Time-Saving and Cost-Effective Innovations for EMI Reduction in Power Supplies

Yogesh Ramadass Distinguished Member Technical Staff Design Manager – Kilby Power, Isolation and Motors Texas Instruments

Ambreesh Tripathi Member Group Technical Staff Systems Manager – Wide Input Buck Switching Regulators Texas Instruments

Paul Curtis Analog Design Engineer Boost & Multi Channel/Phase DCDC Texas Instruments As electronic systems become increasingly dense and interconnected, reducing the effects of electromagnetic interference (EMI) becomes an increasingly critical system design consideration.

EMI can no longer be an afterthought, given its potential to cause significant setbacks late in the At a glance design phase that cost both time and money.

This paper examines EMI in switch-mode One of the most ubiquitous circuits in modern power supplies, and provides technology technology is the switch-mode power supply examples to help designers quickly and easily (SMPS), which provides drastic improvements in pass industry-standard EMI tests. efficiency over linear regulators in most applications. But this efficiency comes at a price, as the switching of power metal-oxide semiconductor field-effect What is EMI? 1 transistors (MOSFETs) in the SMPS causes it to EMI is electromagnetic energy — be a major source of EMI, which in turn can affect produced as an undesirable byproduct reliability. EMI primarily comes from discontinuous of switching currents and voltages — input currents, fast slew rates on switching nodes, that comes from a variety of physical phenomena and manifests during and additional ringing along switching edges caused stringent EMI tests. by parasitic inductances in the power loop.

Conventional methods to Figure 1 on the following page illustrates how 2 reduce EMI each of these elements manifests itself in different Reducing EMI is an endeavor plagued frequency bands, using a buck converter topology with trade-offs. Conventional methods as an example. As pressures mount to increase to reduce EMI include using large, switching frequencies for reduced size and cost, expensive filters or reducing switching as well as to increase slew rates for improved slew rates, a technique that directly efficiency, EMI problems are exacerbated. Thus, it’s impacts efficiency. becoming necessary to incorporate cost-effective and easily integrated EMI mitigation techniques that Innovations in EMI reduction 3 do not compromise the power-supply design. To realize all of the benefits of a switch- mode power supply, it is paramount What is EMI? for EMI reduction techniques to resolve In a system that requires electromagnetic the traditional trade-offs. This requires compatibility (EMC), components acting as creative solutions for both low- and electromagnetic sources are designed to reduce high-frequency EMI, as well as accurate modeling techniques. their interference, and components that are susceptible to interference are designed to

Time-Saving and Cost-Effective Innovations 2 April 2021 for EMI Reduction in Power Supplies SW dV/dt most dominant in 30- to 100-MHz range IIN SW ringing most dominant in >100-MHz range VSW +

Discontinuous current VIN most dominant + + in <30-MHz range

– VSW VOUT IIN – – Figure 1. Example of EMI sources in an SMPS. reduce their susceptibility. When end-equipment into conducted and radiated, which reveal both manufacturers integrate components from various the method of measurement and how the EMI suppliers, the only way to guarantee that the was generated. Although conducted emissions interferers and susceptible circuits can peacefully are typically associated with lower frequencies coexist is through the establishment of a common (<30 MHz) and radiated emissions are typically set of rules, where interference is limited to a associated with higher frequencies (>30 MHz), the certain level, and susceptible circuits are capable of distinction between the two is not quite so simple, handling that level of interference. as conducted and radiated frequency ranges

