Power converter system including active DC link and method of operating power converter

Abstract

A power converter system including an active DC link and a method of operating a power converter. The invention relates to a power converter system (1) comprising: an input converter (3) adapted to provide an output signal; an output converter (4) adapted to convert a DC input signal into an AC or DC output signal; and a DC bus connecting the output signal of the input converter (3) to the input signal of the output converter (4). The system (1) comprises: an active DC link (2) having two switching devices connected in series between first and second lines (L +, L-) of a DC bus; and the resonant circuit is connected in series between the interconnection end between the two switching devices and the second line (L-). The controller operates the two switching devices, in a first mode of operation, the active DC link (2) receives current from the input converter (3) to store energy in the resonant tank, and in a second mode of operation, the active DC link (2) supplies the energy stored in the resonant tank to the output converter (4).

Description

Technical Field

[0001] The present invention generally relates to power converters for feeding a load, such as charging a battery.

[0002] An object of the present invention is to provide a power converter that operates with fewer components than conventional power converters, in particular with a reduced number of electrolytic capacitors, in order to reduce its size, lower its manufacturing costs, and extend its life.

[0003] The present invention is particularly advantageous in application to on-board battery chargers for electric vehicles. Background Art

[0004] Electric and hybrid vehicles are equipped with high-voltage batteries that power the vehicle's propulsion motors. These types of vehicles include an onboard battery charger to control the battery charging process. This includes a power converter that converts alternating current (AC) from the mains into a suitable direct current (DC) to charge the battery.

[0005] Conventionally, such as Figure 1 As shown, these power converters are formed from an AC / DC input converter (3) that converts AC power from the mains into a DC output; a DC / DC output converter (4) that provides a DC output to charge the battery; and a passive DC link that interconnects the output of the AC / DC converter with the input of the DC / DC converter.

[0006] Conventional DC links can be considered passive because they consist of a large number of electrolytic capacitors acting as filters to absorb switching currents and reduce ripple voltage. However, electrolytic capacitors are known to be bulky and have a limited lifespan. Furthermore, their use reduces power density and increases the manufacturing cost of the power converter.

[0007] Therefore, providing a power converter that overcomes the above-mentioned deficiencies of the prior art remains a challenge in the art. Summary of the Invention

[0008] The present invention is defined in the accompanying claims and satisfactorily addresses the above-mentioned disadvantages of the prior art by providing a power converter comprising an active DC link instead of a conventional passive DC link having a large number of capacitors, thereby reducing the manufacturing cost and size of the power converter and extending its service life.

[0009] More specifically, one aspect of the present invention relates to a power converter comprising: an input converter adapted to provide a DC output (which is a DC signal typically including ripple, the ripple being an AC component at a DC level); an output converter adapted to convert the DC input to an AC or DC output; and a DC bus having a first line connecting the output of the input converter to the input of the output converter and a second line for conducting DC power from the input converter to the output converter. The first line may be a positive polarity line, and the second line may be a negative polarity line of the DC bus.

[0010] According to the present invention, the power converter further comprises an active DC link formed by: two semiconductor switching devices, i.e. a top switch and a bottom switch, connected in series between the first line and the second line; a resonant tank, preferably formed by a first capacitor and an inductor, the first capacitor and the inductor being connected in series between an interconnection line between the top switch and the bottom switch and the second line of the DC bus.

[0011] In addition, the power converter further includes a controller configured to turn on (allow current to flow) and turn off (prevent current from flowing) the two semiconductor switching devices. The controller is adapted to operate the two semiconductor switching devices in a first operating mode and a second operating mode. In the first operating mode, the active DC link receives energy from the input converter to store the energy in the resonant tank (e.g., store current in the inductor). In the second operating mode, when the output converter requires additional power, the active DC link supplies the energy stored in the resonant tank to the output converter (e.g., supplies current from the inductor) to meet the power demand of the output converter or a load connected to the output terminal of the output converter.

[0012] The active DC link is a power buffer between the input and output converters, formed by electronic switches and some passive components. They act as an active current filter that supplies the output converter with a current adapted in amplitude and / or frequency to meet its energy requirements for optimal performance.

[0013] The input converter provides a DC output, which typically includes ripple, which is the AC component of the DC signal with positive and negative half-cycles.

