Random pulse-width modulation (RPWM) is a modulation technique introduced for mitigating electromagnetic interference (EMI) of power converters by spreading the energy of the noise signal over a wider bandwidth, so that there are no significant peaks of the noise. This is achieved by randomly varying the main parameters of the pulse-width modulation signal. [1]
Electromagnetic interference (EMI) filters have been widely used for filtering out the conducted emissions generated by power converters since their advent. However, when size is of great concern like in aircraft and automobile applications, one of the practical solutions to suppress conducted emissions is to use random pulse-width modulation (RPWM). In conventional pulse-width modulation (PWM) schemes, the harmonics power is concentrated on the deterministic or known frequencies with a significant magnitude, which leads to mechanical vibration, noise, and EMI. However, by applying randomness to the conventional PWM scheme, the harmonic power will spread out so that no harmonic of significant magnitude exists, and peak harmonics at discrete frequency are significantly reduced. [2]
In RPWM, one of the switching parameters of the PWM signal, such as switching frequency, pulse position and duty cycle are varied randomly in order to spread the energy of the PWM signal. Hence, depending on the parameter which is made random, RPWM can be classified as random frequency modulation (RFM), random pulse-position modulation (RPPM) and random duty-cycle modulation (RDCM). [3]
The properties of RPWM can be investigated further by looking at the power spectral density (PSD). For conventional PWM, the PSD can be directly determined from the Fourier Series expansion of the PWM signal. However, the PSD of the RPWM signals can be described only by a probabilistic level using the theory of stochastic processes such as wide-sense stationary (WSS) random processes. [4]
Among the different RPWM techniques, RFM (random frequency modulation) is the most common method of the three major types, used in many power converter topologies to pass the electromagnetic compatibility (EMC) test. In this type of modulation, the switching frequency of the PWM signal is varied randomly in order to spread the emitted noise of the power converters in which it is applied. RFM is very easy to implement and it offers significant reduction of the noise peaks compared to conventional PWM. However, application is limited to power converters which does not require fixed switching frequency for their normal operation. A greater degree of switching frequency variation can affect the proper functioning of the devices and components inside the power converter circuit. [5]
RPPM is also commonly deployed in power converters to pass the EMC compliance tests. This modulation technique also offers significant reduction of the conducted emission and, consequentially, the radiated emission of power converters. However, compared to RFM, RPPM is less effective in EMI reduction. This is because the PSD of RPPM contains both the density and harmonic components, and the spectrum cannot be fully spread unlike that of RFM where the spectrum has only the density component. However, in this modulation scheme, both the switching frequency and the pulse width are fixed so that the converter components like inductors and capacitors can function properly. [1] [3]
In RDCM, the pulse width or the duty cycle of the PWM signal is varied randomly in order to spread the noise spectrum. This kind of modulation is less common compared to the previous ones. This is because RDCM is less effective at spreading the noise. Moreover, randomly varying the duty cycle may cause output voltage fluctuations and ripples. Besides, in some power converter topologies, the duty cycle variation is the primary means of controlling the input-output voltages and currents using closed loop control systems. [3] An example of this could be the drive for a brushed DC motor. Since the power to the motor is already being "chopped" at a specific frequency to vary the voltage and current, introducing randomization into the process could cause detriments to the system's performance.
This type of modulation is becoming more common in variable-frequency drives of all sizes and applications. in consumer-sized VFDs that include it as a feature, it presents as a user-selectable parameter, often with several different operating levels. Some drives may also use more than one method at once. However, while the term "RPWM" is generally used to label this type of modulation in a technical sense, the technology does not yet have a label in the world of VFD parameter names. For example, Fuji Electric labels its noise-reduction parameter as "motor tone", while both Mitsubishi Electric and Teco Westinghouse label it as "soft PWM". The use of the term "soft PWM" could potentially cause confusion to those not familiar with this technique, as zero crossing control is sometimes labeled as "soft switching".
