Minimizing EMI in the automotive environment

background

The printed circuit board layout determines the success or failure of any power supply, determining its functionality, electromagnetic interference (EMI), and performance under heat. Switching power supply layout is not magic, nor is it difficult, but it may often be overlooked during the initial design stage. However, because both functional and EMI requirements must be met, arrangements that are good for the functional stability of the power supply are often also good for reducing EMI emissions, so it is better to do it sooner rather than later. It should also be mentioned that designing a good layout from the beginning does not increase any costs, and in fact can save costs by eliminating the need for EMI filters, mechanical shielding, time spent on EMI testing, and PC board modifications.

Additionally, potential interference and noise issues can be exacerbated when multiple DC/DC switch-mode regulators are connected in parallel for current sharing and greater output power. If all regulators operate (switch) at similar frequencies, the total energy generated by the multiple regulators in the circuit is concentrated at a single frequency. The presence of this energy can become a concern, especially if the rest of the ICs on the PC board and other system boards are in close proximity to each other and susceptible to this radiated energy. This problem can be particularly troublesome in automotive systems, which are densely packed and often close to audio, RF, CAN bus, and various radar systems.

Addressing the Issue of Noise Radiation from Switching Regulators

In automotive environments, switching regulators are often used to replace linear regulators in areas where heat dissipation and efficiency are important. In addition, the switching regulator is typically the first active component on the input power bus and therefore has a significant impact on the EMI performance of the entire converter circuit.

There are two types of EMI radiation: conducted and radiated. Conducted EMI depends on the wires and circuit traces connected to a product. Since the noise is confined to specific terminals or connectors in the solution design, compliance with conducted EMI requirements can often be ensured early in the development process through good layout or filter design as described above.

Radiated EMI, however, is a different story. Everything on a board that carries current radiates an electromagnetic field. Every trace on the board is an antenna, and every copper plane is a resonator. Any signal other than a pure sine wave or DC voltage generates noise that covers the entire signal spectrum. Even with careful design, a designer never really knows how severe radiated EMI will be until the system is tested. And formal testing for radiated EMI is impossible until the design is substantially complete.

Filters can reduce EMI by attenuating the intensity at a certain frequency or across the entire frequency range. Some of the energy propagates through space (radiation), so metal shielding and magnetic shielding can be added to attenuate it. The part on the PCB trace (conduction) can be controlled by adding ferrite beads and other filters. EMI cannot be completely eliminated, but it can be attenuated to a level acceptable to other communications and digital components. In addition, several regulatory agencies enforce standards to ensure compliance with EMI requirements.

New input filter components using surface mount technology perform better than through-hole components. However, this improvement is offset by the increase in the switching operating frequency of the switching regulator. Faster switching transitions result in higher efficiency, short minimum on and off times, and therefore higher harmonic content. With all other parameters remaining the same, such as switch capacity and transition time, EMI deteriorates by 6dB for every doubling of the switching frequency. Broadband EMI behaves like a first-order high-pass filter, and a 10-fold increase in switching frequency will increase emissions by 20dB.

Experienced PCB designers will design the hotspot loop to be small and keep the shielding ground plane as close to the active layer as possible. However, the device pinout configuration, package construction, thermal design requirements, and the package size required to store sufficient energy in the decoupling components determine the minimum size of the hotspot loop. To further complicate the issue, in a typical planar printed circuit board, magnetic or transformer-type coupling between traces above 30MHz will offset all filter efforts because the higher the harmonic frequency, the more effective the unwanted magnetic coupling becomes.

A new solution to these EMI problems

A reliable and real solution to the EMI problem is to place the entire circuit in a shielded box. Of course, this increases cost, increases required board space, makes thermal management and testing more difficult, and results in additional assembly costs. Another approach often taken is to slow down the switching edges. This has the undesirable effect of reducing efficiency, increasing the minimum on and off times, and creating associated dead time, which detracts from the speed that can be achieved in the current control loop.

Linear Technology recently introduced the LT8614 Silent Switcher™ regulator, which eliminates the above disadvantages by eliminating the need for a shielded box while providing the desired shielded box effect. See Figure 1. The LT8614 also features world-class low IQ, operating at only 2.5µA. This is the total supply current consumed by the device when in regulation with no load.

The device’s ultra-low dropout voltage is limited only by the internal top switch. Unlike other solutions, the RDSON of the LT8614 is not limited by the maximum duty cycle and minimum off-time. The device skips the switch off-cycle when dropout occurs and only performs the minimum off-cycle required to keep the internal top switch boost stage voltage continuously supplied, as shown in Figure 6.

