AN-0971 [ADI]
Recommendations for Control of Radiated Emissions with isoPower Devices; 辐射发射与isoPower器件的控制建议
| 型号： | AN-0971 |
| 厂家： | 
ADI |
| 描述： | Recommendations for Control of Radiated Emissions with isoPower Devices |
| 文件： | 总20页 (文件大小：439K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
AN-0971
APPLICATION NOTE
One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Recommendations for Control of Radiated Emissions with isoPower Devices
by Mark Cantrell
Control of emissions from signal cables and chassis shielding
techniques are outside of the scope of this application note.
INTRODUCTION
iCoupler® digital isolators with integrated isolated power
(isoPower®) employ isolated dc-to-dc converters that switch
currents of ~700 mA at frequencies as high as 300 MHz.
Operation at these high frequencies raises concerns about
radiated emissions and conducted noise. PCB layout and
construction is a very important tool for controlling radiated
emissions and noise from applications containing isoPower
components. This application note identifies the radiation
mechanisms and offers specific guidance on addressing them.
isoPower OVERVIEW
The ADuM5xxx and ADuM6xxx product families represent a
significant step forward in isolation technology. Analog Devices,
Inc., has leveraged their experience in microtransformer design
to create chip scale dc-to-dc power converters. These power
converters are incorporated into Analog Devices signal isolation
products. Power levels of up to one-half watt are available, at
output voltages ranging from 3.3 V to 15 V. isoPower is used to
power the secondary side of the iCoupler data channels as well
as to provide power to off-chip loads.
Several standards for radiated emissions exist. In the U.S.,
the Federal Communications Commission (FCC) controls
the standards and test methods. In Europe, the International
Electrotechnical Commission (IEC) generates standards, and
CISPR test methods are used for evaluating emissions. The
methods and pass/fail limits are slightly different under the
two standards. Although this application note is written with
reference to CISPR standards, all results are applicable to both
standards.
Because the Analog Devices standard data couplers use a similar
magnetic technology, EMI can be an issue in data-only iCouplers
and the data channels of isoPower devices. Emissions from data
channels are addressed in the AN-1109 Application Note, Control
of Radiated Emissions in iCoupler Devices.
Analog Devices uses several power architectures to achieve desired
design goals, such as efficiency, small size, and high output voltage
(see Figure 1). These architectures have three common ele-
ments: a transformer to couple power to the secondary side of
the iCoupler, an oscillator tank circuit that switches current into
the transformer at an optimum frequency for efficient power
transfer, and a rectifier that recreates a dc level on the secondary
side. Several regulation methods are used in these products.
With proper design choices, isoPower devices can easily meet
CISPR Class A (FCC Class A) emissions standards. With care,
these products can also meet CISPR Class B and FCC Part 15,
Subpart B, Class B (FCC Class B) standards in an unshielded
environment. This application note examines PCB-related EMI
mitigation techniques, including board layout and stack-up issues.
The techniques, example layouts, and measurements described in
this application note are highly dependent on the options available
in PCB construction. A 4-layer PCB is required to implement the
evaluated EMI control techniques. Availability of more internal
PCB layers offers the same EMI reductions within smaller
geometry PCBs. For the purposes of this application note, a 4-layer
board was designed and manufactured using materials and
structures well within industry norms.
The physics of the transformer requires that the oscillator
circuit switch current into the transformer at a rate ranging
from between 180 MHz and 300 MHz. The rectifier circuit on
the secondary side effectively doubles this frequency during the
rectification process. These functions are common in switching
power supplies; however, the operating frequency is three
orders of magnitude higher than a standard dc-to-dc converter.
Noise generated by the operation of the converter into the
30 MHz to 1 GHz range is of concern for radiated emissions.
OSC
RECT
REG
OPEN LOOP WITH LINEAR REGULATOR
ADuM524x
OSC
RECT
REG
OSC
RECT
PWM
FULL FEEDBACK PWM CONTROL
ADuM540x, ADuM520x, ADuM5000
ADuM640x, ADuM620x, ADuM6000
OPEN LOOP WITH PWM CONTROL
ADuM5230, ADuM6132
Figure 1. isoPower Architectures
Rev. B | Page 1 of 20
AN-0971
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Recommended Design Practices.................................................. 10
Meeting Isolation Standards ......................................................... 11
Evaluating PCB Structures for EMI............................................. 12
Edge Guarding Results .............................................................. 14
Operating Load and Voltage Dependence.............................. 15
Interplane Capacitance.............................................................. 15
Integrating Techniques .................................................................. 16
Example 1—Basic Insulation Board ........................................ 16
Example 2—Reinforced Insulation Board .............................. 17
Additional Layout Considerations........................................... 18
Conclusions..................................................................................... 19
References.................................................................................... 19
isoPower Overview .......................................................................... 1
Revision History ............................................................................... 2
Sources of Radiated Emissions ....................................................... 3
Edge Emissions ............................................................................. 3
Input-to-Output Dipole Emissions............................................ 3
Sources of Conducted Noise ........................................................... 5
EMI Mitigation Techniques ............................................................ 6
Input-to-Output Stitching........................................................... 6
Edge Guarding .............................................................................. 7
Interplane Capacitive Bypassing ................................................ 8
Power Reduction .......................................................................... 9
Operating Voltage......................................................................... 9
REVISION HISTORY
6/11—Rev. A to Rev. B
Added Operating Voltage Section...................................................9
Changes to Recommended Design Practices Section ............... 10
Changes to Meeting Isolation Standards Section....................... 11
Added Figure 14 ............................................................................. 11
Added Table 1; Renumbered Sequentially .................................. 11
Replaced Example Board Section with Evaluating PCB
Structures for EMI Section............................................................ 12
Changes to Figure 15...................................................................... 12
Added Figure 16 and Figure 17 .................................................... 12
Changes to Table 2.......................................................................... 13
Added Stitching Capacitance Results Section, Figure 18, and
Figure 19 .......................................................................................... 13
Added Figure 20, Edge Guarding Results Section, Table 3, and
Figure 21 .......................................................................................... 14
Added Operating Load and Voltage Dependence Section,
Figure 22, Figure 23, and Interplane Capacitance Section ....... 15
Added Integrating Techniques Section, Figure 24, Example 1—
Basic Insulation Board Section, Table 4, and Figure 25 ............ 16
Added Figure 26 Through Figure 30, Example 2—Reinforced
Insulation Section, and Table 5..................................................... 17
Added Additional Layout Considerations Section and
Changes to Introduction Section, isoPower Overview Section,
and Figure 1....................................................................................... 1
Changes to Edge Emissions Section and Input-to-Output
Dipole Emissions Section ................................................................ 3
Added Figure 3; Renumbered Sequentially .................................. 3
Changes to Sources of Conducted Noise Section ........................ 5
Deleted Figure 9; Renumbered Sequentially................................. 5
Changes to EMI Mitigation Techniques Section and Input-to-
Output Stitching Section ................................................................. 6
Added Safety Rated Capacitor Section and Stitching
Capacitance Built into the PCB Section ........................................ 6
Added Floating Capacitive Structure Section and Gap Overlap
Stitching Section ............................................................................... 7
Replaced Figure 6 and Figure 7 ...................................................... 7
Changes to Edge Guarding Section................................................ 7
Deleted Figure 13.............................................................................. 7
Deleted Table 2; Renumbered Sequentially .................................. 8
Deleted Figure 15.............................................................................. 8
Changes to Figure 9 and Figure 10................................................. 8
Replaced Buried Capacitance Bypassing Section with Interplane
Capacitive Bypassing Section.......................................................... 8
Added Figure 11................................................................................ 8
Changes to Figure 12 and Power Reduction Section................... 9
Replaced Figure 13 ........................................................................... 9
Deleted Appendix A—A Design Example Section, Figure 16,
and Figure 17..................................................................................... 9
Figure 31 .......................................................................................... 18
Changes to Conclusions Section and References Section......... 19
3/09—Rev. 0 to Rev. A
6/08—Revision 0: Initial Version
Rev. B | Page 2 of 20
Application Note
AN-0971
SOURCES OF RADIATED EMISSIONS
There are two sources of emissions in PCBs where isoPower is
used: edge emissions and input-to-output dipole emissions.
driving the dipole. In many cases, this is a very large dipole as
shown in Figure 4. A similar mechanism causes high frequency
signal lines to radiate when crossing splits in the ground and
power planes. This type of radiation is predominantly perpen-
dicular to the gap in the ground planes.
EDGE EMISSIONS
Edge emissions occur when unintended currents meet the edges
of ground and power planes. These unintended currents can
originate from
•
Ground and power noise generated by inadequate bypass
of high power current sinks.
•
Cylindrically radiated magnetic fields coming from
inductive via penetrations radiated out between board
layers eventually meeting the board edge.
Strip line image charge currents spreading from high
frequency signal lines routed too close to the edge of
the board.
•
Edge emissions are generated (see Figure 2) where differential
noise from many sources meets the edge of the board, leaking
out of a plane-to-plane space and acting as a wave guide.
GROUND
POWER
Figure 4. Dipole Radiation Between Input and Output
The ADuM540x device serves as a good example of the issues
involved in generating and mitigating emissions. The oscillator
tank circuit in the ADuM540x runs at approximately 180 MHz.
It can regulate its output to either 5 V or 3.3 V under the control
of a pin on the secondary side. The input voltage can be in the
3 V to 5 V range. The highest power operating mode is 5 V
input and 5 V output and is the primary configuration
examined in this application note.
Figure 2. Edge Radiation From an Edge Matched Ground Power Pair
GROUND
POWER
h
20h
SIGNAL
When operating under a full 100 mA output load, the average
input current is about 290 mA. This means that the peak
current in the tank circuit is about twice that value at a
switching rate of 180 MHz.
Figure 3. Edge Radiation From an Edge Mismatched Power Ground Pair
At the edge boundary, there are two limiting conditions. The
edges of the ground and power planes are lined up, as shown in
Figure 2, or one edge is pulled back by some amount, as shown
in Figure 3. In the first case of lined up edges, there is some
reflection back into the PCB and some transmission of the
fields out of the PCB. In the second case, the edge of the boards
makes a structure similar to the edge of a patch antenna. When
the edges mismatch by <20h, where h is the plane to plane
pacing, the fields couple out of the PCB very efficiently,
resulting in very high emissions. These two limiting cases
become important to discussions of edge treatment of the PCB.
The component’s bypass capacitor is supposed to provide this
high frequency current locally. This is a lot of current for a
bypass capacitor to handle. The capacitor must provide large
charge reserves. At the same time, the capacitor must have a
very low series resistance at 180 MHz. Even with multiple low
ESR capacitors near the pins, an inductively limited bypass
allows voltage transients, and the noise is injected onto the
ground and power planes.
Power transfers to the output side where it is rectified into dc.
The rectification process doubles the tank frequency to 360 MHz.
The input-to-output emissions are both at the rectification
frequency and the tank frequency, along with some higher
harmonics. Figure 5 shows worst-case data collected on a
2-layer evaluation board with a near field probe.
INPUT-TO-OUTPUT DIPOLE EMISSIONS
Input-to-output dipole radiation is generated by driving a
current source across a gap between ground planes. This
is the predominant mechanism for radiation for isoPower
applications. Isolated power supplies, by their very nature,
drive energy across gaps in ground planes. The inability of
high frequency image charges associated with the power signal
to cross the boundary causes differential signals across the gap
Rev. B | Page 3 of 20
AN-0971
Application Note
A board with near field emissions, as shown in Figure 5, and
without a chassis shield would fail FCC Class B emissions
standards by approximately 30 dB at the 360 MHz peak.
180MHz
TANK
360MHz
RECTIFIER
HARMONICS AT
540MHz AND
720MHz
Figure 5. FFT of the Near Field Emissions from a 2-Layer Board
Rev. B | Page 4 of 20
Application Note
AN-0971
SOURCES OF CONDUCTED NOISE
The large currents and frequencies also generate conducted
noise on the ground and power planes. This issue is addressed
along with radiated emissions because the causes and remedies
for both types of EMI are addressed with the same PCB ground
and power structures.
An ideal bypass capacitor of a few microFarads should be ade-
quate to supply the ac component of the current. Real world
bypass capacitors are not ideal, and they connect to one or,
more likely, both of the power planes by an inductive via. In
addition, a large distance between ground and power planes
creates a large inductance between them, restricting their ability
to supply current quickly. These factors contribute to a large
fraction of a volt of high frequency noise on the VDD plane.
The inability of the bypass capacitors and ground/power planes
to provide adequate high frequency current to the isoPower dc-
to-dc converter causes VDD noise. The dc-to-dc converter switches
power in bursts of 2.5 ns pulses with an amplitude of ~700 mA.
Rev. B | Page 5 of 20
AN-0971
Application Note
EMI MITIGATION TECHNIQUES
Many mitigation techniques are available to the designer.
Several techniques that apply directly to the isoPower devices
are identified in this section. The choice of how aggressively
EMI must be addressed for a design to pass FCC/CISPR
emissions levels depends on the requirements of the design as
well as cost and performance tradeoffs. The easiest radiated
EMI mitigation technique to implement is to place the PCB in a
grounded chassis with filter elements limiting noise escaping on
cable shields. Although this application note does not describe
this option, note that, where PCB related techniques are
impractical, this method remains available.
There are at least three options to form a stitching capacitance.
