AN-0971 Datasheet, PDF (17/20 Pages) Analog Devices – Recommendations for Control of Radiated Emissions with isoPower Devices

Application Note
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.
SIGNAL/GND FILL
PWR
GND
SIGNAL/PWR FILL
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/GND FILL
PWR
GND
SIGNAL/PWR FILL
SIGNAL/PWR FILL
GND
PWR
SIGNAL/GND FILL
Figure 27. Edge Guarding Added to the Basic Capacitive Stitching
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.
SIGNAL/PWR FILL
SIGNAL/GND FILL
PWR
GND
SIGNAL/PWR FILL
SIGNAL/GND FILL
SIGNAL/PWR FILL
SIGNAL/GND FILL
PWR
GND
SIGNAL/PWR FILL
SIGNAL/GND FILL
SIGNAL/GND FILL
SIGNAL/PWR FILL
GND
PWR
SIGNAL/GND FILL
SIGNAL/PWR FILL
SIGNAL/PWR FILL
SIGNAL/GND FILL
PWR
GND
SIGNAL/PWR FILL
SIGNAL/GND FILL
Figure 28. Different Methods of Interdigitating Stitching Capacitance to
Maximize Coupling
AN-0971
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}
Figure 29. Minimum Reinforced Insulation, Floating Stitching Capacitor
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).
Table 5. Reinforced 4-Layer Board Emissions
Relative Change
Parameter
Value
180 MHz
peak
360 MHz
Peak
Power Level 10 mA at 3.3 V −10 dBμV/m −22 dBμV/m
Edge Guarding N/A
−11 dBμV/m −4 dBμV/m
Stitching
210 pF
−11 dBμV/m −26 dBμV/m
Total
−32 dBμV/m −52 dBμV/m
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.
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.
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