Power-line modems, power supplies and cleaning up the neighborhood
Power-line modems (PLMs) are increasingly being used to communicate over existing power-distribution networks, in which transformers and OEM-style switching power supplies are already present as part of another product/network. These PLMs are used where it is inappropriate to use RF or to run extra cables for the communication system.
Unfortunately, many OEM-switching power supplies do not meet (or are not required to meet) strict specifications for EMI suppression and, as a result, can pollute the power line with wideband noise. While remaining inoffensive to most appliances, the power-line noise interferes with the modem communication signals.
This article will discuss how to design applications that can use the power line for communication despite the noisy neighbors and their ‘bad’ power supplies and the attenuation of modem signals caused by any transformers in use.
PLMs use a variety of carrier frequencies and modulation techniques. Although this article targets PLMs operating within the four CENELEC bands and the band allowed by the FCC, the ideas presented here can be applied to almost any PLM situation. CENELEC is important because large numbers of modems are already deployed in Europe according to EN 50065-1; therefore the modem ICs available generally comply with the precepts of the standard. The success of so large an installed base provides insight for new applications.
The CENELEC bands A, B, C, and D, correspond to frequency ranges of 3-95 kHz, 95-125 kHz, 125-140 kHz, and 140-148.5 kHz. CENELEC is an acronym for the French phrase: “Comité Européen de Normalisation Électrotechnique”, which in English is “European Committee for Electrotechnical Standardization”. The CENELEC forms the European system for technical standardization and published the standard EN 50065-1, "Signaling on low-voltage electrical installations in the frequency range 3 kHz to 148.5 kHz".
Going farther than CENELEC, the FCC allows frequencies from 3-500 kHz to be used, giving a much-greater bandwidth for the application in the United States. With so large a bandwidth, it is easy to see that a modem signal can be interfered with by a variety of noise sources. These include switching power supplies operating near the modem frequency or having harmonics near it, as well as electrical devices such as light dimmers and motor controllers that may create noise near the zero-crossing of the AC mains voltage waveform.
Because a modem may be integrated into a product that contains a noise source, a power supply, and possibly an iron-core 60Hz transformer, it is necessary to take precautions to prevent the modem signal from being 1) drowned out by switching power-supply noise during receive, 2) "soaked up" by a bridge rectifier and input-filter capacitor of a power supply during transmit, or 3) being excessively attenuated by being forced through an iron-core transformer.
There are several typical kinds of impediments to PLM communication:
1) One kind of noise source is a switching power supply. To avoid interference, one can select a modem carrier frequency that is removed from the dominant noise frequencies created by any switching power supplies.
In some cases, a modem may provide a number of simultaneous carriers covering a wide range, for example, with 97 carriers spread across 3-95 kHz. In this case, a few spikes from the power supply are not troublesome. Other modems operate on a few selectable, fixed frequencies or pairs, such as 50/66 kHz and 56/72 kHz, among others, or provide a single frequency anywhere in the band.
In cases where the operating frequency of the switching power supply cannot be adequately predicted, perhaps due to the use of an OEM module or the desire to leave open future design changes in the switcher, an LC circuit rejecting the modem’s signal bandwidth can prevent switching noise from infiltrating the modem bandwidth (Figure 1). In many cases, this network can provide a second function, in that capacitance (C) dampens switcher transients while inductance (L) supplies power to the switcher’s input.
In some designs, the LC filter may be built-in as the switcher’s EMI filter, thereby saving cost over extra components. The filter need not resonate at the modem transmission frequency, but should isolate the communication band from any noise created by the power supply.
Figure 1 (click here to enlarge)
2) Another kind of noise is caused by zero-crossing interferers, that is, circuits that switch near the point where the mains waveform crosses zero volts. This kind of AC load is primarily represented by modern light dimmers and electric motor drives. Variable speed motor drives, once the province of industrial systems, are frequently found in new appliances and equipment intended for residential use, such as condensing units and furnaces.
Filtering or tuned circuits mentioned previously can be used, but suitable inductors and capacitors will be more costly at HVAC power levels. It is therefore advantageous to select a modem that does not depend on mains-waveform timing to send or receive data, that is, a modem that will communicate on demand without regard to AC phase, Figure 2. Modems that do not require a zero-crossing are also suited for use on DC power lines.
Figure 2 (click here to enlarge)
3) When the modem must be on the secondary side of a large power distribution transformer, the carrier frequencies may be chosen to be lower so that they may pass more easily through the particular transformer.
Each transformer type is different and requires evaluation. With a transformer larger than 3-5 kVA, especially those with widely spaced or “open” windings, signal transmission is not always satisfactory because of excessive attenuation and dissipation of the modem signal in the very-large transformer.
It is more economical to use a series-tuned circuit designed to pass only the bandwidth of the high-frequency modem signal around the transformer while rejecting the mains-frequency signals and their harmonics. A simple narrowband-type circuit is shown bridging the transformer in Figure 3, but there is no reason that a wide band-pass filter would not be useful where a variety of modem designs are to be considered.
Whenever this type of circuit is employed, passing high-frequency signals from a high voltage to a moderate-voltage circuit, it is prudent to take additional care in designing the transient-protection part of the modem circuit, so that any fault-related or lightning-related transients which happen to have energy within the passband do not reach the modem without suitable attenuation.
