UC3901:

Hello Ramkumar,

The values of these components are based on the you design application requirements.
The specifics of these requirements are located here:

www.ti.com/.../technicaldocuments

You will need to design your UCC28740 circuit first. Then you will be able to design the UC3901 feedback components.

There are also modelling tools available for the UC3901 located here:

Regards

John

Hi John,

Yes we have already designed the Controller design it is the same as the reference design of UCC28740.Now can you tell us how to calculate these component values.

Regards

Ramkumar

The UC3901 is a frequency modulated feedback generator and you can set the RC time constant at one tenth of your carrier frequency.
Do you know what you set the carrier frequency at ?
The resistor on the optocoupler sets the LED forward current. The value of the forward current is based on the required emitter current for UCC28740 and the current transfer ratio (CTR) of the optocoupler that you selected.
Do you know what the CTR of the part you selcted and the required emitter current for the UCC28740 is ?

Regards
John

Hi John
Thanks for the immediate reply.
Carrier frequency is 40 to 90KHz.
We have selected the Optocoupler as FOD817D its CTR is 300 Minimum and Maximum is 600%.

Regards

Ramkumar

Hi John,

We have changed the RC values  to 1/10 of the switching frequency.The wave form is attached.

The result shows lot of Pulses,Let us know how to remove this and what is the solution for the same.

Regards

RamkumarUC3901 1A load opto cathode--00000.pdf.

Hi John,

We have changed the RC values to 1/10 of the switching frequency. The waveform is attached.

The result shows a lot of Pulses, Let us know how to remove this and what is the solution for the same.

Regards

Ramkumar8637.UC3901 1A load opto cathode--00000.pdf

Hello Ramkumar,

I am trying to understand your goals with this application.
The UC3901 is an isolated feedback generator that is designed for applications that cannot use an optocoupler because of  long term reliability issues that are requirements in applications such as aerospace . So it replaces the opto with a high frequency (5MHz max) frequency modulated error amplifier.
Your application uses this part  (at a very low carrier 40kHz) in combination with the UCC28740 which is a primary side regulated flyback controller that has the enhanced regulation due to optical feedback.

So my question is why do you wish to use these two parts together ? 
What are your design requirements?

Regards

John

Hi John,

We wanted to design a very ultra low noise Power supply that is why we have selected the UC3901.

We are fine tuning the RC timing values and now able to little reduction in noise.



Regards

Ramkumar