Battery disconnecting device

Abstract

A battery disconnecting device has a first input and a second input to which a battery can be connected, whereby the disconnecting device also has a first output and a second output to which an electric component can be connected, whereby at least one first circuit breaker is arranged between the first input and the first output, and at least one second circuit breaker is arranged between the second input and the second output, whereby the first circuit breaker is at least a transistor and the second circuit breaker is a relay.

Claims

1. A battery disconnecting device, comprising: a first input and a second input to which a battery can be connected, a first output and a second output to which an electric component can be connected, at least one first circuit breaker arranged between the first input and the first output, and at least one second circuit breaker arranged between the second input and the second output, wherein the first circuit breaker includes a plurality of transistors connected in parallel, at least a first transistor of the plurality of transistors and a second transistor of the plurality of transistors consist of different circuit families and/or of different basic materials, and the second circuit breaker is a relay, wherein the battery disconnecting device has a control unit that is configured so as to generate control signals for the transistor or transistors and for the relay, whereby the transistor or transistors and the relay are actuated simultaneously, wherein the first transistor is a MOSFET and the second transistor is an IGBT; and wherein the control unit is configured to change the first circuit breaker's distribution of load current between the first transistor and the second transistor based on a magnitude of the load current.

2. The battery disconnecting device according to claim 1, wherein the transistor for a discharge path from the battery to the component is arranged in the forward direction.

3. The battery disconnecting device according to claim 1, wherein at least one diode is arranged in parallel to the transistor.

4. The battery disconnecting device according to claim 1, wherein the first transistor is a MOSFET and the second transistor is an IGBT.

5. The battery disconnecting device according to claim 1, wherein the control unit is configured to operate the first circuit breaker to distribute more current to the first transistor than the second transistor if the magnitude of the load current is lower a predetermined magnitude, and distribute more current to the second transistor than the first transistor if the magnitude of the load current is above than the predetermined magnitude.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in greater detail below with reference to a preferred embodiment. The individual figures show the following:

(2) FIG. 1 is a block diagram of a traction system having a battery disconnecting device;

(3) FIG. 2a is a partial view of the battery disconnecting device with a depicted discharging current;

(4) FIG. 2b is a partial view of the battery disconnecting device with a depicted charging current;

(5) FIG. 3 is a view of the course-over-time of the blocking of the transistor and the opening of the relay;

(6) FIG. 4 is a block diagram of a traction system having a battery disconnecting device with two relays (state of the art);

(7) FIG. 5 shows various characteristic curves for a circuit according to FIG. 4;

(8) FIG. 6 is a current-voltage characteristic curve for a MOSFET as well as for an IGBT; and

(9) FIG. 7 is a parallel circuit of MOSFETs and IGBTs.

DETAILED DESCRIPTION OF THE INVENTION

(10) Before the invention is elaborated upon, the state of the art will first be explained making reference to FIGS. 4 and 5. The traction system 1 comprises a battery 2 having a plurality of series-connected battery cells 3, power electronics 4 with DC link capacitor C.sub.ZK, an electric machine 5 as well as a battery disconnecting device 6. The battery disconnecting device 6 has a first relay 7 and a second relay 8 by means of which the plus line and the minus line can be switched on or off.

(11) Moreover, the battery disconnecting device 6 has a current sensor 9, a fuse 10, a control unit 11, a precharging relay S.sub.VL and a precharging resistor R.sub.VL. The DC link capacitor C.sub.ZK is charged with a moderate current via the precharging relay S.sub.VL and the precharging resistor R.sub.VL. For this purpose, first of all, the relay 7 is left open and the relay 8 as well as the precharging relay S.sub.VL are closed. Once the DC link capacitor C.sub.ZK is charged, the relay 7 is closed and the precharging relay S.sub.VL is opened. During operation, the current then flows via the low-resistance path via the two relays 7, 8 so that the heat losses are kept within limits. During battery operation (e.g. driving or charging), overloading of the battery cells 3 and of the relays 7, 8 is prevented in that, for instance, the maximum possible current that the battery 2 can deliver under the momentary boundary conditions (e.g. as a function of the temperature of the battery cell 3) is communicated to a high-voltage control unit of the vehicle via a CAN bus. If the high-voltage voltage components exceed this current, the relays 7, 8 are opened by the control unit 11 on the basis of a predefined plausibilization.

