Features and Applications of ON Semiconductor Power Line Carrier Chip

1. Distribution lines Communication

Medium and low voltage AC distribution lines are used for the transmission of electric energy and can also be used as a transmission medium for data communication. Power line carrier communication (PLC) technology is a technology that uses carriers to transmit analog or digital signals on distribution lines at high speed. Using power lines as data transmission media and using existing power distribution networks for communication does not require rewiring. Signals will not be attenuated or even shielded by passing through building walls. The relatively low cost makes this technology popular in many fields such as automatic meter reading systems and lighting control .

Figure 1 is a typical application case of PLC technology - a schematic diagram of a remote electric meter automatic reading system.

The meter exchanges data with the concentrator through the power line. The concentrator is usually located near the transformer and is the core manager of the network. It is responsible for network management, centralized data collection, command transmission, etc. It also exchanges data and transmits information with the main station through the uplink line (PSTN or RF, etc.). One concentrator can manage dozens to hundreds of meters.

In this system, the concentrator will read the operating data of each meter at set time intervals and transmit the data to the main station to achieve automatic remote centralized meter reading.

1.1 EDF Project

Many countries in the world have adopted or are about to deploy smart meter systems and adopt automatic remote meter reading. The Linky meter project of ERDF in France is currently attracting much attention.

ERDF, the largest power distribution network operator in the European Union and a subsidiary of Electricité de France (EDF), has launched a project involving a total of 35 million meters. From 2012 to 2017, the project will replace all traditional meters in France with new Linky smart meters. Smart meter communication uses power line carrier (PLC) technology.

The main technical requirements of the project for PLC are as follows:

Physical layer: IEC 61334-5-1 and EN 50065-1

Modulation: S-FSK

Communication carrier frequency: Fm (signal frequency): 63.3KHz; Fs (space frequency): 74KHz

Communication rate: 2400 Baud

Physical layer is synchronized with the power line at 50Hz

Application layer: IEC 62056-53 and IEC 61334-5-511

1.2 ON Semiconductor Communication with PLC

ON Semiconductor has developed the S-FSK PLC Modem for data communication on low/medium voltage power lines, which has been successfully applied in industrial fields for more than 8 years. The modem is a narrowband PLC transceiver using S-FSK modulation, and is currently the only device that has been verified by the market for many years.

The product has developed from the early AMIS-30585 to the current second-generation product AMIS-49587.

AMIS-49587 fully meets the technical requirements of ERDF and has been selected by Linky smart meter supplier as the core device for PLC communication.

In the following, the features of AMIS-49587 are introduced in combination with the requirements of the EDF Linky meter project.

2. Transceivers covering PHY and MAC Layer

2.1 Linky meter OSI layer reference model:

The Linky project uses a three-layer network structure:

- The physical layer PHY adopts the IEC 61334-5-1 standard.

- Data link layer DLL (including MAC and LLC sublayers) adopts IEC 61334-5-1/IEC 61334-4-32 standards

- Application Layer adopts IEC 62056-53/IEC 61334-4-511 standard

The most prominent feature of AMIS-49587 is that as a PLC transceiver, in addition to completing the transmission, reception, modulation and demodulation of S-FSK signals at the physical layer, it also includes the processing of the MAC sublayer. This feature allows users to focus more on the development of the application layer. When AMIS-49587 is used to exchange data packets at the logical link layer (LLC Layer), the underlying frame header, frame check, etc. will be automatically added. This greatly reduces the workload of customer software development.

2.2 The physical layer uses optimized S-FSK

Power distribution lines are not specially designed for signal transmission. Their impedance is constantly changing, and they are also very susceptible to various electromagnetic interferences from the outside world. The choice of modulation method strives to achieve good communication effects for the special conditions of power lines at a low cost.

FSK (Frequency Shift Keying) is a classic frequency modulation method with low implementation cost: using two independent carrier frequencies to transmit binary 0 and 1. S-FSK (Spread FSK) makes the two frequencies as far apart as possible (>10KHz) so that the transmission quality of the two frequencies is relatively independent, so as to better cope with the impact of common narrowband interference in the power grid.

