RU2251786C2 - Voltage converter and its control process - Google Patents

Abstract

FIELD: electrical engineering; static power converters.

SUBSTANCE: steady mode of sine-wave current resonant excitation is organized in voltage converter power circuit incorporating series LC circuit by means of proposed process, frequency of this sine-wave current being in step with operating clock frequency of converter. To this end automatic operating frequency and phase control unit is inserted in inverter circuit, its phase being synchronous with characteristic oscillations of series resonance-tuned LC circuit formed by isolating capacitor and dissipation inductance of power transformer.

EFFECT: reduced loss at switching members of inverter due to sine-wave type of handled current.

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Description

The invention relates to the field of electrical engineering, in particular to static voltage converters of a DC source of one level to a constant voltage of another level with galvanic isolation between them.

Known "Converter with serial resonance" Japan patent No. 701017084, H 02 M 3/335 (Inventions of the world. Issue 107, No. 18/98). However, the presented Converter does not provide galvanic isolation of the converted voltage.

The essence of this patent, judging by its description, is that in case of voltage dips to maintain power to the load due to reactive energy stored in the resonant circuit, the inverter keys are switched to the resonant current switching mode of the circuit. But in view of the lack of feedback of the resonant circuit with the proposed control circuit, stable synchronous operation of the inverter keys at the resonance frequency is not provided.

Also known is a bridge voltage converter with a series resonant circuit in the diagonal of an alternating current bridge. Such converters are united by a common feature: the implementation in them of transient processes that are close to sinusoidal (Journal of Electronic Components No. 6, 2002, article, Sbrodov A. "The choice of power transistors for voltage converters with a resonant circuit"). But the inverter key management process is different. The switching frequency of the keys does not coincide with the resonant frequency of the LC circuit and is always lower, and the average value of the output voltage of the converter is controlled by changing the pause duration between the switching series of the inverter keys. As a consequence of this pause, in order to ensure the continuity of the current in the load, an inductive-capacitive filter is required after the output rectifier. Resonant oscillations during periods of switching series in this converter are used to reduce dynamic losses on the inverter keys.

The closest in technical essence is a pulse power source (Berezin O.K., Kostikov VG, Shakhnov VA “Sources of power supply of electronic equipment”. Moscow, 2000). Its structural and electrical circuits are presented in Figure 3.1, 3.11.

A direct current energy converter at the input of direct current energy at the output with galvanic isolation from the primary source, consisting of a transistor bridge inverter, the diagonal of the alternating voltage of which includes a series-connected separation capacitor and a primary winding of a power transformer, to the secondary winding of which a half-wave output rectifier is connected, to the output of which a smoothing inductor and a capacitor of the output filter are connected, while the control device Valid PWM control inverter connected to the output and its output connected to the input pre-amplifier, the output of which is connected to the control inputs of the transistor bridge inverter; auxiliary voltage control device and pre-power amplifier provides an additional power source.

However, in this solution there is no mode of resonant excitation of a sinusoidal electric current, the frequency of which coincides with the clock operating frequency of the converter.

A known method of controlling a DC voltage converter of one level to DC voltage of another level, as follows: interrupt the voltage of the primary source with a fixed frequency by means of an electronic switch, rectify the obtained alternating impulse voltage with a half-wave rectifier and average it with an inductive-capacitive filter, while changing the level of the received DC voltage is carried out by pulse width modulation duration openly about the state of the switch keys. The method is described in the article Berezin O.K., Kostikov V.G., Shakhnov V.A. “Power sources of electronic equipment”. Moscow, 2000, Ch. 3.2.

A key element of the novelty of the converter control method is the organization in the power circuit containing a sequential LC circuit by automatically adjusting the stable mode of the resonant excitation of a sinusoidal electric current, the frequency of which coincides with the clock operating frequency of the converter.

The objective of the invention is to provide a DC-DC energy converter, which reduces losses on the key elements of the inverter and rectifier diodes.

