Showing papers on "Pulse-width modulation published in 1993"

Multiple access with time-hopping impulse modulation

Abstract: A time-hopping modulation format employing impulse signal technology has several features which may make it attractive for multiple-access communications. These features are outlined, an estimate of the multiple-access capability of a communication system employing this format under ideal propagation conditions is presented, and emerging design issues are described. >

Apparatus for leakage control and method for its use

Abstract: Apparatus and method find leakage due to tissue load or transients and has active and return electrodes for RF energy sensed by transformers for a circuit that finds leakage at more than two thousand times per second or a frequency dependent on the phase shift between voltage and current. Software and feedback manage V rms to reduce the voltage or increase the crest factor, which is V rms divided by peak voltage, by pulse width modulation of the RF drive. Voltage and current phase shift changes, angle θ, are compared to a threshold, and when greater, the frequency at which the differences between the active and return current signals are examined is increased. The differences between the active and return currents are examined and when greater than maximum leakage while in coagulation mode, the RF drive pulse width is reduced to maintain voltage wave peaks at a preset value and reduce V rms thereby reducing leakage to the allowable maximum. Reducing the duty cycle or the pulse width of the output wave increases the crest factor. When the differences between active and return currents are greater than a maximum for leakage while in cut or bipolar mode, the V rms is reduced to limit leakage to a maximum level. When the phase angle θ is less than a threshold, the frequency with which leakage is examined is reduced.

A new bandwidth efficient transmit antenna modulation diversity scheme for linear digital modulation

Abstract: A system is proposed and analyzed, which uses several transmit antennas to achieve diversity in a flat fading mobile radio environment. The signals at different antennas carry the same digital information, but have different modulation parameters (modulation diversity). In contrast to other proposals, no bandwidth expansion is required. Modulation diversity for quadrature amplitude modulation (QAM) type modulation is achieved by using different partial response type base pulses. Simulation results confirm the expected gain. >

Novel zero-current-transition PWM converters

Abstract: A new family of zero-current-transition (ZCT) pulse-width-modulated (PWM) converters are proposed. The new family of converters implements zero-current turn-off for power transistor(s) without increasing voltage/current stresses and operates at a fixed frequency. The proposed converters are deemed most suitable for high-power applications where the minority-carrier semiconductor devices (such as IGBTs, BJTs, and MCTs) are predominantly used as the power switches. Theoretical analysis is verified on a 100-kHz, 1-kW ZCT-PWM boost converter using an IGBT. >

Digital micro-mirror based image simulation system

Abstract: An image simulation system 20 for testing sensor systems 26 and for training image sensor personnel wherein synthetic image data is generated by a scene generator 21 and projected by an image projector 23. The image projector 23 uses a digital micromirror device array 27 to modulate the incident energy and create an image. Four modulation schemes are discussed including digital pulse-width modulation, phase contrast modulation, full complex modulation, and analog modulation. The digital pulse width modulation technique will typically require synchronizing the image sensor and the image projector. Phase contrast modulation, full complex modulation, and analog modulation do not require synchronizing the image projector 23 and the sensor system 26. Phase contrast modulation and full complex modulation have the capability to produce phase information within the image. The image simulation system 20 can produce high contrast images and is more flexible than prior art system.

Small-signal modeling of average current-mode control

Abstract: A recently proposed average current-mode control method is analyzed. A complete small-signal model for the control scheme is developed. The model is accurate up to half the switching frequency. This control scheme is suitable for applications where the average inductor current needs to be controlled, as in power factor correction circuits and battery charger dischargers. The subharmonic oscillation, commonly found in peak current-mode control, also exists in this method. This subharmonic oscillation can be eliminated by properly choosing the proper gain of the compensation network in the current loop. Model predictions are confirmed experimentally. >

On continuous control of PWM inverters in the overmodulation range including the six-step mode

Abstract: The power output and the dynamic performance of PWM (pulse width modulated) controlled AC motor drives can be improved by increasing the inverter output voltage through overmodulation. Two different solutions are proposed to increase the output voltage in a continuously controllable fashion up to maximum possible value, which is reached in the six-step mode. The solutions differ in their approaches. A space vector strategy is used for high dynamic performance, high switching frequency drives, while a field-oriented PWM method is used for low switching frequency, high power inverters. Experimental results are presented. >

