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 Control of Radiated Emissions With isoPower Devices by Mark Cantrell INTRODUCTION iCoupler® digital isolators with integrated isolated power (isoPower) employ isolated dc-to-dc converters that switch at frequencies as high as 300 MHz. These high frequencies typically raise concerns about radiated emissions. PCB layout and construction is a very important tool for controlling radiated emissions from applications containing these components. This application note identifies the radiation mechanisms and offers specific guidance on addressing them. With proper design choices, these products can easily meet FCC Part 15, Subpart B, Class A (FCC Class A) emissions standards, and with care, may even meet 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. Control of emissions from signal cables and chassis shielding techniques are outside of the scope of this discussion. isoPOWER OVERVIEW The ADuM5xxx product family represents 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 that are incorporated into signal isolation products. Power levels of up to one-half watt are available, which is sufficient to power the secondary side of the iCoupler as well as to provide power to external components. OSC Several power architectures have been implemented to achieve desired design goals, such as efficiency, small size, or high output voltage (see Figure 1). These architectures have three common elements: 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. Supplementary control elements are used to achieve additional control and regulation functions. The physics of the transformer requires that the oscillator circuit switch current into the transformer at a rate ranging between 180 MHz to 300 MHz. The rectifier circuit on the secondary side effectively doubles this frequency during the rectification process. These functions are familiar to those who have constructed a switching power supply; however, the frequency domain is three orders of magnitude higher than a standard dc-to-dc converter, moving noise generated by the operation of the converter into the 30 MHz to 1 GHz range of concern for radiated emissions. RECT REG OPEN-LOOP WITH LINEAR REGULATOR ADuM524x OSC RECT REG OSC RECT OPEN-LOOP WITH PWM CONTROL ADuM5230, ADuM6132 FULL FEEDBACK PWM CONTROL ADuM540x, ADuM520x, ADuM5000 Figure 1. isoPower Architectures Rev. 0 | Page 1 of 8 07541-001 PWM AN-0971 Application Note TABLE OF CONTENTS Introduction ...................................................................................... 1 Additional Practices ..........................................................................6 isoPower Overview ........................................................................... 1 Precautions to Meet Isolation Standards ........................................6 Sources of Emissions ........................................................................ 3 Conclusions ........................................................................................7 EMI Mitigation Techniques ............................................................ 4 Rev. 0 | Page 2 of 8 Application Note AN-0971 SOURCES OF EMISSIONS There are two sources of emissions in PCBs where isoPower is used: edge emissions and input-to-output dipole emissions. • 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. 1 07541-003 Ground and power noise, which is generated by inadequate bypass of high power current sinks. 8 • 9 Edge emissions occur when unintended currents meet the edges of ground and power planes. These unintended currents can originate from: 16 Edge Emissions Figure 3. Dipole Radiation between Input and Output Edge emissions are generated (see Figure 2) where differential noise from many sources meets the edge of the board creating a dipole. Edge emissions should be predominantly in the plane of the board. 07541-002 GROUND POWER Figure 2. Edge Radiation From a Ground Power Pair 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 current 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 driving the dipole. In many cases, this is a very large dipole as shown in Figure 3. 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 perpendicular to the ground planes. 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. This configuration is examined in this application note. 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. 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. Because this is difficult to achieve, even with multiple low ESR capacitors near the pins, the excess noise is injected onto the ground and power planes. Power is transferred 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 predominantly at the rectification frequency with some contribution from the tank frequency and higher harmonics. Worst-case data collected on a 2-layer evaluation board is shown in Figure 4. Emissions in the near field are shown. Rev. 0 | Page 3 of 8 AN-0971 line should be installed across the split to allow high frequency image charges to follow the signal. In isolation applications, this is not a practical possibility. Adding a high frequency capacitor between the ground planes does not allow effective high voltage isolation make it difficult to pass system level isolation requirements. It would likely be ineffective at 300 MHz and above due to via and trace inductance. 