to change SMPS voltage out

Discussion in 'The Projects Forum' started by rakeshm55, May 25, 2015.

  1. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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    Hi,
    i was working on an SMPS .... from ""mean well"" which provides an o/p og 7.5V and 3A....I would like to modify the circuit so that it wold provide 5V.....Please help me to change the voltage....

    SMPS uses TOP223 PWM controller and an PC 817 opto coupler series....In regulation loop has a Az431 Shunt regulator to control the o/p voltage...

    After tracing the circuit , the control loop almost looks like below....
    upload_2015-5-25_15-57-35.png

    I could find R4 = 20k and R3 = 10k so that o/p voltage is 7.5V......I changed R4 to 10K, now the output was stuck at 5.7V...... I tried different values for R4 like 6.8,8.2k but Vout was stuck at 5.7 ......I could observe the opto coupler Forward current to be at abt 25mA at this stage..... In normal case (7.5V out) the opto coupler I forward current was restricted to 3mA.....

    I tried increasing the series resistance but the o/p voltage just increases above 5.7V......

    How to bring down the 5.7V to the range of 4.5V to 5V for an SMPS.........
     
  2. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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  3. DickCappels

    Moderator

    Aug 21, 2008
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    It seems as though something is limiting how far the output can be reduced. Maybe 5.7 volts in where the maximum possible current through the LED in the coupler is not enough to cause the output to reduce further. I would try cutting the resistance of the resistor in series with the LED in half and see if that allows you to get below 5.7 volts.
     
  4. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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    The original series resistance value was 100ohms..... I connected a 500Ohms pot in series with 100Ohms and tried to vary the resistance Here I could observe the output voltage increase from 5.7V and 6.3V...... at 6.3V about 8mA current was flowing thru LED....
     
  5. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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    I forgot to mention that I am trying to get 5V from a 230V AC input.....
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    Could you try reducing the resistor in series with the LED to about 50 ohms? If that doesn't correct the problem, it might require a more "hairy" change to the circuit.
     
  7. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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    Optocoupler data sheet PC817C
    upload_2015-5-25_16-49-55.png

    Absolute maximum rating for current mentioned as 50mA..... For 100Ohms at 5.6V out I already sense 25mA....I guess at this current level opto coupler will reach saturation.....
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The circuit uses the AZ431 as an adjustable zener diode.
    This will reduce the voltage accross the series resitor of the led in the optocoupler.

    Bertus
     
  9. DickCappels

    Moderator

    Aug 21, 2008
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    Remember, when you are at 5.7 volts, you loose 1.3 volts across the LED and 2.5 volts across the AZ-431, leaving only 1.9 volts. 1.90V/50 ma = 38 ohms. It is safe to try 50 ohms.
     
  10. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    If you short out the zener anode to cathode, whats the minimum output then , the zener requires 2.5v to strike, the led needs 1.8v ,thats 4.3v, plus the resistor drop , ?
     
    Last edited: May 25, 2015
  11. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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    how do we judge the voltage across cathode and anode of AZ431??...... When the output was 7.5V the drop across LED resistance 100Ohms was 0.38V, across LED was 1.1V and rest whole 6.0V dropped across AZ431.....

    Also Wont PC817 collector emitter be is saturation for such a high forward LED current.....There or many more confusion i have.... how did the current through the LED increase when the output voltage was set for 5V from 7.5V.....That sounds counter intuitive only reason i could fathom is increase in conduction of AZ431 out transistor but I am still unclear of what controls its conduction......
     
  12. DickCappels

    Moderator

    Aug 21, 2008
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    Consider Dodgydave's post.

    Cutting the resistor in half is a pretty safe experiment. If it doesn't work, then don't leave the thing on very long.

    It is understandable that you might not want to try the experiment, there are other areas in which to look. I just thought you might want to do a check on this aspect this aspect before digging in to the details of the mains side of the coupler.
     
  13. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Have you tried measuring the output voltage with a bit of a load on it? If you are trying to get to 5 volts but hitting a 5.7 volt limit you may simply be up against the lower PWM duty cycle limit of the HV side of the system.

    At worst at 5.7 VDC output you can drop it another .6 volts using common power a diode in series with the output which would get your output voltage well under the typical ~5.5 VDC upper working limit of most logic devices and circuits.
     
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  14. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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    This power supply is used for battery charging......I needed a 2A output....Since there was a goof up on Thermal aspects of charger design higher voltage results in thermal runaways....That was the reason why i tried for a lower voltage configuration of power supply......I tried out diode in series and i am able to charge the batter at 2A..... I need to deliver 25 pcs of this charger ..... I have little idea of SMPS.... My initial intuition was that a simple resistance change would help me achieve the said result.....

    Is there any others means to achieve 5V?? .... I have no knowledge to deal with High voltage stuff....
     
  15. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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    I tried connecting one more 100Ohms in parallel to LED resistance .... But even after the change the o/p voltage remained at 5.7V.... I think with this I have exhausted options available at low voltage side....
     
  16. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Have you measured the output when the zener is shorted out Anode to Cathode, that will give you the minimum voltage, your clearly not listening....
     
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  17. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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    With anode and cathode short the o/p voltage dropped to 5.2V.....
     
  18. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Well there you are, 5V as needed....
     
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  19. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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    I didnt see that one coming....thats it...great guys......

    mmm still I am scratching my head.....
    just for clarity .....SMPS had 90V to 300V spec.....

    So now I can get 5.2V out of this SMPS with 90V to 300V voltage... right.... with this change in input voltage will the control loop compensate for variations in input AC voltage.....
     
  20. DickCappels

    Moderator

    Aug 21, 2008
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    Sadly, No. The control loop is open loop. But I still give Dodgydave the boquet of the day for coming up with that test.

    [​IMG]


    @rakeshm55, how much current is passing through the LED when you short the AZ-431 cathode to anode? Within that answer may lay the solution, and allow you to close the loop again.
     
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