DC / DC voltage converter including safety equipment

Abstract

A DC / DC converter is disclosed, comprising: a. a main input terminal, connected to a first DC voltage source, b. a main output terminal, connected to a second DC voltage source, the voltage provided by the second voltage source being lower than the voltage provided by the first power source, c. a conversion unit or a plurality of conversion units placed in parallel, each unit comprising: i. a step-down chopper, comprising an input terminal and an output terminal, the input terminal of the step-down chopper being connected to the main input terminal, ii. a safety device comprising an input terminal and an output terminal, the safety device comprising a first transistor and a second transistor, each of the transistors comprising a current input terminal, a current output terminal and a control terminal, the input terminal and the output terminal of the first transistor and the second transistor being connected to the main input terminal. The output terminals are respectively connected to the input terminal and the output terminal of the safety device, the input terminal of the safety device is connected to the output terminal of the step-down chopper, and the output terminal of the safety device is connected to the main output terminal, the safety device also includes an input for receiving an inhibition command and is designed to turn on at least one of the first transistor and the second transistor when the inhibition command is received; the DC / DC converter is characterized in that it also includes a monitoring device, which is designed to generate an inhibition command for the control device of a given conversion unit when the first transistor and the second transistor are in the on state for a given conversion unit and the voltage difference between the main output terminal and the input terminal of the safety device is lower than the detection threshold.

Description

Technical Field

[0001] The invention relates to the field of electrical converters, in particular DC / DC voltage converters used in the motor vehicle field. Background Art

[0002] For example, DC / DC voltage converters implemented in the field of motor vehicles can generate short circuits in the event of a failure of one of their power elements embedded on the electronic board, which generate currents with very high amplitudes (for example several hundred amperes). These short circuits can irreversibly damage the voltage converter itself, but also the equipment to which it is connected.

[0003] Therefore, it is necessary to detect this type of high-amplitude current and, if applicable, make the voltage converter safe. To this end, in the prior art, it is known as Figure 1 The technology of the DC / DC voltage converter shown. This type of voltage converter 100 includes:

[0004] a. a main input terminal HV, which is designed to be connected to a first DC voltage source 6, for example, to a 48V battery;

[0005] b. a main output terminal LV, which is designed to be connected to a second DC voltage source 7, for example, to a 12V battery, wherein the voltage provided by the second voltage source is lower than the voltage provided by the first voltage source;

[0006] c. A voltage conversion unit 5 or a plurality of conversion units placed in parallel, each of which comprises:

[0007] i. A step-down chopper 9, comprising an input terminal BE and an output terminal BS, wherein the input terminal of the step-down chopper is connected to the main input terminal;

[0008] ii. A safety device 2, comprising an input terminal D and an output terminal S, the safety device comprising a first T1 transistor and a second T2 transistor, each of the transistors comprising a current input terminal, a current output terminal and a control terminal, the input terminal or the output terminal of the first and second transistors being connected to the input and output terminals of the safety device respectively, the input terminal of the safety device being connected to the output terminal of the step-down chopper, the output terminal of the safety device being connected to the main output terminal, the safety device further comprising a control input G, the control input G being designed to receive an inhibition command, and being designed to turn on at least one of the first and second transistors upon receiving the inhibition command;

[0009] d. A monitoring device 80 designed to send an inhibit signal to the safety device when the current reaching the main output of the voltage converter exceeds a threshold value.

[0010] The monitoring device of the DC / DC converter 100 is usually designed to measure the current supplied by the first power source 7 via a measuring resistor Rs (“shunt resistor”) placed between the safety device and the second DC voltage source, and is designed to send an inhibition signal to the safety device in the event of a short circuit (when the voltage at the ends of the measuring resistor is greater than a threshold value).

[0011] Therefore, in a voltage converter according to the prior art:

[0012] a. When the main input and output terminals are connected to the first and second voltage sources respectively, the measuring resistor is used to detect whether there is a high current in the voltage converter, especially at its output terminal, and when a high current is detected, the safety device will be activated to isolate the main output terminal from other components of the voltage converter.

[0013] However, when the DC / DC voltage converter is operating normally, the use of a measuring resistor has the problem of Joule effect power consumption. Its implementation in the voltage converter also requires corresponding space on the electronic board. Summary of the invention

[0014] The object of the present invention is to at least partially eliminate the above-mentioned problems.

[0015] For this purpose, a DC / DC voltage converter of the type described above is proposed, characterized in that the monitoring device is designed to generate an inhibition command to the safety device of a given conversion unit, such as when, for a given conversion unit, the first transistor and the second transistor are in their conductive state and the voltage difference between the main output terminal and the input terminal of the safety device is below a detection threshold.

