DISCHARGE RESISTANCE

Description

[0001] The invention relates to a discharge resistor according to claim 1.

[0002] These discharge resistors are safety components used, for example, in electric vehicles to discharge buffer capacitors in the event of a fault, during servicing, or in the event of an accident. Such discharge resistors are also used, for example, to discharge DC link capacitors in inverters – generally speaking, the function of discharge resistors is to dissipate dangerous voltages as quickly as possible.

[0003] Accordingly, such discharge resistors must be designed with the lowest possible resistance. A further requirement is that they be intrinsically safe, meaning they must not burn out during normal use, allowing operation without additional monitoring. Discharge resistors must also be able to withstand the respective operating voltage continuously, which in electric vehicles is up to approximately 1200 volts.

[0004] For less demanding applications, discharge resistors can be used in which the resistive element is designed as a wire resistance winding surrounding an insulating body. Cartridge-shaped versions of such wire resistance elements are described, for example, in DE 2 228 460 or DE 37 03 689 C2.

[0005] However, these cartridge-shaped braking resistors have a comparatively complex design and require considerable installation space.

[0006] More compact solutions involve the resistive element having a wire winding wound onto a flat support, which is then inserted into a heat sink designed as a hollow profile. Such a solution is disclosed, for example, in EP 1 711 035 A1 of the applicant.

[0007] To increase "intrinsic safety", the applicant proposes in DE 10 2014 102 601 A1 to better thermally insulate a specific section of the wire winding from the environment compared to the rest of the wire winding, so that in the event of a fault the wire winding element fails in this wire section, so that with appropriate design of this area the risk of damage to adjacent components as well as a dangerous electrical contact to the housing, i.e. a short circuit to the body, can be prevented and thus an "intrinsically safe escape" of the braking / discharging resistor is enabled.

[0008] As mentioned, such load resistors (discharge / braking resistors) based on a wire resistor element have a comparatively complex structure, requiring special measures to increase operational safety / intrinsic safety.

[0009] In electromobility applications, load resistors with PTC resistor elements are therefore preferred. In such load resistors (braking / discharging resistors), the ceramic PTC resistor elements are installed in a housing designed as a heat sink. This housing can, in turn, be made of an extruded profile, preferably from aluminum or an aluminum alloy. Such a load resistor is known from EP 1 225 080 B1 of the applicant.

[0010] However, such discharge resistors are not currently suitable for voltages in the range of 850 to 1200 volts – manufacturer approvals only guarantee a voltage withstand of up to 600 volts. It should be noted that when used as a discharge resistor, the resistance must initially be relatively low in order to ensure rapid discharge of the component through high energy absorption. At the same time, operational reliability must be guaranteed by sufficient impulse withstand capability, which is in the range of more than 3 kV.

[0011] German patent application DE 10 2009 049 404 A1 describes PTC resistors suitable for use with the aforementioned load resistors. These known PTC resistors are designed for improved dielectric strength, for example, at a minimum resistance of 2 ohms and a dielectric strength of 170 volts / mm. A PTC resistor with high dielectric strength is also described in German patent application DE 27 53 766 A1.

[0012] US patent application US 5,733,833 discloses a first embodiment of a discharge resistor with a PTC element having a resistance of 4.5 Ω and a dielectric strength of 530 V. This results in a dielectric strength to resistance ratio greater than 3.

[0013] A method for manufacturing PTC resistor elements is disclosed in DE 32 27 907 A1.

[0014] However, it turned out that even discharge resistors implemented with such PTC resistor elements do not meet the requirements for intrinsic safety, such as those that exist in applications in electromobility.

[0015] In contrast, the invention is based on the objective of creating a discharge resistor with improved intrinsic safety.

[0016] This problem is solved by a discharge resistor according to the preamble of claim 1.

[0017] Advantageous further developments of the invention are the subject of the dependent claims.

[0018] According to the invention, the discharge resistor is designed with at least one PTC component, which is thermally contacted with a heat sink and has connections for electrically contacting the PTC component. According to the invention, the ceramic PTC component is designed such that the ratio of the voltage rating [in volts] to the minimum resistance (Rmin) [in ohms] is > 3, preferably > 4.

