PRINCIPLE OF EMC

ASSIGNMENTName: ARITRA BHADURI

Roll no.: 000910701026

Class: B.E.T.C.E., U.G., 4th year 1st semester

Subgroup in EMC: 1

Teacher concerned: Prof. Sudhabindu Roy

Date of submission: 12/11/2012

QUESTION: Analyze LISN (using LTSpice).

Theoretical Aspect: One of the main objectives of the LISN (Line Impedance Stabilization Network) is to present a constant impedance to the product’s power cord outlet over the frequency range of the conducted emission test.

That is, to present a constant impedance (50 Ω) between the phase conductor and the safety wire (the ‘green wire’) and between the neutral conductor and the safety wire. These wires have been shown in the simulating circuit. Here, instead of a 3-phase power-supply, we have made use of single-phase a.c. power supply, as the equivalent of the commercial supply.

Now, the lower and the upper frequency limits of the FCC regulatory limit are 150 KHz and 30 MHz, respectively.

The LISN specified for use in the conducted emission measurement is shown in the above figure. The purpose of the 1μF capacitors between the phase and the green wires and between the neutral and the green wires on the commercial power side is to divert “external noise” on the commercial power net and prevent that noise from flowing through the measurement device and contaminating the test data. Similarly, the purpose of the 50-μH inductors is to block that noise. The purpose of the other 0.1-μF capacitors is to prevent any d.c. from overloading the input of the test receiver.

Now, the capacitors are low impedances over the measurement frequency range, and the inductor presents a large impedance, as can be seen from the following table:

Element Z150 KHz Z30 MHz

50 μH 47.1 Ω 9.425 KΩ0.1 μF 10.61 Ω 0.053 Ω1 μF 1.06 Ω 0.0053 Ω

receiver

The 1-KΩ resistors act as static charge paths to discharge the 0.1-μF capacitors in the event that the 50-Ω resistors are removed. Resistances of 50 Ω are placed in parallel with these 1-KΩ resistors. One 50-Ω resistor is the input impedance of the test receiver (spectrum analyzer), while the other is a 50-Ω dummy load that insures that the impedance between phase and the safety wire and between the neutral and the safety wires is approximately 50 Ω at all times.

The measured voltages, denoted by V P and V N, are measured between the phase wire and the safety wire and between the neutral wire and the safety wire. Both the phase and the neutral voltages must be measured over the frequency range of the conducted emission limit.

Analysis:

Here we have simulated the LISN circuit by LTSpice version IV, and the product or equipment has been represented by 100 megohm loads, as shown, between the phase and the safety and also between the neutral and the safety wires.

A single-phase power supply of 230 V, 50 Hz (nominal) has been used.

The transient responses of the voltages V P and V N, and also of the phase and the neutral currents I P and IN, have been observed, at the two limiting frequencies, and from the observed values, the approximate impedance between the phase and the safety and also between the neutral and the safety wires has been determined at both frequencies.

o f = 150 KHz Transient response for V P at f = 150 KHz:

Transient response for I P at f = 150 KHz:

Transient response for current through the 1-KΩ between phase and safety wires at f = 150 KHz:

From the transient responses, it is observed that maximum swing for V P

= (228.41 – (−231.72)) mV = 460.13 mV,

that for I P = (4.581 – (−4.655)) mA = 9.236 mA

and that for the 1 KΩ-current = (229.31 – (−232.76)) μA = 462.07 μA.

Therefore, the impedance between the phase and the safety wires on the equipment side

= (460.13 mV) / (9.236 mA + 462.07 μA) = 47.45 Ω, quite close to 50 Ω.

Also, we have (V P) / ( I P) = 49.82 Ω, so the relation V P = 50 I P holds to a very good extent.

Transient response for V N at f = 150 KHz:

Transient response for IN at f = 150 KHz:

Transient response for current through the 1-KΩ between neutral and safety wires at f = 150 KHz:

From the transient responses, it is observed that maximum swing for V N

= (2.092 – (−2.277)) mV = 4.37 mV,

that for IN = (42.03 – (−45.67)) μA = 87.7 μA

and that for the 1 KΩ-current = (2.099 – (−2.277)) μA = 4.38 μA.

Therefore, the impedance between the neutral and the safety wires on the equipment side

= (4.37 mV) / (87.7 μA + 4.38 μA) = 47.46 Ω, quite close to 50 Ω.

Also, we have (V N) / ( IN) = 49.82 Ω, so the relation V N = 50 IN holds to a very good extent.

o f = 30 MHz Transient response for V P at f = 30 MHz:

Transient response for I P at f = 30 MHz:

Transient response for current through the 1-KΩ between phase and safety wires at f = 30 MHz:

From the transient responses, it is observed that maximum swing for V P

= (2.281 – (−0.089)) mV = 2.37 mV,

that for I P = (45.53 – (−1.78)) μA = 47.31 μA

and that for the 1 KΩ-current = (2.289 – (−0.089)) μA = 2.38 μA.

Therefore, the impedance between the phase and the safety wires on the equipment side

= (2.37 mV) / (47.31 mA + 2.38 μA) = 47.70 Ω, quite close to 50 Ω.

Also, we have (V P) / ( I P) = 50.01 Ω, so the relation V P = 50 I P holds to a very good extent.

Transient response for V N at f = 30 MHz:

Transient response for IN at f = 30 MHz:

Transient response for current through the 1-KΩ between neutral and safety wires at f = 30 MHz:

From the transient responses, it is observed that maximum swing for V N = 4.959 μV,

that for IN = 99.02 nA and that for the 1 KΩ-current = 4.951 nA.

Therefore, the impedance between the neutral and the safety wires on the equipment side

= (4.959 μV) / (99.02 nA + 4.951 nA) = 47.70 Ω, quite close to 50 Ω.

Also, we have (V N) / ( IN) = 50.08 Ω, so the relation V N = 50 IN holds to a very good extent.

Thus, it is observed from the transient analyses that the impedance between the phase and the safety wires and also between the neutral and the safety wires on the equipment side of the LISN (network) stays around 47.5 Ω at both frequency-limits (lower and upper) of the FCC regulatory limit, and therefore can be concluded to behave in the same way over the entire range.

In other words, since the capacitors (inductors) of the LISN are essentially short (open) circuits throughout the frequency range of the conducted emission test, hence a constant impedance (close to 50 Ω) between the phase conductor and the safety wire (the “green wire”) and between the neutral conductor and the safety wire is presented.

Therefore the equivalent circuit of the LISN will be 50-Vresistors between the phase wire and the safety wire and between the neutral wire and the safety wire, as shown in the above figure.

At the 50 Hz power frequency the inductors have impedances of 15.7 mΩ, the 0.1-μF capacitors have impedances of 31.8 KΩ, and the 1-μF capacitors have impedances of 3.2 KΩ. Thus at the 50 Hz power frequency the LISN has virtually no effect, and a.c. power for functional operation is provided to the product.

Frequency response for V P (150 KHz to 30 MHz):

. ..

+

VP

−

+

VN

−

Frequency response for V N (150 KHz to 30 MHz):

Lastly, it is easily determined that the impedance between the phase (or the neutral) and the safety wires at a power frequency f Hz

= 1 / (j2πf (0.1 x 10−6 )) Ω + 50 Ω || 1 KΩ

= (−j / (2πf x 0.1 x 10−6) + 47.62) Ω

The magnitude and the angle variation of Z over the conducted emission frequency range have been shown separately on the next page: