Limited and shrinking circuit board space, tight design cycles, and strict electromagnetic interference (EMI) specifications, such as CISPR 32 and CISPR 25, make it difficult to obtain a power source with high efficiency and good thermal performance. Throughout the design cycle, the power design is usually at the end of the design process, and the designer has to work hard to squeeze the complex power into a more compact space, which makes the problem even more complicated and frustrating. In order to get the design done on time, you have to compromise on performance and leave it to the testing and validation process. Simplicity, high performance, and solution size are three considerations that often conflict with each other: give priority to one or two and forgo the third, especially as design deadlines approach. Sacrificing some performance became commonplace; It shouldn’t be like this.
This paper begins with an overview of the serious problem of power supply in complex electronic systems: EMI, often referred to simply as noise. EMI will be generated by the power supply, which must be solved. So what is the root cause of the problem? What are the usual mitigation measures? This paper introduces the strategy of reducing EMI, and proposes a solution that can reduce EMI, maintain efficiency, and put power into a limited solution space.
What is EMI?
Electromagnetic interference is an electromagnetic signal that interferes with the performance of a system. This interference affects a circuit by electromagnetic induction, electrostatic coupling, or conduction. It is a key design challenge for automotive, medical, and test and measurement equipment manufacturers. Many of the limitations mentioned above and increasing power performance requirements (increased power density, higher switching frequency, and higher current) will only amplify EMI’s impact, so solutions are urgently needed to reduce EMI. Many industries require that EMI standards must be met. If they are not taken into consideration in the early design stage, the launch time of products will be seriously affected.
EMI coupling type
EMI is a problem that occurs when the interference source is coupled with the receiver (that is, some components in the electronic system) in an electronic system. EMI can be classified as conduction or radiation according to its coupling medium.
Conducted EMI (low frequency, 450 kHz to 30 MHz)
Conductive EMI is electrically coupled to the element through parasitic impedance and power and ground connections. Noise is transmitted by conduction to another device or circuit. Conducted EMI can be further divided into common mode noise and differential mode noise.
Common-mode noise is transmitted by parasitic capacitance and high dV/dt (C × dV/dt). It travels through a parasitic capacitance along the path from any signal (positive or negative) to GND, as shown in figure 1.
Differential-mode noise is conducted via secondary inductance (magnetic coupling) and a high di/dt (L × di/dt).
Differential mode noise is transmitted by parasitic inductance (magnetic coupling) and high di/dt (L × di/dt).
Figure 1. Differential and common mode noise.
Radiation EMI (high frequency, 30 MHz to 1 GHz)
Radiant EMI is the noise transmitted wirelessly to the device under test by means of magnetic field energy. In a switching power supply, the noise is the result of high di/dt coupled with parasitic inductance. Radiated noise can affect neighboring devices.
EMI control technology
What are the typical methods to solve EMI related problems in power supply? First, EMI is a problem. This may seem obvious, but it can be time consuming to determine the specific situation, as it requires the use of EMI test rooms (not available everywhere) to quantify the electromagnetic energy generated by the power supply and to determine whether the electromagnetic energy meets the system’s EMI standards.
Assuming that EMI problems will be caused by power supply after testing, designers will face the process of reducing EMI through a variety of traditional correction strategies, including:
Achieve high efficiency in as little circuit board space as possible.
Good thermal performance.
Layout optimization: careful power layout is as important as selecting the right power components. The successful layout depends to a large extent on the level of experience of the power supply designer. Layout optimization is essentially an iterative process, and an experienced power supply designer can help minimize the number of iterations, thereby avoiding delays and additional design costs. The problem: insiders often don’t have that experience.
Buffers: some designers plan ahead and provide space for simple buffer circuits (from switching nodes to simple RC filters for GND). This can inhibit the ringing of the switching nodes (a factor that generates EMI), but this technique can lead to increased wear and tear, which has a negative impact on efficiency.
