EMI test setup and noise measurement

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In this chapter, the conductive EMI test setup and test methods are introduced. The theory of noise separation is explained. Some particular EMI testing problems, like the position of LISN and the application of EMI standard, which only exist in this kind of vehicular bi-directional DC/DC converter, are also described. EMI solutions are provided.

Conductive EMI specification for electrical vehicle application

The object of this test is to investigate the EMI performance of the bi-directional DC/DC converter in two topologies, also to study the EMI effect of the hard switch, soft switch, and synchronize rectifier technology. The test procedure will follow the CISPR 25, which is required by Ford Motor Company’s Component Conducted Emissions Requirements (CE 420).
Ford Motor Company’s conducted EMI standard CE 420 is formulated for traditional vehicle with combustion engine, where all the electronic equipment is connected to a 12 V bus refer to same ground. In fuel cell vehicle, the high voltage bus (traction motor drive bus) ground and the low voltage bus ground (12V bus) must be isolated for safety consideration. The two power buses are linked via the isolated bi-directional DC/DC converter. Decided by the particular connection of this converter, it could emit EMI noise to the power lines on both sides. In FCC and CISPR EMI regulations, test method for these kind of bi-directional converters is not clearly defined. Even in Ford Company’s EMC test regulation, more explanation about fuel cell vehicle power management system and bi-directional converter are still needed. Problems like, which side should the available EMI standards apply to? Should both sides have the EMI requirements, need to be answered. New EMI standards for hybrid vehicles need to be made.

Test setup

The test block diagram is shown in the figure 2.1. As required by FCC, two input power lines on high voltage side are connected through two Line Impedance Stabilization Networks (5 mH). The role of LISN here is to reject the possible conducted noise from the main power supply which may contaminate the measurements, and also to present a relatively stable impedance to the unit under test.
On the low voltage side, four 0.675W resistors connected in parallel are used as load.
Two output power lines are connected through two 5 mH LISN (200 A) to the load resistors. The high current LISN (type 8616-5-TS-200-N) rates at 200 A and 5 m. The schematic of LISN is shown in Figure 2.2. The measured impedance of LISN from load terminal to case with 50 W terminator on A.F. jack is shown in figure 2.3. And the phot of the LISN is shown in Figure 2.4.
Spectrum analyzer HP8568B is used as the noise emission receiver. Since the quasi-peak mode is not available. The spectrum analyzer is set to peak detector mode. In this way, the worst noise case will be measured. Frequency sweep range is set from 150 kHz to 108 MHz, which covers the AM and FM broadcasting frequency range. As required by Ford Motor Company’s EMC regulation, resolution bandwidth is set to 10 kHz in frequency range below 30 MHz and 100 kHz in frequency range from 30 MHz to 108 MHz. So every measurement is divided into two frequency ranges, one is 150 kHz to 30 MHz while the other is 30 MHz to 108 MHz. The reference level is set to 100 dBmV with attenuation of 0 dB, an exterior 20 dB attenuator is used this test. The video bandwidth is set to be same as the resolution bandwidth in both frequency ranges.
A HP E3631A triple output linear power supply, which provides positive and negative 12 V, is used as the control circuit power supply. All driver circuits are powered by a battery with voltage of 12 V.
The surface of the test bench is covered by 2 layers of 36 mil thick sheet copper, which works as a electrical ground plane. Ground plane is connected to the electrical pipe via a 4 inch wide copper foil. Electrical pipe is connected to the ground of main utility power net. The converter, spectrum analyzer HP-8568B, and auxiliary power supply (for control circuit) are put on the ground plane. All the equipment cases, the heat sink of the converter are electrically connected to the ground plane. A HP 6843C DC power supply (600 V, 25 A) is used as the main power supply. Limited by the weight and size, the load resistor and the main power supply can not be put on the ground plane. The picture of the test bench with equipment is shown in figure 2.5. [4][5][6].

Noise measurement in the bi-directional converter

This circuit is set up to measure the total noise from low voltage sides of the bi-directional DC/DC converter.
As discussed in the first chapter, all the sensitive electronics equipment are connected to the low voltage bus. In a fuel cell driven vehicle, the converter works in boost mode only during the starting up period, which is less than 200 ms. Most of the time, this converter is working in buck mode to deliver power from high voltage bus to low voltage bus. So overall EMI performance of this converter is mostly decided by the noise generated in buck mode, and decided by the noise on the low voltage side. In this test, the noise level on the low voltage side in buck mode is selected to represent the overall EMI performance of this converter. All EMI analysis and comparison are made for measurement results from low voltage side.

EMI noise separation.

Two different kinds of noise are distinguished as Common Mode (CM) and Differential Mode (DM) noise. Though never required by FCC or CISPR, separate measurement of the Common Mode (CM) and Differential Mode (DM) noise leads to better understanding of noise source and noise propagation in a circuit. An EMI filter can then be designed to treat each mode of noise separately.
The total noise can be divided into common mode and differential mode noise by using differential mode rejection network or common mode rejection network.[7][8] In this test, two noise separators are used to measure the CM noise and DM noise independently.

Chapter I Introduction
1.1 Background and motivation
1.2 Fuel cell power system management
1.3 System Specifications
Chapter II EMI test setup and noise measurement
2.1 Conductive EMI specification for electrical vehicle application
2.2 Test setup
2.3 Noise measurement in bi-directional converter
2.4 Noise separation.
Chapter III Operation of bi-directional converters and their EMI characterization 
3.1 Converter topology selection
3.2 L-type converter
3.3 EMI characterization of L-type converter
3.4 Full-bridge converter
3.5 EMI characterization of full-bridge converter
3.6 EMI Suppression
Chapter IV Conclusion and future works 
4.1 Conclusion
4.2 Future works

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