DE102023202260A1 - Discharge device for an intermediate circuit capacitor - Google Patents

Abstract

A discharge device (230) for an intermediate circuit capacitor (225) of a current transformer (115) comprises the following elements: a resistor (250); a controllable switch (255); connections (240, 245) for connection to the intermediate circuit capacitor (225); and a circuit board (235) with conductor tracks (260) for electrically connecting the elements such that the resistor (250) is connected in series with the switch (255) between the connections (240, 245). The resistor (250) is formed by an arrangement of conductor tracks (260) of the circuit board (235).

Description

The present invention relates to a current transformer with an intermediate circuit capacitor. In particular, the invention relates to a discharge device for the intermediate circuit capacitor.

A motor vehicle comprises a drive train with an electrical energy storage device and an electric drive motor. The electrical energy storage device provides a direct voltage, while the electric drive motor requires an alternating voltage. A current converter is set up to generate the required alternating voltage and to control the behavior of the electric drive machine. The current converter usually comprises a half-bridge, which converts a voltage from the energy storage device to a required level at a high frequency. An intermediate circuit capacitor (ZKK) is provided for decoupling, which temporarily stores electrical energy while the half-bridge is switching.

If the vehicle is parked and the electric drive train is switched off, there may still be a considerable charge in the intermediate circuit capacitor. Since the voltage of the electrical energy storage device can be several 100 V, a charged intermediate circuit capacitor can also have a high voltage, which can be dangerous, for example, for a technician who is servicing the drive train.

An electrical resistor or a transistor can be used to discharge the intermediate circuit capacitor. Transistors that are included in the half-bridge cannot be used for safety reasons (ASIL-B or higher). Discharging using a dedicated transistor has been tested at voltages of up to approx. 400 V and can be considered safe. However, motor vehicles are increasingly being equipped with electrical energy storage devices that have a significantly higher voltage, for example approx. 800 to approx. 1000 V or even more. Discharging an intermediate circuit capacitor with such a high voltage can be prone to errors. A separate resistor must have a high load capacity in order to be able to withstand the required energy quantities at the high voltage. High-performance resistors are often encapsulated in cement and are relatively compact, so that heat generated by them cannot be dissipated well. Resistors with better cooling, on the other hand, can be bulky and expensive.

An object underlying the present invention is therefore to provide an improved technique for discharging an intermediate circuit capacitor of a current transformer. The invention solves this problem by means of the subject matter of the independent claims. Subclaims give preferred embodiments.

According to a first aspect of the present invention, a discharge device for an intermediate circuit capacitor of a current transformer comprises the following elements: a resistor; a controllable switch; connections for connection to the intermediate circuit capacitor; and a circuit board with conductor tracks for electrically connecting the elements such that the resistor is in series with the switch between the connections. The resistor is formed by an arrangement of conductor tracks on the circuit board.

The discharge device can be manufactured inexpensively. The resistance implemented by the circuit board can have a relatively large surface area with a relatively small volume. The heat generated by it can therefore be easily dissipated. At the same time, the discharge device can advantageously require little installation space and can be easily integrated into a system with a current transformer. The controllable switch can be designed to be operationally reliable, so that it can be high-resistance but not low-resistance. The discharge device can achieve a high level of operational reliability and be certified accordingly, for example according to ISO 26262 or ASIL A to D.

The conductor tracks can be laid out in any curve or straight line, so that the available installation space on the circuit board can be used optimally. For example, an I-shaped, L-shaped or O-shaped resistor can easily be formed on the circuit board. More complex shapes can be composed of simple shapes. If there is no continuous surface available for the resistor, it can be divided into two or more sub-surfaces.

The resistor can be scalable to be used at a predetermined operating voltage and to convert a predetermined amount of charge of the intermediate circuit capacitor into heat within a predetermined time. If necessary, the circuit board can be easily cooled, for example by thermally connecting it to a heat sink or a housing. It may be sufficient to thermally connect only part of the area over which the resistor extends.

It is preferred that the resistor comprises a flat, meandering arrangement of conductor tracks. In this way, the length of the conductor tracks through which current flows can be maximized, while at the same time the current density on the circuit board can be kept low. The conductor tracks can be manufactured in a conventional manufacturing process, for example using etching technology based on a copper coating on a carrier material such as hard paper or a fiber-reinforced plastic.

It is further preferred that conductor tracks that are assigned to the resistor and lie in the same plane are arranged at regular mutual distances. This allows optimal heat distribution in the surface of the resistor to be achieved. In a particularly preferred embodiment, several equidistant, parallel conductor tracks are provided, the ends of which are connected to one another in pairs, so that the conductor tracks are electrically connected in series. An electrical resistance and a current load capacity can be controlled depending on the thickness and width of the conductor tracks.

