Introduction of Sensitive Current
Working current greatly impacts the EMI filters’performance. As we all know that  inductor is enamelled wire winded up on a magnetic core. Both magnetic core and wire have their working loss. The higher the working current, the greater the heat generated by working loss. The  in one EMI filter like ferrite cores, capacitors or outlet terminals could stand a limited range of temperature. For example, the curie point of Mn~Zn ferrite core is 120~130℃, and the highest working temperature for polyester film capacitor is +85℃. So maybe it’s better we say sensitive temperature rather than sensitive current.

If no special statement, the rated current listed in specification refers to working current at room temperature 25℃. But because of working loss, EMI filter’s inside temperature would increase until it reaches a balance, and in practice the balance temperature would be higher than RT, like +45℃. At this temperature EMI filter’s performance and reliability will get worse. In order to keep the filter’s performance as it should be, usually there are two ways. One is to improve the filter’s heat dissipation, such as install the filter with metal case on the metal chassis or metal frame of equipment. Another way is to make some allowance of the rated current while you choosing a filter because the maximum working current affected by environment temperature. If Ir stands for rated current at RT +25℃, and Ip stands for allowed maximum working current. Then the relation between Ip/Ir is shown as table 1.

Table1 The relation between environment temperature and the maximum working current.
Environment    20℃  25℃  35℃  45℃  55℃  70℃  75℃  85℃

TemperatureIp/Ir   1.03    1     0.9    0.8    0.7    0.5    0.4     0

Table 1 indicated that when we choose the rated current of EMI filter, we need to consider environment temperature and the maximum working current under that temperature. For example, if the environment temperature is 45℃, the maximum working current is 0.8 of the rated current.

Especially, EMI filters with differential mode inductor are more sensitive to temperature, because different from common mode inductor, of which the two windings are reverse winding. It could neutralize the magnetic field of the ferrite core generated by working current, as a result, the working loss is reduced to the lowest. Whereas common mode inductor only has one single winding, so the working loss would be much higher even the working current is the same. Besides, we should consider if the working current could make the magnetic saturate too. If the magnetic core are saturated, the differential inductor would lost its inductance. That’s why the differential mode filter have lower inductance value and bigger volume.

The other sensitive current is leakage current. Although it’s related with personal safety, still it is ignored by users most of the time. Please refer to figure 1 to have a clear understanding of leakage current.

Fig. 1 Leakage Current

The circuit diagram of EMI filter shown as fig. 1 is the same as FT110 series. Because the midpoint E between common mode capacitors C1 and C2 need to be earthed at FG, while the other end of the capacitors are connected between AC power. So the voltage of E is 1/2 AC voltage. In other words, the power case carries 1/2 AC power voltage. So once human body touches the case, there would be leakage current go through the body to ground (especially when the air is humid). For safety reason, many strict requirements regarding leakage current are stipulated internationally. See table 2.

Table2  Safety Standard for Leakage Current

VDE0804, 3.5mA, 250V, 50Hz (shown as in Table2) tells us the maximum leakage current is 3.5mA. (In order to attain large insertion loss at low frequency range, EMI filters used in shielding rooms usually enlarge the common mode capacitors significantly. So this kind of filters has extremely large leakage current, mostly grade A). From safety point of view, leakage current is the smaller the better. For example, for EMI filters used in medical equipment, maximum allowable leakage current is only from several to dozens of μA.

Special attention should be drawn to the following three questions:

1. International regulations of leakage current is tested under AC 250V 50Hz. But users often confuse it with leakage current in with-stand voltage test, of which the voltage is very high, so the leakage current is mush higher too.

2. Usually, EMI filters applied to 50Hz could also be used at 400Hz, but we ignore that 400Hz is 8 times of 50Hz. So in theory, the testing value under AC 250V 400Hz is also 8 times of the value under AC 250V 50Hz. If beyond limits, we need to choose another EMI filter appropriate for 400Hz.

3. Because the leakage current of three-phase filters is the sum of the value of each phase, so its leakage current is often higher than single phase filter.

How to Test Leakage Current
Leakage current is the current passing through filter ground terminal to phase line or neutral line when rated voltage is applied to the filter. The testing principle is as the following:

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