Specifying Low Leakage Current EMI/EMC Filters for Power supplies
Leakage current is the current flowing through the earth ground connection. This current may be generated not only by the AC-DC power supply or supplies, but also from any additional external line filtering used to further reduce electrical noise.
Most manufacturers of power supplies use “Y” capacitors connected from the line and neutral to ground as part of their integral EMI filter. These specially rated capacitors provide a low impedance path to the ground for high frequency noise to reduce EMI. The larger those “Y” capacitors are, the lower the measured noise.
There are, however, limits for maximum leakage current put in place by the safety standards for electrical and electronic equipment. That limit depends on the application and what ground connection is used. Medical, because sick patients are more vulnerable to electric shock, is the most stringent. Below are the limits for the most common applications.
IEC 60950-1 Information Technology Equipment
Moveable (not handheld) or pluggable 3.5mA
Permanently connected 3.5mA (or higher for some applications)
IEC 60601-1 Medical (Body Float – B category)
0.3mA in USA
Let us assume we are designing a portable piece of medical equipment to be used globally, thus our leakage current “budget” is 300μA. We are also going to assume that we may need an additional external filter because of some additional system noise.
For our power supply we have chosen TDK-Lambda’s EFE300M series and from the datasheet it says that the leakage current is 246μA at 240Vac (60Hz).
According to the manufacturer, the input current rating of the EFE300M is 6.1A. We have a choice of TDK-Lambda filters in the 10A range, allowing for some filter derating.
The two filters that we can use are the RSEN-2010L or the RSAN-2010L with a leakage current of just 10μA at 250VAC. We cannot use a filter with a higher leakage current as the power supply leakage + the filter leakage will be greater than 300μA.
As this is a hospital environment, we can probably go with the RSEN-2010L without spike pulse attenuation. A look at the attenuation characteristic plot shows we will have a good margin for the application and will not need a higher performance two stage filter.
A look at the datasheet for the RSEN-2010L shows that there are no Y capacitors between line/neutral and ground, hence the filter design has very low leakage.
One note of caution – a two power supply scenario is quite common. Often the leakage current budget will be affected by an ATX PC power supply driving a computer, or a power supply driving the flat panel display. A good piece of advice is to always check the system block diagram to see where the AC input is being routed to!
For more visit www.emea.lambda.tdk.com/uk/filter_specify