EMI-Compatible Automotive Switching Regulator Design

It is easy to design an EMI-compliant automotive switching regulator without having to fully understand the complex EMI. This article will discuss the basic factors for successful implementation of switching regulators in an intuitive manner without complex mathematical operations. These include: slew rate control, filter design, component selection, configuration, noise diffusion, and shielding.

The car itself is constantly changing, as is the electronics that drive the car. The most notable of these is plug-in electric vehicles (PEVs), which use 300V to 400V lithium-ion batteries and three-phase propulsion motors instead of gas cans and internal combustion engines. Precise battery charge monitoring, regenerative braking systems, and sophisticated transmission controls optimize battery life and reduce the frequency with which batteries need to be charged. In addition, today's electric cars or other types of automobiles 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 equipment comes the need for higher processing speeds. Therefore, today's automobiles integrate high-performance microprocessors and DSPs, allowing the core voltage to drop to 1V and increase the current by 5A. There are many difficulties in producing such voltages and currents in car batteries ranging from 6V to 40V, one of which is to meet strict standards for electromagnetic compatibility testing (EMC). Linear regulators used to be the primary method used to convert automotive battery voltages to regulated supply voltages, but are now outdated. More precisely, linear regulators reduce the output voltage and increase the load current. Switching regulators are becoming more and more widely used, followed by concerns about electromagnetic interference (EMI) radio frequency and the importance of security systems.

Using a simple method to achieve switching power supply EMC The purpose of this article is to attempt to design an EMI-compliant switching regulator without having to fully understand the complex EMI. In fact, all of the problems associated with EMI stem from the fact that the rate of voltage and current changes in the switching regulator is not fully achieved, as well as the interaction with the parasitic circuit elements on the board signal line or within the component. A 200-kHz step-down switching regulator powered by a car battery rated at 14 V and producing 5 V at 5 A is taken as an example. To achieve considerable efficiency, the voltage slope of the switching node should only be a small fraction of the on-time, for example, 1/12 or less. 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 of the load cycle or PWM signal to the entire time (ton and toff), and Fsw is the switching frequency of the converter.

For a buck converter operating in a CCM, the inductor current is always a non-zero positive current. In this case, the duty cycle is D=Vout/Vin, which is 38% (5V/14V) in this example. When using a switching frequency of 200kHz, we quickly calculated that the on-time was 1.8μs. To support this frequency, the rise/fall time of the control switch must be less than 90 nanoseconds. This makes us notice the first method to reduce noise, that is, slope control. You may not understand it yet, but at this time we understand the harmonics related to the PWM switching node, which is the control waveform of the switching regulator. If this waveform is represented by a trapezoid as shown in Fig. 1(a), the harmonics of the waveform can be represented by the contents in Fig. 1(b), which indicates the driving factors behind EMI. This Fourier envelope defines the amplitude of the harmonics that can be obtained by Fourier analysis or calculating the on time and rise time of the trapezoidal waveform.

When observing the frequency domain, it can be seen that the trapezoidal waveforms with equal rise and fall times are 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 will be reduced to 20dB/dec at the first turning point (on-time) of 1/(π×τ) and subtracted at the second turning point of 1/(π×tr). To 40dB/dec. Therefore, limiting the slope of the switching node waveform has a significant impact on reducing the amount of emissions. Through this discussion, it should be clear that lowering the operating frequency will also help reduce the emission.

One of the difficulties in considering the automotive EMI specification in the AM radio frequency band is related to the AM frequency band. This frequency band starts from 500kHz and continues to 2MHz, which is very suitable for switching regulators. Since the highest energy component of the trapezoidal waveform is the basic component (assuming no circuit board resonance), it can operate up and down the AM band.

Is the duty cycle important?

Another important factor is that if the duty cycle is exactly 50%, all the energy of the complex trapezoidal switching waveform will be presented with odd harmonics (1, 3, 5, 7...). Therefore, operating at 50% duty cycle is the worst case. In the load cycle up to 50%, even if harmonics occur, natural EMI diffusion occurs.

EMI and EMC standards You can think of EMI as inappropriate energy, and this energy does not need to be too much to violate the emission standards. In fact, EMI is a fairly low energy effect. For example, under 1MHz conditions, as long as 20nW of EMI will violate the FCC regulations for conducted emissions. Conducted emissions are measured with a spectrum analyzer monitoring the incoming source high frequency components. The Line Impedance Stabilization Network (LISN) can be used as a low impedance for switching regulators, as well as high-pass filters for spectrum analyzer line noise. Therefore, the input of the switching regulator is the next place to pay attention.