•
•
A safety rated capacitor applied across the barrier.
A floating metal plane spanning the gap between the
isolated and nonisolated sides on an interior layer, as
shown in Figure 6.
•
Extending the ground and power planes on an interior
layer into the isolation gap of the PCB to form a capacitor,
as shown in Figure 7.
Each of these options has its advantages and disadvantages in
effectiveness and area required to implement.
The EMI mitigation practices rely on having relatively
continuous ground and power planes and the ability to specify
their relative positions and distances in the stack-up. This
dictates that the minimum total number of planes is three:
ground, power, and signal planes.
Safety Rated Capacitor
A stitching capacitance can be implemented with a simple ceramic
capacitor across the barrier. Capacitors with guaranteed creep-
age, clearance, and withstand voltage can be obtained from
many major capacitor manufacturers such as Murata, Johansen,
Hitano, and Vishay. Safety rated capacitors are available in
several grades depending on their intended use. The Y2 grade is
used in line-to-ground applications where there is danger of
electric shock and is the recommended safety capacitor type for
a stitching capacitor in a safety rated application. This type of
capacitor is available in surface-mount and radial leaded disk
versions.
For practical considerations in board manufacture, a 4-layer
board is the minimum stack-up. More layers are acceptable and
can be used to greatly enhance the effectiveness of the recom-
mended techniques.
The following techniques are effective in reducing EMI
radiation and on-board noise:
•
•
•
•
Input-to-output ground plane stitching capacitance
Power control
Edge guarding
Because safety capacitors are discrete components, they must
be attached to the PCB with pads or through holes. This adds
inductance to the capacitor on top of its intrinsic inductance.
It also makes the stitching capacitor localized, demanding that
currents flow to the capacitor, which can create asymmetrical
image charge paths and added noise. These factors limit the
effectiveness of discrete capacitors to frequencies below about
200 MHz.
Interplane capacitive bypass
Circuit boards with test structures were prepared to evaluate
each of these EMI mitigation techniques utilizing the ADuM540x.
The layout of each board was varied as little as possible to allow
meaningful comparison of results. Testing was conducted at an
EMI test facility in a shielded chamber. As expected, during
EMI testing, it was determined that the emissions at the tank
frequency (180 MHz) were predominantly in the plane of the
board, suggesting that the primary mechanism for radiation is
from the PCB edges. The rectification emissions (360 MHz)
are primarily perpendicular to the isolation gap in the board,
suggesting input-to-output dipole radiation.
Stitching Capacitance Built into the PCB
The PCB itself can be designed to create a stitching capacitor
structure in several ways. A capacitor is formed when two
planes in a PCB overlap. In this type of capacitor, the
inductance of the parallel plate capacitor formed is extremely
low, and the capacitance is distributed over a large area.
These structures must be constructed on internal layers of a
PCB. The surface layers have minimum creepage and clearance
requirements; therefore, it is not practical to use surface layers
for this type of structure. This requires a minimum of four
board layers.
INPUT-TO-OUTPUT STITCHING
When current flows along PCB traces, an image charge follows
along the ground plane beneath the trace. If the trace crosses a
gap in the ground plane, the image charge cannot follow along.
This creates differential currents and voltages in the PCB
leading to radiated and conducted emissions. The solution is
to provide a path for the image charge to follow along with the
signal. Standard practice is to place a stitching capacitor in
proximity to the signal across the split in the ground plane (see
PCB Design for Real-World EMI Control by Archambeault and
Drewniak in the References section). This same technique
works to minimize radiation between ground planes due to the
operation of isoPower.
A good option is to use a floating metal structure on an interior
layer of the board to bridge between the primary and secondary
power planes. Note that, hereafter, planes dedicated to ground
or power are referred to as reference planes because, from an ac
noise point of view, they behave the same and can be used for
stitching capacitance interchangeably.
Rev. B | Page 6 of 20
Application Note
AN-0971
Floating Capacitive Structure
This architecture is best suited to applications where a larger
amount of board area is available or where reinforced insulation
is required.
An example of a floating stitching capacitance is shown in
Figure 6. The reference planes are shown in blue and green,
and the floating coupling plane is shown in yellow. The capaci-
tance of this structure creates two capacitive regions (shown
with shading) linked by the nonoverlapping portion of the
structure. To ensure that optimum capacitance is created for
the area of the structure, the overlap areas of the primary and
secondary sides should be equal.
Gap Overlap Stitching
A simple method of achieving a good stitching capacitance is to
extend a reference plane from the primary and secondary sides
into the area that is used for creepage on the PCB surface.
w
I
w
w
2
1
I
d
Figure 7. Overlapping Plane Stitching Capacitance
d
The capacitive coupling of the structure in Figure 7 is calculated
with the following basic relationships for parallel plate
capacitors:
Figure 6. Floating Stitching Capacitance
The capacitive coupling of the structure in Figure 6 is calculated
with the following basic relationships for parallel plate capacitors:
Aε
d
C =
and ε = ε0 × εr
(3)
C1 ×C2
C1 +C2
A ε
x
C =
, C1 = C2 =
, and ε = ε0 × εr
d
where:
where:
C is the total stitching capacitance.
AX is the overlap area of the stitching capacitance to each
reference plane.
d is the thickness of the insulation layer in the PCB (see Figure 6).
ε0 is the permittivity of free space, 8.854 × 10−12 F/m.
εr is the relative permittivity of the PCB insulation material,
which is about 4.5 for FR4.
C is the total stitching capacitance.
d is the thickness of the insulation layer in the PCB,
ε0 is the permittivity of free space, 8.854 × 10−12 F/m.
εr is the relative permittivity of the PCB insulation material,
which is about 4.5 for FR4.
lwε
C =
d
(4)
where w, d, and l are the dimensions of the overlapping portion
of the primary and secondary reference planes, as shown in
Figure 7.
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
w1 × w2
w1 + w2
lε
d
C =
×
(1)
where w1, w2, d, and l are the dimensions of the overlapping
portions of the floating plane and the primary and secondary
reference planes, as shown in Figure 6.
The major advantage if this structure is that the capacitance is
created in the gap beneath the isolator, where the top and
bottom layers must remain clear for creepage and clearance
reasons. This board area is not utilized at all in most designs.
The capacitance created is also twice as efficient per unit area as
the floating plane.
If w1 = w2, the equation simplifies to
lw1ε
d
C =
(2)
This architecture has only a single cemented joint and a single
layer of FR4 between the primary and secondary reference
planes. It is well suited to smaller boards where only basic
insulation is required.
There are advantages and disadvantages of this structure in real
applications. The major advantage is that there are two isolation
gaps, one at the primary and one at the secondary. These gaps
are called cemented joints, where the bonding between layers
of FR4 provides the isolation. There are also two sequential
paths through the thickness of the PCB material. The presence
of these gaps and thicknesses is advantageous when creating a
reinforced isolation barrier under some isolation standards.
