Series-connected Transformers Research Articles

ZVS Full Bridge Series Resonant Boost Converter with Series-Connected Transformer

This paper presents a study on a new full bridge series resonant converter (SRC) with wide zero voltage switching (ZVS) range, and higher output voltage. The high frequency transformer is connected in series with the LC series resonant tank. The tank inductance is therefore increased; all switches having the ability to turn on at ZVS, with lower switching frequency than the LC tank resonant frequency. Moreover, the step-up high frequency (HF) transformer design steps are introduced in order to increase the output voltage to overcome the gain limitation of the conventional SRC. Compared to the conventional SRC, the proposed converter has higher energy conversion, able to increase the ZVS range by 36%, and provide much higher output power. Finally, the a laboratory prototypes of the both converters with the same resonant tank parameters and input voltage are examined based on 1 and 2.2 kW power respectively, for veryfing the reliability of the performance and the operation principles of both converters.

Selective Harmonic Elimination in Multi-level Inverters with Series-Connected Transformers with Equal Power Ratings

This study applies the selective harmonic elimination (SHE) technique to design and operate a regulated AC/DC/AC power supply suitable for maritime military applications and underground trains. The input is a single 50/60 Hz AC voltage, and the output is a 400 Hz regulated voltage. The switching angles for a multi-level inverter and transformer turns ratio are determined to operate with special connected transformers with equal power ratings and produce an almost sinusoidal current. As a result of its capability of directly controlling harmonics, the SHE technique is applicable to apparatus with congenital immunity to specific harmonics, such as series-connected transformers, which are specially designed to equally share the total load power. In the present work, a single-phase 50/60 Hz input source is rectified via a semi-controlled bridge rectifier to control DC voltage levels and thereby regulate the output load voltage at a constant level. The DC-rectified voltage then supplies six single-phase quazi-square H-bridge inverters, each of which supplies the primary of a single-phase transformer. The secondaries of the six transformers are connected in series. Through off-line calculation, the switching angles of the six inverters and the turns ratios of the six transformers are designed to ensure equal power distribution for the transformers. The SHE technique is also employed to eliminate the higher-order harmonics of the output voltage. A digital implementation is carried out to determine the switching angles. Theoretical results are demonstrated, and a scaled-down experimental 600 VA prototype is built to verify the validity of the proposed system.

Analysis and Implementation of a New ZVS DC Converter for Medium Power Application

This paper presents a new zero voltage switching (ZVS) converter for medium power and high input voltage applications. Three three-level pulse-width modulation (PWM) circuits with the same power switches are adopted to clamp the voltage stress of MOSFETs at <TEX>$V_{in}/2$</TEX> and to achieve load current sharing. Thus, the current stresses and power ratings of transformers and power semiconductors at the secondary side are reduced. The resonant inductance and resonant capacitance are resonant at the transition interval such that active switches are turned on at ZVS within a wide range of input voltage and load condition. The series-connected transformers are adopted in each three-level circuit. Each transformer can work as an inductor to smooth the output current or a transformer to achieve the electric isolation and power transfer. Thus, no output inductor is needed at the secondary side. Three center-tapped rectifiers connected in parallel are used at the secondary side to achieve load current sharing. Compared with the conventional parallel three-level converters, the proposed converter has less switch counts. Finally, experiments based on a 1.44kW prototype are provided to verify the operation principle of proposed converter.

New ZVS DC--DC Converter With Series-Connected Transformers to Balance the Output Currents

A new zero-voltage switching (ZVS) dc converter for high input voltage and high output current applications is presented in this paper. Two three-level pulse-width modulation (PWM) circuits with the same active switches connected in parallel at the output side are adopted to achieve load current sharing and to decrease current stress of rectifier diodes. The center-tapped rectifiers are adopted at the secondary side to have only one diode conduction loss in the forward current path. The series resonant tank of three-level converter is used to achieve ZVS turn-on for all power switches and zero-current switching for all rectifier diodes within nearly entire load range. Thus, the switching losses of power switches are reduced and the reverse recovery losses of rectifier diodes are eliminated. In order to overcome the drawback of a wide range of switching frequency in conventional series resonant converters, the fixed frequency pulse-width modulation is adopted to regulate the output voltage. The series-connected transformers are used in the primary and secondary sides to share and balance the load current. Finally, experiments based on a laboratory prototype are provided to verify the operation principle and demonstrate the effectiveness of the proposed converter.