(12) FIG. 2 shows a number of characteristic curves, whereby the characteristic curve a represents the current carrying capacity of the fuse 10 while the characteristic curve b represents the current carrying capacity of the relays 7, 8. The figure also shows a characteristic curve c that represents the current disconnecting capacity of the relays 7, 8, in other words, the current that the relays 7, 8 can switch without arc formation. A characteristic curve d is also depicted, which shows the curve of a short-circuit current by way of an example. The characteristic curve e describes a peak current while the characteristic curve f describes a continuous current of the battery 2, whereby the characteristic curves g and h depict the appertaining switching thresholds for the peak current or the continuous current. A peak current arises, for example, when the motor vehicle accelerates. If the peak current stays above the peak current threshold of the characteristic curve g for a period of time t.sub.1, then the relays 7, 8 are opened in order to protect the battery cells. This switch-off procedure does not pose a problem since the current lies below the current disconnecting capacity and the current carrying capacity of the relays 7, 8. The continuous current, in contrast, has to be switched off in a timely manner at point in time t2 since otherwise, there is a risk that the relays 7, 8 will fuse together. A short-circuit current in accordance with the characteristic curve d constitutes a problem because, after a short period of time, the peak of the short-circuit current exceeds the current carrying capacity of the relays 7, 8 so that, for safety reasons, the relays 7, 8 have to be replaced each time a short circuit has occurred. In this context, the fuse 10 does not have to be tripped since the peak current is only present for a few ms and consequently the characteristic curve a is not intersected.

(13) FIG. 1 shows a traction system 1 having a battery disconnecting device 16 according to the invention, whereby identical elements such as those in the embodiment of FIG. 4 are provided with the same reference numerals. The essential difference from the embodiment of FIG. 4 is that the relay 8 was replaced by a parallel connection of several transistors T.sub.E, whereby, thanks to the transistors T.sub.E, the precharging relay S.sub.VL and the precharging resistor R.sub.VL can be dispensed with. In this context, the transistors T.sub.E are connected in such a way that, in the discharging direction, they are connected in the forward direction (also see FIG. 3a). The diodes D.sub.L that, in the charging direction, are connected in the conducting direction, are connected in parallel to the transistors 17. The transistors T.sub.E and the relay 8 are actuated by the control unit 11. The functionality of the precharging relay S.sub.VL and the precharging resistor R.sub.VL can then be implemented by means of a suitable PWM (pulse-width modulation) actuation of the transistors T.sub.E.

(14) In this context, when the transistors T.sub.E are in the discharging direction, that is to say, when current I.sub.ELAD is flowing out of the battery 2, they are actuated so as to be in the conductive state. Since the diodes D.sub.L are polarized in the blocking direction, the current I.sub.ELAD flows exclusively via the transistors T.sub.E. Since the transistors in the conductive state are very low-resistance in the forward direction, the heat losses are low. In the charging direction, the current I.sub.LAD flows into the battery 2. For this purpose, the transistors T.sub.E are blocked since, in the inverse operation, they have a higher resistance than the diodes D.sub.L. As a result, the heat losses only occur at the diodes D.sub.L. As a rule, these heat losses can be managed well so that there is no need for complicated active cooling measures. Moreover, the battery 2 can be galvanically disconnected at a single pole via the relay 8, whereby the relay 8 alone is responsible for the switch-off in the charging direction since the diodes D.sub.L are polarized in the flow direction. In this context, the diodes D.sub.L can be separate diodes D.sub.L, or else, if the transistors are configured as MOSFETs, intrinsic diodes (also known as body diodes) of the transistors T.sub.E can be used. As already elaborated upon, the current flow can only be actively switched by the transistors T.sub.E in the discharging direction. However, this is precisely also the critical current direction in case of a short circuit (see FIG. 5, characteristic curve d, where the current carrying capacity of the relay is exceeded). This problem can now be solved by the transistors T.sub.E since they can switch within the μs range. Therefore, in the case of a short circuit, the current can be switched off via the transistors T.sub.E before the current carrying capacity limit of the relay 8 is reached. This also makes it possible to switch transistors T.sub.E and relays simultaneously, even if the current is greater than the current disconnecting capacity (see characteristic curve c, FIG. 5) of the relay, since the current has already been switched off by the transistors T.sub.E. This is shown schematically in FIG. 3. Here, the switching state of the transistors T.sub.E and of the relay 8 is shown on the Y axis, whereby the number 1 designates a closed relay 8 or a conductive transistor T.sub.E. If the control unit 11 then synchronously actuates the transistors T.sub.E and relays 8 at point in time to, then the transistors T.sub.E are switched off (blocked) after a few μs, whereas the relay 8 only opens after a few ms since the mechanical holding force first has to be eliminated.

(15) FIG. 6 shows the voltage plotted over the current in the conductive state for a MOSFET and for an IGBT. Here, the resistance of the MOSFET at low currents of up to about 15 A is less than that of the IGBTs, whereby, at higher currents of approximately 25 A, the resistance of the IGBTs is less. For this reason, preference should be given to MOSFETs in the case of low load currents and to IGBTs in the case of higher load currents. These different behaviors can now be systematically utilized, which is shown in FIG. 7, whereby, for the sake of clarity, the diodes D.sub.L are not depicted. In this context, the transistors shown on the left-hand side are configured as MOSFETs while the ones on the right-hand side are configured as IGBTs, whereby the load current I.sub.ges is distributed differently, depending on the magnitude. In the lower load range, most of the current flows over the MOSFETs while, in the upper load range, it flows mostly via the IGBTs, which automatically translates into minimum power loss.

(16) As an alternative, only the MOSFETs are rendered connective in the case of low loads, while only the IGBTs are rendered connective in the case of higher loads. It is likewise possible to systematically switch off individual transistors if these have reached a critical temperature range.