single sg3525, maybe i should jsut do the single sg driving two IR2110 of which each IR drives two of the mosfets?

    I've been searching google but most of what i found was IR 2110 driving two mosfets in this configuration , and where a H bridge was used each of the sides was driven with a separate IR2110.
    but if the SG can drive those mosfets alone then what's the use of the IR2110 in the first place since it's not a pwn controller itself nor an oscillator.?
     
  11. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,039
    1,667
    Look at his schematic closer. He has his two switching devices working in a line voltage fed totem pole configuration which is well beyond the designed capabilities of the SG3525's outputs. Given that he needs the IR2110 to drive the upper switching device being how it's being operated on the high side of the DC supply.
     
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