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Transcript
Course Introduction Purpose • This course discusses techniques for analyzing and eliminating noise in microcontroller (MCU) and microprocessor (MPU) based embedded systems. Objectives • Learn about a method for performing a system-level EMI test. • See how to evaluate current balance. • Gain a basic knowledge of tests for measuring the emissions from LSI devices that can be used for product selection. Content • 18 pages Learning Time 30 minutes Reducing EMI EMI reduction is a goal shared by both the semiconductor experts who design MPUs and other LSI devices, and by the engineers who apply those chips in embedded systems Explanation of Terms Anechoic chamber A room designed to block radiation from the outside and to minimize reflections off the room’s walls, ceiling, and floor Balun A passive electronic device that converts between balanced and unbalanced electrical signals CISPR 25 International Special Committee on Radio Interference (CISPR) publication 25: “Limits and methods of measuring radio disturbance characteristics for the protection of receivers on board vehicles.” CISPR is a sub-committee of the International Electrotechnical Commission (IEC). Core A microcontroller chip is composed of a core, I/O ports, and power supply circuitry. The core consists of the CPU, ROM, RAM, and blocks implementing timers, communication, and analog functions. ECU Electronic Control Unit EMI Electromagnetic Interference Harness Cables (wires) connecting a board and power supply or connecting one unit in a system to another LISN Line Impedance Stabilization Network Power supply Two power supplies are applied to the LSI: Vcc and Vss. The core power supply internal to the LSI is VCL (internal step-down). The Vss-based power supply routed through the LSI is VSL. TEM Cell Transverse Electromagnetic Cell WBFC Workbench Faraday Cage Radiation from Wiring Harness • System-level evaluation - Example: performed on three test boards - Test method for measuring emissions from wiring harness: (CISPR 25 equivalent) Board A • Radiation levels ranged from high for board A to low for board C Antenna Anechoic chamber Board B LISN Circuit board with MPU Board C Test setup EMI from Circuit Board • Near-field distribution was measured also, using an EMV-200 test system - A sensor coil on a probe rotates and moves with precision in three dimensions to scan and record the EMI radiated from the circuit board • Data from the CISPR 25 test and the EMV-200 scan was used to examine the correspondence between the field strength and system level evaluation at the connector position EMV-200 Probe with sensor coil Power supply connector MPU Circuit board f = 80MHz Data from near-field scan Emission Measurement Results • With probe above the harness connector, there is a direct relationship between the antenna and near-field probe readings - Using a low-emissions MCU reduces emissions at the wiring harness connector on the board • Moving from a 2-layer board to a 4-layer board further reduces emissions at the wiring harness connector Board A Harness mounting area @80MHz Board B Harness mounting area Directly above MCU Above harness mounting area Board C MCU Harness mounting area @80 MHz Evaluating Current Balance in PCB Near-field measurements show the commonmode radiation caused by unbalanced currents flowing in the circuit board - Test board provides extra pads to which 470Ω resistors can be connected to divert current through loops on left and right, creating differences between the signal and return currents in the area highlighted by the pink oval - Near-field scan data of the entire board was obtained for three test cases: • Case 1: No resistors were connected, so currents in measurement area were balanced • Case 2: a 470Ω resistor was connected on left side of board, creating a 10% current unbalance in the measurement area • Case 3: Two 470Ω resistors were connected on the left and right sides of the board, creating a 20% unbalance in the measurement area Left loop Right loop Area in which a difference between the signal current and return current can be created Termination (50) Pads Line width = 1.3mm Case 