A space vector-based rectifier regulator for AC/DC/AC converters

Abstract: A voltage-sourced rectifier control scheme for use with AC/DC/AC variable speed drives is presented. A control scheme is derived that directly calculates the duration of time spent on the zero state and on each switching state adjacent to the reference vector, over a constant switching interval, in order to drive the line current vector to the reference vector. In addition, under transient conditions, when deadbeat control is not possible, a control scheme is presented that ensures that the line current vector is driven in the direction of the reference current vector. The current reference for the rectifier controller is derived from the bus voltage error and a feedforward term based on the estimated converter output power. The proposed space vector-based rectifier regulator is shown to exhibit improved harmonic and transient performance over existing per-phase duty cycle prediction methods, especially at modulation indices near unity. The deadbeat control of the rectifier input current is accomplished every half-cycle with constant switching frequency while still symmetrically distributing the zero state within the half-cycle period. >

Reduction of power supply EMI emission by switching frequency modulation

Abstract: Electromagnetic interference (EMI) emission is always of grave concern for power electronic circuit designers. Due to rapid switching of high current and high voltage, interference is a serious problem in switching power circuits. Here, the effect of frequency modulation on power supply EMI noise is investigated. Significant reduction of emission is possible with the proposed scheme. >

Analysis of variation of neutral point potential in neutral-point-clamped voltage source PWM inverters

Abstract: The authors present the analysis of the neutral-point potential variation of the neutral-point-clamped voltage source PWM (pulse-width-modulation) inverter (NPC-VSI) for AC motor drives and static VAr compensators (SVC). The potential variation is analyzed with the focus on the current flowing out of or into the neutral point of the DC link. The theoretical minimum capacity of the DC link capacitors is discussed for its application to both a vector-controlled induction motor system of 2.2 kW and a SVC system of 10 MVA, 6.6 kV, 60 Hz. It is shown that the proposed control scheme makes it possible to suppress the neutral-point potential variation within a few percent, so that the total capacity of the DC link capacitors in the NPC-VSI is almost the same as that in the conventional voltage source inverter. >

Random pulse width modulation techniques for converter fed drive systems-a review

Abstract: A review of the state of the art in RPWM (random pulse width modulation) theory and practice is presented. Topics covered include principles of RPWM, means of randomization, a review of the existing RPWM techniques, power spectra, implementation issues, and documented effects of RPWM on electric drive systems. A number of RWPM strategies have been reported, and a beneficial impact on acoustic noise and vibration has been unamiously agreed upon. Published studies have proven that significant improvement of the acoustic noise characteristics of the motors can be achieved at practically no extra expense in comparison with the systems with traditional, deterministic PWM strategies. >

A rule-based fuzzy logic controller for a PWM inverter in a stand alone wind energy conversion scheme

Abstract: The paper presents a rule-based fuzzy logic controller to control the output power of a pulse width modulated (PWM) inverter used in a stand-alone wind energy conversion scheme (SAWECS). The self-excited induction generator used in SAWECS has the inherent problem of fluctuations in the magnitude and frequency of its terminal voltage with changes in wind velocity and load. To overcome this drawback the variable magnitude, variable frequency voltage at the generator terminals is rectified and the DC power is transferred to the load through a PWM inverter. The objective is to track and extract maximum power from the wind energy system and transfer this power to the local isolated load, This is achieved by using the fuzzy logic controller which regulates the modulation index of the PWM inverter based on the input signals: the power error; and its rate of change. These input signals are fuzzified, that is defined by a set of linguistic labels characterized by their membership functions predefined for each class. Using a set of 49 rules which relate the fuzzified input signals to the fuzzy controller output, fuzzy set theory and associated fuzzy logic operations, the fuzzy controller's output is obtained. The fuzzy set describing the controller's output (in terms of linguistic labels) is defuzzified to obtain the actual analog (numerical) output signal which is then used to control the PWM inverter and ensure complete utilization of the available wind energy. The proposed rule-based fuzzy logic controller is simulated and the results are experimentally verified on a scaled down laboratory prototype of the SAWECS.