One of the main factors that cause EMI in a car due to input filter considerations is that the switching regulator supplies AC current to the power supply line. These changing currents themselves have various waveforms of radiation emission and conduction emission. For example, in a non-isolated boost converter, the input capacitor (C2) and the boost inductor (L1) shown in Figure 2(a) form a unidirectional EMI filter that is isolated from the line. However, the input current has the Fourier expanded AC triangle waveform of this waveform, as shown by the green signal line of FIG. 2(b).

As long as L2 and C2 are added, the waveform will become sinusoidal, and the energy will be readjusted to a fairly low high-frequency peak. However, if the input filter is not designed properly, it will amplify the noise and make the control loop unstable. Therefore, understanding the concept of filter design is important for optimizing filter echo and cost. AC analysis using SPICE is a tool for effective understanding of filter behavior.

Whether designing a buck or boost power supply, a differential mode filter or a bidirectional capacitive input filter is quite practical, these can prevent EMI noise from entering the line and radiating and/or conducting noise. It should be noted that the parasitic components related to the filter element, such as the cross-winding termination capacitance and the capacitive ESR, will significantly affect the attenuation of harmonics, and should therefore be used with caution.

Choosing the right component selection is a key part of designing an EMI-compliant switching regulator. For example, the shielded inductance helps to reduce the leakage magnetic field that generates radiation and couples 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 minimize the large inrush currents associated with diodes that change from on-state to off-state. These peak currents can interact with parasitic capacitances, causing oscillations at switching nodes beyond 100 MHz and adversely affecting the EMC test. Although not within the scope of this article, it is important to note that incorrect selection of the loop compensation components of the switching regulator can increase EMI. If power supply is not properly compensated, output ripple and instability will increase noise. Properly compensated power supply is the key to achieving good noise performance.

Keep in mind that the path through which the current is flowing now needs to deal with the necessary aspects of the EMI-compliant switching regulators that are most easily controlled, that is, the circuit signal line paths and component locations. The position of the component will greatly influence the circuit signal line path. Previously it was said that EMI is an inappropriate energy, and that varying currents and voltages are coupled to sensitive circuits (such as high impedance) through parasitic capacitance, mutual inductance, or air. Therefore, it is important to minimize the amount of emission from the source, the component position and the current path.

In the correct configuration of a power supply, the loop portion of the high-current conductor must be minimized. This minimizes the inductance of the antenna source and the transmitted energy. One of the layers is the effective placement of components and the use 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 at Q2 startup. In order to minimize the amount of radiation emitted, C3 must be connected as shown in the figure, where the intrinsic impedance of the capacitor, the circuit signal lines, and the interconnection through the inductor are all minimized. In addition, there is also a need for high quality capacitive dielectrics with high natural frequency such as X7R.

Shielding The last few techniques described in this article are noise masking and noise spreading, which can be used to improve noise margins after applying the techniques discussed earlier. If the EMC standard is not met or the noise margin is not adequate, an external shield is required to divert the radiated electric field emission to prevent transmission to the EMC receiver antenna.

When a switching voltage appears on the surface of a heat sink or magnetic core, an electric field is generated. The electric field is usually shielded by the conductive housing, where the conductive material converts the electric field into a current to isolate the electric field. Of course, there must also be a path for this current (usually grounded). However, the entire conducted noise energy caused by this current needs to be solved with a filter. External magnetic field shielding is more challenging (costly) and does not work well at higher frequencies. Therefore, you should carefully design the relevant magnetic components and circuit board circuit parts.

Using diffused spectrum Finally, this article will explore another increasingly widely used technique that can spread peak harmonic energy over a larger frequency band to effectively reduce this energy. This technique is referred to as Spread Spectrum Frequency Dithering (SSFD), which can change the noise signal from a narrow frequency to a wide frequency by reducing the harmonic peaks to change the noise spectrum. It is necessary to understand the changes in the energy spectrum, and the entire energy remains unchanged. The end result is that the noise level will generally increase, thus damaging the hi-fi system. Figure 4 shows the occurrence of harmonic diffusion and peak reduction. The general reduction is 5 to 10 dB, and subsequent harmonics will increase the peak reduction.

Conclusion You can spend a long time understanding the complexity of EMI, but designing an EMI-compliant switching regulator requires only understanding the application circuit and a few basic circuit design attributes and waveform analysis. Whether designing automotive switching regulators, designing switching regulators without batteries, or complex PEV battery chargers, designing EMI-compliant switching regulators requires knowledge of Maxwell's equations. Fortunately, for most of us, it does not involve partial differential equations. Instead, we only need to pay attention to the magnetic and electric fields that occur when voltages/currents are rapidly changed, and understand the techniques described in this article.