Power converter and operating method for a power converter

Abstract

In a galvanically connected power converter or charging device having an active rectifier and H-bridge DC / DC converter the temporal proportions for the two off-states of the DC / DC converter are determined by the DC / DC converter generating common-mode voltages that correspond to the common-mode voltages of the active rectifier with a frequency of three times the fundamental frequency and compensate for changes.

Description

[0001] 202303222 1 Description Power converter and operating method for a power converter The invention relates to a galvanically non-isolated power converter with an active rectifier, a DC link, and a DC / DC converter. The invention further relates to an operating method for such a power converter. Power converters of the type mentioned are, for example, candidates for charging devices for electrically powered vehicles. They are typically two-stage power converters: in the active rectifier (“Active Front End”, AFE, 1st stage), the mains voltage is converted into DC voltage and stored in an intermediate circuit capacitor; in the DC-DC converter (2nd stage), the intermediate circuit voltage is converted into the voltage required by the load (vehicle battery) (charging voltage). Galvanically non-isolated chargers do not require an isolating transformer between the mains and the vehicle’s high-voltage system.This design is advantageous in terms of the volume, weight, and cost of the charger. However, due to the high number of Y-capacitances installed in the vehicle, as well as parasitic capacitances, the coupling path to PE or earth is also significantly increased in these systems. The reason for this is the elimination of the series connection of the coupling capacitance of the isolation transformer with the aforementioned capacitances of the vehicle. During normal operation, this significantly increases the capacitive leakage currents to earth / PE, which can trigger the mains-side residual current device (RCD) and thus prevent the device from operating. During faulty operation, i.e., if the PE conductor breaks, detrimental touch voltages can build up on the body and / or the housing of the charger. For safety reasons, strict limits exist for the touch currents caused by this.The driving source of the touch currents is the common-mode voltage generated by the active rectifier. It depends on the specific design of the pulse-width control (PWM) and generally has two main components in the spectrum, namely at three times the fundamental frequency, i.e., at 150 Hz if the fundamental frequency is 50 Hz, and at the switching frequency. Common-mode voltages can be counteracted, for example, by active and passive filters. However, these require the use of additional active and / or passive components, which has a negative impact on size and weight. The object of the invention is to provide a power converter of the type mentioned above that reduces or avoids the disadvantages mentioned above. A further object is to provide an operating method for a power converter that reduces or avoids the disadvantages mentioned above.This object is achieved by an operating method having the features specified in claim 1. A further solution consists in the power converter having the features of claim 8. In the operating method according to the invention for a power converter comprising an active rectifier, a DC voltage intermediate circuit, and a DC / DC converter, the active rectifier is controlled such that it switches between a plurality of rectifier switching states. Furthermore, the DC / DC converter is controlled to generate a DC output voltage from the voltage of the intermediate circuit such that it switches between a first switching state and a second switching state at a switching frequency. A first of the two switching states has a plurality of switching options for representing the switching state, in which different common-mode voltages are generated.In a period in which the first switching state exists, a first switching option is used for a first portion of the period and a second switching option is used for the remaining, second portion of the period, wherein the portions are determined such that the common-mode voltage generated by the DC / DC converter is equal in magnitude to a common-mode voltage generated by the rectifier. The power converter according to the invention comprises an active rectifier, a DC voltage intermediate circuit, a DC / DC converter, and a control device for controlling the rectifier and the DC / DC converter. The control device is designed to control the active rectifier such that it switches between a plurality of rectifier switching states.It is further designed to control the DC / DC converter such that it switches between a first switching state and a second switching state in order to generate a DC output voltage from the voltage of the