关键词: power supply design, efficient power supplies, smart sensors, Industry 4.0, IO-Link, DC-DC converter
With tight schedules for their projects, industrial designers often have limited time to design their application's power supply. Proper power supply design, however, is no simple feat, as there are challenges in terms of addressing space constraints, electromagnetic interference (EMI), efficiency, and more. This application note highlights ways to overcome these challenges.
A similar version of this application note originally appeared in November 2018 in Electronic Design's Connected Devices Bootcamp.
Modern systems in application areas including industrial, networking, medical, and consumer have one key thing in common: they all need to collect, synthesize, and act upon data. Sensors used in the industrial internet of things (IIoT), factory automation, defense electronics, and in network infrastructure equipment, for example, have transformed electronic equipment as well as business processes with new intelligence. This intelligence, however, demands more power in continually shrinking spaces without impacting the thermal budget. This challenge makes conventional solutions unviable and complicated.
Designers are under constant pressure to get their products to market quickly, which limits their time to design the power supply. They also do not have much space in their designs to dissipate heat while also meeting shock, vibration, and electromagnetic interference (EMI) requirements. High temperature rises and larger power supplies limit space and opportunities for more product innovation. This application note discusses ways in which designers can power increasingly small equipment and sensors reliably for industrial applications without causing them to overheat.
With Industry 4.0 technologies, cyber-physical systems, the IIoT, cloud computing, and cognitive computing are all operating together to create a "smart factory." Within the smart factory structure, cyber-physical systems monitor physical processes that communicate and cooperate with each other and with humans in real time to make decentralized decisions. Smart sensors monitor physical processes with a higher level of intelligence.
In 2018, roughly 70 million sensors were used in factory automation applications, and 8 million (or 11%) of those were smart sensors with IO-Link® technology. The market for smart sensors with IO-Link is expected to grow to 20 million by 2022, according to analysts1.
Figure 1. IO-Link sensor market trend.1
Analog sensors feature a sensing element and a way to move the sensing data to the programmable logic controller (PLC) in an industrial environment. Data is often transferred unidirectionally (sensor to master only). Analog data communication is prone to noise, and the controller cannot diagnose, re-configure, or recalibrate the sensor directly.
As sensor technologies have advanced, sensor manufacturers have begun integrating more functionality into them, such as reducing noise susceptibility with the introduction of binary sensors. Binary sensors are governed by the IEC 60947-5-2 standard, but data is still limited to unidirectional communication from the sensor to the master. It also has no error control, and a technician is still needed on the factory floor to perform tasks such as manual calibration.
Figure 2. "Old-school" analog and binary sensors.
The IO-Link protocol has given rise to smart sensors, which better meet the demands of Industry 4.0. IO-Link technology enables intelligence at the edge of the factory floor, allowing bidirectional communication between sensors and the controller. As a result, the system can adjust, configure, and diagnose sensors in real time. While traditional factories are generally built and optimized for a single product, smart factories can adapt quickly to market demand fluctuations through efficient, on-the-fly reconfiguration. Real-time diagnostics enable predictive maintenance and also maximize factory uptime.
Figure 3. PLC with IO-Link sensor.
Smart sensors have some key features in common: IO-Link compliance, full-field configurability, real-time diagnostics, and small size (in order to be ubiquitous). These sensors should have intuitive programming to simplify their initial setup and help eliminate logic errors. They should also be fully configurable to reduce device inventory and to support multiple profiles to facilitate flexible manufacturing.
There are some challenges to be aware of when designing smart sensors. All of those smart features come with increased power dissipation at a time when manufacturers are moving toward miniaturization of the components in their systems. So, powering these smart sensors requires overcoming challenges pertaining to heat and size.
As an example, take a look at a smart proximity sensor with IO-Link. Figure 4a provides the sensor block diagram. The microcontroller (MCU) collects data from the sensing element, linearizes and calibrates it, and sends it to the IO-Link transceiver, where the data is then sent to the system PLC. The IO-Link connector also provides 24V to power the sensor.