Analysis and synthesis of randomized modulation schemes for power converters

Abstract: After establishing that the proper objects of study for randomized modulation of converters are the power spectra of signals, we classify such modulation schemes and present associated spectral formulas, several of which are new. We also discuss numerical (Monte Carlo) verification issues for power spectral formulas. A general spectral formula for stationary randomized modulation schemes is presented, and specialized to several modulation schemes of practical interest for DC/DC converters. Analytical results are then given for block-stationary randomized modulation schemes that are suitable for inverter operation. In the process, we present results for several modulation schemes that have been reported in the literature without analytical explanations. Experimental verifications of some of our analytical results are presented. We formulate narrow-band and wide-band synthesis problems in randomized modulation, and solve them numerically. Our results suggest that randomized modulation is very effective in satisfying narrow-band power constraints, but has limited effectiveness in meeting wide-band constraints. >

Circuit topologies for single-phase voltage-doubler boost rectifiers

Abstract: A new family of single-phase voltage-doubler PWM (pulse width modulated) boost rectifiers is presented. By examining the switching states of several standard single-phase boost rectifier circuits, three characteristic PWM voltage switching patterns are identified: unipolar PWM; bipolar PWM; and phase-adjusted unipolar PWM. From this analysis, an equivalent family of voltage-doubler rectifiers is derived. When high output voltages are required, voltage-doubler rectifiers are shown to be able to generate AC line currents with the lowest current distortion. The circuits presented are examined using circuit simulators and experimental results. >

Energy-conservation approach to modeling PWM DC-DC converters

Abstract: A systematic method is presented for including parasitic resistances and offset voltage sources of power switches in averaged dynamic large-signal, DC, and small-signal circuit models of pulse-width modulated (PWM) converters operating in continuous conduction mode (CCM). This method is based on the principle of energy conservation. The approach takes into account the inductor current ripple. For zero-ripple current, the method gives the same results as the state-space averaging method. Reflection rules are introduced and used to simplify the models. As an example, a modeling procedure for the PWM buck converter is detailed. >

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

Abstract: 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. >

An improved full-bridge zero-voltage-switched PWM converter using a saturable inductor

Abstract: The saturable inductor is employed in the full-bridge (FB) zero voltage switched (ZVS) pulse width modulated (PWM) power converter to improve its performance. The current and voltage stresses of the switches as well as parasitic oscillations are significantly reduced compared to those of the conventional FB-ZVS-PWM power converter. The qualitative analysis is presented and is verified on a 500 kHz, 5 V/40 A converter. >

A pulsewidth-modulated inverter with parallel connected transistors using current-sharing reactors

Abstract: A technique for parallel connection of transistors by using current-sharing reactors for the pulse-width-modulated (PWM) transistor inverter is reported. The technique not only increases current capacity, but also decreases the output harmonic contents. The output voltage waveforms of the proposed inverter have certain voltage levels during their half cycles, and thus it is anticipated that it will be difficult to analyze the output harmonics. For such waveforms, a frequency analysis approach is described, and its results are verified by experiments. >

High-frequency link DC/AC converter with suppressed voltage clamp circuits-naturally commutated phase angle control with self turn-off devices

Abstract: High-frequency (HF) link circuit topology is advantageous in realizing compact and light-weight power converters for UPS systems, new energy systems, etc. Among its several circuit configurations, the so called "cycloconverter type" DC/AC converter has its inherent merit of bidirectional power flow and fewer power conversion stages. But, without an additional voltage clamp circuit, a voltage surge occurs at the moment of PWM switching of the cycloconverter due to the stored energy in the leakage inductance at the HF transformer. In this paper, a new approach which adopts only natural-commutation phase angle control for the cycloconverter stage instead of PWM is proposed to remove such voltage clamp circuits. Experimental results of the prototype system which include the application as an AC active filter are also shown.

