How To Reduce EMI in Switching-Converters When Spread Spectrum Is Not an Option
Learn five alternatives to spread spectrum for EMI mitigation in switched-mode power supplies: passive and active EMI filtering, slew rate control, advanced packaging, and PCB layout optimization.
Electronic components crowd our world. Whether in our factories, homes, cars, or pockets, the things we interact with are being crammed with more and more electronic hardware. With these electronics confined to smaller and tighter spaces, electromagnetic interference (EMI) becomes an increasing focus for system design.
Switched-mode power supplies (SMPS) are the most efficient way to supply regulated electrical power, but they generate a significant amount of EMI. Increased switching speeds and switching frequencies result in higher power density but also tend to make EMI worse.
Spread spectrum modulation is one of the best EMI mitigating techniques for SMPS design. What if your application limits you from using spread spectrum? What if you have really strict EMI requirements and you’re not quite meeting them even with spread spectrum?
Thankfully, there are many other ways to improve the EMI performance in SMPS. This article will introduce five other methods for reducing EMI:
- Passive EMI filtering.
- Active EMI filtering.
- Slew rate control.
- Packaging technology.
- PCB layout.
We will also provide links to additional in-depth references to help you learn more about these topics.
What Is Spread Spectrum for EMI Mitigation?
One of the best EMI mitigating techniques for SMPS is spread spectrum. It is a control method of reducing peak EMI by dithering the switch-node frequency over time. Modulating the main switching frequency reduces the fundamental and its harmonics.
There are three primary methods of spread spectrum modulation:
- Dual random.
Generally speaking, these different methods increase in effectiveness in the order they are listed.
Spread spectrum is now a common feature of many SMPS. For example, the Texas Instruments LMQ66430-Q1, which was designed to be the lowest EMI device in its class, uses TI’s most recent and effective spread spectrum technique called Dual Random Spread Spectrum (DRSS). DRSS essentially combines triangular spread spectrum with additional pseudorandom modulation.
Figure 1 illustrates the switch-node frequency variation over time. You can see the overall triangular modulation as well as the higher frequency micro-variations.
Figure 1. DRSS modulation of the switch-node frequency vs time
When Can You Not Use Spread Spectrum?
However, there are some situations in which spread spectrum simply can’t be used. For instance, if a customer is synchronizing one or many of their SMPS to an external signal, the spread spectrum feature of the device is turned off. The result is that the power at the fundamental switch-node frequency and the harmonics will be quite high, and additional filtering will be needed to meet EMI requirements.
Additionally, there may be other reasons why one cannot use spread spectrum. Some applications, such as radio detection and ranging or ultrasound, use particular bandwidths in operation that must be avoided by the surrounding electronics at all costs. In these applications, the SMPS must have a fixed switch-node frequency.
In the following sections, we will investigate multiple other options for EMI noise reduction.
Passive EMI Filtering
The first EMI mitigation technique we will examine is passive EMI filtering. Figure 2 illustrates how to mitigate and filter EMI with passive components.
Figure 2. Typical input filter components for passive EMI filtering (click to enlarge)
A high-frequency ceramic input capacitor (CINHF) helps supply the power MOSFETs with high-frequency energy to improve the switch’s slew rate and edge characteristics, thus reducing switch ringing and high-frequency noise. The bulk capacitance (CINBLK) provides low-frequency damping and prevents resonance between the filter components.
The capacitor-inductor-capacitor (CLC) EMI filter, also known as a π filter, filters differential-mode noise from 10 MHz to 100 MHz, depending on the components selected. In addition, the ferrite bead provides high differential-mode impedance at very high frequencies.
Passive EMI Filter Limitations
There are a few limitations of the passive filter. CF connects to the main power rail and must be rated for the maximum DC voltage. Designs requiring a large CF and a higher DC voltage will necessitate the use of a larger and more expensive capacitor.
Similarly, LIN must be rated for the maximum input current (IIN) at minimum input voltages. In higher-power systems, the inductor size and cost will further increase, given the larger inductance and DC current requirements. Transients such as load dump and cold crank exacerbate the VIN and IIN requirements.
Therefore, as system power requirements increase, it is important to develop methods that will at least match, if not improve upon, the performance of passive filters while meeting size and cost requirements.
Active EMI Filtering
An active EMI filter (AEF) uses an amplifier circuit to sense noise on the input rail and inject an out-of-phase signal to cancel the noise being sensed. An operational amplifier (op amp) network acts effectively to replace a passive capacitor (CF) with an active capacitor. The op amp network requires feedback and compensation components, but these components are much smaller in size and cost than a large passive CF.