A New ZVS DC/DC Converter With Three APWM Circuits

This paper presents a new soft-switching dc/dc converter with series-connected transformers to implement the features of zero-voltage switching (ZVS), load current sharing, low voltage stress for MOSFETs, and low current rating for the rectifier diodes and the output inductors. The proposed converter includes two half-bridge circuits connected in series to limit the voltage stress of MOSFETs at one-half of the input voltage. The output sides of the two circuits are connected in parallel to reduce the current rating on the rectifier diodes and the output inductors and to achieve load current sharing. In order to balance two output inductor currents, series-connected transformers at the low-voltage side are adopted. Thus, the output power is equally distributed by the two half-bridge circuits. The output capacitances of MOSFETs and the resonant inductances are resonant at the transition instant such that MOSFETs can be turned on at ZVS. Finally, experiments based on a laboratory prototype with 1-kW rated power are provided to demonstrate the performance of the proposed converter.

Analysis of an Interleaved Three-Level ZVS Converter With Series-Connected Transformers

This paper presents a new interleaved three-level soft-switching converter with series-connected transformers to achieve zero-voltage switching for all MOSFETs, less transformer secondary windings, and partial ripple current cancellation. Two circuit modules are adopted in the proposed circuit to share the load current and reduce the ripple currents at the input and output capacitors with the interleaved pulse-width modulation (PWM). In each circuit module, two three-level PWM circuits with the same MOSFETs are operated with the phase-shift by one-half of switching period to reduce the input ripple current. The voltage stress of all switches is clamped at Vin/2. The secondary windings of two transformers are connected in series to balance the primary side currents. The current doubler rectifiers are adopted on the secondary side to partially cancel output inductor ripple current. Experiments with a 1-kW prototype, verifying the effectiveness of the proposed converter, are described.

400 W broadband power amplifier using transmission-line transformers with 1:8 impedance transformation ratio

This paper presents a broadband high-power amplifier using transmission-line transformers with a high impedance transformation ratio. Two series-connected transformers with 1:8 and 1:1 impedance transformation ratios are used at the load network of a single-balanced high-power amplifier to match the low load impedance in broadband. The same transformer configuration is also used at the input network. A negative feedback is applied to improve the gain flatness and to increase the stability over the broadband. In order to validate the proposed circuit structure, a broadband high-power amplifier is designed and implemented to achieve an output power of more than 400 W in the frequency band of 30 to 500 MHz. The implemented power amplifier exhibits a flat gain response within 1.80 dB at an average of 19.1 dB and high power-added efficiencies of 28.0 to 56.9 at an output power of 400 W over the entire frequency band.

Implementation of a ZVS Three-Level Converter with Series-Connected Transformers

This paper studies a soft switching DC/DC converter to achieve zero voltage switching (ZVS) for all switches under a wide range of load condition and input voltage. Two three-level PWM circuits with the same power switches are adopted to reduce the voltage stress of MOSFETs at Vin/2 and achieve load current sharing. Thus, the current stress and power rating of power semiconductors at the secondary side are reduced. The series-connected transformers are adopted in each three-level circuit. Each transformer can be operated as an inductor to smooth the output current or a transformer to achieve the electric isolation and power transfer from the input side to the output side. Therefore, no output inductor is needed at the secondary side. Two center-tapped rectifiers connected in parallel are used at the secondary side to achieve load current sharing. Due to the resonant behavior by the resonant inductance and resonant capacitance at the transition interval, all switches are turned on at ZVS. Experiments based on a 1kW prototype are provided to verify the performance of proposed converter.

Analysis and implementation of a new soft switching DC/DC PWM converter

This study presents a new DC/DC converter with series-connected transformers to achieve zero voltage switching (ZVS) for power switches, less transformer secondary winding and partial ripple current cancellation. Two three-level circuits using the same active switches are operated with interleaved half switching cycle. The voltage stress of all switches is clamped at Vin/2. The transformers secondary windings are connected in series to balance the primary side currents. The current-doubler rectifier is adopted on the output side. Thus, the output inductor ripple currents can be partially cancelled each other and the resultant ripple current at the output capacitor is reduced compared with the output ripple current of the centre-tapped rectifier topology. Based on the resonant behaviour by the output capacitance of active switches and the leakage inductance (or external inductance), all power switches are turned on at ZVS. Laboratory experiments with a 1 kW prototype, verifying the effectiveness of the proposed converter, are described.

