CN107425722A - A kind of LLC resonant converter frequency modulation phase shift current-sharing control method in parallel - Google Patents

Abstract

The invention discloses a control method for frequency modulation, phase shift and current sharing of a parallel LLC resonant converter. The control method adopts the combination of the voltage loop as the main and the current loop as the auxiliary, and the voltage loop relies on the PFM (pulse frequency modulation) control method to realize the voltage stable output; the current loop adopts phase shift control so that the generated phase shift angle acts on the pulse signal generated by the voltage loop, so that the system can meet the premise of stable voltage output, realize the current sharing function between parallel modules and prevent a certain phase Excessive current may damage the device. The control strategy combining the voltage loop and the current loop not only realizes the stable output of the voltage, but also eliminates the influence of the current unevenness caused by the parameter error of the electromagnetic device of the parallel module or the uneven load distribution, prolongs the service life of the equipment, and improves the resonance. The conversion efficiency and dynamic stability of the conversion circuit system have important application significance.

Description

A Control Method of Frequency Modulation, Phase Shift and Current Sharing of Parallel LLC Resonant Converter

technical field

The present invention relates to the technical field of power electronics, and more particularly relates to a method for controlling frequency modulation, phase shift and current sharing of parallel LLC resonant converters.

Background technique

With the rapid development of my country's economy, the automobile industry continues to heat up. At the same time, the research and development of electric vehicle charging piles should also be included in the development plan. People put forward higher requirements on the volume, power density, and charging efficiency of charging piles. In order to shorten the charging time of cars and improve work efficiency, it is necessary to increase the operating frequency of the converter in the charging pile.

Due to high frequency requirements, soft switching technology has become a research hotspot. As a feasible way to realize soft switching technology, resonant technology can reduce hard switching loss and improve efficiency. The most commonly used one is the LLC (abbreviation of Lr, Lm, Cr, where Lr is the resonant inductance, Lm the excitation inductance, and Cr is the resonant capacitor) resonant conversion technology, which can realize the primary side switching tube ZVS (zero voltage) in the full load range switch), the secondary side diode can realize ZCS (Zero Current Switching), which not only reduces its voltage stress, but also avoids reverse recovery loss.

However, since the output stage of the resonant converter only has a capacitor filter, it has the disadvantage of large output current ripple. Generally speaking, LLC resonant converter has small switching loss and is suitable for high-frequency applications, but it faces limitations such as capacity and output ripple.

Using a multi-phase interleaved parallel LLC resonant converter can significantly reduce the output ripple in the output capacitor. By calculating the magnitude of the ripple in the case of two-phase, three-phase, four-phase, and five-phase parallel connections, it can be obtained that the more phases are interleaved in parallel, the smaller the ripple rate and the lower the current stress requirements of semiconductor devices, but with the increase in the number of modules However, the degree of improvement becomes less and less obvious, and the topology and control methods become more and more complex. Therefore, two-phase or three-phase parallel connection is usually used, which not only reduces the output current ripple, but also expands the capacity of the equipment. Although some scholars have done research on the current sharing of LLC resonant converters in parallel, there is still room for development. For example, some scholars proposed to use external circuits to control the current sharing of each phase, but the cost is high and the control is complicated; some scholars also proposed to use PFM ( Pulse Frequency Modulation) modulation or PWM (Pulse Width Modulation) modulation method combined with phase shift control method, but it is easy to cause loss of phase shift angle in multi-phase parallel circuit.

Contents of the invention

The present invention proposes a parallel LLC resonant converter frequency modulation phase shift current sharing control method to realize stable voltage output, eliminate the influence of current unevenness caused by the parameter error of the electromagnetic device of the parallel module itself or uneven load distribution, and prolong the service life of the equipment. Improving the conversion efficiency and dynamic stability of the resonant conversion circuit system has important application significance.

The patented technical solution of the present invention is realized through the following steps:

Step 1: First, sample the output voltage U _O of the parallel LLC resonant converter, and make a difference with the given voltage signal U _ref . The output signal is the voltage difference △u, and the output signal after △u is modulated by PI is U _VCO , U As the given signal of the voltage-controlled oscillator, _VCO realizes PFM control through the voltage-controlled oscillator. According to the magnitude of the voltage change, the corresponding frequency change is generated, and the output signal of the voltage-controlled oscillator is counted in the form of counting the pulse sequence. Phase shifting is carried out, and the driving signals of various switching tubes are generated through logical relationships, forming a basic voltage-type control, which can realize the stable voltage output of the resonant converter, and has a fast response speed.