[0019] It follows that the characteristic properties of the PTC component, in particular its thickness, are selected such that, for example, at a normal operating voltage of 1200 V, the initial resistance (Ra) at room temperature (RT) is at most 500 ohms, and the minimum resistance (Rmin) is below this value. According to the invention, these parameters can be adjusted by a person skilled in the art by appropriately selecting the dimensions of the PTC component. This is a surprising solution for those skilled in the art, as there is a common misconception that conventional PTC components can only achieve a voltage withstand of at most 850 V. The invention overcomes this misconception and makes it possible to provide very compact discharge resistors that are easy to manufacture and exhibit the required voltage withstand and pulse resistance.

[0020] According to the invention, the housing of the PTC component, i.e. the volume of the unit consisting of the heat sink and the PTC resistor element, is selected such that the ratio of energy absorption [in Joules J] to volume [cm 3< ] is below 200 J / cm 3< , preferably below 120 J / cm 3< .

[0021] This ensures that the discharge resistor does not heat up to an excessive temperature in the region of the breakdown voltage. In the concept according to the invention, the maximum temperature is in the range of 150°–250° Celsius, preferably a maximum of approximately 180° Celsius. Such inherent safety cannot be achieved with conventional solutions.

[0022] In one embodiment of the invention, the thickness D of the PTC component, i.e., the dimension of the PTC component between its two electrodes / contact plates, is designed to be greater than 4 mm, preferably about 5 mm. Surprisingly, it was found that when designing a PTC component with such a thickness, a dielectric strength in the range of 1200 V can be achieved, while the minimum resistance (Rmin), which is important for rapid discharge of the component, is very low. However, it should be noted that these values ​​can be achieved by suitable housing of the PTC resistor – a finding not considered in the prior art.

[0023] In a particularly preferred embodiment, the ratio between the thickness D [mm] and the minimum resistance R min [ohms] is chosen to be greater than 1 / 90 and less than 1 / 50, preferably about 1 / 60. That is, the manufacturing process of the PTC component is selected such that an initial resistance between 90 Ω and 50 Ω is achieved per mm of thickness of the PTC component.

[0024] It is particularly preferred if the PTC component is approximately cuboid in shape, with a width B2 of more than 8mm, preferably about 11mm, a thickness D of more than 4mm and a length L2 of more than 20mm, preferably about 28mm.

[0025] The intrinsic safety can be further increased by removing the low-resistance edge areas from a blank of the PTC component, so that a flashover in the edge area of ​​the PTC component and a breakdown through the ceramic are reliably prevented, thus increasing the dielectric strength.

[0026] As explained, it is important that the PTC component(s) of the discharge resistor are installed in a suitable heat sink. Press-fitting is particularly suitable for this purpose. The pressing force is selected according to the geometry of the heat sink and the PTC component to ensure sufficient thermal contact within the intended temperature range, which can be, for example, between -20°C and 180°C. Furthermore, this enclosure ensures that the impulse energy generated during discharge can be dissipated without damaging the ceramic (causing it to burst).

[0027] The heat sink can, for example, be designed as an open U-profile into which the PTC component is inserted. This U-profile is then closed using a press plate, which is plastically deformed for compression.

[0028] Alternatively, a heat sink with a closed profile can be used, which is then plastically deformed during pressing.

[0029] According to the invention, the initial resistance at room temperature, with a voltage rating of up to 1200 V, is in the range of 300 Ω to 500 Ω. Room temperature is typically defined as RT = 25°C ± 5K.

[0030] The discharge resistor can also be implemented with more than one PTC component. These can be arranged in parallel or in series.

[0031] According to the invention, the maximum energy absorption of the discharge resistor is higher than 1500 J and is preferably in the range of about 2000 J to 6000 J and more.

[0032] Preferred embodiments of the invention are explained in more detail below with reference to schematic drawings. These show: Figure 1a three-dimensional representation of a discharge resistor according to the invention; Figure 2 a top view of the discharge resistor according to Figure 1 ; Figure 3 a section through the discharge resistor along line AA in Figure 2 ; Figure 4 Characteristic curves of the discharge resistance according to the Figures 1 to 3 and a heating element and Figure 5 a variant of the embodiment according to the Figures 1 to 3 .

[0033] Figure 1Figure 1 shows a three-dimensional representation of a first embodiment of a discharge resistor 1. This resistor has a heat sink 2, which consists of a U-shaped extruded aluminum profile 4 and an associated press plate 6. The extruded profile 4 and the press plate 6 are advantageously made of aluminum or an aluminum alloy. These two components (extruded profile 4, press plate 6) form a receiving space for a PTC component 8, which is connected via two wires 10, 12 to an electronic component of an electrical circuit, for example, an electric vehicle, whereby this electronic component must be discharged in the event of a malfunction or during servicing.

[0034] This electrical component can be a buffer capacitor or an intermediate circuit capacitor, whose voltage is to be dissipated by the discharge resistor 1 as quickly as possible. This dissipation occurs through the conversion of the electrical voltage into heat, which, possibly buffered by the PTC component, is released into the environment via a heat sink. The impulse withstand capability is in the range of several kV, for example, between 3000 and 4000 volts.

[0035] The discharge voltage is preferably in the range between 850 and 1200 volts (DC).

[0036] Figure 2 shows a top view of the discharge resistor 1. Figure 1 and Figure 3 a section along line AA in Figure 2 .

[0037] Accordingly, the PTC component 8 is in the form of a cuboid with a length L2, a width B2 and a thickness D. In the illustrated embodiment, the length L2 is approximately 28 mm, the width B2 approximately 11 mm and the thickness D 5 mm.

[0038] This ceramic PTC component 8 is manufactured from a blank with a plate-like contour, the dimensions of which are slightly larger than those of the previously described PTC component 8. During manufacturing, edge areas of this blank are cut away, as these are often lower in resistance than the central areas. In this way, a PTC component with a homogeneous resistance characteristic is produced, which is ensured by the optimal edge properties of the component. In principle, however, it is also possible to manufacture a PTC component from the outset with a sufficiently uniform resistance characteristic without deviations towards the edge areas.

[0039] Electrical contact is made via electrodes / connection plates which are electrically connected to the strands 10, 12.

[0040] The PTC component 8, which has a homogeneous resistance characteristic and its electrodes, is embedded in an electrical insulating shell 14, which prevents direct electrical contact with the heat sink 2 and achieves an insulation strength of up to 4,000 V.

[0041] As can be seen in particular from section AA in Figure 3 As can be seen, the extruded profile 4 has an approximately U-shaped cross-section with a base 16 and two legs 18, 20. The latter are each flanged in the area of ​​their free end sections to form a U-shaped recess 22, 24 into which the press plate 6 can be inserted, so that it is pressed along its edge sections (perpendicular to the plane of the drawing in Figure 3) is shown in the recordings 22, 24. The press plate 6 and the extruded profile 4 define - as explained - a receiving space 26 into which the encased PTC component 8 is inserted.

[0042] To thermally contact the PTC component 8 with the heat sink 2, the pressure plate 8 is subjected to a pressing force F in the direction of the arrow, so that the PTC component 8 is pressed into the receiving space 26. The pressure plate 6 is supported by the legs 18, 20. The pressing is carried out in such a way that the thermal contact is maintained over the entire operating temperature range of the discharge resistor 1. In automotive applications, this operating temperature range is between approximately -20°C and approximately 180°C (see Figure 4 ).

[0043] This means that the crimping must be designed in such a way that any dimensional changes of the components caused by the temperature are compensated for and sufficient thermal contact between PTC component 8 and heat sink 2 is always ensured.

[0044] According to the invention, the dimensions of the heat sink 2 are selected such that the ratio between the maximum energy absorption in joules (J) and the installation volume (cm³) is below 200 J / cm³, preferably approximately 120 J / cm³, wherein the discharge resistance is designed for a very high maximum energy absorption of more than 1500 J. Preferably, the maximum energy absorption is in the range of 2000 J to 6000 J or more.

