RU2661495C1 - Resonant converter with switching frequency automatic phase tuning width-pulse adjustment method - Google Patents

Abstract

FIELD: electrical equipment.

SUBSTANCE: invention relates to the field of electrical equipment and can be used in one level direct current into the different level direct current conversion devices, in particular in power supply sources. Method of pulse-width adjustment of the output voltage of a resonant converter is that a predetermined voltage level is maintained on the load of the resonant converter, for which purpose the voltage value is measured on the load of the resonant converter and a constant voltage signal proportional to the load value is formed. Setting the setpoint voltage proportional to the supported voltage level on the load, and generating the difference signal (DS) using the proportional-integral-differentiating controller. AC signal proportional to the loop current is formed, it is converted into an AC voltage signal (ACVS) and the instant of the AC voltage signal transition through zero is monitored. Depending on the DS size, setting the PWM controller generation frequency, clocked by the voltage controlled oscillator (VCO), using which generating the certain width pulses. Pulses are supplied to the inverter key elements control electrodes, at that, providing the pulse width phase self-tuning, setting the filling factor so, at which the signal front VCO formation instant coincides in time with the instant of transition through the ACVS zero.

EFFECT: technical result consists in the pulse width phase auto-tuning by the loop current feedback introduction to provide the zero phase offset between the circulating in the loop current and control signals.

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Description

The invention relates to the field of electrical engineering, in particular to devices for converting direct current of one level into direct current of another level, and in particular to methods of controlling these devices. The invention is applicable to power supplies with output voltages from tens of volts to hundreds of kilovolts and an output power from units of watts to hundreds of kilowatts. Power supplies incorporating this invention can be used in the construction of power systems for radio transmitting devices, x-ray machines, lasers, as well as chargers, welding machines and other devices.

Resonant DC converters are known from the general prior art, in particular, a series resonant converter with the load connected in series to the LC resonant circuit or parallel to the resonator circuit capacitor (Meleshin V.I. Transistor converter technology. M .: Publishing House Technosphere, 2005, 623 p.) . Traditionally, the voltage at the output of the resonant converters is regulated by changing the switching frequency of the key elements of the inverter, which can be performed according to the bridge or half-bridge circuit. For regulation, operating frequencies are usually used above the resonant frequency of the LC circuit, since in this range the key elements operate in the “soft” switching mode, namely the switching mode at zero voltages, which provides a significant reduction in dynamic losses and an increase in the efficiency coefficient (Efficiency).

The main disadvantage when using the method of controlling pulse frequency modulation (PFM) is as follows: for a resonant converter circuit with a load connected in series to a resonant LC circuit, the range of adjustment of the output voltage is limited; for a circuit with a load connected in parallel to the resonant circuit capacitor, it preserves, and even increases, the power of losses with a decrease in the output current of the converter, which means low efficiency in such operating modes.

A known voltage converter and a method for controlling it (RF patent No. 2251786), which provides reduction of losses on the key elements of the inverter due to the sinusoidal nature of the switched current. In the voltage converter and the method of controlling it in a power circuit containing a series LC circuit, a stable mode of resonant excitation of a sinusoidal electric current is maintained, the frequency of which coincides with the clock operating frequency of the converter.

A technical solution is also known (RF patent No. 2427068), in which the aforementioned disadvantages are eliminated by applying the control of the output power of a resonant converter based on pulse-width modulation (PWM). A feature of the known method of regulation is the use of three control modes - PFM, PFM together with PWM and PWM. The PWM + PWM mode is used in case of light load (under light load in this case we mean the output current is much lower than the rated current), and the PWM mode in case of idle. This solution allows you to expand the range of output voltages of the region of small loads.

A disadvantage of the known technical solution is the threshold mode of operation, which negatively affects the efficiency for some combinations of output voltage and output current.

The technical problem, the solution of which is provided by the invention, is to increase the efficiency of the resonant converter, in particular, to increase the efficiency when working with output power levels much lower than the maximum.

The technical result consists in phase-locked loop switching frequency due to the introduction of feedback on the loop current to ensure zero phase shift between the current circulating in the loop and control signals.