[0014] The controller is adapted so that in a first operating mode and during a positive half-cycle of the ripple, the active DC link receives and stores energy by setting the top switch to on and the bottom switch to off, so that the active DC link receives energy from the input converter, such as current for charging the inductor. In this case, the active DC link does not provide power to the output converter.

[0015] Furthermore, in the first operating mode and during the negative half-cycle of the ripple, the top switch is off and the bottom switch is on, so that the inductor is connected in parallel with the first capacitor and the inductor discharges to charge the first capacitor. In this case, the active DC link does not provide power to the output converter.

[0016] Additionally, the controller is adapted so that in a second operating mode and during a negative half cycle of the ripple, the top switch is off and the bottom switch is on, so that the inductor is connected in parallel with the first capacitor and the inductor is charged by the first capacitor. In this case, the active DC link does not provide power to the output converter.

[0017] Furthermore, in the second operating mode and during the positive half cycle, the top switch is on and the bottom switch is off, causing the inductor to discharge to supply current to the output converter. In this case, the active DC link powers the output converter.

[0018] In practice, the power converter is electrically connected to the system to be operated (load), such as a battery of an electric / hybrid vehicle, or an electric motor of an electric / hybrid vehicle.

[0019] When the output converter (or the operated system connected thereto) demands more power, the controller operates the active DC link so that the output converter meets the power demand of the operated system. However, because the input converter requires some time to increase its power output, in the second operating mode described above, the active DC link of the present invention compensates for this delay in the input converter's power supply by transferring the energy stored in the active DC link inductor as described above. The effect achieved by the active DC link is twofold: first, the power required by the load is supplied at least partially without delay, and second, the AC signal (ripple) is smoothed, resulting in a clean or nearly clean DC voltage being received by the output converter.

[0020] When the power required by the output converter (or the operated system (load) connected thereto) is less than the power supplied by the input converter, the controller will operate the active DC link to store energy as described above for the first operating mode.

[0021] For the controller to determine whether the operated system demands more or less power, a first control loop is executed in which the voltage at the DC bus, ie the voltage between the first and second lines, is measured and compared with a reference voltage.

[0022] The reference voltage can be a fixed value or a variable value; for example, it can be varied based on the requirements of the output converter. Comparing the measured voltage with the reference voltage enables a determination of whether more or less power is required. Thus, if the voltage measurement is higher than the reference voltage, this means the operating system is not demanding more power. Conversely, if the voltage measurement is lower than the reference voltage, this means the operating system is demanding more power, and based on this comparison, the controller will operate the top and bottom switches in either the first or second operating mode described previously.

[0023] The voltage comparison determines a voltage difference, which in turn can be used to determine a reference current intensity, for example, at the inductor, or at the output of the input converter, or at the input of the output converter. The controller is further adapted to execute a second control loop in which the measured current is compared with the reference current to operate the switch in either the first operating mode or the second operating mode.

[0024] Preferably, the active DC link comprises a second capacitor connected between the first line and the second line and downstream of the top switch and the bottom switch. The second capacitor stores energy and further filters the input voltage of the output converter.

[0025] Preferably, the controller is adapted to operate the respective semiconductor switching devices by means of a pulse width modulated PWM signal.More preferably, the two semiconductor switching devices are implemented as MOSFETs.

[0026] In a preferred embodiment of the invention, the input converter is a full-bridge converter having an input port adapted to be connected to an AC power source, and the output port of the output converter is adapted to be connected to a battery of an electric vehicle.

[0027] The output converter can be implemented as a galvanically isolated DC / DC converter. In this case, due to the inclusion of an active DC link, the input waveform of the DC / DC converter can be continuous, which in turn allows the DC / DC converter to operate normally, namely:

[0028] -Achieve zero voltage switching (ZVS);

[0029] - Improve efficiency;

[0030] - Reduce component self-heating; and

[0031] - Avoid possible resonance between the AC component and the inductive / capacitive elements of the converter.

