TI Engineers: How to design EMI-compliant automotive switching regulators

It is easy to design EMI-compatible automotive switching regulators without a complete understanding of the complexities of EMI. This article shares the basic factors for successfully implementing a switching regulator in an intuitive way without complex mathematical operations, mainly including: slope control, filter design, component selection, configuration, noise diffusion and shielding.

　　The automobile itself is constantly changing, and so are the electronics that drive it. The most notable of these is the plug-in electric vehicle (PEV), which uses a 300V to 400V lithium-ion battery and a three-phase propulsion motor to replace the gas tank and internal combustion engine. Sophisticated battery pack charge monitoring, regenerative braking systems, and complex transmission control optimize battery life, requiring less frequent recharging. In addition, today's electric or other types of vehicles have many electronic modules that can improve performance, safety, convenience, and comfort. Many mid-range cars are equipped with advanced global positioning systems (GPS), integrated DVD players, and high-performance audio systems.

　　With these advanced devices comes the need for higher processing speeds. As a result, today's cars integrate high-performance microprocessors and DSPs, driving core voltages down to 1V and currents up to 5A. Generating such voltages and currents from a car battery that ranges from 6V to 40V presents many challenges, one of which is meeting the stringent standards for electromagnetic compatibility testing (EMC). Linear regulators were once the primary method used to convert car battery voltage to a regulated supply voltage, but are no longer relevant. More precisely, linear regulators reduce the output voltage, resulting in increased load current. Switching regulators are becoming more and more widely used, and with them come concerns about electromagnetic interference (EMI), wireless radio frequencies, and an emphasis on safety systems.

　　This article will explore the basic factors for successfully implementing a switching regulator in an intuitive way without complex mathematical operations, mainly including: slew rate control, filter design, component selection, configuration, noise diffusion and shielding.

　　A simple way to achieve EMC in switching power supplies

　　The goal of this article is to try to design EMI-compliant switching regulators without having to fully understand the intricacies of EMI. Virtually all problems with EMI stem from not fully understanding the rates at which voltages and currents change within the switching regulator and how they interact with parasitic circuit elements on board signal traces or within the component. For example, in a 200kHz step-down switching regulator powered by a 14V rated car battery producing 5V at 5A, the voltage slope at the switch node should be a small fraction of the on-time, say less than 1/12, to achieve respectable efficiency. The on-time of a buck converter operating in continuous conduction mode (CCM) is D/fsw, where D is the ratio of the duty cycle or the percentage of the pulse-width modulated (PWM) signal on time to the total time (ton and toff), and fsw is the converter's switching frequency.

　　For a buck converter operating in CCM, the inductor current is always non-zero and positive. In this case, the duty cycle is D = Vout/Vin, which in this case is 38% (5V/14V). Using a switching frequency of 200kHz, we quickly calculate that the on-time is 1.8μs. To support this frequency, the rise/fall time of the control switch must be less than 90ns. This brings us to the first method of reducing noise, which is slope control. You may not understand it yet, but at this point we have a good understanding of the harmonics associated with the PWM switching node, which is the control waveform of the switching regulator. If this waveform is represented as a trapezoid as shown in Figure 1(a), the harmonics of the waveform can be represented as shown in Figure 1(b), which shows the driving factor behind EMI. This Fourier envelope defines the harmonic amplitudes that can be obtained through Fourier analysis or by calculating the on-time and rise time of the trapezoidal waveform.

　　When looking at the frequency domain, it can be seen that the trapezoidal waveform with equal rise and fall times is composed of different harmonic signals that exist at integer multiples of the fundamental frequency of the periodic signal. It is worth noting that the energy of each harmonic is reduced to 20dB/dec at the first turning point (on time) of 1/(π×τ) and to 40dB/dec at the second turning point of 1/(π×tr). Therefore, limiting the slope of the switch node waveform can have a significant impact on reducing emissions. From this discussion, it should be clear that reducing the operating frequency also helps reduce emissions.

　　AM RF Band Considerations

　　One of the challenges with automotive EMI regulations is related to the AM band. This band starts at 500kHz and continues to 2MHz, making it a good fit for switching regulators. Since the highest energy element of the trapezoidal waveform is the fundamental element (assuming there are no board resonances), it can operate above and below the AM band.

　　Does duty cycle matter?

　　Another important factor is that if the duty cycle is exactly 50%, all the energy of the complex trapezoidal switching waveform will appear as odd harmonics (1, 3, 5, 7, ...). Therefore, operation at a 50% duty cycle is the worst case. At duty cycles around 50%, even if harmonics are present, natural EMI diffusion will occur.

　　EMI and EMC Standards

　　You can think of EMI as undesirable energy, and it doesn’t take much to violate emissions standards. In fact, EMI is a fairly low energy effect. For example, at 1MHz, just 20nW of EMI will violate the FCC’s conducted emissions regulations. Conducted emissions are measured by monitoring the high frequency components of the input source with a spectrum analyzer. The line impedance stabilization network (LISN) acts as a low impedance for the switching regulator and a high pass filter for the line noise on the spectrum analyzer. Therefore, the input of the switching regulator is the next area of ​​concern.

