阅读说明：本技术 电源单元和使用该电源单元的电源系统 (Power supply unit and power supply system using the same ) 是由 袁权发 李天河 田凯 于 2018-09-27 设计创作，主要内容包括：本发明的目标因此是提供一种电源系统的电源单元和使用该电源单元的电源系统。电源单元包括：第一功率转换电路,可操作以跨其第一输出正端子与第一输出负端子输出第一DC电压；第二功率转换电路,可操作以跨其第二输出正端子与第二输出负端子输出第二DC电压；第一可控单向半导体开关,可操作以生成从第一功率转换电路的第一输出正端子到第二功率转换电路的第二输出负端子的第一传导路径；第一单向半导体开关,可操作以生成从第一功率转换电路的第一输出正端子到第二功率转换电路的第二输出正端子的第二传导路径；第二单向半导体开关,可操作以生成从第一功率转换电路的第一输出负端子到第二功率转换电路的第二输出负端子的第三传导路径；第一低通滤波器；以及控制单元,可操作以向第一可控单向半导体开关发出接通信号或关断信号,使得第一功率转换电路和第二功率转换电路经由第一传导路径或经由第二传导路径和第三传导路径,向低通滤波器提供电流。第一低通滤波器可以有助于平滑电源单元的输出电压和电流,以便实现相对较宽的线性恒定输出功率范围。通过由开关装置控制开关频率的开关周期内串联和并联连接的比率,在第一低通滤波器的帮助下,电源单元的输出电压和电流范围都可以平滑且线性地扩大。(An object of the present invention is therefore to provide a power supply unit of a power supply system and a power supply system using the same. The power supply unit includes: a first power conversion circuit operable to output a first DC voltage across its first output positive terminal and first output negative terminal; a second power conversion circuit operable to output a second DC voltage across its second output positive terminal and second output negative terminal; a first controllable unidirectional semiconductor switch operable to generate a first conduction path from a first output positive terminal of the first power conversion circuit to a second output negative terminal of the second power conversion circuit; a first unidirectional semiconductor switch operable to generate a second conductive path from the first output positive terminal of the first power conversion circuit to the second output positive terminal of the second power conversion circuit; a second unidirectional semiconductor switch operable to generate a third conductive path from the first output negative terminal of the first power conversion circuit to the second output negative terminal of the second power conversion circuit; a first low-pass filter; and a control unit operable to issue an on signal or an off signal to the first controllable unidirectional semiconductor switch such that the first power conversion circuit and the second power conversion circuit provide current to the low pass filter via the first conductive path or via the second conductive path and the third conductive path. The first low-pass filter may help to smooth the output voltage and current of the power supply unit in order to achieve a relatively wide linear constant output power range. By controlling the ratio of the series and parallel connections within the switching period of the switching frequency by the switching means, both the output voltage and the current range of the power supply unit can be smoothly and linearly expanded with the aid of the first low-pass filter.)

1. A power supply unit of a power supply system, comprising:

a first power conversion circuit operable to output a first DC voltage across a first output positive terminal and a first output negative terminal of the first power conversion circuit;

a second power conversion circuit operable to output a second DC voltage across a second output positive terminal and a second output negative terminal of the second power conversion circuit;

a first controllable unidirectional semiconductor switch operable to generate a first conduction path from the first output positive terminal of the first power conversion circuit to the second output negative terminal of the second power conversion circuit;

a first unidirectional semiconductor switch operable to generate a second conductive path from the first output positive terminal of the first power conversion circuit to the second output positive terminal of the second power conversion circuit;

a second unidirectional semiconductor switch operable to generate a third conductive path from the first output negative terminal of the first power conversion circuit to the second output negative terminal of the second power conversion circuit;

a first low-pass filter; and

a control unit operable to issue an on signal or an off signal to the first controllable unidirectional semiconductor switch such that the first and second power conversion circuits provide current to the low pass filter via the first conductive path or via the second and third conductive paths.

