question about DCM and FCCM

Zhihong Huang

Intellectual 1550 points

Hi team,

I have some questions about the FCCM and DCM.

The formula of the VIN/Vout in the DCM mode is shown below, but I don’t understand how this formula is came out. Do you know how to deduce this formula?
What kind of application scenario will we recommend customers to use DCDC with FCCM mode and why?
Is there a big difference between DCDC efficiency of FCCM and DCM? Will this difference in efficiency increase with frequency?
If you have relevant materials such as application note or slides comparing DCM mode with FCCM mode in terms of efficiency or other aspects, please share them with me.

Best Regards

Wesley Huang

wesley-huang@ti.com

over 4 years ago

0 Lishuang Zhao over 4 years ago

TI__Expert 7500 points

Hi ZhiHong,

1. For the formula, you can refer the below link analysis page 10-18. The analysis technology is based on inductor volt-second balance and capacitor charge balance.

https://ecee.colorado.edu/~ecen5797/course_material/Ch5slides.pdf

3. If the customers want to fix frequency, lower Vout ripple and don't care about efficiency at light load, we can recommend FCCM.

4. Yes. FCCM efficiency is lower than DCM at light load. It will increase the switching loss as well as conduction loss because it is always switching.

5. You can look up any relevant documents you want online. For the questions above, you can refer to below application note.

http://www.ti.com/lit/an/slva388/slva388.pdf

Thanks,

Lishuang

0 Zhihong Huang over 4 years ago in reply to Lishuang Zhao

TI__Intellectual 1550 points

Hi Lishuang,

Thanks for your reply, it benefits me a lot. I have a question about your reply.

1. Do you mean that DCDC with FCCM must be working under fixed frequency? Why can't it work in Variable frequency mode such as ECO and Sleep mode?

Best Regards

Wesley Huang

wesley-huang@ti.com

0 Lishuang Zhao over 4 years ago in reply to Zhihong Huang

TI__Expert 7500 points

Hi Wesley,

Yes. FCCM is fixed frequncy from 0A to full loading.

Which part did you use? You can submit a thread with your part name in title, then it will have dedicate engineer to support you according to the part's performance.

Thanks,

Lishuang

Power management

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question about DCM and FCCM