Step 2: Sampling the resonant current i _Lr of each parallel LLC resonant circuit. Because the resonant current changes according to the resonant frequency, it is difficult to realize the real-time tracking and timely adjustment of the resonant current of each phase directly by PI modulation control, so this design uses the effective value of the resonant current as the feedback signal.

Step 3: Determine the magnitude of the effective value of the resonant current of each phase, and find out the parallel module M _LLCi where the maximum effective value of the resonant current is located (if there are m-phase resonant converters connected in parallel, then 0<i≤m); by calculating the resonant current of each phase The difference between current effective values |△I _{errLk-j_} | (if there are m-phase resonant converters connected in parallel, then 0<k≤m, 0<j≤m, k≠j), and find the maximum difference And let it be △I _{err_max} .

Step 4: Through the resonant module M _LLCi , make the _i -th corresponding switch ki jump from the ground terminal to the a terminal, and use the current difference △I _{err_max} to convert it into phase The corresponding phase shift angle θ.

Step 5: The angle range of the phase shift is Because the phase shifting effect is on the driving pulse of the resonant circuit where the maximum resonant current is located, it can suppress the excessive resonant current of a certain phase and avoid equipment damage due to overcurrent.

Step 6: Apply the generated phase shift angle θ to the driving signal of the resonant circuit of the i-th phase of the resonant current to combine it with the PFM pulse frequency. Due to the limitation of the phase shift angle θ, it is possible to realize the uniformity on the basis of voltage. stream function.

Instructions attached

Figure 1. Main circuit topology of three-phase parallel LLC resonant converter

Figure 2. Block diagram of current sharing control

Figure 3. Drive pulse phase shift waveform

Figure 4. FM phase shift phase current sharing control block diagram

detailed description

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the patents of the present invention will be clearly described below in conjunction with the drawings in the patents of the present invention. Obviously, the described embodiments are the embodiment of the present invention Some, but not all, embodiments.

The control method of the present invention is suitable for series resonant conversion circuits, specifically including: parallel connection of LC series resonant conversion circuits (SRC), parallel connection of series-parallel resonant conversion circuits (SPRC) and parallel connection of LLC series resonant conversion circuits, the main topology of the circuit can be half bridge, full bridge, etc.

This embodiment uses a three-phase LLC resonant converter for illustration.

Table 1 Ripple rate of output capacitor ripple in each phase

Io represents the output current, Imi represents that there are i modules connected in parallel, the peak value of each module, △Ic is the maximum ripple value

The circuit topology involved in this embodiment is shown in Figure 1. The parallel structure of the three-phase LLC resonant converter is adopted. From Table 1, it can be seen that the three-phase parallel connection of the LLC resonant converter has a small output ripple and expands the entire system. capacity, so the parallel connection of three-phase LLC resonant converters is used as the research object. The resonant converter can be mainly divided into: square wave generator, resonant unit, transformer, rectifier unit and output filter unit.

The square wave generator is specifically: the switch network is mainly composed of a half-bridge and a switch tube; the resonant network adopts an LLC resonant cavity, including a resonant capacitor C _r , a resonant inductance L _r and a transformer excitation inductance L _m ; the primary side of the transformer adopts a Y-type connection , the secondary side is parallel output; the rectifier unit adopts switching tube device or rectifier diode to realize the transmission of electric energy; the output adopts capacitor filter.

First, the output voltage Uo is sampled, and compared with the given signal Uref to get △u, the value of which is modulated by PI and the output signal is used as the given signal of the voltage-controlled oscillator. The voltage-controlled oscillator has a high frequency modulation bandwidth and can Higher frequency adjustment is achieved with very small voltage fluctuations. The voltage-controlled oscillator generates a sawtooth wave, which is then converted into a square wave through a logical relationship, and the three-phase phase shift control is realized by counting the pulse sequence; finally, the driving signal of each switching tube is generated by using the logical relationship to make the three-phase parallel resonant converter Realize the basic voltage-type PFM voltage regulation control.

