RU2265948C1 - Pulse transformer of network voltage and method for controlling said transformer - Google Patents

Abstract

FIELD: technology for improving technical-economical coefficients of single-clock pulse transformers, receiving power directly from alternating voltage network via rectifier without use of smoothing capacitors.

SUBSTANCE: decreased commutation losses and over-voltages in force transistor circuit are achieved due to draining of current of this transistor immediately prior to disabling into parallel circuits, containing previously charged damping capacitor of comparably low capacity, splitting diode and auxiliary transistor key of comparably low power.

EFFECT: decreased commutation losses and over-voltages in force transistor circuit.

2 cl, 7 dwg

Description

The invention relates to a conversion technique and can be used to improve the technical and economic indicators of pulse converters that are powered directly from the AC network, by reducing the number of components, as well as reducing the current load of power transistor switches at the time of switching and the resulting overvoltage.

Converters of this type are traditionally performed according to a structure comprising a mains voltage rectifier, a capacitor filter for smoothing the rectified voltage, a matching transformer and power transistor switches [see, for example, 1) Moin B.C. Stabilized transistor steam converters .: M. Energoatomizdat, 1986. 2) Severns R., Bloom G. Pulse DC-DC converters for secondary power supply systems: M. Energoatomizdat, 1988.]. The transfer of the transformer from the input power circuits with an industrial frequency of 50 Hz to the output circuits of the converter with a higher modulation frequency up to tens and hundreds of kHz can significantly reduce the overall dimensions of this element, which is the main advantage of this structure. Its further improvement is possible by reducing the capacity or even completely eliminating the smoothing capacitor from the converter, which, as a rule, has rather large dimensions and cost. Such an exception is possible on the basis of direct high-frequency conversion of the mains voltage, leading to the elimination of the low-frequency network component in its composition in a modulation manner without smoothing capacitors [see Sidorov S.N. A method for controlling a transistor converter with a single-phase high frequency link. RF patent No. 2227958, publ. in BI No. 12, 2004]. It is shown that such elimination is possible by leveling the volt-second area of voltage pulses in the primary windings of the transformer, which leads to a significant decrease in the low-frequency component in its composition to a level of several% in circuits with preliminary single-phase two-half-wave rectification or even to complete elimination of this component in schemes with three-phase rectification . However, in most cases, the converters of this type must meet not one, but a set of requirements, which include, first of all, the minimum level of switching losses and overvoltages in the circuits of power transistors. The proposed technical solution is an attempt to solve these problems by installing at the output of the rectifier, instead of a smoothing capacitor, a similar element of another purpose - a damping non-polar capacitor of much lower capacity and cost. The closest solution, based on the use of capacitors in pulse converters, is contained, for example, in articles 1) S. Clemente, B. Pelly, R. Ruttonsha A universal power source with a frequency of 100 kHz on one MOSFET. - Power semiconductor devices. Per. from English under the editorship of V. Tokareva. Voronezh, 1995. 2) P. Ugrinov Voltage limitation on a key transistor in single-cycle voltage converters. - Power Electronics, No. 1, 2004. Thematic supplement to the journal "Components and Technologies".

The known solution assumes the presence in the circuit of a single-cycle pulse converter of the primary power source in the form of a rectifier of the mains voltage on the diodes, as well as a matching transformer, the primary winding of which, shunted in the opposite direction by a discharge diode, is connected in the conducting direction of the current to the poles of the rectifier using a power transistor switch, as well as the presence of a damping capacitor, one of the plates of which is connected to the positive pole of the rectifier, and the other plate under lyuchena in a direction to charge the capacitor to the negative pole of the rectifier via a separating diode.

