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UC3843 Issue

Amruth
Intellectual 290 points
Other Parts Discussed in Thread: UC3843, UCC28740, UCC28711

Hi,

       I'm using UC3843 for a Flyback converter, if I load more than 80ma, then output is dropping and reflecting back on the input.

Input Voltage = 100-200 VDC

Output = 24V, 0.5 A

Frequency = 100 Khz

Inductance = 2.3 mH

Output remains 24V as long as output current is < 80ma, but when the load on the output increases, then 12.3 V is there both on input and output.

Thanks,

Amruth

  • 0
    Genius 3295 points
    Hi Amruth,
    Could you provide a schematic of your design? Especially need to see how the PWMIC is set up.
    -Chuck
  • 0 in reply to Chuck Sampson
    Intellectual 290 points

    Hi,

            We are using VIPER22 to generate 15V VCC to UC3843

    When the UC3843 is remove, VIPER generates proper 15V, but when we have UC3843, the voltage drops to 8.2V

    U18 is VIPER circuit, which bucks the 200-500VDC input to 15VDC output to power the Vcc of UC3843.  +12_BUCK is the problem point. When UC3843 is removed, with everything else, VIPER produces clean 15V.  When we put the UC3843, it suddenly drops to 8.2V. Anything rings here ?

  • 0 in reply to Amruth
    TI__Expert 4015 points

    Hi Amruth

    1. Your transformer winding polarity is wrong. The primary and secondary should have opposite polarity and your schematic shows them having the same polarity.
    2. You opto couple appears to be connected upside down, the collector should be connected to COMP and the emitter to ground.
    3. This looks like a complicated and expensive solution. The Flyback can provide it's own bias power after startup and remove the need for the buck circuit. Is the buck required for something else as well, or is it just to improve the standby power. If it is for standby power, a flyback controller like the UCC28740 has High voltage startup and good standby power and could reduce your BOM count considerably.

    Thanks

    Billy 

  • 0 in reply to Billy Long
    TI__Expert 4015 points
    Hi Amruth

    Your transformer winding polarity is wrong. The primary and secondary should have opposite polarity and your schematic shows them having the same polarity.
    You opto couple appears to be connected upside down, the collector should be connected to COMP and the emitter to ground.
    This looks like a complicated and expensive solution. The Flyback can provide it's own bias power after startup and remove the need for the buck circuit. Is the buck required for something else as well, or is it just to improve the standby power. If it is for standby power, a flyback controller like the UCC28740 has High voltage startup and good standby power and could reduce your BOM count considerably.
    Thanks

    Billy
  • 0 in reply to Billy Long
    Intellectual 290 points

    Hi Billy,


    Thanks for replying, finally we are able make it work without Viper. Presently we are using voltage drop with 12V Zener at the end to do the start-up, it is working fine.

    We are contemplating on using UCC28711 or UCC28740. Kindly recommend which one to go for.

    I see the primary difference is that UCC28711 is primary side regulation, with max duty of 0.45.   I'm designing  200 to 600 VDC input, 24V 0.5A dual isolated output. Kindly help me to make the choice.

    Note: we are using this for Solar inverter, so that input voltage can vary from 100V to 600V , under different conditions. It will be haphazard during morning, evening and cloudy conditions.

    Thanks,

    Amruth

    ThingsCloud.com

  • 0 in reply to Amruth
    TI__Expert 4015 points
    I would say it depends on your regulation requirements. If one of the outputs requires tight regulation, then the UCC28740 is the better option, but if both have the same regulation requirments the the UCC28711 allows you to simpfly the system by removing the opto coupuler which will also improve the reliability of the system. Cross regulation of < 5% may be difficult to achieve.
    Thanks
    Billy