A Circulating-Current-Free Frequency Conversion Modulation Method Applicable to LC Series Resonant Full-Bridge Converters

Abstract

The present invention discloses a non-circulating frequency conversion modulation method applicable to an LC series resonant full-bridge converter. In the converter to which the present invention is applicable, an LC series resonant cavity is placed on the secondary side of a high-frequency transformer. The first switching tube Q1 leads the third switching tube Q3 by half a switching period, i.e., 0.5T. s The duty cycles of the two are the same and not less than 0.75; the second switching tube Q2 leads the fourth switching tube Q4 by half a switching period, i.e., 0.5T. s The duty cycles of the two are the same and the conduction time is half of the series resonance period, i.e., 0.5T. r The upper and lower switching tubes of the same bridge arm are complementary in conduction. The voltage or power of the converter is controlled by adjusting the switching frequency. Under the condition of the same output voltage, the maximum value of the magnetic flux density of the high-frequency transformer in the present invention is not affected by the changes in the switching frequency and power, solving the problem that the transformer is prone to saturation under light load; and there is no circulating current flowing back to the input power supply, reducing the current stress of semiconductor devices and the reactive power loss of the converter, and improving the efficiency and power density.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics and relates to a non-circulating frequency conversion modulation method applicable to an LC series resonant full-bridge converter. Background Art

[0002] Photovoltaic (PV) power generation, as the world's richest renewable energy power generation, has attracted much attention with the continuous reduction of costs. For PV, energy aggregation is one of the important technologies. The LC resonant converter has been widely concerned because of its wide voltage gain range, high power density and other characteristics, which can well adapt to the characteristics of low and fluctuating PV power generation voltage. According to different modulation methods, the LC resonant converter can be divided into four types. The first type is the half-cycle discontinuous current mode (HC-DCM) LC resonant converter, which is suitable for high-power applications. However, since it does not have voltage regulation ability, it can usually be regarded as a DC transformer. The second type uses constant duty cycle frequency conversion modulation (Pulse Frequency Modulation, PFM) to achieve controllable voltage gain. However, when at the lowest switching frequency, the maximum magnetic flux density of the high-frequency transformer will be too large and cannot be avoided, which will increase the cost and volume of the transformer. The third type introduces a pulse removal technology to avoid excessive magnetic flux density at low frequencies, where the LC series resonant cavity is located on the secondary side of the transformer. However, it is more suitable for controlling the output power rather than the voltage gain. The fourth type is PFM with a constant conduction time, where the voltage gain and output power are controllable, and a wide voltage gain range can be obtained. The literature (G. Ning, W. Chen, Y. Ling, M. Su, and G. Xu, "Magnetic flux density analysis of series resonant converter operating in discontinuous conduction mode for high-voltage high-power applications." IET Power Electronics, vol. 13, no 18, pp. 4386-4394, December 2020) studied the fourth type of LC series resonant full-bridge converter. However, when the switching frequency is greater than a certain critical point, the maximum magnetic flux density of the high-frequency transformer will increase sharply. This phenomenon can be avoided by increasing the transformer boost ratio. However, as the boost ratio increases, the current peak of the converter will also increase. And since the LC series resonant cavity is placed on the primary side of the high-frequency transformer, the circulating current flowing back to the input power supply will further increase the current stress of the switching tube and the reactive power of the converter, which will lead to a decrease in the efficiency and power density of the converter.

[0003] In addition, a Chinese patent with the patent name "Frequency Conversion Phase-Shifted Asymmetric Duty Cycle Modulation Method for Series Resonant Full-Bridge Converter" and the patent number CN108075668A studied a modulation method that can solve the problem of easy saturation of the transformer under light load through a modulation method including three control steps: phase shift, asymmetric duty cycle, and frequency conversion. However, in the actual operation process, since the actual voltage fluctuates when the primary voltage of the transformer is clamped to zero, the magnetic flux density of the transformer is still not ideal under light load.

[0004] A Chinese patent with the patent name "Transformer Magnetic Flux Density Control Method for Series Resonant Full-Bridge Converter" and the patent number CN108667305A also studied a modulation method to improve the characteristics of the transformer under light load. However, since some switching tubes need to be turned on and off twice within a switching cycle, the volume and cost of the transformer will increase significantly. Summary of the Invention