These rules are established in industry-standard do overlap. specifications such as Comité International Spécial Conducted emissions measurements are designed des Perturbations Radioélectriques (CISPR) 25 to quantify the EMI generated from a device and for the automotive industry, and CISPR 32 for returned to its power source. It’s important to multimedia equipment. CISPR standards are critical reduce these emissions for many applications, as it for EMI design, as they will dictate the targeted is common for other sensitive circuits to connect to performance of any EMI mitigation technique. This the same power-supply lines. Reducing conducted paper’s focus will be on reducing interference, as EMI is particularly important when dealing with long the SMPS is typically the electromagnetic interferer. wire harnesses, which are increasing in number in For a comprehensive list of EMI standards, see the modern automobiles. white papers, “An overview of conducted EMI Figure 2 on the following page shows a generic test specifications for power supplies” and setup for conducted emissions, including a power “An overview of radiated EMI specifications for source, line impedance stabilization network (LISN), power supplies.” EMI receiver, supply wires and a device under In addition to knowing the appropriate standard test (DUT). The LISN plays a key role, acting as a for a given application, it is also important low-pass filter that ensures the repeatability and to understand how to measure EMI, as this comparability of EMI measurements, and providing knowledge will give you insight into reducing a precise impedance to the DUT. Figure 2 also EMI. EMI measurements are typically divided illustrates an important subdivision of conducted

Time-Saving and Cost-Effective Innovations 3 April 2021 for EMI Reduction in Power Supplies emissions into common-mode (CM) and differ- magnetic fields also generate DM conducted ential-mode (DM) currents. DM currents flow emissions, and the same surfaces that generate between the power-supply line and its return path, radiated electric fields also generate CM conducted and are the dominant factor at lower frequencies. emissions, many EMI mitigation techniques reduce CM currents flow between each of the power both conducted and radiated emissions, but may lines and ground, and are the dominant factor at be targeted specifically for one or the other.

higher frequencies. In general, lower-frequency emissions are mitigated by using large passive filters, which add board EMI receiver area and cost to the solution. High-frequency LISN V1 i i CM /2 DM V DC/DC V + IN + OUT+ + V regulator emissions present different challenges in terms of VIN + – DM DM loop area Load – VIN– VOUT– i i (DUT) CM /2 DM measurement, modeling and mitigation, primarily LISN V2

V +V V 1 2 as a result of their parasitic nature. Common CM = 2 CM loop area mitigation techniques for high-frequency emissions i — CM include controlling slew rates and reducing Reference GND plane parasitics. Figure 3 summarizes the mitigation Figure 2. Generic test setup for conducted emissions techniques contained in this paper, the frequency measurements, with DM and CM loops highlighted in teal and bands in which they are most beneficial and an red, respectively. example of the frequency ranges covered by the Radiated measurements have a setup similar to CISPR 25 standard. conducted measurements; the main difference is that the EMI receiver is not connected directly to Conventional methods to the LISN but to a nearby antenna. Radiated energy reduce EMI in the low- and in an SMPS comes from fast transient current high-frequency ranges loops generating magnetic fields, and fast transient Input voltage ripple generated by discontinuous voltage surfaces generating electric fields. Because currents in an SMPS can conduct to other systems the same current loops that generate radiated when the systems share common physical

Conducted Conducted Conducted Very low frequency CISPR 25 low frequency CISPR 25 high frequency

Radiated Radiated Radiated CISPR 25 rod antenna CISPR 25 bicon antenna CISPR 25 log antenna

1 150 30 108 200 1 kHz kHz MHz MHz MHz GHz

Active EMI ﬁltering Cancellation windings Package optimization Slew rate control Integrated capacitors Spread spectrum EMI modeling ﬂows

Figure 3. A summary of the EMI mitigation techniques presented in this paper.

Time-Saving and Cost-Effective Innovations 4 April 2021 for EMI Reduction in Power Supplies 100 Without ﬁlter 3.3µH 90 With passive EMI ﬁlter CISPR 25 peak limit + 80

60 HO LO 4.7µF 50 VIN 100nF 4.7µF 100nF 4.7µF 4.7µF 68 µF Controller + 40

DM noise (dBµV) LO 30 CO VOUT

20 – – 10

0 106 107 108 Frequency (Hz) Figure 4. A typical LC-based passive filter for EMI reduction, along with the attenuation obtained.

contacts. Without proper mitigation, excessive Another conventional approach to mitigating EMI input or output voltage ripple can compromise is to use spread spectrum (or clock dithering) to operation of the source, load or adjacent system. modulate the switching frequency of an SMPS, Traditionally, you could minimize the input ripple by which will reduce peaks in the frequency spectrum using a passive inductor-capacitor (LC)-based EMI associated with the fundamental switching filter, as shown inFigure 4. An LC filter offers the frequency and its harmonics — but at the expense required attenuation necessary to meet EMI speci- of an increased noise floor, as shown inFigure 5.

