High voltage startup circuits of flyback controllers (NCP1239 & NCP1219)

[Flyback]

Hi,
In AND8393 page 2 it shows a diode (D13) and two 1206 resistors (R41=R42= 1k82) being used in the HV startup circuit to NCP1219.
The startup current is only 12mA and only flows for some few hundred milliseconds at startup, so why bother to use all these components?
Why not just have say two 0805 resistors (ie not 1206) coming down from the HVDC bus, and not bother with the diode?
The power in each of the R41 and R42 resistors is a mere 260mW for a few hundred milliseconds at startup, so surely an 0805 would be fine?

Also, in AND9296 page 3 it shows two diodes (D1 and D2) and two resistors (R5 and R6) in the HV startup circuit of NCP1239.
The startup current of NCP1239 is only 3mA, and this only flows for a few hundred milliseconds at startup. During this few hundred milliseconds, the
dissipation in the R5 and R6 would only be 24mW. As such, why have 0805 resistors been used when 0603 would suffice? Also, why bother to connect them to the AC lines with D1 and
D2?...They could simply be connected to the HVDC bus without bothering with the diodes. The dissipation is so small and limited only to startup time.


AND9296

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwi397mOzquIAxUiVUEAHRYlFJsQFnoECBQQAQ&url=https%3A%2F%2Fwww.onsemi.jp%2Fpub%2FCollateral%2FAND9296-D.PDF&usg=AOvVaw1p9MsI_9dNFeWzqVHY100L&opi=89978449

AND8393

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjL9JaBzquIAxWPWEEAHQ4HDtgQFnoECBMQAQ&url=https%3A%2F%2Fwww.onsemi.com%2Fdownload%2Fapplication-notes%2Fpdf%2Fand8393-d.pdf&usg=AOvVaw0tg6Sjh_pZMrl4JfJhu0jN&opi=89978449

 

[Nigel Goodwin]

You need to consider the voltage ratings of the resistors, generally higher wattage resistors have higher voltage ratings.

You also need to consider reliability, designers with only theoretical experience often put resistors in series as shown - these tend to be VERY, VERY unreliable. Resistors in series in this way commonly tend to go high or O/C, as new TV's, VCR's etc. were released you could predict which ones would have failing PSU's because of the use of resistors in series.

If they are specifying particular sizes of resistors it would be foolhardy to make them smaller!.