In Figure 3, we can see that under broadband interference with relatively average noise energy, the signal-to-noise ratio of the received signals of the two carrier frequencies is similar. The receiver filters out other frequencies and generates two demodulated signals - dS and dM at f0 (idle frequency) and f1 (transmission frequency). If dS>dM, it is considered that the received data is "0"; otherwise, it is considered that the received data is "1". In this case, the receiver works in FSK mode; if narrowband interference is encountered, which makes the signal-to-noise ratio of one of the carrier frequencies very poor, the receiver will ignore this channel and use the demodulated signal of another better channel to compare with an internal threshold T to determine whether it receives "1" or "0". At this time, the receiver works in amplitude shift keying ASK mode.

In addition, the demodulation algorithm of the internal processor of the modem is particularly important. It has a great impact on the receiving sensitivity. The Linky project requires that the receiver can normally identify the S-FSK signal within the effective value of 2mV to 2V.

Flexible modulation and demodulation modes and advanced and reliable demodulation algorithms enable AMIS-49587 to have excellent performance in resisting interference on power lines.

2.3 Physical layer frame format

AMIS-49587 transmits data according to the IEC61334-5-1 physical frame format.

2.4 MAC Frame and Physical Frame

The physical frame is sent at time slots (or time slots). The frame start point is called the time slot indicator, which corresponds to the zero crossing point of the power line voltage of 50Hz. The client (that is, the host) must start sending the physical frame at the zero crossing point. The entire system of IEC61334-5-1 is synchronized based on time slots, and it is very important to understand this.

Taking 2400bps as an example, it takes 150mS to transmit one time slice or physical frame.

The physical frame consists of a preamble, a start subframe delimiter, a MAC subframe (Data), and a pause field.

The physical frame always starts at an integer multiple of the basic time slot, which is called the time slot indicator. After the time slot is synchronized, the physical layer of each device can independently track the time slot indicator through its internal clock.

The preamble and start subframe delimiter (AAAAh and 54C7h) are of great significance. During the reception of these 4 bytes, the receiver can:

1) Adjust and determine the receiving gain

2) Measuring signal-to-noise ratio

3) Determine the demodulation mode FSK or ASK

4) Frame check: whether it is the start of a legal physical frame

5) Adjust the synchronization between the server (Server, also known as the slave) and the client (Client)

As shown in Figure 5: The physical frame "packs" the MAC frame before sending. A physical frame has a 38-byte data field and can send one MAC subframe at a time. A long MAC frame can be composed of up to 7 MAC subframes. A long MAC frame with multiple MAC subframes will be split into several subframes and sent sequentially by the corresponding number of physical frames. After the receiver receives all of them, they will be integrated.

The MAC frame header consists of the number of subframes, the initial trusted value IC, the current trusted value CC, the difference trusted value DC, the source address, the destination address, and the padding length Pad Length. The use of trusted values ​​will be described in detail in the relay section below. The LLC frame is included in the MAC frame as data.

3. ON Semiconductor PLC Solution

The solution mainly consists of PLC Modem, AMIS-49587, driver amplifier NCS5650 and coupling transformer.

Transmission path of PLC signal (red arrow): The S-FSK signal modulated by AMIS-49587 is amplified by NCS5650 and coupled to the power line through the transformer. The transformer realizes voltage conversion and impedance matching, and is also used for isolation of strong and weak electricity. In addition to amplifying the power of the signal, the two-stage op amp structure of NCS5650 also forms a 4th-order low-pass filter with a very steep attenuation characteristic. In Europe, where there are strict restrictions on power line access equipment, only by adding similar filters can the system ensure that the high-frequency interference injection into the power line meets the requirements of the EN 50065 specification.

The blue arrow marks the receiving path: the signal coupled from the power line by the transformer passes through the low-pass filter formed by the built-in amplifier of AMIS-49587 and is demodulated and analyzed by FSK in the internal ARM.

The black arrow in Figure 6 is the 50Hz zero-crossing detection signal pin. The system relies on this signal for synchronization timing.

