Switch mode power supplies
Switch mode power supplies
(OP)
In very simple terms, why do SMPS cause currents in the earth conductor?
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Switch mode power supplies
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RE: Switch mode power supplies
They shouldn't really.
How is it arranged?
If its fed from a battery, sometimes the low voltage
return is the same connection as earth.
If thats the case then the earth return may include
part of the battery current, have a look at the layout
if thats the prob, then you could re-arrange the
connections so that the current path doesn't go along
the earthing. Sometimes that is not possible, such as
where the battery supply has only one feed wire, and
returns via local metalwork (like on a car).
Even on cars high current items like starter motors
sometimes have extra cabling to avoid excess current
through local metalwork.
The output from a battery fed power supply shouldn't
involve or cause earth currents in its output circuits,
and you should be able to use normal type safety trips.
(i think). Using the output will mean that around ten
times the output current of the unit will be drawn from
the battery so it could be the return path that is
showing up.
John
RE: Switch mode power supplies
RE: Switch mode power supplies
RE: Switch mode power supplies
Just to clarify, I refer to units in IT equipment served by 230volts in the uk. I am an inspection engineer and I have been able to measure considerable currents in protective earthing conductors at the origin of installations.
Please see response also to Electricuwe
RE: Switch mode power supplies
Where Cmax is the total capacitance from live-to-earth (or neutral-to-earth).
Example: SMPS with 4700pf (+/- 20% tolerance) safety class Y capacitor fitted from live-to-earth (and 4700pf from neutral to earth) Operating from UK 240VAC 50Hz (note: 264VAC max)
Ileakage = 264 x 50 x 2 x PI x 5.64E-09 = 0.468mA
If you had say 20 pcs of equipment like this then the total leakage current on the single 240VAC utility supply would be additive, so = 4.68mA
(for 100 psc equipment then = 46.8mA etc.)
RE: Switch mode power supplies
In SMPS input filtering, there are Safety Class Y capacitors installed from both the live and neurtal terminals. There is a leakage current flow from the live-to-earth (also from the neutral-to-earth). The magnitude of which can be calculated very accurately by integrating the AC sinewave input voltage etc. but can be estimated as Ileakage = Vac(max) x 2 x PI x frequency x Cmax
Where Cmax is the total capacitance from live-to-earth (or neutral-to-earth).
Example: SMPS with 4700pf (+/- 20% tolerance) safety class Y capacitor fitted from live-to-earth (and 4700pf from neutral to earth) Operating from UK 240VAC 50Hz (note: 264VAC max)
Ileakage = 264 x 50 x 2 x PI x 5.64E-09 = 0.468mA
If you had say 10 pcs of equipment like this then the total leakage current on the single 240VAC utility supply would be additive, so = 4.68mA
(for 100 psc equipment then = 46.8mA etc.)
By the way - not all SMPS utilize the earth conductor as part of their overall filtering and EMC solutions. Example: Some notebook computer AC/DC adapters.
RE: Switch mode power supplies
Also is there an equivalent explanation earth current due to the capacitor used to couple the DC output an isolated supply to earth? I assume it is a factor of the switching frequency and the shape and magnitude of the output ripple.
RE: Switch mode power supplies
Decopuling capacitors on the DC side, equivalent earth current flow? Don't believe there is due to the nature of the DC. However, maybe there is someone out there can answer this question.
RE: Switch mode power supplies