The problem is solved due to the fact that the inverter control circuit additionally includes an automatic adjustment unit for the operating frequency of the converter, synchronous with the natural oscillations of the series LC resonance circuit formed by the separation capacitance and dissipation inductance of the power transformer. In series with the separation capacity and the primary winding of the transformer, an inductor is additionally included, which reduces switching losses from overcharging the stray capacitance of this winding, and also creates an inductance that complements the dissipation inductance of the transformer.

The Converter is built according to the circuit shown in Fig.1.

U1 is the input voltage of the converter, U2 is the output voltage of the converter. The inverter 1, the transistor bridge 2 of which, like the prototype, is equipped with shunt diodes, is controlled by the control device 10 through a preliminary power amplifier (PUM) 11. The PUM provides galvanic isolation with the control inputs of the inverter transistors, as is done in the prototype [1] figure 3.26, figure 3.27. A power resonant circuit consisting of a series-connected capacitor 3, an inductor 4 and a primary winding of the transformer 5 is included in the diagonal of the alternating current of the bridge.

The primary winding of the current transformer 6 is also sequentially included in the circuit as a current sensor. The output rectifier 7 is connected to the output filter 12. For the operation of the converter, it is crucial that the rectifier is half-wave (bridge or with a midpoint on the secondary winding of the transformer), the output capacitive filter must be connected directly to the output of the rectifier without a choke.

The control device is built on the basis of a microprocessor or using specialized microcircuits (see [2]), for example, 1156ЕУ2, TL494CN.

The control device generates control signals to the input of the preliminary power amplifier and has a feedback loop "a" with the converter output by PWM regulation of the output voltage.

Two additional feedback loops have been introduced into the converter control circuit. The first of them "b" closes with the control device through the node of the automatic frequency and phase 9.

The phase locked loop (PLL) is built according to one of the schemes used in television and radio communication equipment. In this case, the phase of switching the inverter keys according to the signal from the control device output is compared with the current phase in the power circuit controlled by current sensor 6. Alternatively, the voltage on the secondary winding of the transformer can be integrated in the closed state interval of the inverter keys. The impact on the switching frequency of the inverter keys is made by connecting the output of the PLL to the frequency input of the control device. Thus, the converter clock frequency is captured by the frequency of the resonant current oscillations in the LC circuit.

To improve the dynamic characteristics of the converter, which determine the reaction rate to disturbing influences in terms of input voltage and load, a second additional feedback circuit "c" is formed through the switching unit of the recovery mode 8 with a current sensor 6 and with the output of the control device. The output of the mode switching device is connected to the input of the preliminary power amplifier at the point of control of a pair of, for example, lower transistors at different shoulders of the inverter bridge.

One of the two modes is the mode with partial recovery of the resonant current energy in the power circuit. At the moments when all the keys are closed during the operation of the inverter, the loop current flows through the shunt diodes to the primary source to meet its voltage.

The second mode is characterized by the absence of a recovery phase, in which the inverter keys connect the power circuit in-phase to the oscillations of its current either to the voltage of the primary source (phase of energy extraction from the source), or close the loop current through itself, preserving its energy.

With a sharp load shedding or an abrupt change in the setting parameter, a transient process with overshoot is possible due to the energy inertia of the oscillating circuit, characterized by a damping decrement.

The switching unit of the recovery mode 8 generates a transition signal from the first mode to the second and vice versa. At start-up and with reduced power supplied to the load, the converter operates in a recovery mode.

With an increase in the load, which is reflected in an increase in the current in the power circuit, and, accordingly, the signal of the current sensor 6, the switching unit of the recovery mode 8 generates a signal to the control device 10 to switch to the operation mode without recovery, in which the upper pair of inverter keys is still controlled latitudinally -modulated signals, and the lower pair is switched according to the signals of the current sensor 6 in phase with the direction of the current in the power circuit at the clock frequency of the converter in the same way as in the prototype (see diagram moo O 3 and O 4 in Figure 3.11 [1]). For this key pair in the non-regenerative mode, the duty cycle of the pulse-width modulated pulses (modulation coefficient K), which determine the open state of the inverter keys, is unity (K = 1).