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 aggressive EMI must be addressed for a design to pass FCC 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 discuss this option, note that where PCB related techniques are not possible, or impractical, this method remains available. The following techniques are effective in reducing EMI radiation and on-board noise: • Input-to-output ground plane stitching • Edge guarding • Buried capacitive bypass • Power control Circuit boards with test structures were prepared to evaluate each of these EMI mitigation techniques utilizing the ADuM540x. The layout of each board was kept as uniform as possible to allow meaningful comparison of results. Testing was conducted at an EMI test facility under standard conditions for FCC Class B certification. 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 board suggesting input-tooutput dipole radiation. Input-to-Output Stitching The primary cause of radiation across a split in the ground plane is the lack of a return path for an image charge. In designs where crossing a split in a reference plane with a high frequency data line is required, a stitching capacitor paired with the signal VDDI VISO GND1 GNDISO 07541-005 EMI MITIGATION TECHNIQUES 16 A board with near field emissions, as shown in Figure 4, and without a chassis shield would fail FCC Class B emissions standards by approximately 40 dB at the 360 MHz peak. 9 Figure 4. Near Field Emissions From a 2-Layer Board However, a stitching capacitance can be constructed with copper patterns on the PCB. Figure 5 illustrates a metal pattern built on a buried power plane. The ground planes, shown in Figure 5 as dotted outlines, overlap the floating metal structures creating a very low inductance capacitive bridge between the GND1 and GNDISO. In actual applications, the ground planes should have the same outer perimeter as the power planes; however, they are shown larger to highlight their shape. These structures should be constructed on inner PCB planes to preserve the integrity of the clearance distance on the surface of the PCB. High frequency image charges can now couple to the secondary side drastically reducing the dipole radiation. 1 HARMONICS AT 540MHz AND 720MHz 8 360MHz RECTIFIER 07541-004 180MHz TANK Application Note Figure 5. Input-to-Output Capacitive Stitch When varying amounts of capacitance were added between the input and output planes, the dipole radiation at 360 MHz and above was dramatically reduced. Test results for varying amounts of input-to-output capacitance are shown in Figure 6. Note that the largest reduction in emissions occurs with a stitching capacitance of about 100 pF. This is likely because the capacitance near to the ADuM540x has the greatest effect in reducing the emissions. Extending the capacitive structures far from the ADuM540x has less effect. The effect of a 100 pF stitching structure is a 25 dB decrease in radiated emissions. The capacitance can be easily estimated with the following relationship. This equation assumes that the area of overlap between the floating structure and the ground plane is the same on each side of the clearance gap of the PCB. C= Aε 2d (1) where: C is the coupling capacitance between the input and output sides. A is the total area of overlap between the buried metal structure and the local ground planes. ε is the dielectric constant of the PCB material. d is the spacing between the ground plane and the buried structure. Rev. 0 | Page 4 of 8 Application Note AN-0971 TANK PEAK EMISSIONS vs. SPANNING CAPACITANCE @ 180MHz 65 RECTIFICATION PEAK vs. STITCHING CAPACITANCE @ 360MHz 90 NO FENCE 2mm FENCE 85 60 EMISSIONS (dB µV/m) EMISSIONS (dB µV/m) 80 75 70 65 60 55 50 45 0 50 100 150 200 250 300 SPANNING CAPACITANCE (pF) 350 400 40 07541-006 50 0 Figure 6. Emissions vs. Input-to-Output Coupling Capacitance 100 200 CAPACITANCE (pF) 300 400 07541-009 55 Figure 9. Tank Frequency Emissions Edge Guarding Buried Capacitance Bypassing Noise on the power and ground planes that reaches the edge of a circuit board can radiate as shown in Figure 2. If the edge is treated with a shield structure, the noise is reflected back into the interplane space. This can increase the voltage noise on the planes, but reduces edge radiation. Buried capacitance bypassing is a technique intended to reduce the 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 effective between 300 MHz and 1 GHz. 07541-007 GROUND POWER GROUND VIA EDGE FENCE AND GUARD RINGS Figure 7. Via Fence Structure, Side View GROUND FENCE 07541-008 POWER PLANE Power and ground noise reduction provides a better operating environment for noise sensitive components near the isoPower device. Emissions are reduced proportionate to the reduction in power and ground noise. Although this reduction is not as significant as that achieved with the stitching or edge guarding techniques, it significantly improves the power environment of the board. The stack-up used for EMI test boards was G-P-S-S-G-S as shown in Figure 10. A 4 mil core layer was used for the top power and ground planes. These tightly coupled planes provide the buried capacitance layer. Having the ground plane on the top layer allows the isoPower device to couple its ground pins directly to the copper plane providing the added benefit of good heat removal from the package. GROUND POWER BURIED CAPACITIVE LAYER SIGNAL SIGNAL Figure 8. Via Fence and Guard Ring, Shown on the Power Plane Layer GROUND SIGNAL The edge treatment was expected to have its most pronounced effect on the edge emissions from the tank circuit at 180 MHz. Measurements of the tank frequency peak are shown in Figure 9. This plot shows the reduction of tank frequency emissions as stitching capacitance is added. The curve is relatively flat; this confirms that the tank frequency does not radiate predominantly from the input-to output-dipole. However, when a 2 mm via fence is added to the same type of board, the emissions drop by almost 10 dB. This confirms that edge treatments significantly reduce edge emissions. 