[0005] In view of the phenomenon that the magnetic flux density of the high-frequency transformer in the existing LC modulation method changes with frequency and there is circulating current, and after fully considering factors such as cost, system reliability, and loss, the present invention provides a non-circulating current frequency conversion modulation method applicable to the LC series resonant full-bridge converter. This modulation method can achieve that the magnetic flux density of the high-frequency transformer T hf is only related to the resonant frequency and independent of the switching frequency on the premise of ensuring zero-current turn-on and turn-off of all semiconductor devices, effectively reducing the cost and volume of the high-frequency transformer T hf Meanwhile, the circulating current flowing back to the input power supply is eliminated, effectively reducing the current stress of the switching tubes and the reactive power loss of the converter, and having the characteristics of high efficiency and high power density.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A non-circulating current frequency conversion modulation method applicable to the LC series resonant full-bridge converter. The full-bridge converter based on this method includes a first switching tube Q1 located in the upper left bridge arm, a second switching tube Q2 located in the lower left bridge arm, a third switching tube Q3 located in the upper right bridge arm, and a fourth switching tube Q4 located in the lower right bridge arm. It is characterized in that:

[0008] This method includes: within a switching cycle, controlling the first switching tube Q1 in the series resonant full-bridge converter to lead the third switching tube Q3 by half a switching cycle of 0.5T s , and their duty cycles are the same and not less than 0.75; the second switching tube Q2 leads the fourth switching tube Q4 by half a switching cycle of 0.5T s , and their duty cycles are the same and not greater than 0.25, and the conduction time is half a series resonant cycle of 0.5T r; The upper and lower switching tubes of the same bridge arm conduct complementarily; within the switching period, the control of the converter voltage or power magnitude is achieved by adjusting the switching frequency.

[0009] As a preferred technical solution of the present invention: the high-frequency transformer T hf The voltage v at both ends of the primary side AB Is V in Or –V in , is 0 when the second switching tube Q2 conducts or the fourth switching tube Q4 disconnects. In this case, the high-frequency transformer T hf The maximum magnetic flux density is only related to the resonance frequency and has nothing to do with the switching frequency.

[0010] In the above structure: the high-frequency transformer T hf The maximum magnetic flux density is only related to the resonance frequency and has nothing to do with the switching frequency. The switching frequency adjustment range can be allowed to be very wide, so as to obtain a wide voltage range.

[0011] As a preferred technical solution of the present invention: in the series-resonant full-bridge converter, the LC series-resonant cavity is placed on the secondary side of the high-frequency transformer T hf The secondary side of the high-frequency transformer T hf There is no circulating current flowing back to the input power supply on the primary side.

[0012] In the above structure: the LC series-resonant cavity is placed on the secondary side of the high-frequency transformer T hf The secondary side of the high-frequency transformer T hf There is no circulating current flowing back to the input power supply on the primary side, thereby reducing the current stress of semiconductor devices and the reactive power loss of the converter, and improving the efficiency and power density.

[0013] As a preferred technical solution of the present invention: the primary switching tubes and the secondary diodes are all turned on and off with zero current.

[0014] Compared with the prior art, the beneficial effects of the present invention are:

[0015] 1. Under the same parameters, the current stress of semiconductor devices is reduced by at least 50%, which will effectively reduce the experimental cost and improve the efficiency;

[0016] 2. The maximum magnetic flux density of the high-frequency transformer is independent of the switching frequency and only related to the resonance frequency. Therefore, a very wide switching frequency range can be adopted to achieve a wide voltage gain without considering the sharp increase in the cost and volume of the transformer;

[0017] 3. Eliminates the influence of reactive power on energy transmission, effectively improving the transmission efficiency and power density;

[0018] 4. All semiconductor devices can achieve zero-current turn-on and turn-off, eliminating turn-on losses and turn-off losses, and significantly improving the efficiency of the converter. Description of the Drawings

[0019] Figure 1 is the main circuit of the present invention;

[0020] Figure 2 is the main waveform diagram of the converter;

[0021] Figure 3 is the current flow diagram of the converter in Mode 1;

[0022] Figure 4 is the current flow diagram of the converter in Mode 2;

[0023] Figure 5 is the current flow diagram of the converter in Mode 3;

[0024] Figure 6 is the equivalent circuit diagram of the LC series resonant circuit in Mode 1;

[0025] Figure 7 is the equivalent circuit diagram of the LC series resonant circuit in Mode 2. Detailed Embodiment

[0026] The present invention will be further described in detail below in conjunction with the drawings and specific embodiments:

[0027] The present invention proposes a non-circulating frequency conversion modulation method applicable to an LC series resonant full-bridge converter. First, the voltage stress of semiconductor devices can be reduced by more than 50%; second, the maximum magnetic flux density of the high-frequency transformer is independent of the switching frequency and only related to the resonant frequency. Therefore, a very wide switching frequency range can be adopted to achieve a wide voltage gain without considering the sharp increase in the cost and volume of the transformer; third, the influence of reactive power on energy transmission is eliminated, effectively improving the transmission efficiency and power density; fourth, all semiconductor devices can achieve zero-current turn-on and turn-off, eliminating turn-on losses and turn-off losses, and significantly improving the efficiency of the converter.