A known method of controlling a converter of this type involves the formation and pulse-width regulation of the voltage in the load circuit by cyclic connection with a high frequency of the primary winding of the matching transformer using a power transistor switch to the source of the rectified mains voltage with subsequent reduction or elimination of the low-frequency component in the composition of this voltage, for due to the proper selection of switching moments, leveling the volt-second area of the pulse voltage in the primary windings of said transformer during each half-wave of mains rectified voltage without its smoothing and use the capacitor at the source output as a damping element, serving for limiting switching surges and losses in the power transistors by reducing the magnitude and rate of change of current prior to switching off.

To solve the above problems, it is proposed to additionally introduce an auxiliary transistor switch of relatively low power into the circuit of a single-cycle pulse converter, connected by the first output to the connection point of the damping capacitor in the direction of its discharge with the anode of the isolation diode, and the second output connected to the connection point of the transformer primary winding with the power transistor the key. The proposed method for controlling an auxiliary transistor switch is aimed at reducing current and power losses in the power transistor switch when they are locked and consists in short-term periodic switching on of the auxiliary transistor at times immediately preceding the switching off of power transistors, followed by simultaneous switching off of power and auxiliary transistor switches.

Figure 1, 2 shows the results of computer simulation using the program "Electronics Workbench" of electromagnetic processes in the Converter, adopted as a prototype;

figure 3 presents a simplified model of the Converter with a one-time on-off power transistor, confirming the validity of the proposed solution; figure 4 - the results of computer simulation of a single-cycle pulse converter, the operation of which occurs in accordance with the proposed technical solution.

To familiarize yourself with the operation of the device adopted as a prototype, Fig. 1 shows a diagram of a single-cycle pulse converter, the power of which is supplied from a single-phase AC source by means of a rectifier, in the output circuits of which there is no smoothing capacitor. It is seen that in the case of traditional control of a power transistor with a constant duty cycle of control pulses, the presence in the supply voltage of low-frequency ripples with a frequency of 100 Hz leads to significant undesirable ripples with the same frequency of the load current at the converter output (see Fig. 1a). Nevertheless, by applying a control method leading to equalization of the volt-second areas of the output pulse voltage, according to the RF Patent No. 2227958, the indicated low-frequency component in the current can be significantly reduced (see Fig. 1b), and even completely eliminated when powered by a three-phase source (see figb). Thus, in cases of the formation of a current of a given shape, the implementation of a pulse converter and its supply from an alternating voltage network is possible in a modulating manner without the use of smoothing capacitors of significant capacity.

However, in practice, converters of this type must meet several requirements, which, in addition to the lack of overall and expensive smoothing capacitors, include the minimum level of switching losses and overvoltages in power transistor circuits. The solution to this problem is known, for example, from the practice of resonant inverters and involves turning off the power transistor when the current and voltage in the power circuit reach a minimum, in particular zero, level. However, pulse-width regulation under resonance conditions is difficult, and therefore the application fields of this solution are limited. Therefore, it is proposed to reduce the current of the power transistor before turning it off by creating parallel circuits, in one of which a damping capacitor is installed, which has the necessary excess voltage compared to the voltage in the power transistor circuit. The principal possibility of such a withdrawal of current in parallel circuits is illustrated by the simplified model of the converter in Fig. 3, which operates, for the sake of clarity, in a one-time on-off mode. The converter contains a network power supply with a rectifier on diodes 1, a matching transformer 2, the primary winding of which is connected to the rectifier using a power transistor switch 3, made in this case by an uncontrolled composite circuit of connecting two bipolar transistors. In parallel to the rectifier poles, a damping capacitor 4 is connected with a diode 5 connected in series in the direction of the capacitor charge. To accomplish this task, the collector circuit of the power transistor switch is proposed to be connected to the connection point of the capacitor and diode using a switching element 6, the role of which in the model under consideration is played by the switch Space A current sensor Iv is installed in the output circuit of the power transistor, which allows you to observe the process of switching the power transistor using a virtual oscilloscope. It is believed that in the initial state, Space is in the open lower position. When a voltage is applied, the current Iv begins to increase, while the capacitor 4 is charged to a value not less than the amplitude value of the supply voltage by the time moment coinciding with the middle of the oscilloscope screen. Putting Space in the turned on position leads to the attachment of the negative capacitor plate to the output circuit of the power transistor, which helps to drive the current of the primary winding of the transformer from the circuit with the power transistor into two parallel circuits with a capacitor 4 and a diode 5. As a result, as can be seen from the waveform , the current of the power transistor switch almost instantly decreases to zero, and the voltage on its collector remains negligible, equal to the voltage drop across the isolation diode. In this case, conditions are created for the subsequent switching off of the power transistor in almost de-energized state at a low voltage at the power terminals. In real conditions, this means a significant reduction in switching power losses in the transistor, and along with switching overvoltages on this main element of the circuit. It can be seen that the implementation of this task requires the use of a damping capacitor of relatively small capacity, the choice of which should ensure the equality or some excess of half the period of natural oscillations of the LC circuit in which the capacitor is recharged, in comparison with the proper turn-off time of the power transistor.