2. The power supply unit of claim 1, wherein:

the first low pass filter includes a first inductive element across the first output negative terminal and the second output positive terminal and an input port.

3. The power supply unit of claim 2, wherein:

the first low pass filter further includes an output port for providing a third DC voltage across the positive and negative output port terminals.

4. The power supply unit of any one of the preceding claims, further comprising:

a third unidirectional semiconductor switch electrically connected in anti-parallel with the first controllable unidirectional semiconductor switch.

5. The power supply unit of any one of the preceding claims, wherein:

the control unit is further operable to issue control signals to the first power conversion circuit and the second power conversion circuit such that the first DC voltage is substantially equal to the second DC voltage.

6. A power supply system comprising:

a first power supply unit according to any one of claim 3;

a second power supply unit according to any one of claim 3;

a second controllable unidirectional semiconductor switch operable to generate a fourth conduction path from the positive output port terminal of the first power supply unit to the negative output port terminal of the second power supply unit;

a fourth unidirectional semiconductor switch operable to generate a fifth conductive path from the positive output port terminal of the first power supply unit to the positive output port terminal of the second power supply unit;

a fifth unidirectional semiconductor switch operable to generate a sixth conduction path from the negative output port terminal of the first power supply unit to the negative output port terminal of the second power supply unit;

a second low-pass filter; and

a controller operable to issue an on-signal or an off-signal to the second controllable unidirectional semiconductor switch such that the first and second power supply units provide current to the second high frequency filtering device via the fourth conductive path or via the fifth and sixth conductive paths.

7. The power supply system of claim 6, wherein:

the second low pass filter includes a second inductive element and an input port across the negative output port terminal of the first power supply unit and the positive output port terminal of the second power supply unit.

8. The power supply system of claim 6, further comprising:

a sixth unidirectional semiconductor switch electrically connected in anti-parallel with the second controllable unidirectional semiconductor switch.

9. The power supply system according to any one of claims 6 to 8, wherein:

the controller is further operable to issue control signals to the first power supply unit and the second power supply unit such that the DC output voltage from the first power supply unit is substantially equal to the DC output voltage from the second power supply unit.

Technical Field

The present invention relates to converting DC power input to DC power output, and more particularly to power conversion of DC-DC without intermediate conversion to AC power.

Background

To reduce cost and power consumption and improve efficiency of the power supply, a DC power supply unit electrically switched between a series connection and a parallel connection is provided. By using series-parallel configurable switching cells of relatively low rate power switching devices, the conversion process can achieve a relatively wide range of voltage outputs or current outputs. Thus, the cost spent on the power supply is reduced.

This is particularly useful in the field of Electric Vehicle (EV) charging technology. The charging voltage of the EV is increasingly higher to increase the charging power. For some super EVs or passenger cars, charging voltages may require up to 1000V. Therefore, to meet the charging requirements of most EVs, the charging voltage range will now be from 200V to 1000V. In order to output such a wide range of voltages, conventional topologies such as single LLC, phase-shifting topologies need to sacrifice a lot of power efficiency because their operation is far from the optimum operating point under a wide output voltage regulation range. A conventional solution for extending the output voltage range is to apply a modular series or parallel connection method to two or more isolated/individual power modules (power supply units). For example, patent CN 204538972U discloses a wide-range output switching power supply having a power supply module connected to a switching circuit connected to a power supply, and a sub-power supply module connected to an end of a positive electrode. By changing the connection mode of the power modules in series or parallel connection by using the relay of the power switch, the range of the output voltage and current can be enlarged. When power modules are connected in parallel, the output current of the system is spread to the sum of all connected power modules. The system voltage is expanded by the series connection of the power modules.

Disclosure of Invention

However, the constant output power (CP) range of conventional systems cannot be achieved by switching between series and parallel connections because the maximum output current of conventional power supply systems can experience a step-down as the connection of the power supply modules is changed from parallel to series to increase the voltage.