With a sharp load drop on the converter, the PWM controller responds to this by reducing the pulse width to a minimum. This serves as a signal for the transition to the first mode of operation with recovery, in which there is a return of energy to the primary source. Return to the second mode of operation of the converter occurs automatically according to the algorithm described above.

The control of the mode, in fact, consists in switching the duty cycle of the width-modulated signals that control in this case the lower key pair, from the current value, which is identical with the modulation of the signals of the upper pair, to K = 1 and vice versa.

The power of the PLLs and switching the recovery mode, as well as the control device and the preliminary power amplifier, is provided by an additional power supply (DIEP), which is not galvanically connected with the primary power source U1 and with the power circuit as a whole. To simplify the drawing, the DIEP is not shown in FIG. 1.

The equivalent electrical circuit of the power circuit of the Converter and voltage diagrams on the sections of the circuit in this mode are presented in figure 2 and 3, where:

G1 - a generator of bipolar voltage pulses with an internal resistance equal to the internal resistance of the primary source, taking into account the voltage drop on the public keys of the inverter and its inverse diodes in the conduction state;

C1 is the resonant capacitance;

L1 - transformer dissipation inductance, total with complementary inductance;

VD1 - a half-wave rectifier;

1, 2, 3 - characteristic points of the equivalent circuit.

Between points 2 and 3, the equivalent of the secondary side of the transformer is shown, reduced through the transformation coefficient to the primary side, where:

C2 is the equivalent of the capacity of the output filter,

Rн is the equivalent load;

Ia is the sinusoidal current in the power circuit;

Ua is the bipolar voltage in the diagonal of the alternating current of the inverter bridge;

Ua1 is the voltage corresponding to the open state of one and the other pair of power switches, width-modulated in duration;

Ua2 is the voltage corresponding to the state of direct conduction of the reverse diodes of the inverter bridge (phase of recovery of the current of the power circuit);

Uв - voltage between points 2-3 in this circuit.

A sinusoidal current of a resonant frequency and odd harmonics currents many times attenuated by the loop inductance flow through a series LC circuit. The inverter clock frequency is determined by the fundamental harmonic of the circuit. The voltage of this frequency from the frequency spectrum of the equivalent generator on the circuit itself between points 1-2 in the measure of its quality factor is small and drops on the rectifier and the load.

The voltage Uv is a symmetrical bipolar meander, which is a characteristic result of the operation of the converter in the mode of resonant excitation of the power circuit at the inverter clock frequency.

The diagrams of figure 3 shows the role of the PLL, controlling and synchronizing the transitions through zero current Ia and voltage Ua at the phasing points t _f .

The meander voltage across the transformer depends on the amplitude of the sinusoidal current in the power circuit, which is modulated in accordance with pulse-width modulation of the open state of the inverter keys. Pulse-width modulation is carried out by changing the duration of the pause of the inverter voltage in the push-pull converter circuit with a constant conversion frequency.

The adjusting characteristic of the converter — the dependence of the output voltage U2 on the modulation coefficient K with an open feedback circuit — is presented in Fig. 4, where U1 is the input voltage of the converter.

Point “k” on characteristic Y corresponds to the equality of the energy supplied to the power circuit at each period (half period) of fluctuations in its current from the primary source, and the energy returned to the source in the recovery phase. Line X in figure 4 depicts the adjustment characteristic of the converter when operating without recovery of the resonant circuit current. As can be seen from the graph, the control of the output voltage of the converter in the recovery mode is more dynamic due to the greater slope of the characteristic Y compared to X.

The present method of controlling energy transformations using resonance in a power circuit and a PLL for synchronizing processes in a key converter is applicable for both full-bridge and half-bridge inverter circuits.

Several experimental samples were manufactured with converted voltage levels from 28 V to 400 V with a power of up to 1.5 kW. To control the inverter, controllers based on specialized microcircuits, in particular 1156ЕУ2, are used.