[0016] Therefore, it is no longer necessary to use a measuring resistor, which can eliminate Joule losses associated with the measuring resistor and reduce the size of the voltage converter.

[0017] According to a particular embodiment of the invention, the safety device is also designed to switch on the first and second transistors, such as upon receipt of said inhibit command.Thus, when the first and second transistors are switched on, the first and second voltage sources are insulated from each other.

[0018] According to a particular embodiment of the invention, at least one conversion unit of the DC voltage converter comprises a plurality of step-down choppers connected in parallel.

[0019] According to a particular embodiment of the invention, the monitoring device is only active when the first and second transistors of each cell are in their conducting state.

[0020] According to a particular embodiment of the invention, the monitoring device is also designed to generate an inhibition command to the safety device of each conversion unit.

[0021] According to a specific embodiment of the invention, the monitoring device comprises:

[0022] a. Input stage, which includes:

[0023] i. The first voltage divider bridge and the second voltage divider bridge;

[0024] ii. Current source or voltage source;

[0025] iii. as many diodes as there are conversion units, the anodes of the diodes of the input stage being connected to each other and to a current source or a voltage source, and each conversion unit being connected to the cathode of one of the diodes by means of the input terminal of its safety device;

[0026] iv. a voltage-shift diode having its cathode connected to the main output terminal and its anode connected to the voltage source or current source,

[0027] The voltage comparator has a non-inverting terminal connected to the anode of the diode of the input stage by means of a first voltage-dividing bridge, and an inverting terminal connected to the anode of the voltage-shifting diode by means of a second voltage-dividing bridge.

[0028] Optionally, the voltage converter further includes a third resistor connected between the non-inverting input terminal and the output of the voltage comparator.

[0029] Also optionally, the output of the voltage comparator is also connected to the cathode of a trigger diode, the anode of the trigger diode is connected to the control input of a safety device of the conversion unit, and each control input of the safety device is directly or via a resistor connected to the control terminal of the first transistor and / or the second transistor.

[0030] According to other embodiments, it is also conceivable that the voltage converter has a combination of some or all of the above-mentioned features. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A DC / DC voltage converter according to the prior art is shown.

[0032] Figure 2 A DC / DC voltage converter implementing the invention is shown according to a first embodiment of the invention.

[0033] Figure 3 A monitoring device for a DC / DC voltage converter of the present invention is shown according to a first embodiment of the present invention.

[0034] Figure 4 A DC / DC voltage converter implementing the invention is shown according to a second embodiment of the invention.

[0035] Figure 5 A monitoring device for a DC / DC voltage converter of the present invention is shown according to a second embodiment of the present invention. DETAILED DESCRIPTION

[0036] refer to Figure 2 , a DC / DC voltage converter 1 implementing the present invention will now be described. For example, the DC / DC voltage converter 1 is used in a motor vehicle. For simplicity, the reference numerals given in this figure are the same as those in the Figure 1 The common elements of the voltage converter according to the prior art are represented in FIG. 1 with the same reference numerals.

[0037] The DC / DC voltage converter 1 comprises:

[0038] a. a main input terminal HV, connected to a first DC voltage source 6, such as a 48V battery;

[0039] b. The main output terminal LV is connected to a second DC voltage source 7, such as a 12V battery, which provides a voltage lower than the voltage provided by the first voltage source 6;

[0040] c. Conversion unit 5, comprising:

[0041] i. an input terminal BE connected to the main input terminal HV;

[0042] ii. Output terminal BS;

[0043] iii. a step-down chopper 9, comprising an inductor L, a transistor T3, the drain of which is connected to the input terminal BE and the source of which is located at the first end of the inductor, and a transistor T4, the drain of which is connected between ground and the first end of the inductor L. The combination of the inductor L and the two transistors T3 and T4 forms a step-down chopper of the "Buck" type, the operation of which is well known to those skilled in the art;

[0044] iv. a safety device 2 comprising an input terminal D1 connected to the second end of the inductor, an output terminal S1 connected to the main output terminal LV, and a control terminal G1;

[0045] v. Monitoring equipment 8; and

[0046] vi. Control equipment (in Figure 2 not shown).

