“Limited and shrinking circuit board space, tight design cycles, and strict electromagnetic interference (EMI) specifications (such as CISPR 32 and CISPR 25) these limiting factors have all made it difficult to obtain power supplies with high efficiency and good thermal performance. In the entire design cycle, the power supply design is usually basically in the final stage of the design process, designers need to work hard to squeeze the complex power supply into a more compact space, which makes the problem more complicated and very frustrating. In order to complete the design on time, we can only make some concessions in terms of performance, and leave the problem to the test and verification link for processing.Simple, high performance and solution size three
Authors: Bhakti Waghmare and Diarmuid Carey
Limited and shrinking circuit board space, tight design cycles, and strict electromagnetic interference (EMI) specifications (such as CISPR 32 and CISPR 25) these limiting factors have all made it difficult to obtain power supplies with high efficiency and good thermal performance. In the entire design cycle, the power supply design is usually basically in the final stage of the design process, designers need to work hard to squeeze the complex power supply into a more compact space, which makes the problem more complicated and very frustrating. In order to complete the design on time, we can only make some concessions in terms of performance, and leave the problem to the test and verification link for processing. The three considerations of simplicity, high performance, and solution size often conflict with each other: Only one or two can be given priority, and the third should be discarded, especially when the design deadline is approaching. It has become commonplace to sacrifice some performance; it shouldn’t be the case.
This article first outlines the serious problems caused by power supplies in complex Electronic systems: EMI, usually abbreviated as noise. The power supply will produce EMI, which must be solved, so what is the source of the problem? What are the usual mitigation measures? This article introduces strategies to reduce EMI and proposes a solution that can reduce EMI, maintain efficiency, and put the power supply in the limited solution space.
What is EMI?
Electromagnetic interference is an electromagnetic signal that can interfere with system performance. This interference affects the circuit through electromagnetic induction, electrostatic coupling or conduction. It is a key design challenge for automotive, medical, and test and measurement equipment manufacturers. Many of the above mentioned limitations and increasing power supply performance requirements (increased power density, higher switching frequency, and higher current) will only amplify the impact of EMI, so solutions are urgently needed to reduce EMI. Many industries require that EMI standards must be met. If they are not considered at the initial design stage, it will seriously affect the time to market of the product.
EMI coupling type
EMI is a problem generated when the interference source in the electronic system is coupled with the receiver (that is, some components in the electronic system). EMI can be classified as conducted or radiated according to its coupling medium.
Conducted EMI (low frequency, 450 kHz to 30 MHz)
Conducted EMI is conductively coupled to the components through parasitic impedance and power and ground connections. The noise is transmitted to another device or circuit by conduction. Conducted EMI can be further divided into common mode noise and differential mode noise.
Common mode noise is conducted through parasitic capacitance and high dV/dt (C × dV/dt). It transmits along the path of any signal (positive or negative) to GND through parasitic capacitance, as shown in Figure 1.
Differential mode noise is conducted through parasitic inductance (magnetic coupling) and high di/dt (L × di/dt).
Figure 1. Differential mode and common mode noise.
Radiated EMI (high frequency, 30 MHz to 1 GHz)
Radiated EMI is the noise that is transmitted wirelessly to the device under test through magnetic field energy. In a switching power supply, this noise is the result of high di/dt coupling with parasitic inductance. Radiated noise will affect neighboring devices.
EMI control technology
What are the typical methods to solve EMI-related problems in power supplies? First of all, determining EMI is a problem. This may seem obvious, but determining its specific conditions can be very time-consuming, because it requires the use of an EMI test room (not available everywhere) in order to quantify the electromagnetic energy generated by the power supply and determine whether the electromagnetic energy meets the EMI of the system standard requirement.
Assuming that after testing, the power supply will bring EMI problems, then the designer will face the process of reducing EMI through a variety of traditional correction strategies, including:
・ Layout optimization: Careful power layout is as important as choosing the right power components. The successful layout largely depends on the experience level of the power supply designer. Layout optimization is essentially an iterative process. Experienced power supply designers can help minimize the number of iterations, thereby avoiding time delays and additional design costs. The problem is: insiders often don’t have this experience.
・ X Buffer: Some designers plan ahead and provide a footprint for a simple buffer circuit (simple RC filter from switch node to GND). This can suppress the ringing phenomenon of the switching node (a factor that produces EMI), but this technique will increase the loss, which will have a negative impact on efficiency.