Therefore, it is desirable to provide a power supply unit and a power supply system using the same to widen a constant output power range for a wide output range application by linearly expanding output voltage and current ranges.

It is therefore an object of the present invention to provide a power supply unit of a power supply system. The power supply unit includes: a first power conversion circuit operable to output a first DC voltage across its first output positive terminal and first output negative terminal; a second power conversion circuit operable to output a second DC voltage across its second output positive terminal and second output negative terminal; a first controllable unidirectional semiconductor switch operable to generate a first conduction path from a first output positive terminal of the first power conversion circuit to a second output negative terminal of the second power conversion circuit; a first unidirectional semiconductor switch operable to generate a second conductive path from the first output positive terminal of the first power conversion circuit to the second output positive terminal of the second power conversion circuit; a second unidirectional semiconductor switch operable to generate a third conductive path from the first output negative terminal of the first power conversion circuit to the second output negative terminal of the second power conversion circuit; a first low-pass filter; and a control unit operable to issue an on signal or an off signal to the first controllable unidirectional semiconductor switch such that the first power conversion circuit and the second power conversion circuit provide current to the low pass filter via the first conductive path or via the second conductive path and the third conductive path.

The first low-pass filter may help to smooth the output voltage and current of the power supply unit in order to achieve a relatively wide linear constant output power range. By controlling the ratio of the series and parallel connections within the switching period of the switching frequency by the switching means, both the output voltage and the current range of the power supply unit can be smoothly and linearly expanded with the aid of the first low-pass filter.

Preferably, the first low pass filter comprises a first inductive element across the first output negative terminal and the second output positive terminal and an output port.

Preferably, the first low pass filter further comprises an output port for providing a third DC voltage (power supply unit output voltage) across the positive and negative output port terminals.

According to another object of the present invention, a power supply system is provided. The power supply system includes: a first power supply unit; a second power supply unit; a second controllable unidirectional semiconductor switch operable to generate a fourth conduction path from the positive output port terminal of the first power supply unit to the negative output port terminal of the second power supply unit; a fourth unidirectional semiconductor switch operable to generate a fifth conductive path from the positive output port terminal of the first power supply unit to the positive output port terminal of the second power supply unit; a fifth unidirectional semiconductor switch operable to generate a sixth conduction path from the negative output port terminal of the first power supply unit to the negative output port terminal of the second power supply unit; a second low-pass filter; and a controller operable to issue an on signal or an off signal to the second controllable unidirectional semiconductor switch such that the first and second power supply units provide current to the second high frequency filtering device via the fourth conductive path or via the fifth and sixth conductive paths. The power supply unit may be used as a submodule to form a power supply system with a wider output voltage and constant power range.

Drawings

The subject matter of the invention will be explained in more detail below with reference to preferred exemplary embodiments shown in the drawings, in which:

FIG. 1 is a schematic circuit block diagram illustrating a power supply unit according to an embodiment of the present invention;

FIGS. 2A and 2B illustrate two modes of operation of an apparatus according to an embodiment of the present invention;

FIG. 3A shows a V-I curve according to a power conversion circuit providing a DC voltage;

FIG. 3B shows a V-I curve for an exemplary operation of a solution according to the prior art using the power conversion circuit from FIG. 3A;

FIG. 3C shows a V-I curve for an exemplary operation of a solution according to the invention using the power conversion circuit from FIG. 3A;

fig. 4A and 4B show electrical waveforms relating to components of a power supply unit providing a constant power of 30kW at two output DC voltage levels at 600V and 700V, respectively, according to the invention; and

fig. 5 is a schematic circuit block diagram showing a power supply system using a power supply unit according to an embodiment of the present invention.

The reference symbols used in the drawings and their meanings are listed in abstract form in the list of reference symbols. In principle, identical components are provided with the same reference symbols in the figures.