intermediate circuit at a switching frequency, wherein a first of the two switching states comprises a plurality of switching options for representing the switching state, in which different common-mode voltages are generated. The DC / DC converter is further controlled such that, in periods in which the first switching state exists, a first switching option is used for a first portion of the period and a second switching option is used for the remaining, second portion of the period. The control device determines the portions such that the common-mode voltage generated by the DC / DC converter is of the same magnitude as a common-mode voltage generated by the rectifier.The invention advantageously utilizes the fact that there is a degree of freedom in the control of the DC / DC converter. 202303222 4 The DC / DC converter can, for example, be an H-bridge DC / DC converter with two half-bridges connected in parallel. Under the condition that exactly one of the two semiconductor switching elements of each half-bridge is always switched on, four different switching options result. In the two switching options that belong to an on position, the diagonally arranged semiconductor switching elements of the two half-bridges are switched on. In this switching option, the intermediate circuit voltage is present at the output terminals, i.e. between the centers of the two half-bridges. However, there is no common-mode voltage in these two switching options.In the first switching state, which represents an off position, in the first switching option the two upper semiconductor switching elements of the two half-bridges are switched on, and in the second switching option the two lower semiconductor switching elements of the two half-bridges are switched on. This offers the advantageous use of a degree of freedom, which consists in two different switching options for the first switching state, which differ in the common-mode voltage generated. The active rectifier can be a two-point rectifier which, in a manner known per se, comprises three half-bridges connected in parallel, each with two semiconductor switching elements such as IGBTs or MOSFETs. The semiconductor switching elements can comprise intrinsic and parallel freewheeling diodes implemented as discrete components. However, the active rectifier can also have a different design.The DC intermediate circuit comprises one or more capacitors and is expediently connected in parallel to the external terminals of the half-bridges of the active rectifier. 202303222 5 The control device expediently comprises a programmable control module, for example a microcontroller. It is connected to a higher-level controller, from which it receives signals that determine the operating state for the power converter. The control device calculates times for switching operations for the semiconductor switching elements of the rectifier and the DC / DC converter and sends corresponding signals to gate driver units assigned to the semiconductor switching elements. Advantageous embodiments of the operating method and power converter according to the invention emerge from the dependent claims.The embodiment of the independent claims can be combined with the features of one of the subclaims or, preferably, with those of several subclaims. Accordingly, the following additional features can be provided: The portions of the time period during which the first and second switching options are used are preferably determined from the switching period of the DC / DC converter, the intermediate circuit voltage, the DC output voltage, and an instantaneous value for the common-mode voltage generated by the active rectifier. Advantageously, the switching period and the intermediate circuit voltage are fixed or known to the control device. Likewise, the DC output voltage—i.e., the charging voltage for a battery of an electrically powered vehicle—is known to the control device, since it must adjust this charging voltage. The instantaneous value for the common-mode voltage generated by the active rectifier can be determined from available control data.The control device can thus calculate the components from known and available data and perform this calculation again for each switching period, so that the components in each switching period are adapted to the instantaneous value of the common-mode voltage. 