fications. The trade-off is a size and cost penalty for Spread spectrum is an attractive technique, given the system depending on the required attenuation, its ease of implementation and the fact that you which will lower the overall power density. Also, can use it in conjunction with other EMI reduction large-sized inductors for input EMI filter design lose methods. It is not a panacea, however, as it can attenuation at frequencies greater than 30 MHz only provide relative reductions to existing EMI, because of their lower self-resonant frequency, and by nature its performance diminishes with necessitating additional components like ferrite lower switching frequencies. Furthermore, you can beads to handle the high-frequency attenuation. traditionally only apply spread spectrum to a single

50 Before spread spectrum 45 After spread spectrum

Energy (dBµV)

0 0510 15 20 25 30 Frequency (MHz) Figure 5. Example of the frequency spectrum of an SMPS with and without spread spectrum.

Time-Saving and Cost-Effective Innovations 5 April 2021 for EMI Reduction in Power Supplies frequency band, for reasons that we will explore in High slew rates will additionally result in the next section. high-frequency switch-node ringing, due to the

To minimize the size of the filter inductors, you presence of parasitic inductances in the power can choose higher switching frequencies for your path of DC/DC converters, which further increases SMPS design. It is important to avoid sensitive emissions at the ringing frequency and above. frequency bands for switcher operation, however. Figure 7 on the following page shows how the For example, the preferred switching frequency for slew rate and the associated ringing on the switch automotive power solutions has traditionally been node affect emissions. The traditional way to limit in the sub-AM bands (approximately 400 kHz). EMI emissions caused by the switching transition Choosing a higher switching frequency to minimize is to slow them down by adding intentional inductor size means that you must avoid the entire resistance in the gate-drive path of the switching AM band (525 to 1,705 kHz) so as to not have device. This causes the transitions to happen more fundamental switching spurs in the more stringent slowly, leading to faster roll-off of the emissions automotive EMI frequency bands. and an 8- to 10-dB reduction in emissions at the ringing frequency. This slowdown of the Switching converters from Texas Instruments (TI) switching edges comes with a penalty of 2% to have a switching frequency above 1.8 MHz to 3% in the peak current efficiency of the switching satisfy the EMI band requirements. The push to converter, however. higher switching frequencies imposes a severe restriction on the switching transition rise and fall Innovations in reducing times to reduce switching losses. However, a switch low-frequency emissions node with very short rise and fall times maintains Let’s look at a few techniques that TI uses when high energy content even at high frequencies close building its converters and controllers to address the to its 100th harmonic, as shown in Figure 6, again fundamental trade-offs involving efficiency, EMI, size highlighting the trade-off between high efficiency and cost. and low EMI.

–20 Light red: ³ 1-ns rise time Dark red: 40-ns rise time –40

–60 >20 dB

–80

Spectrum / dB

–100

–120 500k 650k 800k 1M 1.5M 2M 2.5M 3M 4M 5M 6M 7M 8M 10M 15M 20M 25M 30M 40M 50M 60M 80M 100M 150M 200M250M 400M 600M 800M 1G Frequency / Hz 100 MHz / div

Figure 6. EMI plots of square waveforms with different rise times.

Time-Saving and Cost-Effective Innovations 6 April 2021 for EMI Reduction in Power Supplies SW (RB = 0)W 20.0 13.0 FFT of SW node SW (RB = 500 W) 12.0 10.0 11.0 0.0 10.0

9.0 –10.0 ~10 dB¯ @ 8.0 –20.0 160 MHz 7.0 ~8 dB¯ @

dB –30.0 400 MHz V (V) 6.0 Slew rate

5.0 3.8 V/ns Slew rate –40.0 4.0 1.6 V/ns –50.0 3.0

2.0 –60.0 1.0 –70.0 0.0

–80.0 1.005 1.01 1.015 1.02 1.025 106 107 108 109 Time (µs) Hz

Figure 7. The effect of different switch-node slew rates and associated ringing on high-frequency emissions. Reduced slew rates impact EMI roll-off in the 30- to 200-MHz band while the reduced ringing impacts EMI at the ringing frequency around 400 MHz.