Detailed Description

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. It should be noted that the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Further, it should be noted that the word "may" is used in this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must). The word "include" and its derivatives mean "including, but not limited to". The term "connected" means "directly or indirectly connected", and the term "coupled" means "directly or indirectly connected".

Fig. 1 is a schematic circuit block diagram showing a power supply unit according to an embodiment of the present invention. As shown in fig. 1, the power supply unit 1 includes a first power conversion circuit 10, a second power conversion circuit 11, a first controllable unidirectional semiconductor switch 12, a first unidirectional semiconductor switch 13, a second unidirectional semiconductor switch 14, a first low-pass filter 15, and a control unit 16. The first power conversion circuit 10 is electrically isolated from the second power conversion circuit 11.

By the first power conversion circuit 10, its input voltage is converted into a first DC voltage V1, which behaves as a DC voltage source. The first power conversion circuit 10 includes a first output positive terminal 100 and a first output negative terminal 101. The input voltage thereof is converted into a second DC voltage V2, which behaves as a DC voltage source, by the second power conversion circuit 11. The second power conversion circuit 11 includes a second output positive terminal 110 and a second output negative terminal 111. For example, if their input voltage is an AC voltage, either of the first power conversion circuit 10 and the second power conversion circuit 11 may be an AC-DC power converter using a conventional AC-DC power conversion circuit topology, such as an AC-DC power converter with transformer isolation. Alternatively, they may be two separate AC-DC power converters each without transformer isolation but electrically coupled to a respective secondary winding of the transformer, and the transformer having a primary winding electrically coupled to the AC power source. Their input AC voltages may be supplied directly from one AC power source or from both AC power sources, respectively, or through one or both AC-to-AC power converters and converted to AC bus voltages, which in turn supply the input terminals of the respective first and second power conversion circuits 10, 11. Alternatively, if their input voltage is a DC voltage, either of the first power conversion circuit 10 and the second power conversion circuit 11 may be a DC-DC power converter using a conventional DC-DC power conversion circuit topology, such as a DC-DC power converter with transformer isolation. Alternatively, they may be two separate DC-DC power converters each without transformer isolation, but with their input sides electrically isolated. Their input DC voltages may be supplied directly from one or respectively two DC power supplies, or through one or two DC-DC power converters, and converted to DC bus voltages, which in turn supply the input terminals of the respective first and second power conversion circuits 10, 11. The power converter circuit is electrically coupled to an AC power source or a DC power source and converts an AC voltage or a DC voltage to an AC or DC bus voltage.

The first controllable unidirectional semiconductor switch 12 is operable to generate a first conduction path P1 from the first output positive terminal 100 of the first power conversion circuit 10 to the second output negative terminal 111 of the second power conversion circuit 11. The first controllable unidirectional semiconductor switch 12 may be, for example, a power MOSFET or an IGBT, the state of which depends not only on its external power circuit but also on the signal on its drive terminal (this terminal is called gate or base). In the case of using MOSFETs, the drains and sources thereof are electrically coupled to the first output positive terminal 100 of the first power conversion circuit 10 and the second output negative terminal 111 of the second power conversion circuit 11, respectively; in the case of using the IGBT, the collector and emitter thereof are electrically coupled to the first output positive terminal 100 of the first power conversion circuit 10 and the second output negative terminal 111 of the second power conversion circuit 11, respectively. The control unit 16 may issue an on-signal or an off-signal to the first controllable unidirectional semiconductor switch 12.

