Minimize EMI in automotive environments
Printed circuit board layout determines the success or failure of every power supply. It sets functional, electromagnetic interference (EMI) and thermal behaviour. While switching power supply layout is not a “black” art, it can often be overlooked in the initial design process. Nevertheless, since functional and EMI requirements have to be met, what is good for functional stability of the power supply is also usually good for its EMI emissions as well. It should also be note that good layout from the beginning does not add any cost, but can actually provide cost savings, eliminating the need for EMI filters, mechanical shielding, EMI test time and PC board revisions.
Moreover, the potential problem for interference and noise can be exasperated when multiple DC/DC switchmode regulators are paralleled for current sharing and higher output power. If all are operating (switching) at a similar frequency, the combined energy generated by multiple regulators in a circuit is then concentrated at one frequency. Presence of this energy can become a concern especially if the rest of ICs on the PC board as well as other system boards are close to each other and susceptible to this radiated energy. This can be particularly troubling in automotive systems which are densly populated in and are often in close proximity to audio, RF, CAN bus and various radar systems.
Addressing Switching Regulator Noise Emissions
In an automotive environment, switching regulators usually replace linear regulators in areas where low heat dissipation and efficiency are valued. Moreover, the switching regulator is typically the first active component on the input power bus line, and therefore has a significant impact on the EMI performance of the complete converter circuit.
There are two types of EMI emissions; conducted and radiated. Conducted emissions ride on the wires and traces that connect up to a product. Since the noise is localized to a specific terminal or connector in the design, compliance with conducted emissions requirements can often be assured relatively early in the development process with a good layout or filter design as already stated.
Radiated emissions, however, are another story. Everything on the board that carries current radiates an electromagnetic field. Every trace on the board is an antenna, and every copper plane is a resonator. Anything, other than a pure sine wave or DC voltage, generates noise all over the signal spectrum. Even with careful design, a designer never really knows how the bad the radiated emissions are going to be are until the system gets tested. And radiated emissions testing cannot be formally performed until the design is essentially complete.
Filters are often used to reduce EMI by attenuating the strength at a certain frequency or over a range of frequencies. A portion of this energy that travels through space (radiated) is attenuated by adding metallic and magnetic shields. The part that rides on PCB traces (conducted) is tamed by adding ferrite beads and other filters. EMI cannot be eliminated but can be attenuated to a level that is acceptable by other communication and digital components. Moreover, several regulatory bodies enforce standards to ensure compliance.
Modern input filter components in surface mount technology have better performance than through-hole parts. However, this improvement is outpaced by the increase in operating switching frequencies of switching regulators. Higher efficiency, low minimum on- and off-times result in higher harmonic content due to the faster switch transitions. For every doubling in switching frequency, the EMI becomes 6dB worse while all other parameters, such as switch capacity and transition times, remain constant. The wideband EMI behaves like a first order high pass with 20dB higher emissions if the switching frequency increases by 10 times.
Savvy PCB designers will make the hot loops small and use shielding ground layers as close to the active layer as possible. Nevertheless, device pin-outs, package construction, thermal design requirements and package sizes needed for adequate energy storage in decoupling components dictate a minimum hot loop size. To further complicate matters, in typical planar printed circuit boards, the magnetic or transformer style coupling between traces above 30MHz will diminish all filter efforts since the higher the harmonic frequencies are the more effective unwanted magnetic coupling becomes.
A New Solution to these EMI issues
The tried and true solution to EMI issues is to use a shielding box for the complete circuit. Of course, this adds costs, increases required board space, makes thermal management and testing more difficult, as well as introduces additional assembly costs. Another frequently used method is to slow down the switching edges. This has the undesired effect of reducing the efficiency, increasing minimum on-, off-times, and their associated dead times and compromises the potential current control loop speed.
Linear’s recently introduced LT8614 Silent Switcher™ regulator delivers the desired effects of a shielded box without using one, and so eliminates the above mentioned drawbacks. See Figure 1. The LT8614 also has a world class low IQ of only 2.5µA operating current. This is the total supply current consumed by the device, in regulation with no load.
Figure 1. The LT8614 Silent Switcher minimizes EMI/EMC emissions while delivering high efficiency at frequencies up to 3 MHz.
Its ultralow dropout is only limited by the internal top switch. Unlike alternative solutions, the LT8614’s RDSON is not limited by maximum duty cycle and minimum off-times. The device skips its switch-off cycles in dropout and performs only the minimum required off cycles to keep the internal top switch boost stage voltage sustained, as shown in Figure 6.
Figure 6. Ch1: LT8610, Ch2: LT8614 switch mode dropout behaviour.
