“Limited and shrinking board space, tight design cycles, and stringent electromagnetic interference (EMI) specifications such as CISPR 32 and CISPR 25 are limiting factors in producing power supplies with high efficiency and good thermal performance. As design cycles often push power supply designs toward the end of the design flow, things get more complicated — a recipe for frustration as designers try to squeeze complex power supplies into more compact locations.
Authors: Bhakti Waghmare, Diarmuid Carey
Limited and shrinking board space, tight design cycles, and stringent electromagnetic interference (EMI) specifications such as CISPR 32 and CISPR 25 are limiting factors in producing power supplies with high efficiency and good thermal performance. As design cycles often push power supply designs toward the end of the design flow, things get more complicated — a recipe for frustration as designers try to squeeze complex power supplies into more compact locations. Performance suffers to complete the design on time, pushing the can to testing and validation. Traditionally, simplicity, performance, and number of solutions have been at odds: prioritize one or two required features over a third—especially when design deadlines are looming. Sacrifices are accepted as normal; they shouldn’t.
This article begins with an overview of an important issue posed by power supplies in complex Electronic systems: EMI, often referred to simply as noise. Power sources generate it and must be addressed, but what are the sources and what are typical mitigation strategies? This article presents strategies for reducing EMI, proposing a solution to reduce EMI, maintain efficiency, and fit the power supply into a limited solution volume.
What is Electromagnetic Interference?
Electromagnetic interference is an electromagnetic signal that interferes with system performance. This disturbance affects circuits through electromagnetic induction, electrostatic coupling or conduction. This is a key design challenge for automotive, medical, and test and measurement equipment manufacturers. Many of the constraints mentioned above and the ever-increasing performance demands on power supplies—increased power density, higher switching frequencies, and higher currents—only amplify the impact of EMI, so solutions are needed to reduce it. In many industries, EMI standards must be met and, if not considered early in the design cycle, can significantly impact time-to-market.
EMI coupling type
EMI is a problem in electronic systems when the source of interference couples with the receiver (that is, some component in the electronic system). EMI is classified by its coupling medium: conducted or radiated.
Conducted EMI (low frequency, 450 kHz to 30 MHz)
Conducted EMI is conducted to components through parasitic impedances and power and ground connections. Noise is transmitted to another device or circuit by conduction. Conducted EMI can be further divided into common mode noise or differential mode noise.
Common mode noise is conducted through parasitic capacitance and high dV/dt (C × dV/dt). It follows a path from any signal (positive or negative) through parasitic capacitance to GND, 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 noise that is wirelessly transmitted to the device under test through magnetic energy. In switching power supplies, noise is the result of high di/dt plus parasitic inductance. This radiated noise can affect nearby equipment.
EMI Control Technology
What is the typical way to troubleshoot EMI related problems in power supplies? First, determine that EMI is a problem. This may seem obvious, but acquiring this knowledge can be time-consuming as it requires the use of EMI chambers (not available in every corner) to quantify the electromagnetic energy produced by the power supply and whether it is adequately compliant with the system.
Assuming that after testing, the power supply will have EMI issues, we will be faced with reducing it through a number of traditional correction strategies, including:
High efficiency with minimal board area.
good thermal properties.
Layout Optimization: Careful power supply layout is as important as choosing the correct components for the power supply. A successful layout depends largely on the experience level of the power supply designer. Layout optimization is iterative in nature, and an experienced power supply designer can help minimize the number of iterations, avoiding time delays and additional design costs. Here’s the thing: this experience isn’t common within companies.
Snubbers: Some designers plan ahead and provide packages for simple snubber circuits (simple RC filters from switch node to GND). This suppresses switch node ringing (a factor in EMI), but this technique results in increased losses that negatively impact efficiency.
Reduced edge rate: Reduced switch node ringing can also be achieved by reducing the gate turn-on slew rate. Unfortunately, like the snubber, this negatively affects overall system efficiency.
Spread Spectrum Frequency Modulation (SSFM): Implemented as an option in many Analog Devices Power by Linear™ switching regulators, this feature helps designs pass stringent EMI testing standards. In SSFM, the clock used to drive the switching frequency is modulated within a known range (eg, ±10% variation around the programmed fSW). This helps spread the peak noise energy over a wider frequency range.
Filters and Shields: Filters and shields are always expensive in money and space. They also complicate production.
All of the above contingencies can reduce noise, but they all have drawbacks. Minimizing noise in a power supply design is often the cleanest path, but it is difficult to achieve. ADI Silent Switcher® and Silent Switcher 2 regulators achieve low noise on the regulator without additional filtering, shielding, or significant layout iterations. Avoiding costly countermeasures can speed time-to-market and result in significant cost savings.
Minimize current loops
To reduce EMI, hot loops (high di/dt loops) in power circuits must be identified and their effects reduced. The thermal loop is shown in Figure 2. During one cycle of a standard buck converter, AC flows through the blue loop, M1 is closed and M2 is open. During the off cycle when M1 is on and M2 is off, current flows through the green loop. It’s not entirely intuitive that the loop producing the highest EMI is neither the blue loop nor the green loop — only the purple loop conducts a fully switched AC, switching from zero to IPEAK and back to zero. This loop is called the hot loop because it has the highest AC and EMI energy.
High di/dt and parasitic inductance in the switch hot loop can cause electromagnetic noise and switch ringing. To reduce EMI and improve functionality, the radiated effects of the purple loop need to be minimized. The radiated emission of a hot loop increases with its area, so if possible, reducing the PC area of the hot loop to zero and using an ideal capacitor with zero impedance can solve the problem.
Figure 2. Buck converter thermal loop.
