Unipolar Pulse Width Modulation Research Articles

Comparative study on various unipolar PWM strategies for single phase five-level cascaded inverter

A single phase cascaded inverter consisting of two full bridges creates a five-level AC output voltage. Due to switch combination redundancies, there are certain degrees of freedom to generate the five-level AC output voltage. This paper presents the different types of unipolar pulse width modulation (PWM) strategies for the chosen inverter. The effectiveness of the developed strategies are demonstrated by simulation and experimentation. The results indicate that the multilevel inverter triggered by the developed unipolar PWM strategies exhibits reduced harmonics and higher DC bus utilisation. PWM strategies developed are implemented in real time using dSPACE/real time interface (RTI). The simulation and experimental results closely match with each other validating the strategies presented.

Switching Characterization of Cascaded Multilevel-Inverter-Controlled Systems

In this paper, the method of triangular carrier switching control of two-level inverters is extended to cascaded multilevel inverters using phase-shifted multicarrier unipolar pulsewidth modulation (PWM). The condition for smooth modulation is obtained using the Bessel's function representation of the PWM output and the switching condition of the multilevel-inverter-controlled system. A method is proposed for the determination of the minimum amplitude of the triangular carrier for smooth modulation at fixed switching frequency. It is shown that the multilevel modulation based on the phase-shifted carriers significantly reduces the ripple magnitude in the switching function and allows the use of a smaller carrier amplitude under closed loop. This increases the forward gain and, hence, improves the tracking characteristics. The proposed cascaded multilevel inverter control is implemented for the operation of a distribution static compensator (DSTATCOM) in the voltage control mode. The experimental verification of the theoretical and simulation results is provided through a field-programmable gate array (FPGA) based control of a laboratory model of a single-phase DSTATCOM.

A new adaptive hysteresis current control with unipolar PWM used in active power filters

This paper presents a new adaptive hysteresis current control with unipolar pulse width modulation based on magnitude and time errors. Variable switching frequency due to fixed hysteresis bands is known as a drawback of hysteresis current control used in power system applications, which generates sub-harmonics and low order harmonics, and affects the quality of the power systems. Adaptive hysteresis band is a classical method to control the switching frequency. Several issues and solutions have been discussed and proposed in this paper to keep the switching frequency constant. Simulations have been carried out to verify the proposed adaptive hysteresis band controller and the results are discussed in this paper.

A Carrier-Based Unipolar PWM Current Controller That Minimizes the PWM-Cycle Average Current-Error Using Internal Feedback of the PWM Signals

A carrier-based unipolar pulsewidth modulation (PWM) current controller is described using a one-phase PWM rectifier bridge as a test circuit. The controller PWM signal generator uses a sawtooth carrier signal to obtain both synchronized PWM waveforms and a constant switching frequency. A current-error signal and an amplitude modulation depth signal are used to create the input reference signal to the PWM signal generator. The amplitude modulation depth signal is obtained from the PWM-cycle average of the PWM signal generator output signals. This internal feedback is illustrated using a second-order low pass Butterworth filter. With an appropriately designed low-pass filter, the current-error signal is forced to be centered on zero over a PWM cycle. The maximum peak-peak magnitude of the current-error signal is used as a design guideline for deciding an appropriate gain constant in the controller current feedback loop. The low pass filter design is examined to determine a suitable gain and 20-dB attenuation frequency to minimize the current-error signal. The steady-state and transient operation of the proposed current controller is compared with a proportional-integral controller and an average current-error controller. SPICE simulations and experimental results are used to demonstrate the characteristics of the controller.

Elimination of the Output Voltage Imbalance in a Half-Bridge Boost Rectifier

In this paper, a four-switch voltage-doubler boost rectifier is analyzed. A new control scheme is proposed to eliminate the output voltage imbalance when the load is unevenly distributed across the two output dc rails. Unipolar pulsewidth modulation switching patterns are adopted to slow down the discharging rate of the output capacitor with the lower voltage. No dc or out-of-phase component is injected in the source current. Thus, the unity input power factor can be genuinely achieved. Two hysteresis comparators are employed in the presented controller. One is used for the source current to track its command. The other comparator is to confine the output voltage imbalance within a reasonable level. Modified averaged circuit models are adopted to derive the limits of the ratio of the dual loads for output voltage balance. Experimental results on prototype circuits confirm tightly with the theoretical analysis.

A comparative study of shunt hybrid and shunt active power filters for single-phase applications: Simulation and experimental validation

The aim of this paper is to compare the performance of the single-phase shunt active power filter (SPSAPF) and the single-phase shunt hybrid power filter (SPSHPF) that adopt both an indirect current control scheme with a unipolar pulse width modulation (U-PWM) strategy. The SPSHPF topology includes, in addition to the components of the SPSAPF, a power factor correction capacitor connected in series with a transformer. The primary winding of the transformer is connected to the single-phase voltage–source inverter, which is the main part of the filter. The indirect current control technique that is implemented for both filters is based on extracting the source current reference from the distorted waveform of the load current. The U-PWM control technique is based on comparing simultaneously a triangular high frequency carrier signal with a slow-varying regulation signal and its opposite. The double comparison process results in the gate signals for the semiconductors. A laboratory prototype for each filter is built. It is demonstrated that the rating of the inverter used in the SPSHPF is three to four times lower than the one corresponding to the SPSAPF. In addition, the performance of the SPSHPF is found to be much better than that of the SPSAPF as far as the line current distortion is concerned.