Reducing edge rate: reducing the ringing of the switch nodes can also be achieved by reducing the clamping pendulum rate of gate conduction. Unfortunately, like buffers, this can have a negative impact on the overall system’s efficiency.
Spread frequency (SSFM) : many ADI Power by Linear? Switching regulators are provided with this feature, which helps the product design pass the stringent EMI testing standards. Using SSFM technology, the clock of the driving switch frequency is modulated within a known range (e.g., the range of programming frequency fSW up and down ±10%). This helps distribute peak noise energy over a wider range of frequencies.
Filters and shielding: filters and shielding always take up a lot of cost and space. They also complicate production.
All of these constraints can reduce noise, but they are also flawed. Minimizing noise in a power supply design usually solves the problem completely, but it is difficult to achieve. ADI’s Silent Switcher? The Silent Switcher 2 voltage regulator implements low noise on the voltage regulator side, eliminating the need for additional filtering, shielding, or massive layout iterations. By not having to use expensive countermeasures, it speeds up the time to market and saves a lot of cost.
Minimize the current loop
In order to reduce EMI, the thermal circuit (high di/dt circuit) in the power supply circuit must be identified and its impact reduced. The thermal circuit is shown in figure 2. During one cycle of a standard step-down converter, when M1 is off and M2 is on, ac current flows along the blue circuit. During the closing cycle when M1 is on and M2 is off, the current flows along the green circuit. The circuit that generates the highest EMI is not completely intuitive. It is neither a blue circuit nor a green circuit, but a purple circuit that conducts the full switching ac current (switching from zero to IPEAK and back again to zero). This circuit is called a thermal circuit because it has the highest ac and EMI energy.
It is the high di/dt and parasitic inductance in the thermal circuit of the switching voltage regulator that cause the electromagnetic noise and the switch ringing. To reduce EMI and improve functions, the radiation effect of the purple loop should be minimized. The electromagnetic radiation disturbance of the thermal circuit increases with the increase of its area. Therefore, if possible, reducing the PC area of the thermal circuit to zero and using the ideal capacitance of zero impedance can solve this problem.
Figure 2. Thermal circuit of buck converter.
Use a Silent Switcher regulator to achieve low noise
Magnetic field offset
Although it is impossible to completely eliminate the thermal circuit region, we can divide the thermal circuit into two circuits with opposite polarity. This effectively creates local magnetic fields that cancel each other out at any point in the distance from the IC. This is the concept behind the Silent Switcher voltage regulator.
Figure 3. Magnetic field cancellation in a Silent Switcher voltage regulator.
Flip chips replace bonding wires
Another way to improve EMI is to shorten the wires in the thermal circuit. This can be done by abandoning the traditional bonding method of connecting the chip to the encapsulated pin. Inside the package, flip the silicon chip and add the copper column. The range of the thermal circuit can be further narrowed by shortening the distance from the internal FET to the encapsulating pin and input capacitance.
Figure 4. Disassembly diagram of LT8610 bonding line.
Figure 5. Flip chip with copper column.
Silent Switcher and Silent Switcher 2
Figure 6. A typical Silent Switcher application schematic and its appearance on a PCB.
Figure 6 shows a typical application using a Silent Switcher regulator that can be identified by symmetrical input capacitors on two input voltage pins. Layout is important in this scenario, because the Silent Switcher technique requires that these input capacitors be arranged as symmetrically as possible to take advantage of counterbalanced fields. Otherwise, the advantages of Silent Switcher technology will be lost. The question, of course, is how to ensure the correct layout in the design and throughout the production process. The answer is a Silent Switcher 2 regulator.
Silent Switcher 2
A Silent Switcher 2 regulator can further reduce EMI. By integrating the capacitors (VIN, INTVCC, and boost capacitors) into the LQFN package, the sensitivity of EMI performance to PCB layout is eliminated so that it can be placed as close to the pin as possible. All thermal loops and connections are internal, minimizing EMI and reducing the total plate area of the solution.
