Per Ranstad

Automated Design of a High-Power High-Frequency LCC Resonant Converter for Electrostatic Precipitators

This work presents an automated design procedure for series parallel resonant converters (LCC) employed in electrostatic precipitator (ESP) power supplies, which reduces the designer effort significantly. The requirements for the power supplies in ESP applications and means to derive an accurate mathematical model of the LCC converter, such as the power loss from commercial insulated-gate bipolar transistors, are described in detail in this paper. The converter parameters, such as resonant tank elements, are selected in order to improve the overall efficiency of the system, when a typical ESP energization operation range is considered. The analysis comprises two different control strategies: the conventional variable frequency control and the dual control. Both control strategies are analyzed by comparing semiconductor losses of five commercial modules. Finally, the circuit operation and design are verified with a 60 kW LCC resonant converter test setup.

On Dynamic Effects Influencing IGBT Losses in Soft-Switching Converters

Two different dynamic effects influencing the insulated gate bipolar transistor (IGBT) losses in soft-switching converters are demonstrated. The first one, the Dynamic tail-charge effect shows that the tail charge is dependent not only on the absolute value of the current at turn-off, but also on the dynamics of the current. This effect may have a significant impact on the optimization of zero-current-switching converters. The Dynamic conduction losses originate from the conductivity modulation lag of the IGBT. It is shown by experiments that the on-state losses depend on the operating frequency. Different methods to accurately determine the on-state losses are evaluated. It was found that the best method is an indirect measurement, where the stray inductance is identified by the use of an oscillating circuit. The experiments are performed under a sinusoidal current excitation at a fixed amplitude (150 A) for different frequencies (up to 104 kHz). The switching devices used are IGBT modules rated 300-400 A/1200 V in a bridge-leg configuration. From the experiments performed, it is found that IGBTs of a modern punch-though (PT) designs have the lowest losses in the series-loaded resonant converters studied in this paper.

An Experimental Evaluation of SiC Switches in Soft-Switching Converters

Soft-switching converters equipped with insulated gate bipolar transistors (IGBTs) in silicon (Si) have to be dimensioned with respect to additional losses due to the dynamic conduction losses originating from the conductivity modulation lag. Replacing the IGBTs with emerging silicon carbide (SiC) transistors could reduce not only the dynamic conduction losses but also other loss components of the IGBTs. In the present paper, therefore, several types of SiC transistors are compared to a state-of-the-art 1200-V Si IGBT. First, the conduction losses with sinusoidal current at a fixed amplitude (150 A) are investigated at different frequencies up to 200 kHz. It was found that the SiC transistors showed no signs of dynamic conduction losses in the studied frequency range. Second, the SiC transistors were compared to the Si IGBT in a realistic soft-switching converter test system. Using a calorimetric approach, it was found that all SiC transistors showed loss reductions of more than 50%. In some cases loss reductions of 65% were achieved even if the chip area of the SiC transistor was only 11% of that of the Si IGBT. It was concluded that by increasing the chip area to a third of the Si IGBT, the SiC vertical trench junction field-effect transistor could yield a loss reduction of approximately 90%. The reverse conduction capability of the channel of unipolar devices is also identified to be an important property for loss reductions. A majority of the new SiC devices are challenging from a gate/base driver point-of-view. This aspect must also be taken into consideration when making new designs of soft-switching converters using new SiC transistors.

On the Distribution of AC and DC Winding Capacitances in High-Frequency Power Transformers With Rectifier Loads

In this paper, a method to adjust the ac winding capacitance of high-voltage high-frequency transformers by means of winding-rectifier integration is described. First, a theoretical background to the method is given. From the theory, an equivalent circuit describing the characteristics of the combination of the transformer and the rectifier is derived. The derived circuit introduces the concept of a dc capacitance. Finally, the equivalent circuit and the method itself are verified by means of experiments on a transformer-rectifier system from an industrial application with the ratings 70 kV, 30 kW, and 25 kHz. The results from the experiments show that it is possible to vary the ac component of the winding capacitance from a few percent up to 95% of the total winding capacitance. This means that it is virtually free to choose between ac and dc capacitances during the design stage. This is very important in applications such as resonant converters with transformers having secondary windings connected to rectifiers with capacitive output filters.

