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Earth interacting with a AC/DC converter

Winsu

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Hi All,

I have designed a converter using the FL7701MX to step down the voltage from 230V AC to 12V. It is actually a LED driver and current is set at 500mA.

The driver works fine when the Neutral and the Live is connected, but when the earth cable is also connected it start to make a noise. The earth cable is connected to two Cy capacitor of 1nf ( live and neutral). It is also connected to a mounting hole to make connection between the mechanical earth and the electrical earth.

Between the mounting hole mentioned before and the the electrical earth connection there is a 0ohm resistor. The application keep making noise with and without that 0 ohm resistor.

The driver is placed on a metallic tray and the LED board is placed on a metallic tray as well. There is a cable from the driver to the LED board to make the connection. There are also ferrites at the output stage of the PCB driver for high frequency noise( this may not be very relevant for this problem).

I don't what is causing this issue. I think this is a bit weird, I don't actually know what could be causing this and what is worst I don't know what investigate further, because I have never faced a similar problem.

Could anyone point me the right direction?

Thanks all!
 

treez

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It may be unstable without the earth connection too...but unstable at a frequency above audible.
Show a scope shot of the drain node pulses...at a timebase of say 20us, then 100us.
I presume it is flyback.
Remember to use isol txfmr or diff probe.
If you are not used to probing live mains then please do not do it.
Lets see if it is stable even without that earth connection.
 

Winsu

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It is switching at about 75kHz and it seems stable. The light output is very solid and bright, and the current is what is expected to be. It just make noise when the earth is connected.

There is another driver on the same PCB which shares the neutral for a less brighter output. So, there are two drivers on that PCB , both share the neutral and the earth but individual lives, just one live is connected at the same time to have just one driver output. I think they are completely isolated between each other.

Could it be case of leaking current through the Cy capacitors and dragging current?. Then the driver get a bit "lost"?
 

Winsu

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Capacitance between PCB driver and metallic tray could affect to the earth connection?
 

mtwieg

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The FL7701MX is a pretty odd controller, the datasheet says very little about how it works internally, and only a few pins you can probe for useful information.

Can you post a schematic showing how the Y capacitors are connected?

Also, are you using the ADIM pin at all?
 

Winsu

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Hi mtwieg,

The Adim is being used. I have putted a 100nf in that pin and I put a constant 4V with a zener circuit to keep the driver working at full power.

There is an application note called AN-9744 which can be found in the onsemi website. It basically explains that when dimming as there is less demand of power the EMI capacitor may not reach 0V ( for ZCD pin) and it can start working on DC mode. It can lead to flickering ( possible noise as well I suppose), to avoid that the AN-9744 recommend to put the HV pin before the diode bridge with two diodes ,then the IC can register every half cycle and it wont get in DC mode anymore. I have tried that and unfortunately it didn't fix the issue.

The Y capacitors are connected between L and earth and N and earth. The neutral and earth connections are shared by both drivers, so there are 4 Y capacitors in total, 2 for each driver. Both drivers never work at the same time ( they are set for different power levels) , but when connecting just one driver ( because the earth and neutral are shared) one Y capacitor of the second driver is also connected because the earth and the neutral are shared by both drivers.

I hope tis make sense. I am planning to remove Y capacitors and separate neutral and earth from both drivers, but after that no idea what else I could think to fix the issue.
 

mtwieg

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I just noticed you mentions two drivers, that makes things a lot more complicated. Is there only one rectifier then? A schematic would help a lot.
 

Winsu

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No, there are two rectifiers, one for each driver....

Unfortunately I can not share schematics...
 

FvM

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Unfortunately I can not share schematics...
In other words you are looking forward to solve the issue on your own.

The Y-capacitor effect would make sense if the capacitors are connected on the DC side of the bridge rectifier, not to phase and neutral as stated in post #6.
 

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