The control method of the current sharing loop for the three-phase parallel LLC resonant circuit is shown in Figure 2. First, the resonant current of the three-phase resonant circuit is sampled to obtain i _L1 , i _L2 , and i _L3 . Since i _L1 , i _L2 , and i _L3 change according to the resonant frequency, it is difficult to realize real-time tracking and timely adjustment of i _L1 , i _L2 , and i _L3 by directly using PI modulation control, so this design adopts the effective resonant current Values I _L1 , I _L2 , and I _L3 are used as feedback signals to determine that the maximum effective value of the resonant current is I _max2 , and the parallel module where it is located is M _LLC2 (assuming that the resonant current in the second-phase LLC converter in each parallel resonant converter is The effective value is the largest), calculate I _L1 , I _L2 , and I _L3 to make the difference between each other to get |△I _errL1-2 |, |△I _errL2-3 |, |△I _errL1-3 |, and select the largest difference value, and make its maximum absolute value △I _{err_max} . Because of the maximum current I _max2 , the second corresponding switch k2 jumps to terminal a and closes, and performs PI modulation of the current loop on the RMS value I _max2 of the resonant current of the second phase to generate a corresponding phase shift angle. Select the range of the phase shift angle, and the phase shift angle is 0≤θ<60°. The generated phase shift angle is applied to the drive signal of the phase resonant circuit with the largest resonant current, and the drive signal obtained at this time is shown in FIG. 3 .

Combining the voltage-type PFM control with the three-phase LLC current sharing control, the system's interleaved parallel LLC resonant converter high-efficiency charging control strategy is obtained. The control method implemented is shown in Figure 4. It can be seen that the voltage loop is mainly realized by the PFM control method Stable output of voltage. When the voltage fluctuates, the pulse frequency is adjusted in time according to the voltage-controlled oscillator to stabilize the output voltage of the converter; the current loop mainly uses phase-shift control to make the generated phase-shift angle act on the pulse signal generated by the voltage loop. Widely realize the current sharing between the three phases, so that the system can meet the premise of stable voltage output, realize the current sharing function between parallel modules and prevent the equipment from being damaged by excessive current in one phase. The control strategy combining the voltage loop and the current loop not only realizes the stable output of the voltage, but also eliminates the influence of the uneven current caused by the parameter error of the electromagnetic device of the parallel module or the uneven load distribution, prolongs the service life of the equipment, and improves the resonance conversion The conversion efficiency and dynamic stability of the circuit system have important application significance.

The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed in the present invention can easily think of changes or Replacement should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims.

Claims (4)

1. A parallel LLC resonant converter frequency modulation phase shift current sharing control method is characterized in that, adopting the control method that takes the voltage loop as the main and the current loop as the auxiliary combination, the voltage loop relies on the PFM (pulse frequency modulation) control method to produce The modulation signal with constant duty cycle and frequency change realizes the stable output of voltage; the current loop adopts the phase shift control method to make the generated phase shift angle act on the pulse signal generated by the voltage loop, and realizes the uniformity among the three phases by adjusting the pulse width. flow. 2. The current sharing control method of combining frequency modulation and phase shifting of parallel LLC resonant converter according to claim 1, characterized in that, the current loop calculates the magnitude of its effective value respectively by sampling the resonant current of each phase, and By comparison, the parallel module M _LLCi where the maximum resonant current is located is determined, and the maximum current difference △I _{err_max} between the maximum current I _maxi and the effective value of each resonant current is calculated. 3. The current sharing control method of combining frequency modulation and phase shift of parallel LLC resonant converter according to claim 2, characterized in that, by selecting the maximum current I _maxi , it is judged to turn on the i-th corresponding switch ki, and through PI modulation , so that a corresponding phase shift angle θ is generated, and the generated phase shift angle is applied to the drive signal of the resonant circuit of the phase with the largest resonant current. 4. The current sharing control method combining frequency modulation and phase shifting of parallel LLC resonant converters according to claim 1, characterized in that, the system meets the voltage requirements by using the voltage loop as the main control method and the current loop as the auxiliary control method. On the premise of stable output, the current sharing function between parallel modules can be realized and the equipment can be prevented from being damaged by excessive current of a certain phase.