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• The DC side Positive and Negative outputs of these power supplies are isolated from
the chassis. As explained earlier, the noise currents are filtered to the chassis of the unit
and the chassis is connected to the Earth Ground through the Earth Ground Pin of the
AC power outlet receptacle. Avoid connecting (referencing) the DC Negative output
terminal of the power supply to the Earth Ground.
• Connect a ¼” wavelength of wire on the Negative terminal of the power supply.
Connect one end of the wire to the Negative terminal and leave the other end free.
The wavelength corresponds to the wavelength of the interfering frequency. (May not
be practical for long wave lengths).
[Formula: Wave length (Meters) = 300 / frequency in MHz]
5.6 COMBINED FILTERED NOISE CURRENTS FROM MULTIPLE SMPS
ON A BRANCH CIRCUIT MAY TRIP GROUND FAULT CIRUIT IN-
TERRUPTER (GFCI)
During malfunction or an accident, the metal chassis of an electrical device may get energized
to unsafe voltage due to internal high voltage section coming in contact with the chassis. If a
person standing on Earth touches this energized chassis, a leakage current proportional
to the person’s skin resistance will flow through the person’s body to Earth Ground. The
leakage current through the body is higher when the skin contact resistance is lower i.e. if
the skin is wet or wounded. This leakage current does not return to the power source but
is dissipated in Earth Ground. A leakage current of > 4-6mA passing through the human
body could produce lethal electrical shock. Ground Fault Circuit Interrupter (GFCI) is used
for safety against electrical shock due to leakage. GFCI measures the difference between
the current sent to the load and returned from the load and will trip and disconnect
the power circuit if the difference is > 4-6 mA. GFCIs are normally installed in AC Branch
Circuits feeding power outlets in wet areas like marine craft, RVs, spas, hot-tubs, kitchens,
washrooms, etc.
As explained earlier at Section 5.2, RF noise filtration circuits in SMPS generate intentional
Earth Leakage Current. SMPS are used extensively as DC power sources in modern day
electrical / electronic devices e.g. Audio / Video / Computing devices, power supplies, battery
chargers etc. A single GFCI outlet / GFCI breaker may be serving multiple SMPS loads and
therefore, will be sensing the sum of all the Earth Leakage Currents and, if the sum is
> 4-6mA after connecting this unit, the GFCI will trip. In such a case, disconnect other SMPS
based device(s) being served by this GFCI one by one till the net leakage current is reduced
to < 4mA and the GFCI does not trip. Other solution is to power this unit from a GFCI outlet
/ GFCI breaker that does not have any SMPS load or power from an outlet that is not
protected by GFCI.
SECTION 5
| Limiting Electro-Magnetic
Interference (EMI)
Summary of Contents for SEC-1212P
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