The disadvantage of this type of structure is that the capacitance
is formed under active circuit area so there can be via penetra-
tions and traces that run across the gaps. Equation 2 also shows
that the capacitance formed is half as effective per unit area
used compared to a simple parallel plate capacitor.
EDGE GUARDING
Noise on the power and ground planes that reaches the edge of
a circuit board can radiate as shown in Figure 2 and Figure 3.
If the edge is treated with a shielding structure, the noise is
reflected back into the interplane space (see Archambeault and
Drewniak in the References section). This can increase the
voltage noise on the planes but reduces edge radiation.
Making a solid conductive edge treatment on a PCB is possible,
but the process is expensive. A less expensive solution that
Rev. B | Page 7 of 20
AN-0971
Application Note
works well is to treat the edges of the board with a guard ring
structure laced together by vias. The structure is shown in
Figure 8 for a typical 4-layer board. Figure 9 shows how this
structure can be implemented on the power and ground layers
of the primary side of a circuit board.
provide the interplane capacitance layer that supplements the
bypass capacitors required for proper operation of the device.
SIGNAL/POWER
GROUND
POWER
BURIED
CAPACITIVE
LAYER
SIGNAL/GROUND
Figure 10. PCB Stack-Up for Interplane Capacitance
GROUND
POWER
In addition to the ground and power planes, the capacitance can
be increased even further by filling signal layers with alternating
ground and power fill. The top and bottom layers in Figure 10
are labeled signal/power and signal/ground to illustrate the fills
on those particular layers. This has the added benefit of creating
additional shielding for EMI that leaks around the edges of a via
fence structure, keeping it in the PCB. Care must be taken when
making ground and power fills. These fills must be tied back to
the full reference plane because a floating fill can act as a patch
antenna and radiate instead of shielding. Some recommended
practices for fills include
GROUND VIA EDGE FENCE
AND GUARD RINGS
Figure 8. Via Fence Structure, Side View
•
Fills should be tied to their appropriate reference plane
along the edges with vias every 10 mm.
Thin fingers of fill should be removed.
If the fill has an irregular shape, put vias at the extreme
edges of the shape.
Figure 9. Via Fence and Guard Ring,
Shown on the Primary Power Plane Layers
•
•
There are two goals in creating edge guarding. The first is to reflect
cylindrical emissions from vias back into the interplane space, not
allowing it to escape from the edge. The second is to shield any
edge currents flowing on internal planes due to noise or large
currents.
POWER FILL
The spacing of the vias used to create the edge guard is difficult to
determine without extensive modeling. Analog Devices’ test boards
used 4 mm via spacing for the evaluation boards. This spacing is
small enough to provide attenuation to signals less than 18 GHz,
and it conformed to general guidance from other sources. The
number of vias required is reasonable. Further investigation into
the via density was not conducted.
AVOID SMALL
FILL ISLANDS
INTERPLANE CAPACITIVE BYPASSING
Interplane capacitance bypassing is a technique intended to
reduce both the conducted and radiated emissions of the board
by improving the bypass integrity at high frequencies. This has
two beneficial effects. First, it reduces the distance that high
frequency noise can spread in the ground and power plane pair.
Second, it reduces the initial noise injected into the power
ground planes by providing a bypass capacitance that is effec-
tive between 200 MHz and 1 GHz (see Archambeault and
Drewniak in the References section). Power and ground noise
reduction provides a better operating environment for noise
sensitive components near the isoPower device. Conducted
emissions are reduced proportionate to the reduction in power
and ground noise. The reduction in radiated emissions is not as
significant as that achieved with the stitching or edge guarding
techniques; however, it significantly improves the power
environment of the board.
VIA TO REFERENCE
PLANE
GROUNDED
VIA FENCE
Figure 11. Features of Fill
The effectiveness of interplane capacitance is shown in Figure 12.
It shows the noise generated by the PWM controlling the
primary side oscillator in an ADuM5xxx series part. In the top
section is the noise on the VDD1 pin generated in a 2-layer board.
The center section shows a substantial improvement in a PCB
with ground and power planes separated by 24 mil. Finally, in
the lower pane, a tightly spaced ground and power plane with a
4 mil separation shows noise much smaller than the supply ripple.
The stack-up used for EMI test boards was signal-ground-
power-signal, as shown in Figure 10. A thin core layer is used
for the power and ground planes. These tightly coupled planes
Rev. B | Page 8 of 20
Application Note
AN-0971
2-LAYER BOARD NO POWER PLANE
tank circuit (180 MHz peak) is not directly proportional to the
average current.
∆460mV
33
31
29
27
24MIL SPACING BETWEEN POWER AND GROUND
5V
25
∆104mV
4MIL SPACING BETWEEN POWER AND GROUND
3.3V
23
21
19
17
15
∆64mV
Figure 12. VDD Voltage Noise for Various Stack-Up Options
POWER REDUCTION
0
10
20
30
40
50
60
70
In isoPower devices with active feedback architecture, emissions
can be reduced by keeping the tank circuit off as long as possi-
ble. Running isoPower with light loads achieves this for the
ADuM5xxx and ADuM6xxx devices. Load reduction proves to
be a very significant factor in emissions levels.
PWM DUTY FACTOR (%)
Figure 13. Emissions at the 180 MHz Tank Frequency vs. PWM duty Factor
Emissions at 360 MHz is proportional to the average load
current. In practical application, this means that choosing
whether it is better to operate at 3.3 V or 5 V from an EMI point
of view depends on which peak needs to be controlled and the
required load current. See the Operating Load and Voltage
Dependence section of this application note for additional
information.
OPERATING VOLTAGE
The operating voltage is the last parameter to choose when
designing with isoPower. It is not as straightforward as simply
selecting a low power or low voltage operating condition. As
shown in Figure 13, the emissions at 180 MHZ correlate closely
with the duty factor of the PWM regulation signal and are
largely independent of operating voltage. The PWM duty factor
controls the proportion of time that the tank oscillator is
actively switching. It implies that the noise generated by the
Rev. B | Page 9 of 20
AN-0971
Application Note
RECOMMENDED DESIGN PRACTICES
Consider the following general design practices for minimizing
EMI issues on PCBs. These methods do not introduce any
additional isolation boundaries in the PCB that need
certification review.
goes through the closest via, even if multiple paths are
present.
•
Be very careful to route signal lines against a single refer-
ence plane. It is vital to maintain the image charge path so
that image charges do not have to travel by circuitous routes
to meet back with the original signal on another plane.
Do not route high speed lines close to the edges of the PCB.
Routing data or power off boards, especially through
cables, can introduce an additional radiation concern.
