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    Power supply output voltage problem

    Hello,
    I have used PI expert to design a dual output voltage power supply 5V and 12V. 5V is the one being regulated and it needs to be fairly stable as it supplies sensitive circuits. The 12 V supply is only feeding a relay, so it is allowed to vary. The output is 5.6V, where I need to get 5V. For flyback loop, I have used TL431 and optocoupler PC817.
    I have also used zener diode 5.5V instead of the TL431. But when I used zener the output voltage is 6.12V
    I am struggling to understand what is wrong..?.
    Any suggestions would be most welcomed.

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    Re: Power supply output voltage problem

    Quote Originally Posted by rohithadpad View Post
    Hello,
    I have used PI expert to design a dual output voltage power supply 5V and 12V. 5V is the one being regulated and it needs to be fairly stable as it supplies sensitive circuits. The 12 V supply is only feeding a relay, so it is allowed to vary. The output is 5.6V, where I need to get 5V. For flyback loop, I have used TL431 and optocoupler PC817.
    I have also used zener diode 5.5V instead of the TL431. But when I used zener the output voltage is 6.12V
    I am struggling to understand what is wrong..?.
    Any suggestions would be most welcomed.
    The TL-431 is an adjustable three terminal regulator. There are two resistors that can be coordinated in order to fine tune the output voltage. You should be able to get 5v to be very accurate.

    The zener approach would require you to use a part with less than 5v in order to account for the drop across the zener. A 4.5v zener might work good for you.


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    Re: Power supply output voltage problem

    Hi,

    I have also used zener diode 5.5V instead of the TL431. But when I used zener the output voltage is 6.12V
    The zener voltage depends on zener current. Read zener diodedatasheet.

    Klaus
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    Re: Power supply output voltage problem

    You must take into account the voltage drop over the opto-led as well.



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    Re: Power supply output voltage problem

    Quote Originally Posted by PowerSupplyGuy View Post
    The TL-431 is an adjustable three terminal regulator. There are two resistors that can be coordinated in order to fine tune the output voltage. You should be able to get 5v to be very accurate.
    Thanks for your reply. Zener of 4.3V has helped me. Now I am getting 5.1V as output.

    But in case of TL431, as you can see in the attachment that I have used 10K resistors in voltage divider circuit which should adjust TL431 and give me the output voltage as 5V. But my output is not 5V. Can you help me understand why the output is not 5V??



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    Re: Power supply output voltage problem

    hello,
    now the output of the circuit is 5V. but when I am doing load regulation voltage across load changes with the resistance. IS there any problem in the circut?

    I have attached my load regulation readings.Click image for larger version. 

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    Re: Power supply output voltage problem

    you are measuring at end of long leads?...(conduction volt drop).
    Also, have you ensured you can always get enough led current through the opto?
    What current does your pwm controller need extracted from its FB pin in order for your reg circuit to be able to fully brake the smps.....can your opto ctr ensure this current is reachable?
    Is you layout poor?
    Are you getting switch on current spikes which are so large that the dead time cannot handle it (LEB).

    Is you transformer saturating.

    Is the controller stopping the primary peak current from going high enough?

    Do you have a lot of resistance in your post filter?

    Is it the diode drop getting bigger as current increases?

    How have you done the primary clamp (RCD)?
    You are getting lots of loss in your clamp?
    Are you using a fast enough diode (low trr)?
    Did you use a big enough primary bus capacitor?



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    Re: Power supply output voltage problem

    Can anyone explain me the role of Capacitor C15 in the given schematic??



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    Re: Power supply output voltage problem

    That is part of the feedback loop stabilization.
    My batteries are recharged by "Helpful Post" ratings.
    If you feel that I've helped you, please indicate it as a Helpful Post


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    Re: Power supply output voltage problem

    Hello rohithadpad,

    But in case of TL431, as you can see in the attachment that I have used 10K resistors in voltage divider circuit which should adjust TL431 and give me the output voltage as 5V. But my output is not 5V. Can you help me understand why the output is not 5V??
    It could be due to components not being 100% in their tolerance values.
    You could replace R17 with a 20k trimpot and adjust it to give you exactly 5V under load.
    If you do insert a trimmer, make sure its set to its mid position before firing the PSU up.
    You can then adjust it (under load) to give an accurate 5 volts.
    Regards,
    Relayer



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    Re: Power supply output voltage problem

    Hello Sir,
    Thanks for your reply on my post. I am having some more questions so I am messaging privately with you.

    I am facing one problem right now. I have the dual output of the power supply as shown in the circuit. I am checking the output voltage for 4 different conditions, as:
    1.When there is no load connected at either of output.
    2.When there is full load (0.1A) at 12V and no load at 5V.
    3.when there is full load at both the outputs.
    4.When there is No load at the 12V output and full load (0.5A) at the 5V output.

    I have attached my output readings. they are taking at 2 conditions when C15 is connected in circuit and removed from the circuit. There is huge fall in Voltage at the 12V terminal. I am unable to understand why this is happening.

    Note:
    1. All 12V output readings are taken when the pre-load of 1KOhm is connected.
    2. The transformer used has two windings for 5V and 12V output.Click image for larger version. 

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