Designing an EMC-Compliant Automotive Switching Buck Regulator
The automobile is changing and so, too, are the electronics that make them run. The most radical example is the plug-in electric vehicle (PEV) where a 300-400V Lithium-Ion (Li-Ion) battery replaces the gas tank, and a three-phase propulsion motor replaces the combustion engine. Sophisticated battery pack monitoring, regenerative braking systems, and complex transmission control attempt to optimize battery utilization and life, and extend range between charge cycles. A modern day car, electric or otherwise, has dozens of electronic modules to facilitate performance, safety, convenience and security. It is not uncommon to find mid-range cars with advanced global positioning systems (GPS), integrated DVD players, and high-performance audio systems.
With these advancements comes the need for greater processing speed. As such, today’s automobiles incorporate high-performance microprocessors and DSPs requiring core voltages down to 1V, and currents upwards of 5A. Generating these voltages and currents from a car battery that can vary from 6V to 40V is fraught with challenges, not the least of which is satisfying strict standards for electromagnetic compatibility (EMC). The linear regulator, once the primary method for converting the car battery to a regulated source voltage, has run out of “steam.” Or more accurately, it generates too much “steam” as output voltages drop and load currents increase. Instead, the switching regulator is seeing increased usage, and with a proliferation of switching regulators, comes an increased concern of electromagnetic interference (EMI) disturbing radios and safety critical systems.
This article investigates the basic considerations for successfully implementing a switching regulator, presenting it in an intuitive way without complex mathematics. The principle considerations we will investigate include: 1) slew-rate control, 2) filter design, 3) component selection, 4) layout, and 5) noise spreading and shielding.
SMPS EMC the easy way
For a buck converter operating in CCM, the inductor current is always positive and non-zero. Under this condition a good approximation of the duty cycle is D=Vout/Vin, or in our case 38 percent (5V/14V). Using the switching frequency of 200 kHz we quickly calculate the on-time to be 1.8 µs. To support this frequency the rise/fall time of the control switch would have to be less than 90 ns. This brings us to our first noise mitigation awareness – slew rate control. You may not realize this, but at this point we have a very good idea of our harmonic content associated with the PWM switch node – the control waveform of a switching regulator. If we approximate this waveform as the trapezoidal shown in Figure 1a, the harmonic content of the waveform can be expressed as in Figure 1b, which represents the driving force behind EMI. This “Fourier Envelope” defines the harmonic content amplitudes that can be obtained via Fourier analysis, or more simply by calculating the on-time and rise-time of the trapezoidal waveform.
When viewed in the frequency domain, a trapezoidal waveform with equal rise and fall times is composed of a set of discrete harmonic signals that exist at integer multiples of the periodic signal’s fundamental frequency. Notice that the energy in each harmonic falls off at 20 dB/dec after the first break point at 1/(p × t), where t is the on-time and at 40 dB/dec after the second located at 1/(p × tr). Consequently, limiting the slew-rate of the switching waveforms can have a profound impact on reducing emissions. From this discussion it should be clear that operating at lower frequencies can also facilitate a reduction in emissions.
AM radio band considerations
Does duty cycle matter?
EMI and EMC standards
Conducted emissions can become radiated emissions: input filter considerations
By simply adding L2 and C2, the waveform becomes more sinusoidal and the energy is re-distributed with significantly lower high-frequency peaks. The input filter, however, if improperly designed, can actually amplify noise and destabilize the control loop. So understanding the concepts of input filter design are important in optimizing filter response and cost. Using the AC analysis of SPICE is an effective tool in comprehending the filter behavior. For a detailed discussion on EMC and filter design as it applies to switching regulators, see Unitrode Design Seminars  archived under “training” on the TI website.
Whether you are designing a buck or boost supply, the “differential mode” filter or a bi-directional pi-filter can be your best friend by keeping EMI noise from getting onto the line and radiating, and/or conducting noise. Note that parasitic elements associated with the filter components, including inter-winding terminal capacitance and capacitor ESR, can significantly affect attenuation of harmonic content and should be considered carefully.
Selecting the right components
Diodes with soft or low-reverse recovery characteristics minimize high-current spikes associated with a diode going from a conducting state to a blocking state. These peak currents react with parasitic inductances to create ringing on the switching nodes that can exceed 100 MHz and wreak havoc in the EMC test chamber. Although beyond the scope of this paper, EMI can be aggravated by the improper selection of a switching regulator’s loop compensating components. If the power supply is not properly compensated, output ripple and instabilities will manifest themselves as increased noise. A properly compensated power supply is essential to achieve good noise performance.
Keep in mind the path that current flows
Fundamental to the proper layout of a power supply is minimizing loop area of high-current caring conductors. In doing this you minimize inductance that can act as an antenna source and radiate energy. One aspect of this is effectively placing the component and selecting the decoupling capacitor. Figure 3 illustrates the output power stage and filter of a synchronous buck converter. C3 decouples the power stage, providing a low-impedance source when Q2 turns on. To minimize radiated emissions, C3 must be connected as illustrated where the intrinsic impedances of the capacitor, circuit trace and interconnect via inductance are minimized. A high-quality capacitor dielectric with a high self-resonant frequency like X7R is also necessary.
An electric field is produced when switched voltages are present on surfaces such as heat sinks or magnetic cores, causing them to act as antennas. Electric fields usually can be shielded relatively easily by conductive enclosures, where the conductive material terminates the field by converting it to current. Of course, there must be a path for this current, which is normally ground. But this current merely contributes to overall conducted noise energy where it needs to be addressed with filters. External magnetic field shielding is more challenging (costly) and largely ineffective at higher frequencies. As such, leakage fields should be controlled by carefully designing the magnetics and circuit board loop areas (see the Unitrode Design Seminars).
When all else fails – spread the spectrum
1. Unitrode Design Seminars, 1980 – 2003: http://www.ti.com/2003powerseminar-ca2 .
For more information on these and other power solutions, visit: www.ti.com/power-ca