Electro-thermal simulation of an IGBT PWM inverter

Abstract: A recently developed electro-thermal network simulation methodology is used to analyze the behavior of a full-bridge, pulse-width-modulated (PWM), voltage-source inverter which uses insulated gate bipolar transistors (IGBTs) as the switching devices. The electro-thermal simulations are performed using the Saber circuit simulator and include the control logic circuitry, the IGBT gate drivers, the physics-based IGBT electro-thermal model, and the thermal network component models for the power device silicon chips, packages, and heat sinks. It is shown that the thermal response of the silicon chip determines the IGBT temperature rise during the device switching cycle. The thermal response of the device TO247 package and silicon chip determines the device temperature rise during a single phase of the 60-Hz sinusoidal output. Also, the thermal response of the heat sink determines the device temperature rise during the system start-up and after load impedance changes. It is also shown that the full electro-thermal analysis is required to accurately describe the power losses and circuit efficiency. >

Power spectra of a PWM inverter with randomized pulse position

Abstract: Random pulse width modulation in static power converters results in the partial transfer of power from the discrete spectrum of the output voltage to the continuous spectrum, with advantageous effects on the operation of the supplied electromechanical systems. In this paper, a random PWM technique with randomized pulse position for three-phase voltage-controlled inverters is analyzed. Closed-form equations for the discrete and continuous power spectra of the line-to-line and line-to-neutral voltages of the inverter have been derived and confirmed by experiments. Presented theory opens the way to numerical optimization of the voltage spectra of randomly modulated inverters. >

Space vector PWM technique with minimum switching losses and a variable pulse rate

Abstract: A novel PWM technique for three-phase voltage-controlled power inverters, based on the concept of voltage space vectors, is described. The minimum-loss, variable pulse rate (MLVPR) strategy, employing an implicit asymmetrical modulating function, results in the switching losses in the inverter reduced by half in comparison with the classic space vector PWM method. The pulse rate is varied from one 60/spl deg/ sector of the vector space to another, which produces spread, partly continuous spectra of the output voltage of the inverter. Theoretical analysis and results of experimental investigation of a prototype modulator are presented. >

Combined motor drive and battery recharge system

Abstract: A combined battery recharge and motor drive system employs an essentially conventional polyphase pulse width modulated (PWM) inverter and a polyphase motor which may be reconnected to operate in a battery recharge mode. Single phase or three-phase AC power from an external source is applied across the reconfigured circuit, and the inverter switches are then controlled to operate as a boost switching regulator. Electrostatic (Faraday) shielding techniques and construction allow extremely small ground currents for improved safety an GFI outlet compatibility.

Unity PF current-source rectifier based on dynamic trilogic PWM

Abstract: One remaining step in perfecting the stand-alone, unity power factor, regulated current-source pulse-width-modulated (PWM) rectifier is to reduce cost, by bringing the twelve-valve converter (consisting of three single-phase full bridges that operate with two-level or bilogic PWM) to the six-valve bridge. However, the six-valve topology requires a three-level or trilogic PWM strategy that can handle feedback signals. A general method of translating three-phase biologic PWM signals into three-phase trilogic PWM signals is presented. The method of translation retains the characteristics of the bilogic PWM, including the frequency bandwidth. Experiments show that the trilogic PWM signals produced by the method can handle not only stabilizing feedback signals but also signals for active filtering. >

Microprocessor control of multiple peak power tracking DC/DC converters for use with solar cell arrays

Abstract: A method and an apparatus for efficiently controlling the power output of a solar cell array string or a plurality of solar cell array strings to achieve a maximum amount of output power from the strings under varying conditions of use. Maximum power output from a solar array string is achieved through control of a pulse width modulated DC/DC buck converter which transfers power from a solar array to a load or battery bus. The input voltage from the solar array to the converter is controlled by a pulse width modulation duty cycle, which in turn is controlled by a differential signal comparing the array voltage with a control voltage from a controller. By periodically adjusting the control voltage up or down by a small amount and comparing the power on the load or bus with that generated at different voltage values a maximum power output voltage may be obtained. The system is totally modular and additional solar array strings may be added to the system simply be adding converter boards to the system and changing some constants in the controller's control routines.