Figure 7. Block diagram of the Silent Switcher application and the Silent Switcher 2 application.
Figure 8. Unsealed LT8640S Silent Switcher 2 voltage regulator.
Lighter Module Silent Switcher regulator
Leverage the knowledge and experience you gain from developing a Silent Switcher product portfolio, and use the existing ubiquitous mobile Module? The product portfolio makes the power supply products we provide easy to design and meet certain important requirements of the power supply, including thermal performance, reliability, accuracy, efficiency and good EMI performance.
The LTM8053 shown in figure 9 integrates two input capacitors for magnetic field cancellation with some other passive components required for the power supply. All of this is accomplished with a 6.25mm × 9mm × 3.32mm BGA package, allowing the customer to focus on the design of the rest of the circuit board.
No need for LDO voltage stabilizer — power supply case study
A typical high-speed ADC requires many voltage rails, some of which must be very low noise to achieve the highest performance in the ADC data sheet. In order to strike a balance between high efficiency, small plate space, and low noise, the generally accepted solution is to combine the switching power supply with the LDO rear voltage stabilizer, as shown in figure 10. The switching regulator can achieve a higher buck ratio with higher efficiency, but the noise is relatively large. Low noise LDO rear voltage regulators are relatively inefficient, but they can suppress most of the conduction noise generated by switching voltage regulators. Reducing the step-down ratio of the LDO rear regulator as much as possible is helpful to improve efficiency. This combination produces a clean power supply that enables the ADC to run at maximum performance. The problem is that multiple regulators make the layout more complex, and LDO rear regulators may cause heat dissipation problems at higher loads.
Figure 10. Typical power supply design for AD9625 ADC.
The design shown in figure 10 clearly requires some trade-offs. In this case, low noise is a priority, so efficiency and circuit board space must be compromised. But maybe not. The latest Silent Switcher light-up Module combines the low-noise switching regulator design with the light-up Module package, enabling easy design, high efficiency, small size and low noise. These regulators not only minimize the amount of space taken up by the circuit board, but also realize extensibility by using a light-bulb regulator to power multiple voltage rails, further saving space and time. Figure 11 shows the power tree alternative that USES the LTM8065 Silent Switcher light Module stabilizer to power an ADC.
Figure 11. A space-saving solution that USES the Silent Switcher light Module stabilizer to power AD9625.
These designs have been tested and compared with each other. ADI recently published an article that tested and compared ADC performance using the power supply design shown in figure 10 and figure 11. The test consists of the following three configurations:
Standard configuration for ADC power supply using switching voltage regulator and LDO voltage regulator.
LTM8065 is used to power the ADC directly without further filtering.
Further purify the output using the LTM8065 and additional output LC filters.
The measured SFDR and SNRFS results show that LTM8065 can be used to power ADC directly without affecting ADC performance.
The core advantage of this implementation is that it greatly reduces the number of components, thereby increasing efficiency, simplifying production and reducing the space occupied by the circuit board.
In summary, as more system-level designs need to meet more stringent specifications, it is critical to make the most of modular power supply designs, especially when professional power design experience is limited. Because many market segments require that the system design must meet the requirements of the latest EMI specification, applying Silent Switcher technology to small size design and using the easy to use feature of the light-user Module regulator can greatly reduce the time to market and save circuit board space.
The advantages of Silent Switcher mobile Module regulators
Save PCB layout design time (no need to redesign the circuit board to solve the noise problem).
No extra EMI filter (save space cost of components and circuit boards).
It reduces the requirement of internal power supply expert to debug the power supply noise.
Provides high efficiency over a wide range of operating frequencies.
It is not necessary to use LDO rear voltage regulator when supplying power to noise-sensitive devices.
Shorten the design cycle.
Achieve high efficiency in as little circuit board space as possible.
Good thermal performance.