Hans Ertl

Influence of the modulation method on the conduction and switching losses of a PWM converter system

The dependency of the conduction losses of a bridge leg of a PWM power converter system with a high pulse rate on the shape of the phase modulation functions is explored. This is done for modulation methods which are optimized with respect to minimum harmonic current RMS values. The results are compared to the results gained for simple sinusoidal modulation. Besides the conduction losses, the switching losses of the electric valves are calculated. Deviations from the classical sinusoidal modulation are only obtained for modulation methods for which the output voltage is formed by a cyclic change via only two active and a third nonswitching bridge leg. As the calculations show, these modulation methods allow a significant increase of the effective switching frequency. This effect is dependent on the phase angle between the fundamental of the converter output phase voltage and the converter output phase current; for this comparison, equal switching losses, as for the simple sinusoidal modulation, are assumed.<<ETX>>

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

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.<<ETX>>

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

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. >

EMI filter design for high switching frequency three-phase/level PWM rectifier systems

The actual attenuation characteristic of EMI filters in practice often differs from theoretical predictions and minor changes could result in a significant performance improvement. Whereas the performance of the differential-mode (DM) filter stage usually can be well predicted, the common-mode (CM) behavior is more difficult to handle. This is especially true for three-phase PWM rectifier systems, which show a large high-frequency CM voltage at the rectifier output. In this work the possible CM noise current paths of a three-phase/level PWM rectifier are analyzed where parasitic capacitances to the heat sink and to earth are considered. Additionally, a concept to significantly reduce CM emissions is discussed in detail. Based on the proposed models an EMI filter design for a system with 1 MHz switching frequency is shown. Experimental verification of the designed EMI filter is presented by impedance and conducted emission (CE) measurements taken from a 10 kW hardware prototype. Several practical aspects of filter realization like component arrangement, shielding layers, magnetic coupling etc. are discussed and verified by measurements.

A novel single-switch three-phase AC/DC buck-boost converter with high-quality input current waveforms and isolated DC output

A new three-phase single-switch AC-DC flyback power converter system is presented. The system operates in the discontinuous mode. The simple structure of its power and control circuit, low mains current distortion and resistive fundamental behavior as well as the high-frequency isolation of the controlled output voltage are pointed out. Besides the analysis of the stationary operating behavior the dependencies of the peak values, average values and RMS values of the device currents and of the maximum blocking voltages across the power electronic devices on the circuit parameters are given as analytic approximations. The theoretical analysis is verified by digital simulation.<<ETX>>

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

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.<<ETX>>