Power management: more than just designing a power supply

No matter what type of product electronic engineers are designing, power management has become one of the most pressing challenges they face. From designing a single battery pack for electric vehicles to achieve the maximum mileage, to the smallest battery-powered IoT sensor, by extending battery life to maintaining the operational efficiency of the factory, these are all vital. Power is no longer just a set of static power rails that must be designed. Today's power architects must adapt to rapidly changing load conditions, provide transient-free power rails to achieve strict tolerances, and try to fit all devices into one space increasingly restricted in the shell. In this technical article, we - China PCBA manufacturer - SysPCB will focus on some of the important challenges faced by power architects, focusing on managing converter noise, production and certification challenges, and the need to further reduce PCB size.

Introduction

Nowadays, there are a variety of power requirements that need to be addressed. It is not only necessary to consider a wide range of available energy sources, such as solar energy, energy harvesting technology, batteries, power over Ethernet, inductive power, line power, etc., but also need to consider the specification for each power rail. Increasingly complex semiconductor innovations have created equally diverse power budget requirements, from energy-harvesting ultra-low-power wireless SoC devices to high-current, sequencing multiple power rails for computationally intensive FPGAs and inference processors.

Deal with transients and EMI

Transients can appear on the power rail from a variety of sources. High dv/dt switching (such as the method used in industrial motor drives) is a common cause of large transients. If not suppressed by a filter composed of passive components, these transients may cause permanent damage to the switching transistor and its related drivers and circuits. Many power supplies use switching topologies such as buck, boost, or buck-boost to convert the input power to the desired output voltage. Although this power conversion method is popular, efficient, and well-proven, but the switching process itself generates electromagnetic interference (EMI), which is induced to the power rail and radiates out. 

Traditional filtering techniques can be used to deal with switching transients on the power rail, but, as we will discuss, for some sensitive monitoring applications, transients can still interfere with the normal operation of the circuit. Radiated noise will bring higher circuit design complexity and potential additional costs. For example, it may be necessary to shield the surroundings of the converter circuit with metal or metal foil, which requires additional production processes and increases component costs. Many switching regulator ICs have a fixed switching frequency of 1.5 to 1.8 MHz, which is the top of the AM broadcast radio frequency band. In certain application scenarios such as car infotainment system receivers, it may cause some trouble. Another method is to select a switching frequency of the device that is unlikely to cause problems.

Even with the best filtering technology, but the smallest interference from the switching converter can still affect sensitive measurement results, such as in patient vital signs monitors or applications for test and measurement purposes. A good device suitable for such applications is Texas Instruments TPS62840, which is a 1.8-6.5V, 750 mA step-down converter. The device has an extremely low quiescent current of 60nA, and the converter can be temporarily stopped by using the STOP pin to eliminate any switching noise. The holding capacitor at the output of the converter is used to power the specific application, so it can continue to work without being affected by any noise (see Figure 1). This technology can be used not only to achieve sensitive measurement functions, but also to improve the signal-to-noise ratio when the wireless link conditions are marginal.

Figure 1: Description of the STOP function of the Texas Instruments TPS62840 step-down converter. (Source: Texas Instruments)

Reduce design volume

The available space that can accommodate electronic product systems is shrinking, whether for industrial automation equipment or compact consumer electronics. The area of the factory workshop is very precious, and all the control equipment usually used for specific production tasks need to be compressed into a single control cabinet. Many power supply designers and engineering teams are considering using a module-based approach to configure power supplies. The electronics industry is no stranger to discrete device solutions and modular solutions, and of course power management is no exception. 

In addition to achieving a higher degree of functional integration, the module also has the advantage of shortening the time to market and eliminating the need for more and more professional power supply designers within the engineering team and the obstacles that they bring. For example, the DC-DC converter has long been a densely packaged device that meets the industry standard size. Power module design engineers are not only good at integrating switching controller ICs into compact modules, but also integrating many related components. BOM cost and heat dissipation characteristics have been optimized. Texas Instruments takes this concept a step further by integrating one of the larger components (the inductor) into the module in the design. The size of the TPSM82822 module is only 2.0 x 2.5 x 1.1 mm, and it is constructed using the industry standard 10-pin MicroSIP package format. These synchronous pulse-width modulation (PWM) mode step-down converters are available in 1A and 2A versions with power saving mode To improve light load efficiency, the typical quiescent current can be as low as 4µA. The module can tolerate an input voltage of 2.4 ~ 5.5VDC, and provide an adjustable output voltage of 0.6 ~ 4VDC, the operating efficiency is usually as high as 95%.

To help prototype design based on TPSM82822, an evaluation board TPSM82822EVM is now available, as shown in Figure 2.

Figure 2: Texas Instruments TPSM82822EVM evaluation board for the high-efficiency step-down converter module TPSM82822 with integrated inductor. (Source: Texas Instruments)

Challenges in production testing and inspection

Many conventional switching converter ICs are manufactured in the industry standard QFN package. Although this is a convenient format used for a long time, it is not suitable for the appearance inspection technology required by the automotive industry during the assembly process, see Figure 3. As can be seen from the left, top and bottom of the figure, the solder joints on the standard QFN package are often located on the PCB under the device and are not visible. 

Only a small amount of solder can be directly seen from the side, which poses a challenge to the visual inspection test. Are the components underneath fully soldered or are there dry joints? In order to resolve this uncertainty, Texas Instruments has developed an enhanced QFN package that incorporates a plated cavity carved from the side of the package, thereby increasing the area of the solder joint can be visually inspected. These "wettable flanks" (see Figure 3 on the right) can provide a larger, easier-to-see solder joint, which eliminates the question of whether the device has been completely and reliably soldered to the PCB.

Figure 3: The wettable flanks of Texas Instruments TPS62810 automotive-grade step-down converter. (Source: Texas Instruments)

Figure 4: TPS62810-Q1 and TPS62810-EP. (Source: Texas Instruments)

Meet safety power supply design certification requirements

With the deployment of industrial IoT applications and other factory automation programs, electronic systems are increasingly used in industrial and commercial equipment. Since some working environments may contain dangerous or explosive liquids or gases, the equipment in them needs to comply with mandatory safety standards. For power engineers, this is a technical challenge, because the power conversion process usually generates a certain degree of heat, and depending on the specific application, the voltage may arc between components, and the components may explode under fault conditions.

ATEX directive can minimize or eliminate the risk of fire in hazardous environments. For any given gas, vapor or mist hazard, it can be classified according to three different areas. In the power IC environment, possible ignition sources are considered to be electric sparks and high surface temperatures. For example, for a smart gas meter, the maximum temperature allowed is 244°C or 275°C depending on the package size. The design adopts a converter IC with a larger pitch and leads, even in a humid environment, it helps to reduce the possibility of electrical stress and sparks. Another requirement is that a package with efficient heat dissipation can be selected to prevent the device from reaching the maximum allowable temperature. Texas Instruments' TPS62840 uses a high thermal conductivity HVSSOP-8 package with a size of 3 x 5 mm. It uses a copper plate bonded to the IC substrate, which can dissipate all the heat of the main body IC, so it does not exceed the maximum temperature.

In conclusion

Power management is a rapidly changing field, and power is the core of every design. The most important goal of power configuration is not to affect the performance of specific applications.