The blue dotted box in the figure is the application processor in the meter, which is responsible for communication application layer processing and metering. Its interface with the PLC Modem is a simple SCI serial port.

The power supply of the solution is very simple: one 12V-supply line amplifier for driving the PLC signal coupling transformer; one 3.3V power supply for AMIS-49587.

3.1 AMIS-49587 Functional Block Diagram

Let's take a look at the internal structure of AMIS-49587.

The core of AMIS-49587 is a 32-bit ARM processor, which completes the processing of the physical layer and MAC layer, runs the S-FSK modulation and demodulation algorithm, and also manages the communication with the external MCU. The embedded software is stored in the on-chip ROM.

On the left side of the chip is the analog part: FSK signal synthesis output, receive demodulation, system clock and 50Hz phase-locked loop.

The chip includes all analog and digital parts such as S-FSK signal processing and MCU interface management. The transformer driver becomes a heat source on the transceiver board because it is the power amplifier part. In order to prevent high heat from affecting the system accuracy, AMIS-49587 does not incorporate the signal power driver into this IC, but adopts an external solution.

3.2 Unique system relay solution

In network communication, long-distance information transmission requires relaying. AMIS-49587 from ON Semiconductor supports relaying using the Repetition with Credit algorithm. In this relay solution, the system does not have a repeater that needs to be pre-set. The core concept is that each server (i.e., electricity meter) can be a repeater for other servers to help transmit information or commands. Even if the destination address of the received frame does not match, the server will forward it if forwarding is required. Forwarding is done in a time-slice chorus manner, which relies on the entire system to be synchronized with the time slice.

The Repetition with Credit relay algorithm uses a method called credit value management. The credit value is divided into 7 levels and is managed by the client (concentrator). The system stipulates: If the server is configured as a Repeater, if the current credit value of the received MAC frame is greater than 0, the server must repeat the frame when the next time slice arrives, and the current credit value will be reduced by 1. The frame repetition process ends when the current credit value reaches 0. Under this mechanism, there may be many servers repeating and forwarding at the same time in the same time slice, which is harmony.

The following (see Figure 8) takes the forwarding process of a single MAC frame as an example to illustrate the Repetition with Credit mechanism.

1) The concentrator sends a frame to meter 5 in time slot K and sets the initial trust value in the MAC frame header to 2. Meters (Module PLC) 1 and 2 receive the frame correctly in time slot K because they are close to each other.

2) Since the credit value of this frame is greater than 0, the concentrator, meter 1 and meter 2 start to repeat this frame in time slice K+1 after receiving it, and the current credit value decreases by 1 and becomes 1. Meters 3 and 4 receive this frame in this time slice. However, meter 5 still does not receive it because the line is too far away.

3) Meters 3 and 4 repeat the same frame at K+2. The current credibility value decreases by one, and the concentrator, meters 1 and 2 also repeat at the same time, "harmoniously" with 3 and 4. Meter 5 receives this frame correctly. Since the current credibility value has become 0, all meters will no longer repeat this frame in the next time slice.

The maximum initial trust value that can be set in the system is 7. Assuming that the communication distance between a concentrator and an electric meter is 300m, if there is a relay with 7-level trust value management, the communication distance will be up to 2400m.

In this relay mechanism, there are three variables IC, CC, and DC, which represent the initial trusted value, the current trusted value, and the difference trusted value, respectively. The concentrator sets the initial trusted value according to the algorithm. The current trusted value CC decreases one by one with each forwarding during the frame forwarding process. The difference trusted value DC has no meaning to the intermediate forwarder. Only at the destination address meter, IC minus CC is used to obtain the difference trusted value DC. The meter will send the DC value back to the concentrator in the reply frame, and the concentrator can revise the initial trusted value of the next visit to the meter based on this value.

Since the power line impedance and interference conditions are constantly changing, the quality of PLC communication is also constantly changing. The trusted value algorithm enables the client to dynamically manage the network communication status in real time to achieve reliable data transmission.

The PLC network composed of AMIS-49587 abandons the traditional routing solution with many disadvantages. There is no complicated routing table, and no need for manual setting and adjustment of relay transponders. The network will automatically find the best routing line and continue to make dynamic adjustments.