Feedthrough filter capacitors or similar filter structures can
be used to minimize cable radiation.
•
•
Use a stack-up of at least four layers.
Space the power and ground plane as closely as practical to
optimize bypass.
•
•
•
All vias in the power path should be as large as practical.
Small vias have high inductance and generate noise. Using
multiple small vias is not as effective in reducing via induct-
ance as a single large via because the bulk of the current
Rev. B | Page 10 of 20
Application Note
AN-0971
MEETING ISOLATION STANDARDS
The techniques described in this application note do not affect
board isolation, with the exception of input-to-output coupling
through a stitching capacitance. When stitching is implemented
with a safety capacitor, the capacitor has rated working and
transient voltages, as well as specified creepage and clearance.
This makes the safety capacitor relatively easy to deal with from
a certification point of view. However, its performance as an
EMI suppression element is limited.
In PCB insulation, it is important to certification agencies that
materials have an adequate dielectric breakdown to pass the
transient test requirements and that they are constructed in a
way that the insulation does not break down over time. Table 1
compares four standards to what is required to make a basic or
reinforced insulation barrier inside a PCB.
In the case of basic insulation in printed circuit boards, there is
no minimum specification for distance through the insulation.
Thus, the designer has a great deal of flexibility in board layout.
Materials such as FR4 must be thick enough to withstand the
required overvoltage for the life of the product.
The PCB stitching capacitor, by its nature, is most effective when
conductors are located as close to each other as possible. To
obtain maximum performance from these elements, it is neces-
sary to push the internal spacing requirements as close to the
limit as possible, while maintaining safety. Different standards
can have completely different approaches to PCB construction and
the applicable standard must be applied.
Reinforced insulation requires a minimum distance of 0.4 mm
(about 16 mil) of insulation along a bonded surface, such as the
gap between copper structures on an internal PCB layer or
directly through the insulation from layer to layer in most cases.
In addition, there can be type testing requirements for circuit
boards unless multiple layers of insulation are used between
active structures. Although this requirement necessitates careful
board design and possibly more than four layers, it should not
be burdensome if taken into account at the start of a design.
Certification agencies treat the surface layers of a multilayer
PCB differently than interior layers. The surface has creepage
and clearance requirements that are driven by air ionization and
breakdown along dirty surfaces. Interior layers are treated as
solid insulation or permanently cemented joints between solid
insulation.
Capacitive coupling across the isolation barrier allows ac
leakage and transients to couple from one ground plane to the
other. Although 300 pF seems small, high voltage, high speed
transients can inject significant currents across the barrier
through this capacitance. Take this into account if the
application is to be subjected to these environments.
CREEPAGE/
CLEARANCE
THROUGH
INSULATION
CEMENTED
JOINT
Figure 14. Critical Distances in PCB Design
Table 1. Comparison of Isolation Creepage in Isolation Standards
IEC 60950
Along a
Through insulation cemented
IEC 61010 2nd Edition
IEC 61010 3rd Edition
Along a
IEC 60601
Through
Along a
Type of
Insulation
insulation
cemented joint
(6.7.2.2.3)
Through
cemented
joint (6.7)
Cemented and
solid insulation
(2.10.6.4)
joint (2.10.6.3) (6.7.2.2.3)
insulation (6.7)
Functional
Insulation
No requirement
No
No
No requirement
No requirement
No requirement
0.4 mm minimum
0.4 mm
minimum
Verified by test
Verified by test
Verified by test
requirement
requirement
Basic Insulation
No requirement
No
No
0.4 mm minimum
0.4 mm
minimum
requirement
requirement
Supplemental/
Reinforced
insulation
0.4 mm minimum or 0.4 mm
multiple layers of
insulation, precured
No
0.4 mm minimum
or multiple layers
of insulation,
0.4 mm
minimum
minimum
(2.10.5.2)
requirement
precured
Rev. B | Page 11 of 20
AN-0971
Application Note
EVALUATING PCB STRUCTURES FOR EMI
Choosing a combination of PCB structures and techniques can
achieve the desired system radiated EMI goal. There are two
sets of standards for radiated emissions, one from the United
States Federal Communications Commission (FCC) and a
second from Comité Internationale Spécial des Perturbations
Radioelectrotechnique (CISPR), a special committee of the IEC.
60
50
40
30
20
FCC CLASS B
FCC CLASS A
CISPR 22 CLASS B
10
CISRR 22 CLASS A
0
10
100
FREQUENCY (MHz)
1000
10000
Figure 16. Control Board
Figure 15. FCC and CISPR Limits Corrected to 10 m Antenna Distance
Referring to Figure 15, the emissions at these frequencies must
be below 30 dBμV/m at 180 MHz and below 37 dBμV/m at
360 MHz normalized to 10 m antenna distance to achieve
Class B emissions levels. The configuration of the control board
is considered a standard PCB layout for isolation application.
Emissions at a 5 V operating condition and full load represent
the worst case for radiated emissions. Figure 17 shows the
facility data collected for the control board. The features to note
are the tank frequency peak at 180 MHz and the rectification
frequency of 360 MHz. The harmonics at higher frequencies
usually disappear when EMI mitigation is applied.
In this application note, the CISPR22 emissions standards are
used for evaluating the PCB results. Figure 15 shows the relation-
ship between FCC and CISPR levels. Over most of the spectrum,
the CISPR levels are more conservative than the FCC levels,
and because many products for the international markets must
address both standards, only the CISPR pass limits are refer-
enced in this application note. Refer to Figure 15 for the relevant
FCC levels if they are required for analysis.
Effectiveness of the EMI mitigation techniques was verified by
creating a set of evaluation boards with different combinations
of stitching capacitance, edge guarding, and size. The control
for these experiments was a 4-layer PCB with interior ground
and power planes separated by 4 mil of FR4. This gives a good
amount of interplane capacitance on each side of the boundary,
no edge guarding, and no stitching capacitance, as shown in
Figure 16. Testing was conducted at an EMI facility in a 3 m
screen room. The goal was to view the broad spectrum of
emissions rather than to focus on individual peaks. Peaks from
this test correlate well to the results from the 10 m far field
results.
360MHz
75
70
720MHz
65
180MHz
60
55
540MHz
50
45
40
35
900MHz
30
50
100
300
500
1k
EMISSIONS FREQUENCY (MHz)
Figure 17. Emissions from the Control Board at 5 V and 90% Load
Table 2 shows that emissions from this board are significant and
must be reduced by 32 dB for the 360 MHz peak and 36 dB for
the 180 MHz peak to bring them into compliance with CISPR
Class B. Reducing that 36 dB of emissions is the goal of the design.
