WO2011006703A1

WO2011006703A1 - Electronic switch

Info

Publication number: WO2011006703A1
Application number: PCT/EP2010/057054
Authority: WO
Inventors: Thomas Walker; Matthias Renner; Marcel Rudolph
Original assignee: Robert Bosch Gmbh
Priority date: 2009-07-17
Filing date: 2010-05-21
Publication date: 2011-01-20
Also published as: DE102009027814A1

Abstract

The invention relates to an electronic switch (100) for switching an electronic load, comprising a control input (110) for receiving a control signal, a transistor (120) for controlling a load current flowing through the load as a function of the control signal, power determining means (140) for determining a loss power of the transistor (120), and a limiter (130) for actuating the transistor (120), such that the load current flowing through the load is limited to a first positive maximum value if the loss power is below a predetermined value, and otherwise is limited to a second positive maximum value.

Description

description

title

Electronic switch The invention relates to an electronic switch. In particular, the invention relates to an electronic switch integrated with a control logic for switching an electrical load.

State of the art

In many areas of electronics, integrated circuits are used to turn electrical loads on and off. These may be resistive, inductive or capacitive loads or even complex loads, which are a combination of the mentioned. Some loads show a complex turn-on behavior that a load-driving switch must be designed to work efficiently to avoid peak or continuous load damage. In particular, such loads that can accommodate a high inrush current have proven to shorten the life of electronic drive circuits. For example, a capacitor, a cold light bulb or a stationary DC motor allow a very high current flow when switched on, to which a load-controlling switch is to be designed.

Known in the prior art are those electronic switches which limit a current through the switch, and thus a current through the load, to a maximum value in order to avoid damage from short-term peak loads. Furthermore, it is known to use thermal overload protection in electronic switches. Since the current flowing through the electronic switch causes a power dissipation proportional to the power loss in the switch, which is discharged as heat, an overload of the electronic switch can be done by monitoring the temperature of the switch.

If the temperature exceeds a predetermined value, the Switch open, and the current flow through the load and interrupted by the switch. If the electronic switch cools down far enough, it can be closed again and a current flow through the load can be made possible. It has been shown that frequent thermal aging causes electronic components to age prematurely, which can affect their performance and service life. It is therefore an object of the invention to provide an electronic switch, which is robust compared to a switch-on variable electrical loads.

Disclosure of the invention

This object is achieved by an electronic switch according to claim 1. Subclaims indicate expedient or advantageous embodiments.

In one embodiment, an electronic switch is configured such that a power dissipation of a transistor through which a load current flows is determined, and depending on the power loss with respect to a predetermined threshold value, the load current is limited to one of two different values.

Such an electronic switch may have a life which is independent of any short circuits at an output of the switch. As a result, the average life of the electronic switch can be increased, which can result in cost advantages. Further, an average and / or maximum power dissipation of the electronic switch can be reduced, which may reduce a required chip area and the electronic switch can be particularly well integrated with other electronic components without negatively affecting the other components.

In the following, various embodiments of an electronic switch are set forth and described with reference to the attached figures, wherein:

Figure 1 is a block diagram of an electronic switch;

FIG. 2 shows illustrative structures of a high-side switch and a

Low-side switch; Figure 3 is a block diagram of a two-stage high-side switch according to Figures 1 and 2; and

Figure 4 shows exemplary waveforms of currents of an electronic switch of Figure 3 in comparison to an electronic switch according to the prior art.

In Figure 1, an electronic switch 100 comprises a control input 1 10, a transistor 120, a limiter 130, power determining means 140 and an output 150. Further, the electronic switch 100 is connected to an electrical supply voltage see.

In response to a control signal applied to the control input 1 10, the transistor 120 is turned on to allow a current flow at the output 150 by a load L (not shown). As a transistor 120 are particularly well suited bipolar transistors, since they have characteristics that allow precise control of a current flowing through the transistor 120 current. In principle, other transistor types, for example FET or IGBT, are also possible. Power determining means 140 are connected to the transistor 120. The

Power determining means 140 may include, for example, a voltmeter that determines a voltage that drops across those terminals of transistor 120 between which flows a load current provided by output 150 of the load. Other ways of determining a power dissipation of the transistor 120 are also possible, for example by means of inductive current measurement or thermal monitoring.

Depending on the power dissipation of the transistor 120 determined by the power determining means 140, the limiter 130 drives the transistor 120 such that the load current described above is limited. The limitation is thus dependent on the power that is converted by the transistor 120 into heat. The power loss of the transistor 120 is dependent on its internal resistance, which can be influenced by the limiter 130, and the magnitude of the load current, which depending on the load L, in particular immediately after a switch-on, may be time-dependent. FIG. 2 shows alternatives of the wiring of the transistor 120 of the electronic switch 100 from FIG. 1 in a circuit as a high-side switch (also: "high-side switch", HSS, left) and low-side switch (also: " Low-side switch ", LSS, right). The choice of a PNP transistor 120 in the case of the high-side switch or of an NPN transistor 120 in the case of the low-side switch is an expedient embodiment and should not be understood as limiting the technique shown. Of the essential elements of the electronic switch 100, only the transistor 120 is shown in FIG. When high-side switch (left in Figure 2) is a connection of the electrical

Last L at ground potential, and the other is connected to the output 150 of the electronic switch 100. When the electronic switch 100 is turned on, the transistor 120 is driven so as to apply a positive supply voltage + U _b to the output 150 so that the load current from the terminal + U _b can flow through the transistor 120, the output 150 and the load L. (considered technical direction of current).