With an op-amp integrated into a DC/DC controller package, the overall solution size can be smaller when compared to that of a fully passive EMI filter. This is exactly what TI has done with its LM5149-Q1 and LM25149-Q1 buck controllers.
Figure 3 compares the PCB layout of passive and active EMI filters. The passive filter layout uses an inductor that has a significantly larger volume. The op amp is integrated into the controller IC (not shown in Figure 3).
Figure 3. (a) a passive EMI filter with two 1210 capacitors (b) an AEF with significantly smaller and cheaper components (click to enlarge)
The AEF layout uses smaller and cheaper components while achieving improved filter performance (Table 1).
Table 1. Cost, area, and height comparison between passive and active EMI filters
Figure 4 compares the EMI performance of three different buck regulator configurations–without a filter, with a passive EMI filter, and with an AEF. The AEF uses smaller components for a 50% area reduction and shows improved EMI attenuation.
Figure 4. Buck regulator noise spectrum with (a) no filter, (b) a passive EMI filter, and (c) an active EMI filter (click to enlarge)
True Slew-Rate Control for EMI Reduction
Slew-rate control is the ability to slow the transitions of the switch-node voltage and current in a buck regulator to reduce high-frequency emissions. By increasing the transition times, the switch-node square wave will begin to look like a trapezoid wave with harmonics rolling off at 40 dB/dec, as shown in Figure 5.
Figure 5. EMI spectrum of a square wave
The ability to slow down the slew rate by controlling the drive strength of the high-side field-effect transistor’s (FET) driver enables the harmonics to roll off at lower frequencies, thereby reducing the overall noise and lowering EMI.
One conventional way to control the slew rate in a buck regulator is to place a resistor (RBOOT) in series with the boot capacitor, as shown in Figure 6. Adding RBOOT may cause the bootstrap voltage of the high-side driver to drop below its undervoltage lockout as peak gate-drive currents flow through it, affecting the normal operation of the buck converter.
Figure 6. Conventional slew-rate control method in a buck converter
Implemented in buck converters such as the LM61460-Q1 and LM61495-Q1, an improved slew-rate control method uses a dedicated RBOOT pin. Figure 7 shows a true slew-rate control feature with a dedicated boot resistor that controls the drive strength of the high-side FET’s driver.
Figure 7. The true slew-rate control method in a buck converter
A higher RBOOT resistance yields slower switch-node rise times. Accurately controlling the rise time of the switch-node voltage makes it possible to precisely control the switch-node harmonics’ roll-off frequency, which effectively improves the noise amplitude measurement.
In some applications, true slew-rate control can eliminate the need for shielding and common-mode chokes, which further reduces total solution size. Depending on the application, there is a slight trade-off in that efficiency, but true slew-rate control can yield a solution that is CISPR 25 Class 5-compliant.
Improved Package Technology for Reduced EMI
The packaging of SMPS ICs is an incredibly important factor in its EMI performance. Packages optimized for EMI have reduced power-loop parasitic inductance, which reduces switch-node ringing. TI’s buck regulator portfolio provides a multitude of different package technologies that can help meet design expectations. The two most common package technologies available are standard wire-bond quad flat no-lead (QFN) and flip-chip-on-leadframe (the HotRod package).
QFN Packaging and EMI
As shown in Figure 8, standard QFN packages connect the die to the leadframe through wire bonds. This wire bond adds parasitic inductance to the power loop.
Figure 8. Standard QFN package construction
Figure 9 shows the internal parasitic inductance (Lpara3 and Lpara4) in the simplified buck schematic. The parasitic inductance resonates with the parasitic capacitance of the switching node at every switch edge, causing undamped switch ringing, which can increase EMI.
Figure 9. Typical buck converter input parasitic elements
HotRod Packaging and EMI
TI’s HotRod package, as shown in Figure 10, provides a lower-noise solution by eliminating the use of bond wires. HotRod packages connect the copper bumps of the silicon die directly to the leadframe. Step-down converters such as the LMR43620-Q1 and LM63460-Q1 are available in HotRod packaging to help meet system EMI requirements. Flip-chip-on-leadframe is also used in some TI buck modules, including the TPSM365R3, which also integrates the inductor within the package.
Figure 10. HotRod flip-chip-on-leadframe package construction
Optimizing the PCB To Reduce EMI
In this section, we will look at several methods of optimizing the board layout to improve EMI performance.
