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Reduce Conducted EMI in Automotive Buck Converter Applications

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Reduce Conducted EMI in Automotive Buck Converter Applications Application Report SNVA886–July 2019 Reduce Conducted EMI in Automotive Buck Converter Applications Neal Zhang, Daniel Li, Vincent Zhang, Andy Chen ABSTRACT Compliance to Electromagnetic Interference (EMI) standards often presents a significant challenge for many automotive design engineers. Adherence to EMI standards is a requirement for automotive electronic control units (ECUs), and automotive EMI standards are more stringent than those in the industrial and communication market segments. The buck converter is continuously switching during operation, making it one of the primary sources of noise in the system. Usually, it becomes easy to pass radiated EMI after conducted EMI passes. This article introduces how to reduce the conducted EMI of the buck converter in automotive applications through schematics, layout-optimization, and shielding. Section 1 introduces automotive conducted EMI test standards. Section 2 provides the conducted emission model of a buck converter. Section 3 provides several methods to reduce EMI based on the noise model, along with test results to validate the effectiveness of each method. Section 4 summarizes tips to reduce automotive conducted EMI. Contents 1 Automotive Conducted EMI Test .......................................................................................... 2 2 Conducted Emission Model of Buck Converter.......................................................................... 5 3 Reduce Conducted EMI of Buck Converter .............................................................................. 9 4 Summary of Tips to Reduce Buck Converter Conducted EMI ....................................................... 18 5 References .................................................................................................................. 19 List of Figures 1 Conducted Emission Test Setup in CISPR 25........................................................................... 2 2 Test Setup Photo for Buck Converter TPS560430-Q1 ................................................................. 3 3 Equivalent Circuit of a Conducted Emission Test in CISPR 25 ....................................................... 4 4 Ideal Conducted Emission Model of a Buck Converter................................................................. 5 5 Capacitor Equivalent Circuit................................................................................................ 5 6 Variation of Impedance with Capacitance ................................................................................ 6 7 Inductor Equivalent Circuit.................................................................................................. 6 8 Impedance Comparison of Inductors and Bead ......................................................................... 7 9 Electrical Coupling Example ............................................................................................... 8 10 Magnetic Coupling Example ............................................................................................... 8 11 Buck Converter Schematic with 3-Stage EMI Filter..................................................................... 9 12 Buck Converter PCB with 2-Stage EMI Filter ............................................................................ 9 13 Noise with 2-Stage EMI Filter Against the CISPR 25 Class 3 Standard............................................ 10 14 SW Voltage, Input Current, and Inductor Current Waveforms ....................................................... 11 15 FFT Analysis of SW Voltage, Input Current, and Inductor Current ................................................. 12 16 Impact of a Boot Resistor to SW Voltage ............................................................................... 13 17 EMI Testing Result with a 75 Ω Boot Resistor ......................................................................... 13 18 Impedance Characteristics of the First-Stage Bead ................................................................... 14 19 Improper Input Filter with Ferrite Bead Not Placed Near the IC ..................................................... 14 SNVA886–July 2019 Reduce Conducted EMI in Automotive Buck Converter Applications 1 Submit Documentation Feedback Copyright © 2019, Texas Instruments Incorporated Automotive Conducted EMI Test www.ti.com 20 EMI Testing Result with the Ferrite Bead not Placed Near the IC................................................... 14 21 Wide Copper Allows Noise to Bypass the Filter Capacitor ........................................................... 15 22 Force Noise to Pass the Filter Capacitor ............................................................................... 