“It goes without saying that the layout design of the PC circuit board determines the success or failure of each power supply design. It determines the function, electromagnetic interference (EMI) and thermal behavior of a power supply. Although the switching power supply layout is not black magic, it is often overlooked in the design process, and it is too late to find that it is essential. Therefore, an effective method is needed to weaken these potential EMI threats from the beginning to ensure that the power supply is quiet and stable.Although many switch-mode power supply designers are well aware of the design complexity and nuances of switch-mode power supplies, many companies simply do not have enough designers to meet all project requirements to complete the design.
It goes without saying that the layout design of the PC circuit board determines the success or failure of each power supply design. It determines the function, electromagnetic interference (EMI) and thermal behavior of a power supply. Although the switching power supply layout is not black magic, it is often overlooked in the design process, and it is too late to find that it is essential. Therefore, an effective method is needed to weaken these potential EMI threats from the beginning to ensure that the power supply is quiet and stable. Although many switch-mode power supply designers are well aware of the design complexity and nuances of switch-mode power supplies, many companies simply do not have enough designers to meet all project requirements to complete the design. Many designers will retire and leave the industry! So, how to solve this problem?
First of all, because of the shortage of analog power supply designers, more and more digital designers are required to design switch mode power supplies! Although most digital designers know how to use a simple linear regulator, not all designs require step-down (buck mode). In fact, many are in boost mode (boost) or even buck-boost topology (combined buck and boost modes).
Obviously, many Electronic system manufacturers face a problem: how to implement all the switch-mode power supply circuits required by the system?
Solve the problem of shortage of design resources
In this article, I will introduce some basic principles of the working of a buck regulator, including how high di/dt and parasitic inductance in the thermal loop of a switching regulator can cause electromagnetic noise and switch ringing. Then we will look at how to reduce high frequency noise. I will also introduce ADI’s Power by Linear™ Silent Switcher®Technology, including how it is structured, and demonstrate how it can help solve EMI problems without affecting performance in the slightest. It also includes how the Silent Switcher device works.
I will also outline the packaging and layout of Silent Switcher and discuss how these packages and layouts can improve the overall performance of the buck converter. In addition, I will demonstrate how to incorporate this technology into our μModule®Voltage regulator to improve the integration of Silent Switcher devices. For designers who are not familiar with switch-mode power supply design techniques, these simple and easy-to-use solutions can be very useful.
Basic buck regulator circuit
One of the most basic power supply topologies is a buck regulator, as shown in Figure 1. EMI starts from the high di/dt loop. The power supply line and load line should not have high AC current components. Therefore, the input capacitor C2 should transfer all the AC components of the relevant current to the output capacitor C1, where all the AC components of the current end.
Figure 1. Schematic diagram of a synchronous buck regulator.
Referring to Figure 1, during the on period when M1 is closed and M2 is open, alternating current flows in the solid blue loop. In the off period, when M1 is on and M2 is off, AC current flows in the green dashed loop. It is difficult for most people to understand that the loop that produces the highest EMI is neither the solid blue loop nor the dashed green loop. It is the fully switched AC current flowing in the red loop of the dashed line that switches from zero to peak I and then back to zero. The dashed red loop usually refers to the hot loop because it has the highest AC current and EMI energy.
What causes electromagnetic noise and switch ringing is the high di/dt and parasitic inductance in the thermal loop of the switching regulator. To reduce EMI and improve functions, it is necessary to minimize the radiation effect of the dashed red loop. If we can reduce the PC circuit board area of the dashed red loop to zero, and can buy ideal capacitors with zero impedance, this problem can be solved. However, in the real world, all a design engineer can do is find the best compromise!
So, where does this high-frequency noise come from? In electronic circuits, through parasitic resistance, inductance and capacitive coupling, high frequency harmonics are generated during the switching process. Knowing where the noise is generated, how to reduce high-frequency switching noise? The traditional way to reduce noise is to slow down the MOSFET switching edge. This can be achieved by slowing down the internal switch driver or adding a buffer from the outside.
Figure 2. How to convert the LT8610 into a Silent Switcher device-LT8614.
However, this will reduce the efficiency of the converter because of increased switching losses-especially when the switching regulator is operating at a high switching frequency (such as 2 MHz). Having said that, why should we operate at a frequency of 2 MHz? There are actually several reasons:
► It allows the use of smaller (size) external components such as capacitors and inductors. For example, every time the switching frequency is doubled, the inductance value and output capacitance value will be halved.
► In automotive applications, switching at 2 MHz can avoid noise in the AM frequency band.
To reduce radiation, filters and shields can also be used, but this requires more external components and circuit board area. Spread spectrum (SSFM) technology can also be used, but this will make the system clock jitter within a known range. SSFM helps meet EMI standard requirements. EMI energy is scattered and distributed in the frequency domain. Although the switching frequency selected by ordinary switching power supplies is usually outside the AM frequency band (530 kHz to 1.8 MHz), in the AM frequency band, unmodulated switching harmonics may still not meet the strict automotive EMI requirements. Adding the SSFM function can significantly reduce EMI in the AM band and other areas.
Or just use ADI’s Silent Switcher technology, which can meet all the above requirements:
► High efficiency
► High switching frequency
► Low electromagnetic radiation (EMI)
Silent Switcher technology
Silent Switcher devices do not need to slow down the switching edge rate, which solves the trade-off between EMI and efficiency. So how can it be achieved? Consider using the LT8610, as shown on the left side of Figure 2. This is a single-chip (with internal FET) synchronous buck converter that supports 42 V input and can provide up to 2.5 A of output current. Please note that there is an input pin (VIN).
