CHAPTER 5 EMI FILTER STUDIES

Transcription

1 139 CHAPTER 5 EMI FILTER STUDIES 5.1 INTRODUCTION The EMI can be effectively reduced using filtering techniques in close association with other hardening EMI methodology. The tracked vehicle EMI countermeasures includes modified system design, wherein the filtering techniques were effectively used to mitigate onboard conducted EMI.One such case study was made on a blower motor with emission levels much higher than stipulated by MIL STD 461C causing malfunction of control console. The preliminary vehicle measurements were carried out for EMI performance in Radiated emission (RE) and conducted emission (CE). The EMI conducted emission at subsystem level was identified cause as the main source of noise. Hence it was decided to design and instal suitable filter to counter the same for the tracked vehicle. The choice of feed through was made after technical studies and measurements were taken to confirm the same. 5.2 TEST SETUP The test setup as shown in Figure 5.1 was used to reduce the conducted EMI levels to the MIL STD 461C to as applicable with filter connected to the motor. EMI filter box has a 50A relay in its housing, also the protection element that is used to power the motor.

2 140 D C MOTOR B E MI FILTE R A +24 volts ground ground E MI ME TE R 50 Ω attenuator Figure 5.1 EMI Filter Test set up EMI before filter at A- EMI after filter at B = Insertion loss measure 5.3 DESIGN CONSIDERATION The motor was identified as a possible interference source that caused malfunction of the electronic modules. A line filter was used to power the DC motor from a 28 V power supply which reduced the interference. The EMI filter assembly was housed in a metallic box that further reduced the radiation from the lead wires interconnection. Military connectors were used to reduce the EMI and ensure the connections are intact and leak proof. Figure 5.2 EMI Filter Box

3 141 The use of EMI protection gaskets in the intermediate mounting surface (rear side) of the connector were used to bring down EMI levels (Williams 1995). The contact resistance of the 200A connectors is 1.5 m. The feed through filter of polyphaser corporation was chosen rated at 100 V peak, 50A, 1.5µF power supply with attenuation up to 2GHz. 5.4 EMI FILTERS The tracked vehicle is generally powered by a 28 V DC source that meets the MIL 1275 standards. The measured ripple factor is of the order of about ± 300 mv. However the special systems fitted in the vehicle for crew survivability may produce EMI interference that may cause disruption of normal function of electronics system. Filtering (Prasad 1999) is an important mitigation technique for suppressing undesired conducted electromagnetic interference (EMI). Conducted EMI currents that might get into the DUT from the power supply lines may cause malfunction. The signal input and output lines have filters that bypass the noise ensuring clean power supply to critical system. The Conventional filter analysis and design assumes idealised and simplified conditions. These assumptions are not completely valid in many EMI filters because of unavoidable and severe impedance mismatch. Classical passive filter theory is well developed for communication circuits, where one can operate under impedance matched conditions. The filter characteristics in such cases are evaluated with 50 terminations, and experimentally measured by test methods such as those prescribed by MIL

4 142 STD 220 A (Military standards). A filter evaluated with this procedure may behave differently when used in a circuit, where the impedances presented by the circuits to the filter are not exactly 50. The effectiveness of filtering is greatly influenced by the impedances the filter faces looking into the generator and the load. In power lines, these impedances vary over a wide range because of frequent load switching. The interferences appearing on the line may be due to two components: common mode (CM) and Differential mode (DM) currents. The selection and Types of filters (Figure 5.3) are dependent on mutual impedances of source and load as indicated below. Impedances Filter type Source load Low Low T Low High L High Low π FIGURE 5.3 Powerline feedthrough schematic alongwith hardware. 5.5 FILTER EVALUATION No-load insertion-loss measurements are performed in two steps as shown in Figure 5.1. An EMI filter box was fabricated as indicated in Figure 5.2. First, the input voltage V 1 of the receiver is recorded when the

5 143 filter is not connected in the circuit. The receiver input voltage V 2 is next recorded when the filter is included in the circuit. The insertion loss of the filter is obtained for a range of frequency extending up to several hundred MHz. The insertion loss as a function of frequency is the most fundamental characteristic of a filter, and is defined by IL (db) = 20 log 10 V 1 /V L (5.1) Where V 1 = the output voltage of the signal source without the filter being connected in the circuit V L = the output voltage of the signal source at the output terminal of the filter with the filter in the circuit The results are tabulated in Table 5.1 and shown in Figure 5.4. Table 5.1 Insertion loss measurements sample feed though capacitor based on MIL STD 220A Frequency Insertion loss (db) 10 Hz Hz Hz 18 1KHz KHz 28 1 MHz MHz MHz 93 1 GHz 96

6 144 INSERTION LOSS Insertion loss (db) Frequency 10 Hz 100 Hz 500 Hz 1KHz 100 KHz 1 MHz 150 MHz 400 MHz 1 GHz Figure 5.4 Insertion loss vs frequency for a 28 V system for AFV 5.6 EMI FILTER STUDY INFERENCE EMI filter effectiveness is evaluated by measuring the insertion loss or attenuation offered by the filter for EMI frequencies that gets emitted from the motor terminals as conducted interference. The MIL STD 461 CE03 as applicable with the filter is indicated in Figures 5.3and 5.4. The measurements were carried out before and after the insertion of filters on board the vehicle as indicated in Figure 5.5. and Figure 5.6. The recordings are shown to reduce the EMI to acceptable levels of the order of 20 db / MHz. The use of shielded box further reduces the radiations emitted by the interconnecting cables. It should be noted that adopting methodology such as bonding, use of better alloys, improved grounding reduced the contact resistance. This takes care of any surges that are likely to damage the equipment. These measurements clearly indicate that the filter has ensured the requisite performance.

7 CONCLUSION The emission levels were found to be well within the limits prescribed by the military standards after the EMI hardening and use of online filters. The recordings are shown to reduce the EMI to acceptable levels of the order of 20 db / MHz. The unit was found to function satisfactorily within the vehicle. These filters should be kept as close the load as possible to ensure enhanced EMI levels of performance. The filters can be placed inside, as integral part of load housing itself. The conducted EMI is perennial problem in tracked vehicle as system is mounted in closely due to space constraints. The power supply is kept as clean as possible from transients. The power and signals to critical modules such as firing circuits are isolated using techniques such as optoisolators, current transformers, use of backup battery etc. In case of electrical loads and sources the EMI is isolated near to the source itself. The added advantage is that the core goes to saturation in case of short circuit, faults, etc. thus it limits the bus currents to finite values.

8 146 Emission level db µa Emission level db µa Frequency (MHz) Figure 5.5 CE-03 recorded from power supply line to critical device Figure 5.6 CE-03 after installation of filter in the power supply to critical device Frequency (MHz)