In a further preferred embodiment, the circuit board has several levels, with conductor tracks assigned to the resistor lying on different levels. In this way, the effective length of conductor tracks that lie electrically between the connections can be increased. The circuit board can have any number of layers; two, four, six or eight layers are common in the industry, for example.

In a further preferred embodiment, conductor tracks assigned to the resistor are arranged offset from one another in adjacent planes. A current density within the circuit board can thus be more homogeneous. Heat can be generated more evenly distributed within the circuit board, so that thermal overloading of an individual area can be better prevented.

It is further preferred that the circuit board has a predetermined section in which no electrical elements other than the conductor tracks assigned to the resistor extend. The section can be used exclusively by the resistor, so that another element can neither electrically nor thermally interact with the resistor. The section can be selected so that it is close to an element that is suitable for absorbing heat, for example a housing shell or a heat sink. Another section of the circuit board can, however, be used for the electrical connection or accommodation of other elements of the current transformer. For example, semiconductor modules that work as current valves can be electrically connected to one another and/or to a control device by means of the circuit board. The control device can also be built on the circuit board or electrically connected via the circuit board.

It is further preferred that a surface of the portion of the circuit board is free of other elements. The surface can be used to abut against an element for absorbing thermal energy.

The discharge device may further comprise means for determining a temperature of the resistor based on a current flowing through the resistor and a voltage applied to the resistor.

The resistor formed by conductor tracks can have a PTC characteristic, whereby its resistance increases with increasing temperature. By determining its resistance, its temperature can be determined. The temperature can be evaluated for diagnostic purposes. This can prevent the resistor from being thermally overloaded.

According to a further aspect of the present invention, a current converter comprises an intermediate circuit capacitor and a discharge device described herein. The current converter is preferably designed for use on board a motor vehicle. In particular, the current converter can be designed to convert a direct voltage of several 100 V into one or more alternating voltages. The intermediate circuit capacitor can be connected to the direct voltage.

In a further preferred embodiment, the circuit board of the discharge device is designed to electrically connect other elements of the current transformer to one another. For example, the circuit board can carry active or passive components, power components or electrical leads. Electrical or electronic elements can be attached to the circuit board both electrically and mechanically.

The electric drive axle comprises a current converter described herein and preferably an electric drive machine. The drive axle can be used to drive a motor vehicle.

According to yet another aspect of the present invention, a motor vehicle comprises an electric drive axle as described herein. The motor vehicle can be electrically driven and preferably comprises a motorcycle, a passenger car, a truck or a bus.

• 1 einen Antriebsstrang in einem Kraftfahrzeug;
• 2 einen Stromwandler mit einem Zwischenkreiskondensator und einer Entladevorrichtung; und
• 3 einen Widerstand einer Entladevorrichtung

darstellt.The invention will now be described in more detail with reference to the accompanying figures, in which:

• 1 a drive train in a motor vehicle;
• 2 a current transformer with an intermediate circuit capacitor and a discharge device; and
• 3 a resistance of a discharge device

represents.

1 shows a drive train 100 on board a motor vehicle 105. The drive train 100 comprises an electric drive machine 110, a current converter 115 and an electric energy storage device 120. The drive train 100 can additionally comprise further components, for example a transmission or another drive motor, in particular an internal combustion engine. The electric energy storage device 120 provides a direct voltage in the range of several 100 V. In different embodiments, a nominal voltage of the energy storage device 120 can be, for example, approximately 400 V, approximately 800 V, approximately 1000 V or more. The current converter 115 is designed to convert the direct voltage into phase-shifted alternating voltages in order to control the electric drive machine 110. In particular, frequencies and voltages can be controlled such that the electric drive machine 110 converts a predetermined torque or assumes a predetermined speed.

2 shows a current transformer 115 for use in a drive train 100 of a motor vehicle 105.

In the presentation of 2 On the left, the current transformer 115 can be connected to the electrical energy storage device 120. In a right-hand area, connections for the electrical drive machine 110 are shown. The current transformer 115 comprises, for example, three half-bridges 205, each of which comprises an upper current valve 210 and a lower current valve 215. A control device 220 is provided for controlling the current valves 210, 215 of the half-bridges 205. An intermediate circuit capacitor (ZKK) 225 is provided parallel to connections of the electrical energy storage device 120. In the embodiment shown, one ZKK 225 is provided for the entire current transformer 115; in another embodiment, several ZKK 225 can also be used; in particular, each half-bridge 205 can be assigned a ZKK 225. Usually, a ZKK 225 is located in spatial proximity to a half-bridge 205.

