Tool/software:
Hello,
we are working on a flyback design using UCC28750 controller.
the secondary which is being controlled, is on the same ground as primary. So, we do not need any isolation and want to avoid opto in the feedback loop.
Could you recommend a way to regulate the output in that case?
Thanks,
Adnan
Hi Adnan,
Thank you for reaching out.
Since the feedback is current on FB pin it is internally pulled up by a resistor it requires a current feedback.
There a few steps which we can take in going from optocoupler to without using one.
Shown below is the implementation for UCC28750 using TL431. In all cases GNDs can be changed for non isolated configuration.
Refer image-1
Now in the above we can replace TL431 with zener diode as shown below:
Refer image - 2
The series drops across Vz, R1 and the forward drop of the LED of opto determine the output voltage. A diode is optional depending on the availability of a suitable Zener voltage. Resistor R2 provides a 0.5 mA bias current so that Vz is operated close to its knee voltage. Resistor R1 adjust the output voltage and limits the feedback current during output load transient.
Vout = Vz+Vopto(forward drop)+VR1
Output regulation is fully dependent upon the choice of these components but I thinkTL431 based scheme offers better regulation.
Lastly,
Refer image - 3
R2 basically is selected to set the bias current in Vz. R3 limits the base current to the transistor, the dotted components are optional which decide the roll off of high frequency gain.
Please refer another concept implementation below
The above are a concept but needs to be verified by you on the actual design.
Regards,
Harish