Spread spectrum Clock frequency Energy Spread spectrum uses the principle of conservation Dfc fm of energy to reduce EMI peaks by spreading energy 2fD c across multiple frequencies. However, the peak fc energy that a susceptible circuit “sees” may not 1 T = m f reduce; it depends on the relationship between the m susceptible circuit’s bandwidth and the method Time Frequency Unmodulated fc of frequency modulation. When measuring EMI, Modulated the spectrum analyzer is acting as the susceptible Figure 8. Spread-spectrum parameters fm and ΔfC in both the time and circuit, and industry standards set the resolution frequency domains. bandwidth (RBW). Thus, it is important to modulate having a significant impact on the conducted the frequency in the most effective way for the and radiated EMI performance. Figure 9 on the standard you are targeting. A general rule of following page illustrates this modulation profile both thumb is to have the modulation frequency, f , m in the time and the frequency domains, a feature approximately equal to your targeted RBW, with the present on the TPS55165-Q1, a synchronous spreading bandwidth, Δf , around ±5% to ±10%. C buck-boost converter. Figure 8 illustrates these parameters in both the The fact that EMI is not limited to a single band (and time and frequency domains. thus a single RBW) but multiple bands presents It is common to set f around 9 kHz to optimize for m a predicament, as spread spectrum can typically the low-frequency band in standards such as CISPR only target improvements in a single band. A new 25, but this happens to also be in the audible solution to this problem is a digital spread-spectrum range. To combat this, you can further modulate technique called dual random spread spectrum the triangular modulation in a pseudorandom (DRSS). The basic principle behind DRSS is fashion to spread the audible energy without to superimpose two modulation profiles, each

Time-Saving and Cost-Effective Innovations 7 April 2021 for EMI Reduction in Power Supplies equency

equency

Clock fr

Time

70 70 Time 65 65

60 60 Figure 10. Time domain modulation profile of DRSS.

55 55 50 50 Figure 11 shows the conducted emissions 45 45 40 40 performance of the LM5156-Q1, a nonsynchronous 35 35

30 30 boost controller, with and without DRSS. You can

Conducted emissions (dBµV) 25 Conducted emissions (dBµV) 25 see significantly reduced spectral peaks in both the 20 20 56789101112131415 56789101112131415 150-kHz to 30-MHz band, as well as the 30- to Frequency (kHz) Frequency (kHz) 108-MHz band, which are the two key bands for the Figure 9. Audible tone reduction achieved by pseudorandomly CISPR 25 automotive standard. The LM5157-Q1 modulating the triangle waveform at the conclusion of every nonsynchronous boost converter also features modulation cycle. DRSS and achieves similar performance.

targeting a different RBW. For more information, Spread-spectrum techniques are applicable see the application report, “EMI Reduction for both non-isolated and isolated topologies, Technique, Dual Random Spread Spectrum.” as the EMI sources are similar for both, and Figure 10 shows a DRSS modulation profile in the frequency spreading provides the same benefit. time domain, with a triangular envelope targeting The UCC12040 and UCC12050 isolated DC/DC lower RBWs, and a superimposed pseudorandom converters with integrated transformers are able to sequence targeting higher RBWs.

80 9-kHz RBW 120-kHz RBW 70

60 AM radio

40 FM

20 Conducted Emissions (dB µV) CISPR 25 peak limit

10 Conducted emissions (no SS) Conducted emissions (DRSS) 0 0.1 110 100 Frequency (MHz)

Figure 11. EMI performance of the LM5156-Q1 boost controller before and after spread spectrum, using a printed circuit board (PCB) not specifically designed for lower EMI.