The control device 220 is designed to periodically open and close the two current valves 210, 215 of a half-bridge 205 alternately, so that only one current valve 210, 215 is open at a time. By selecting a suitable duty cycle of on and off times, a predetermined potential can be set at a center tap that is connected to the electric drive machine 110, which lies between a high and a low potential of the electrical energy storage device 120. Voltage peaks that can occur when opening and closing current valves 210, 215 are smoothed out by means of the ZKK 225. By varying the duty cycle, the electric drive machine 110 can be provided with a suitable voltage or current curve. In this way, phase-shifted alternating currents with the same voltages and frequencies can be provided by means of the three half-bridges 205 in order to control the electric drive machine 110.

A discharge device 230 is provided in parallel with a ZKK 225, which comprises a circuit board 235, a first connection 240, a second connection 245, a resistor 250 and a controllable switch 255. The first connection 240 is connected to a high potential and the second connection 245 to a low potential of the electrical energy storage device 120. The connections 240, 245 are usually arranged or formed on the circuit board 235.

The circuit board 235 comprises a number of conductor tracks 260 for electrically connecting elements 240 to 255, which can be attached to the circuit board 235. A controllable switch 255 preferably comprises a semiconductor, in particular a transistor, for example a field effect or bipolar transistor. The resistor 250 and the controllable switch 255 are connected in series between the first connection 240 and the second connection 245 by means of the conductor tracks 260. If the controllable switch 255 closes, the ZKK 225 located at the connections 240, 245 can be discharged via the resistor 250. At this time, the ZKK 225 is preferably not electrically connected to the electrical energy storage device 120. The electrical energy stored in the ZKK 225 can be converted into heat in the resistor 250. Optionally, part of the energy can also be converted into heat in the controllable switch 255. An interface 265 is provided to control the switch 255.

Further elements than just those of the discharge device 230 can be mounted on the circuit board 235. For example, a flow valve 210, 215 or a part of the control device 220 be attached to the circuit board 235 and electrically contacted. Control or power lines between elements on the circuit board 235 can be electrically connected to one another by means of conductor tracks 260.

It is proposed to form the resistor 250 by an arrangement of conductor tracks 260 on the circuit board 235.

3 shows views of an exemplary circuit board 235. In an upper area, a schematic top view is shown, in a lower area, a schematic longitudinal section through the circuit board 235.

The circuit board 235 comprises several levels L1-Ln, on each of which conductor tracks 260 can be formed. Instead of levels, one can also speak of layers. In the embodiment shown, six levels L1-L6 are provided. Conductor tracks 260 of different levels L can be connected to one another by means of a through-plating 305, which can be implemented, for example, galvanically or in the form of a rivet. A through-plating 305 can be made between adjacent levels L or levels further apart from one another.

A meandering arrangement 310 of conductor tracks 260 is formed in a plane L. The arrangement 310 comprises several parallel and preferably equidistant conductor tracks 260, the ends of which are connected to one another in pairs so that they are electrically in series. A connection between two straight conductor tracks 260 is preferably made by means of a further conductor track 260, so that an arrangement 310 can effectively consist of a continuous conductor track 260.

Corresponding arrangements 310 can be provided on further levels L2-Ln. The arrangements 310 can be connected to one another by means of vias 305 in such a way that they are also electrically connected in series with one another, as in 3 is indicated by interrupted connections. In this way, the illustrated arrangements 310 of different levels L1-Ln can lie on top of one another on the circuit board 235 and practically form a stack. Preferably, a section is reserved on the circuit board 235 on which no elements other than the arrangement 310 for forming the resistor 250 are provided. In particular, this section is preferably free of electrical or electronic components that are attached to the circuit board 235. In this area, the circuit board 235 can rest against an object that is designed to dissipate thermal energy from the circuit board 235. This object can comprise, for example, a heat sink, a housing or a heat exchanger.

A typical ZKK 225 for a current transformer in the context mentioned has a capacity of approx. 400 µF to 1.3 mF. An example ZKK 225, which was charged to approx. 900 V, could be discharged to a voltage of less than approx. 50 V within approx. 1 s using a resistor of approx. 230 Ω, so that it can be considered harmless. During discharging, a maximum current of approx. 4.5 A flowed through conductor tracks 260 of resistor 250. The heating of resistor 250 remained low enough to rule out damage to conductor tracks 260.