[0047] The safety device 2 comprises a first transistor T1 and a second transistor T2 connected end to end. In the example described here, the first transistor T1 and the second transistor T2 are N-type doped MOSFETs. The drain of the first transistor T1 is connected to the input terminal D1 of the safety device 2, and the drain of the second transistor T2 is connected to the output terminal S1 of the safety device 2. The first transistor T1 and the second transistor T2 are connected by means of their respective sources so that the intrinsic diodes of these transistors are connected through their anodes. In other words, the cathode of the intrinsic diode of the second transistor T2 is connected to the main output terminal of the voltage converter 1, while the cathode of the first transistor T1 is connected to the input terminal of the safety device 2. In this way, when the first transistor T1 and the first transistor T2 are turned on, no current flows between the input terminal D1 and the output terminal S1 of the safety device 2. The control terminals of the first transistor T1 and the second transistor T2 are also connected to the control terminal G1 of the safety device 2. Therefore, receiving an inhibition command at the control terminal G1 of the safety device 2 can trigger the opening of the transistors T1 and T2, thereby interrupting any circulating current in both directions between the input terminal D1 and the output terminal S1 of the safety device 2.

[0048] The control device makes it possible to command the opening or closing of transistors T3 and T4 so as to cause the step-down chopper to switch. In addition, the control device can also control the opening or closing of transistors T1 and T2 of the safety device 2. To this end, the control device supplies the control terminals of transistors T1, T2, T3 and T4 with the voltages required to activate these transistors respectively.

[0049] The monitoring device 8 is also designed to generate an inhibition command for the safety device 2 to the conversion unit 5 when the first transistor T1 and the second transistor T2 are in their conducting state and when the voltage difference between the input terminal D1 of the safety device 2 and the main output terminal LV is lower than the detection threshold Vseuil.

[0050] In addition, the threshold voltage Vseuil is equal to the voltage difference Vb-Va, which is equal to the sum of the source-drain resistances of the transistors T1 and T2 in the on state multiplied by the current flowing through these transistors, as shown in equation [Math. 1].

[0051] [Mathematical formula 1]

[0052] Vseuil=(Rds T1 +Rds T2 ).I

[0053] In the example described here, the threshold voltage Vs is equal to -0.3V.

[0054] Monitoring equipment 8 Figure 3 and includes:

[0055] a. A measuring circuit 3, comprising:

[0056] i. The voltage comparator 12 includes an inverting terminal, a non-inverting terminal and an output terminal. When the voltage on the positive inverting terminal is greater than the voltage on the non-inverting terminal of the voltage comparator 12, its output potential VS is positive. In the opposite case, its output potential VS is negative.

[0057] ii. An input stage, which comprises a current source SC1 supplied by a voltage Vin and which is connected in series with an activation switch Ta. The current source SC1 provides a first and a second electrical branch. The activation switch Ta has a control terminal connected to, for example, a microcontroller. Thus, by means of the activation switch Ta, the monitoring device 8 can be activated (switch Ta closed) or deactivated (switch Ta open) depending on the microcontroller. Thus, the monitoring device is only activated when current actually passes through the safety device 2. In the description hereinafter, it is assumed that the monitoring device 8 is activated. The first electrical branch comprises a diode D3, the cathode of which is connected to the main output terminal LV, and the second electrical branch comprises a diode D2 having the same threshold voltage as the diode D3, the cathode of which is connected to the input terminal D1 of the safety device 2;

[0058] iii. The first voltage divider bridge is formed by resistor R3 and resistor R1, which are connected to the non-inverting terminal of the voltage comparator 12. Resistor R3 is connected between the non-inverting terminal and the anode of diode D2, and resistor R1 is connected between the non-inverting terminal and electrical ground.

[0059] iv. A second voltage divider bridge of resistors R5 and R6 is connected to the inverting terminal of the voltage comparator 12. The resistor R5 is connected between the inverting terminal of the voltage comparator 12 and the anode of the diode D3, and the resistor R6 is connected between the inverting terminal and electrical ground. The value of the resistor R5 is equal to the value of the resistor R3, and the value of the resistor R6 is equal to the value of the resistor R1.

[0060] b. A control circuit 4, comprising:

[0061] i. A diode D5 , whose anode is connected to the control terminal G1 of the safety device and whose cathode is connected to the output Vs of the voltage comparator 12 .

[0062] The association of resistors R1, R3, R4, R5 and R6 with the voltage comparator 12 compares the voltages present at the anodes of diodes D3 and D2. The voltage V2 present at the anode of diode D3 is equal to the threshold voltage of the voltage shift diode D3 present at the main output. The voltage V1 present at the anode of diode D2 is equal to the threshold voltage of the voltage shift diode D2 present at the input of the safety device 2. In other words, the voltage comparator 12 compares the voltage at the input of the safety device 2 with the voltage at the main output within the threshold voltage range of diodes D3 and D2.