However, comparing the LT8610 with the LT8614 (a single-chip synchronous buck converter that supports 42 V input, which can provide up to 4 A output current), we can see that the LT8614 has two V on the other side of the package.INPins and two ground pins. This is important because it is part of the realization of ultra-low noise switching!
How to make a switching regulator with ultra-low noise
How to achieve this goal? V on the other side of the chipINPlacing two input capacitors between the pin and the ground pin can eliminate the magnetic field. This is highlighted in the slide, and red arrows are used to point to the location of the capacitor on both the schematic and the demo board, as shown in Figure 3.
Figure 3. LT8614 diagram, showing the V where the filter capacitor is placed on the other side of the ICINAnd the ground pin.
LT8614 includes Silent Switcher function. Using this feature, we can reduce parasitic inductance by using copper pillar flip-chip packaging. In addition, there is a reverse VIN, Grounding and input capacitance, can eliminate the magnetic field (applicable to the right-hand rule) to reduce EMI radiation.
Since there is no need to use the long bonding wires required by the wire bonding assembly technology, large parasitic resistance and inductance will not be generated, thereby reducing the parasitic inductance of the package. The opposite magnetic fields generated by the two symmetrically distributed input thermal circuits cancel each other out, and the electric circuit has no net magnetic field.
We compare the LT8614 Silent Switcher regulator with the current advanced switching regulator LT8610. In the GTEM room, the standard demonstration boards of the two devices were tested with the same load, the same input voltage, and the same inductance. We found that compared with using the LT8610 with very good EMI performance, the LT8614 can also be increased by 20 dB, especially in areas where it is more difficult to manage higher frequencies. In the overall design, compared with other sensitive systems, the LT8614 switching power supply requires less filtering and shorter distance, which enables a simpler and more compact design. In addition, in the time domain, the LT8614 performs well at the edge of the switch node.
Figure 4. LT8614 radiated EMI performance can meet the most stringent CISPR 25 Class 5 limit requirements.
Further enhancement of Silent Switcher devices
Although the LT8614 has excellent performance, but we have not stopped the pace of improvement. Therefore, the LT8640 buck regulator adopts the Silent Switcher architecture, which is designed to minimize EMI/EMC radiation while providing high efficiency at frequencies up to 3 MHz. It adopts 3 mm × 4 mm QFN package, adopts integrated power monolithic structure, and provides all necessary circuit functions at the same time, forming a solution with the smallest PCB footprint. The transient response performance is still excellent, and the output voltage ripple at any load (from zero current to full current) is less than 10 mV p-pat. LT8640 allows high V at high frequencyINTo low VOUTConversion, the shortest switch on-time is 30 ns.
To improve EMI/EMC, LT8640 can work in spread spectrum mode. This function adjusts the clock with 20% triangular frequency modulation. When the LT8640 is in the spread spectrum modulation mode, use the triangular frequency modulation function to adjust the switching frequency between the RT setting value and approximately 20% higher than this value. The modulation frequency is approximately 3 kHz. For example, when the LT8640 is set to 2 MHz, the frequency at a rate of 3 kHz will vary from 2 MHz to 2.4 MHz. When the spread spectrum working mode is selected, the burst mode (Burst Mode®) The operation will be disabled and the device will operate in pulse skip mode or forced continuous mode.
However, even though we have instructions in the Silent Switcher data sheet, such as providing schematics and layout suggestions, and placing the input capacitors as close as possible to both sides of the IC-some customers still make mistakes. In addition, our in-house engineers also spent too much time to solve customer PCB layout problems. Therefore, our designers have proposed the best solution to this problem-Silent Switcher 2 architecture.
Silent Switcher 2
Using Silent Switcher 2 technology, we only need to integrate the capacitor in the new LQFN package: VINCapacitance, IntVCCAnd boost capacitor-placed as close to the pin as possible. The advantage is that all thermal loops and ground planes are included, thereby reducing EMI. The fewer external components, the smaller the solution size. In addition, we have eliminated PCB layout sensitivity.
As shown in Figure 5, you can see the difference between the LT8640 and LT8640S schematics. The marketing breakthrough is to put the “S” suffix on the new version that contains internal capacitors with a higher degree of integration. Because it is more “quiet” than the first generation!
Figure 5. LT8640S is a Silent Switcher 2 device with higher capacitance integration.
Silent Switcher 2 technology improves heat dissipation performance. The multiple large-size ground exposed pads on the LQFN flip-chip package help the package and PCB to dissipate heat. Since we eliminated high-resistance bonding wires, we also improved the conversion efficiency. The EMI performance of the LT8640S easily meets the radiated EMI performance CISPR 25 Class 5 peak limit requirements and has a large margin.
Next step: All components are integrated with Silent Switcher 2 μModule regulator
The Silent Switcher technology is so compelling, we chose to integrate it into our μModule regulator product line. All components are integrated in a small size package, providing users with a simple, reliable, high-performance and high-power density solution. LTM8053 and LTM8073 are miniature modular regulators that integrate almost all components, with only a few capacitors and resistors connected to the outside.
Figure 6. LTM8053 Silent Switcher 2 μModule.
In summary, the functions and advantages of Silent Switcher will make it easier for your switch-mode power supply design to meet the requirements of various noise immunity standards such as CISPR 32 and CISPR 25. They can do this easily and effectively due to the following characteristics:
► Able to perform efficient conversion at a switching frequency greater than 2 MHz with minimal impact on conversion efficiency.
► The internal bypass capacitor reduces EMI radiation and provides a more compact solution, occupying board space.
► Using Silent Switcher 2 technology basically eliminates the sensitivity of PCB layout.
► Optional spread spectrum modulation helps reduce noise sensitivity.
► Using Silent Switcher devices can not only save PCB area, but also reduce the number of layers required.
Tony Armstrong [[email protected]