UCC28742EVM-001: Secondary Side filter Capacitor C5

Christian Grau28

Intellectual 375 points

Part Number: UCC28742EVM-001

Dear All

On the low voltage side of the Flyback reference design UCC28742-EVM there's a filter circuit.

I understand what the ferrite L3 and the capacitors C6 und C7 do.

Can anyone explain what C5 is for?

Any help appreciated

Chris

over 4 years ago

0 Mike O' over 4 years ago

TI__Guru 70795 points

Hello,

C5, L3, C6 and C7 form a Pi filter on the output of the fly back converter.

If you don't have capacitor C5 you would discharging the magnetizing inductance of the fly back transformer into L3.

Inductors will resist a sudden change in current and need to conduct current. V = L*di/dt

If C5 was not used this would cause excessive voltage on the secondary output winding, which would be reflected to the primary and possible damage the primary FET or RCD clamp.

Regards,

0 Christian Grau28 over 4 years ago in reply to Mike O'

Intellectual 375 points

Hi Mike

Thank you for your answer. I did not think of that but now that I read it it's perfectly clear.

Kind regards

Chris

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UCC28742EVM-001: Secondary Side filter Capacitor C5