pi filter design for the conducted emission

Pi filter design for the conducted emission series#

An “L” circuit filter is best suited for filtering lines when the source and load impedances exhibit large differences. This type of filter network has attenuation characteristics that increases at a rate of 40 dB per decade (20 dB at 100 kHz, 60 dB at 1MHz).

Pi filter design for the conducted emission series#

L-Circuit Filter – A two element network consisting of a series inductive component connected to a shunt feedthrough capacitor.A feedthrough capacitor filter is usually the best choice for filtering lines that exhibit very high source and load impedances. Feedthrough Capacitor – A single element shunt feedthrough capacitor has attenuation characteristics that increases at a rate of 20 dB per decade (10 dB at 10 kHz, 30 dB at 100 kHz).It should be pointed out that these are only general guidelines due to the fact that most impedance conditions and EMI profiles are dynamic, complex, and change with frequency. The following is a brief description of the most popular types of EMI Filter circuits and their application. The results are that the insertion loss curves “levels off” at approximately 80 to 90 dB. For all practical purposes it becomes an “L” circuit.Īdditionally, the amount of filtering achievable is limited by the inductance (ESL) and resistance (ESR) in the capacitor and the parasitic capacitance in the inductors. In the case of a “PI” circuit that exhibits maximum or load impedance is reduced the filter still has two active elements. If the circuit impedances varies with frequency, as most circuits do, then it is advantageous to use multiple element filters such as a “PI” or “T” circuit. As these impedances become lower, the insertion loss for the “PI” filter also becomes lower. A “PI” circuit type filter, for example, is best suited when the source and load impedances are of similar values and relatively high. More importantly, when the source and load impedance of the circuit changes, the slope of the insertion loss curve also changes. Increasing or decreasing the individual elements values does not change the slope of the curve but does affect the cutoff frequency. That is, the reject-band will be reached must faster with each section, or element, added. They are equally effective in reducing EMI emissions as well as protecting a device from unwanted EMI entering via the power lines.Įach additional element improves the slope of the insertion loss curve. They do not differentiate between EMI generated inside or outside the subsystem or system. They are all low-pass brute force networks, passing DC and power line frequencies with very low losses while attenuating the unwanted signals at higher frequencies. Circuit ConfigurationsĮMI line filters are passive devices and their effect are bidirectional.

This intentional mismatch, which is widely practiced throughout the industry, only affects the very low frequencies by introducing ripple in the pass-band and has little, if any, negative effect in the reject band. In most cases the ideal inductor for a given response has been greatly reduced in value to accommodate the operating current and reduce the DCR therefore the capacitors have to be increased in value to achieve the required insertion loss. That is to say the ideal design value of the individual components that make up the network have been modified, or intentionally mismatched, in order to accommodate operating line voltages, operating line currents, and reasonable packaging schemes. This is particularly true, and becomes more pronounced, when you consider that most EMI line filters are not matched filter networks. Passive inductive and capacitive filters are impedance sensitive devices by nature and therefore source and load conditions must be taken into consideration when selecting a filter circuit. However, the results can be misleading when it comes to selecting the proper filter circuit that must function in a complex impedance setting. MIL-STD-220 insertion loss tests are well defined, universal, and are excellent for monitoring filter manufacturing consistencies. In this matched 50 ohm impedance condition, various types of filter circuit configurations, single capacitor, “L’s”, “PI’s”, and “T’s”, will exhibit the same response for that given circuit regardless of the relationship between the input, output, and RF signal source. Insertion loss, the term used to express a filter’s ability to reduce or attenuate unwanted signals, has traditionally been measured in a 50 ohm source and 50 ohm load impedance condition, as standardized in MIL-STD-220.