202303222 6 A value averaged over a switching period of the rectifier can be determined as the instantaneous value for the common-mode voltage generated by the rectifier. This advantageously determines a single value for a switching period, which can also be used to control the DC / DC converter in a switching period. This avoids having to set values ​​for the common-mode voltage that vary within a switching period. The instantaneous value can be determined using the switching period duration, the intermediate circuit voltage, and components of the switching period duration that are assigned to switching states of the active rectifier.The instantaneous level of the common-mode voltage can be advantageously determined and fixed using these values. The intermediate circuit voltage and the switching period of the active rectifier are known in the control unit. The components of the switching period of the rectifier are determined in the controller, as they are required to control the rectifier. It is expedient to separate the first component of the period and the second component of the period in time by placing a second period between them in which the second switching state is used. This avoids direct switching, and possibly even multiple switching between the switching options of the first state. The active rectifier and the DC / DC converter can be operated at the same switching frequency. The switching frequency used is significantly higher than the fundamental frequency of the AC input voltage.For example, the switching frequency can be 16 kHz, 25 kHz, or 48 kHz or more. The power converter is particularly preferably a charger for an electrically powered vehicle. 202303222 7 The invention is described and explained in more detail below with reference to the exemplary embodiments illustrated in the figures.Shown are: Figure 1 an electrical circuit diagram of a galvanically non-isolated charger with an active rectifier and a DC / DC converter, Figure 2 a time profile of the switching states of the active rectifier within a switching period, Figure 3 a time profile of a resulting touch current in the event of a fault without compensation, Figure 4 a time profile of the switching states of the DC / DC converter without setting a common mode voltage, Figure 5 a time profile of the switching states of the DC / DC converter with setting a common mode voltage, Figure 6 a time profile of a resulting touch current in the event of a fault with compensation, Figure 1 an electrical circuit diagram of a galvanically non-isolated charger 10 with an active rectifier 12 and a DC / DC converter 14 is shown. The charger 10 is connected on the input side to a three-phase supply voltage 16, for example the 400 V local network.The connection can be made, for example, via a so-called wall box, i.e. a high-voltage connection specifically designed for charging an electrically powered vehicle. The active rectifier 12 comprises, for each phase on the mains side, an inductor 18a…c and three half-bridges, each with two semiconductor switching elements 20a…f, for example IGBTs or MOSFETs. The half-bridges are connected in parallel with their outer terminals in a known manner and connected with their center terminals to the inductors 18a…c. The active rectifier 12 thus formed is a two-point rectifier. The outer terminals of the half-bridges are connected to a DC intermediate circuit 22. In this example, the DC intermediate circuit 22 comprises two series capacitors 23a, 23b, but in other examples it can also have one or more capacitors.Between the capacitors 23a, 23b is a node 24, which is subsequently used as a reference point for common-mode voltages. The DC intermediate circuit 22 is also connected to the external terminals of two further half-bridges, each with two semiconductor switching elements 15a...d, which form the DC / DC converter 14. The DC / DC converter 14 is thus an H-bridge DC / DC converter. The center terminals of the further half-bridges are connected to an output filter 26. The output filter 26 comprises two inductors 27a, b and a capacitor 27c, which are connected between the two center terminals as an LCL series. The load, in this case the vehicle's accumulator, commonly referred to as the battery, is connected in parallel to the capacitor 27c. The charger 10 also comprises a control device, which is not shown in Figure 1.The control device is connected, on the one hand, to a higher-level controller, which forwards commands regarding the overall status of the charger 10 to the control device. The control device itself calculates switching times for the semiconductor switching elements 15a...d, 20a...f used in the charger 10. At the determined switching times, signals are sent to gate driver units (also not shown in Figure 1), which then, in turn, cause the semiconductor switching elements 15a...d, 20a...f to switch. The control device performs the calculation steps described below. These are the calculation steps for the pulse width modulation for the active rectifier 12. Their results are incorporated into the further calculation steps for the switching processes of the DC / DC converter 14. Parasitic couplings to ground 28 are symbolized in Figure 1 by the capacitive elements 29a, 29b.The vehicle-side capacitive couplings via Y capacitors to PE are symbolized by capacitive elements 30a and 30b. The body and the housing of the charger are assumed to be perfectly grounded, i.e., the PE conductor and ground 