Comparison of 2- and 3-level active filters with enhanced bridge-leg loss distribution

In this paper an efficiency comparison between 3-phase shunt active filters derived from the 2-level VSC, the 3-level NPC, Active NPC (A-NPC) and the T-type VSC is presented. In order to address the loss distribution issue of the 3-level topologies, while keeping the efficiency of the system high, a space vector modulation scheme incorporating a special clamping of the phase is proposed. It is shown that 3-level active filters can have their losses well distributed over the chip dies, leading to only a small difference in their operating temperatures. Additionally, a semiconductor area based comparison is used to further evaluate the studied active filter systems. Finally, experimental results obtained with a 12kVAr/48kHz 3-level NPC based shunt active filter employing custom SiC power modules are presented in order to demonstrate the performance and feasibility of this solution.

A novel control strategy applied to the series loaded resonant converter

An novel control strategy of the series loaded resonant converter is presented. The main objective of the control strategy is to minimize the switching losses in the main switching elements (IGBT). The results are experimentally verified on a 60 kW/25 kHz prototype converter. The IGBT losses obtained with the proposed control strategy are compared with those of the commonly used frequency control and phase-shift control strategies. With the proposed control strategy it is found that the losses are reduced in the entire operating range compared to phase-shift control. The same statement is valid also for frequency control, except for the highest output voltage

Optimal design of resonant converter for Electrostatic Precipitators

This work presents a design optimization procedure for Series Parallel Resonant Converters (LCC) employed in Electrostatic Precipitator (ESP) power supplies. The system parameters, such as resonant tank elements, are selected in order to reduce semiconductor losses when a typical ESP energization operation range is considered. Here, the sum of the power losses of the switches are predicted for a set of parameters by mathematical models of the LCC resonant converter, and also by loss characteristics of suitable commercially available IGBTs obtained from experimental analysis and datasheet values. The analysis comprises two different control strategies: the conventional Variable Frequency (VF) control and the Dual Control (DC). Finally, the circuit operation and design are verified with a 60kW charging capability LCC resonant converter test set-up. Both control strategies are analyzed by comparing semiconductors losses for five commercial modules.

Comparison of electrostatic precipitator power supplies with low effects on the mains

In this article, the distortions caused by the power supplies employed in Electrostatic Precipitators (ESP) are investigated, and means for improving the line power quality are proposed. Multi-pulse and PWM rectifier topologies and other concepts including hybrid systems and active filters are evaluated in order to identify suitable systems for ESP applications. A comparison of the studied systems rated to 60 kW and fully designed employing commercial components is shown. The ESP systems efficiency, power density, current harmonic THD, among others features are used for the assessment. The loss calculations are extended to a variable chip area to allow a fair comparison between the studied systems. Finally, the VIENNA 6-switches rectifier and active filter concepts are chosen and experimental analyses are carried out, verifying the performance and feasibility of the proposed systems.

Three elements resonant converter: the LCC topology by using MATLAB

The behaviour of a series-parallel resonant power converter is analysed by means of MATLAB simulations. PSPICE and experimental results are also presented. Stress parameters are calculated and compared with those of the series-loaded resonant power converter.

Investigation of long-term parameter variations of SiC power MOSFETs

Experimental investigations on the gate-oxide and body-diode reliability of commercially available Silicon Carbide (SiC) MOSFETs from the second generation are performed. The body-diode conduction test is performed with a current density of 50 A/cm2 in order to determine if the body-diode of the MOSFETs is free from bipolar degradation. The second test is stressing the gate-oxide. A negative bias is applied on the gate oxide in order to detect and quantify potential drifts.