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Tying the output of an isolated offline PSU to a voltage..(stopping it floating)

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treez

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We have a 60W offline , isolated , Flyback LED driver. The LEDs are mounted on MCPCB, which is screwed into an earthed heatsink. The output ground of the LED bank is not connected to earth. We believe that the output “ground” could unfortunately float up to a high voltage (Mains peak voltage ) , and we don’t want this. Therefore, we thought about connecting the output “ground” to Earth. However, this means having another connector and its too expensive. Therefore, we wish to have a compromise…..what we want to do , is to simply connect a 1 Megohm axial resistor , with a suitable voltage withstand, across the isolation barrier under the Flyback transformer. We will connect it between output “ground” and [Primary DC bus “ground”]. I admit that that is not the same as tethering the output “ground” to earth, but at least it should stop the output “ground” from floating up to mains peak.
As you know [Primary DC bus “ground”] is actually connected to Mains Neutral for 10ms, then to the negative_going mains live for the next 10ms , repeating. As such, we believe that the average voltage of [Primary DC bus “ground”] is actually around minus 107V…..this is better than mains peak. Do you think that this is a good idea?
 

FvM

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An isolated output doesn't float to a high voltage without a reason. Which reason do you particularly expect?

If the output circuit is exposed to the outside and you expect it to catch ESD, then you need a capacitor in combination with a discharge resistor. Without the capacitor, a single ESD pulse will cause a flashover somewhere inside the circuit, and without the resistor, multiple ESD events can charge up the output above the capacitor breakdown voltage.

But if the circuit is kept internally, you can just leave it floating, unless EMI control requires different treatment.
 

treez

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If the output circuit is exposed to the outside and you expect it to catch ESD
..Thanks, well, the MCPCB is inside a waterproof enclosure...its not like human hands can toucn it...is that what you meant?
 

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Yes, don't see a problem with floating output.
 
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    treez

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