UC3842: Vcc of UC3842 not steady

Sachin Devassy

Part Number: UC3842

Dear All,

I am making a flyback converter using UC3842 (Output 12V, 12W, Input 100V-220V). The desired voltage is 12V however, the initial output when the feedback resistor was 9.5k was 21 V . When the Feedback resistor Rf was changed to 3.3k, then output is 15V. The Vcc voltage is sort of a square wave. I am attaching the waveforms (Ch1=Vcc, Ch2=Output Voltage).

what could be the reason for this. The polarity of the transformers is correct. The led biasing resistor was also changed but issue was not solved. Is this feedback issue or startup issue.

over 1 year ago

0 Steven Mappus1 over 1 year ago

TI__Mastermind 32165 points

VCC should not be a square wave. Try powering VCC directly from DC lab supply and make sure your current and voltage control loop are stable. Make sure the polarity of the AUX winding is correct with respect to the VOUT winding. Check to make sure the AUX diode is not damaged.

Regards,

Steve

0 Sachin Devassy over 1 year ago in reply to Steven Mappus1

Intellectual 425 points

Thanks Steve for the reply. So should I power on vcc using dc supply and then ramp the input supply to check output regulation or should I get get some voltage at output and check pwm signal

0 Steven Mappus1 over 1 year ago in reply to Sachin Devassy

TI__Mastermind 32165 points

Power VCC with external DC supply
Check VREF and oscillator ramp to make sure IC is alive and ready.
slowly increase VIN and check for regulation at VOUT

Steve

0 Sachin Devassy over 1 year ago in reply to Steven Mappus1

Intellectual 425 points

Hi Steven,

I powered Vcc with external DC supply. Then when I am increasing input, the fuse (1A) is getting blown. The output resistance kept is 50Ohms.

0 Steven Mappus1 over 1 year ago in reply to Sachin Devassy

TI__Mastermind 32165 points

Is the converter starting up in current limit? If your current sense is set up properly, the average current should be less than thew rated fuse current? try starting up Can you show some waveforms and maybe a schematic showing how you are biasing the converter with the external DC lab supply? Your external DC bias should look like below:

Steve

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UC3842: Vcc of UC3842 not steady