Input Filter Considerations

　　One of the main causes of EMI in automobiles is the AC current introduced by the switching regulator on the power bus. These changing currents themselves have various waveforms of radiated and conducted emissions. For example, in a non-isolated boost converter, the input capacitor (C2) and boost inductor (L1) shown in Figure 2 (a) form a unidirectional EMI filter that isolates line emissions. However, the input current has an AC triangle waveform that is Fourier expanded from this waveform, as shown by the green signal line in Figure 2 (b).

　　As long as L2 and C2 are added, the waveform will become a sine curve, and the energy will be readjusted to a relatively low high-frequency peak. However, if the input filter is not designed correctly, the noise will be amplified and the control loop will be unstable. Therefore, understanding the concept of filter design is very important for optimizing filter return and cost. AC analysis using SPICE is an effective tool to understand the behavior of the filter.

　　Whether designing a buck or boost power supply, differential mode filters or bidirectional capacitor input filters are quite practical to prevent EMI noise from entering the line and radiating and/or conducting noise. It should be noted that parasitic elements such as the cross-winding terminal capacitance and capacitor ESR associated with the filter components will significantly affect the attenuation of harmonics and should be used with caution.

　　Choosing the right components

　　Component selection is a critical part of designing an EMI-compliant switching regulator. For example, a shielded inductor helps minimize leakage magnetic fields that can radiate and couple into mutual inductance and high-impedance circuits, such as the input error amplifier of a PWM controller.

　　Diodes with soft reverse or low reverse recovery characteristics can minimize the large surge currents associated with diodes changing from on to off. These peak currents can interact with parasitic capacitances to cause oscillations at the switching node in excess of 100MHz and adversely affect EMC testing. Although beyond the scope of this article, it is important to note that improper selection of loop compensation components for switching regulators can exacerbate EMI. If the power supply is not properly compensated, output ripple and instability can increase noise. A properly compensated power supply is key to achieving good noise performance.

　　Keep in mind the path that current takes

　　Now we need to deal with the most easily controlled essential aspects of EMI compliant switching regulators, which are the circuit signal line path and component location. Component location will greatly affect the circuit signal line path. As mentioned earlier, EMI is inappropriate energy, and the changing current and voltage will be coupled to sensitive circuits (such as high impedance) through parasitic capacitance, mutual inductance or air. Therefore, in order to minimize the emission from the source, component location and current path are of great utility.

　　In a proper configuration of a power supply, the loop portion of the high current conductor must be minimized. This minimizes the inductance that acts as an antenna source and radiates energy. One aspect is the effective placement of components and selection of decoupling capacitors. Figure 3 shows the output power stage and filter of a synchronous buck converter. C3 decouples the power stage to provide a low impedance source when Q2 is turned on. To minimize radiated emissions, C3 must be connected as shown, where the inherent impedance of the capacitor, the circuit signal line, and the interconnection through the inductor are minimized. In addition, a high-quality capacitor dielectric with a high self-resonance frequency such as X7R is also required.

　　shield

　　The final techniques that will be described in this article are noise shielding and noise diffusion, which can be used to improve noise margins after applying the techniques discussed previously. If EMC standards are not met or noise margins are insufficient, external shielding is required to divert radiated E-field emissions from being transmitted to the EMC receiver antenna.

　　When switching voltage is present on a surface such as a heat sink or magnetic core, an electric field is generated. This is usually shielded by a conductive housing, where the conductive material isolates the electric field by converting it into a current. Of course, there must be a path for this current (usually ground). However, the entire conducted noise energy caused by this current needs to be addressed by a filter. External magnetic field shielding is more challenging (costly) and is not as effective at higher frequencies. Therefore, the associated magnetic components and circuit board loops should be designed with care.

　　Diffuse spectrum

　　Finally, this article will explore another increasingly common technique that effectively reduces peak harmonic energy by spreading it over a larger frequency band. This technique, called spread spectrum frequency dithering (SSFD), changes the noise spectrum by reducing the harmonic peaks, changing the noise signal from narrowband to wideband. It is important to understand that the energy spectrum is changed while the overall energy remains the same. The end result is that the noise level is generally increased, which is detrimental to high-fidelity systems. Figure 4 shows the harmonic spreading and peak reduction that occurs. The reduction is typically 5 to 10 dB, with subsequent harmonics increasing the peak reduction.

　　Conclusion

　　You can spend a long time understanding the complexities of EMI, but designing an EMI-compliant switching regulator requires only an understanding of the application circuit and a few basic circuit design properties and waveform analysis. Whether you are designing a switching regulator for an automobile, a switching regulator that does not use a battery, or a complex PEV battery charger, designing an EMI-compliant switching regulator requires an understanding of the concepts of Maxwell's equations. Fortunately for most of us, there are no partial differential equations involved, only attention to the magnetic and electric fields that appear when changing voltage/current rapidly and understanding the techniques described in this article.