[0032] The power converter of the present invention includes an active DC link that can be used as a power buffer between any two power electronic converters. For example, it can serve as a buffer between a 450V-48V DC / DC converter and a 48V-12V DC / DC converter. When two power electronic converters are connected in series, the power consumed or supplied and the switching frequency may be out of sync. In this case, when the input converter cannot provide the required power, the active DC link of the present invention can provide supplemental power for the output converter.

[0033] Another aspect of the invention relates to an on-board charger (OBC) for a vehicle, comprising a power converter as previously defined. In this application of the invention, an active DC link is placed after a power factor corrector (PFC) operating with a DC voltage input, enabling the output converter to operate in an optimal manner.

[0034] Another aspect of the present invention relates to a method of operating a power converter to supply power to a load. The method comprises the following steps:

[0035] generating a DC output signal with the aid of an input converter, the DC output signal comprising an AC component having a positive half-cycle and a negative half-cycle,

[0036] conducting the DC output signal to an output converter by means of a DC bus having a first line and a second line, wherein a load is connected to an output port of the output converter,

[0037] The voltage at the DC bus is measured and compared with a reference voltage to determine the power demand of the load, such that if the voltage measurement result is higher than the reference voltage, energy from the input converter is stored in a resonant tank (formed by the first capacitor and the inductor) connected to the DC bus during a positive half-cycle of the AC component, and if the voltage measurement result is lower than the reference voltage, the energy previously stored in the resonant tank is supplied to the output converter during the positive half-cycle of the AC component to enable the output converter to meet the power demand of the load. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to complete this specification and provide a better understanding of the present invention, a set of drawings is provided. These drawings constitute an integral part of the specification and illustrate embodiments of the present invention. However, they should not be interpreted as limiting the scope of the present invention, but rather as examples of how the present invention may be implemented. These drawings include the following:

[0039] Figure 1 A power converter including a passive DC link according to the prior art is shown.

[0040] Figure 2A power converter according to the invention comprising an active DC link instead of a passive DC link is shown.

[0041] Figure 3 An electrical diagram of an exemplary implementation of a power converter including the active DC link of the present invention is shown.

[0042] Figure 4A and Figure 4B The power converter of the present invention is shown to supply power to the output converter in the second operation mode ( Figure 4A ), and storing power in the DC link in the first operating mode ( Figure 4B ). The arrows indicate the flow of current.

[0043] Figure 5 A schematic diagram showing the dual control loops executed by the controller is shown.

[0044] Figure 6 Several graphs of current and PWM control signals are shown. The two areas marked as "Zoom 1" and "Zoom 2" in the above figure are respectively Figure 8 and Figure 9 Medium zoom.

[0045] Figure 7 A diagram illustrating the dead time between PWM control signals is shown.

[0046] Figure 8 Shown Figure 6 A magnified view of the Zoom 1 area in Figure 1. The figure includes the power converter circuit, with the states of the two MOSFETs marked by dashed lines within the figure area.

[0047] Figure 9 Shown Figure 6 A magnified view of the Zoom 2 area in Figure 2. The figure includes the power converter circuit, with the states of the two MOSFETs marked by dashed lines within the figure area. DETAILED DESCRIPTION

[0048] Figure 2 A power converter 1 comprising an active DC link 2 according to the invention is shown, wherein the power converter 1 comprises an input converter 3, which is an AC / DC or DC / DC converter adapted to provide a DC output, and an output converter 4, which is a DC / AC or DC / DC converter adapted to convert a DC input into an AC or DC output.

[0049] Power converter 1 has a DC bus with a positive line L+ and a negative line L− connecting the output of input converter 3 to the input of output converter 4 for conducting DC current from input converter 3 to output converter 4 .

[0050] The active DC link 2 is connected to the DC busbar lines L+, L− and is formed by two semiconductor switches, in particular a top switch Qtop and a bottom switch Qbot, for example, which are implemented as MOSFETs and are connected in series between the positive line L+ and the negative line L−. In addition, the active DC link 2 has a first capacitor Cac and an inductor L connected in series between the interconnection between the two switches Qtop, Qbot and the negative line L−, as shown in FIG. Figure 2 The second capacitor Cdc is connected between the positive line L+ and the negative line L− and downstream of the two switches Qtop and Qbot.

