Issue with voltage sag on flyback converter when load is attached.

Discussion in 'Homework Help' started by MCTG, Apr 11, 2015.

  1. MCTG

    Thread Starter New Member

    Nov 25, 2014
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    I am an electrical engineering student working on the power supply component of my senior design. I am running into an issue with my flyback converter.

    I am currently using an TI's LM5022 chip with the PMP7760 reference design also provided by TI. All information on this can be found at http://www.ti.com/tool/pmp7760. I am not altering their design at all, using all the same components as provided by their BOM. My input is the wall voltage sent through a transformer and rectified to create 75V.

    TI's design states that the output can be up to 1.5A 21V. However, whenever i attach a load to the output the voltage sags significantly to reduce the current output. For example, i have attached a 60 ohm load to the power supply to draw 1/3 amp and the voltage output reduces from 21V to 5V. I have verified that my rectifier stage does not sag through measurements and by swapping out my rectifier with a lab DC voltage supply, so the issue is definitely within the flyback converter.

    This is my first usage of a flyback converter, so please forgive my ignorance. I am using the transformer suggested by TI in their reference design, so i would hope that the transformer would not be saturating. The only other thing i can see that may be an issue is the fact that their is no feedback from the main output back to the switching chip. The feedback comes from another winding of the transformer that is not loaded. So i do not understand how the chip is supposed to react to a change in the load without any feedback.

    Does anyone have experience with this type of voltage sag or have any suggestions on how to attack this problem? I am running out of ideas and would greatly appreciate any help provided.
    upload_2015-4-11_16-8-22.png
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    And your schematic shows a Vin of 30-800V. Either way, the datasheet for the 5022 says the absolute max Vin is 65V, 60V recommended. I foresee magic smoke if Q100 ever had problems :(.
    I'm curious as to why you start off with a 75V supply if ultimately you want 25V?
     
    Last edited: Apr 11, 2015
  3. MCTG

    Thread Starter New Member

    Nov 25, 2014
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    I see your point with Q100. It is a fairly robust transistor though. It is the FQD2N100. Its drain to source voltage is 1000V and its continuous drain current is 1.6A, so you would hope nothing would hurt it in this schematic. I do agree though, it might be bad if a transient or some ESD destroyed it. There is not a great reason i am starting with 75V. I started working on this reference design trying to get 170VDC on the input. I had issues pulling too much current from the wall while my capacitors were charging, so i decided to bring my voltage down to a point that this design will still work with since i already became invested in this design. I had already purchased all of the components and assembled the circuit. Looking back, i think i may have been able to get away with using smaller caps on the input, feel free to correct me if i am wrong. I admit, it doesn't make much sense at this point, i am probably going to switch to starting with 25V now. I am still curious though on why i am not able to keep the voltage up on the output when current is being pulled. I was hoping to walk away from my project with a good understanding of flyback converters, but i feel like i am missing something that should be obvious.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    MOD NOTE: Moved from The Projects Forum to Homework Help.

    This should increase the number of people that see your thread and who routinely help students. It also helps ensure that the type of assistance you receive is appropriate for a graded assignment.
     
  5. MCTG

    Thread Starter New Member

    Nov 25, 2014
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    Thank you, i apologize if i did not follow the proper etiquette.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    Not a problem at all. The distinction between a project and homework is not that obvious, particularly to newbies. Hope you get some good assistance. This is an area that I'm not familiar enough with to even try to help.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Do you have a spec for that? I can't find it anywhere on the web, so can't run a sim to check performance under load.
     
  9. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Your two Zener diodes, D100 and D110, do not have anywhere near enough current going through them for satisfactory operation: their voltage is likely to be substantially lower than their rated Zener voltage, they will provide poor regulation due to their higher than normal impedance, and their voltage is likely to be VERY noisy. As a general rule of thumb with a Zener diode (and it's ONLY a rule of thumb), design the circuit so that at minimum input voltage and maximum load current, the Zener still has at least 1 milliamp going through it. Again, that's only a rule of thumb: consult the device data sheet.
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    As I read the circuit, neither zener is critical to regulating the output voltage. They come into play at power-up. D100 limits the gate voltage on Q100, and D110 limits the Q101 base voltage for managing under-voltage lock-out. So the rather vague zener voltages due to current starvation shouldn't matter in this instance.
     
  11. #12

    Expert

    Nov 30, 2010
    16,261
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    My rule of thumb is: A zener is pretty stable at 1/2 the wattage it was designed for.
    Of course, this is old wisdom, like 40 years old. I have already received news about the improvements in zener technology because of this thread.
     
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