Time-Saving and Cost-Effective Innovations 8 April 2021 for EMI Reduction in Power Supplies pass CISPR 32 Class B EMI testing limits in part of the passive filter to about 43% and 27% of the due to internal spread-spectrum techniques. original values, respectively. For higher-current converters, it is possible to obtain further benefits Active EMI filtering in cost and efficiency from a reduction in inductor To substantially improve emissions in the DC resistance. low-frequency spectrum, the LM25149-Q1 buck controller incorporates an active EMI filtering 100 Bare noise 90 With passive EMI ﬁlter approach. The integrated active EMI filter reduces With AEF DM conducted emissions at the input by acting as 80 CISPR 25 peak limit an effective low-impedance shunt. Figure 12 shows 70 how the active EMI filter of the buck controller 60 connects to the input line. The sense and inject 50 pins hook up to the input through their respective 40

DM noise (dBµV) capacitors. The active element within the active 30 EMI filter block amplifies the sensed signal and 20 injects an appropriate anti-polarity signal through 10 0 the inject capacitor in order to minimize the overall 106 107 108 Frequency (Hz) disturbance on the input line. This reduces the 3.3-µH DM inductor filtering burden on the passive elements needed, with traditional 1-µH DM inductor passive EMI ﬁlter with AEF thereby reducing their size, volume and cost.

L + Csen Cinj Ccomp Rcomp Figure 13. EMI attenuation obtained using passive and active Ccomp1 INJ HO filtering and comparison of the passive inductor needed for RDC_fb LO VIN Controller Cin + filtering in both approaches for a 12-V input, 5-V/5-A output + AEF R comp1 SEN LO buck converter. CO VOUT AGND – – Cancellation windings Figure 12. Active EMI filter showing the sense, inject capacitors Unlike non-isolated converters, an additional and components for compensation. emissions path across the isolation boundary is a

Figure 13 shows the EMI measurement results of key cause of common-mode (CM) EMI in isolated a buck converter operating at a 400-kHz switching converters. Figure 14 on the following page shows frequency, comparing the active and passive EMI the presence of the parasitic capacitances across filtering approaches. To effectively meet the CISPR the isolation transformer in a standard flyback 25 Class 5 spectral mask, the passive EMI filter converter. CM currents can flow directly from needs a 3.3-µH DM inductor with a 10-µF DM primary to earth through the parasitic capacitance capacitor. The active filtering approach can achieve associated with each switched node. The CM the same effective attenuation with a DM inductor currents also flow from primary to secondary of only 1 µH along with 100-nF sense and inject because of parasitic capacitance between the capacitors. This helps reduce the size and volume windings, causing an increase in the measured

Time-Saving and Cost-Effective Innovations 9 April 2021 for EMI Reduction in Power Supplies CM EMI. Conventionally, you could attenuate this and helps generate a canceling CM voltage to additional disturbance by using a large CM choke in null the CM injection from the outer half-primary the input power path. layer. Equalizing the parasitic capacitances to the secondary layer from the auxiliary winding and primary outer layers helps null the CM current + injected into the secondary layer from the outer

– VAC half-primary layer, by pushing an opposite-phase CM EMI Z voltage CM current from the cancellation layer. The net effect — close to zero CM current flowing into

Earth the secondary layer — reduces CM emissions, Figure 14. CM EMI generating parasitics in a flyback converter. significantly helping the design meet EMI spectral standards with minimal CM filtering.

To help minimize the size of the passive filtering, Innovations in reducing the 65-W active clamp flyback with silicon FETs high-frequency emissions reference design for a high-power-density 5- to 20-V AC/DC adapter employs cancellation The EMI mitigation techniques we’ve described winding- and shielding-based approaches specific so far generally reduce low-frequency emissions to isolated converters. As shown in Figure 15, an (<30 MHz), with a corresponding reduction in the improved internal transformer structure has an extra amount of passive filtering required and associated auxiliary winding layer (shown in black) inserted in size, volume and cost benefits. Now, let’s look at between the inner primary and secondary layers techniques designed to mitigate high-frequency for CM balance. The auxiliary CM balance layer emissions (>30 MHz). shields the inner half-primary to secondary interface

Auxiliary bias Primary (outer layer) Secondary

CM balance Primary auxiliary (inner layer)

NP NS NAUX dBµV 115 V–Conducted EMI QP and AVG with earthed load 100 EN55032 QP-8 90 CP–S CS-AUX EN55032 AV-8 80 QP result ICM1 ICM2 AVG result 70

0 0.1 1 10 MHz Figure 15. Using shielding and cancellation windings to reduce EMI in a flyback converter.