Low Noise Using Silent Switcher Regulators
It is impossible to reduce the thermal cycle area to zero, but we can divide the thermal cycle into two cycles of opposite polarity. This effectively contains the local magnetic fields, which effectively cancel each other out at any distance from the IC. This is the concept behind Silent Switcher regulators.
Figure 3. Magnetic cancellation in a Silent Switcher regulator.
Flip chip replaces wire bonding
Another way to improve EMI is to shorten the wires in the hot loop. This can be done by eliminating the traditional wire bonding method of connecting the die to the package pins. In the package, the silicon was flipped and copper pillars were added. This further reduces the area of the hot loop by reducing the distance from the internal FET to the package pins and input capacitors.
Mute Switcher and Mute Switcher 2
Figure 6. Typical Silent Switcher application schematic and its appearance on the PCB.
Figure 6 shows a typical application using a Silent Switcher regulator, identified by the symmetrical input capacitors on the two input voltage pins. Layout is important in this scheme because Silent Switcher technology requires these input capacitors to be placed as symmetrically as possible to provide mutual field cancellation benefits. Otherwise, the benefits of Silent Switcher technology will be lost. Of course, the question is how to make sure the design and layout throughout the production process are correct? The answer is the Silent Switcher 2 regulator.
mute switcher 2
Silent Switcher 2 regulators further reduce EMI. By integrating capacitors into the LQFN package (VIN cap, INTVCC, and boost caps), EMI performance sensitivity to PCB layout is eliminated, allowing placement as close to the pins as possible. All thermal loops and ground planes are internal, which minimizes EMI and results in a smaller overall solution footprint.
Figure 7. Silent Switcher application and Silent Switcher 2 application diagram.
Figure 8. LT8640S Silent Switcher 2 regulator with cover removed.
Silent Switcher 2 technology also improves thermal performance. The large multi-ground exposed pad on the LQFN flip-chip package helps to extract heat from the package into the PCB. Higher conversion efficiency also stems from the elimination of high-resistance bond wires. When testing EMI performance, the LT8640S passed the CISPR 25 Class 5 peak limit with a large margin.
µModule Silent Switcher Regulator
Leveraging the knowledge and experience gained while developing the Silent Switcher portfolio, and combining it with the already large µModule® product portfolio, allows us to provide power products that are easy to design while meeting some of the most important metrics for power supplies – thermal, Reliability, accuracy, efficiency and excellent EMI performance.
Figure 9 shows that the LTM8053 integrates two input capacitors, allowing magnetic field cancellation, as well as many other passive components required for the operation of this supply. All of this is implemented in a 6.25 mm × 9 mm × 3.32 mm BGA package, which allows customers to focus on other areas of board design.
Figure 9. LTM8053 Silent Switcher exposed die and EMI results.
LDO Regulators No Longer Needed – Power Supply Case Study
A typical high-speed ADC requires multiple voltage rails, some of which must be very quiet to achieve the highest performance listed in the ADC’s data sheet. A generally accepted solution to achieve the balance of high efficiency, small board area, and low noise is to combine a switching power supply with an LDO post-regulator, as shown in Figure 10. Switching regulators can achieve relatively high step-down ratios with high efficiency, but are relatively noisy. The low-noise LDO post-regulator is relatively inefficient, but it suppresses most of the conducted noise generated by switching regulators. Minimizing the step-down ratio of the LDO post-regulator helps improve efficiency. This combination produces a clean power supply that allows the ADC to operate at the highest level of performance. The problem lies in the complex layout of many regulators,
Figure 10. Typical power supply design for powering the AD9625 ADC.
In the design shown in Figure 10, several tradeoffs are apparent. In this case, low noise is a priority, so efficiency and board space must suffer. Or maybe not. The latest generation of Silent Switcher µModule devices combine a low-noise switching regulator design with a µModule package – a hitherto unattainable combination of simple design, high efficiency, compact size and low noise. These regulators minimize board area, but also enable scalability – a single µModule regulator can power multiple voltage rails, further saving area and time. Figure 11 shows an alternative power tree for powering the ADC using the LTM8065 Silent Switcher µModule regulator.
Figure 11. Space-saving solution for powering the AD9625 using a Silent Switcher µModule regulator.
These designs have been tested against each other. A recent ADI article tested and compared the performance of ADCs designed using the power supplies in Figure 10 and Figure 11. 1 Three configurations were tested:
Standard configuration using switching regulator and LDO regulator to power ADC.
The ADC is powered directly using the LTM8065 without further filtering.
The output is further cleaned up using the LTM8065 which adds an output LC filter.
The measured SFDR and SNRFS results show that the LTM8065 can be used to power the ADC directly without affecting the ADC performance.
The core benefit of this implementation is a significant reduction in component count, resulting in increased efficiency, easier production, and reduced board area.
In conclusion, as we see a shift towards more system-level designs with tighter specifications, it is important to use modular power supply designs where possible, especially when power supply design expertise is minimal. As many market segments require system designs to pass the latest EMI specifications, the use of Silent Switcher technology is integrated into a small form factor, and the ease of use of µModule regulators can greatly reduce your time-to-market while saving board area.
Benefits of Silent Switcher µModule Regulators
Save PCB layout design time (no need to re-spin the board to correct for noise issues).
No need for additional EMI filters (saving component and board area costs).
Reduces the need for in-house power experts to debug power supply noise.
High efficiency is achieved over a wide operating frequency range.
No LDO post-regulator is required when powering noise-sensitive devices.
Shorten the design cycle.
High efficiency with minimal board area.
good thermal properties.
The Links: W25Q128FVSIG G084SN03-V8 IGBTMODULE