The first unidirectional semiconductor switch 13 is operable to generate a second conduction path P2 from the first output positive terminal 100 of the first power conversion circuit 10 to the second output positive terminal 110 of the second power conversion circuit 11. For example, the first unidirectional semiconductor switch 13 may be a power diode, a body diode of a power MOSFET or a freewheeling diode of an IGBT, the state being completely dependent on the external power circuit to which it is connected. The forward voltage turns on the first unidirectional semiconductor switch 13 and the reverse voltage turns off the first unidirectional semiconductor switch 13. In the case of using a power diode, the anode and the cathode of the power diode are electrically coupled to the first output positive terminal 100 of the first power conversion circuit 10 and the second output positive terminal 110 of the second power conversion circuit 11, respectively. In the case of using the body diode of the power MOSFET or the freewheel diode of the IGBT, the anode and the cathode of the body diode or the freewheel diode are also electrically coupled to the first output positive terminal 100 of the first power conversion circuit 10 and the second output positive terminal 110 of the second power conversion circuit 11, respectively.

The second unidirectional semiconductor switch 14 is operable to generate a third conduction path P3 from the first output negative terminal 101 of the first power conversion circuit 10 to the second output negative terminal 111 of the second power conversion circuit 11. For example, the second unidirectional semiconductor switch 14 may be a power diode, a body diode of a power MOSFET or a freewheeling diode of an IGBT, the state being completely dependent on the external power circuit to which it is connected. The forward voltage turns on the second unidirectional semiconductor switch 14 and the reverse voltage turns off the second unidirectional semiconductor switch 14. In the case of using a power diode, the anode and cathode of the power diode are electrically coupled to the first output negative terminal 101 of the first power conversion circuit 10 and the second output negative terminal 111 of the second power conversion circuit 11, respectively. In the case of using the body diode of the power MOSFET or the freewheel diode of the IGBT, the anode and the cathode of the body diode or the freewheel diode are also electrically coupled to the first output negative terminal 101 of the first power conversion circuit 10 and the second output negative terminal 111 of the second power conversion circuit 11, respectively.

The first low pass filter 15 is operable to pass signals having frequencies below a selected cut-off frequency and to attenuate signals having frequencies above the cut-off frequency. The cut-off frequency may be set based on the quality requirements of the DC voltage output. For example, the first low-pass filter 15 may be an LC filter and an LCL filter. In this embodiment, the first low-pass filter 15 uses an LC filter in which the first inductance element 150 and the first capacitance element 151 are connected in series and the input port is across the first output negative terminal 101 of the first power conversion circuit 10 and the second output positive terminal 110 of the second power conversion circuit 11. The first low pass filter 15 may further comprise an output port for providing the third DC voltage V3 across the positive and negative output port terminals.

By issuing an on signal or an off signal to the first controllable unidirectional semiconductor switch 12, the control unit 16 may control the first power conversion circuit 10 and the second power conversion circuit 11 to supply a current to the low-pass filter 15 via the first conductive path P1 or the second conductive path P2 and the third conductive path P3.

Fig. 2A and 2B show two modes of operation of an apparatus according to an embodiment of the invention. The control unit 16 issues a control signal PWM to the first controllable unidirectional semiconductor switch 12 depending on the first DC voltage V1 and the second DC voltage V2. Preferably, the control unit 16 is further operable to issue control signals to the first power conversion circuit 10 and the second power conversion circuit 11 such that the first DC voltage V1 is substantially equal to the second DC voltage V2. For example, the magnitudes of the first DC voltage V1 and the second DC voltage V2 are adjusted to be equal to V. As shown in fig. 2A, when the first controllable unidirectional semiconductor switch 12 is turned on, the first unidirectional semiconductor switch 13 and the second unidirectional semiconductor switch 14 are turned off due to reverse bias, and then the outputs of the first power conversion circuit 10 and the second power conversion circuit 11 are connected in series through the first controllable unidirectional semiconductor switch 12. As shown in fig. 2B, when the first controllable unidirectional semiconductor switch 12 is turned off, the outputs of the first power conversion circuit 10 and the second power conversion circuit 11 are connected in parallel through the first unidirectional semiconductor switch 13 and the second unidirectional semiconductor switch 14, which are forward biased. By switching the first controllable unidirectional semiconductor switch 12 on and off at a certain ratio (PWM method), the device 1 is able to regulate the third DC voltage V3 between V and 2V with the aid of the first low-pass filter 15. Here, a power conversion circuit must be used to output the DC voltage because PWM modulation requires a power input from a DC voltage source. The first low-pass filter 15 operates to smooth the third DC voltage V3 as an output voltage and current in order to achieve a relatively wide linear constant output power range. By controlling the ratio of the series and parallel connections in the switching period of the switching frequency by the switching means, with the aid of the first low-pass filter 15, the current range across the output voltage (the third DC voltage V3) of the first low-pass filter 15 and the power supply unit can be smoothly and linearly extended. Further, no modification is required for the first power conversion circuit 10 and the second power conversion circuit 11.