The claimed technical result is achieved by the fact that in the method of pulse-width regulation of the output voltage of the resonant converter, including an inverter with key elements and a series oscillatory LC circuit, maintain a given voltage level at the load of the resonant converter, which measures the current voltage value at the load of the resonant converter and forms DC voltage signal proportional to the load, based on which using proportions Onal-integral-differentiating regulator provides the formation of a differential signal with respect to the set voltage proportional to the supported voltage level on the load, generates an AC signal proportional to the current strength in the oscillatory circuit, converts the AC signal into an AC voltage signal and controls the moment of transition of the AC voltage signal through zero, and depending on the value of the difference signal, pulses of variable width are formed using PWM-k an ontroller clocked by a controlled voltage generator, which is supplied to the control electrodes of the key devices of the inverter, while providing phase-locked loop switching frequency, setting the repetition rate of the pulses generated by the PWM controller, clocked by the said generator included in the phase-locked loop, such that the moment of formation mentioned the signal edge generator coincides in time with the moment of transition through zero of the said alternating voltage signal, proportional to nogo current strength in the resonant circuit.

The invention uses the principle of PWM + PWM control over the entire range, which will allow for higher efficiency.

One of the key advantages of resonant converters in comparison with other switching power supplies is the “soft” switching of key elements, which ensures a decrease in dynamic power losses and an increase in the efficiency of the converter. In a resonant converter with PFM control in the switching frequency range above the resonant frequency of the LC circuit, the conditions for ensuring the soft switching mode of key elements are fulfilled automatically. On the other hand, the advantages of a resonant converter are best shown in the vicinity of the operating point corresponding to the rated output voltage and current, as well as the maximum output power of the converter.

To solve the technical problem posed with obtaining the claimed technical result, PWM-based control is used, as well as combined PWM + PWM control. The technical complexity of the implementation of these types of control lies in the lack of automatic fulfillment of the conditions for ensuring the soft switching mode, which can lead to an increase in dynamic losses in key elements, a decrease in their reliability and MTBF, as well as directly to failure. The presence or absence of switching mode at zero voltage is affected by a combination of factors: switching frequency, filling width and output current of the converter (load). The consequence is the need for adjusting the parameters of the Converter. The control system must not only adjust the output voltage of the converter according to known mathematical formulas, but also "adjust the coefficients" in these formulas depending on the output current.

The claimed invention uses a simple criterion for the presence or absence of the mode of "soft" switching. To ensure “soft” switching of key elements in a resonant converter with a series LC circuit, it is necessary and sufficient to ensure phase delay of the circulating current in the circuit relative to the key element control signals. Increasing the frequency increases the phase difference, but also increases the level of static and dynamic losses in the converter. To improve the efficiency of the converter, it is advisable to provide a zero phase shift between the current circulating in the circuit and the control signals. It also means that key elements must switch at the moment the loop current passes through zero.

Therefore, the set of essential features given in the claims ensures the achievement of the claimed technical result and allows us to solve the technical problem.

The claimed method is explained further with reference to the drawings, where

FIG. 1 shows a diagram of a resonant converter with a series resonant circuit and connecting the load in series to the circuit capacitor;

FIG. 2 shows a diagram of a resonant converter with a series resonant circuit and connecting the load parallel to the circuit capacitor;

FIG. 3 shows diagrams of control signals for key elements circulating in the current circuit ik and voltage at the inverter output Uab when the current ik is delayed in phase;

FIG. 4 shows diagrams of control signals for key elements circulating in the current circuit ik and voltage at the inverter output Uab when the current ik is ahead in phase;

FIG. 5 shows a functional diagram with feedback on the loop current, ensuring the implementation of the claimed method;

FIG. 6 is a structural diagram of a control system indicated in FIG. 5 as SU.

The following notation is used in the drawings:

V1, V2, V3, V4 - key elements;

D1, D2, D3, D4 - antiparallel diodes;

Lp, Cp - elements of the resonant circuit;

T, T1 - transformer;

BB - rectifier;

VF - output filter;

RFN is the equivalent load resistance;

DN - voltage sensor;

DT - current sensor;

SU - control system;

ZC - zero signal transition detector;

PD - phase detector (phase comparison scheme);

Low-pass filter - low-pass filter;

VCO - voltage controlled oscillator;

1/2 - frequency divider;

PWM - pulse width modulation scheme, PWM controller;

PID - proportional-integral-differential controller;

SFUS - a scheme for the formation of control signals.

The method of pulse-width regulation of the output voltage of the resonant converter is implemented as follows.

