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I am working on a flyback AC:DC switching converter design which converts wall power to DC. The design is working but I wanted to probe it for some characterization data.
I do have Earth ground in my design and it is wired up exactly like this TI EVA schematic.
http://focus.ti.com/lit/ug/sluu263c/sluu263c.pdf
So I plugged the design and the scope into the same power strip, both use the third prong. Told the scope to use Earth ground as its reference and as soon as I connect the probe ground to the design the input fuse popped.
This is exactly the behavior described in the app note below, page 2 warning 2. But the National design has no Earth ground (but maybe this is irrelevant because my design also has no Earth ground per se during the rectifier dead time).
http://www.national.com/an/AN/AN-2097.pdf
"The ground connection on the evaluation board is NOT referenced to Earth ground. If an oscilloscope ground lead is connected to the evaluation board ground test point for analysis and the mains AC power is applied (without any isolation), the fuse (F1) will fail open. For bench evaluation, either the input AC power source or the bench measurement equipment should be isolated from the Earth ground connection. Isolating the evaliation board (using 1:1 line isolation transformer) rather than the oscilloscope is highly recommended."
I do have access to a line isolation transformer, and following the warning I isolated my design which does work. But I am trying to figure out what happened in the first setup and why using the scope with the Earth ground setting failed.
Maybe the scope's probe ground is not actually physically connected to Earth ground but only some copy? I guess it must be because while typing this I happened to think that this signal has to move from the primary to secondary side inside the scope somehow, no?
Also, why do you think isolating the design rather than the scope is recommended? Shouldn't it be equivalent from a measurement point of view? I figured maybe it was a safety thing as the scope presumably has isolation but if one is probing the primary (w/o the iso transformer) then there is no isolation.
Does anyone know of a general reference on how to safely take accurate measurements on systems connected to the line with a scope?
I do have Earth ground in my design and it is wired up exactly like this TI EVA schematic.
http://focus.ti.com/lit/ug/sluu263c/sluu263c.pdf
So I plugged the design and the scope into the same power strip, both use the third prong. Told the scope to use Earth ground as its reference and as soon as I connect the probe ground to the design the input fuse popped.
This is exactly the behavior described in the app note below, page 2 warning 2. But the National design has no Earth ground (but maybe this is irrelevant because my design also has no Earth ground per se during the rectifier dead time).
http://www.national.com/an/AN/AN-2097.pdf
"The ground connection on the evaluation board is NOT referenced to Earth ground. If an oscilloscope ground lead is connected to the evaluation board ground test point for analysis and the mains AC power is applied (without any isolation), the fuse (F1) will fail open. For bench evaluation, either the input AC power source or the bench measurement equipment should be isolated from the Earth ground connection. Isolating the evaliation board (using 1:1 line isolation transformer) rather than the oscilloscope is highly recommended."
I do have access to a line isolation transformer, and following the warning I isolated my design which does work. But I am trying to figure out what happened in the first setup and why using the scope with the Earth ground setting failed.
Maybe the scope's probe ground is not actually physically connected to Earth ground but only some copy? I guess it must be because while typing this I happened to think that this signal has to move from the primary to secondary side inside the scope somehow, no?
Also, why do you think isolating the design rather than the scope is recommended? Shouldn't it be equivalent from a measurement point of view? I figured maybe it was a safety thing as the scope presumably has isolation but if one is probing the primary (w/o the iso transformer) then there is no isolation.
Does anyone know of a general reference on how to safely take accurate measurements on systems connected to the line with a scope?
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