In contrast, in the case of the low-side switch (on the right in FIG. 2), one terminal of the load L is connected to the positive supply voltage + U _b and the other to the output 150 of the electronic switch 100. In the case of switch-on of the electronic switch 100, the transistor 120 is driven such that the output 150 is connected to ground potential. Thus, the load current of + U _b through the load L, the output 150 and the transistor 120 can flow to ground.

Both the high-side switch and the low-side switch, the output 150 is high impedance when the electronic switch 100 is turned off and the transistor 120 is not driven. The output 150 is high-impedance and there is no significant load current I _L.

The proposed electronic switch is suitable both for a configuration as a high-side switch and as a low-side switch. A person skilled in the art will have no difficulty in combining one of the interconnections shown in FIG. 2 with the block diagram of FIG. 1 in order to implement the corresponding variant. FIG. 3 shows a more detailed block diagram 300 of a high-side switch on the basis of the electronic switch 100 from FIG. 1. A corresponding low-side switch results from corresponding, mentioned above with reference to Figure 2 interconnection. The load L is modeled by a capacitor C and a resistor R, receives a high inrush current (due to the capacitor C) and a constant continuous current (due to the resistor R). In an expansion of the block diagram in FIG. 1, the electronic switch 100 in FIG. 3 comprises the transistor 120, operational amplifiers OV1 and OV2, a constant voltage source U2, the output 150, current sources I _L1 and I _2, and the switches S1 and S2. The resistor RT models an internal resistance between an emitter and a collector of the transistor 120 and is comprised by the transistor 120.

A latch comprises an operational amplifier OV3, a constant voltage source U4 and a gate G1. A short-circuit monitor comprises a signal output 310, a timer 320, a switch S3, a constant voltage source U1, an operational amplifier OV4 and a gate G2.

In the following, for the consideration of the logical interconnections, logically "high" corresponds to a positive voltage or "1" during "low" to a negative one

In the following, a basic functionality of the electronic switch 100 will first be described, therefore, it is initially assumed that an output of the operational amplifier OV3 is such that an output of the gate G1 corresponds to the control signal at the control input 110 follows.

If the control signal at the control input 1 10 "low", the switch S1 is open and a base of the transistor 120 connected to no potential, so that the transistor 120 blocks and no current flows between its emitter and its collector Output 150 in a high-resistance state and a load current I _L is 0. However, the control signal at the control input 1 10 "high", the switch S1 is closed, so that in any case the predetermined by the constant current source Iu current through the base of the transistor 120 to flow can, whereby the transistor 120 turns on so far, that the load current I _{L is} limited to a first value. The operational amplifier OV1 determines a voltage difference U3 between the emitter and the collector of the transistor 120 (the corresponding feedback resistances). stands at OV1 are not shown). The operational amplifier OV2 compares this voltage difference U3 with the constant voltage U2 and switches its output to "high" if and only if the voltage difference U3 is less than U.sub.2, thereby closing the switch S2 so that the base of the transistor 120 additionally switches off the constant current source \ _L2 flowing current can flow, and the load current I _{L is} limited to a second value which is greater than the first value.

After the voltage difference determined by the operational amplifier OV1 is representative of a power loss converted in the transistor 120, the load current I _L available at the output 150 is initially limited to a first value with the described circuit during a switch-on operation until the power loss of the transistor 120 has fallen below the predetermined value expressed by U2 and only then is a second, higher load current I _L , the maximum permanent from the electronic switch

100 available power equivalent, provided at the output 150 of the load L. During a switch-on process, the internal resistance of the load L, due to the capacitor C, increases from an initially small value, so that the power loss, and thus also the differential voltage U3 of the transistor 120, decreases over time. Thus, while the load L is low-ohmic immediately after being turned on, it is operated at a lower load current I _L than at a later time when its internal resistance has already risen. Such a "slow start-up" has a life-prolonging effect on the transistor 120 and possibly on other components of the electronic switch 100 connected to it. In many cases, this slow start-up also increases the lifetime or reliability of the load L.