[0051] Furthermore, the power converter 1 has a controller (not shown), for example implemented as a microcontroller, for switching the two switches Qtop, Qbot on and off, preferably by means of pulse width modulated (PWM) signals, such as Figure 6 and Figure 7 shown.

[0052] like Figure 7 As shown, the PWM signals for the individual MOSFETs are coordinated with a dead time that ensures both MOSFETs are never on simultaneously. That is, when one MOSFET is on, the other is off. The dead time prevents cross-conduction, which would short-circuit the high-voltage DC bus if both Qtop and Qbot were on simultaneously. Therefore, during a switch position change, the on-position switch is first turned off, followed by the off-position switch, to prevent a short circuit. Therefore, during the dead time, both switches are in the off position.

[0053] exist Figure 3 In the practical implementation shown, input converter 3 is a full-bridge AC / DC converter having an input port connected to an AC source 5. Output converter 4 is a galvanically isolated DC / DC converter having a transformer 6 and a DC / AC full-bridge 7 connected to the primary winding of transformer 6, and an AC / DC full-bridge 8 connected to the secondary winding of transformer 6. The output of AC / DC full-bridge 8 is connected to a battery Batt, such as the battery of an electric vehicle, for charging when necessary. An active DC link 2 is connected between input converter 3 and DC / AC full-bridge 7.

[0054] The controller is adapted to Figure 4B The first operating mode shown operates the two switches Qtop, Qbot, wherein the active DC link 2 receives current from the input converter 3 via the DC bus to store current in the inductor L. When the output converter 4 does not require additional power, the inductor L is charged.

[0055] Additionally, the controller is adapted to Figure 4A The second operating mode shown operates the two switches Qtop, Qbot, wherein the DC link 2 supplies the current previously stored in the inductor L to the output converter 4. Thus, in the second operating mode, the output converter 4 receives power from the active DC link 2 in addition to the power received from the input converter 3 via the DC bus, thereby meeting the power requirement for optimal operation of the output converter 4.

[0056] When the active DC link 2 operates in the second operating mode, Figure 4A and Figure 4B as well as Figure 6 、 Figure 8 and Figure 9 The current I shown in 转换器 is the sum of the current supplied by the input converter 3 and the current supplied by the active DC link 2 .

[0057] from Figure 6 As can be seen from the figure, the active DC link 2 injects current in the form of a half-sine waveform, so that when the current I 转换器 In the positive half cycle, the DC link 2 injects current into the output converter 4, but this is not always the case, because there are moments when the transmission is zero, such as Figure 8 During the negative half-cycle, the converter absorbs current from the input converter, but this is not always the case, as there are moments when the current transfer is zero, as shown in Figure 8 As shown. Due to the series connection with the first capacitor Cac, the current I through the inductor L 电感 Specific current I 转换器 Closer to a sinusoidal shape.

[0058] Figure 8 Shown Figure 6 The enlarged view of the Zoom 1 region of FIG. 1 corresponds to the second operating mode, in which the DC link 2 injects current into the output converter 4 via the DC bus lines L+, L- as a pulsating signal. In the negative half cycle corresponding to the ripple ( Figure 8 During the time period of the second operation mode (the left circuit in FIG), the controller sets the top switch Qtop to OFF and the bottom switch Qbot to ON by means of PWM control, so that the first capacitor Cac and the inductor L are connected in parallel, and the inductor L is charged by the first capacitor Cac. During this time period of the second operation mode, the current I 转换器 is zero, and the current I through the inductor L 电感 Increases as the inductor L is charged.

[0059] During the second operating mode and during the positive half cycle of the ripple ( Figure 8The controller sets the top switch Qtop to ON and the bottom switch Qbot to OFF by means of PWM control, so that the current stored in the inductor L can be supplied to the output converter 4 through the DC bus lines L+ and L-. During this period of the second operating mode, as the inductor L releases current to the output converter 4, the current I 电感 Continuously decreasing, and I 转换器 Above zero.

[0060] Figure 9 Shown Figure 6 2 corresponds to the first operating mode, in which the DC link 2 receives current from the input converter 3 via the DC bus lines L+, L-. During the time period corresponding to the positive half cycle of the ripple ( Figure 9 The controller sets the top switch Qtop to ON and the bottom switch Qbot to OFF, so that current can flow through the inductor L and the first capacitor Cac to charge the inductor L. During this period of the first operation mode, the DC link 2 absorbs power from the DC bus, and the current I 电感 It keeps increasing, but it is negative, and I 转换器 It is also decreasing.