Random telegraph signals from proton-irradiated CCDs

Abstract: It has been shown that proton-induced defects in charge couple devices (CCDs) can demonstrate the classic phenomenology of random telegraph signals (RTSs). These fluctuations take the form of RTSs with well-defined amplitudes and time constants for the high and low dark current states. The time constants are strongly temperature-activated and the evidence suggests the presence of a bistable defect whose structural reconfigurations cause changes in the dark current. Though an important noise source for room-temperature systems, the RTS pulse width is increased on cooling and the effect is not likely to be important below approximately -20 degrees C. Annealing of the RTS defect was found to occur at approximately 100 degrees C. >

Single stage, high power factor, gas discharge lamp ballast

Abstract: A gas discharge lamp ballast provides near unity power factor simultaneously with high-frequency lamp ballasting in a single switching power conversion stage resulting in efficiency improvements, reduction in size and weight and reduced component count and cost. The single conversion stage comprises a fast-recovery diode connecting in series the input inductor, energy transfer capacitor and the resonant matching network, and switching means alternately connecting the first junction between the input inductor and energy transfer capacitor, or the second junction between the matching network and the other side of said capacitor to the return current path. The switching means comprises two current bidirectional switches driven out of phase, thus producing a square-wave high frequency voltage source, which is in turn converted by the resonant matching network into a sine-wave ac current source required by the gas discharge lamp. The fast-recovery input diode in conjunction with the input inductance chosen to be less than the critical inductance value forces the input inductor current into a new discontinuous inductor current mode (DICM). The average input inductor current is shown to closely follow the rectified line voltage when the ballast is operated at the constant duty ratio and at the constant switching frequency either open-loop or with the slow feedback loop of conventional PWM control. Zero voltage switching for the two current bidirectional switches is achieved by introducing two transition intervals during which both switches are OFF and utilizing the negative value of the lagging current of the matching network above resonance.

Wide load range three-level ZVS-PWM DC-to-DC converter

Abstract: A new three-level ZVS-PWM DC-DC converter operating in a wide load range is introduced. It is used for high-voltage and high-power applications and has as its main features: zero-voltage-switching (ZVS) from no load up to full load, operation at constant frequency, regulation by pulse-width modulation (PWM), and low RMS current stress upon power switches. Its greatest attribute is that the voltage across the switches is half the value of the input voltage. A prototype operating at 100 kHz, rated at 600 V input voltage, 1.5 kW output power, and 25 A output current, has been fabricated and successfully tested in the laboratory. The principle of operation, theoretical analysis, design procedure, and an example, along with experimental results, are presented. >

Soft-switching converter DC-to-DC isolated with voltage bidirectional switches on the secondary side of an isolation transformer

Abstract: A soft-switching dc-to-dc converter for providing regulated voltage to a load combines the functions of loss-less soft switching at zero voltage and at a constant switching frequency on the primary side and soft switching at zero current on the secondary side of an isolation transformer for all load currents from full load to no-load and PWM output voltage regulation on the secondary side. This is made possible by using voltage bidirectional switching element in series with a low-pass filter on the secondary side and current bidirectional switching element on the primary side driven at close to 50% duty ratio with adjustable dead time between switching. The adjustable dead-time is realized by sensing voltages across paired primary side current bidirectional switching element in order to enable alternate turn ON of said pair only at zero voltage, thereby eliminating or greatly reducing switching losses under all load current conditions. The output voltage is regulated by PWM control of the voltage bidirectional switching element comprising one of the following: a bipolar transistor, or a MOSFET in series with a diode, or preferably a saturable reactor in series with a diode to form a magnetic amplifier on the secondary side.

Force commutated HVDC and SVC based on phase-shifted multi-converter modules

Abstract: The authors examine how presently available gate-turn-off thyristors (GTOs), which are still relatively slow, can be used in force-commutated high-voltage direct-current (HVDC) and static VAR compensator (SVC) converters by employing multiconverter modules in conjunction with a phase-shifting principle which cancels the undesirable switching harmonics. They point out that incorporating the sinusoidal pulse-width modulation (SPWM) technique enables feedback control, active filtering and regulatory functions to be performed by the converters. This is because a reasonable bandwidth of the modulating signal is transmitted by the multiconverter station in spite of the low switching rates of the GTO valves. >