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Earth problem?

Winsu

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Hi All,

I have designed a buck converter to power up some LEDs. I have used the IC FL7701MX to design the buck converter. The wattage that the buck converter delivers is about 6W. This IC has also dimming funcionality that I have included in my design. The design seems to work just fine. It delivers the right amout of power/current when working at full power or at dimming level. The application notes for this design includes a PI filter like the one incuded in the below document:


The configuration of the IC is the same than the above document apart for the value of some components to adjust the current needed and apart from the inductor that in my design it is located before the LEDs. My design also inlcudes a PI filter like in the design notes is described ( the values differes a bit from the desing notes).

My design also includes a line filter before the diode bridge.The line filter includes common mode choke, Cx capacitors, Cy capacitors and a RC filter.Below is a drawing where I summarize the whole design.


1618764711581.png


This design is resting in a metallic housing with a LED PCB.

The problem comes when someone touches with the bare hands any metallic external part of the housing causing that the lamp starts to dim a bit. When the lamp is touched by plastic, wood or something else it works just fine. It seems like the human capacitance /static electric is coupling noise in somehow.

I think the problem is earth related . Initially I had in my design the electrical earth and the metallic earth connected by a link resistor, but I have removed the resistor and the problem has improved a bit but it still persits.

The Cy capacitosr are connected between live/ neutral and electrical earth and their value is 1nf each. My next step would be to removed the Cy capacitor or reduce their value to see if it resolved the issue.

What could I try to eliminate the problem?. What is causing the problem?, is it noise being coupled on the earth lines or somewhere else?, what else could it be? Any help is very welcome?

Thanks,
Winsu
 

KlausST

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Hi,

You don't give detailed informations about your circuit.
Just vague informations like: "the circuit is like this but some values are changed".
I feel unable to give useful help. I guess it's not a problem of wrongly working devices nor wrong application note.

Try to show your exact schematic. With all part values.
Show your PCB layout.
Show some pictures of your circuit including wiring.

Klaus
 

Winsu

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Hi Klaus,

I am sorry but unfortunately I can't provide detailed information. I understand it is difficult to provide accurate help with partial information. I was hoping to get some guidelines on how to fit a problem which looks like an earth problem.

Earth noise could cause this type of issues?
 

KlausST

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Earth noise could cause this type of issues?
Hi,
What is Earth noise? Noise is voltage ... and voltage needs to be referred to a reference potential. Often Earth is the reference.
..then Earth noise is zero.

Honestly ... there are many possible reasons ... just guessing is a waste of time.
Did you follow all the recommendations given in datasheets and application notes?

Klaus
 

Easy peasy

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is your LED power supply isolated? if not is the metal case earthed properly ? - if it is there will be no "hand" effect

the case should be properly earthed in any event.

Have you read in detail safety standards that apply to the equipment you are designing ?
 

Winsu

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Hi,
What is Earth noise? Noise is voltage ... and voltage needs to be referred to a reference potential. Often Earth is the reference.
..then Earth noise is zero.

Honestly ... there are many possible reasons ... just guessing is a waste of time.
Did you follow all the recommendations given in datasheets and application notes?

Klaus
Hi Klaus,

I think I followed all the recommendations of the application notes and I also added a line filter with Cx, Cy, common mode choke and a RC filter.

I think all this is related with the earth because initially I have a resistor joining the electrical earth and the mechanical earth. Basically the PCB has 4 mounting holes to allow the PCB to fitted on a metal tray. Once of those holes is connected to the electrical earth ( the earth cable) through a cero ohms resistor, so if the 0 ohm resistor is fitted there is contact between the electrical earth and the housing through the mounting hole, then metal tray , then housing then external earth stud.

By experience connecting the electrical earth to the mechanical earth sometimes improves EMC results but in this case the IC was making funny things. It was flickering/dimming the LEDs and the 0ohm was removed the light was completely stable, so it resolved the issue.

We though the problem was fixed as it wasn't flickering anymore outside and inside the housing. But after handling the unit while on it started to dim when someone touches the bare metal ( most of the housing is painted, apart from cable gland, earth stud and a few nuts).

Also we have noticed that when the external earth stud is connected to an external earth ( which is actually its purpose to allow the mechanical earth path to be completed ) and when somebody touches the metal work it dims even more than when it is not connected to an external earth.

I think the problem is related with the earth, there must be some sort of earth loop where the earth find two different path and then there is a differential of potential.

Would it make sense to test the unit removed the Cy capacitors and/or the earth cable?

If this doesn't work what other things I could consider? for example could the dimming track be picking noise just when someone touches the metal work?. There is a dimming feature, and it is done with a divider resistor and a switch. The switch is relative far from the IC so the tracks is long but I don't think this would be the case...

Any ideas on what to test or think would be helpful.
--- Updated ---

is your LED power supply isolated? if not is the metal case earthed properly ? - if it is there will be no "hand" effect

the case should be properly earthed in any event.

Have you read in detail safety standards that apply to the equipment you are designing ?
Hi Easy Peasy,

It is not isolated, there is not transformer in this design. If this a possible reason for what is happening?

The metal case is supposed to be earthed properly. There should be continuity between every point of the metal case and the external earth stud has to be connected to the earth wherever it is going to be installed.

I am aware that there has to be a complete earth path for safety reasons, so if I cable get loose the earth would conducted the current from the loose cable.

Why is the case that if the metal case is properly earthed then there shouldn't be any hand effect?
 
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mtwieg

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That app note makes no mention of earth connection, but in that application the electronics are buried in a plastic housing. If you have a metal enclosure (not sure what you mean by a metal "tray") then it should be earthed for safety and EMC reasons (and the earth path should be capable of withstanding fault currents, so don't use a little jumper resistor).

You also mention adding a filter before the rectifier. I have a hunch that this is related to the strange dimming behavior. It's possible the combination of filters and rectifier are causing severe distortions at the input of the FL7701MX, especially near the zero crossings, which can confuse the controller. This problem is usually most severe at light load.

But it's not clear why touching the earthed enclosure produces an effect. If the enclosure is properly earthed (meaning the impedance to earth is very low), then introducing additional capacitance between earth and the enclosure should have no effect.

Where is this link resistor in your diagram? Does removing it leave the line filter's GND connected to your enclosure?
 

treez

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you say the problem is that it dims when someone touches the metal enclosure.
You also say the dimming track is long.
Long tracks cause problems.
Either cut or disconnect the long track from the diming pin...put a capacitor right on the dimming pin.
Ensure that the Vcc cap and the RT resistor are right next to the FL7701.

....Now having done all this lot...again touch the metal enclosure...tell us if it dims or not......then we see where to go from there.

BTW, have you put and rc filter on the CS input...say 510R and 220pF......the 220pF right next to the cs pin...this sometimes helps.

Also, please find my pcb layout of smps doc, which will help to reduce these kind of ghost noise problems....by good pcb layout
 

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