A term that could be potentially used as a standard label could be "scrambled PWM" or "carrier scrambling", as the word "scramble" in this sense is nonspecific to the method of RPWM used, yet informs the user that there is indeed a specialized process occurring which affects the shape and properties of the output waveform. In addition, the word has a home in the telecommunications field, where a scrambler is any device (typically analog) that is used to encode a signal so that it will be unintelligible if intercepted before it can reach the intended recipient without an appropriately tuned de-scrambler. That is to say that "scramble" would not be out of place if used to label RPWM in a more general sense.
Regardless of the label, upon observation of the manual in consumer-sized VFDs that include it, the focus of the parameter description does appear to be on acoustic noise reduction, rather than EMI reduction and motor health.
In modern rail traction converters, this method presents in a change in the sound that the motors emit when driven by inverters which utilize it. As opposed to the normally steady, carrier-based whine of a classic SPWM convertor, the sound is more of a hiss, akin to white noise. Because rail traction convertors operate at such high power levels, EMI is more readily created in such systems. In these applications RPWM is highly beneficial to the motor's health and the level of emitted EMI.
RPWM techniques are very effective in reducing the EMI of power converters. However, when power converters with this special type of modulation coexist with communication systems, there may be a severe electromagnetic interference conflict between the power system and the communication system. This detrimental effect can be observed in power line communication (PLC) systems, where both power converters and communication systems coexist. Indeed, recent studies have confirmed that RPWM applied to power converters to minimize conducted emissions can detrimentally interfere with the PLC system. [6] [7]
The interference can be worsened when the switching frequency and the bandwidth of the PLC system overlaps with that of the power system. Most power converters use a switching frequency that is below 150 kHz, which is in the low frequency electromagnetic compatibility range. This could cause coexistence issues mainly in narrow band PLC systems, (specialized PLC protocols which are being used for smart grid application, such as Prime PLC and G3-PLC, in frequencies below 150 kHz.) In conventional PWM, the noise from power converter overlaps with the PLC frequency band at discrete multiples of the switching frequency only. This results in less interference to the PLC system. However, In RPWM, the noise is more evenly distributed and both the PLC and the noise from power converter shares wider bandwidth. This creates more disturbance to the PLC system. Therefore, it is advisable to carefully observe the properties of any coexisting systems in order to choose a switching frequency for randomly modulated power converters that does not overlap with that of a coexisting PLC system. [6]
Electromagnetic compatibility (EMC) is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment, by limiting the unintentional generation, propagation and reception of electromagnetic energy which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage to operational equipment. The goal of EMC is the correct operation of different equipment in a common electromagnetic environment. It is also the name given to the associated branch of electrical engineering.
In telecommunication, especially radio communication, spread spectrum are techniques by which a signal generated with a particular bandwidth is deliberately spread in the frequency domain over a wider frequency band. Spread-spectrum techniques are used for the establishment of secure communications, increasing resistance to natural interference, noise, and jamming, to prevent detection, to limit power flux density, and to enable multiple-access communications.
Pulse-width modulation (PWM), also known as pulse-duration modulation (PDM) or pulse-length modulation (PLM), is any method of representing a signal as a rectangular wave with a varying duty cycle.
A power inverter, inverter, or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opposite of rectifiers which were originally large electromechanical devices converting AC to DC.
A switched-mode power supply (SMPS), also called switching-mode power supply, switch-mode power supply, switched power supply, or simply switcher, is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently.
This is an index of articles relating to electronics and electricity or natural electricity and things that run on electricity and things that use or conduct electricity.
Power electronics is the application of electronics to the control and conversion of electric power.
PWM rectifier is an AC to DC power converter, that is implemented using forced commutated power electronic semiconductor switches. Conventional PWM converters are used for wind turbines that have a permanent-magnet alternator.
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The disturbance may degrade the performance of the circuit or even stop it from functioning. In the case of a data path, these effects can range from an increase in error rate to a total loss of the data. Both human-made and natural sources generate changing electrical currents and voltages that can cause EMI: ignition systems, cellular network of mobile phones, lightning, solar flares, and auroras. EMI frequently affects AM radios. It can also affect mobile phones, FM radios, and televisions, as well as observations for radio astronomy and atmospheric science.