A real diagram of the proposed device is depicted in figure 4. In addition to the above elements, the circuit contains an auxiliary transistor switch 6 instead of the Space switch, which is connected for a short time using a special Regul 3 control device with some lead relative to the moments when the power transistor switch turns off. Since the electromagnetic switching processes are independent of the modulation method, consideration of the operation of this device was carried out with a constant duty cycle of the control pulses. To monitor current changes in circuits with power and auxiliary transistors, as well as in a circuit with a capacitor, current sensors DT1 - DT3 are used. The waveforms in FIG. 4a illustrate the shape of the current in the power transistor switch circuit (upper beam) and the auxiliary transistor current (lower beam). It can be seen that short-term switching on of the auxiliary transistor does occur with a lead with respect to the moments when the power transistor is turned off and are accompanied by the removal of the current of the latter into a parallel circuit. Subsequent shutdowns of the power and auxiliary transistors occur simultaneously and for each - at a lower current compared to the amplitude value of the load current. Studies of the circuit showed that the degree of decrease in the current of the power transistor before turning off depends both on the lead time when the auxiliary transistor is turned on and on the degree of saturation of the latter. In any case, the average current of the auxiliary transistor is less than the switched load current, and therefore the power of this element is relatively small. The oscillograms in Fig. 4b illustrate the current shape of the damping capacitor (lower beam), which confirms the oscillatory nature of the recharging of this capacitor and the correctness of the recommendation for choosing its capacitance.

Claims (2)

1. A pulsed voltage converter containing a primary power source in the form of a rectifier of the mains voltage on the diodes, as well as a matching transformer, the primary winding of which is shunted in the opposite direction by a discharge diode, is connected in the conductive direction to the poles of the rectifier using a power transistor switch, and containing a damping capacitor, one of the plates of which is connected to the positive pole of the rectifier, and the other plate is connected in the direction of charge of the condenser at the negative pole of the rectifier using an isolation diode, characterized by the presence of an auxiliary transistor switch of relatively low power, which is connected to the connection point of the damping capacitor with the anode of the separation diode by the first output, and connected to the connection point of the transformer primary winding to the discharge terminal of the damping capacitor transistor key. 2. A method of controlling a single-cycle pulse converter, which provides for the formation and pulse-width regulation of voltage by cyclic connection with a high frequency of the primary winding of the matching transformer using a power transistor switch to a source of rectified mains voltage, as well as eliminating the low-frequency component with a frequency in the voltage of the primary winding of the transformer network due to the proper selection of switching moments, leveling volt-second squares of pulses during each half-wave of the rectified voltage without prior smoothing it and using a capacitor at the output of the power source as a damping element, which serves to limit switching overvoltages and losses in power transistors by reducing the magnitude and rate of change of current before switching them off, characterized in that the specified current reduction is provided by parallel connection to the power transistor pre-charged on the previous extra-switching nnom range snubber capacitor towards its discharge through the auxiliary transistor, followed by turning off the power transistor and removing the gate on pulse to the control input of the auxiliary transistor.

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