Figure 3 (click here to enlarge)
4) When the modem must be on the far side of an electrically isolated low-power circuit, such as on the 12 VAC side of a UL-listed 120V/12V 300W outdoor low-voltage lighting transformer, the carrier frequencies may be chosen so that they may pass more readily through the transformer.
As long as the wire distance is not too great, the attenuation through this smaller type of transformer will not be the factor that ultimately prevents communication. Reliable communications were achieved using a 132.5 kHz carrier impressed on a 12 VAC line successfully passed through 200 feet of 14-gauge power cable, and then through such an isolating transformer to another modem, Figure 4. In a situation where the modem’s driver amplifier must work with a low-voltage line such as 12VAC, the first concern may be that the turns ratio of the 1:1 output transformer in the modem should be changed to more accurately match the characteristics of the low-voltage installation.
A closer look at this application shows that the impedance on the 12V/300W line is around 0.5 ohms, which is high enough to allow an adequate signal to be developed on the 120V side of the transformer. In an experiment, this setup was used with a 132.5 kHz carrier and another PLM on the 120V side of the transformer, and data was exchanged with no difficulty.
Figure 4 (click here to enlarge)
5) In the application of low-voltage lighting, a “solid-state transformer” might be used. This is an inexpensive and efficient alternative to an iron-core transformer. The solid-state transformer can generate an extreme amount of noise. The output waveform is not an inverter-like 60-Hz square or sine wave, but a line-amplitude-modulated square-ish wave at 20-40 kHz. When this is observed at mains-frequency sweep rates, it resembles a two-tone SSB (single sideband) radio-frequency transmission.
The most constant characteristic of the solid-state transformer’s output is an RMS value of 12 Volts into a resistive load which, apart from cost saving and improved efficiency, is the sole purpose of the transformer. A typical high-quality unit recently examined used no line or load filtering and created a broad band of random switching noise around 20 kHz, with rich harmonics extending up to more than 100 kHz.
The noise is usually not an issue until power-line communication is to be used. The solid-state transformer is already extremely popular for outdoor lighting, especially new installations and high-end feature-rich applications capable of color-changing, artistic, or “mood” lighting, Figure 5. In these systems, with many luminaires separated by considerable lengths of cable, it is necessary to use a PLM to control each luminaire.
Figure 5 (click here to enlarge)
6) Low-impedance DC lines and large-reservoir rectifier circuits can present an issue due to attenuation of the modem signal. A simple L or LC network can isolate the signal from low-impedance components, such as solar panels and DC/DC converters as well as capacitor-input rectifier circuits. In these applications, noise is not a limiting factor except for extremely long cable runs in the presence of RF interference.
Small power supplies do not always use power factor correction (PFC), and many large ones are not required to do so. Using a full-wave bridge rectifier directly with a large filter capacitor can present a very low impedance to the AC power source, whether that source is the AC power line or another voltage, such as 48VAC from a transformer, Figure 6.
The modem signal will pass through any forward-biased rectifiers and be partially dissipated in the filter capacitor. The modem signal will then become distorted because the rectifiers are not always forward-biased in a rectifier-type (non-PFC) power supply.
Use of an inductor in a rectifier-type power supply will help prevent modem-signal loss in the filter capacitor. The inductor may be an inexpensive toroid in the 50/60Hz AC part of the circuit, before the rectifier, designed to reject the high-frequency modem signal, or it may be a larger power-supply component placed directly after the bridge rectifier designed for the role of a power-supply choke.
Challenges for communications via DC systems are also represented by battery banks and decoupling capacitors. An effective method of isolation is an inductance. Its value may be the same as for an AC power circuit. It is important to use inductors cautiously in high-current DC circuits that may have abrupt switching characteristics because of the transients that can be generated.
Automotive circuits and large photovoltaic arrays are two types of systems subject to frequent load connections and disconnections. When using PLMs and isolating the modem-signal bandwidth from the DC load with an inductor, some care is necessary to provide overvoltage protection for the modem input.
Figure 6 (click here to enlarge)
As PLMs become more commonplace, it will be necessary to prevent interference with neighbors’ systems, both at work and at home. There is every reason to believe that simple filters will also be effective.
Almost every system specifying a PM will come up against at least two of the issues presented. Fortunately, simple L or LC filters and common-mode chokes will greatly improve the quality of the communications. The six very general situations diagrammed here account for the great majority of challenges to the use of PLMs in electrically noisy environments. Because every application is different, it is not useful to present completely worked-out solutions, but rather to show generally how easily the impediments may be overcome.
Selecting an appropriate PLM, such as the ST7580 for DC power lines and non-zero-crossing applications, or the ST7590 for AC power lines, is only the first step in assuring a reliable signal path at great distances. The final step to complete success in a power line communications system is to take a few simple and inexpensive actions to remove known disturbances, and create paths the modem signals can easily traverse.
About the author
Patrick Jankowiak studied Laser and Electro-Optics Technology at Texas State Technical College in 1979. He owned and operated a service facility for 11 years, and worked for manufacturers such as Sony Broadcast Division with digital and HD systems and automation.
Since 2001, he has been with STMicroelectronics as a Technical Marketer and a Field Application Engineer working with a spectrum of products from ADSL Modems to Industrial and Power Control devices. He has two patents, one of which is MEMS, with others pending. Mr. Jankowiak enjoys a wide range of technology and served in the Texas State Guard for nine years.