[0028] Embodiment: Refer to Figure 1 and Figure 2 , an LC series resonant full-bridge converter and its non-circulating frequency conversion modulation method. Specifically, within one switching period, the first switching tube Q1 in the series resonant full-bridge converter is controlled to lead the third switching tube Q3 by half a switching period of 0.5T s , and their duty cycles are the same and not less than 0.75; the second switching tube Q2 leads the fourth switching tube Q4 by half a switching period of 0.5T s , and their duty cycles are the same and not greater than 0.25, and the conduction time is half of the series resonant period of 0.5T r; The upper and lower switching tubes of the same bridge arm conduct complementarily; the voltage or power of the converter is controlled by adjusting the switching frequency within the switching period. High-frequency transformer T hf The voltage v across the primary side ends AB Is V only when the second switching tube Q2 or the fourth switching tube Q4 conducts in Or –V in And remains 0 at other times. In this case, the high-frequency transformer T hf The maximum magnetic flux density is only related to the resonance frequency and independent of the switching frequency. Therefore, the design of the high-frequency transformer is independent of the switching frequency, and the switching frequency adjustment range can be allowed to be very wide, so as to obtain a wide voltage range. In the series resonant full-bridge converter, the LC series resonant cavity is placed on the secondary side of the high-frequency transformer T hf The secondary side of, the high-frequency transformer T hf There is no circulating current flowing back to the input power supply on the primary side, thus reducing the current stress of semiconductor devices and the reactive power loss of the converter, and improving the efficiency and power density.

[0029] In this embodiment: The primary switching tubes and the secondary diodes are all turned on and off with zero current.

[0030] Working principle: Refer to Figure 1 And 2 The main circuit and typical control waveforms of, the proposed converter can be divided into 6 switching modes in one switching period T s It can be divided into 6 switching modes. Due to symmetry, the following will detail the 3 switching modes within half of the switching period [t0~t3].

[0031] Mode 1 (t0~t1): The current path of this mode is as shown in Figure 3 . Before t0, the current i flowing through the switching tube Q3 Q3 Is zero, so the switching tube Q3 can be turned off with zero current at t0. At the same time, since the current i flowing through Q4 Q4 Starts to rise sinusoidally from zero at t0, so the switching tube Q4 can be turned on with zero current. In this mode, the voltage v across the primary side of the high-frequency transformer T hf =V AB =V in , therefore, the magnetic flux density B of the high-frequency transformer T hf From –B t (B m (B m Is the maximum magnetic flux density of the high-frequency transformer T hf Increases linearly to B m . The equivalent circuit of the LC series resonant band circuit is as shown in Figure 6 , where V t =nV in -V o。Starting from t0, the resonant capacitor C r begins to resonate positively with the resonant inductor L r Therefore, the voltage v r across the resonant capacitor C Cr increases from the initial value v Cr (t0), and the resonant current i r flowing through the resonant inductor L r starts to vary sinusoidally from zero.

[0032] Mode 2 (t1 - t2): The current path in this mode is as shown in Figure 4 . At time t1, the positive resonance ends. At this time, the primary current i p and the resonant current i r are both zero, and the voltage v r across the resonant capacitor C Cr reaches the peak value v Crmax , and v Crmax > V o . Since the currents i Q3 and i Q4 flowing through the switching transistors Q3 and Q4 are respectively rising sinusoidally from zero and falling sinusoidally to zero at time t1, zero-current turn-on of the switching transistor Q3 and zero-current turn-off of the switching transistor Q4 can be achieved. Within this mode, the primary voltage v hf of the high-frequency transformer T AB = 0. Therefore, the magnetic flux density B hf of the high-frequency transformer T t remains unchanged at B m . At the same time, since v Crmax > V o , the resonant cavity can achieve reverse resonance, and the equivalent circuit of the resonant circuit is as shown in Figure 7 . And the resonant current does not pass through the input terminal of the converter. In other words, there is no circulating current flowing back to the input terminal of the transformer.

[0033] Figure 5 Mode 3 (t2 - t3): The current path in this mode is as shown in Figure 5 . At time t2, the reverse resonance ends. At this time, the primary current i p and the resonant current i r are both zero, and the voltage v r across the resonant capacitor C Cr (t2) < V o . Within this mode, the input capacitor C in is charged, the output capacitor C o powers the load, and the resonant cavity does not work.