An HVDC converter station is a specialised type of substation which forms the terminal equipment for a high-voltage direct current (HVDC) transmission line. It converts direct current to alternating current or the reverse. In addition to the converter, the station usually contains:
A variable-frequency drive is a type of AC motor drive that controls speed and torque by varying the frequency of the input electricity. Depending on its topology, it controls the associated voltage or current variation.
A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices operate as electronic switches, and not as linear gain devices as in other amplifiers. They operate by rapidly switching back and forth between the supply rails, using pulse-width modulation, pulse-density modulation, or related techniques to produce a pulse train output. A simple low-pass filter may be used to attenuate their high-frequency content to provide analog output current and voltage. Little energy is dissipated in the amplifying transistors because they are always either fully on or fully off, so efficiency can exceed 90%.
Pulse-frequency modulation (PFM) is a modulation method for representing an analog signal using only two levels. It is analogous to pulse-width modulation (PWM), in which the magnitude of an analog signal is encoded in the duty cycle of a square wave. Unlike PWM, in which the width of square pulses is varied at a constant frequency, PFM fixes the width of square pulses while varying the frequency. In other words, the frequency of the pulse train is varied in accordance with the instantaneous amplitude of the modulating signal at sampling intervals. The amplitude and width of the pulses are kept constant.
Space vector modulation (SVM) is an algorithm for the control of pulse-width modulation (PWM), invented by Gerhard Pfaff, Alois Weschta, and Albert Wick in 1982. It is used for the creation of alternating current (AC) waveforms; most commonly to drive 3 phase AC powered motors at varying speeds from DC using multiple class-D amplifiers. There are variations of SVM that result in different quality and computational requirements. One active area of development is in the reduction of total harmonic distortion (THD) created by the rapid switching inherent to these algorithms.
The Vienna Rectifier is a pulse-width modulation rectifier, invented in 1993 by Johann W. Kolar at TU Wien.
Stochastic Signal Density Modulation (SSDM) is a novel power modulation technique primarily used for LED power control. The information is encoded - or the power level is set - using pulses that have pseudo-random widths. The pulses are produced so that, on average, the produced signal will have the desired ratio between high and low states. The main benefit of using SSDM over, for example, Pulse-width modulation (PWM), which is usually the preferred method for controlling LED power, is reduced electromagnetic interference. Figure 1 illustrates a SSDM signal and demonstrates how the average signal density approaches desired value. The pseudo-random pulses in the signal are visible.
Switching Noise Jitter (SNJ) is the aggregation of variability of noise events in the time-domain on the supply bias of an electronic system, in particular with a voltage regulated supply bias incorporated with closed-loop (feedback) control, for instance, SMPS. SNJ is measurable using real-time spectral histogram analysis and expressed as a rate of occurrence in percentage. The existence of SNJ was firstly demonstrated and termed by TransSiP Inc in 2016 and 2017 at the Applied Power Electronics Conference (APEC), and reviewed with experts at Tektronix prior to be featured as a case study published by Tektronix. The discovery of SNJ was also featured in multiple articles published by Planet Analog magazine and EDN Network. Difficult to filter using conventional LC networks due to variability in both time and frequency domains, SNJ can introduce random errors in analog to digital conversion, affecting both data integrity and system performance in digital communications and location-based services
Conducted emissions are the effects in power quality that occur via electrical and magnetic coupling, electronic switch of semiconductor devices, which form a part of electromagnetic compatibility issues in electrical engineering. These affect the ability of all interconnected system devices in the electromagnetic environment, by restricting or limiting their intentional generation, propagation and reception of electromagnetic energy.
Switching Control Techniques address electromagnetic interference (EMI) mitigation on power electronics (PE). The design of power electronics involves overcoming three key challenges:
Low-frequency electromagnetic compatibility is a specific field in the domain of electromagnetic compatibility (EMC) and power quality (PQ), which deals with electromagnetic interference phenomena in the frequency range between 2 kHz and 150 kHz. It is a special frequency range because it does not fit in the PQ problems, with range of up to 2 kHz, where relative levels of voltage and current can have massive impact on efficiency and integrity of electric systems, and neither in the conducted EMC range, which starts at 150 kHz and influences mainly informational systems, and already too far from radiated EMC range, which starts at 30 MHz and goes up to 1 GHz.