Figure 4a. A smart proximity sensor with IO-Link.
To facilitate our power design discussion, consider the following:
See Figure 4b and Figure 4c.
Figure 4b. A smart proximity sensor with IO-Link.
Figure 4c. A simplified block diagram.
Figure 5 shows a traditional power solution using an low drop-out linear regulator (LDO).
Figure 5. LDO - Traditional sensor power solution.
Old-school analog sensor circuitry typically consumes about 15mA. The 24V industrial power rail can reach 30VDC maximum. The power dissipations are as follows:
In this example, only 75mW is used to do real work (i.e., powering the sensor circuitry), while 375mW is lost in the LDO due to its inefficiency. The total power that our device must dissipate is 450mW.
Increasing the intelligence of the sensor calls for more features, which demands more current at the expense of the device's power dissipation. If the sensor circuitry current increases to 30mA, using the provided power dissipation calculations the following is calculated:
900mW exceeds the power dissipation limit of most small proximity sensors.
Heat presents the biggest challenge for sensor designs. Figure 6 shows power dissipation figures of the last two examples where LDO power dissipation dominates. The LDO's excessive power dissipation limits how much current the sensor circuitry can have, which, in turn, inhibits innovation. At 15mA sensor current, the power dissipation barely fits within an M8 connector size, which has about a 450mW thermal dissipation limit. At 30mA sensor current, a larger M12 still cannot tolerate the heat dissipation.
Figure 6. A comparison of power dissipation with sensor thermal capability.
Clearly, a more efficient power solution is required in order to meet the thermal dissipation limit. Figure 7 shows a DC-DC converter solution. At 15mA sensor current, and with a conservative 75% efficiency, the DC-DC converter has only 25mW of power loss and helps reduce the total device power loss from 450mW to 100mW, a 4.5x reduction in power dissipation.
Figure 7. A comparison of power supply dissipation in an LDO and a DC-DC converter.
Repeating the same calculation for 30mA sensor current, we can see that the high efficiency of the DC-DC converter reduces heat and enables more sensor current. As such, the sensor can become smarter, supporting more circuitry, features, and innovation, as shown in Figure 8.
Figure 8. High-efficiency DC-DC converter reduces heat and enables more sensor current and innovation.
Smart systems require a larger number of smart sensors, which must be small so that they can be ubiquitous. Even though they must fit in increasingly smaller spaces, smart sensors also contain more circuitry than their analog counterparts. Figure 9 shows an M12 connector on the left, which has an outer diameter of 12mm and a PCB board maximum width of 10.5mm. While this M12 connector can fit a typical DC-DC converter solution, a smaller size (M6) shown on the right has only a 4.5mm PCB board width. Accommodating a power supply solution in such a small space is a real challenge. With its high level of integration, Maxim's MAXM17532 uSLIC™ power module, which operates from 4.0V to 42V input voltage and is available in a 2.6mm x 3mm x 1.5mm solution size, fits very well into this miniature sensor.
Figure 9. MAXM17532 DC-DC converter power module fits into tiny sensor packages.
The MAXM17552, another part in the uSLIC family, can operate up to 60V input voltage. Figure 10 shows the MAXM17532/MAXM17552 typical application circuit schematic.
Figure 10. MAXM17532/MAXM17552 typical application circuit schematic.
The smart sensors demanded by Industry 4.0 applications require higher power, but also need to fit into smaller spaces. Traditional power solutions using LDOs create heat problems because of their excessive dissipation. Heat and size are, indeed, two major challenges facing sensor designers. The solution lies in a power supply that features highly efficient and miniaturized components. With advancements in modern DC-DC converter integration, a highly integrated DC-DC power module provides an ideal solution.
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应用笔记 7044,AN7044, AN 7044, APP7044, Appnote7044, Appnote 7044