UC2845: The duty ratio of UC2845

Kun Yuan

Part Number: UC2845
Other Parts Discussed in Thread: TL431

Hi E2E,

My customer use UC2845 to make a flyback. Vin=24V,6*Vout=24V,One 12V auxiliary winding output, oscillation frequency 220 khz, output frequency 110 khz, optocoupler feedback. They met a problem.

At present, the duty ratio of the transformer's input terminal is 4-11%, and the input and output power is small. We want to increase the duty ratio to increase the input and output power. However, several transformers have been replaced, and the duty ratio can not be adjusted (want to be adjusted to about 30%), basically maintained at about 9%, and the input power is about 8.6W. The schematic is as attachment.

7824.POWER.pdf

over 6 years ago

0 Peter Meaney over 6 years ago

TI__Mastermind 31580 points

Hi Kun,

Can you send some waveforms of the COMP pin, ISEN pin as well as the input voltage, output voltage and loading. I also need the transformer primary inductance.

Thanks

Peter

0 Peter Meaney over 6 years ago in reply to Peter Meaney

TI__Mastermind 31580 points

Hi Kun,

Its been seven days since the last activity on this post, I hope you have made some progress in the meantime. I am going to close this post. If you have more questions please open a new post.

Regards

Peter

0 Kun Yuan over 6 years ago in reply to Peter Meaney

Intellectual 945 points

Hi Peter,

Transformer output 24V and Vin 24V waveforms as attachments. Other pins waveforms have been damaged, please wait a little longer time.

BRs

KUN

0 Kun Yuan over 6 years ago in reply to Peter Meaney

Intellectual 945 points

hi Peter,

The waveforms as attachments. Please refer ,thanks!

COM pin

ISEN pin waveform

PWM OUT Pin waveform

VI waveform

VREF waveform

Oscillation waveform

Any you want, please let me know.

BRs

KUN

0 Peter Meaney over 6 years ago in reply to Kun Yuan

TI__Mastermind 31580 points

Hi Kun,

The COMP pin voltage is about 1.6V, this voltage level is been set by the voltage control loop on the secondary side, based on the TL400 (TL431).

As the load on the power supply increases there should be a small drop in the output voltage this should cause the TL431 to shunt less current so the voltage on pin 1 of the opto-coupler should rise as the output load is increased.

Regards

Peter

0 Peter Meaney over 6 years ago in reply to Peter Meaney

TI__Mastermind 31580 points

Hi Kun,

I have seen that you opened a new post, please contniue to reply to this post if you have more questions.

Have to checked to see if the voltage on the COMP pin varies with load?

Thanks

Peter

0 Peter Meaney over 6 years ago in reply to Peter Meaney

TI__Mastermind 31580 points

Hi Kun,

Since there has been no activity on this post for over a week now I am going to close it. Please open a new post if you have more questions.

Thanks

Peter

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UC2845: The duty ratio of UC2845