1: Current Balanced With no 470Ω resistors connected, current was balanced, so minimum levels of EMI were detected when the EMV-200’s probe scanned the measurement area of the printed circuit board 100% Case A 100% h = 2.5mm f = 80MHz No 470Ω resistors (Both loops open) Case 2: Current Unbalanced by 10% With a 470Ω resistor connected, a 10% current unbalance was created, which caused the EMI to grow to moderate levels in the area of the unbalance Additional resistor (470) 1/10 100% Case A 100% 100% Case B 90% h = 2.5mm f = 80MHz Case 3: Current Unbalanced by 20% With both 470Ω resistors connected, a 20% current unbalance created; this caused the EMI to becomes high in the area of the unbalance Two additional resistors (470) 1/10 1/10 Near-field scans can help locate the cause of EMI problems 100% Case A 100% 100% Case B 90% 100% Case C 80% h = 2.5mm f = 80MHz Board Layout Affects Emissions An ideal microstrip line shows a fairly uniform current distribution and minimum emissions Reference Microstrip Line Terminated Signal input: 100MHz sine wave, 1.0Vp-p ) Microstrip line Pitch: 5mm; Scan height: 10mm Scanned from bottom side (reference plane) @100MHz Layout Affects Emissions — 2 Emissions increase as the width of the pc board becomes more narrow Symmetric Pattern Pitch: 5mm; Scan height: 10mm Scanned from bottom side (reference plane) @100MHz Layout Affects Emissions — 3 The asymmetric pc board causes even more emissions Asymmetric Pattern Pitch: 5mm; Scan height: 10mm Scanned from bottom side (reference plane) @100MHz Emission Measurement Standards The international standards listed here are used to measure electromagnetic emissions* from MCUs and other ICs Latest Standard Document Issue Date Remarks IEC 61967-1: General conditions and definitions [IEC 61967-1] 2002-03-12 IS IEC 61967-2: Measurement of radiated emissions, [IEC 61967-2] 2005-09 IS [IEC TS 61967-3] 2005-06 TS IEC 61967-4: Measurement of conducted emissions, [IEC 61967-4] 2002-04-30 IS 1-ohm/50-ohm Direct Coupling Method [IEC 61967-4 Ed. 1.1] 2006-2007 Edition 1.1 IEC 61967-5: Measurement of conducted emissions, [IEC 61967-5] 2003-01-17 IS [IEC 61967-6] 2002-06-25 IS Standard No.: Title TEM-cell and wideband TEM-cell Method IEC 61967-3: Measurement of radiated emissions, Surface Scan Method (Technical Specifications) Workbench Faraday Cage Method IEC 61967-6: Measurement of conducted emissions, Magnetic Probe Method *Measurement range: 150kHz to 1GHz IS: IEC International Standard TS: Technical Specification Supply Current Measurement The VDE probe and magnetic probe methods are international standards; the resistor-divider probe method is not VDE Probe Magnetic Probe IS Vcc IC IS [IEC 61967-4] 49Ω Vcc [IEC TS 61967-6] 1Ω Vcc 950Ω in 50Ω in Resistor-Divider Probe 50Ω IC vn IC 1K Ohm ex. 50Ω EM Radiation, CM Voltage Testing These three methods are also good for emissions testing; the Faraday Cage method can measure common-mode voltage for each part of the circuit board TEM Cell Faraday Cage IS Loop Probe IS [IEC 61967-2] TS [IEC 61967-5] [IEC TS 61967-3] Vcc 50Ω vn 50Ω u Problem with Normal TEM Cell When measuring emissions from LSI devices, the combined EM field data are almost identical to that of the magnetic field measurement alone; the electric field data is difficult to see Electric field + Magnetic field (combined result produced by a normal TEM cell measurement) TEM cell output level (dB) Frequency (MHz) TEM Cell Terminator 50Ω TEM cell method (normal) Measuring system 50Ω Magnetic field Electric field Applying the TEM Cell Method With the “hybrid balun” that Renesas has adopted, voltages proportional to a pure electric field and a pure magnetic field can be obtained • Photo shows an electric field coupling • Changing the terminator and output port results in a magnetic field coupling 50Ω terminator (magnetic field coupling) Electric field coupling (50Ω terminator) Output Renesas “Hybrid Balun” Course Summary System-level evaluation techniques Importance of circuit board layout Methods for evaluating emissions from LSI devices For more information on specific devices and related support products and material, please visit our Web site: http://america.renesas.com