Rev. B | Page 12 of 20
Application Note
AN-0971
In the Stitching Capacitance Results and Edge Guarding Results
sections, the data presented is normalized to the 5 V/5 V 90%
load condition with no stitching capacitance, so that values
from Figure 20, Figure 22, Figure 23, and Table 3 can be sub-
tracted from the baseline operation of the application board
directly.
Table 2. Class B Emission Limits
Requirements
180 MHz
62 dB
360 MHz
73 dB
2-Layer PCB Emissions
Class B Limit
30 dB
37 dB
Required EMI Reduction
32 dB
36 dB
Stitching Capacitance Results
The addition of stitching capacitance has proven to be the most
effective way to reduce emissions across the entire spectrum. It
is most effective when it has very low inductance and is spread
across the entire length of the barrier. The optimum geometry
to achieve a level of stitching capacitance is dependent on available
space and requirements of the regulatory requirements control-
ling the design. For this evaluation, a gap overlap capacitance
was implemented because it develops a large capacitance and
uses a part of the PCB that is usually cleared of all traces and
components. Several other options are available and are
described in the Integrating Techniques section.
Figure 18. Overlap Stitching Capacitance
62
NO STITCHING
300pF STITCHING
57
52
47
Figure 18 shows the PCB layout with the internal planes outlined.
The board is built on a 4 mil core with power and ground planes
extending from each side to overlap in the middle. The overlap
is l = 114 mm by w = 6.5 mm and the separation, d = 0.1 mm.
Applying Equation 4, the stitching capacitance is 300 pF. An
additional PCB was made with a shorter length to generate
150 pF of stitching capacitance. A modification allowed 75 pF
capacitance measurements as well.
42
37
32
27
22
17
12
7
An example data set is shown in Figure 19. The peaks can
clearly be seen at the expected frequencies. The two curves
represent the control board with no stitching capacitance and
the 300 pF overlap stitching capacitor configuration. There is a
dramatic 25 dB to 30 dB drop in emissions due to the stitching
capacitance depending on the peak. The peak values vary with
load and voltage, but the reductions are independent of
operating voltage and load current.
40
70
100
200
400
700
1k
FREQUENCY (MHz)
Figure 19. Effect of 300 pF of stitching capacitance on Emissions of a 10%
Loaded ADuM540x
Figure 20 summarizes the emissions as a function of stitching
capacitance. Note that the shape of the curves is dependent on
the frequency range of the emissions peak. Lower frequency
emissions (<200 MHz) are only slightly reduced until the capaci-
tance is greater than 150 pF. Higher frequency emissions
(>200 MHz) have the majority of their reduction at less
than 150 pF.
Rev. B | Page 13 of 20
AN-0971
Application Note
0
–5
–10
–15
–20
–25
–30
180MHz
360MHz
–35
0
50
100
150
200
250
300
STITCHING CAPACITANCE (pF)
Figure 20. Reduction in Emissions Due to Stitching Capacitance for 5 V/5 V
Operation at 10% Load
The difference in the shape of the curves seems to be primarily
related to inductance in the capacitive coupling and the required
amount of capacitance to achieve the optimum result. At
360 MHz the low inductance stitching has sufficiently coupled
the input and output planes with 150 pF. The 180 MHz emis-
sions are primarily from the input plane and more bulk
capacitance is required to reduce emissions. This makes some
options available; because most component capacitors are still
very effective as bypass below 200 MHz, the stitching capaci-
tance can be made from a combination of PCB based stitching
for low inductance and discrete component capacitors to
increase the total capacitance. Several examples of options
are examined in the Integrating Techniques section.
Figure 21. Overlap Stitching Capacitance, Edge Guarding, and
Ground/Power Fills
Guarding is installed on both sides, but it is less effective on the
secondary planes. A guard ring was applied to each layer, except
the reference layer (see Figure 8 and Figure 9), and the layers
linked together with vias every 4 mm. There are a few options
for fencing near the iCoupler. If spacing is tight, the fence can
be interrupted on all layers under the device, as shown in Figure
21. Guard rings can also be interrupted only on the top and bottom
layers and can continue on interior layers. The more disruption
there is in the edge fence, the more EMI leakage is possible.
The shape of the curves shows that when the amount of
stitching capacitance is limited due to available board space,
other measures must be taken to reduce the low frequency
emissions, such as adding component capacitance or edge
guarding.
In some system isolation requirements, there can be a large
distance required along cemented joints, up to the full surface
creepage. In these cases, stitching and edge guarding is still
possible and even more desirable. If long internal PCB creepage
is required, in many cases, the stitching capacitance structures
appear very similar to the offset edge case shown in Figure 3,
which can be an efficient radiator. An example of using edge
guarding for offset edges is shown in Figure 27.
EDGE GUARDING RESULTS
In isoPower systems, the bulk of the current flows in the
primary side ground and power planes and the vias that
connect them to the active pins. This results in the majority
of the edge radiation occurring on the primary side. Edge
guarding is most effective when applied to the planes of the
primary side of the converter. Figure 21 illustrates adding edge
guarding to the test vehicle.
Results for the edge guarded board are presented in Table 3.
Because most of the edge emissions are generated on the
primary side by the large primary side currents, the largest
reductions are in the 180 MHz peak, typically about −11 dBμV/m.
The results for the 360 MHz peak are less than half as large.
Table 3. Edge Guarding Reduction in Emissions
Board Type
180 MHz
360 MHZ
Edge Guard
−11 dBμV/m
−4.5 dBμV/m
Rev. B | Page 14 of 20
Application Note
AN-0971
0
OPERATING LOAD AND VOLTAGE DEPENDENCE
Emissions are directly related to the length of time the tank
oscillator is on. Figure 22 and Figure 23 show how emissions at
the tank frequency and the rectification frequency vary with
load. The emissions are nearly linear with the current at higher
loads. The emissions drop significantly at very light loads where
the tank circuit may not turn on completely. With both a light
load and a low output voltage, it is possible to reduce tank and
rectification emissions by over 20 dB.
–5
–10
–15
5V
3.3V
–20
–25
–30
Because of the relationship of the tank duty factor to the load
current at different voltages, the 180 MHz radiation when
operating at 60 mA and 3.3 V load contains as much energy as
operating at 100 mA and 5 V (see Figure 22). However, the
power transferred at 5 V is more than twice as high.
2
0
0.02
0.04
0.06
0.08
0.10
I
CURRENT (A)
ISO
Figure 23. Emissions at 360 MHz vs. Load Current
When comparing the 180 MHz and 360 MHz responses at differ-
ent voltages, these results show that operating at 3.3 V and low
load, there is only a small penalty at 180 MHz and a benefit at
360 MHz. If there is a high load current, it is best to run at 5 V.
0
–2
3.3V
–4
INTERPLANE CAPACITANCE
–6
5V
The use of interplane capacitance in these designs has a mar-
ginal effect on emissions but a significant effect on power supply
noise. The same layers used for stitching capacitance also make
interplane capacitance on each side of the barrier. The same
characteristics that produce good coupling for a stitching capaci-
tance also make good interplane capacitance, that is, thin layers
and continuous planes.
