Power Line Communication Implementation for DC Applications

Introduction

Power line communication (PLC) is a communication technology that sends data over existing power cables. The technology uses half-duplex to transmit power and data between PLC nodes. Because power and data can be transmitted simultaneously over the same line, PLC technology eliminates the need for additional wiring to interconnect devices. PLC can provide a low-cost communication medium for a wide range of applications, fully meeting the needs of environments where it may be too expensive to network using other technologies. As a communication technology, PLC can be divided into two categories:

● Broadband PLC is suitable for high-speed broadband network connections such as the Internet. It generally operates at higher frequencies (1.8 to 250MHz)

and high data rates (up to hundreds of Mbps), mostly for shorter distance applications.

● Narrowband PLC is suitable for applications that require narrowband control or low-bandwidth data acquisition, where low cost and high reliability are important. It generally operates at lower frequencies (3 to 500KHz) and lower data rates (hundreds of kbps), with a larger coverage range (up to several kilometers), which can be extended by repeaters.

PLC can be further subdivided into AC power line PLC and DC power line PLC based on the underlying power line characteristics.

Many utilities around the world have chosen AC line narrowband PLC for their smart grid projects. They monitor power usage by day or even by device or application, and offer pricing structures that incentivize consumers to adjust their usage, thereby reducing peak loads and avoiding the construction of new power plants. The

widespread adoption of PLC in smart grid applications has led to a high level of attention for AC power line PLC. However, DC power line narrowband PLC is also emerging in home networking, lighting and solar applications, and transportation vehicles (electronic controls in airplanes, cars, and trains). Using PLC in these applications can reduce wiring complexity, weight, and ultimately the cost of communications.

In this article, we will focus on the use of DC power line PLC and provide reference designs that can help customers quickly and effectively adopt this technology.

System integrators often ask the question: How do you compare the advantages and disadvantages of DC power line PLC and low-power wireless technology? Although neither DC line PLC nor low-power wireless requires new wiring, when using PLC, the connection is either underground, through walls, or in corners. The communication channel is owned by the operator or utility, so there is no risk of sharing bandwidth. PLC has no line of sight and is not affected by weather.

High flexibility of DC PLC solutions

Developing an effective PLC solution is challenging. Power lines are typically noisy, requiring a robust system architecture to ensure data reliability. Each end application and operating environment is different, so a flexible design is needed to adapt to a variety of conditions. System designers need a highly flexible platform that not only helps them optimize the design according to the specific requirements of each application, but also allows the design to adapt to new standards and new market opportunities that emerge in the future. In this way, intellectual property can be reused in multiple applications, accelerating development and product launch, and continuously expanding market opportunities.

The key to achieving high flexibility is the modular architecture of hardware and software. By splitting a complex PLC system into multiple independent subsystems, developers can modify an aspect of the design (such as the modulation scheme or network protocol used) without having to completely redesign the entire system. Some examples of various achievable applications include:

● Modulation scheme: High flexibility at the hardware and software level helps developers implement the most efficient modulation scheme for a specific application. For example, several modulation schemes are provided for narrowband communications, including spread frequency shift key (S-FSK) and orthogonal frequency division multiplexing (OFDM) modulation;

● Communication protocol: To achieve interoperability, devices may need to comply with specific protocol standards. Developers can easily implement common PLC standards (including S-FSK (IEC61334), PRIME and G3) or custom protocols that meet the specific needs of their applications by using a highly flexible platform.

Figure 1: Comparison of PLC communication protocols

Existing standards have few provisions for narrowband DC PLC applications, and in many applications the network is standalone. In these cases, a simpler communication protocol stack can be used. A specific example of such a simpler proprietary protocol stack is PLC-Lite from Texas Instruments (TI) (Figure 1). This protocol stack is particularly suitable for low-cost environments and applications that do not require the complexity of G3 and PRIME, but still require a robust communication channel. PLC-Lite

is an ideal solution for simple light bulbs or wall switches in building networks that only require a few Kbps. PLC-Lite not only provides a maximum data rate of 21Kbps, but also supports full-band and half-band modes. It provides higher robustness against certain types of interference, including narrowband interference that can affect G3 links. PLC-Lite includes a simple carrier sense multiple access with collision avoidance (CSMA/CA) media access control (MAC) layer that can be integrated with any application protocol stack.

Due to its simple structure and low data rate, PLC-Lite has a significantly lower implementation cost per link. It also provides great flexibility, allowing designers to customize channel links outside the limitations of industry standards. Figure 2 is an overview of the complete feature set of PLC-Lite.

Figure 2: TI PLC-Lite features

Power Line Interface Challenges

For DC PLC implementations, interfacing the power lines in the system presents another challenge that needs to be addressed. Some specific problem areas include:

● Impedance control for multiple node support;

● PLC filtering of any power switches;

● Power line coupling protection circuits required to achieve reliable AC coupling.

(1) Multi-node support

Most DC PLC implementations need to support a large number of nodes (tens to hundreds) connected via a single power line bus to be practical. For the transmitted signal to reach all nodes without significant attenuation, the main requirement is reflected in the familiar equation:

source impedance << load impedance (Equation 1).