Time-Saving and Cost-Effective Innovations 10 April 2021 for EMI Reduction in Power Supplies HotRod™ package higher efficiency while making a smaller solution One of the main approaches to reducing size possible. high-frequency emissions is to minimize the An additional benefit of devices in the HotRod power-loop inductance. Step-down converters package is that they facilitate parallel input path from TI such as the LM53635-Q1, LMS3655-Q1, pinouts — the layout arrangement of a DC/DC LM61495-Q1, LMR33630-Q1 and LM61460-Q1 converter’s input capacitors. By optimizing the move away from bond-wire packages to pinout of the DC/DC converter so that there is leadframe-based flip-chip (HotRod) packages that symmetry in the input capacitors’ layout, the help lower the power-loop inductance and in turn opposing magnetic fields generated by the input reduce switch-node ringing. power loops are within the symmetric loops, thereby

HotRod packages flip the silicon die and place it minimizing emissions to nearby systems. A parallel directly on a lead frame in order to minimize the input path further minimizes high-frequency EMI, parasitic inductance caused by bond-wires on particularly in the most stringent FM band, as shown pins running switching currents. Figure 16 shows in Figure 17 on the following page. the construction and benefit of HotRod packages. Enhanced HotRod™ QFN Along with an improvement in power-loop The Enhanced HotRod quad flat no-lead (QFN) inductance, HotRod-style packages also help package offers all of the EMI reduction capabilities lower resistance in the power path, leading to of the HotRod package and has an added

Standard bond-wire QFN HotRod™ interconnect package with exposed pad (Flip-chip-on-lead) QFN Low interconnect parasitics

Bond-wire device HotRod-packaged device

T T

1 (a) (b) Figure 16. Standard QFN with bond-wires to electrically connect to the die (a); HotRod package with copper pillars and flip-chip interconnect between the leadframe and die (b).

Time-Saving and Cost-Effective Innovations 11 April 2021 for EMI Reduction in Power Supplies Switch

GND 11 7 GND GND 11 7 GND 10 10 12 8 6 12 8 6

13 5 13 5 CIN 9 CIN 9 CIN HF1 HF1 HF2

14 4 14 4 VIN VIN VIN VIN

15 3 15 3

16 LM53635 2 16 LM53635 2 17 1 17 1 1 1 2 2 2 1 1 2 2 2 8 9 0 1 2 8 9 0 1 2

VHF1-PK5 VHF1-PK5

VHF2-PKFM-PK5 40 VHF2-PKFM-PK5 TVI-PK5 TVI-PK5

VHF1-AV5 TVI-AV5 VHF1-AV5 TVI-AV5

VHF2-AVFM-AV5 20 VHF2-AVFM-AV5

Fails the limits Passes the limits –10

–20

30 MHz 108 MHz 30 MHz 108 MHz Figure 17. Effect of the parallel input path on EMI in an SMPS.

advantage of even lower switch-node capacitance, of the package. The DAP facilitates better thermal resulting in much lower ringing. The resistor- dissipation through the PCB and reduces the rise in inductor-capacitor (RLC) parasitic on the junction temperature by more than 15% compared

input-voltage (VIN) and ground (GND) pins is also to the HotRod package. In addition, lower RLC lower in devices with the Enhanced HotRod QFN parasitics on the VIN, GND and switch-node pins compared to the HotRod package. also result in better efficiency and lower EMI. As

The LM60440-Q1 step-down converter comes expected this results in better EMI, particularly in an Enhanced HotRod QFN, and Figure 18 on around the switch-node ringing frequency band as the following page shows the pinout and board shown in Figure 19 on the following page. layout. The Enhanced HotRod QFN not only Integrated input bypass capacitor improves efficiency but also includes a footprint As we described earlier, a large input power loop that has a large die-attach pad (DAP) at the center results in higher emissions at high-frequency