At the same time, the minimum operating input voltage is only 2.9V typical (3.4V maximum), enabling it to supply a 3.3V rail with the part in dropout. The LT8614 has higher efficiency than the LT8610/11 at high currents since its total switch resistance is lower. It can also be synchronised to an external frequency operating from 200 KHz to 3MHz.
The AC switch losses are low, so it can be operated at high switching frequencies with minimal efficiency loss. In EMI-sensitive applications, such as those commonly found in many automotive environments, a good balance can be attained and the LT8614 can run either below the AM band for even lower EMI, or above the AM band. In a setup with 700 kHz operating switching frequency, the standard LT8614 demo board does not exceed the noise floor in a CISPR25, Calls 5 measurement.
The Figure 2 measurements were taken in an anechoic chamber under the following conditions: 12V in, 3.3V out at 2A with a fixed switching frequency of 700 kHz.
Figure 2. Blue trace is the noise floor; red trace is the LT8614 board at CISPR25 radiated measurement in an anechoic chamber.
To compare the LT8614 Silent Switcher technology against another current state-of-the-art switching regulator, the part was measured against the LT8610. The test was performed in a GTEM cell using the same load, input voltage and the same inductor on the standard demo boards for both parts.
One can see that up to a 20dB improvement is attained using the LT8614 Silent Switcher technology compared to the already very good EMI performance of the LT8610, especially in the more difficult to manage high frequency area. This enables simpler and more compact designs where the LT8614 switching power supply needs less filtering compared to other sensitive systems in the overall design.
In the time domain, the LT8614 shows very benign behaviour on the switch node edges, as shown in Figure 4. Even at 4ns/div the LT8614 Silent Switcher regulator shows very low ringing (see Ch2 in Figure 3). The LT8610 has a good damped ringing (Ch1, Figure 3) but one can see the higher energy stored in the hot loop compared to the LT8614 (in Ch2).
Figure 3. Blue trace is the LT8614, purple trace is the LT8610; both 13.5Vin, 3.3V out at 2.2A load.
Figure 4. Ch1: LT8610, Ch2: LT8614 switch node rising edge both at 8.4V in, 3.3Vout at 2.2A.
Figure 5 shows the switch node at 13.2V input. One can see the extremely low deviation from the ideal square wave of the LT8614, shown in Ch2. All time domain measurements in Figures 3, 4 and 5 are done with 500MHz Tektronix P6139A probes with close probe tip shield connection to the PCB GND plane, both on the standard demo boards.
Figure 5. Ch1: LT8610, Ch2: LT8614, both at 13.2V in, 3.3V 2.2A out.
Besides their 42V absolute maximum input voltage ratings for automotive environments, their dropout behaviour is also very important. Often critical 3.3V logic supplies need to be supported through cold crank situations. The LT8614 Silent Switcher regulator maintains the close to ideal behaviour of the LT861x family in this case. Instead of higher undervoltage lockout voltages and maximum duty cycle clamps of alternative parts, the LT8610/11/14 devices operate down to 3.4V and start skipping off cycles as soon as necessary, as shown in Figure 6. This results in the ideal dropout behaviour as shown in Figure 7.
Figure 7. LT8614 dropout behaviour.
The LT8614’s low minimum on-time of 30ns enables large step-down ratios even at high switching frequencies. As a result, it can supply logic core voltages with a single step-down from inputs up to 42V.
Conclusion
It is well known that EMI considerations for automotive environments require careful attention during the initial design process in order to ensure that they will pass EMI testing once the system is completed. Until now, there was not a sure way to guarantee that this could easily be attained with the right power IC selection. This has now changed due to the introduction of the LT8614. This LT8614 Silent Switcher regulator reduces EMI from current state-of-the-art switching regulators by more than 20dB, while increasing conversion efficiencies with no drawbacks. A 10 time improvement of EMI in the frequency range above 30MHz is attained without compromising minimum on- and off-times or efficiency in the same board area. This is accomplished with no special components or shielding, representing a significant breakthrough in switching regulator design. This is just the sort of breakthrough product that allows automotive-system designers to take their products to the next level of noise performance.
About the author:
Tony Armstrong, Director of Product Marketing Manager for Linear Technology’s power product group, joined the Company in May of 2000. He is responsible for all aspects of the power conversion and management products from conception through obsolescence. Prior to joining Linear, Tony held various positions in marketing, sales and operations at Siliconix Inc., Semtech Corp., Fairchild Semiconductors and Intel Corp. (Europe). He attained a BS (Honors) in Applied Mathematics from the University of Manchester in England in 1981.
This article by courtesy of EDN.
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