SMPS PWM controllers self-destruct at power down?

[Flyback]

Hello,
Pages 14 and 15 of the NCP1217 PWM controller datasheet show that PWM controllers with an internal high voltage startup regulator have a definite chance of destroying themselves with destructive latch-up at power down.
The NCP1217 shows how to add external components in order to prevent this from happening.

(Eg with a flyback SMPS When switched off at the mains, the FET keeps switching as the primary DC bus collapses, and then the primary DC bus capacitor resonates with the primary inductance and actually swings negative due to the decaying rail voltage getting lower….since the primary DC bus is directly connected to the High voltage startup regulator of the chip, the negative swing wrecks the control chip).
NCP1217 PWM controller datasheet:
https://www.onsemi.com/pub_link/Collateral/NCP1217-D.PDF
Why for example does the L6566B pwm controller datasheet say nothing of this real and important problem?
L6566B pwm controller datasheet:
https://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00167474.pdf

 

[mvs sarma]

i suggest to use two resistors of 270K in series across the electrolytic capacitor and they help discharging the capacitor after shutting down and thus help protecting your smps.

 

[Flyback]

thankyou, though I fear that even that will not stop the dreadful ringing...the pages of the datasheet say we must put series resistance in the HVS line, and also put a diode from Vcc to the primary DC bus. Why I wonder does the L6566B datasheet make no mention of this problem?