Fig. 3A shows a V-I curve according to a power conversion circuit providing a DC voltage. Assume that the two power conversion circuits each have parameters and that they both operate with substantially the same amplitude to output a DC output voltage:

power capacity: 15 kW;

output voltage range: 200V to 500V;

the maximum output current 40A.

The output voltage of the power conversion circuit can be continuously adjusted in the range of 200V to 500V, with a maximum output power of 15kW and a maximum output current of 40A.

Fig. 3B shows a V-I curve for an exemplary operation of a solution according to the prior art using the power conversion circuit from fig. 3A. According to the prior art, two power conversion circuits providing two DC output voltages are switched between a series connection and a parallel connection, but their supply power is fed to the load without passing through a low frequency filter with inductive elements. Its output voltage can be continuously adjusted in the range of 200V to 1000V, with a maximum rated output power of 30kW (15kW x 2) and a maximum rated current of 80A (40A x 2).

In order to output a DC voltage at a maximum amplitude of 1000V, two power conversion circuits must be electrically coupled in series by appropriately setting the on and off modes of the switching mechanism. For example, if the load resistance is reduced while the output voltage reference is still set to 1000V, the operating point may change from point a1(1000V, 0A) to point B1(1000V, 30A). If the load resistance continues to decrease, the maximum output power of 30kW and the maximum current of 40A must be satisfied because there is no low-pass filter interposing an inductance element between the output side of the power conversion circuit and the load, and therefore the operation point should be changed from point B1(1000V, 30A) to point C1(750V, 40A). If the load resistance continues to decrease, a limit is imposed on moving the operating point in the direction of the current increase for the maximum current 40A of each power conversion circuit. Therefore, the electrical coupling relationship must be changed from the series connection to the parallel connection, and the operation point is changed from the point C1(750V, 40A) to the point D1(500V, 40A). From the operation point D1(500V, 40A), the operation point may be changed from the point D1(500V, 40A) to the point E1(500V, 60A) as the load resistance decreases. When the point E1(500V, 60A) is reached, and as the load resistance decreases, the operating point may continue to move along the curve in the direction of increasing current, with constant power output at a maximum output power of 30kW, at which time the operating point may move from point E1(500V, 60A) to point F1(375V, 80A). When reaching the point F1(375V, 80A), if the load resistance increases, the operation point may move from the point F1(375V, 80A) to the point G1(200V, 80A) and further move from the point G1(200V, 80A) to the point H1(200V, 0A) in the direction in which the current decreases. It will be appreciated by those skilled in the art that when their conditions are changed for each of the portions of the V-I curve, the direction of movement of the operating point may change, for example, the load resistance changes from decreasing to increasing, or the electrical coupling relationship changes from a parallel connection to a series connection. Further, by varying the output DC voltage/output current from the power conversion circuit, the operating point can reach any of the points encompassed by the I-V curve.

In contrast, fig. 3C shows a V-I curve for an exemplary operation of the solution according to the invention using the power conversion circuit from fig. 3A. According to an embodiment of the invention as shown in fig. 1, the two power conversion circuits 10, 11 providing the two DC output voltages are switched between a series connection and a parallel connection, wherein their supply power is fed to the load through a low frequency filter 15 having an inductive element 150. The key principle is that the inductive element resists the tendency of current changes by generating and destroying magnetic fields. The electrical coupling between the power conversion circuits 10, 11 can be switched between series and parallel connection by continuously applying a PWM control signal to the control terminal of the controllable unidirectional semiconductor switch 12. When the electrical coupling relationship changes from a parallel connection to a series connection, current flows through the inductive element in a clockwise direction, and the inductive element 150 stores some energy by generating a magnetic field. The polarity of the left side of the inductor is positive. When the electrical coupling relationship changes from a series connection to a parallel connection, a reverse voltage will be applied across the inductive element 150. The previously generated magnetic field will be destroyed to maintain the current flow to the load. So the polarity will be reversed (meaning that the left side of the inductor is now negative). Therefore, the inductive element 150 will be connected in series, thereby generating a higher voltage to charge the capacitive element 151 of the first low pass filter 15.

If the controllable unidirectional semiconductor switch 12 is cycled fast enough, for example by PWM, the inductive element 150 will not fully discharge between charging phases and the load will always see a voltage greater than that of the power conversion circuit 10, 11 when the switch 12 is open. Further, when the switch 12 is closed, the capacitive element 151 connected in parallel with the load is charged to a combined voltage of the output voltage of the power conversion circuits 10, 11 and the voltage induced across the inductive element 150. Then, when the switch is turned off, the capacitive element 151 can thus supply voltage and energy to the load. During this time, blocking diodes 13, 14 prevent capacitive element 151 from discharging through switch 12. Of course, the switch must be opened again at a sufficiently fast speed to prevent the capacitive element 151 from discharging too much.

In order to keep the output DC voltage at a maximum amplitude of 1000V, two power conversion circuits must be coupled in series by appropriately setting the on and off modes of the switching mechanism. For example, if the load resistance is reduced while the output voltage reference is still set to 1000V, the operating point may change from point a2(1000V, 0A) to point B2(1000V, 30A).

If the load resistance continues to decrease, the requirements of a maximum output power of 30kW and a maximum current of 40A must be met. Due to the presence of the low-pass filter 15, which low-pass filter 15 has the inductive element 150 and the capacitive element 151 interposed between the output side of the power conversion circuits 10, 11 and the load, the operating point can be smoothly changed from point B2(1000V, 30A) to point F2(375V, 80A) with frequent switching of the electrical coupling relationship between the power conversion circuits 10, 11 by continuously applying the control signal of PWM to the control terminal of the controllable unidirectional semiconductor switch 12. It will be appreciated by those skilled in the art that the switching frequency of the PWM is relatively high and its duty cycle varies from 0 to 1, which meets the technical requirements as described above. When F2(375V, 80A) is reached, if the load resistance increases, the operating point may shift from point F2(375V, 80A) to G2(200V, 80A) and further from G2(200V, 80A) to H2(200V, 0A) in the direction of current decrease. It will be appreciated by those skilled in the art that when their condition is changed for each of the portions of the V-I curve, the direction of movement of the operating point may change, for example, the load resistance changes from decreasing to increasing, or the electrical coupling relationship changes from a parallel connection to a series connection. Further, by varying the output DC voltage/output current from the power conversion circuit, the operating point can reach any of the points enclosed by the I-V curve.

By comparing the prior art solution and the solution according to the invention, it can be observed that the output current suddenly drops from 60A to 40A with an increase in the output voltage, wherein the output voltage reaches 500V, and according to fig. 3B the output voltage remains at 500V until the output current increases to 60A. In contrast, according to fig. 3C, since the first low-pass filter 15 operates according to the present invention to smooth the output voltage (the third DC voltage V3 as a result of PWM modulation of the first DC voltage V1 and the second DC voltage V2) and the output current, the portion of the output V-I curve between the output currents of 40A and 60A becomes smoother, and thus it can be observed that a relatively constant output power can be achieved. By using the power supply unit according to the invention it was observed that a continuous constant full power (CP) output of from 375V to 1000V, 30kW could be provided. In contrast, if conventional solutions are used, a constant full power output ranging from 500V to 750V is lost.

