Notes On EMI Power Filter Selection
The main performance indicators of EMI power filter generally include insertion loss, frequency characteristics, impedance matching, rated current value, insulation resistance value, leakage current, physical size and weight, use environment and its own reliability. In the use of the most considered is the rated voltage and current value, insertion loss, leakage current three.
When we choose the power filter, we should mainly consider three aspects of the indicators:
1.The voltage/power supply
Power supplies can be divided into AC power supplies and DC power supplies. Accordingly, many manufacturers of power filters can also be divided into AC and DC.
In principle, the AC power filter can be used for both AC and DC power supplies, but the DC filter can not be used for communication, mainly due to the low voltage resistance of the capacitor in the DC filter, and may have high AC loss and cause overheating. Even the voltage resistance of the DC filter is not a problem because the DC filter uses a large capacity common-mode filter capacitor if the leakage current will exceed the leakage current problem.
Therefore, DC power filters should never be used in AC situations.
Ac filter used in DC situation, from the point of view of safety is no problem, but to pay the cost and volume of the price; In the prototype phase, if you happen to have an AC filter, you can replace the DC filter.
When the working current of the power supply filter exceeds the rated current, the filter will not only be overheated, but also the low frequency filtering performance will be reduced. This is because the inductance in the filter will saturate the magnetic core and reduce the actual inductance in the case of a large current. Therefore, when determining the rated working current of the filter, the maximum working current of the equipment shall prevail to ensure that the filter has good performance under the maximum current state. Otherwise, when the interference appears under the maximum working current state, the equipment will be interfered or the conduction emission will exceed the standard.
When determining the rated current of the filter, a certain margin should be left; In particular, it is customary for people to call the AC "effective value", rather than the AC "peak", leaving a certain margin is very necessary. The rated current value of the general filter should be 1.5 times the actual current value.
2.Followed by insertion loss
From the point of view of interference suppression, insertion loss is the most important index. ? The insertion loss is divided into differential mode insertion loss and common mode insertion loss.
How to use the power filter to determine the required insertion loss?
Firstly, the filter is not installed at the power supply entrance of the equipment, and the conduction emission and conduction sensitivity of the equipment are measured, and compared with the standards to be met, to see how much difference between the two decibels. The role of the filter is to make up for this gap.
3.The last is the structure size
Since the inside of the filter is generally potting, environmental characteristics are not the main problem. However, the temperature characteristics of all potting materials and filter capacitors have certain influence on the environmental characteristics of power filter.
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