–8
–10
–12
–14
–16
0
0.02
0.04
0.06
0.08
0.10
I
CURRENT (A)
ISO
The board shown in Figure 21 has alternating ground and
power fills implemented in unused sections of signal layers.
This can add to the interplane capacitance and has no effect on
isolation. The islands of fill are connected to their respective
planes with vias every 10 mm, and the layout is careful to avoid
fingers of fill or isolated islands.
Figure 22. Emissions at 180 MHz vs. Load Current
When the 360 MHz behavior is examined in Figure 23, it
behaves more as expected with the 3.3 V emissions lower over
almost the entire operating range. There is a significant benefit
to running at 3.3 V at low load conditions.
Use of signal layers in high layer count boards can also allow
increased stitching capacitance by interdigitation of multiple
layers (see the Integrating Techniques section for additional
information).
Rev. B | Page 15 of 20
AN-0971
Application Note
INTEGRATING TECHNIQUES
Determining exactly which of the methods described in this
application note should be combined to achieve the emissions
target requires an expected baseline emissions measurement.
The behavior of the control board can be used if no other esti-
mate is available. Many of the options do not have a large impact
on cost or area and can be implemented on any multilayer PCB.
The process of choosing the PCB layout and application tech-
niques to minimize EMI is illustrated in Figure 24. This breaks
the process down into three categories: items that do not affect
isolation, items that have isolation impacts, and, finally, a
system level approach, such as shielding.
EXAMPLE 1—BASIC INSULATION BOARD
For the first layout example, the 60 mA load is assumed to be
the maximum load required for the application. This reduces
the emissions by several decibels without any design effort.
However, because this is still a relatively heavy current applica-
tion from an isoPower perspective, edge guarding of the primary
side is recommended. Edge guarding reduces the 180 MHz peak
by 10 dB and provides some reduction of the 360 MHz peak as
well. In addition, with the large power requirement, choosing a
5 V/5 V operating condition gives the lowest emissions (see
Figure 22). Optimizing interplane capacitance should be consid-
ered if there are sensitive analog circuits or long cables attached
to the application PCB, but it is optional.
STEPS TO MINIMIZE
isoPOWER EMI
The emissions have been reduced to 11 dB to 15 dB from the
baseline with the methods already used, but to obtain the 32 dB
and 36 dB reductions required for Class B, stitching capacitance
must be employed. Because this is not a medical application,
the user has the flexibility to add cross barrier capacitance. Only
basic insulation is required; therefore, the smallest size of the
stitching capacitor is the single overlap option. Adding up the
reductions thus far yields 15 dB in the 180 MHz peak and 11 dB
in the 360 MHz peak. This leaves 17 dB in the lower frequency
and 21 dB at the higher frequency to obtain. As shown in Figure
140 pF of stitching is required to achieve the goal for the 360 . ) [
peak, but 250 pF is required to make the 180 MHz peak NFFU
the desired level. Using Equation 4, assuming a 4 mil EJFMFDUSJD
and an 8 mm overlap, a 77 mm long capacitor TUSVDUVSF JT
required to produce a 250 pF capacitance.
MINIMIZE LOAD
THESE FEATURES
CAN BE
IMPLEMENTED
WITH MINIMAL
EDGE GUARDING
COST AND BOARD
SPACE AND NO
IMPACT ON
3.3V OPERATION
ISOLATION.
BURIED
CAPACITANCE
MEDICAL
APPLICATION?
YES
NO
REINFORCED
BASIC
PROTECTION
LEVEL
THESE FEATURES
CAN IMPACT
ISOLATION
FLOATING
STITCHING
CAPACITOR
OVERLAPPING
STITCHING
CAPACITOR
Table 4. Basic Insulation 4-Layer Board
RATINGS.
Relative Change
180 MHz
Peak
360 MHz
Peak
SAFETY
STITCHING
CAPACITOR
SAFETY
STITCHING
CAPACITOR
Parameter
Value
Power Level
60 mA at 5 V −4 dBμV/m
−7 dBμV/m
Edge Guarding N/A
−11 dBμV/m −4 dBμV/m
−17 dBμV/m −27 dBμV/m
−32 dBμV/m −38 dBμV/m
Stitching
Total
250 pF
SYSTEM
SHIELDING
IS UNDESIRABLE
IN MOST
SYSTEM
SHIELDING
Table 4 shows a summary of the reductions achieved with a
250 pF stitching capacitance with edge guarding and 60 mA of
current at 5 V. The capacitive stitch is built as shown in Figure 25.
Note that, with a 4-layer board, the full PCB creepage must be
observed on the top and bottom layers, but the spacing can be
much smaller on the internal layers.
CASES.
Figure 24. Selecting EMI Mitigation Options
To illustrate this process, two examples are examined, which
cover most aspects of the process. To start with, create two types
of application boards.
CEMENTED JOINTS
•
A PCB capable of 60 mA of average current that must meet
basic insulation
SIGNAL/GND FILL
PWR
GND
•
A PCB that supplies less than 10 mA of current but is
capable of reinforced insulation
SIGNAL/PWR FILL
The control board is used as the baseline. Its emissions at full
load are shown in Table 2 along with the CISPR Class B limits.
SURFACE CREEPAGE
Figure 25. Basic Insulation Single Overlap Stitching Capacitor
Rev. B | Page 16 of 20
Application Note
AN-0971
This design may be acceptable if 77 mm of overlap length is
available. If space is at a premium, an additional option is
shown in Figure 26. Because the required capacitance for the
360 MHz peak is smaller than what is required at 180 MHz,
design the PCB for the 140 pF stitching capacitance and
supplement with a safety rated capacitor. This reduces the
length of the stitching capacitor to 44 mm and adds a 160 pF
safety capacitor.
EXAMPLE 2—REINFORCED INSULATION BOARD
The second board requires reinforced insulation. The analysis
of edge guarding remains the same. Because the current level is
low, there is an advantage in the 360 MHz peak to run the
supply at 3.3 V/3.3 V (see Figure 23).
CEMENTED JOINTS
SIGNAL/GND FILL
PWR
GND
SIGNAL/PWR FILL
SURFACE CREEPAGE
2-LAYER BARRIER
SIGNAL/GND FILL
PWR
GND
Figure 29. Minimum Reinforced Insulation, Floating Stitching Capacitor
SIGNAL/PWR FILL
If the standards for this example allow a 2-layer reinforced
structure, a stitching capacitor can be constructed as shown
in Figure 29. Performing a calculation similar to the previous
example, the minimum stitching capacitance is 210 pF. Because
the floating stitch in reinforced applications uses twice the area
as the single overlap, this results in the capacitor being very
large (see Equation 2).
