Why 1206 resistors used for HV startup pin

[cupoftea]

Hi,
On page 17 of AND9240-D, R100 and R101 are 1206....why?....they could be 0603. All they do is carry the 3mA startup current for the NCP1239 Flyback controller

https://www.onsemi.com/download/application-notes/pdf/and9240-d.pdf

 

[KlausST]

Hi,

Regarding R101, R100, it´s unclear why they arer there at all.

Either there are many mistakes in the schematic of page#17, or I´m too dumb to read it:
* Datasheet of NCP1236 says Pin#8 = HV and should be connected to the bulk capacitor.
* Rectifier "-" (D103, D105) should be connected to primary_GND .. I guess
* RD100, RD102 are 820k each, how can there be useful current flow

Klaus

 

[FvM]

2x 1206 is required due to standard resistor voltage rating.

 

[cupoftea]

Thanks, ayk, these resistors will see a max voltage of 0.003A * 2700R = about 10V.....no need for 1206....they just port the current to the (3mA) current source inside the NCP1239...one of the App notes calls for these resistors so as to make the HV pin less noisy, since it also does HVDC detection for brownout purposes (UVLO)


NCP1239

https://www.onsemi.com/pdf/datasheet/ncp1239-d.pdf

 

[FvM]

You are right, I overlooked that HV pin is 650 V rated. The two series resistors look like they want to provide additional sure protection though. The other point is that the design doesn't use resistors smaller than 1206 at all, it's not necessarily on purpose in each instance.

 

[dick_freebird]

Maybe somebody picked values based on pin ratings rather than pin
application voltages? Or maybe there is expected a fault condition
which differs greatly from the normal (like an easily field-miswirable
" threat-port").

 

[Easy peasy]

For UL you have to allow for single point failure - so if a resistor is shorted you have to work out the volts and power in the remaining resistors ....

 

[cupoftea]

Thanks, if say R100 goes short, then R101 will dissipate 24mW. If R100 stays good, R101 still dissipates 24mW. These resistors are in the path of a 3mA current source (which only works at startup)
I am a bit baffled why they even need two resistors....one 0603 would have been fine.
The datasheet says nothing of these resistors being required at all.

One of the App Notes says they are needed for "filtering noise off the HV pin".

I have a suspicion they are actually only needed when the HV pin is diode connected to the AC side of the mains bridge, to reduce the size of reverse recovery current spikes in the NCP1239 HV pin, but cannot be sure of this.

None of the App Notes say this.

 

[Easy peasy]

You have overlooked what happens if the pin in the IC is shorted to local ground - again - a single point failure

 

[cupoftea]

thanks, ok thats for UL...for us, we not bothered for UL, fuse would blow...datasheet shows no resistors there....just HV pin direct to rail.