15 23 Layout of a Larger SW Area Leads to Worse EMI..................................................................... 16 24 EMI Testing Result with the Layout of Larger SW Area .............................................................. 16 25 Shielding Box Reduces Electrical Feld Coupling ...................................................................... 17 26 Multi-Wound Inductor Cross-Section Image ............................................................................ 17 27 Position of the Shielding Box on PCB ................................................................................... 18 28 Noise with Shielding Box Against the CISPR 25 Class 5 Standard ................................................. 18 List of Tables 1 Conducted Emissions Peak and Average Limits in CISPR 25 ........................................................ 4 Trademarks All trademarks are the property of their respective owners. 1 Automotive Conducted EMI Test Among the various automotive standards for conducted EMI, CISPR 25 is the essential international test standard. This chapter introduces the test setup and limit. 1.1 Test Setup Figure 1 is the test setup specified by CISPR 25, and Figure 2 is the test configuration photo for buck converter TPS560430-Q1. The power supply is a 12-V battery. The EUT is a TPS560430-Q1 board with an input filter. The board is placed 50 mm above the metal ground plane. The Artificial Network (AN), also known as a line impedance stabilization network (LISN), is inserted in the power supply to measure the noise emission from the buck converter. Side view Power AN Power supply lines EUT Supply Ground plane Top view Power AN Supply AN EUT Load Simulator Shielded enclosure 7 = Low relative permittivity support, eR < 1.4 Measuring 8 = High-quality coaxial cable, e.g. double-shielded 50: Instrument 11 = 50: load 12 = Bulkhead connector Figure 1. Conducted Emission Test Setup in CISPR 25 2 Reduce Conducted EMI in Automotive Buck Converter Applications SNVA886–July 2019 Submit Documentation Feedback Copyright © 2019, Texas Instruments Incorporated www.ti.com Automotive Conducted EMI Test Figure 2. Test Setup Photo for Buck Converter TPS560430-Q1 1.2 Equivalent Circuit Figure 3 is the equivalent circuit of the test setup with the internal schematic of the LISN. The LISN allows DC power into the harness of the EUT. The LISN also strips out RF noise coming from the EUT, and directs it to the RF-measurement equipment. The measurement equipment is configured for 50 Ω input. SNVA886–July 2019 Reduce Conducted EMI in Automotive Buck Converter Applications 3 Submit Documentation Feedback Copyright © 2019, Texas Instruments Incorporated Automotive Conducted EMI Test www.ti.com 5mH 0.1mF Buck Battery 1mF AN1 Converter 1k: Measurement Port Board Load DC Power 50W Supply (EMI Source) 5mH 0.1mF 1mF AN2 Measurement 1k: Port 50W Copper GND plane Figure 3. Equivalent Circuit of a Conducted Emission Test in CISPR 25 1.3 Test Standard Limit Table 1 lists the peak and average limits for conducted EMI according to CISPR 25 . Class 5 is the most stringent. This test covers 150 kHz to 108 MHz in specific frequency bands (AM and FM radio, and mobile service bands). For CISPR 25, higher noise spikes are allowed in the gaps between the concerned frequency bands. However, some customers limit the noise in the frequency band gaps to the adjacent band limit due to special requirements. The measurement frequency is up to 108 MHz, and the limit is quite tight. Parasitic capacitance and inductance, as well as near-field radiation, greatly influences the test results. These constraints make it much harder to pass conducted EMI standards for automotive applications than standards for industrial and communication applications. Table 1. Conducted Emissions Peak and Average Limits in CISPR 25 Levels in dB (μV) Frequen Class 1 Class 2 Class 3 Class 4 Class 5 Band cy (MHz) Peak Average Peak Average Peak Average Peak Average Peak Average Broadcast 0.15 - LW 110 90 100 80 90 70 80 60 70 50 0.3 0.53 - MW 86 66 78 58 70 50 62 42 54 34 1.8 SW 5.9 - 6.2 77 57 71 51 65 45 59 39 53 33 FM 76 - 108 62 42 56 36 50 30 44 24 38 18 TV Band 41 - 88 58 48 52 42 46 36 40 30 34 24 I Mobile Services CB 26 - 28 68 48 62 42 56 36 50 30 44 24 VHF 30 - 54 68 48 62 42 56 36 50 30 44 24 VHF 68 - 87 62 42 56 36 50 30 44 24 38 18 4 Reduce Conducted EMI in Automotive Buck Converter Applications SNVA886–July 2019 Submit Documentation Feedback Copyright © 2019, Texas Instruments Incorporated www.ti.com Conducted Emission Model of Buck Converter 2 Conducted Emission Model of Buck Converter 2.1 Ideal Conducted Emission Model The Ideal Conducted EMI Model does not consider parasitic parameters or coupling effects. Instead, the noise level is calculated directly from the schematic. The result is only accurate
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