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Original question:

UC3843A: Flyback Converter Design

UC3843A: Flyback Converter Design

Piyush Hirapara
Prodigy 115 points
Part Number: UC3843A
Other Parts Discussed in Thread: UC1844A

Hello Everyone,

Currently, we are working on a flyback converter of 120W output. We are using UC3843A in which we are getting output but we are facing many problems which are listed below:-

1. The below circuit diagram is used for the startup of the IC , the IC gets to start at 8.8V but gets immediately gets turn off after giving a small pulse off output whereas we have referred the same application in datasheet of UC3843A, moreover as per the datasheet the IC should get turned off at 7.6V but we are getting turned off at 8.4V (the analysis for the same is attached in the below video).

2. Can you please help us the better/best way to power up the iC apart of the tertiary winding as we don't want to go with it. 

Thanks in Advance

Regards

  • 0
    TI__Mastermind 34465 points

    If you look at the device comparison table in the data sheet for this part then you will find it is better to use the UC1844A than the UC3843A because of its greater hysteresis between UVLO(on) and UVLO(off). Also this other part has a Dmax of 50% which is all that is required for a flyback.

    Also in the data sheet you will see that it may take 0.5mA to startup this ic and 17mA to operate it.

    Therefore you need to replace your 380k resistor with a (50-16)/0.5 = 68k

    Your 600uF capacitor then has 21mS to startup your 12V output.

    If it takes longer than 21mS then the power supply will turn on and off continuously.

    You can fix this by increasing the capacitance.

    Regards

    John

  • 0 in reply to John Griffin1
    Prodigy 115 points

    can this circuit damage my ic if i let it ON OFF for some time

    John Griffin1 said:

    If you look at the device comparison table in the data sheet for this part then you will find it is better to use the UC1844A than the UC3843A because of its greater hysteresis between UVLO(on) and UVLO(off). Also this other part has a Dmax of 50% which is all that is required for a flyback.

    Also in the data sheet you will see that it may take 0.5mA to startup this ic and 17mA to operate it.

    Therefore you need to replace your 380k resistor with a (50-16)/0.5 = 68k

    Your 600uF capacitor then has 21mS to startup your 12V output.

    If it takes longer than 21mS then the power supply will turn on and off continuously.

    You can fix this by increasing the capacitance.

    Regards

    John

    is it possible that my ic damages if I let it ON OFF for some time? because my converter not working anymore.

  • 0 in reply to Piyush Hirapara
    TI__Mastermind 34465 points

    Sounds like something is broken so.

    I can really say what the cause may be

  • 0 in reply to John Griffin1
    TI__Mastermind 31580 points

    Hi Piyush,

    Since there has been no resposne to this post for some time I am going to close this post. Please open a new post if you have more questions.

    Regards

    Peter