28 have the same potential. A break in the PE conductor corresponds to the opening of symbolic switch 31. Such a break allows the generation of capacitively coupled touch voltages and thus touch currents. The human body, or its equivalent electrical circuit, is arranged parallel to symbolic switch 31 to represent touch voltages and touch currents. One component of such contact voltages arises as a beat of common-mode voltages at three times the fundamental frequency, i.e. in this example at 150 Hz, starting from a fundamental frequency (mains frequency) of 50 Hz and is caused by the switching operations of the active rectifier 12.The time-dependent common-mode voltage and CM0 is defined for the active rectifier 12 as: u CM0 = (u R0 +u S0 +u T0 ) / 3 This includes R0 , and S0 , and u T0 Components of the common-mode voltage, which are also shown in Figure 1. These components are each either +U DC / 2 or –U DC / 2, depending on whether the upper or lower switching element of the respective half-bridge is switched on. In total, the common-mode voltage is therefore u CM0 = + / -U DC / 2, if all upper or all lower switching elements are switched on or + / -U DC / 6, when a mixture of upper and lower switching elements is switched on in the active rectifier 12. 202303222 10 If the switching state with the upper switch switched on is designated as 1 and the switching state with the lower switch switched on as 0 and the switching states for the three half-bridges are written one after the other, the possible overall states, referred to as vectors, are as follows: Vector Switching states Common mode RST Voltage / U DCV0 000 -1 / 2 V1 100 -1 / 6 V2 110 +1 / 6 V3 010 -1 / 6 V4 011 +1 / 6 V5 001 -1 / 6 V6 101 +1 / 6 V7 111 +1 / 2 The currently used switching state changes several times within a switching period, as described below. The exact course of the switching operations depends on the selected pulse width modulation for the active rectifier 12, and the course of the beat of the common-mode voltage with three times the frequency of the mains voltage is also determined by the implementation of the PWM. For the specific example of a symmetrical space vector PWM, a course within a switching period results, which is shown in simplified form in Figure 2. Figure 2 shows the individual components of the common-mode voltages u R0 , and S0 , and u T0 and the total common-mode voltage u CM0 over the time of a switching period. The switching period of length T = 1 / f with f = 48 kHz is divided into switching times, which are denoted by τ a , τ band τ0. In reality, these are not of equal length and depend on the current operating point of the rectifier 12. They result from the pulse width modulation used. 202303222 11 The switching period is generally described as consisting of two zero vectors V0 and V0' as well as two active vectors Va and Vb. In the case of the 1st sector, these are specifically the vectors V7 and V0 as well as V1 and V2. The switch-on times τa, τb and τ0 of the vectors are recalculated for each period by the PWM unit of the rectifier 12. The instantaneous value of the common-mode voltage e for the current switching period is calculated from the pulse pattern of the rectifier 12. This is done by comparing the phase output voltages u R0 , and S0 and u T0 of the rectifier 12 for each switching period as a voltage-time product u R0,Vt , and S0,Vt and u T0,Vt According to the definition for u CM0the mean is then calculated: After insertion and dissolution we get: Where S stands for the sector in the pulse width modulation and T AFEfor the period of one switching period of the active rectifier 12. These equations, as well as the other considerations, apply to symmetrical space vector PWM. For other PWM methods, the equations must be adapted. The touch current, i.e., for the simulation, the current through a body simulation with low-pass weighting, in the event of a fault (PE conductor break), i.e., when the symbolic switch 31 is open, manifests itself in symmetrical space vector PWM as a roughly triangular wave, as shown in Figure 3. It can be seen that the amplitude is slightly more than 40 mA. The period is 1 / 150 Hz, i.e., approximately 6 ms. 202303222 12 A similar scheme can be set up for the switching operations of the H-bridge DC / DC converter 14, although it only has four different states, since only two half-bridges are present. The common-mode voltage u CM,DC is calculated for the DC / DC converter 14 as follows: u CM,DC = (u A0 +uB0 ) / 2 The following switching states are possible with the DC / DC converter: State Switching states Common mode AB Voltage / U DC DC010 0 DC100 -1 / 2 DC211 +1 / 2 DC301 0 The DC / DC converter 14 only forms a common-mode voltage amplitude in the two off states DC1 and DC2, i.e., when both half-bridges are switched up or down simultaneously. The use of these off states represents a degree of freedom. If the off time of a switching period is evenly divided between the switching states DC1 and DC2, the DC / DC converter does not, on average, form a common-mode voltage for this switching period. An uneven distribution, on the other hand, produces a corresponding common-mode voltage value that exists on average over the switching period. This can be used to counteract and compensate for the low-frequency common-mode beat of the active rectifier 12. The DC / DC converter 14 sets the intermediate circuit voltage U DCdown to the voltage required by the battery. This occurs by switching between the on-state DC0 and an off-state at a switching frequency, which in this example corresponds to that of rectifier 12. In the on-state DC0, the intermediate circuit voltage U DC at the output filter 26. In the off state, a voltage of 0 V is applied to the output filter 26. The required voltage is set by dividing the switching period T DC between a duration for the on state T ON and a duration for the off state T OFF (It is T DC = T ON + T OFF ). The share of ON by T DC , in which the on state is used, is calculated as d ON = U BAT / U DC , the share for the off state to d OFF = 1 – d ON . The durations are T ON = d ON * T DC and T OFF = d OFF * T DC. In the on state, no common-mode voltage can be generated. In the off state, however, the common-mode voltage is set by selecting the off state used. Figure 4 shows a circuit diagram for the DC / DC converter 14, in which within one switching period T DC each of the off states DC1 and DC2 is used equally (T OFF1 = T OFF2 = T OFF / 2). In this case, no common-mode voltage results on average. This control is therefore the starting point and is suitable for the case where the amplitude of the common-mode beat is e = 0. In addition, for Figure 4, the assumption has been made that U BAT = 0.5 U DC , ie it applies d ON = d OFF = 0.5. In other words, the switching period T DC evenly distributed over ON and OFF times, ie T OFF = 0.5 T DC and T OFF1 = T OFF2 = 0.25 T DC . Would U BAT = 0.4 U DC , then d would beON = 0.4 and d OFF = 0.6. In order to avoid generating a common-mode voltage on average, T OFF1 = T OFF2 = T OFF / 2 = d OFF * T DC / 2 = 0.3 T DC If e is not 0, then a suitable common-mode voltage is set so that the time periods T OFF1 and T OFF2 for the off states, whereby the summed time for the off state (d OFF * T DC = T OFF = T OFF1 + T OFF2 ) must be maintained, as it is determined by the battery voltage. The proportion of CM of the period that should be used to generate the required level of common-mode voltage is d CM = e / (0.5 * U DC ). The frequency of the beat is 150 Hz, which is very low compared to the switching frequency of, for example, 1 / T DC= 48 kHz. Therefore, the common-mode beat has a practically fixed instantaneous voltage value e for each switching period, which can be negative or positive. Depending on the sign of the common-mode voltage, the off-state DC1 or DC2 must be used for the component d CM of the period duration. The remaining part of the off state d OFF,RES = d OFF – d CM is divided equally between the two off states DC1 and DC2. This results in the following times for the two off states for a positive common-mode voltage to be balanced: ^^ைிி^ ൌ 0.5 ^^ ൌ ൫ ^^ ^ 0,5 ∙ ^^ ൯1 ^^^^ െ ^^^^் ^ 2 ^^ைிிଶ ^ெ ைிி,ோாௌ ∙ ^^^^ ൌ2 ∙ ^^^^ ∙ ^^^^ For a negative common-mode voltage to be compensated, the two off states must be exchanged, ie the following times for the two off states result: ^^ைிிଶ ൌ 0.5 ^ ^ ிி^ ൌ ൫ 1 ^^^^ െ ^^^^் ^ 2 ^^ை^^^ெ ^ 0,5 ∙ ^^ைிி,ோாௌ൯ ∙ ^^^^ ൌ2 ∙ ^^^^ ∙ ^^^^Figure 5 shows a circuit diagram for the DC / DC converter 14, in which within a switching period T DC the off states DC1 and DC2 are used to different extents. The value of e shown in Figure 5 corresponds approximately to e = -1 / 6 * U DC . The switch-off time T OFF2 for the DC2 state is shortened and the switch-off time T OFF1 for the state DC1 is extended by the same amount, so that T OFF = d OFF * T DC = 202303222 15 T OFF1 + T OFF2 applies. This results in the curve of the DC output voltage 51 shown. Furthermore, Figure 5 shows the common-mode voltage 52 generated in the DC / DC converter 14. As can be seen from the hatching below the curve of the common-mode voltage 52, the circuit diagram shown causes, on average over the switching period T DCa common-mode voltage that counteracts the common-mode beat caused by the active rectifier 12. The result of the compensation is shown in Figure 6, again using the touch current in the event of a fault, i.e., with symbolic switch 31 open. The current flows through a hypothetical human body, i.e., through its equivalent circuit in the simulation. Compared to Figure 3, the low-frequency common-mode beat is almost no longer detectable in the current waveform. The RMS value of the current is minimized from 22.8 mA to 1.8 mA with active 150 Hz compensation, which corresponds to a reduction of 92%.