Time-Saving and Cost-Effective Innovations 12 April 2021 for EMI Reduction in Power Supplies SW VOUT 1 12 11 Shielded inductor with terminations underneath the package. PGND PGND LIND Butterﬂy arrangement of output caps minimizes VIN 2 13 10 VIN effective loop inductance DAP through H-ﬁeld cancellation. SW 3 9 EN SW 12 L2 12 L1 BOOT 4 8 PG PGND 1 11 PGND

VCC 576 FB VINVIN 2 13 1010 VINVIN

Input caps located close SW 3 DAP 9 EN to IC pins and in butterﬂy AGND arrangement for H-ﬁeld cancellation. BOOT 4 8 PG LM604x0 pin-out VCC 5 6 7 FB The boot and VCC caps locate as close as possible to the IC pins, resulting in AGND low parasitic inductance for the MOSFET gate loops.

LM604x0 layout with parallel input loops Figure 18. The pinout and PCB layout on an Enhanced HotRod™ QFN package device. bands because of increased switch-node ringing. Figure 21 on the following page compares the Integrating high-frequency input decoupling radiated EMI — identical conditions on identical capacitors inside the device package helps minimize boards — of the LMQ62440-Q1, with and without the input loop parasitic and thus reduces EMI. the bypass capacitors integrated. The results show This technique is used in the LMQ62440-Q1, a a 9-dB reduction in emission in the most stringent step-down converter, as shown in Figure 20 on TV band (200 to 230 MHz), which helps the system the following page. Beyond reducing the input remain under EMI limits set by industry standards power-loop inductance, the package integration of without the need for additional components on the input high-frequency capacitors also helps make the board. the solution more immune to changes in the board layout of the end system.

–10.0 spectrum_SW spectrum_SW

–20.0

Red: HotRod™ package Teal: Enhanced HotRod™ package –30.0

–40.0

–50.0

–60.0 100 400 500 1,000 MHz Figure 19. SW node FFT comparison of HotRod- vs. Enhanced HotRod-packaged device.

Time-Saving and Cost-Effective Innovations 13 April 2021 for EMI Reduction in Power Supplies the RBOOT pin (the teal dotted loop) is multiplied and drawn through from CBOOT (the red dashed line) to turn on the high-side power MOSFET. By doing this, the resistor can control the slew rate, but not suffer the efficiency loss that happens when a series BOOT resistor runs the majority of the current. With RBOOT short-circuited to CBOOT, the rise time is rapid; switch-node harmonics will Figure 20. Two high-frequency input bypass capacitors not roll off until above 150 MHz. If CBOOT and integrated in the LMQ62440-Q1 device. RBOOT remain connected through 700 Ω, the True slew-rate control slewing time increases to 10 ns when converting Despite the aforementioned techniques, in some 13.5 V to 5 V. This slow rise time enables the energy designs high-frequency EMI (60 to 250 MHz) may in switch-node harmonics to roll off near 50 MHz still not fall under specified standard limits. One way under most conditions. to mitigate and improve the margin in order to pass EMI modeling capabilities industry standards is to use a resistor in series with the boot capacitor of the switching converter. Using Modeling any circuit is an important way to evaluate a resistor reduces the switching-edge slew rates, a design’s performance in the early stages, and thus which reduces EMI, but comes with the expected plays a critical role in reducing design cycle times. penalty of reduced efficiency. EMI modeling is a complex process that involves time-domain circuit analysis as well as frequency- Switching converters such as the LM61440-Q1 domain electromagnetic simulations of the PCB. and LM62440-Q1 are designed such that a resistor Modeling EMI emissions makes it easier to meet can be used to select the strength of the high-side EMI standard limits more quickly by reducing the FET’s driver during turn-on. As shown in Figure 22 number of design iterations. on the following page, the current drawn through

Figure 21. Radiated EMI performance of the LMQ62440-Q1 device without and with integrated bypass capacitors.