At the same time, the LT8614's minimum input operating voltage is typically only 2.9V (3.4V maximum), allowing the device to provide a 3.3V rail in dropout. The LT8614 is more efficient than the LT8610/11 at high currents because its total switch resistance is lower. The device can also be synchronized to external frequencies from 200kHz to 3MHz.

The device has low AC switching losses, so it can operate at high switching frequencies with minimal efficiency loss. A good balance can be achieved in EMI-sensitive applications (such as those common in many automotive environments), and the LT8614 can operate at frequencies below the AM band (for even lower EMI) or above the AM band. In a setting with an operating switching frequency of 700kHz, the standard LT8614 demo board does not exceed the noise floor of CISPR25 - Calls 5 measurements.

The measurement results shown in Figure 2 were obtained in an anechoic chamber under the following conditions: 12Vin, 3.3Vout/2A, and a fixed switching frequency of 700kHz.

Figure 2: The blue curve is the noise floor; the red curve is the CISPR25 radiated measurement result of the LT8614 board in an anechoic chamber.

The LT8614 and LT8610 were tested to compare the LT8614 with Silent Switcher technology and another state-of-the-art switching regulator, the LT8610. The testing was performed in a GTEM cell, with both devices measured using a standard demo board with the same load, input voltage and the same inductor.

As can be seen, the LT8614 with LT8614 Silent Switcher technology achieves up to 20dB improvement over the already very good EMI performance of the LT8610, especially in the more difficult to manage high frequency region. This allows for simpler, more compact designs, with the LT8614 switcher requiring less filtering overall than other sensitive systems.

In the time domain, the LT8614 behaves very well on the switch node edges, as shown in Figure 4. Even at 4ns per division, the LT8614 Silent Switcher regulator shows very little ringing (see Channel 2 in Figure 3). The LT8610’s ringing is also well attenuated (Figure 3 Channel 1), but it can be seen that the LT8610 hotspot loop stores higher energy than the LT8614 (Channel 2).

Figure 3: The blue curve is the test result of LT8614, and the purple curve is the test result of LT8610. The test conditions are 13.5Vin, 3.3Vout/2.2A load.

Figure 4: Channel 1: LT8610, Channel 2: LT8614, switch node rising edge, both tested at 8.4Vin, 3.3Vout/2.2A.

Figure 5 shows the switch node for a 13.2V input. It can be seen that the LT8614 has minimal deviation from an ideal square wave, as shown in Channel 2. All time domain measurements in Figures 3, 4, and 5 were made with a 500MHz Tektronix P6139A probe with the closed probe tip shield connected to the PCB GND plane on a standard demo board.

Figure 5: Channel 1: LT8610, Channel 2: LT8614, all tested at 13.2Vin, 3.3Vout/2.2A. In addition to the 42V absolute maximum input voltage rating for automotive environments, the dropout performance of the devices is also very important. It is often necessary to support the critical 3.3V logic supply to cope with cold crank conditions. In this case, the LT8614 Silent Switcher regulator remains close to the ideal performance of the LT861x family. Instead of providing higher undervoltage lockout voltages and maximum duty cycle clamps like other devices, the LT8610/11/14 devices operate down to 3.4V and skip cycles whenever necessary, as shown in Figure 6. This results in ideal dropout performance, as shown in Figure 7.

Figure 6: Channel 1: LT8610, Channel 2: LT8614, switch node dropout performance

Figure 7: LT8614 dropout performance

The LT8614's very short 30ns minimum on-time allows large step-down ratios even at high switching frequencies. Thus, the device can provide logic core voltages from inputs as high as 42V in a single step-down.

in conclusion

It is well known that EMI issues in the automotive environment require careful attention during the initial design phase to ensure that the system will pass EMI testing once it is developed. Until recently, there was no surefire way to ensure that EMI issues could be easily addressed by properly selecting a power IC. Now, with the introduction of the LT8614, that has changed. The LT8614 Silent Switcher regulator offers more than 20dB lower EMI than the latest switching regulators available today, while also offering a dramatic improvement in conversion efficiency. That is, EMI is improved by a factor of 10 over a frequency range above 30MHz without sacrificing minimum on and off times or efficiency for the same board area. This improvement can be achieved without the need for special components or shielding, which represents a major breakthrough in switching regulator design. This is a breakthrough device that enables automotive system designers to push the noise performance of their products to a whole new level.