Showing papers by "Johann W. Kolar published in 1991"

Minimizing the current harmonics RMS value of three-phase PWM converter systems by optimal and suboptimal transition between continuous and discontinuous modulation

Abstract: A given converter voltage space vector can be realized by a three-phase PWM converter system by switching only two bridge legs. There, the third phase is clamped to the positive or negative DC link voltage. This method is called discontinuous modulation, due to the discontinuous shape of the generating phase modulation functions. As a closer analysis shows, the discontinuous method allows an increase in the effective system pulse frequency, dependent on the phase angle between converter output voltage and output current. The basis for a comparison with the continuous method is chosen as equal average switching losses of any converter bridge leg. For harmonic-optimal operation one must change between continuous and discontinuous modulation (or between various variants of the discontinuous modulation), depending on the load status. The harmonic losses of the control methods are calculated directly in the time domain. For this purpose, the space vector calculus is applied and approximations are used which are sufficiently exact for PWM converter systems with high pulse frequency. >

Analysis of a Smith-predictor-based-control concept eliminating the right-half plane zero of continuous mode boost and buck-boost DC/DC converters

Abstract: It is pointed out that the small-signal transfer function of a boost or a buck-boost converter shows a zero located in the right-half complex plane for continuous operation This zero makes the stabilization of the system considerably difficult The authors use a boost converter as an example to show that application of the principle of linear prediction (as originally proposed for the control of systems with dead times) yields the mirror image of the control circuit zero This shift of the zero into the left-half s-plane is achieved with unchanged location of the poles Thereby the small signal transfer function is split into two parts: a phase-minimum system and a nonphase-minimum system The latter can be interpreted as a linear Pade approximation of a dead-time element For the design of a robust control system it is necessary to investigate the shift of the poles of the closed system depending on the change of the operating point How the predictor influences the small- and large-signal system disturbance responses for changes of the load and for input voltage changes is also analyzed >

Analysis of on- and off-line optimized predictive current controllers for PWM converter systems

Abstract: The problem of on-off current control for coupling of a DC voltage system with a three-phase (polyphase) AC voltage system via a pulsewidth modulated (PWM) converter is discussed. The AC voltage represents either the counter EMF (electromotive force) of an AC machine or the three-phase power supply system (mains). The following control concepts are investigated by digital computer simulation: a simple hysteresis controller; a predictive controller with online optimization (optimization with respect to minimum switching frequency); and a controller based on offline optimization (using a switching table). It is shown that the relatively involved predictive controller can be replaced by a switching table of very limited size. For rating of the treated controllers the switching frequency as a function of the RMS voltage of the AC system and the other system parameters is used. >

Analysis of turn-off behavior and switching losses of a 1200 V/50 A zero-voltage or zero-current switched IGBT

Abstract: The authors describe the development of the basics for the determination of the maximum obtainable switching frequencies of a 20 kW, 820 V/50 V full-bridge DC-DC converter designed with (second-generation) 1200 V/50 A IGBTs (insulated-gate bipolar transistors). Because the turn-on losses can be almost completely avoided by phase-shift PWM (pulse-width modulation), emphasis is placed on the determination of the turn-off losses. The dependencies of the switching power losses per switching cycle on the load condition and on the junction temperature are investigated for the following cases: simple PWM (switching without snubber), switching with capacitive parallel branch (turn-off snubber network), and force commutation of the power electronic switches using a commutation circuit (zero-current switching). The results obtained indicate the possibility of rating the overall effort linked to a given operating mode of the power electronic devices. The limits of the operating region of the converter system can also be determined. >

A Bidirectional DC-DC Converter—Analysis and Control Design

Abstract: A system for DC-DC power conversion based on a buck-boost converter topology is presented which makes power flow in both directions possible. The possibility of bidirectional power flow is useful for certain applications, such as uninterruptable power supplies (UPS) etc. The structure is compared with the well known unidirectional buck-boost converter. Open-loop control is treated based on simulation using duty cycle averaging. The system behaviour of the bidirectional converter is analyzed; a structure diagram is given and the transfer function of the system is derived. The validity of the duty cycle averaging is proven by comparison to a switched model. The controller for the converter is then realized as simple voltage controller, as voltage controller with an inner-loop current controller (cascade control) and with two kinds of state space control. The transfer functions of the different system parts are derived and dimensioning guide-lines for the controller sections are presented. The closed-loop be...