UCC28730: UCC28730-Valley Switching

Hello, 

Our office is closed Monday for the US Martin Luther King, Jr holiday. We will get back with you on your question early next week.

Best regards,

Don

Thanks for your interest. And I will be waiting your answer asap. Thanks again.

Hi Sahin,

Thank you for the query on UCC28730.

As you correctly say, when  the output rectifier has reduced to zero there is a small dead time which allows for low/no reverse recovery diode loss and low turn on loss but there is a component of Coss (switch node cap.) related loss which depends on the square of voltage and this has significant effect on the efficiency at light load. Valley switching mechanism helps reduce this loss by switching as close to the minimum value (less than the input voltage).

The following video (around 7th min) explains this.

https://training.ti.com/understanding-basics-flyback-converter#:~:text=Valley%20switch%20flybacks%20have%20all,at%20the%20current%20sense%20resistor.

Regards,

Harish

I really appreciate for your help and explanation. The knowledge and video which you've shared with me was also superb. But,I still have a question about valley switching. Even though we use valley switching there is still a certain amount of energy which is caused by this parasitic capacitance and mosfet drain-source stress voltage which is reflected from the output. As you said,valley switching helps to reduce losses caused by Coss. If can help about my question that would be perfect. Where is this power dissipated during operation? Thanks from now.

Best regards.

Hi Sahin,

Thanks for the reply.

The energy will be dissipated across the switch during turn-on. Many manufacturers include this number in the (Eon coefficient). You can refer to this long useful post which discusses this in detail.

https://www.edaboard.com/threads/coss-related-switching-losses-eoss.375240/

Regards,

Harish

Hi Harish,

Thanks for your answers and help.The things you've shared with me are more than knowledge to me. But I have one more question about the control law used in UCC28730. Is there any possibility that I can see what is inside of this control law? How does it realize AM and FM? I would appreciate if you could share your opinions with. Thanks from now.

Best regards

Şahin.