Course Introduction Purpose Objectives Content Learning Time

Transcription

1 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

2 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 Anechoic chamber Balun Explanation of Terms A room designed to block radiation from the outside and to minimize reflections off the room s walls, ceiling, and floor A passive electronic device that converts between balanced and unbalanced electrical signals CISPR 25 Core ECU EMI Harness LISN Power supply TEM Cell WBFC 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). 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. Electronic Control Unit Electromagnetic Interference Cables (wires) connecting a board and power supply or connecting one unit in a system to another Line Impedance Stabilization Network 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. Transverse Electromagnetic Cell Workbench Faraday Cage

3 Radiation from Wiring Harness System-level evaluation - Example: performed on three test boards - Test method for measuring emissions from wiring harness: (CISPR 25 equivalent) Radiation levels ranged from high for board A to low for board C Antenna Anechoic chamber Board A Board B LISN Circuit board with MPU Harness Board C Test setup

4 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 MPU Circuit board Probe with sensor coil Power supply connector f = 80MHz Data from near-field scan

5 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 MCU Board A Harness mounting Moving from a 2-layer board to a 4- layer board further reduces emissions at the wiring harness connector Directly above MCU Above harness mounting area MCU Board B Harness mounting area Board C MCU Harness mounting MHz

6 Evaluating Current Balance in PCB Near-field measurements show the common-mode 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 470Ωresistor was connected on left side of board, creating a 10% current unbalance in the measurement area Case 3: Two 470Ωresistors were connected on the left and right sides of the board, creating Left loop Pads Line width = 1.3mm Right loop Area in which a difference between the signal current and return current can be created Termination (50Ω) 100, 90 or 80%

7 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 No 470Ω resistors (Both loops open) Case A h = 2.5mm f = 80MHz

8 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 (470Ω) 1/10 h = 2.5mm f = 80MHz Case A Case B 90%

9 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 Near-field scans can help locate the cause of EMI problems Two additional resistors (470Ω) 1/10 1/10 h = 2.5mm f = 80MHz Case A Case B 90% Case C 80%

10 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

11 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

12 Layout Affects Emissions 3 The asymmetric pc board causes even more emissions Asymmetric Pattern Pitch: 5mm; Scan height: 10mm Scanned from bottom side (reference

13 Emission Measurement Standards The international standards listed here are used to measure electromagnetic emissions* from MCUs and other ICs Latest Standard Standard No.: Title Document Issue Remarks Date IEC : General conditions and definitions [IEC ] IS 12 IEC : Measurement of radiated emissions, [IEC ] IS TEM-cell and wideband TEM-cell Method IEC : Measurement of radiated emissions, Surface Scan Method (Technical [IEC TS ] Specifications) IEC : Measurement of conducted emissions, [IEC ] 1-ohm/50-ohm Direct Coupling Method [IEC Ed. 1.1] TS Edition 1.1 IS IEC : Measurement of conducted emissions, Workbench Faraday Cage Method [IEC ] IS IEC : Measurement of conducted emissions, Magnetic Probe Method [IEC ] IS *Measurement range: 150kHz to 1GHz IS: IEC International Standard TS: Technical Specification

14 Supply Current Measurement The VDE probe and magnetic probe methods are international standards; the resistor-divider probe method is not VDE Probe Magnetic Probe Resistor-Divider Probe IS IS IC Vcc 49Ω in 1Ω [IEC ] 50Ω in IC Vcc [IEC TS ] 50Ω vnic Vcc 950Ω 1K Ohm ex. 50Ω

15 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 Loop Probe IS IS TS [IEC ] Vc c [IEC ] [IEC TS ] 50Ω 50Ω vn u

16 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 TEM Cell TEM cell output level (db) Electric field + Magnetic field (combined result produced by a normal TEM cell measurement) Frequency (MHz) Magnetic field Terminator 50Ω TEM cell method (normal) Measuring system 50Ω Electric field

17 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) Renesas Hybrid Balun Output

18 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: