A capacitor bootstrap type DC transformer

Abstract

The present invention provides a capacitor bootstrap DC transformer, comprising a primary capacitor bootstrap half-bridge circuit, a secondary capacitor bootstrap half-bridge circuit, an AC inductor La and a high-frequency transformer, wherein the primary capacitor bootstrap half-bridge circuit comprises a bootstrap capacitor Cp, a switch device Sp1 and a switch device Sp2, and the secondary capacitor bootstrap half-bridge circuit comprises a bootstrap capacitor Cs, a switch device Ss1 and a switch device Ss2. The capacitor bootstrap DC transformer of the present invention adopts a square wave inverter circuit and a square wave rectifier circuit of a capacitor bootstrap half-bridge structure, and compared with the scheme of adopting a full-bridge circuit, the number of switch devices of the DC transformer is reduced by half, the number of active components required is reduced, the integration of the topology is effectively improved, the power density of the DC transformer is improved and the cost is reduced, and the power transmission performance is consistent with the full-bridge circuit scheme.

Description

Technical Field

[0001] The present invention relates to the field of direct current power distribution, and more specifically, to a capacitor bootstrap type direct current transformer. Background Art

[0002] DC power distribution technology has broad application prospects in many fields such as ship power systems, data centers, and electric vehicles due to its advantages of high efficiency, low cost, and simple control. In order to improve the power supply reliability of the DC power distribution system, the interconnection between DC distribution networks of different voltage levels needs to meet the requirements of electrical isolation, so transformers are indispensable. However, transformers can only achieve AC-AC voltage isolation conversion. In order to achieve isolated voltage conversion from DC to DC, the basic design idea is to first use power semiconductors to form a "DC-AC" power conversion link, then use a high-frequency transformer for electrical isolation, and finally use the power conversion link to achieve "AC-DC" conversion. Since the frequency of the converted AC power is much higher than the mains frequency, the size of the AC transformer can be significantly reduced. In addition, since power semiconductor devices are used for power conversion, port voltage, current, power, etc. can be controlled. A DC-DC converter that meets the above conditions is called a DC transformer.

[0003] In the case of medium and low voltage DC distribution network interconnection, the main topological schemes of DC transformers include dual active bridge (DAB) conversion circuit, CLLC resonant conversion circuit, etc. Among them, the DAB scheme has a small number of passive components, a simple circuit structure, relatively simple control, and fast dynamic response. It is widely used in medium and high power occasions. The DAB type DC transformer consists of four main parts: square wave inverter circuit, AC inductor, high-frequency transformer and square wave rectifier circuit. The AC side of the square wave inverter circuit and the square wave rectifier circuit is connected through the AC inductor and the high-frequency transformer. The high-frequency square wave generated by the square wave inverter circuit and the square wave rectifier circuit acts on the AC inductor and the high-frequency transformer.

[0004] The square wave inverter (rectifier) ​​circuit of the DAB type DC transformer can be a full-bridge circuit or a half-bridge circuit. The DC voltage utilization rate of the full-bridge square wave inverter (rectifier) ​​circuit is 100%, that is, the amplitude of the square wave is equal to the amplitude of the port DC voltage, but four switching devices are required. The half-bridge square wave inverter (rectifier) ​​circuit only requires two switching devices and two voltage-dividing capacitors, but the DC voltage utilization rate is only 50%, that is, the amplitude of the square wave is only half of the amplitude of the port DC voltage. Since the number of devices is halved, the number of corresponding supporting auxiliary circuits such as the drive circuit and the isolated drive power supply is also halved, so the cost of the half-bridge DAB type DC transformer is lower. However, limited by the 50% DC voltage utilization rate, under the same current stress, the half-bridge DAB type DC transformer only has 25% of the transmission power capacity of the full-bridge DAB type DC transformer. Therefore, in medium and high power scenarios, how to reduce the number of switching devices of the DAB type DC transformer and thus improve the integration and power density of the topology needs further research and solution. Summary of the invention

[0005] The present invention aims at the technical problems existing in the prior art and proposes a capacitor bootstrap type DC transformer, comprising a primary capacitor bootstrap half-bridge circuit, a secondary capacitor bootstrap half-bridge circuit, an AC inductor La and a high frequency transformer; the primary capacitor bootstrap half-bridge circuit comprises a bootstrap capacitor Cp, a switch device Sp1 and a switch device Sp2, and the secondary capacitor bootstrap half-bridge circuit comprises a bootstrap capacitor Cs, a switch device Ss1 and a switch device Ss2; the first end of the bootstrap capacitor Cp is connected to the positive electrode of the DC port 1, and the second end of the bootstrap capacitor Cp is connected to the positive electrode of the DC port 1 through the switch device Sp1 and the switch device Sp2 in sequence. The first end of the bootstrap capacitor Cp and the common end of the switch device Sp1 and the switch device Sp2 are connected to the first end of the AC inductor La, and the second end of the AC inductor La is connected to the negative electrode of the DC port 1 through the primary winding of the high-frequency transformer; the first end of the bootstrap capacitor Cs is connected to the positive electrode of the DC port 2, the second end of the bootstrap capacitor Cs is connected to the first end of the bootstrap capacitor Cs through the switch device Ss1 and the switch device Ss2 in turn, and the common end of the switch device Ss1 and the switch device Ss2 is connected to the negative electrode of the DC port 2 through the secondary winding of the high-frequency transformer;

[0006] On the primary side of the capacitor bootstrap DC transformer, the primary side capacitor bootstrap half-bridge circuit is connected in series with the DC port 1 to form a primary side square wave inverter circuit for generating a high-frequency square wave voltage vtp(t); on the secondary side of the capacitor bootstrap DC transformer, the secondary side capacitor bootstrap half-bridge circuit is connected in series with the DC port 2 to form a secondary side square wave rectifier circuit for generating a high-frequency square wave voltage vts(t); the primary side high-frequency square wave voltage vtp(t) and the secondary side high-frequency square wave voltage vts(t) act together on the two ends of the AC inductor La and the high-frequency transformer to realize power transmission between the DC port 1 and the DC port 2.

[0007] Furthermore, the switching device Sp1, the switching device Sp2, the switching device Ss1 and the switching device Ss2 are fully controlled switching devices, including but not limited to Metal Oxide Semiconductor Field Effect Transistor (MOSFET), Insulate-Gate Bipolar Transistor (IGBT) and Gallium Nitride High Electron Mobility Transistor (GaN HEMT), etc.

[0008] Furthermore, the switch devices Sp1 and Sp2 in the primary capacitor bootstrap half-bridge circuit are complementary turned on. The switch devices Ss1 and Ss2 in the secondary capacitor bootstrap half-bridge circuit are complementary turned on.

[0009] Furthermore, there is a certain phase shift angle between the turn-on time of the switch device Sp1 in the primary capacitor bootstrap half-bridge circuit and the turn-on time of the switch device Ss1 in the secondary capacitor bootstrap half-bridge circuit. ,in It is called the shift ratio and is generally controlled at to avoid large reactive circulating current losses.

[0010] Furthermore, phase shift modulation is adopted between the primary and secondary capacitor bootstrap half-bridge circuits, and the current ip(t) of the AC inductor La is a bipolar trapezoidal wave, thereby realizing zero voltage turn-on of the switch devices in the primary and secondary capacitor bootstrap half-bridge circuits.

[0011] Furthermore, when DC port 1 transmits power to DC port 2 (forward transmission), that is, , the capacitor bootstrap DC transformer includes four circuit working modes: mode 1, mode 2, mode 3 and mode 4. When DC port 2 transfers power to DC port 1 (reverse transmission), that is, The capacitor bootstrap DC transformer includes four circuit working modes: mode 4, mode 3, mode 2 and mode 1.

[0012] The present invention provides a capacitor bootstrap DC transformer, which adopts a square wave inverter (rectifier) ​​circuit with a capacitor bootstrap half-bridge structure. Compared with the traditional DAB type DC transformer, the capacitor bootstrap DC transformer has the following advantages:

[0013] (1) The square wave inverter (rectifier) ​​circuit adopts a new capacitor bootstrap half-bridge structure. Compared with the solution using a full-bridge circuit, the number of switching devices in the DC transformer is reduced by half; the number of active components required is reduced, which effectively improves the integration of the topology and promotes the improvement of the power density of the DC transformer and the reduction of the cost;

[0014] (2) The square wave on the AC side of the square wave inverter circuit and the square wave rectifier circuit is generated by superimposing the DC port voltage and the bootstrap capacitor voltage, which can achieve 100% DC voltage utilization and thus achieve the same power transmission capacity as a full-bridge DAB type DC transformer;

[0015] (3) The bipolar trapezoidal inductor current waveform is retained, which has good soft switching characteristics, effectively reduces the switching loss of the DC transformer, is conducive to the increase of switching frequency, and thus promotes the improvement of power density. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A circuit schematic diagram of a capacitor bootstrap type DC transformer provided by the present invention;

[0017] Figure 2 This is the circuit schematic diagram of the capacitor bootstrap DC transformer working in mode 1;

[0018] Figure 3 It is the circuit schematic diagram of the capacitor bootstrap DC transformer working in mode 2;

[0019] Figure 4 It is the circuit schematic diagram of the capacitor bootstrap DC transformer working in mode 3;

[0020] Figure 5 It is the circuit schematic diagram of the capacitor bootstrap DC transformer working in mode 4;

[0021] Figure 6 It is a schematic diagram of the working waveform of the capacitor bootstrap DC transformer;

[0022] Figure 7 It is the equivalent circuit schematic diagram of the capacitor bootstrap DC transformer;

[0023] Figure 8 It is a schematic diagram of a forward transmission power simulation waveform of a capacitor bootstrap type DC transformer embodiment;

[0024] Fig. 9It is a schematic diagram of reverse transmission power simulation waveform of a capacitor bootstrap DC transformer embodiment. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. In addition, the technical features in the various embodiments or single embodiments provided by the present invention can be arbitrarily combined with each other to form a feasible technical solution. This combination is not subject to the constraints of the sequence of steps and / or the structural composition mode, but must be based on the ability of ordinary technicians in this field to achieve. When the combination of technical solutions is contradictory or cannot be achieved, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0026] The circuit structure of the capacitor bootstrap DC transformer provided by the present invention is as follows: Figure 1 As shown, it is used to realize the interconnection between DC port 1 and DC port 2. Figure 1 The capacitor bootstrap DC transformer circuit structure shown specifically includes the following elements or circuit components: a primary capacitor bootstrap half-bridge circuit, a secondary capacitor bootstrap half-bridge circuit, an AC inductor La, and a high-frequency transformer. The primary capacitor bootstrap half-bridge circuit includes a bootstrap capacitor Cp, a switch device Sp1, and a switch device Sp2; similarly, the secondary capacitor bootstrap half-bridge circuit includes a bootstrap capacitor Cs, a switch device Ss1, and a switch device Ss2. The first end of the bootstrap capacitor Cp is connected to the positive electrode of the DC port 1, and the second end of the bootstrap capacitor Cp is connected to the first end of the bootstrap capacitor Cp through the switch device Sp1 and the switch device Sp2 in sequence, the common end of the switch device Sp1 and the switch device Sp2 is connected to the first end of the AC inductor La, and the second end of the AC inductor La is connected to the negative electrode of the DC port 1 through the primary winding of the high-frequency transformer; the first end of the bootstrap capacitor Cs is connected to the positive electrode of the DC port 2, and the second end of the bootstrap capacitor Cs is connected to the first end of the bootstrap capacitor Cs through the switch device Ss1 and the switch device Ss2 in sequence, and the common end of the switch device Ss1 and the switch device Ss2 is connected to the negative electrode of the DC port 2 through the secondary winding of the high-frequency transformer.

[0027] On the primary side of the capacitor bootstrap DC transformer, the primary side capacitor bootstrap half-bridge circuit is connected in series with the DC port 1 to form a primary side square wave inverter circuit for generating a high-frequency square wave voltage vtp(t); on the secondary side of the capacitor bootstrap DC transformer, the secondary side capacitor bootstrap half-bridge circuit is connected in series with the DC port 2 to form a secondary side square wave rectifier circuit for generating a high-frequency square wave voltage vts(t); the primary side high-frequency square wave voltage vtp(t) and the secondary side high-frequency square wave voltage vts(t) act together on the two ends of the AC inductor La and the high-frequency transformer to realize power transmission between the DC port 1 and the DC port 2.

[0028] The switch device Sp1, the switch device Sp2, the switch device Ss1 and the switch device Ss2 are fully controlled switch devices, for example, they may be MOS tubes or other switch devices.

[0029] Assume that the ratio of the primary and secondary turns of the high-frequency transformer, that is, the transformer ratio, is K. The working principle of the capacitor bootstrap DC transformer is as follows:

[0030] (1) The switch devices Sp1 and Sp2 in the primary capacitor bootstrap half-bridge circuit are turned on and off complementary to each other (ignoring the extremely short dead time between the switching states of Sp1 and Sp2). Taking Sp1 as the main switch device for analysis, the turn-on time of Sp1 and Sp2 is half of the switching period Ts, that is, the turn-on duty cycle is 50%; similarly, the switch devices Ss1 and Ss2 in the secondary capacitor bootstrap half-bridge circuit are turned on and off complementary to each other (ignoring the extremely short dead time between the switching states of Ss1 and Ss2). Taking Ss1 as the main switch device for analysis, the turn-on time of Ss1 and Ss2 is half of the switching period Ts, that is, the turn-on duty cycle is 50%.

[0031] (2) The driving signal of Sp1 in the primary capacitor bootstrap half-bridge circuit is gp1, and the driving signal of Ss1 in the secondary capacitor bootstrap half-bridge circuit is gs1. There is a certain phase shift angle between the driving signal gp1 and the driving signal gs1. ,in It is called the shift ratio and is generally controlled at To avoid large reactive circulating current loss. Take the example of power transmission from DC port 1 to DC port 2 (forward transmission), that is, , the capacitor bootstrap DC transformer includes four circuit working modes: mode 1, mode 2, mode 3 and mode 4, respectively. Figure 2 ,like Figure 3 ,like Figure 4 He Ru Figure 5 As shown, the working waveform is as follows Figure 6 shown.

[0032] When the capacitor bootstrap DC transformer is in operating mode 1, Figure 2 As shown, corresponding to Figure 6 The period [t1, t2]. The switch device Sp1 in the primary capacitor bootstrap half-bridge circuit is closed and Sp2 is opened, and the switch device Ss1 in the secondary capacitor bootstrap half-bridge circuit is opened and Ss2 is closed. At this time, the amplitude of the primary high-frequency square wave voltage vtp(t) is obtained by subtracting the DC port 1 voltage Vdc1 and the bootstrap capacitor Cp voltage vcp, that is, vtp(t)=Vdc1-vcp. The secondary bootstrap capacitor Cs is bypassed, so the amplitude of the secondary high-frequency square wave voltage vts(t) is equal to the DC port 2 voltage Vdc2, that is, vts(t)=Vdc2. Therefore, the current ip(t) of the AC inductor La decreases linearly, specifically:

[0033]

[0034] When the capacitor bootstrap DC transformer is in operating mode 2, Figure 3 As shown, corresponding to Figure 6 [t2, t3] period. The switch device Sp1 in the primary capacitor bootstrap half-bridge circuit is closed and Sp2 is opened, and the switch device Ss1 in the secondary capacitor bootstrap half-bridge circuit is closed and Ss2 is opened. At this time, the amplitude of the primary high-frequency square wave voltage vtp(t) is obtained by subtracting the DC port 1 voltage Vdc1 and the bootstrap capacitor Cp voltage vcp, that is, vtp(t)=Vdc1-vcp; the amplitude of the secondary high-frequency square wave voltage vts(t) is obtained by subtracting the DC port 2 voltage Vdc2 and the bootstrap capacitor Cs voltage vcs, that is, vts(t)=Vdc2-vcs. Therefore, the change of the current ip(t) of the AC inductor La is:

[0035]

[0036] When the capacitor bootstrap DC transformer is in operating mode 3, Figure 4 As shown, corresponding to Figure 6 [t3, t4] period. The switch device Sp1 in the primary capacitor bootstrap half-bridge circuit is disconnected and Sp2 is closed, and the switch device Ss1 in the secondary capacitor bootstrap half-bridge circuit is closed and Ss2 is disconnected. At this time, the primary bootstrap capacitor Cp is bypassed, so the amplitude of the primary high-frequency square wave voltage vtp(t) is equal to the DC port 1 voltage Vdc1, that is, vtp(t)=Vdc1; the amplitude of the secondary high-frequency square wave voltage vts(t) is obtained by subtracting the DC port 2 voltage Vdc2 and the bootstrap capacitor Cs voltage vcs, that is, vts(t)=Vdc2-vcs. Therefore, the change of the current ip(t) of the AC inductor La is:

[0037]

[0038] When the capacitor bootstrap DC transformer is in operating mode 4, Figure 5 As shown, corresponding to Figure 6 [t4, t5] period. The switch device Sp1 in the primary capacitor bootstrap half-bridge circuit is disconnected and Sp2 is closed, and the switch device Ss1 in the secondary capacitor bootstrap half-bridge circuit is disconnected and Ss2 is closed. At this time, the primary bootstrap capacitor Cp is bypassed, so the amplitude of the primary high-frequency square wave voltage vtp(t) is equal to the DC port 1 voltage Vdc1, that is, vtp(t)=Vdc1; the secondary bootstrap capacitor Cs is also bypassed, so the amplitude of the secondary high-frequency square wave voltage vts(t) is equal to the DC port 2 voltage Vdc2, that is, vts(t)=Vdc2. Therefore, the change of the current ip(t) of the AC inductor La is:

[0039]

[0040] The volt-second product of the primary high-frequency square wave voltage vtp(t) and the secondary high-frequency square wave voltage vts(t) within one switching cycle is equal to zero to ensure the normal operation of the high-frequency transformer. Therefore, the values ​​of the primary bootstrap capacitor Cp voltage vcp and the secondary bootstrap capacitor Cs voltage vcs are:

[0041]

[0042]

[0043] Furthermore, by analyzing the working mode of the capacitor bootstrap DC transformer, the expressions of the primary high-frequency square wave voltage vtp(t) and the secondary high-frequency square wave voltage vts(t) in one switching cycle can be obtained:

[0044]

[0045]

[0046] Therefore, vtp(t) and vts(t) are square waves with equal positive and negative amplitudes and a duty cycle of 50%. Further, the equivalent circuit of the capacitor bootstrap DC transformer is as follows: Figure 7 As shown, vtp(t) and vts(t) act together on the AC inductor La and the two ends of the high-frequency transformer.

[0047] Furthermore, the expression of the current ip(t) of the AC inductor La in one switching cycle is:

[0048]

[0049] Where, G=KVdc2 / Vdc1.

[0050] Furthermore, the transmission power between DC port 1 and DC port 2 can be obtained by integrating and averaging the primary high-frequency square wave vtp(t) and the current ip(t) of the AC inductor La:

[0051]

[0052] Therefore, the power transmission characteristics of the capacitor bootstrap DC transformer proposed in the present invention are consistent with those of the traditional DAB DC transformer, and the shift ratio between the primary high-frequency square wave voltage vtp(t) and the secondary high-frequency square wave voltage vts(t) is controlled. The positive and negative and size of the DC port can realize the power transmission between DC port 1 and DC port 2.

[0053] A specific implementation example is given below:

[0054] In order to verify the basic principle of the capacitor bootstrap DC transformer, a simulation model of the capacitor bootstrap DC transformer was built in PLECS. The specific circuit parameters of the capacitor bootstrap DC transformer are shown in Table 1.

[0055] Table 1 Parameters of capacitor bootstrap DC transformer embodiment

[0056]

[0057] Figure 8 The simulation waveform of the forward transmission power of the capacitor bootstrap DC transformer embodiment is given. It can be seen that the voltage of DC port 1 Vdc1=400V, the voltage of the primary bootstrap capacitor Cp vcp=800V, which is exactly 2Vdc1; the voltage of DC port 2 Vdc2=48V, the voltage of the secondary bootstrap capacitor Cs vcs=96V, which is exactly 2Vdc2. In addition, the primary high-frequency square wave voltage vtp(t) is a square wave with a duty cycle of 50% and an amplitude of 400V, and the secondary high-frequency square wave voltage vts(t) is a square wave with a duty cycle of 50% and an amplitude of 48V. Therefore, the DC voltage utilization rate of the primary and secondary sides of the capacitor bootstrap DC transformer is 100%. At this time, the primary high-frequency square wave voltage vtp(t) leads the secondary high-frequency square wave voltage vts(t) by 0.1366μs, that is, the shift is , the power transmitted from DC port 1 to DC port 2 is 1000W. At the same time, the current ip(t) of the AC inductor La is a bipolar trapezoidal wave. Taking Sp1 in the primary capacitor bootstrap half bridge as an example, before its drive signal gp1 becomes high, the current flowing through Sp1 is positive, so its freewheeling diode is naturally turned on. When the drive signal gp1 becomes high, Sp1 achieves zero voltage turn-on. Similarly, Sp2 in the primary capacitor bootstrap half bridge and Ss1 and Ss2 in the secondary capacitor bootstrap half bridge can achieve zero voltage turn-on.

[0058] Fig. 9The simulation waveform of the reverse power transmission of the capacitor bootstrap DC transformer embodiment is given. It can be seen that the voltage of DC port 1 is Vdc1=400V, and the voltage of the primary bootstrap capacitor Cp is vcp=800V, which is exactly 2Vdc1; the voltage of DC port 2 is Vdc2=48V, and the voltage of the secondary bootstrap capacitor Cs is vcs=96V, which is exactly 2Vdc2. In addition, the primary high-frequency square wave voltage vtp(t) is a square wave with a duty cycle of 50% and an amplitude of 400V, and the secondary high-frequency square wave voltage vts(t) is a square wave with a duty cycle of 50% and an amplitude of 48V. Therefore, the DC voltage utilization rate of the primary and secondary sides of the capacitor bootstrap DC transformer is 100%. At this time, the secondary high-frequency square wave voltage vts(t) leads the primary high-frequency square wave voltage vtp(t) by 0.1366μs, that is, the shift is , the power transmitted from DC port 2 to DC port 1 is 1000W. At the same time, the current ip(t) of the AC inductor La is a bipolar trapezoidal wave. Taking Sp1 in the primary capacitor bootstrap half bridge as an example, before its drive signal gp1 becomes high, the current flowing through Sp1 is positive, so its freewheeling diode is naturally turned on. When the drive signal gp1 becomes high, Sp1 achieves zero voltage turn-on. Similarly, Sp2 in the primary capacitor bootstrap half bridge and Ss1 and Ss2 in the secondary capacitor bootstrap half bridge can achieve zero voltage turn-on.

[0059] From the above simulation waveforms, it can be seen that although the capacitor bootstrap DC transformer proposed in the present invention requires a reduced number of active components, it can still achieve 100% DC voltage utilization, thereby achieving the same power transmission capacity as the full-bridge DAB DC transformer; at the same time, it has good soft switching characteristics, effectively reducing the switching loss of the DC transformer. In summary, the capacitor bootstrap DC transformer proposed in the present invention can effectively improve the integration of the topology, promote the improvement of the power density of the DC transformer and reduce the cost.

[0060] The advantages of the capacitor bootstrap DC transformer provided by the present invention mainly include the following points:

[0061] Advantage 1: The capacitor-bootstrap DC transformer proposed in the present invention adopts a novel capacitor-bootstrap half-bridge structure. Compared with the solution using a full-bridge circuit, the number of switching devices of the DC transformer is reduced by half; the number of required active components is reduced, which effectively improves the integration of the topology and promotes the improvement of the power density of the DC transformer and the reduction of the cost.

[0062] like Figure 1As shown, the primary side capacitor bootstrap half-bridge circuit of the capacitor bootstrap DC transformer is composed of a bootstrap capacitor Cp, a switch device Sp1 and a switch device Sp2, and only contains two switch devices; the secondary side capacitor bootstrap half-bridge circuit is composed of a bootstrap capacitor Cs, a switch device Ss1 and a switch device Ss2, and also only contains two switch devices. Therefore, the capacitor bootstrap DC transformer uses a total of 4 switch devices. The traditional full-bridge DAB type DC transformer requires 4 switch devices on the primary and secondary sides to form a full bridge, so a total of 8 switch devices are required. Therefore, the capacitor bootstrap DC transformer proposed in the present invention is compared with the full-bridge DAB type DC transformer, and the number of switch devices required is reduced by half. The number of switch devices is halved, and the number of supporting components such as the corresponding drive circuit, isolation drive power supply, etc. is also halved. At the same time, the primary and secondary side capacitor bootstrap half-bridge circuits are newly added with bootstrap capacitors Cp and Cs to support the negative level voltage of the primary high-frequency square wave voltage vtp(t) and the secondary high-frequency square wave voltage vts(t). At high switching frequencies, the capacitance of bootstrap capacitors Cp and Cs is small, so the volume space required by bootstrap capacitors Cp and Cs is much smaller than the volume space required by active components such as switching devices and their drive circuits. Therefore, the capacitor bootstrap DC transformer effectively improves the topology integration by halving the number of active components, promoting the improvement of DC transformer power density and the reduction of cost.

[0063] Advantage 2: The AC side square wave of the square wave inverter circuit and the square wave rectifier circuit is generated by the superposition of the DC port voltage and the bootstrap capacitor voltage, which can achieve 100% DC voltage utilization, thereby achieving the same power transmission capacity as the full-bridge DAB type DC transformer.

[0064] The expression of the primary high-frequency square wave voltage vtp(t) and the secondary high-frequency square wave voltage vts(t) in one switching cycle

[0065]

[0066]

[0067] It can be seen that the primary high-frequency square wave voltage vtp(t) is a square wave with an amplitude of Vdc1, a duty cycle of 50%, and a DC voltage utilization rate of 100%; similarly, the secondary high-frequency square wave voltage vts(t) is a square wave with an amplitude of Vdc2, a duty cycle of 50%, and a DC voltage utilization rate of 100%.

[0068] Furthermore, the transmission power between the DC port 1 and the DC port 2 of the capacitor bootstrap DC transformer is:

[0069]

[0070] The transmission power characteristics of the full-bridge DAB DC transformer are consistent, so under the same switching period Ts and AC inductance La, the maximum transmission power Pmax of the two is also the same, that is:

[0071]

[0072] The amplitudes of the high-frequency square wave voltages on the primary and secondary sides of the half-bridge DAB DC transformer are 0.5Vdc1 and 0.5Vdc2 respectively, and its maximum transmission power Phmax is only:

[0073]

[0074] It can be seen that, thanks to the 100% DC voltage utilization, the capacitor bootstrap DC transformer proposed in the present invention has the same power transmission capacity as the full-bridge DAB DC transformer, which is four times the power transmission capacity of the half-bridge DAB DC transformer.

[0075] Advantage 3: It has good soft switching characteristics, which effectively reduces the switching loss of the DC transformer, is conducive to the increase of switching frequency, and thus promotes the improvement of power density.

[0076] Under voltage matching conditions, that is, G=1, the current ip(t) of the AC inductor La is simplified to:

[0077]

[0078] At t1, ip(t1)>0. During the dead time, the freewheeling diode of Sp1 in the primary capacitor bootstrap half bridge is naturally turned on, and Sp1 achieves zero voltage turn-on. At t2, is(t2)=Kip(t2)<0. During the dead time, the freewheeling diode of Ss1 in the secondary capacitor bootstrap half bridge is naturally turned on, and Ss1 achieves zero voltage turn-on. At t3, ip(t3)<0. During the dead time, the freewheeling diode of Sp2 in the primary capacitor bootstrap half bridge is naturally turned on, and Sp2 achieves zero voltage turn-on. At t4, is(t4)=Kip(t4)>0. During the dead time, the freewheeling diode of Ss2 in the secondary capacitor bootstrap half bridge is naturally turned on, and Ss2 achieves zero voltage turn-on. At t5, the next switching cycle begins and the above process is repeated. Therefore, the capacitor bootstrap DC transformer proposed in the present invention has good soft switching characteristics under voltage matching conditions, effectively reduces the switching loss of the DC transformer, is beneficial to the increase of switching frequency, and further promotes the improvement of power density.

[0079] It should be noted that in the above embodiments, the description of each embodiment has its own emphasis, and for parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0080] Although the preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0081] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A capacitor bootstrap DC transformer, characterized in that: The invention comprises a primary capacitor bootstrap half-bridge circuit, a secondary capacitor bootstrap half-bridge circuit, an AC inductor La and a high-frequency transformer; the primary capacitor bootstrap half-bridge circuit comprises a bootstrap capacitor Cp, a switch device Sp1 and a switch device Sp2, and the secondary capacitor bootstrap half-bridge circuit comprises a bootstrap capacitor Cs, a switch device Ss1 and a switch device Ss2; the first end of the bootstrap capacitor Cp is connected to the positive electrode of the DC port 1, the second end of the bootstrap capacitor Cp is connected to the first end of the bootstrap capacitor Cp through the switch device Sp1 and the switch device Sp2 in sequence, and the switch device Sp1 is connected to the positive electrode of the DC port 1, and the second end of the bootstrap capacitor Cp is connected to the first end of the bootstrap capacitor Cp through the switch device Sp1 and the switch device Sp2 in sequence. The common end of the device Sp1 and the switch device Sp2 is connected to the first end of the AC inductor La, and the second end of the AC inductor La is connected to the negative electrode of the DC port 1 through the primary winding of the high-frequency transformer; the first end of the bootstrap capacitor Cs is connected to the positive electrode of the DC port 2, and the second end of the bootstrap capacitor Cs is connected to the first end of the bootstrap capacitor Cs through the switch device Ss1 and the switch device Ss2 in turn, and the common end of the switch device Ss1 and the switch device Ss2 is connected to the negative electrode of the DC port 2 through the secondary winding of the high-frequency transformer; On the primary side of the capacitor bootstrap DC transformer, the primary side capacitor bootstrap half-bridge circuit is connected in series with the DC port 1 to form a primary side square wave inverter circuit for generating a high-frequency square wave voltage vtp(t); on the secondary side of the capacitor bootstrap DC transformer, the secondary side capacitor bootstrap half-bridge circuit is connected in series with the DC port 2 to form a secondary side square wave rectifier circuit for generating a high-frequency square wave voltage vts(t); the primary side high-frequency square wave voltage vtp(t) and the secondary side high-frequency square wave voltage vts(t) act together on the two ends of the AC inductor La and the high-frequency transformer to realize power transmission between the DC port 1 and the DC port 2.

2. The capacitor bootstrap DC transformer according to claim 1, characterized in that: The switch device Sp1 , the switch device Sp2 , the switch device Ss1 , and the switch device Ss2 are fully controlled switch devices.

3. The capacitor bootstrap DC transformer according to claim 1, characterized in that: The switch device Sp1 and the switch device Sp2 in the primary capacitor bootstrap half-bridge circuit are complementary turned on, and the switch device Ss1 and the switch device Ss2 in the secondary capacitor bootstrap half-bridge circuit are complementary turned on.

4. The capacitor bootstrap DC transformer according to claim 1, characterized in that: Phase-shift modulation is adopted between the primary capacitor bootstrap half-bridge circuit and the secondary capacitor bootstrap half-bridge circuit, and the current ip(t) of the AC inductor La is a bipolar trapezoidal wave, so as to realize zero-voltage turning-on of the switching devices in the primary capacitor bootstrap half-bridge circuit and the secondary capacitor bootstrap half-bridge circuit.

5. The capacitor bootstrap DC transformer according to claim 1, characterized in that: When the DC port 1 transmits power to the DC port 2, that is, in the forward transmission, the capacitor bootstrap DC transformer includes four circuit working modes, namely, mode 1, mode 2, mode 3 and mode 4; When the DC port 2 transmits power to the DC port 1, that is, reverse transmission, the capacitor bootstrap DC transformer includes four circuit working modes, namely, mode 4, mode 3, mode 2 and mode 1; Wherein, mode 1 refers to that the switch device Sp1 in the primary capacitor bootstrap half-bridge circuit is closed and the switch device Sp2 is disconnected, and the switch device Ss1 in the secondary capacitor bootstrap half-bridge circuit is disconnected and the switch device Ss2 is closed; Mode 2 refers to that the switch device Sp1 in the primary capacitor bootstrap half-bridge circuit is closed and the switch device Sp2 is disconnected, and the switch device Ss1 in the secondary capacitor bootstrap half-bridge circuit is closed and the switch device Ss2 is disconnected; Mode 3 refers to the switch device Sp1 in the primary capacitor bootstrap half-bridge circuit being disconnected and the switch device Sp2 being closed, and the switch device Ss1 in the secondary capacitor bootstrap half-bridge circuit being closed and the switch device Ss2 being disconnected; Mode 4 means that the switch device Sp1 in the primary capacitor bootstrap half-bridge circuit is disconnected and the switch device Sp2 is closed, and the switch device Ss1 in the secondary capacitor bootstrap half-bridge circuit is disconnected and the switch device Ss2 is closed.

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