TFS7703 PIXLS Design Sheet Query
I have a requirement for a 2-switch forward design, specifically using the TFS7703, but no experience using this topology. The output is a simple dual, 40V /5A and 12V /1.7A. It will be based as closely as possible to the schematic example given in figure 30 of the TFS770x datasheet. The PIXLS design sheet has many variables that are not clear to me, and no data (that I could find) about calculating the Cout, Lout, and opto-isolator feedback component values. I need to rapidly close the design without extensive engineering research (that is why we chose the PI part). Can PI provide more specific details about the use of the PIXLS spreadsheet and quickest way to calculate the mentioned component values for a starting point for our first design.
Tod
Comments
Thanks Wesley. Can you tell me one other thing: The output load range placed on the design will be 200W maximum, and down to around 5 to 10W minimum. I did not see any mention in datasheet of a skip mode for vey light loads. Can you tell me what to expect with this at the very light load level? Will it go out of regulation, etc.?
Tod
There is no explicit skip cycle mode, however it is possible that you will see the high side Mosfet skipping cycles every now and then in very light load. This is because the bootstrap that supplies the high side mosfet driver needs a minimum on time that may not be reached in very light load. This is not a problem as output regulation will be maintained. (check out page 24 of the data sheet).
Low side mosfet will keep on going normally.
Hi Tod.
Sorry that you are struggling a bit, and thanks for the feedback. We try to make the spreadsheets as self explanatory as we can, but I see we need to improve on this for the Hiper family.
Regarding the data you are asking:
Lout (assuming you mean L1 on Figure 30), is calculated by the spreadsheet (starting on line 185 until line 240).
There is some info about Cout on page 16 of the datasheet.
Page 17 offers some info on the feedback loop design, although in both cases I agree that the information is not very detailed.
My suggestion to you is to basically use the standby section to do the 12V design (this shouldn't give you much trouble because the TFS2 uses a TNY engine that is digital). You will get a high power warning because you are slightly above 20W. Use the increased current limit and optimize the efficiency (as you would on any flyback design). You will need to test on the bench for thermal performance.
Since you don't have much experience with designing this type of supply I suggest you asking specific questions in the high power forum: http://www.powerint.com/en/forum/technical-forums/high-power-design especially after reading the datasheet sections I was suggesting.
I hope this brings you forward.