UCC28600-Q1: Quasi resonant operation

Hi Rohan,

Thank you for the query on UCC28600.

You can refer to the following low power designs with UCC28600.

PMP8471, 

PMP7148, 

PMP7033

Regards,

Harish

Harish, thank you for your response. I have 2 follow-up queries on this design:

1. I need it to be a non-isolated Flyback. How do I modify the FB  pin circuitry for the same? Should I assume fixed 5V FB pin voltage?

2. I followed UCC28600 datasheet Rev. K (Pg. 26), but Fsw(VinMax) I was getting is around 781kHz which is unrealistic. Is there a typo or am I missing something?

My design parameters are as follows: Vin: 60 to 470V

Pout=4W

Efficiency=80%; Vf=0.3

Nps=7.84 (calculated using datasheet formulae)

Lp=1mH (calculated using datasheet formulae)

Note: I am replacing VPFC(max) in formulae with VinMax=470V

Hi Rohan,

1. It will need FB information to change operating modes depending on the load condition. Tying it to 5V would not help this. If you do not want isolated, you could connect the grounds through through low resistances (like 50 millionhm)

2. I thnk you should use the excel calculator and use values as recommended by the excel calculator to start with. Putting approximate values there, I did not get such high frequency you mentioned (see screenshot below). Please specify the output voltage also to the values you listed above.

Thank you

Regards,

Harish

Harish Ramakrishnan5 said:

1. It will need FB information to change operating modes depending on the load condition. Tying it to 5V would not help this. If you do not want isolated, you could connect the grounds through through low resistances (like 50 millionhm)

Harish, I won't be tying it to 5V. I meant for output voltage calculation. Can I use a simple divider to the FB pin without using an opto-isolator,

Harish Ramakrishnan5 said:

2. I thnk you should use the excel calculator and use values as recommended by the excel calculator to start with. Putting approximate values there, I did not get such high frequency you mentioned (see screenshot below). Please specify the output voltage also to the values you listed above.

Regarding the calculator, this looks like its for Ac-dc application. Can I set Vac(min) and Vac(max) to match "Vbulk min and max" with "Vdc min and max" values?

I'll share the rough schematic or requirements with you over chat so you can get a clearer picture.

Thanks,

Rohan

Hi Rohan,

1. Connecting a simple resistive voltage divider will not work. Please refer PMP 4043 schematic for a non isolated implementation example. https://www.ti.com/lit/df/slur912/slur912.pdf?ts=1666073233445&ref_url=https%253A%252F%252Fwww.google.com%252F

You can also try the following advice mentioned in another e2e thread which recommends using an external amplifier to drive the FB pin.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/499720/ucc28600-in-non-isolated-feedback/1809909?tisearch=e2e-sitesearch&keymatch=UCC28600%252525252525252520AND%252525252525252520FB%252525252525252520pin#1809909

2. The calculator can be modified for DC use by just scaling the by a factor (1/1.414) and feeding the relevant parameters.

Please let us know if you have any questions.

Thank you

Regards,

Harish

Hi Harish, I went through the thread, but there is no mention of the calculations in it. Is there a design guide, or calculator, or even formulae to design this? That would be really helpful

My parameters are: 

Vin: (DC) 60 to 470V

Outputs: 15V, 200mA -8V, 200mA, 5V, 100mA

Hi Rohan,

Thread which I pointed to shouws a way to implement non isolated case. We do not have any calculator to demonstrate this. You can refer to the PMP-4043  design which I mentioned in the previous thread as an example implementation.

Thank you

Regards,

Harish