In the embodiment shown, a conductor track 260 is approximately 10 cm long, approximately 100 µm wide and approximately 30 µm thick. Distances between adjacent, parallel and straight conductor tracks 260 are also approximately 100 µm, for example. 86 conductor tracks 260 can therefore be used per level L1-L6, which corresponds to a length of approximately 8.6 m. In six levels L1-L6, there are therefore approximately 50 m of conductor tracks 260 with a total resistance value of approximately 230 Ω. The entire resistor 250 occupies an area of approximately 17.2 cm ² on the circuit board 235.

In the embodiment mentioned, a cross section of a conductor track 260 is approximately 300 µm ² and the resistance value per length is approximately 4.46 Ω/m. It is preferred that the cross section and thus the resistance value of the conductor tracks 260 per length is as large as possible everywhere. If the copper coating in one of the levels L has a greater thickness, the width of the conductor tracks 260 in this level L can be reduced in order to equalize the cross section. Optionally, distances between the conductor tracks 260 in this level L1-L6 can be increased at the same time so that a power loss in W/m ² is as large as in one of the other levels L1-L6.

In a further embodiment, the fact that the arrangement 310 of conductor tracks 260 can exhibit a PTC characteristic, i.e. its resistance value can increase with increasing temperature, can be used to determine a temperature of the resistor 250. The actual resistance value of the resistor 250 can be determined as the quotient of a voltage drop across the resistor 250 and a current flowing through the resistor 250. The temperature can be determined from this by multiplying by a predetermined factor that is characteristic of the resistor 250. A corresponding procedure can be used if the resistor 250 comprises a material that has an NTC characteristic.

The current flowing through resistor 250 can be determined by providing another resistor, the resistance of which is known and not dependent on the temperature of resistor 250, in series with switch 255 and resistor 250 between terminals 240 and 245. A voltage drop across this resistor is proportional to the current flowing through resistor 250.

The temperature can be determined, for example, by means of the control device 220. The controllable switch 255 can also be controlled by the control device 220. However, one or both tasks can also be performed by another component.

Reference symbols

100

Drivetrain

105

Motor vehicle

110

electric drive machine

115

Current transformer

120

electrical energy storage

205

Half bridge

210

upper flow valve

215

lower flow valve

220

Control device

225

Intermediate circuit capacitor (ZKK)

230

Unloading device

235

Circuit board

240

first connection

245

second connection

250

Resistance

255

controllable switch

260

Conductor track

265

interface

L

Level, location

305

Through-hole plating

310

arrangement

Claims (12)

Discharging device (230) for an intermediate circuit capacitor (225) of a current transformer (115), wherein the discharging device (230) comprises the following elements: a resistor (250); a controllable switch (255); terminals (240, 245) for connection to the intermediate circuit capacitor (225); a circuit board (235) with conductor tracks (260) for electrically connecting the elements such that the resistor (250) is connected in series with the switch (255) between the terminals (240, 245); wherein the resistor (250) is formed by an arrangement of conductor tracks (260) of the circuit board (235). Unloading device (230) according to Claim 1 , wherein the resistor (250) comprises a planar, meander-shaped arrangement of conductor tracks (260). Unloading device (230) according to Claim 1 or 2 , wherein conductor tracks (260) associated with the resistor (250) and lying in the same plane (L1-Ln) are arranged at regular mutual distances. Discharge device (230) according to one of the preceding claims, wherein the circuit board (235) has several levels (L1-Ln) and conductor tracks (260) assigned to the resistor (250) lie on different levels (L1-Ln). Unloading device (230) according to Claim 4 , wherein conductor tracks (260) associated with the resistor (250) are arranged offset from one another in adjacent planes (L1-Ln). Discharge device (230) according to one of the preceding claims, wherein the circuit board (235) has a predetermined section in which no electrical elements other than the conductor tracks (260) associated with the resistor (250) extend. Unloading device (230) according to Claim 6 wherein a surface of the portion of the circuit board (235) is free of other elements. A discharge device (230) according to any one of the preceding claims, further comprising means for determining a temperature of the resistor (250) based on a current flowing through the resistor (250) and a voltage applied to the resistor (250). Current converter (115) comprising an intermediate circuit capacitor (225) and a discharge device (230) according to one of the preceding claims. Current transformer (115) to Claim 9 , wherein the circuit board (235) is adapted to electrically connect further elements of the current transformer (115) to one another. Electric drive axle comprising a current transformer (115) according to Claim 9 or 10 . Motor vehicle (105) comprising an electric drive axle according to Claim 11 .

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