Figure 26. Augmented Stitching with Safety Capacitor
Finally, Figure 27 shows the basic insulation structure with edge
guards in place. Note that the ground planes used for the edge
guard are also used for the stitching capacitor. This avoids the
20h patch antenna effect.
SIGNAL/PWR FILL
GND
SIGNAL/GND FILL
PWR
GND
PWR
SIGNAL/GND FILL
SIGNAL/PWR FILL
Table 5. Reinforced 4-Layer Board Emissions
Figure 27. Edge Guarding Added to the Basic Capacitive Stitching
Relative Change
180 MHz
peak
360 MHz
Peak
An additional technique to increase capacitance without large
PCB space is to use interdigitated stitching capacitors, as shown
in Figure 28. Because the extra layers that overlap are connected to
the primary planes with vias, the extra planes are more induc-
tive than the primary overlap. This is not typically a problem
because the lower frequency peak requires the largest capacitance
and more inductance can be tolerated. As noted previously,
when edge guarding is used, it works best to use the edge guard
ground planes for coupling.
Parameter
Value
Power Level
Edge Guarding N/A
Stitching
Total
10 mA at 3.3 V
−10 dBμV/m −22 dBμV/m
−11 dBμV/m −4 dBμV/m
−11 dBμV/m −26 dBμV/m
−32 dBμV/m −52 dBμV/m
210 pF
In Example 1, a safety capacitor was used to supplement
stitching at low frequencies so that the PCB stitching capacitor
can be reduced. This can be done in this context as well, but
high voltage safety capacitors are relatively expensive, and there
may be regulatory restrictions; therefore, it is undesirable to use
safety capacitors. There are, however, alternatives for both safety
and size.
SIGNAL/PWR FILL
SIGNAL/GND FILL
PWR
SIGNAL/PWR FILL
SIGNAL/GND FILL
PWR
GND
GND
SIGNAL/PWR FILL
SIGNAL/GND FILL
SIGNAL/PWR FILL
SIGNAL/GND FILL
SIGNAL/GND FILL
SIGNAL/PWR FILL
GND
SIGNAL/PWR FILL
SIGNAL/GND FILL
PWR
If more layers are available, a safety rated stitching capacitor can
be constructed, as shown in Figure 30. This structure places at
least four layers of PCB material between the active input and
output structures. For most agencies, it is acceptable without
test or certification. The capacitance can be complicated to
calculate because there can be coupling to several layers, but the
spacing is larger as well. Usually, this highly reinforced PCB has
the largest area requirements.
PWR
GND
SIGNAL/GND FILL
SIGNAL/PWR FILL
SIGNAL/PWR FILL
SIGNAL/GND FILL
Figure 28. Different Methods of Interdigitating Stitching Capacitance to
Maximize Coupling
4-LAYER BARRIER
SIGNAL/GND FILL
PWR
SIGNAL
GND
SIGNAL/PWR FILL
SIGNAL/GND FILL
SURFACE
CREEPAGE
CEMENTED JOINTS
Figure 30. Reinforced Insulation, Floating Stitching Capacitor
Rev. B | Page 17 of 20
AN-0971
Application Note
expose the planes, this is a prime location for high voltage
breakdown.
ADDITIONAL LAYOUT CONSIDERATIONS
In the previous examples, the boards have small internal
distances between the primary and secondary planes to
maximize coupling and minimize radiation. The design
guidelines apply to the well-controlled areas away from the
edges of the PCB. However, care must be taken at the edges of
the circuit board to maintain the same level of robustness at the
edges as in the field. The two properties of breakdown that must
be addressed are the relatively low breakdown of air and the
intensification of electric fields at metallic corners or points.
Planes that are exposed at the edge of the PCB or even close to
the edge of the PCB can provide a breakdown path.
The lower diagram in Figure 31 shows how rounding (exagger-
ated for emphasis) or beveling the corners and pulling the
overlap portion of the plane back from the edge keeps this from
being a weak point in the isolation design. It eliminates sharp
corners and moves an edge of one of the planes back far enough
to ensure sufficient high integrity PCB material is present near
the edge.
POTENTIAL BREAKDOWN
Boards are manufactured in large sheets, then cut or scribed
and snapped into individual boards. The cutting operations can
be inaccurate or cause cracking and fraying of the FR4 material.
If the tightly spaced internal layers come near the edge of the
board, they can be exposed to low breakdown air by inaccurate
cutting or microscopic cracking of the FR4. This creates problems,
especially if the gap between the input and output layers termi-
nates at the edge of a PCB at a sharp corner, which enhances
electric fields and becomes the most likely location for arcing.
BETTER DESIGN
AVOIDS BREAKDOWN
It is highly recommended that the inside corners of the interior
planes be beveled such that they have the full creepage of the
top layers where they intersect the edges of the PCB.
Figure 31 shows a poor and a good layout where interior planes
meet the edge of a board. The top drawing illustrates two planes
that are closely spaced in the field of the PCB. The corners near
the top and bottom of the gap are closely spaced and extend all
the way to the edge of the PCB. Because the PCB integrity can
be lower at this point, or the PCB sectioning process may even
Figure 31. Designs for Close Planes Meeting the PCB Edges
Rev. B | Page 18 of 20
Application Note
AN-0971
CONCLUSIONS
Each method outlined in this application note addresses specific
radiation sources and can be combined with the other techniques
described to achieve the desired reductions in the associated
emissions. Test boards easily meet CISPR or FCC Class B
standards with no external shielding by using interplane
stitching capacitors and edge fencing. In addition, use of interplane
decoupling capacitance in the ground and power planes
produces a very quiet environment for precision measurement
applications.
Where low ac leakage is required, as in some medical
applications, stitching capacitance may not be a viable solution.
In these applications, grounded metallic chassis enclosures may
be the most practical solution for minimizing emissions.
REFERENCES
Archambeault, Bruce R. and James Drewniak. 2002. PCB
Design for Real-World EMI Control. Boston: Kluwer
Academic Publishers.
Gisin, Franz and Pantic-Tannr, Zoric, 2001. Minimizing EMI
Caused by Radially Propagating Waves Inside High Speed
Digital Logic PCBs, Mikrotalasna Revija (December).
Although this application note relies on data collected on the
ADuM540x devices, the techniques are applicable across the
isoPower line. All isoPower products contain similar tank and
rectification circuits. The largest currents flow on the primary
side of the devices, causing these devices to behave in a similar
manner from a radiated emissions point of view.
Rev. B | Page 19 of 20
AN-0971
NOTES
Application Note
©2008–2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN07541-0-6/11(B)
Rev. B | Page 20 of 20
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