The latching device ensures that the transistor 120 is driven only when a sufficiently high input voltage + U _{b is} present, by comparing the input voltage + U _b with the constant voltage U4, and in the case when + U _{b is} less than U4 is, the output of the operational amplifier OV3 is set to "low", so that the output of the gate G1 is independent of the control input 1 10 applied control signal "low" and the switch S1 is not closed. Another embodiment of the electronic switch 100 relates to the generation of a short-circuit signal. As soon as the two switches S1 and S2 are closed, that is to say the maximum permanently deliverable load current I _{L is available} at the output, the gate G2 triggers the timer 320 by means of its output. After a predetermined time has elapsed, the timer 320 closes the switch

S3, so that the operational amplifier OV4 compares the voltage applied to the output 150 with the constant voltage U1. If U1 is greater than the output voltage at the output 150, the operational amplifier OV4 supplies a "high" value to the signal output 310 in order to signal the presence of a short circuit Set "low" to turn off the electronic switch 100.

Of course, the electronic switch 100 can alternatively or additionally be combined with a conventional thermal fuse of the prior art (not shown).

FIG. 4 shows a diagram 400 of two time profiles 410 and 420 of load currents. The curve 410 corresponds to a load current flowing at an output of a high-side switch of the prior art during the course

420 corresponds to the course of the load current I _L at the high-side switch of Figures 1 and 3. In the horizontal direction, a time t is plotted, in the vertical direction, a current. I _L1 denotes a first value to which the load current I _L can be limited, while lι_ _{2 describes} a second, larger value to which the load current I _L can be limited. The curves 410 and 420 assume a load whose internal resistance is low immediately after a switch-on process and increases with time, similar to the load in the illustration of FIG. 3. In the case of the load current curve 410 of the high-side switch of the prior art flows immediately after switching on at the time t _0, the maximum permissible current I _L2 . At a time ti, the internal resistance of the load L has come increased that the load current is below the value of \ _L2 falls and up to the time t ₃ asymptotically approaching a final value. In contrast, the trace 420 of the load current I _L of the load current _{L available} at the output 150 of the electronic switch 100 of FIG. 3 begins at a value I _L1 that is less than the value \ _L2 . Shortly before a time X ₂ , which is later than the time ti, the curve 420 begins to sink below the limit Iu. At time t ₂ , the power loss of the transistor 120 of the electronic switch 100 in FIGS. 1 and 3 has fallen below the predetermined value, so that the maximum current intensity I _L2 at the output 150 is enabled. The internal resistance of the load L, however, is already so great that the load current profile 420 increases abruptly, but does not reach the value \ _L2 . As a result, the curve 420 decreases asymptotically from the time X ₂ to the time t ₃ , and from the time t ₃ corresponds to the value of the curve 410.

In diagram 400, only the load current curves 410 and 420 are plotted, but not the power loss of the transistor 120, whose falling below a predetermined threshold at time t ₂ full driving of the transistor 120, and thus releasing the maximum permissible current lι_ ₂ at the time X ₂ causes.

Claims

claims

1 . An electronic switch (100) for switching an electrical load (L), the switch (100) comprising:

- A control input (1 10) for receiving a control signal;

- A transistor (120) for controlling a through the load (L) flowing

Load current (I _L ) in response to the control signal;

- A limiter (130) for driving the transistor (120) such that the load current flowing through the load (L) (I _L ) is limited to a first positive maximum value;

marked by

- Power determining means (140) for determining a power loss of the transistor (1 10), wherein

- The limiter (130) is adapted to limit the load current (I _L ) to a second positive maximum value, which is smaller than the first maximum value, if the power loss exceeds a predetermined value.

2. Switch (100) according to claim 1, wherein the limiter (130) is adapted to the load current (I _L ) in response to a position of the power loss with respect to a plurality of different predetermined values on a

Limit many different maximum values.

A switch (100) according to any one of claims 1 or 2, wherein the power determining means (140) comprises means for detecting a voltage drop across the transistor (120).

4. Switch (100) according to one of claims 1 to 3, wherein the limiter (130) comprises at least one constant current source (Li, L ₂ ) for driving the transistor (120) with a constant control current.

5. Switch (100) according to claim 4, wherein the limiter is adapted to form the control current of the transistor of a plurality of constant current sources (I _L1 , L ₂ ) additively.

6. Switch (100) according to one of claims 1 to 5, further comprising a

Short-circuit monitor for outputting a short-circuit signal, if a voltage applied across the load (L) during a predetermined period of time after activation of the control signal does not exceed a predetermined voltage.

7. A switch (100) according to any one of claims 1 to 6, further comprising a locking device, which prevents driving of the transistor (120) if a voltage to be applied to the load (L) is less than a certain voltage.

8. Switch (100) according to one of claims 1 to 7, wherein the switch in the manner of a high-side switch (HSS) or a low-side switch (LSS) is constructed.

9. Integrated electronic circuit, which has an electronic switch

(100) according to any one of claims 1 to 9.

10. A method for switching an electrical load (L) by means of a transistor

(120) in the presence of a control signal, comprising the steps of:

- determining a power loss of the transistor (120);

- comparing the power loss with a predetermined threshold value;

- defining a through the load (L) flowing load current (I _L) if the power loss is at a first positive maximum value below the threshold value, and

otherwise limiting the load current (IL) flowing through the load (L) to a second positive maximum value that is less than the first maximum value.