UC3845: UC3845 Gate Signal Problem

Behtan Umut Mercan

Part Number: UC3845

Hello,

I have designed a flyback circuit and I have used UC3845 for the following configuration input 12-18 V. Output 48 V 1 A. I attached the Altium schematic pdf file below.

However, I could not get any gate signal from to UC3845. Therefore my circuit does not working.

Can you please tell me what could be the problem? How can I solve it?

Thank you in advance.

Sincerely,

Behtan Umut Mercan

UC3845_schematic_v2.pdf

over 2 years ago

0 Behtan Umut Mercan over 2 years ago

Prodigy 60 points

Also I would like to add that I am using UC3845 with the voltage mode.

+1 Hong Huang over 2 years ago in reply to Behtan Umut Mercan

TI__Guru 72935 points

Hi,

It looks your schematics should produce the gate signal. So try the below:

1) Measure UC3845 pin 7 to make sure its VCC does get > 9V from you input voltage source

2) If (1) confirmed, measure UC3845 pin 8 (VREF) to make sure its voltage at about 5V

3) If (1) and (2) confirmed, measure the waveform of pin 4 (RTCT) to make sure a triangle waveform present

4) if you get RTCT waveform with frequency 200kHz, then measure pin 1 (COMP) which should be above 4V if pin 2 < 2.3V

Please let me know above test results.

0 Behtan Umut Mercan over 2 years ago in reply to Hong Huang

Prodigy 60 points

Hello Hong Huang,

Thanks for the message. It is resolved with the putting decoupling capacitor to the Vfb pin.

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UC3845: UC3845 Gate Signal Problem