Analysis, Design and Implementation of an Interleaved DC/DC Converter with Series-Connected Transformers

An interleaved DC/DC converter with series-connected transformers is presented to implement the features of zero voltage switching (ZVS), load current sharing and ripple current reduction. The proposed converter includes two half-bridge converter cells connected in series to reduce the voltage stress of the switches at one-half of the input voltage. The output sides of the two converter cells with interleaved pulse-width modulation are connected in parallel to reduce the ripple current at the output capacitor and to achieve load current sharing. Therefore, the size of the output chokes and the capacitor can be reduced. The output capacitances of the MOSFETs and the resonant inductances are resonant at the transition instant to achieve ZVS turn-on. In addition, the switching losses on the power switches are reduced. Finally, experiments on a laboratory prototype (24V/40A) are provided to demonstrate the performance of the proposed converter.

Implementation of parallel zero-voltage switching converter with series-connected transformers

SUMMARY This paper presents a parallel zero-voltage switching (ZVS) DC–DC converter with series-connected transformers. In order to increase output power, two transformers connected in series are used in the proposed converter. Two buck-type converters connected in parallel have the same switching devices. The primary windings of series-connected transformers can achieve the balanced secondary winding currents. The current doubler rectifiers with ripple current cancellation are connected in parallel at the output side to reduce the current stress of the secondary winding. Thus, the current ripple on the output capacitor is reduced, and the size of the output choke and output capacitor are reduced. Only two switches are used in the proposed circuit instead of four switches in the conventional parallel ZVS converter to achieve ZVS and output current sharing. Therefore, the proposed converter has less power switches. The ZVS turn-on is implemented during the commutation stage of two complementary switches such that the switching losses and thermal stresses on the semiconductors are reduced. Experimental results for a 528-W (48 V/11 A) prototype are presented to prove the theoretical analysis and circuit performance. Copyright © 2011 John Wiley & Sons, Ltd.

ZVS Resonant Converter With Series-Connected Transformers

This paper presents a new series-resonant converter with series-connected transformers in order to realize zero voltage switching (ZVS) for power switches, zero current switching (ZCS) for rectifier diodes at full load, less transformer secondary winding with a full-wave rectifier, and load current sharing. Two series-resonant converters using the same switches are operated with interleaved half switching cycle. The secondary windings of transformers are connected in series in order to ensure that the primary side currents are balanced to share load current. Thus, the sizes of the transformer core and bobbin are reduced. The full-wave diode rectifier is adopted on the output side such that the voltage stress of the rectifier diodes is equal to the output voltage, rather than being two times the output voltage as in a conventional center-tapped rectifier topology. Based on the resonant behavior, all switches are turned on at ZVS, and rectifier diodes are turned off at ZCS if the switching frequency is less than the series resonant frequency. The proposed converter can be applied for the power supply unit of all-in-one PCs, liquid crystal display (LCD) TVs and servers, and dc-dc converters for fuel-cell power system. Laboratory experiments with a 1000-W prototype, verifying the effectiveness of the proposed converter, are described.

The Effect of Series-Connected Transformer in DVR Applications

W. Chankhamrian and K. Bhumkittipich Abstract— This paper presents the effect of Series-Connected Transformer in DVR Applications. According to the DVR operation, the comparison of with and without series-connected transformer is studied. The simulation results show that the magnitude of load voltage without the transformer about 1.0 p.u. (per unit) more than with transformer. Moreover, it has not effect of phase shift and no voltage drop. Therefore, it can be ensured that the effect of transformer, that occurred on results, has to consider for DVR applications.

Multilevel Optimal Predictive Dynamic Voltage Restorer

This paper presents an optimal predictive controller for a multilevel converter-based dynamic voltage restorer (DVR), which is able to improve the voltage quality of sensitive loads connected to the electrical power network. The optimal predictive controlled multilevel DVR can restore sags and short interruptions while reducing the total harmonic distortion (THD) of the ac line voltages to values lower than 1%. The DVR is based on a three-phase neutral point clamped converter to dynamically inject a compensation voltage vector in series with the line voltage, through series-connected transformer secondary windings. To assure high-quality voltages for sensitive loads, we devise optimal predictive control laws for the injected compensation ac voltages. A suitable quadratic weighed cost functional is used to choose the voltage vector, minimizing both the ac voltage errors through current injection and the dc side capacitor voltage unbalancing. The performance of the proposed predictive controller is compared to classical proportional integral (PI): synchronous frame and stationary frame (P+resonant) controllers. The line-side filter capacitor topology is compared to the regular converter-side filter capacitor. Obtained experimental results show that the ac voltages are almost sinusoidal in steady-state operation when facing balanced and unbalanced sags and short interruptions with unbalanced loads. Voltage THD is reduced to values lower than 1%; the DVR is behaving also as a series active power filter for the ac voltages.