In addition, the Linky project also introduced a Repeater Call mechanism. This mechanism, which runs regularly, adjusts the settings of the repeaters in the transmission path through advanced algorithms, reduces unnecessary meters participating in the "harmony", and reduces possible crosstalk or echo, which is a further optimization of network routing.

3.3 Network establishment

The network composed of AMIS-49587 adopts a master-slave structure, with a client (also called a host or concentrator) and multiple servers (slaves or meters) forming a network. The initiator of general communication is the client. According to IEC 61334-5-511, the client runs the "Discovery" and "Register" services. "Discovery" searches for servers that have newly joined or rejoined the network for some reason. If the server responds correctly, it enters the registration process and will be assigned an independent MAC address.

The client will periodically run discovery and registration services to implement system Plug & Play. It will also periodically perform point-to-point Ping services to determine whether the server is online and eliminate possible address conflicts.

3.3.1 Smart synchronization

In a master-slave network, the server must first bind with the client (after binding, it will only reply to the client) to complete the registration and then communicate with the client normally. This process is called synchronization with the client. After the server synchronizes with a client, it will no longer reply to other clients. When the client access times out or the server actively cancels synchronization, it re-enters the client search state.

After the server is powered on and locked to 50Hz, it will continuously analyze the channel to search for the preamble (AAAAh) and the start subframe delimiter (54C7h). If it detects them and then correctly receives the physical frame sent by the client, it can synchronize with the client and accept registration.

During the synchronization process, AMIS-49587 uses a more intelligent Smart Synchronization: within a certain period of time (settable), the newly connected server can synchronize with multiple clients and then actively cancel synchronization, during which the signal strength (SNR) of each client is recorded. When the set time arrives, the server will finally choose to synchronize with the client with the strongest signal.

This mechanism effectively solves the common multi-zone/multi-phase signal crosstalk problem in meter reading systems. Since the meters will automatically find the nearest concentrator to synchronize with it, no manual intervention is required, the network path is automatically optimized, and the workload in construction is greatly reduced.

3.3.2 Alarm Mechanism

The PLC network composed of AMIS-49587 also has an alarm mechanism. When a meter fails, it is required to report an alarm through the network so that the management personnel can be informed and handle it in time. Active reporting is equivalent to realizing two-way communication in the network. The Linky meter will issue an Alarm in the 3 bytes of the Pause time period of the physical frame. After receiving it, the host will initiate a Discovery service to investigate the specific cause of the failure.

Another very useful function of Alarm is that when a new meter is connected, it will prompt the concentrator to initiate Discovery through Alarm. This will speed up the connection process of the new meter. There is no need to wait for the host's routine Discovery service to arrive.

IV. Conclusion

AMIS-49587 from ON Semiconductor is a power line carrier transceiver developed in full compliance with the IEC-61334 standard. In addition to completing the physical layer modulation and demodulation, the unique feature of this chip is the embedded MAC layer processing, making it a PLC transceiver with protocol parsing function. When using this transceiver to transmit or receive data, customers do not need to pay too much attention to the details of the protocol.

The trusted value "harmony" relay mode eliminates the complicated routing table. Under the control of the concentrator, the system automatically finds and continuously adjusts the best routing line, making long-distance communication simple and reliable while greatly reducing the construction and maintenance workload. The addition of intelligent synchronization and Repeater Call mode further optimizes the network in real time and dynamically.

ON Semiconductor has also developed a line driver specifically for PLC applications, the NCS5650. It integrates a high-bandwidth operational amplifier and a power amplifier with an output current of up to 2A. The two-stage operational amplifier structure makes it very easy to configure a 4th-order low-pass filter to meet the strict restrictions on high-frequency injection into power lines in various regulations (such as EN 50065).

ON Semiconductor's PLC solution is very suitable for automatic meter reading, lighting control, household appliances and other regional centralized control occasions. The solution is simple and easy to use, and its implementation effect and reliability have been verified in European industrial fields for more than 8 years.