[0063] refer to Figure 3 , and conforming to equations [Math. 2] and [Math. 3], the voltages of the non-inverting terminal V+ and the inverting terminal V- are expressed according to the voltages V1, V2 and the output voltage Vs of the voltage comparator 12.

[0064] [Mathematical formula 2]

[0065]

[0066] Consider that resistor R3 is equal to resistor R5 and resistor R1 is equal to resistor R6.

[0067] [Mathematical formula 3]

[0068]

[0069] According to equation [Math. 2], when the resistor R4 is selected to be significantly larger than the resistor R3, for example, ten times larger, equation [Math. 2] is equivalent to equation [Math. 4].

[0070] [Formula 4]

[0071]

[0072] When the voltage V+ becomes lower than the voltage V-, the output voltage of the voltage comparator 12 switches from +Vsat to -Vsat. In other words, the switching occurs when the condition of equation [Math. 5] applies.

[0073] [Formula 5]

[0074]

[0075] According to equation [Math 5], when voltage V1-V2 becomes lower than switching threshold SB (provided by equation [Math 6]), the output voltage VS of the voltage comparator changes from its positive saturation value +Vsat to its negative saturation value -Vsat.

[0076] [Mathematical formula 6]

[0077]

[0078] By appropriately selecting the values ​​of the resistors R1, R3, R4, R5 (equal to R3) and R6 (equal to R1) according to the saturation voltage Vsat, a switching threshold SB equal to the voltage Vseuil can be obtained.

[0079] Therefore, when the voltage V1-V2 is lower than the voltage Vseuil, the output voltage of the voltage comparator 12 becomes its negative saturation value -Vsat.

[0080] Thus, during a fault in the DC / DC converter 1 , for example when the transistor T4 is short-circuited, the current flowing through the two transistors T1 and T2 increases until a potential drop occurs at the input of the safety device, which is detected by switching of the voltage comparator 12 .

[0081] When the voltage Vs is equal to the negative saturation value -Vsat, the potential -Vsat is applied to the control terminal G1.

[0082] When the voltage Vs is equal to the negative voltage -Vsat, the diode D5 can cancel / short-circuit the voltage provided to the control terminals of the switch tubes T1 and T2 by the control device, so that the switch tubes are turned on.

[0083] In other words, when the output voltage Vs is equal to the negative saturation value -Vsat, the safety device 2 receives a suppression command at its control terminal G1.

[0084] According to a variation of the first embodiment, the DC / DC converter 1 has a plurality of step-down choppers 9, each step-down chopper having an input terminal BE and an output terminal BS, and the plurality of step-down choppers are connected in parallel, the terminals BE are connected to each other and the terminals BS are connected to each other. The plurality of step-down choppers are connected in parallel to convert a larger current.

[0085] Now refer to Figure 4 and Figure 5 A second embodiment of the present invention is described. Figure 2 and Figure 3 The same elements as in the first embodiment retain the same reference numerals and will not be described repeatedly.

[0086] In this second embodiment, Figure 4 The DC / DC voltage converter 1 ′ represented in EMBODIMENT 1 comprises a second conversion unit 5 ′ connected in parallel with the first conversion unit 5 .

[0087] The conversion unit 5' comprises:

[0088] a. Input terminal BE ', which is connected to the main input terminal HV;

[0089] b. a step-down chopper 9' comprising an inductor L', a transistor T3', the drain of which is connected to the input terminal BE', the source of which is located at the first end of the inductor, and a transistor T4', the drain of which is connected between ground and the first end of the inductor L'. The combination of the inductor L' and the two transistors T3' and T4' provides a step-down chopper of the "step-down" type, the operation of which is well known to those skilled in the art;

[0090] c. A safety device 2', comprising an input terminal D1' connected to the second end of the inductor, an output terminal S1' connected to the main output terminal LV, and a control terminal G1'.

[0091] The DC / DC voltage converter 1 ' further comprises a monitoring device 8' and a device (not shown) for controlling the transistors of the DC / DC voltage converter 1 '.