Analysis and Implementation of a ZVS-PWM Converter With Series-Connected Transformers

This brief presents the analysis, design, and implementation of zero-voltage switching (ZVS) active clamp converter with series-connected transformer. A family of isolated ZVS active clamp converters is introduced. The technique of the adopted ZVS commutation will not increase additional voltage stress of switching devices. In the adopted converter with series-connected transformer, each transformer can be operated as an inductor or a transformer. Therefore, no output inductor is needed. To reduce the voltage stress of the switching device in the conventional forward converter, the active clamp technique is used to recycle the energy stored in the transformer leakage back into the input dc source. Finally, experimental results are presented taken from a laboratory prototype with 100-W rated power, input voltage of 155 V, output voltage of 5 V, and operating at 150 kHz.

Analysis and implementation of soft switching converter with series-connected transformers

The system analysis and design consideration of a zero voltage switching (ZVS) converter with series-connected transformers are presested. Based on the operational behaviour, each transformer in the adopted converter can be operated as an inductor or a transformer. Therefore no output filter inductor is needed in the adopted converter. To reduce the voltage stress of the switching device in the conventional forward converter, an active snubber based on a clamp switch and a clamp capacitor is used to recycle the energy stored in the transformer leakage. During the transition interval, the resonance based on the junction capacitance of switches and transformer leakage inductance can achieve ZVS operation of switches. The centre-tapped rectifier is used at the secondary side to achieve full-wave rectification. The operating principles, steady-state analysis and design equations of the proposed converter are provided. Finally, experimental results for a 100 W (5 V/20 A) prototype circuit are provided to verify the converter performance.

A single-stage power factor correction AC/DC converter based on zero voltage switching full bridge topology with two series-connected transformers

A single-stage power factor correction ac/dc converter based on zero voltage switching (ZVS) full bridge topology with two series-connected transformers is proposed in this paper. The proposed converter offers a very wide ZVS range due to the configuration of two series-connected transformers. It features a high efficiency over wide load ranges. Furthermore, it shows the low voltage stress on a dc link capacitor. The proposed converter also gives the high power factor and low input current harmonics complied with IEC 61000-3-2 Class D requirements by integrating a boost stage operated in a discontinuous current mode. The ZVS conditions, large signal modeling, and design procedure are discussed in detail. Experimental results are presented to show the validity of the proposed converter.

Analog equivalent circuit aided design of ferroresonant transformers and circuits

This paper shows how an analog equivalent circuit of the ferroresonant series-connected secondary capacitor and high-leakage inductance transformer type may be used to design ferroresonant type regulating circuits for supplying power to nonlinear loads. The functions of a high-leakage inductance transformer are provided by two linear variable inductors and an adjustable highly nonlinear inductor. The functions of the nonlinear loads are provided by a biased diode type of load consisting of a bridge rectifier loaded by a large capacitor, shunted by a variable resistor, and in series with a small resistor or linear inductor. The series resistor simulates the dynamic resistance of a magnetron and the linear inductor simulates the dynamie characteristics of high pressure mercury vapor type of lamp loads. The parameters of the analog circuit are varied until the desired degree of regulation of output current and power vs. percent of supply line input voltage is had without excessive values of peak output current. The parameters are then reduced by calculation to a set of dimensionless parameters for use in designing the required high-leakage-inductance transformer and for specifying the value of the capacitor required in series with the transformer secondary.

Three-phase distribution and electric welding and furnace loads

The paper deals with the methods of connecting single-phase equipment to 3-phase mains. The single-phase transformer and the open-delta series-connected transformer are compared, both as regards current distribution and the voltages on the substation busbars. Mention is made of the power-factor correction of single-phase loads, from which there has been developed a method of obtaining balanced 3-phase/single-phase conversion by static means. The remainder of the paper gives full details of this phase-converting system for arc and resistance welding machines. Some actual operational results are tabulated.