Everyone knows that the layout of the printed circuit board determines the success or failure of all power supplies, and determines the function, electromagnetic interference (EMI) and performance when heated. Switching power supply layout is not magic, it is not difficult, but in the initial design stage, it may often be overlooked. However, because both the function and EMI requirements must be met, arrangements that are beneficial to the stability of the power supply function are often helpful to reduce EMI radiation, so it is better to do it later than early. It should also be mentioned that designing a good layout from the beginning will not increase any costs, in fact, it can also save costs, because there is no need for EMI filters, mechanical shielding, time-consuming EMI testing and modification of the PC board.
In addition, when multiple DC / DC switch-mode regulators are connected in parallel to achieve current sharing and greater output power, potential interference and noise problems may worsen. If all voltage regulators work at similar frequencies (switching), then the total energy produced by multiple voltage regulators in the circuit will be concentrated on one frequency. The presence of this energy may become a worrying problem, especially if the PC board and the remaining ICs on other system boards are close to each other and are susceptible to this radiated energy. In automotive systems, this problem can be particularly troublesome because automotive systems are densely packed and often close to audio, RF, CAN bus, and various radar systems.
Dealing with noise radiation problems of switching regulators
In the automotive environment, switching regulators are often used to replace linear regulators in areas where heat dissipation and efficiency are important. In addition, the switching regulator is generally the first active component on the input power bus, so it 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 limited to the specific terminals or connectors in the scheme design, through the aforementioned good layout or filter design, it is often early in the development process to ensure compliance with conducted EMI requirements.
However, radiated EMI is a different matter. All components carrying electrical current on the circuit board radiate an electromagnetic field. Each trace on the circuit board is an antenna, and each copper plane is a resonator. In addition to pure sine waves or DC voltages, any signal generates noise that covers the entire signal spectrum. Even after careful design, the designer will never really know how serious radiated EMI will be before the system is tested. And before the design is basically completed, it is impossible to formally conduct radiated EMI testing.
The filter can attenuate the intensity at a certain frequency or the entire frequency range to reduce EMI. Part of the energy is transmitted through space (radiation), so metal shielding and magnetic shielding can be added to attenuate. The part of 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 some standards to ensure compliance with EMI requirements.
The performance of new input filter components using surface mount technology is 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 components. With all other parameters such as switching capacity and conversion time remaining unchanged, every time the switching frequency doubles, EMI deteriorates by 6dB. Broadband EMI behaves like a first-order high-pass filter, if the switching frequency increases 10 times , Will increase the 20dB radiation.
Experienced PCB designers will design hotspot loops as small as possible and keep the shield ground layer as close to the active layer as possible. However, the device pinout configuration, package configuration, thermal design requirements, and package size required to store sufficient energy in the decoupling component determine the minimum size of the hotspot loop. To complicate matters further, in a typical flat printed circuit board, magnetic or transformer-type coupling above 30MHz between traces will cancel out all filter efforts, because the higher the harmonic frequency, the unwanted magnetic coupling Become more effective.
New solutions to these EMI problems
The reliable and true solution to the EMI problem is to place the entire circuit in a shielded box. Of course, doing so increases costs, increases the required board space, makes thermal management and testing more difficult, and results in additional assembly costs. Another commonly used method is to slow down the switching edges. Doing so will produce an undesirable result, which is to reduce efficiency, increase the minimum on and off time, and produce related dead time, which is detrimental to the speed that the current control loop may reach.
Linear Technology recently introduced the LT8614 Silent Switcher voltage regulator. This device does not require a shielding box, but can provide the desired shielding box effect, thus eliminating the above disadvantages. See Figure 1. The LT8614 also has a world-class low IQ, and the operating current is only 2.5? A. This is the total power supply current consumed by the device in the no-load regulation state.
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 the minimum off time. The device skips the switch off cycle when a dropout occurs, and only performs the shortest required off cycle to keep the internal top switch boost stage voltage continuously provided, as shown in Figure 6.
At the same time, the minimum input operating voltage of the LT8614 is typically 2.9V (up to 3.4V), which allows the device to provide a 3.3V rail when there is a voltage drop. The LT8614 is more efficient than the LT8610 / 11 at high currents because its total switching resistance is small. The device can also be synchronized to external frequencies from 200kHz to 3MHz.
The AC switching loss of this device is very low, so it can work at high switching frequency 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 can operate at frequencies below the AM band (to achieve even lower EMI) or above the AM band jobs. In a setting with an operating switching frequency of 700kHz, the standard LT8614 demonstration circuit board does not exceed the noise floor of the CISPR25-Calls 5 measurement result.
In order to compare the LT8614 with Silent Switcher technology and another current latest switching regulator LT8610, the LT8614 and LT8610 were tested. The test was conducted in a GTEM unit, and the two devices were measured using a standard demonstration circuit board with the same load, input voltage, and same inductor.
It can be seen that the LT8614 with the LT8614 Silent Switcher technology achieves an improvement of up to 20dB compared to the already very good EMI performance of the LT8610, especially in the more difficult to manage high-frequency regions. This makes it possible to achieve a simpler and more compact design. Compared with other sensitive systems, the overall design of the LT8614 switching power supply requires lower filtering.
In the time domain, the LT8614 performs very well on the edge of the switch node, 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 ringing of the LT8610 also attenuated well (Figure 3, channel 1), but it can be seen that compared to the LT8614 (channel 2), the LT8610 hotspot loop stores higher energy.
It can be seen that the deviation of the LT8614 from the ideal square wave is extremely small, as shown in channel 2. All time domain measurements in Figures 3, 4, and 5 were measured with a 500MHz Tektronix P6139A probe. The closed probe tip shield is connected to the PCB GND plane. The tests are performed on a standard demonstration circuit board.
In addition to the 42V absolute maximum input voltage rating for automotive environments, the device's dropout performance is also very important. It is often necessary to support the critical 3.3V logic power supply to cope with the cold car start situation. In this case, the LT8614 Silent Switcher regulator maintains close to the ideal performance of the LT861x series. LT8610 / 11/14 devices do not provide higher undervoltage lockout voltage and maximum duty cycle clamping like other devices, but operate at voltages as low as 3.4V, and skip several cycles as necessary, such as Figure 6 shows. This produces an ideal differential pressure performance, as shown in Figure 7.
The minimum on-time of the LT8614 is a very short 30ns, which allows a large step-down ratio even at high switching frequencies. Therefore, the device can provide a logic core voltage from an input up to 42V after a single step-down.
in conclusion
As we all know, EMI in the automotive environment requires careful attention during the initial design stage to ensure that it can pass EMI tests once the system development is complete. Until recently, there was no sure way to ensure that by properly selecting the power IC, EMI problems could be easily solved. Now, due to the introduction of LT8614, the situation has changed. Compared with the latest switching regulators, the LT8614 Silent Switcher regulator has an EMI of more than 20dB. At the same time, the LT8614 also perfectly improves the conversion efficiency. That is to say, without sacrificing the shortest on and off time or efficiency of the same circuit board area, in the frequency range above 30MHz, EMI has been improved by 10 times. No special components or shielding is required to achieve such a large improvement, which means a major breakthrough in the field of switching regulator design. This is a breakthrough device that enables automotive system designers to advance the noise performance of their products to a whole new level.
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