[0034] In summary, the present invention proposes a non-circulating frequency conversion modulation method applicable to an LC series resonant full-bridge converter. On the basis of realizing the regulation of the transmission power and boost ratio of the converter, it can not only make the maximum magnetic flux density of the high-frequency transformer T hf independent of the switching frequency, but also eliminate the circulating current flowing back to the input end, thereby reducing the current stress of semiconductor devices and eliminating reactive power. Finally, according to the symmetry of the circuit and working principle, all primary side switching tubes and secondary side rectifier diodes can achieve zero-current turn-on and turn-off.

[0035] The above are only preferred embodiments of the present invention, and do not limit the present invention in any other form. Any modification or equivalent change made according to the technical essence of the present invention still belongs to the scope protected by the present invention.

Claims

1. A non-circulating frequency conversion modulation method applicable to an LC series resonant full-bridge converter. The full-bridge converter based on this method includes a first switching tube located in the upper left bridge arm Q 1, a second switching tube located in the lower left bridge arm Q 2, a third switching tube located in the upper right bridge arm Q 3, and a fourth switching tube located in the lower right bridge arm Q 4, and is characterized in that: The method includes: within one switching period, controlling the first switching transistor in the series resonant full-bridge converter Q 1 to lead the third switching transistor Q 3 by half a switching period of 0.5 T s , with the same duty cycle for both and not less than 0.75; the second switching transistor Q 2 to lead the fourth switching transistor Q 4 by half a switching period of 0.5 T s , with the same duty cycle for both and not greater than 0.25, and the conduction time is half of the series resonant period of 0.5 T r ; the upper and lower switching transistors of the same bridge arm conduct complementarily; within the switching period, the control of the converter voltage or power magnitude is achieved by adjusting the switching frequency; In the series resonant full-bridge converter, the LC series resonant cavity is placed on the secondary side of the high-frequency transformer T hf , and there is no circulating current flowing back to the input power supply on the primary side of the high-frequency transformer T hf . The converter within one switching period T s can be divided into six switching modes. Due to symmetry, the three switching modes within half of the switching period t 0 to t 3] are as follows: Mode 1 t 0 to t 1: Before t 0, the current Q flowing through the switching transistor i Q3 is zero. Therefore, at t time 0, the switching transistor Q 3 can be turned off with zero current, and at the same time, since the current Q flowing through i Q4 starts to increase sinusoidally from zero at t time 0, the switching transistor Q 4 can be turned on with zero current. In this mode, the primary voltage T hf of the high-frequency transformer v AB = V in . Therefore, the magnetic flux density T hf of the high-frequency transformer B t increases linearly from – B m , B m where T hf is the maximum magnetic flux density of the high-frequency transformer B m . In the equivalent circuit of the LC series resonance band circuit V t = nV in - V o . Starting from t 0, the resonant capacitor C r begins to resonate positively with the resonant inductor L r . Therefore, the voltage C r across the resonant capacitor v Cr increases from the initial value v Cr ( t 0), and the resonant current L r flowing through the resonant inductor i r varies sinusoidally starting from zero; Mode 2 t 1 to t 2: At t At time 1, the forward resonance ends. At this time, the primary current i p and the resonance current i r are both zero. The voltage of the resonance capacitor C r reaches the peak value v Cr , and v Crmax > v Crmax . Since at V o time 1, the currents t flowing through switch Q 3 and switch Q 4 i Q3 and i Q4 respectively rise sinusoidally from zero and drop sinusoidally to zero, zero-current turn-on of switch Q 3 and zero-current turn-off of switch Q 4， are realized. In this mode, the primary voltage T hf of the high-frequency transformer v AB = 0. Therefore, the magnetic flux density T hf of the high-frequency transformer B t remains B m unchanged. At the same time, since v Crmax > V o , the resonance cavity realizes reverse resonance, and the resonance current does not pass through the input end of the converter; Mode Three t 2 to t 3: At t At time 2, the reverse resonance ends, and at this time the primary current i p and the resonance current i r are both zero, and the voltage of the resonance capacitor C r is v Cr ( t 2) < V o， Within this mode, the input capacitor C in is charged, the output capacitor C o powers the load, and the resonant cavity does not work.

2. A non-circulating frequency conversion modulation method applicable to an LC series resonance full-bridge converter according to claim 1, characterized in that: High-frequency transformer T hf Voltage across the primary side terminals v AB At the second switching transistor Q When 2 is conducting or the fourth switching transistor Q When 4 is conducting, it is V in Or – V in That is, at the second switching transistor Q When 2 is conducting or the fourth switching transistor Q When 4 is off, it is 0. In this case, the maximum magnetic flux density of the high-frequency transformer T hf Is only related to the resonance frequency and independent of the switching frequency source.

3. A non-circulating frequency conversion modulation method applicable to an LC series resonant full-bridge converter according to claim 2, characterized in that: Both the primary side switching transistor and the secondary side diode are turned on and off with zero current.

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