Transcript Circuit board - Renesas e

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