We will show how to achieve this in the reference design. For the following analysis, the modulation frequency of PLC-Lite is assumed to be approximately 40KHz. We can then calculate the source impedance of the PLC node (Equation 2).

(Equation 2)

Where:

c = C6 = 22 μF (Figure 3)

Figure 3: Input coupling stage circuit

Assume that the load impedance for a given receiver node is the same as seen by the transmitter node, approximately 30 Ohms. As more nodes are added, this load impedance can be reduced because the load is a parallel combination of impedances. For example, if there are nine nodes in the system, the total load impedance seen by a transmitter node can be calculated as shown in Equation 2.

Below are two examples. One case: one transmitter (master) node and four receiver (slave) nodes. The other case: one master node and nine slave nodes. Based on the calculations in Equation 2, the source impedance requirements will change with the number of slave nodes.

● 9 (receivers) + 1 (transmitter) = 10 PLC nodes, load impedance = 30/10 = 3 Ohms;

● 4 (receivers) + 1 (transmitter) = 5 PLC nodes, load impedance = 30/5 = 6 Ohms
 

Figure 4: Test results with one transmitter and no receiver

Figure 5: Test results for 1 transmitter and 4 receivers

As shown in Figure 4 and Figure 5, the amplitude of the modulated signal changes significantly as the number of slave nodes increases. In the previous setup, the DC line was connected to the oscilloscope in a probe manner (AC coupling). Pressing an external switch on the PLC node triggers the oscilloscope, which then generates a PLC communication burst.

(2) PLC filtering of any power switch

Another challenge in the DC PLC design process is that the PLC node must use a DC power supply to generate local voltages (15V, 3.3V) and modulate the same DC power supply. If appropriate filtering technology is not used, the DC/DC switching power supply will interfere with the PLC modulation in this case.
 

Figure 6: Power supply filter circuit

As shown in Figure 6, a low-pass filter can separate the PLC modulation signal from the switching regulator. The Fc of the low-pass filter can be calculated based on the frequency band occupied by the PLC modulation. Since PLC Lite occupies 42KHz to 90KHz, when L=360μH (180μH + 180μH) and C=1μF, the Fc of the low-pass filter is:

(3) Reliable AC-coupled power line protection circuit The

APLC analog front end (AFE) will be affected by DC power surges. Therefore, an AC coupling stage must be designed so that the PLC node can work reliably in harsh environments.

Figure 7: First stage AC coupling

Figure 8: Second stage AC coupling

To ensure overall system reliability, the DC line is not directly AC coupled to the AFE device. The line goes through a two-stage AC coupling process (Figure 7, Figure 8). In the first stage, the DC line is AC coupled to an intermediate stage with TVS protection, which limits the voltage surge to 9.2V for a surge peak current of 43.5A. The common mode is biased to ground in this stage. In the second stage of AC coupling, the data is AC coupled to the AFE device at a 7.5V DC bias voltage.

Reference Design

The DC (24V nominal) Power Line Communication (PLC) reference design is intended as an evaluation board to help users develop end products for industrial applications that take advantage of this feature to achieve power transmission and communication on the same DC power line. The reference design provides a complete design guide for the hardware and firmware design of the master and slave nodes in an extremely small (approximately 1 inch diameter) industrial size. The design files include schematics, bill of materials, layouts, Altium files, Gerber files, a complete software package with application layers, and an easy-to-use graphical user interface (GUI).

The application layer supports not only slave nodes, but also communication with a host processor (such as a PC or Sitara ARM MPU) (Figure 9). The host processor communicates with the master node only through the USB-UART interface. The master node then communicates with the slave nodes through the PLC. The evaluation board (EVM) also provides an easy-to-use GUI (Figure 10), which not only runs through the host processor, but also provides address management and slave node status monitoring and user control functions.

The reference design is optimized for the source impedance of each slave node, and we can connect multiple slave devices to the master device. The analog front end (AFE) has added protection circuitry to enable reliable AC coupling to the 24V line. The layout of this reference design is optimized according to the layout requirements of the AFE031 high-current traces.

Figure 9: System block diagram of the reference design

Figure 10: Screenshot of the GUI tool

Conclusion

In this article, we have reviewed why narrowband DC power line PLC is an effective tool for networking a variety of industrial applications. This approach can leverage the advantages of the successful use of PLC for AC power line applications in existing smart grid deployments. Supporting the wide range of operating conditions for PLC requires highly flexible hardware and software solutions. In addition, DC power line connectivity also requires careful design considerations to ensure that the system can robustly scale to multiple network nodes.

To address the design challenges and help system designers successfully implement DC power line PLC in their applications, TI has introduced a DC PLC reference design [1] based on TI's analog front end AFE031 and C2000 microcontroller [2]. The reference design comes with a complete set of hardware design files, MCU firmware, GUI-based application software, and extremely detailed laboratory test results documentation. Designers can easily evaluate the platform and effectively build their own end application.