Ferrite bead in Drain of offline Flyback and Forward SMPS's?

[HighTechPower]

Hi in reference to a power supply design book by author Mr Sanjaya Maniktala, I would like to say that there might be seen this mentioned product failures after mass production due to high di/dt or other reason so due to short delivery time & to make the solution less costly they simply inserted a ferrite bead. This type of examples are mentioned in one of Sanjaya's book. Plz go through them if you can.

--- Updated Feb 25, 2022 ---

I think the book title is Troubleshooting switching power converters. Their are examples where manufacturer used ferrite beads as quick & cheapest solution to the problems in hand.

 

[Easy peasy]

What " problems in hand " did the ferrite beads solve ? exactly ?

 

[HighTechPower]

What " problems in hand " did the ferrite beads solve ? exactly ?

Well bro I gone through the book quickly & it's mentioned in chapter 7 with figures 7.3 & 7.4. They inserted ferrite bead in cathode of output freewheeling diode of flyback power supply to limit high dv/dt that was causing it's failure. The diode used was cheaper & of low dv/dt type and parasitics were creating high dv/dt and causing it's failure.
In another example at page 246 they inserted ferrite bead in leg of Y capacitor in input EMI filter to cover the FCC etc standards.

--- Updated Feb 26, 2022 ---

Also see figure 7.5 where they say the use of bead to damp the ringing of output diode.
Yet on page 240 they recommend the use of ferrite bead to kill the energy associated with CM noise.

 

[HighTechPower]

However on page 177 figure 8.2 he mentions not to use beads at common input bus of parallel converters to avoid dangerous inductive spikes that will be generated in this case.