TFS7708H , main converter issue Ref : DER 472

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Good day!

We are working on Two switch forward family of PI Part # TFS7708H without PFC ,
I/P AC range : 180VAC to 285VAC
O/P : 30V 10A and 60V 5A

Sample reference design we are following is DER - 472 (61V 4.59A ) and
Design Example: 12 V / 15 A Main Output, 12 V, 0.83 A Standby

Now the standby converter is working fine (12V / 1A ) transformer EF20
Problem is with Main Converter, upon injecting current on by-Pass(BP) of TFS7708H
the whole converter with standby converter switches off and the then takes several minutes
for the standby converter to start again, meanwhile the primary bias of standby converter drops to 3V ~ 5V.

Hereby, i am attaching PI design excel file containing our exact design .

Please suggest how can we address this issue.


30V_10A_highline.pixls77 KB

Did you measure how much current you are injecting into the bypass pin? As I said in a previous post, if you inject to much current, the device will disable both the main and standby supplies. This is covered in the data sheet, and also mentioned in the data sheet description of the bypass pin.

BTW, a schematic would be far more useful in determining the cause of your problem rather than the *.pixls file. If you feel uncomfortable showing the schematic in its entirety, the portion showing the primary bias circuit and the current injection circuit used for the bypass pin would be a decent start.