The regulation task is to maintain a given voltage level at the load 1 of the resonant transducer (Fig. 5). To accomplish this task, using the voltage sensor 2, measure the current voltage value at the load 1 of the resonant transducer and generate a constant voltage signal proportional to the load value. Based on the indicated signal, a difference signal is generated using the proportional-integral-differentiating controller 3 (PID controller), the input of which also supplies the set voltage U _set . The setpoint voltage is selected proportional to the supported voltage level at the load of the resonant converter. To implement feedback on the loop current, an alternating current signal is generated proportional to the current strength in the oscillating circuit, which is measured by the current sensor 4, and the alternating current signal is converted into an alternating voltage signal. These operations are necessary to provide the ability to control the moment of transition of the AC signal through zero. Next, form a control signal, which is fed to the control electrodes of the key elements 5 of the inverter. The control signal is generated using a PWM controller 6, clocked by a voltage controlled oscillator (VCO) 7. The generation frequency of the VCO 7 is determined by the magnitude of the difference signal generated using the PID controller 3. The phase-locked loop of the switching frequency is provided by setting the repetition rate of the pulses generated by the PWM controller 6, which is clocked by the VCO 7 included in the phase-locked loop (PLL), such at which the moment of formation of the VCO 7 of the signal front coincides in time with the moment of transition through zero of the AC signal proportional to the current strength in the oscillatory ntur. This ensures zero phase shift between the current circulating in the circuit and the control signals.

Therefore, to implement the claimed method, it is necessary:

1) measure the alternating current circulating in the resonant circuit;

2) control the key elements by changing the repetition frequency of the control signals, so that the moment of switching on the key element coincides with the moment of passage of the loop current through zero.

The method can be implemented in a device including: an inverter, which can be half-bridge or bridge; resonant circuit, consistent with the output of the inverter; a current sensor based on any known physical principles that allow converting alternating current into a signal proportional to it, convenient for processing by the control system; the load of the circuit included in the circuit in series or parallel to the capacitor of the circuit, which is a rectifier, a low-pass filter and directly the load of the resonant converter; a voltage sensor based on any known physical principles that converts the voltage at the load of the resonant transducer into a signal proportional to it, convenient for processing by the control system; a control system whose function is to generate control signals for the key elements of the inverter to stabilize the voltage at the output of the resonant converter, such that the phase shift between the current circulating in the circuit and the control signals of the key elements is minimal.

The physical implementation of these algorithmic principles is carried out by introducing a phase locked loop (PLL) into the control system. The control system must contain a unit that monitors the instant of transition of the current circulating in the circuit through zero, forming a pulse signal at its output. The resulting pulse signal, if necessary, is converted and fed to the phase comparison circuit of the PLL system. The PLL includes a voltage controlled oscillator (VCO) that synthesizes a signal clocking a PWM circuit. The principle of the PLL is the presence of feedback, adjusting the frequency of the signal at the output of the VCO so that the phase difference between this signal and the input is minimal. Adjustment and stabilization of the output voltage is carried out using the principles of PWM control, and the PLL system seeks to establish the minimum operating frequency at which the soft switching mode is still provided.

These control principles are also applicable to well-known topologies of resonant converters of a higher order, such as LCC, LLC, LLCC and other topologies, which include at least one inductive element connected in series with the load relative to the output of the inverter.

Shown in FIG. 1 and 2, known circuits of resonant converters with a series resonant circuit and connecting the load in series (Fig. 1) and parallel to the capacitor circuit (Fig. 2) are given for explanation. The explosive rectifier can be made according to a bridge circuit, a current doubler circuit, or another. The output filter VF can be capacitive, inductive-capacitive, or performed in a more complex scheme. The choice of rectifier circuit and output filter from the point of view of the claimed invention is not significant. In FIG. 3 shows the case of the current delay ik in phase and the fulfillment of soft switching conditions; in FIG. 4 - the case of the advance of the current ik in phase. A characteristic change in the shape of the Uab reveals a lack of soft switching. This is an undesirable (in extreme cases - emergency) mode of operation.

The functional diagram shown in FIG. 5 illustrates the difference realized in the invention, which consists in the use of a circulating current sensor (DC) 4, the signal of which is fed to a control system 9 (CS), i.e. feedback on loop current. The loop current measurement can be carried out, for example, using a current transformer and is not of practical complexity at the present level of technology.

The ability to adjust and stabilize the output voltage of the resonant converter by changing the duty cycle of the control signal (PWM) and the repetition frequency of the control signal (PFM) is known. From the point of view of improving efficiency, it is desirable that the duty cycle and repetition rate be as low as possible, however, some of their combinations lead to the loss of an important useful property of the traditional resonant transducer - “soft” switching.

The control method allows you to automatically select the minimum operating frequency and duty cycle at which the conditions for ensuring "soft" switching of key elements are still fulfilled. The criterion is proposed to consider the presence of a delay in the current circulating in the circuit relative to the control signals of key elements.