[0061] During the first operating mode and during the negative half cycle of the ripple ( Figure 9 The controller sets the top switch Qtop to OFF and the bottom switch Qbot to ON by means of PWM control, so that the first capacitor Cac and the inductor L are connected in parallel and the first capacitor Cac can be charged by the inductor L. During this period of the first operation mode, I 转换器 is zero, and as the inductor L is discharged to charge the first capacitor Cac, the current I through the inductor L 电感 Decrease to zero.

[0062] from Figure 8 and Figure 9 It can be seen that during the positive half cycle of the ripple, in the first operating mode and the second operating mode, the inductor L and the first capacitor Cac are connected in series between the positive line and the negative line to charge the inductor L in the first operating mode and discharge the inductor L toward the output converter 4 during the second operating mode; while during the negative half cycle of the ripple, in the first operating mode and the second operating mode, the inductor L and the first capacitor Cac are connected in parallel therebetween and are disconnected from the positive line and the negative line.

[0063] It should be noted that, to simplify the illustration, Figure 8 and Figure 9The dead time in the PWM control signal for the switches is not shown, but a practical implementation would include dead time as previously described.

[0064] Figure 5 A dual-loop control diagram is shown, which can be used to control an active DC link. The controller uses the DC link's DC voltage as a control variable, for example, the voltage at the second capacitor Cdc. To obtain this voltage measurement, a voltmeter is connected in parallel with the second capacitor Cdc, and the controller processes the voltage measurement. The controller aims to maintain a certain DC voltage level at the DC capacitor. To achieve this, the active DC link injects or sinks current to compensate for the AC component caused by the current demand of the output converter 4. In other words, voltage control is achieved indirectly through current compensation.

[0065] More specifically, if Figure 5 As shown, the controller is adapted to execute a first control loop in which the voltage measured in the second capacitor Cdc is compared to a reference voltage, such that the operation of the top and bottom semiconductor switching devices is controlled to maintain the energy stored in the second capacitor Cdc at a desired level. The voltage comparison between the measured voltage and the reference voltage allows a determination to be made as to whether more or less power is required. Therefore, if the voltage measurement result is higher than the reference voltage, this means that the system being operated does not require more power. Conversely, if the voltage measurement result is lower than the reference voltage, this means that the system being operated requires more power, and based on this comparison, the controller will operate the top and bottom switches in either the first or second operating mode described above.

[0066] The voltage comparison determines a voltage difference, which in turn can be used to determine a reference current intensity, for example, at the inductor, or at the output of the input converter, or at the input of the output converter. The controller is further adapted to execute a second control loop in which the measured current is compared with the reference current to alternately operate the switch in the first operating mode or the second operating mode, depending on the demand of the operating system (load).

Claims

1. A power converter system (1), comprising: an input converter (3) adapted to provide an output signal comprising a DC output signal, an output converter (4) adapted to convert a DC input signal into an AC or DC output signal, a DC bus having a first line (L+) and a second line (L-) connecting the output signal of the input converter (3) to the input signal of the output converter (4), Characterized in that the power converter system (1) further comprises: an active DC link (2), said active DC link comprising two semiconductor switching devices (Qtop, Qbot) connected in series between said first line (L+) and said second line (L-), a resonant circuit comprising a first capacitor (Cac) and an inductor (L), the first capacitor and the inductor being connected in series between an interconnection end between the two semiconductor switching devices (Qtop, Qbot) and the second line (L-); and A controller for switching the states of the two semiconductor switching devices (Qtop, Qbot), wherein the controller is adapted to operate the two semiconductor switching devices (Qtop, Qbot) in a first operating mode and a second operating mode, wherein in the first operating mode, the active DC link (2) receives energy from the input converter (3) to store the energy in the resonant tank, and in the second operating mode, the active DC link (2) supplies the energy stored in the resonant tank to the output converter (4).