[0045] It is by no means a prerequisite that the discharge resistor 1 comprises only one PTC component 8. In principle, several such PTC components can also be arranged axially one behind the other in a correspondingly extended heat sink 2. The dimensions of the PTC component 8 are selected, for example, according to the above specifications – when designing the overall volume, i.e., the volume enclosed by the heat sink, the aforementioned characteristic value calculated from the ratio between energy absorption and volume [J / cm³] should be taken into account. In this case, the required intrinsic safety is ensured, so that, for example, even when the maximum breakdown voltage (approximately 1200 volts) is reached, the operating temperature remains within the predetermined range, i.e., below 200° Celsius, preferably at a maximum of approximately 180° Celsius.

[0046] The table below shows examples of the aforementioned parameters for different numbers of PTC components, where L1 is the total length of the heat sink (see Figure 2 ) and dimension L2 is the length of a PTC component. The volume is calculated from the aforementioned length L1 multiplied by the height H and the width B1 (see Figure 3 ) of the heat sink 2. This table also lists the energy absorption in joules; with all these parameters, an approximate value of approximately 120 J / cm³ is obtained at a maximum operating temperature of 180° Celsius. Taking these parameters into account, a discharge resistor with optimal intrinsic safety can thus be created. Number of PTCs L1 [mm] L2 [mm] Volume [cm³] (L1xB1xH) Energy input [J] Key figure 1 60 28 18 2000 2 88 56 26 3000 3 116 84 35 4000 < approximately 120 J / cm³ < 4 144 112 43 5000

[0047] By choosing the dimensions of the PTC component 8 as described above and pressing it with the heat sink 2, it is possible to use the discharge resistor at voltages up to 1200 volts.

[0048] In the characteristic curve diagram according to Figure 4 The component resistance is shown as a function of temperature, with the top line (dashed) representing the characteristic curve of a heating resistor and the bottom line representing the characteristic curves of a discharge resistor. The middle line (dashed-dotted) is the so-called "no-load characteristic," while the bottom line (solid) is the typical characteristic curve of a discharge resistor according to the invention at 900 volts.

[0049] Heating resistors are typically designed to convert electrical energy into heat energy during continuous operation – a high initial resistance is advantageous for this application. In the illustrated embodiment (characteristic curve above), the initial resistance is, for example, significantly more than 10,000 ohms. The resistance then drops to approximately 3,000 ohms as it heats up to the operating temperature (approx. 140°C) and then increases sharply with rising temperature – this is the typical behavior of a PTC heating resistor.

[0050] In contrast, a discharge resistor requires that the initial resistance be as low as possible.

[0051] According to the diagram in Figure 4In both the typical characteristic curve (below) and the no-load characteristic curve (slightly above the typical curve), the initial resistance Ra at room temperature (RT) is very low, for example, around 500 ohms. As the temperature rises to the operating temperature, this initial resistance decreases further to, for example, around 300 to 350 ohms (Rmin). This minimum resistance Rmin allows for very high energy absorption and thus ensures a discharge in a very short time. The slightly higher initial resistance Ra compared to the minimum resistance Rmin ensures a "damped" initial discharge. This damped initial discharge reduces the stress on the contacts of the circuit with the electronic component being discharged, while still allowing for a high energy input, since the resistance initially decreases with increasing temperature.

[0052] As the discharge resistor 1 heats up, its resistance rises sharply, ensuring intrinsic safety even during continuous discharge without the need for additional electronic safety elements. According to the in Figure 4 In the characteristic curve shown, the operating temperature remains below 180° Celsius even within the intrinsic safety range – this maximum temperature is significantly higher in heating applications. This intrinsic safety is necessary, for example, when using regenerable capacitors (so-called double-layer capacitors), as these can build up a dangerous voltage again through self-regeneration.

[0053] This inherent safety feature also allows the discharge resistor 1 to remain permanently connected. Such a discharge resistor 1, with a low initial resistance Ra and extremely high dielectric strength, is unprecedented in the prior art. As explained at the outset, however, it must be noted that the thermal load on the discharge resistor 1 is very high due to the extremely high pulse power (high start peak). Therefore, the crimping must, on the one hand, allow for the expansion of the PTC component 8 at the beginning of the discharge, while on the other hand, ensuring sufficient contact throughout the entire discharge of the electronic component.