A multi-loop voltage feedback filterless class-D switching audio amplifier using unipolar pulse-width-modulation

Voltage feedback is frequently used in class-D switching audio power amplifiers. This paper discusses the design and implementation of a low-cost filterless class-D, unipolar pulse-width modulation switching audio amplifier having a multi-loop voltage feedback scheme. Classical frequency-compensation techniques are used to design and stabilize the three voltage feedback loops implemented in this application. This design method proves to be a cost-effective solution for designing high-fidelity (hi-fi) audio amplifiers. The cost is reduced because no output filter is used, the required switching frequency is half of the one needed if bipolar PWM was used, and no current sensor is needed for feedback purposes. The output impedance is extremely low due to the reduction of the successive voltage loops, making the amplifier less load dependent. Simulation results show that a total harmonic distortion (THD) of 0.005% can be achieved using this topology, as well as a flat frequency response, free of phase distortion in the audio band. Experimental results show the feasibility of this control scheme, since a THD of 0.05% was achieved with a laboratory prototyped amplifier. A comparison of the performance of this audio amplifier with that of some commercial class-D audio amplifiers, reveals that our design can seriously compete with some of the ICs leading the market at a lower cost.

Single-phase half-bridge rectifier with power factor correction

A single-phase half-bridge switching mode rectifier is presented to draw a sinusoidal line current, to achieve power factor correction and to maintain the DC-link voltage constant. Four active switches are used in the proposed rectifier to generate a unipolar pulse width modulation (PWM) voltage waveform on the AC terminal voltage. There is no clamping diode in the proposed rectifier (as compared with the neutral point clamped (NPC) converter) so as to achieve three-level PWM operation. Two control loops are used in the proposed control scheme. In the outer control loop, a proportional integral voltage controller is used to regulate the DC-link voltage. A phase lock loop circuit is adopted to generate a sinusoidal waveform in phase with mains voltage to achieve power factor correction. In the inner control loop, a carrier-based current controller is used to track the line current command. To compensate the neutral point voltage due to the load variation, a neutral point voltage compensator is used in the control scheme. Three voltage levels are generated on the AC terminal of the adopted rectifier. Computer simulations and experimental results are presented to verify the effectiveness of the proposed control algorithm.

Implementation of a single-phase AC/AC converter based on neutral-point-clamped topology

A single-phase ac/ac converter based on neutral-point-clamped scheme is proposed to perform unity input power factor and to provide a stable ac voltage to the critical loads. The ac/dc rectifier part is controlled to generate a unipolar pulsewidth modulation (PWM) waveform on the ac terminal by using four power switches with voltage stress of half the dc-link voltage. The carrier-based current control scheme is employed in the inner control loop to track the line current command. To regulate the dc bus voltage, a proportional-integral (PI) control is adopted in the outer control loop. The dc/ac inverter part of the system with four power switches is employed to generate a stable and clean sinusoidal output voltage to the critical load. The instantaneous current control scheme is used to track the output voltage command. To verify the effectiveness of the proposed control algorithm, the simulation and experimental results based on a laboratory prototype were implemented and discussed.

A composite soft-switching inverter configuration with unipolar pulsewidth modulation control

This paper presents a new composite soft-switching configuration for single-phase inverters where power bridge leg modules are used. The presented configuration consists of only one inductor and one capacitor as well as two low-power-rated switches/diodes for full-bridge circuits. It can realize snubber functions and/or resonant zero-current switching at any load current for switches in power inverters with unipolar sinusoid pulse width modulation control. The idea presented here is that soft-switching processes at turn-on and turn-off for each active switch in inverters can be different. The detailed circuit operational processes, simulation waveforms, and experimental results are included.

Dual hysteresis loops for a high-performance four-switch boost rectifier

A new current controller adopting dual hysteresis loops for a high-performance four-switch boost rectifier is proposed in this letter. The four-switch boost rectifier is operated under the so-called phase-adjusted unipolar pulsewidth modulation strategy. The inner loop confines the current ripple within a preset band. The outer loop determines the instants to change the switching patterns. Experimental results are satisfactory.

Analysis of pulse width modulated systems and their saturated modes of oscillation

The paper studies the modes of oscillations in control systems which employ pulse width modulation power dispatch techniques in their loops for a wide class of linear time invariant systems. Particular care is taken of the mode of oscillation under which instability can occur. Both bipolar and unipolar pulse width modulation is considered and conditions which produce chaotic behaviour in the plant response are investigated.

Low-distortion variable-level PWM technique

A multicarrier disposition pulse width modulation (PWM) technique that results in low harmonic distortion for a three-phase PWM inverter with a 5-level line-to-line output voltage waveform is discussed in the paper. Comparison with the conventional 5-level unipolar PWM technique for three-phase inverters reveals that the proposed strategy results in a high performance converter with minimised filter requirements since the amplitude of all harmonics is reduced. The resulting line-to-line voltage waveform has five levels at high and three levels at low amplitude modulation indexes (variable-level). Harmonic analysis is presented, along with simulated and experimental results to support the theoretical considerations.

Techniques for minimizing the input current distortion of current-controlled single-phase boost rectifiers

Techniques for minimizing the input current distortion of current-controlled single-phase boost rectifiers are described. The switching patterns of several boost rectifiers are examined to identify the nature of their input current waveforms. This analysis is used to examine the low-frequency current distortion levels, and hence the power quality, associated with the rectifiers. A PWM (pulse width modulation) strategy that selectively switches between positive unipolar PWM and negative unipolar PWM, called phase-adjusted unipolar PWM, is shown to produce the lowest current distortion levels. A novel two-switch asymmetrical half-bridge rectifier is presented that draws an input current at a unity fundamental power factor and with the same low distortion as obtained with the four-switch H-bridge rectifier. The operation of the various rectifiers is examined with reference to theoretical predictions, circuit simulations, and experimental results. This analysis is used to compare the performances of the various rectifier switching patterns. >