An explanation of the principle of operation is more convenient to begin with a description of the operation of the resonant converter assembled according to the circuit in FIG. 2. In FIG. 3 and 4 are diagrams of the operation of the resonant converter assembled according to the circuit of FIG. 2, and the diagrams indicated by the letter “a)” (Figs. 3, 4) show antiphase control signals of key elements 5 of the resonant transducer. Key elements 5 are controlled in pairs by antiphase signals, so that either the keys (key elements) V1, V4, or V2, V3 in FIG. 2. The diagrams marked with the letter “b)” (Figs. 3 and 4) show the current ik circulating in the circuit and the voltage at the inverter output Uab. It is clearly seen that in FIG. 3, the loop current passes through zero a little later than the edge of the enable signal, which ensures the presence of a switching mode at zero voltage. In FIG. 4, the loop current passes through zero a little earlier than the edge of the enable signal, which leads to a specific distortion of the shape of Uab, which means that there is no switching mode at zero voltage. Thus, the fundamental possibility of both a qualitative determination of whether the resonant transducer is operating in soft switching mode and a quantitative determination, in particular, how much it is permissible to reduce the delay of the loop current in phase with respect to the control signals, is obvious.

The loop current measurement can be carried out, for example, using a current transformer and is not of practical complexity at the present level of technology.

The input signals of the control system 9 is the signal from the voltage sensor 2 Uoc, proportional to the voltage at the output of the converter, as well as the signal from the current sensor 4 ik, proportional to the current circulating in the circuit. According to the claimed method, the control system must track the transition moment of the loop current ik through zero. In FIG. 6, the detector 10 of the signal crossing through zero, denoted as ZC, generates a pulse signal at its output, whose edges coincide with the moments when the input signal passes through zero. The physical implementation of such a device is known and can be performed in both analog and digital form. Among the analog options, the performance based on the operational comparator is widely known.

The signal received from the detector 10 is fed to the phase-locked loop PLL 8. The PLL is a feedback system that automatically adjusts the frequency at its output so that the phase of the signal at the input and output of the PLL coincides. Since the delay of the loop current in phase depends, among other factors, on the operating frequency, it is possible to select the working frequency so that this delay is absent, regardless of the combination of other factors. From the point of view of the theory of automatic control, the PLL is a classical solution to problems of this type. The detailed principles of the PLL and the technical implementation are known to specialists.

Since the PLL already includes a voltage controlled oscillator (VCO) 7, it is advisable to use this VCO to clock the PWM circuit (PWM controller). Currently, there are PWM integrated circuits, also known as PWM controllers, with the possibility of external clocking. The combination of a VCO and such a PWM controller allows the generation of pulsed signals with a controlled duty cycle and frequency.

Regulation and stabilization of the output voltage is carried out by changing the repetition rate of the control pulses. The proportional-integral-differential controller 3 compares the feedback signal Uoc and the reference signal Uust, after which it outputs the difference signal (error signal) to the PWM 6 circuit (PWM controller). The operating principles and physical implementation of PID controllers are well known.

The specified sequence of actions implemented in the device, a diagram of which is shown in FIG. 5 provides regulation of pulse-width regulation of the output voltage of the resonant converter by generating control signals of variable frequency (switching frequency), which are supplied to the control electrodes of the key elements 5 of the inverter. Moreover, to increase the efficiency of the resonant converter, a phase-locked loop of the switching frequency is provided by introducing feedback on the loop current to provide zero phase shift between the current circulating in the loop and the control signals.

Claims (1)

A method of pulse-width regulation of the output voltage of a resonant converter, including an inverter with key elements and a series LC oscillatory circuit, characterized in that a predetermined voltage level is maintained at the load of the resonant converter, for which the voltage value at the load of the resonant converter is measured and a constant voltage signal is generated, proportional to the load, based on which using proportional-integral-differentiating its regulator provides the formation of a differential signal with respect to the setpoint voltage proportional to the supported voltage level at the load, generates an AC signal proportional to the current strength in the oscillatory circuit, converts the AC signal into an AC voltage signal and controls the moment the AC voltage signal passes through zero, and depending on the value of the difference signal, pulses of variable width are formed using a PWM controller, a clocked oscillator m, controlled by voltage, which are supplied to the control electrodes of the key elements of the inverter, while providing phase-locked loop switching frequency, setting the repetition rate of the pulses generated by the PWM controller, clocked by the mentioned generator, included in the loop phase-locked loop, such that the moment of formation by the said front generator of the signal coincides in time with the moment of crossing through zero the aforementioned AC voltage signal, proportional to the current strength in the oscillatory nture.

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