Long mains cable to product filters noise out, or brings more noise in?

Hello,
We have an open collector comparator (TS391ILT) in our non-isolated offline LED lamp.
It sometimes trips due to noise. We wish to try and assess how to reduce the noise.

The Lamp has no AC input filter, as it is mostly linear regulator based.

The comparator is connected as per the attached schematic.
(The input signal comes from a 220R resistor in series with the output of an opamp. The 8V3 reference is from the output of an opamp connected as a noninverting amplifier)
Even when the input signal to the comparator is continuously high (ie 12V), the comparator still sometimes trips (presumably due to noise) and gives a high pulse at its output. In fact, it’s a little more involved than this, because when we connect the product to the mains via an isolation transformer, the comparator does **not** trip and go output_high when its input signal is continuously high. So in other words, powering the product through a mains isolation transformer makes the product less noisy. (and thus prevents noise tripping of the comparator)
Do you believe that the leakage inductance of the mains isolation transformer, and its parasitic Live/Neutral capacitance, is actually filtering out the noise from the mains and stopping it from pervading around the LED lamp? Also, do you believe that a very long length of mains cable to the lamp would have (to a degree) the same parasitic effect as the mains isolation transformer?...ie, in terms of having a stray inductance and capacitance (albeit smaller than that of the isolation transformer) which acts as a filter?….or alternatively, do you think that a long length of mains cable leading to the lamp would actually bring more mains bourne noise to the product, by acting as an “antenna”?

TS391ILT comparator:
https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.CD00001660.pdf

Hi,

I don´t blame on the comparator.

There is no input filter, neither against noise (and for EMC), nor against ESD.
And there is no hysteresis.

For every external input I recommend to use filters.
The hysteresis just ensures clean output signals when the input level is close to the threshold level.

To choose another comparator won´t bring much benefit (as long as the new one isn´t much slower or has internal hysteresis)

Klaus

Thanks, i appreciate and see the logic of your kind comments.
I do recognise that adding filters will make it less noise susceptible.
Though i must admit we are really trying to kill cost and totally reduce components with this design.

-the thing is the reference input to the comparator is directly from the output of an noninverting opamp which is only 3cm away.
Also, the signal input to the comparator is from a 220R series resistor in series with an opamp output which is only 1cm away from the comparator.
The comparator'ss Vdd pin has a 100n ceramic capacitor just 5mm away from its vdd pin.
Also, i wasnt sure how adding any more filtering was going to help the situation with ESD?
We thought the comparator's internal ESD diodes would help with ESD?

Long mains cable to product filters noise out, or brings more noise in...

Click to expand...

It can be both; but under normal operation, the cables carry more noise than they take up from outside sources...

Power supply lines tend to be noisy but they soon get thermally equilibrated with the local environment. At that time it will pick outside noise.

Perhaps a simple capacitor filter will do; but the noise is all locally generated- perhaps in the same building.

Modern comparators are fast and hence more susceptible to noise. Good old slow comparators are perhaps less sensitive to noise.

Just have a small cap to the input to slow down the signal...