Printed circuit board layout determines the success or failure of all power supplies, determining functionality, electromagnetic interference (EMI), and performance when exposed to heat. Switching power supply layout is not magic, it’s not difficult, but it can often be overlooked in the initial design stage. However, because both functional and EMI requirements must be met, arrangements that are beneficial to the functional stability of the power supply are often beneficial to reduce EMI emissions, so it is better to do it later. It should also be mentioned that designing a good layout from the outset adds no cost and actually saves money as there is no need for EMI filters, mechanical shielding, time spent on EMI testing and PC board modifications.
Additionally, potential interference and noise problems can be exacerbated when multiple DC/DC switch-mode regulators are paralleled for current sharing and greater output power. If all the regulators are operating (switching) at similar frequencies, the total energy generated by multiple regulators in the circuit will be concentrated at one frequency. The presence of this energy can be a concern, especially if the PC board and the rest of the ICs on other system boards are in close proximity to each other and are susceptible to this radiated energy. This problem can be especially troublesome in automotive systems, which are densely packed and often in close proximity to audio, RF, CAN bus, and various radar systems.
Addressing Switching Regulator Noise Radiation Issues
In automotive environments, switching regulators are often used instead of linear regulators in areas where heat dissipation and efficiency are important. In addition, switching regulators are typically the first active component on the input power bus and therefore have 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 noise is limited to specific terminations or connectors in the solution design, compliance with conducted EMI requirements can be guaranteed, often early in the development process, through the aforementioned good layout or filter design.
Radiated EMI, however, is a different story. All components on a circuit board that carry current radiate 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 produces noise that covers the entire signal spectrum. Even with careful design, designers never really know how severe radiated EMI will be until the system is tested. Moreover, it is impossible to formally conduct radiated EMI testing until the design is basically completed.
Filters can attenuate intensity at a certain frequency or across a range of frequencies to reduce EMI. Part of the energy travels through space (radiation), so metallic and magnetic shielding can be added to attenuate it. The part on the PCB trace (conducting) can be controlled by adding ferrite beads and other filters. EMI cannot be completely eliminated, but can be attenuated to levels acceptable to other communications and digital components. Additionally, several regulatory agencies enforce some 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 an increase in the switching regulator switching frequency. Faster switching transitions result in higher efficiency, shorter minimum on and off times, and therefore higher harmonic content. With all other parameters held constant, such as switching 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, with a 20dB increase in radiation if the switching frequency is increased by a factor of 10.
An experienced PCB designer will design the hotspot loop to be small and keep the shield ground as close to the active layer as possible. However, device pinout configuration, package construction, thermal design requirements, and package size required to store sufficient energy in decoupling components determine the minimum size of the hotspot loop. Compounding the problem is that in a typical planar printed circuit board, magnetic or transformer-type coupling between traces above 30MHz will cancel all filter efforts, since the higher the harmonic frequencies, the more unwanted magnetic coupling is. become more effective.
A new solution to these EMI problems
A reliable and real solution to the EMI problem is to put the entire circuit in a shielded box. Of course, doing so increases cost, increases required board space, makes thermal management and testing more difficult, and results in additional assembly costs. Another frequently used method is to slow down the switching edges. Doing so has the undesired effect of reducing efficiency, increasing the minimum on and off times, creating an associated dead time, and detracting from the speed at which the current control loop can be achieved.
Linear Technology recently introduced the LT8614SilentSwitcher regulator, which eliminates the above drawbacks by providing the desired shielding box effect without the need for a shielding box. See Figure 1. The LT8614 also features a world-class low IQ with an operating current of only 2.5µA. This is the total supply current consumed by the device in the no-load regulation state.
Figure 1: LT8614 SilentSwitcher minimizes MW EMI/EMC emissions
The device’s ultra-low dropout voltage is limited only by the internal top-side switch. Unlike other solutions, the RDSON of the LT8614 is not limited by the maximum duty cycle and minimum off time. The device skips switch off cycles when a dropout occurs and performs only the shortest off cycles required to keep the internal top switch boost stage voltage continuously available, as shown in Figure 6.
At the same time, the LT8614’s minimum input operating voltage is only 2.9V typical (3.4V maximum), allowing the device to deliver a 3.3V rail in dropout conditions. The LT8614 is more efficient than the LT8610/11 at high currents due to its lower total switch resistance. The device can also be synchronized to external frequencies from 200kHz to 3MHz.
The device’s low AC switching losses allow it to operate at high switching frequencies with minimal loss of efficiency. Good balance can be achieved in EMI sensitive applications such as those common in many automotive environments, and the LT8614 is capable of operating at frequencies below the AM band (for even lower EMI) or above the AM band Work. The standard LT8614 demo board does not exceed the noise floor of the CISPR25-Calls5 measurement in a setting operating at a switching frequency of 700kHz.
The measurements shown in Figure 2 were taken in an anechoic chamber under the following conditions: 12Vin, 3.3Vout/2A, and a fixed switching frequency of 700kHz.
The LT8614 and LT8610 were tested in order to compare the LT8614 with SilentSwitcher technology and another current state-of-the-art switching regulator, the LT8610. The test was performed in a GTEM cell, and both devices were measured using a standard demo board with the same load, input voltage, and the same Inductor.
It can be seen that the LT8614 with LT8614 SilentSwitcher technology achieves up to 20dB improvement compared to the already very good EMI performance of the LT8610, especially in the more difficult to manage high frequency region. This enables simpler, more compact designs, and the LT8614 switching power supply requires less filtering in the overall design than other sensitive systems.
In the time domain, the LT8614 performs very well on switch node edges, as shown in Figure 4. Even at 4ns per division, the LT8614 SilentSwitcher regulator shows very little ringing (see Channel 2 in Figure 3). The ringing of the LT8610 is also well attenuated (Figure 3, channel 1), but it can be seen that the LT8610 hotspot loop stores higher energy compared to the LT8614 (channel 2).
Figure 5 shows the switch node for a 13.2V input. It can be seen that the LT8614 deviates very little from the ideal square wave, as shown in Channel 2. All time domain measurements in Figures 3, 4, and 5 were made with a 500MHz TektronixP6139A probe with the enclosed probe tip shield connected to the PCB GND plane, and tests were performed on a standard demo board.
In addition to the 42V absolute maximum input voltage rating for the automotive environment, the dropout performance of the device is also important. It is often necessary to support the critical 3.3V logic supply for cold crank conditions. In this case, the LT8614SilentSwitcher regulator remains close to the ideal performance of the LT861x family. Instead of offering the higher undervoltage lockout voltage and maximum duty cycle clamping like other devices, the LT8610/11/14 devices operate down to 3.4V and skip cycles whenever necessary, such as shown in Figure 6. This produces the ideal differential pressure behavior, as shown in Figure 7.
The minimum on-time of the LT8614 is a very short 30ns, which allows large step-down ratios even at high switching frequencies. Therefore, the device can supply the logic core voltage with a single step-down from an input up to 42V.
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 EMI test will pass once the system is developed. Until recently, there was no sure-fire way to guarantee that EMI problems could be easily resolved with proper selection of power ICs. Now, with the introduction of the LT8614, things have changed. Compared with the latest switching regulators, the EMI of the LT8614SilentSwitcher regulator is more than 20dB lower, and the LT8614 also perfectly improves the conversion efficiency. That is, EMI is improved by a factor of 10 in the frequency range above 30MHz without sacrificing minimum on and off times or efficiency on the same board area. This large improvement can be achieved without 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.
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