Place the Capacitor Close to the Package
Another benefit of HotRod package technology is the optimization of the input path pinouts. The input-high transient current (di/dt) loop is critical in a step-down converter. Figure 11 illustrates the importance of minimizing the area of this loop.
Figure 11. Placing the input capacitor near the converter IC (a) results in 7 dBµV less noise than (b) placement farther away (click to enlarge)
The HotRod package technology enables you to easily place and route the input decoupling capacitors close to the input and ground pins. Reducing switching power-loop parasitic inductances and minimizing input trace routing contribute to a lower EMI signature. Input pinout optimization also reduces switch-node ringing, output voltage noise, and EMI.
Mirror the Currents
Current flowing through a copper trace generates a magnetic field, which results in an overall increase in EMI noise measurements. The HotRod package pinout is designed to have the input current loop split onto either side of the device, as shown in Figure 12.
Figure 12. Symmetrical HotRod package pinout creates opposing magnetic fields
This symmetrical input and ground pins create an equal and opposite magnetic field, which provides a self-containing effect on the magnetic field and further reduces EMI.
Integrate the Capacitors Into the Package
The input current loop is a high di/dt loop that affects EMI at higher frequency ranges. Devices that integrate a high-frequency input decoupling capacitor within the device package effectively reduce the high di/dt inductive loop area and further reduce EMI.
Figure 13 provides images of the LMQ61460-Q1 in which integrated capacitors are soldered directly onto the internal leadframe of the device, which minimizes the parasitic inductance on the input loop.
Figure 13. Capacitors integrated inside the package for reduced EMI
The lab EMI measurements shown in Figure 14 demonstrate that, without an input ferrite bead, a device with an integrated capacitor provides approximately 8 dBµV of margin compared to a device without an integrated capacitor. A device with an integrated capacitor provides an approximate 2 to 3 dBµV improvement over a device with an input ferrite bead but without an integrated capacitor.
Figure 14. EMI advantages of buck converters with integrated capacitors (click to enlarge)
The TI LMQ61460-Q1 and LM62440-Q1 step-down converters use this device package construction method. Integrating the input decoupling capacitors inside the device provides a solution that is resistant to EMI and makes the PCB layout easier.
Resources for Learning More About EMI Reduction
The challenge of designing an all-in-one solution that is compact and complies with rigorous EMI standards can be a great hurdle for systems engineers. When designing a switching buck regulator to meet industry EMI standards, mindful consideration of devices that use advanced EMI suppression techniques ensures that the overall end equipment is safe, operable, and reliable, even in a noisy environment.
When spread spectrum cannot be used, it certainly can be more challenging to meet these EMI standards, but with the techniques considered here, compliance can still be achieved. Many of these techniques are now features within many switching converter ICs. Therefore, component selection is increasingly important. Depending on the IC selected, the difficulty of meeting EMI standards could vary significantly. In addition to simply meeting standards, the cost and size of an SMPS solution is also of great importance. Some of the features mentioned here are more helpful in this regard, such as AEF and integrated capacitors.
To learn more about reducing EMI in switched-mode power supplies, here are six helpful resources.
- Hegarty, Tim. “An Engineer’s Guide to Low EMI in DC/DC Regulators.” Texas Instruments e-book, literature No. SLYY208, 2021.
- Martin, Alan. 2013. “AN-2162 Simple Success With Conducted EMI From DC-DC Converters.” Texas Instruments application report, literature No. SNVA489C, April 2013.
- Texas Instruments. n.d. Buck Regulator Input Filter Capacitor for Conducted EMI Compliance. Accessed Nov. 30, 2021.
- Ramadass, Yogesh, Ambreesh Tripathi, and Paul Curtis. “Time-Saving and Cost-Effective Innovations for EMI Reduction in Power Supplies.” Texas Instruments white paper, literature No. SLYY200, April 2021.
- Ramadass, Yogesh, and Ambreesh Tripathi. “Advanced EMI Mitigation Techniques for Automotive Converters.” Texas Instruments Analog Design Journal article, literature No. SLYT789, 1Q 2020.
- Murray, Orlando, and Hua, Jimmy. “Advanced Power-Electronic Features for Reducing EMI
This article was co-authored by Richard Fu, Applications Engineer for Texas Instrument’s Wide VIN Buck Switching Regulators group.
Featured image (modified) used courtesy of TI and Adobe
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