Fig. 4A and 4B show electrical waveforms relating to components of a power supply unit according to the invention providing a constant power of 30kW at two output DC voltage levels of 600V and 700V, respectively. As shown in fig. 4A and 4B, the output voltage from either of the two power conversion circuits of the power supply unit must be a DC voltage. Otherwise, if the power conversion circuit does not function as a DC voltage source, but for example a DC current source. Further, by changing the ratio of the PWM signal applied to the first controllable unidirectional semiconductor switch 12, the output voltage of the power supply unit can be controlled by a constant power output. For example, for constant output power, a PWM signal with a ratio of 0.5 obtains an output voltage at 600V, and a PWM signal with a ratio of 0.75 obtains an output voltage at 700V.

Furthermore, the device 1 has a third unidirectional semiconductor switch 17 electrically connected in anti-parallel with the first controllable unidirectional semiconductor switch 12. The third unidirectional semiconductor switch 17 may be a power diode or a body diode of a power MOSFET of the first controllable unidirectional semiconductor switch 12.

Fig. 5 is a schematic circuit block diagram showing a power supply system using a power supply unit according to an embodiment of the present invention. The power supply unit may be used as a submodule to form an even wider output voltage and constant power range power supply system. As shown in fig. 5, the power supply system 4 has a topology similar to the power supply unit according to fig. 1, except that the first power conversion circuit 10 and the second power conversion circuit 11 of the power supply unit 1 are replaced by two power supply units each using the topology according to fig. 1, respectively. The power supply system 4 includes a first power supply unit 40 and a second power supply unit 41. Either of the two units may use the unit according to fig. 1. The power supply system 4 further includes: a second controllable unidirectional semiconductor switch 42 operable to generate a fourth conduction path P4 from the positive output port terminal of first power supply unit 40 to the negative output port terminal of second power supply unit 41; a fourth unidirectional semiconductor switch 43 operable to generate a fifth conduction path P5 from the positive output port terminal of first power supply unit 40 to the positive output port terminal of second power supply unit 41; and a fifth unidirectional semiconductor switch 44 operable to generate a sixth conduction path P6 from the negative output port terminal of first power supply unit 40 to the negative output port terminal of second power supply unit 41; a second low-pass filter 45; and a controller 46 operable to issue an on-signal or an off-signal to the second controllable unidirectional semiconductor switch 42 such that the first and second power supply units 40, 41 provide current to the second low-pass filter 45 via the fourth conductive path P4 or the fifth and sixth conductive paths P5, P6. By using this power supply system, it is possible to output a voltage from V to 4V by changing the state of the switch with a constant output power.

Further, the second low-pass filter 45 includes a second inductive element 46 and an input port across the negative output port terminal of the first power supply unit 40 and the positive output port terminal of the second power supply unit 41.

Furthermore, the power supply system 4 further comprises a sixth unidirectional semiconductor switch 47, the sixth unidirectional semiconductor switch 47 being electrically connected in anti-parallel with the second controllable unidirectional semiconductor switch 42.

Furthermore, the controller is further operable to issue control signals to the first power supply unit and the second power supply unit such that the DC output voltage from the first power supply unit is substantially equal to the DC output voltage from the second power supply unit.

Although the present invention has been described based on some preferred embodiments, it should be understood by those skilled in the art that these embodiments should not limit the scope of the present invention in any way. Any alterations and modifications to the embodiments should be within the purview of those having ordinary skill in the art and understanding the principles of the invention and the scope of the invention as defined by the appended claims without departing from the spirit and intended scope of the invention.