[0054] Figure 5 shows a variant of the discharge resistor 1 according to Figure 1In this embodiment, a closed extruded profile 4 is used instead of a two-part heat sink 2, in the receiving space 26 of which the PTC component 8, contained in the insulating shell 14, is inserted. The extruded profile 4 according to Figure 5 The side walls 28, 30 are approximately U-shaped and transition into a top wall 32, which runs approximately parallel to the base 16.

[0055] For pressing, the pressing force F is applied to the top wall 32, whereby the bulging side walls 28, 30 are deformed outwards, so that the clear width of the U-shaped areas is reduced and accordingly a pressing surface 34 of the top wall 32 lies flat against the PTC component 8 or the insulation shell 14.

[0056] The crimping is designed according to the same criteria as those shown in the exemplary embodiment according to Figure 4As explained, the crimping is chosen to allow expansion of the PTC component 8 during the high voltage peak at the beginning of the discharge, while ensuring contact throughout the entire discharge process.

[0057] Disclosed is a discharge resistor with at least one PTC component, which is pressed onto a heat sink in such a way that the discharge resistor can be used at voltages up to 1200 volts. The discharge resistor is preferably designed such that the ratio of volume [cm³] to energy absorption [J] is below 200 J / cm³, preferably below 120 J / cm³. Reference symbol list:

[0058] 1 Discharge resistor 2 Heat sink 4 Extruded profile 6 Press plate 8 PTC component 10 Stranded wire 12 Stranded wire 14 Insulating sleeve 16 Base 18 Leg 20 Leg 22 Mounting 24 Mounting 26 Mounting chamber 28 Side wall 30 Side wall 32 Top wall 34 Pressing surface L1 Length of the heatsink L2 Length of a PCT component B1 Width of the heatsink B2 Width of a PTC component H Height of the heatsink

Claims

1. A discharge resistor having a PTC component (8) which is thermally contacted to a heat sink (2) and having terminals for electrically contacting the PTC component (8), characterized in that the PTC component (8) is configured in such a way that the ratio of the dielectric strength [in volts] to the minimum resistance (Rmin) [in ohms] is greater than 3, preferably greater than 4, wherein the ratio of the maximum energy consumption [J] of the discharge resistor to the volume [cm3] of the heat sink (2) is in the range below 200 J / cm3, preferably below 120 J / cm3, wherein the minimum resistance (Rmin) is between 300 Ω and 500 Ω, and the maximum energy consumption being more than 1500 J.

2. The discharge resistor according to patent claim 1, wherein a thickness (D) of the PTC component being ≥ 4 mm, preferably approximately 5 mm.

3. The discharge resistor according to one of patent claims 1 and 2, wherein the ratio of the thickness (D) in mm to the minimum resistance (Rmin) in ohms is ≥ 1:90 and ≤ 1:50, preferably approximately 1:60.

4. The discharge resistor according to one of the preceding patent claims, wherein the PTC component (8) is of approximately cuboidal design, having a width (B1) of more than 8 mm, preferably approximately 11 mm, a thickness (D) of ≥ 4 mm, and a length (L2) of more than 20 mm, preferably of 28 mm.

5. The discharge resistor according to one of the preceding patent claims, wherein the PTC component (8) is produced from a plate-shaped blank, wherein lowresistance edge / outer regions of the blank are removed.

6. The discharge resistor according to one of the preceding patent claims, wherein the PTC component (8) is pressed to the heat sink (2).

7. The discharge resistor according to one of the preceding patent claims, wherein the heat sink (2) is formed with an open, approximately U-shaped extruded profile (4) which has a receptacle (22, 24) for a pressing plate (6).

8. The discharge resistor according to one of patent claims 1 to 6, wherein the heat sink (2) is formed by an extruded profile (4) which is closed on the circumferential side.

9. The discharge resistor according to one of the preceding patent claims, wherein a plurality of PTC components (8) is arranged in the heat sink (2).