The technical result of the present invention is to reduce losses on the key elements of the inverter due to the sinusoidal nature of the switched current. In the absence of a choke in the output filter and a sinusoidal current in the power circuit, switching of the rectifier diodes occurs at zero current values (see diagrams of figure 3). Tests of experimental samples showed a more than twofold decrease in energy losses in the converter compared to the prototype when using the same elemental base.

Literature

1. Berezin O.K., Kostikov V.G., Shakhnov V.A. “Power sources of electronic equipment”. Ed. Moscow, 2000, UDC 621.314, pp. 121-128.

2. “Microcircuits for switching power supplies and their application”, Dodeka Publishing House, 2000, UDC 621.375 (03), pp. 239-246.

3. The journal "Practical Power Electronics", No. 7, 2002

4. IVSC Rospatent. Inventions of the countries of the world. Vol. 107, No. 9/98, p. 36, “A method for powering a transformer to prevent distortion in the shape of the supply voltage”.

5. IVSC Rospatent. Inventions of the countries of the world. Issue 107, No. 18/98, p. 36, “Converter with series resonance”.

6. Semenov B.Yu., “Power Electronics”, SOLON-R Publishing House, - 2001, Moscow.

7. Sbrodov A., “The choice of power transistors for voltage converters with a resonant circuit”, journal “Electronic components” No. 6, 2002, p. 68.

Claims (4)

1. A voltage converter of a direct current source at the input to a constant voltage at the output with galvanic isolation from the primary source, consisting of a transistor bridge inverter, the diagonal of the alternating voltage of which includes a series-connected separation capacitor and a primary winding of a power transformer, to the secondary winding of which a half-wave output is connected a rectifier, to the output of which a smoothing choke and an output filter capacitor are connected, while the device entry board for the PWM control inverter connected to the output, and its output is connected to the pre-amplifier input, the output of which is connected to the control inputs of the transistor bridge inverter; with auxiliary voltages, the control device and the preliminary power amplifier provide an additional power source, characterized in that the inverter control circuit additionally includes an assembly for automatically adjusting the operating frequency of the converter synchronous with the natural oscillations of the series LC resonant circuit formed by the separation capacitance and dissipation inductance of the power transformer, this is the secondary winding of the bridges connected to the diagonal of the alternating current circuit of the current inverter of the current transformer is connected to the first input of the auto-tuning unit, the second input of the auto-tuning unit is connected to the output of the control device, the output of the auto-tuning unit is connected to its frequency setting input. 2. The converter according to claim 1, characterized in that the current transformer is additionally included in the diagonal of the alternating current of the inverter bridge in series with the primary winding of the power transformer and the separation capacitance, and the power circuit current recovery mode switching device is added to the converter control circuit, the first input of which connected to the secondary winding of the current transformer, the second input is connected to the output of the control device, and the output of the recovery mode switching device is connected to the input do pre power amplifier. 3. The Converter according to claim 1, characterized in that in series with the separation capacitance and the primary winding of the transformer, an inductor is additionally included, which reduces switching losses from overcharging of the parasitic capacitance of this winding, as well as creating an inductance that complements the dissipation inductance of the transformer. 4. A method for controlling a DC-DC energy converter at the input of DC-energy at the output, as follows: they interrupt the voltage of the primary source with a given frequency using electronic keys, convert the obtained alternating pulse voltage to an alternating pulse voltage of a given level with a transformer, which is rectified by a half-wave rectifier and averaged using an inductive-capacitive filter, while regulating the voltage at the output of the converter They are carried out by pulse-width modulation of the duration of the open state of the keys at a constant conversion clock frequency, characterized in that they carry out an intermediate conversion of the energy of a pulsed alternating voltage into the energy of resonant oscillations in a serial LC circuit, for which electronic keys are controlled by automatic adjustment synchronously and in phase with the frequency of natural oscillations of this circuit, in this case, the resonant current is transformed, resulting in They obtain the voltage of the symmetric bipolar meander of the clock frequency, and using pulse-width modulation of the duration of the open state of the keys, they modulate the amplitude of the sinusoidal current in the LC circuit and, accordingly, the voltage level of the meander, after a biannual rectification of which they obtain an adjustable constant voltage at the output of the converter.

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