User:Douglas Aguiar do Nascimento/sandbox

Switching Control Techniques address the Electromagnetic Interference (EMI) mitigation on power electronics (PE). The designing of power electronics involves three key challenges to be overcome, they are : 1. power losses, 2. EMI and 3. harmonics. Also, the use of PE brings some drawbacks into the electrical grid regarding the EMI, in which necessary to consider during its desing and operation, especially when is desirable to meet the EMC constraits (e.g., CISPR 22). Dealing with static converters designed from PE, for example, can causes signal disturbances in the electrical environment (either near or distant from), e.g. radio reception signal, vehicle navigation system, avionics, etc.

Those disturbances are caused mainly by the high level of high frequency interference from the switching activities on semiconductor components inside PE and it has been challenging to handle with once filtering and shielding techniques demands for cost and size increasing for its implementation, along with efficiency issues. Therefore, the switching mode power suppy is used instead in order to obtain a higher efficiency.

Mitigation of Electromagnetic Interference
The semiconductor technologies breakthrough and its adoption on power electronics devices provide fast power switching devices off the shelf with progressively increasing efficiency and high density power technology in electronics systems. In order to reach the EMC requirements it is important to better understand the dynamics behavior of the switching devices in PE and the factors which causes modifications on output waveform shaping and rate. Therefore, it is possible to distinguish two ways of EMI suppression regarding the PEI (see Fig. 1): power source and propagation path. In one hand, the intervening on the source it is possible by using switching control techniques (increasing the efficiency), redesigning of the circuit (costly and time demanding) and using soft switching transition. On another hand, if one applies externally or internally filters (also costly) it possible to address the propagation path.



Considering that handling the electric supply it is not necessary to modify the internal circuitry of the electronic device, e.g. inverter, converter, rectifier, so on (then cutting off costs), by using switching control techniques it is possible to increase the efficiency of the PE. Although the use of switching transistors, can impact negatively the supply as primary source of electromagnetic emanations, it can enhance the efficiency of the controllers ( e.g. as used by high-efficiency controllers). Once the waveforms fundamental component is associated to conversion of energy (eiter DC or AC) and the switching frequency (even dozens of kiloHertz or above), the choice of convenient waveforme profile is made regarding to the target and, the PE converter constraints.

Thus, the high efficiency reached by the switching power converters is related to the use of switching devices, energy storage elements and transformers, through proper modulation activity of the switches to convert the available DC or AC and voltage or current signal waveforms of the power source into the AC or DC waveforms needed by the load. Those switches devices are mostly semiconductores such as: transistors, diodes, thyristors, Field-effect transistor etc. The high perfomance of the switching devices are the main reason for search a appropriate switching control technique. The two most popular methods are :
 * Deterministic, in which pulse-width modulation (PWM) is applicable as programmed switching method and;
 * Non-deterministic (or random modulation), characterized by the random PWM (RPWM) method.

The key distinction of these techniques is attributed to the fact of randomness introduces a continuous EMI noise, i.e. a uniform power across the frequency band.

Deterministic Modulation


The PWM is considered the most common method of deterministic technique. Considering the example of DC-DC converter, the controlled switch is designed to “cut-off” the DC waveform into a pulse-shaped waveform. These signal continuity between the values of specific voltage and zero value at the switching frequency, and this DC-DC converter is controlling the duty cycle (𝐷), that is, the time frame in which the switch device is turned off in each cycle, it is possible to control the time frame that the pulse waveform takes the value. Generally, the waveform of the power electronics interface (PEI) are steady-state periodic time functions.

By the middle of 1990s, some researches started to evaluate the frequency modulation techniques to reduce EMI emissions, focusing on communications and microprocessor systems. The main concern with these latter approaches is that EMI is equally spread along the whole frequency spectrum, and these approaches do not provide any control over the bands where EMI energy is spread. This feature is crucial for applications in the context of telecommunications, telematics, and automation systems where EMI at specific selective frequencies must be avoided. Investigations of such techniques applied to EMI reduction of digital systems is a subject of significant concern, inclusive opening a new area of research, with modulation techniques to power electronics converters with randomized modulation.

As examplification of use, the Fig. 2.A side shows the spectrum and Fig. 2.B shows the spectrogram of EMI shaped-noise voltage output for a programmable PWM with switching frequency in Buck-Converter with 𝐷 = 0.50, in accordance with CISPR A standard. According to Fig. 2.A, the programmable switching frequency creates a significative impact by the EMI noise shape as well as the high sideband. In Fig. 2.B, shows the high peaks amplitude of EMI noise, at the switching frequency and their multiple harmonics.

Non-deterministic Modulation


The randomness process can be performed by spreading the harmonics power existing at well-defined frequencies (i.e. discrete harmonics) through a wide range of frequencies in order to remove harmonic components of significant magnitude. With that, discrete harmonics are substantially decreased, and the harmonic power is extended across the whole spectrum as noise.

The procedure behind of most randomized modulation is related to the schemes of successive randomizations of the switching pulse train (or its segments), which are independent statistically and ruled by  probabilistic rules. So, the randomized modulation procedure must enable accurate control of the time-domain performance of randomized switching, in addition to spectral shaping in the frequency domain. The elementary analysis problem in randomized modulation regards the spectral characteristics of the signal (and associated waveforms) in a converter to the probabilistic structure that governs the dithering of an underlying deterministic nominal switching pattern. In this case, the suitable approach to analyse the randomized switching setup is the power spectrum, computed from the Fourier Transform (FT) of the original signal autocorrelation. Note that the FT of a random signal is itself a random function, i.e., it is a random variable at each frequency. The power spectrum, on the other hand, owns convergence properties and can be estimated reliably from the available signal.

Therefore, it is possible to categorize the randomized modulation strategies as stationary.

The Fig. 3.A shows the spectrum of EMI noise shape of voltage output for RPWM with the switching frequency implemented in one Buck-Converter, and in accordance with CISPR A standard. It shows the spectrogram of EMI noise shape of voltage output for an RPWM, where is possible to note (also in Fig. 3. A) that aleatory process inserts the continuous EMI noise shape, in low-frequencies, the EMI noise shape follows oscillatory mode with their noise value decreasing across the spectrum. Fig. 3.B, shows the spectrogram of EMI noise shape of voltage output for an RPWM, where is possible to note (also in Fig. 3.A) that randomization process introduces the continuous EMI noise shape, and in low-frequencies, the EMI noise shape follows oscillatory mode with their noise value decreasing across the spectrum.