・X Reduced edge rate: Reducing the ringing of the switching node can also be achieved by reducing the gate turn-on slew rate. Unfortunately, similar to the buffer, this can have a negative impact on the efficiency of the overall system.
* Spread Spectrum (SSFM): Many ADI’s Power by Linear™ switching regulators provide this feature, which helps product designs pass strict EMI test standards. Using SSFM technology, the clock driving the switching frequency is modulated within a known range (for example, a ±10% variation range of the programming frequency fSW). This helps distribute the peak noise energy to a wider frequency range.
・ X filters and shields: Filters and shields always take up a lot of cost and space. They also complicate production.
All the above control measures can reduce noise, but they also have drawbacks. Minimizing the noise in the power supply design usually solves the problem completely, but it is difficult to achieve. ADI’s Silent Switcher® and Silent Switcher 2 regulators achieve low noise on the regulator side, eliminating the need for additional filtering, shielding, or a large number of layout iterations. Because it is not necessary to adopt expensive countermeasures, it speeds up the time to market and saves a lot of costs.
Minimize the current loop
In order to reduce EMI, it is necessary to determine the thermal loop (high di/dt loop) in the power circuit and reduce its impact. The thermal circuit is shown in Figure 2. In one cycle of a standard buck converter, when M1 is closed and M2 is open, AC current flows along the blue loop. In the off period when M1 is on and M2 is off, current flows along the green loop. The loop that produces the highest EMI is not completely visible. It is neither a blue loop nor a green loop, but a purple loop that conducts a fully switched AC current (switching from zero to IPEAK and then back to zero). This loop is called a thermal loop because it has the greatest AC 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 purple loop. The electromagnetic radiation disturbance of the thermal loop increases as its area increases. Therefore, if possible, reducing the PC area of the thermal loop to zero and using a zero impedance ideal capacitor can solve this problem.
Figure 2. Thermal circuit of a buck converter
Use Silent Switcher regulator to achieve low noise magnetic field cancellation
Although it is impossible to completely eliminate the thermal loop area, we can divide the thermal loop into two loops with opposite polarities. This can effectively form a local magnetic field, which can effectively cancel each other out at any position from the IC. This is the concept behind the Silent Switcher regulator.
Figure 3. Magnetic field cancellation in a Silent Switcher regulator.
Flip chip replaces bonding wire
Another way to improve EMI is to shorten the wires in the thermal loop. This can be achieved by abandoning the traditional bonding wire method of connecting the chip to the package pins. Flip silicon chips in the package and add copper pillars. By shortening the distance from the internal FET to the package pin and the input capacitor, the range of the thermal loop can be further reduced.
Figure 4. Schematic diagram of disassembly of LT8610 bonding wire.
Figure 5. Flip chip with copper pillars.
Silent Switcher and Silent Switcher 2
Figure 6. Typical Silent Switcher application schematic diagram and its appearance on the PCB.
Figure 6 shows a typical application using the Silent Switcher regulator, which can be identified by the symmetrical input capacitance on the two input voltage pins. The layout is very important in this scheme, because the Silent Switcher technology requires that these input capacitors be arranged symmetrically as much as possible in order to take advantage of the field cancellation. Otherwise, the advantages of SilentSwitcher technology will be lost. Of course, the question is how to ensure the correct layout in the design and the entire production process. The answer is the Silent Switcher 2 regulator.
Silent Switcher 2
Silent Switcher 2 regulator can further reduce EMI. By integrating the capacitor VIN capacitor, INTVCC and boost capacitor) into the LQFN package, the sensitivity of EMI performance to the PCB layout is eliminated, and it can be placed as close to the pin as possible. All thermal loops and ground planes are internal to minimize EMI and make the total board area of the solution smaller.
Figure 7. Silent Switcher application and Silent Switcher 2 application block diagram.
Figure 8. The unsealed LT8640S Silent Switcher 2 regulator.
Silent Switcher 2 technology can also improve thermal performance. The multiple large-size ground exposed pads on the LQFN flip-chip package help the package dissipate heat through the PCB. Eliminating high-resistance bonding wires can also improve conversion efficiency. In the EMI performance test, the LT8640S can meet the CISPR 25 Class 5 peak limit requirements and has a large margin.
µModule Silent Switcher Regulator
With the knowledge and experience gained from the development of the Silent Switcher product portfolio, and the use of the existing extensive µModule® product portfolio, the power supply products we provide are easy to design, while meeting certain important requirements for power supplies, including thermal performance and reliability. , Precision, efficiency and good EMI performance.
