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

It sounds like you are injecting too much current into the BP pin, triggering latching shutdown. This is covered on page 12 of the Hiper-TFS datasheet. If you are going to use an injected current to enable the main, then the current should be controlled so as not to hit this shutdown threshold.. As an example, the DER-472 circuit uses a 2-transistor current source enabled by an optocoupler to inject the right amount of current into the BP pin to activate the main.