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Original question:

UC3842: SOFT-START CIRCUIT

UC3842: Starting issue

Venkatesh B
Intellectual 783 points
Part Number: UC3842

Tool/software:

Hi Ti Team,

I am using UC3842 IC for 250W Flyback converter for charging application

when i turned ON giving input from Variac(0 to 230V )AC converter works fine,but when i give direct 230V AC converter Fails,shunt ,Uc3842 IC ,Mosfet Blown 

What could be the reason for this 

Attached the schematics

5a_phoenix_v2.3.pdf

Thank you

venkatesh B

  • 0
    TI__Mastermind 32775 points

    When you use the variac to apply VIN, you are slowly introducing the input voltage which is limiting the inrush current at startup. Also, the variac inductance may be helping to limit the inrush current. When you apply VIN from 220Vac, there is essentially no current limiting. 

    1. Try using an NTC and relay inrush current limiting circuit and insert this before the input bridge rectifier
    2. Try measuring the voltage at U1-3 to see what is happening at the I_SEN pin during startup. Use single event trigger at startup. Measure the voltage at the pin using tip and barrel scope probe technique as shown below:

    Regards,

    Steve