07541-009 Making a solid conductive edge treatment on a PCB is possible, but expensive. A less expensive solution that works well is to treat the edges of the board with a guard ring structure laced together by vias. In the ADuM540x test boards, the via spacing was 2 mm. This structure is shown in Figure 7 and Figure 8. Figure 10. PCB Stack-Up for Buried Capacitance Results of testing (see Figure 11) compare VDD noise for three board stack-ups. The top chart in this figure shows a 2-layer board without a continuous power plane. The high frequency noise is very large, approaching 0.5 VP-P. The second chart in Figure 11 shows a 6-layer stack-up with a common S-G-S-S-P-S stack-up with about 24 mil of spacing between the ground and power planes. The noise was reduced by about a factor of four. The third chart shows a 6-layer stack-up with the buried Rev. 0 | Page 5 of 8 AN-0971 Application Note EMISSIONS vs. LOAD @ 360mhZ capacitance configuration, about 4 mils between power and ground planes. This reduces the noise to a peak amplitude of about 60 mV. 60 55 EMISSIONS (dB µV/m) 2-LAYER BOARD NO POWER PLANE Δ 460mV 50 45 +5V –5V +3.3V –3.3V 40 6-LAYER BOARD 24mm SPACING POWER-TO-GROUND Δ 104mV 30 07541-011 6-LAYER BOARD 24mm SPACING POWER-TO-GROUND Δ 64.0mV Figure 11. VDD Voltage Noise for Various Stack-Up Options 40 60 80 LOAD CURRENT (mA) 100 120 Figure 13. Emissions at the Rectifier Frequency for Different Loads and Voltage Configurations ADDITIONAL PRACTICES • 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 inductance as a single large via since the bulk of the current goes through the closest via, even if multiple paths are present. • Be very careful to rout signal lines against a single reference plane. It is vital to maintain the image charge path so that image charges do not have to travel by circuitous routs to meet back up with the original signal on another plane. • Do not rout 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. In isoPower devices with active feedback or a controllable PWM control architecture, emissions can be reduced by keeping the tank circuit off as much as possible. Running isoPower with light loads achieves this for the ADuM540x. In addition, devices like the ADuM540x can operate at 3.3 V input and output. Operation at the lower voltage range reduces generated noise as well as production of radiated emissions. Figure 12 and Figure 13 show how emissions at the tank frequency and the rectification frequency vary with load and supply voltage, respectively. The tank frequency emissions are nearly linear with the current load, while the rectification emissions drop significantly at very light loads. With both light load and low output voltage, it is possible to reduce tank and rectification emissions by up to 20 dB. PRECAUTIONS TO MEET ISOLATION STANDARDS TANK EMISSIONS vs. LOAD @ 180MHz 70 Most of the techniques discussed in this application note do not effect board isolation, with the exception of the buried stitching capacitor. The structure allows coupling of ac transients across the isolation barrier. In addition, if reinforced insulation is required across the barrier, then special rules for insulation thickness and spacing apply. 60 50 40 NO FENCE 2mm FENCE 30 20 10 0 20 40 60 LOAD (mA) 80 100 120 Figure 12. Emissions at the Tank Frequency for Different Load and Fence Options 07541-012 EMISSIONS (dB µV/m) 20 The following general practices should also be considered: Power Reduction 0 0 07541-013 35 Special care must be taken when laying out structures for a buried stitching capacitor across an isolation gap. Insulation in printed circuit boards does not have any minimum “distance through the insulation” specification for functional or basic insulation standards, thus the designer has a great deal of flexibility in board layout. If reinforced insulation is required, then 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 must be maintained. Although this requirement necessitates careful Rev. 0 | Page 6 of 8 Application Note AN-0971 board design, it should not be burdensome if taken into account at the start of a design. Capacitive coupling across the isolation barrier allows ac leakage and transients to couple from one ground plane to the other. While 100 pF seems small, high voltage high speed transients can inject significant currents across the barrier through this capacitance. This must be taken into account if the application will be subjected to these environments. CONCLUSIONS Different EMI mitigation techniques do not interact strongly with each other. The methods outlined in this application note address specific radiation sources, and can be combined to achieve the desired reductions in the associated emissions. Test boards easily meet FCC Class A standards with no external shielding by utilizing buried stitching capacitors and edge fencing. Use of buried decoupling capacitance in the ground and power planes makes a very quiet environment for precision measurement applications. While this application note relies on data collected on the ADuM540x devices, the techniques are applicable across the isoPower line. All isoPower couplers 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. 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. Rev. 0 | Page 7 of 8 AN-0971 Application Note NOTES ©2008 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN07541-0-6/08(0) Rev. 0 | Page 8 of 8