2. The power converter system (1) according to claim 1, wherein: The controller is adapted to alternately switch the two semiconductor switching devices (Qtop, Qbot) on and off, and determine a power demand of the output converter (4) or a power demand of a load connected to an output terminal of the output converter (4), and based on the power demand, operate the active DC link (2) in the first operating mode or the second operating mode to store energy in the resonant tank or supply energy to the output converter (4).

3. The power converter system (1) according to claim 2, wherein: The controller is adapted to compare a measured voltage at the output of the active DC link (2) with a predefined reference voltage in order to determine a voltage difference, and wherein the controller is further adapted to determine a power requirement of the output converter (4) based on the voltage difference.

4. The power converter system (1) according to claim 3, wherein: The controller is further adapted to calculate a reference current intensity at the inductor (L) based on the voltage difference and to compare the current measured at the inductor (L) with the reference current intensity at the inductor (L), and wherein the controller is further adapted to generate a control signal for switching the two semiconductor switching devices (Qtop, Qbot) on and off.

5. The power converter system (1) according to claim 4, wherein: The control signal is a pulse width modulation (PWM) signal.

6. A power converter system (1) according to any one of the preceding claims, wherein: The controller is adapted such that in a first operating mode and during a positive half-cycle of a ripple of a DC output signal generated by the input converter (3), the top switch is set to ON and the bottom switch is set to OFF such that the active DC link (2) receives current from the input converter to charge the inductor (L).

7. The power converter system (1) according to claim 5, wherein: The controller is adapted so that in a first operating mode and during a negative half cycle of the ripple of the DC output signal generated by the input converter (3), the top switch is set to OFF and the bottom switch is set to ON, so that the inductor (L) and the first capacitor (Cac) are connected in parallel to enable the inductor (L) to charge the first capacitor (Cac).

8. A power converter system (1) according to any one of the preceding claims, wherein: The controller is adapted so that in the second operating mode and during the negative half cycle of the ripple of the DC output signal generated by the input converter (3), the top switch is set to OFF and the bottom switch is set to ON so that the inductor (L) and the first capacitor (Cac) are connected in parallel and the inductor (L) can be charged by the first capacitor (Cac).

9. The power converter system (1) according to claim 8, wherein: The controller is adapted such that in the second operating mode and during a positive half cycle of the ripple of the DC output signal generated by the input converter (3), the top switch is set to ON and the bottom switch is set to OFF, so that the inductor (L) can discharge to supply current to the output converter (4).

10. The power converter system (1) according to any one of the preceding claims, wherein The active DC link (2) comprises a second capacitor (Cdc) connected between the first line (L+) and the second line (L-) and downstream of the top semiconductor switching device (Qtop) and the bottom semiconductor switching device (Qbot), wherein the first line (L+) is a positive line and the second line (L-) is a negative line.

11. The power converter system (1) according to any one of claims 5 to 10, wherein: The two semiconductor switching devices (Qtop, Qbot) are MOSFETs, and the PWM signals of the respective MOSFETs are coordinated to have dead time, so that the two MOSFETs are never turned on at the same time.

12. A power converter system (1) according to any one of the preceding claims, wherein The input converter (3) is adapted to convert an AC or DC input to a DC output.

13. A power converter system (1) according to any one of the preceding claims, wherein: The output port of the output converter (4) is adapted to be connected to a system being operated or to a load, which preferably comprises a battery of an electric vehicle.

14. An on-board battery charger (OBC) for a vehicle, comprising a power converter system (1) according to any one of the preceding claims.

15. A method of operating a power converter system (1) to supply power to a load, the method comprising the steps of: generating a DC output signal by means of an input converter (3), said DC output signal comprising an AC component having a positive half-cycle and a negative half-cycle, The DC output signal is conducted to an output converter (4) by means of a DC bus having a first line (L+) and a second line (L-), wherein the load is connected to an output port of the output converter (4), The voltage at the DC bus is measured and compared with a reference voltage to determine the power demand of the load, so that if the voltage measurement result is higher than the reference voltage, energy from the input converter is stored in a resonant tank connected to the DC bus during a positive half-cycle of the AC component, and if the voltage measurement result is lower than the reference voltage, energy previously stored in the resonant tank is supplied to the output converter (4) during a positive half-cycle of the AC component to enable the output converter to meet the power demand of the load.

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