The LTM8053 shown in Figure 9 integrates two input capacitors for magnetic field cancellation and some other passive components required by the power supply. All of these are achieved through a 6.25 mm × 9 mm × 3.32 mm BGA package, allowing customers to concentrate on the design of other parts of the circuit board.
Figure 9. LTM8053 Silent Switcher bare chip and EMI results.
No need for LDO regulator-power supply case study
A typical high-speed ADC requires many voltage rails, some of which must have very low noise to achieve the highest performance in the ADC data sheet. In order to strike a balance between high efficiency, small board space and low noise, the generally accepted solution is to combine the switching power supply with an LDO post regulator, as shown in Figure 10. The switching regulator can achieve a higher step-down ratio with higher efficiency, but the noise is relatively large. The low-noise LDO post-regulator has relatively low efficiency, but it can suppress most of the conducted noise generated by the switching regulator. Minimizing the step-down ratio of the LDO post-regulator helps to improve efficiency. This combination can produce clean power, allowing the ADC to operate at maximum performance. But the problem is that multiple regulators will make the layout more complicated, and the LDO post regulator may cause heat dissipation problems under higher loads.
Figure 10. Typical power supply design for AD9625 ADC.
The design shown in Figure 10 obviously requires some trade-offs. In this case, low noise is a priority, so efficiency and board space must be compromised. But maybe it doesn’t have to be so. The latest generation of Silent Switcher µModule devices combines low-noise switching regulator design and µModule packaging to achieve the goals of easy design, high efficiency, small size and low noise at the same time. These regulators not only minimize the space occupied by the circuit board, but also achieve scalability. A µModule regulator can be used to power multiple voltage rails, further saving space and time. Figure 11 shows an alternative power tree that uses the LTM8065 Silent Switcher µModule regulator to power the ADC.
Figure 11. Using the Silent Switcher µModule regulator to power the AD9625, a space-saving solution.
These designs have been tested and compared with each other. An article recently published by Analog Devices tested and compared ADC performance using the power supply designs shown in Figure 10 and Figure 11. The test includes the following three configurations:
・A standard configuration that uses a switching regulator and LDO regulator to supply power to the ADC.
・ Use LTM8065 to directly power the ADC without further filtering.
・ Use LTM8065 and additional output LC filter to further purify the output.
The measured SFDR and SNRFS results show that LTM8065 can be used to directly supply power to the ADC without affecting the performance of the ADC.
The core advantage of this implementation is to greatly reduce the number of components, thereby improving efficiency, simplifying production and reducing the space occupied by the circuit board.
In short, as more system-level designs need to meet more stringent specifications, it is important to make full use of modular power supply designs as much as possible, especially when professional experience in power supply design is limited. Because many market segments require system design to comply with the latest EMI specifications, the Silent Switcher technology is applied to small-size designs, and with the easy-to-use characteristics of µModule regulators, the time to market for products can be greatly shortened, while saving Circuit board space.
Advantages of Silent Switcher µModule Regulator
・ Save PCB layout design time (no need to redesign the circuit board to solve the noise problem).
・ No need for additional EMI filters (saving components and board space costs).
・ Reduced the need for internal power supply experts to debug power supply noise.
・ Provide high efficiency in a wide operating frequency range.
・ No need to use LDO post regulator when powering noise-sensitive devices
・ Shorten the design cycle.
・ Achieve high efficiency in the smallest possible board space.
・ Good thermal performance.
1 Aldrick Limjoco, Patrick Pasaquian and Jefferson Eco. “Silent Switcher µModule regulator provides low-noise power supply for GSPS sampling ADC and saves half of the space” ADI, October 2018.
Bhakti Waghmare is currently a product marketing engineer for μModule regulators in the Power by Linear product division. He is based in Santa Clara, California, USA. She is responsible for the marketing support of μModule regulator power products. Bhakti joined ADI in 2018. She holds a bachelor’s degree in mechanical engineering and a master’s degree in industrial engineering from Wayne State University (located in Detroit, Michigan, USA).
Diarmuid Carey is an application engineer at the European Central Application Center, based in Limerick, Ireland. He has been an application engineer since 2008 and joined ADI in 2017 to provide design support for the Power by Linear product portfolio for customers in many European markets. He holds a bachelor’s degree in computer engineering from the University of Limerick.