[0002] 202303222 16 Reference numerals 10 Charger 12 Active rectifier 14 DC / DC converter 15a…d Semiconductor switch 16 Supply voltage 18a…c Inductance 20a…f IGBTs 22 DC link 23a, b Intermediate link capacitor 24 Node 26 Output filter 27a, b Filter inductance 27c Filter capacitor 28 Ground 29a, b Parasitic capacitance 30a, b Y capacitor 51 DC output voltage 52 Common mode voltage PE Protective Earth U DC DC link voltage U BAT Battery voltage

Claims

202303222 17 patent claims 1. Operating method for a power converter (10) comprising an active rectifier (12), a DC voltage intermediate circuit (22), and a DC / DC converter (14), in which - the active rectifier (12) is controlled such that it switches between a plurality of rectifier switching states, - the DC / DC converter (14) switches between a first switching state and a second switching state to generate a DC output voltage from the voltage of the intermediate circuit (22) at a switching frequency, - a first of the two switching states comprises a plurality of switching options for representing the switching state, in which different common-mode voltages are generated, - in a period in which the first switching state exists, a first switching option is used for a first portion of the period and a second switching option is used for the remaining, second portion of the period,- the components are determined such that the common-mode voltage generated by the DC / DC converter (14) is equal in magnitude to a common-mode voltage generated by the rectifier (12).

2. Operating method according to claim 1, wherein the components are determined from the switching period of the DC / DC converter (14), the intermediate circuit voltage, the DC output voltage, and an instantaneous value for the common-mode voltage generated by the active rectifier (12).

3. Operating method according to one of the preceding claims, wherein a value averaged over a switching period of the rectifier (12) is determined as the instantaneous value for the common-mode voltage generated by the rectifier (12). 202303222 18 4. Operating method according to claim 3, wherein the instantaneous value is determined using the switching period, the intermediate circuit voltage, and portions of the switching period that are assigned to switching states of the active rectifier (12).

5. Operating method according to claim 1, wherein the first portion of the period and the second portion of the period are separated from one another in time by placing a second period between them, in which the second switching state is used.

6. Operating method according to claim 1, wherein the active rectifier (12) is operated with symmetrical space vector pulse width modulation.

7. Operating method according to claim 1, wherein the active rectifier (12) and the DC / DC converter (14) are operated with the same switching frequency.Power converter (10) with an active rectifier (12), a DC voltage intermediate circuit (22), a DC / DC converter (14) and a control device for controlling the rectifier (12) and the DC / DC converter (14), wherein the control device is designed to - control the active rectifier (12) in such a way that it changes between a plurality of rectifier switching states, - control the DC / DC converter (14) in such a way that it changes between a first switching state and a second switching state in order to generate a DC output voltage from the voltage of the intermediate circuit (22) at a switching frequency, wherein a first of the two switching states comprises a plurality of switching options for representing the switching state, in which different common mode voltages are generated, - further control the DC / DC converter (14) in such a way that in time periods in which the first switching state is present, 202303222 19 a first switching option is used for a first portion of the period and a second switching option is used for the remaining, second portion of the period, - determining the portions such that the common-mode voltage generated by the DC / DC converter (14) is equal in magnitude to a common-mode voltage generated by the rectifier (12).

9. Power converter (10) according to claim 8, wherein the active rectifier (12) is a two-point rectifier (12).

10. Power converter (10) according to claim 8 or 9, wherein the DC / DC converter (14) is an H-bridge DC / DC converter with two half-bridges connected in parallel.

11. Power converter (10) according to claim 10, wherein in the first switching state, in the first switching option, the two upper semiconductor switching elements (15a, c) of the two half-bridges are switched on, and in the second switching option, the two lower semiconductor switching elements (15b, d) of the two half-bridges are switched on. 12.Power converter (10) according to claim 10 or 11, wherein in the second switching state diagonally arranged semiconductor switching elements (15a…d) of the two half-bridges are switched on.

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