Time-Saving and Cost-Effective Innovations 14 April 2021 for EMI Reduction in Power Supplies VIN LM62440-Q1 VCC

CBOOT

HS HS FET RBOOT driver

SW LS FET

(a)

(b) Figure 22. True slew-rate control implementation in the LM62440 (a); reduction in switch-node ringing using true slew-rate control (b).

Let’s review some of the options available to you for ensures filter stability and converter-loop stability modeling EMI. while designing the filter. This online tool supports over 100 TI power devices. Low-frequency EMI designs using WEBENCH® design tool It’s a common mistake to leave an input EMI filter inductor undamped, which negatively affects The WEBENCH input filter design tool helps you overall design stability. The WEBENCH design tool automatically design a proper input filter to mitigate performs impedance analysis on the input filter lower-frequency (<30 MHz) conducted EMI noise for and SMPS (as shown in Figure 23 on the following compliance standards like CISPR 32 and CISPR 25. page) and suggests the appropriate damping The tool optimizes filter size while ensuring that the component to ensure stability. design complies with a particular standard. It also

Time-Saving and Cost-Effective Innovations 15 April 2021 for EMI Reduction in Power Supplies Filter vs. converter impedance comparison Filter components 60 Rz Lz Lf_inpﬂt 40 1 µH Lf 1 µ H 511.4 mW 1 µH 190 mW Rd_inpﬂt 20 + 100 mW

V Gain (dB) – IN Power Cf 7.8 µ F 0 Cf_inpﬂt supply 10 µF –20 Cb_inpﬂt 1 mW 1 100 10 k1 M Qty = 2 1 µF Cb 4 µ F 5 W Frequency (Hz) Qty = 4 Filter output impedance Line impedance Filter Rd 100 m W Converter input impedance open loop

Converter input impedance ideal loop

Ripple frequency spectrum (before ﬁlter) Ripple frequency spectrum (after ﬁlter) 120 80

100 60 80 40 60 20 40 Ripple magnitude (dBµV) Ripple magnitude (dBµV) 20 0 0 510152025 0 510152025 Frequency (kHz) Frequency (kHz)

Ripple magnitude (before ﬁlter) Ripple magnitude (after ﬁlter)

CISPR 25 Class 5 noise limit CISPR 25 Class 5 noise limit Figure 23. Input EMI suggestion with impedance analysis in the WEBENCH® design tool.

Conducted and radiated EMI results page of the device data sheet. The EMI report in published in data sheets the LM62440-Q1 data sheet includes complete SMPS device evaluation modules are tested data for a CISPR 25 Class 5 conducted and against most stringent industrial and automotive radiated setup. EMI standards, and results are published in In addition, TI can perform system-level EMI the data sheet to help you understand the EMI modeling and measurements in-house to help performance of the devices in advance. You can you validate EMI performance and accelerate access a detailed EMI report by clicking “Optimized cycle times. for Ultra-Low EMI Requirements” on the first

Time-Saving and Cost-Effective Innovations 16 April 2021 for EMI Reduction in Power Supplies Conclusion Employing a combination of techniques with TI’s EMI-optimized power-management devices ensures The rapid growth of electronics has put tremendous that designs using TI components will pass industry strain on the design of power converters, where standards without much rework. TI products enable complex systems are crammed into ever-smaller you to remain under end-equipment EMI limits spaces. The close proximity of sensitive systems without sacrificing power density or efficiency. makes it challenging to suppress EMI. You must take extreme care when designing power converters See ti.com/lowemi to learn more about TI products to comply with the limits set forth by standards that use these technologies, including buck-boost bodies to ensure that critical systems can operate and inverting regulators, isolated bias supplies, safely in a noisy environment. multichannel integrated circuits (PMIC), step-down (buck) regulators and step-up (boost) regulators. Designing for low EMI can save you significant development cycle time while also reducing Key product categories for low EMI board area and solution cost. TI offers multiple Buck-boost & inverting regulators features and technologies to mitigate EMI such as spread spectrum, active EMI filtering, cancellation Isolated bias supplies windings, package innovations, integrated Multi-channel ICs (PMIC) input bypass capacitors, and true slew-rate Step-down (buck) regulators control methodologies. Step-up (boost) regulators

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