[0092] The safety device 2' comprises a first transistor T1' and a second transistor T2' connected end to end. In the example described here, the first transistor T1' and the second transistor T2' are N-type doped MOSFETs. The drain of the first transistor T1' is connected to the input of the safety device 2', and the drain of the second transistor T2' is connected to the output of the safety device 2. The first transistor T1' and the second transistor T2' are connected by their respective sources so that the intrinsic diodes of these transistors are connected by their anodes. In other words, the cathode of the intrinsic diode of the second transistor T2' is connected to the main output of the voltage converter 1', and the cathode of the first transistor T1' is connected to the input of the safety device 2'. In this way, when the first transistor T1' and the first transistor T2' are turned on, no current flows between the input terminal D1' and the output terminal S1' of the safety device 2'. The control terminals of the first transistor T1' and the second transistor T2' are also connected to the control terminal G1' of the safety device 2'. Therefore, receiving an inhibition command at the control terminal G1' of the safety device 2 enables the transistors T1' and T2' to be triggered to turn on.

[0093] Monitoring equipment such as Figure 5 As shown, it includes a measuring circuit 3' and a control circuit 4'. When the first transistor T1, T1' and the second transistor T2, T2' are in the on state, and when the voltage difference between the input terminal D1' of the safety device 2' and the main output terminal LV and the voltage difference between the input terminal D1 of the safety device 2 and the main output terminal LV are lower than the detection threshold Vseuil, the monitoring device is designed to generate an inhibition command to the safety device 2' and to the safety device 2.

[0094] In this second embodiment, the monitoring device 8' differs from the monitoring device 8 in that:

[0095] a. The measuring circuit 3' further comprises a third electrical branch, the third electrical branch comprising a diode D4, the cathode of the diode D4 being connected to the input terminal D1' of the safety device 2', and the anode of the diode D4 being connected to the anode of the diode D2. In other words, the diodes D2 and D4 form a diode AND. In addition, the threshold voltages of the diodes D2 and D4 are the same;

[0096] b. The control circuit 4 ′ further comprises a diode D6 , an anode of the diode D6 is connected to the control terminal of the safety device 2 ′, and a cathode of the diode D6 is connected to the output terminal Vs of the voltage comparator 12 .

[0097] The voltage V1 present at the anodes of the diodes D2 and D4 is equal to the minimum voltage among the threshold voltages of the voltage-shift diodes D2 and D4 present at the input terminals D1 and D1'. The voltage V2 present at the anode of the diode D3 is equal to the threshold voltage of the voltage-shift transistor D3 present at the main offset terminal. In other words, within the threshold voltages of the diodes D3, D4, and D2, the voltage comparator 12 compares the minimum voltage present at the input terminals D1 and D1' with the voltage present at the main output terminal.

[0098] Therefore, according to equation [Math. 6], when the voltage V1-V2 is lower than the switching threshold SB, the output voltage VS of the voltage comparator changes from the positive saturation value +Vsat to the negative saturation value -Vsat.

[0099] When the voltage Vs is equal to the negative voltage -Vsat, the diode D5 can eliminate / short-circuit the voltage provided to the control terminals of the switch tubes T1 and T2 by the control device, so that the switch tubes are turned on. Similarly, when the voltage Vs is equal to the negative voltage -Vsat, the diode D6 enables the voltage provided to the control terminals of the switch tubes T1' and T2' to be canceled / short-circuited by the control device, so that the switch tubes are turned on.

[0100] In other words, when the voltage V1-V2 becomes lower than the switching threshold SB, the output voltage Vs is equal to -Vsat, which results in the opening of the transistors T1, T1', T2 and T2'.

[0101] Therefore, during a fault in the DC / DC converter 1', for example when the transistor T4' is short-circuited, the current flowing through the two transistors T1' and T2' increases until a potential drop occurs at the input of the safety device 2', which is detected by the switching of the voltage comparator 12, which causes the transistors T1, T1', T2 and T2' of the safety devices 2 and 2' to open and make the DC / DC voltage converter 1' safe.

[0102] In addition and optionally, the control device can scan the voltage at the output of the comparator 12, and when the voltage is equal to -Vsat, the control device turns on the switches T3, T3', T4 and T4', thereby putting the DC / DC voltage converter 1' into a completely safe state.

Claims

1. A DC / DC converter, comprising: a. a main input terminal (HV) which is designed to be connected to a first DC voltage source (6); b. a main output terminal (LV) designed to be connected to a second DC voltage source (7), the voltage provided by the second DC voltage source (7) being lower than the voltage provided by the first DC voltage source (6); c. A conversion unit (5) or a plurality of conversion units (5, 5') placed in parallel, each unit (5) comprising: i. a step-down chopper (9), comprising an input terminal (BE) and an output terminal (BS), wherein the input terminal of the step-down chopper (9) is connected to the main input terminal; ii. A safety device (2) comprising an input terminal (D1) and an output terminal (S1), wherein the safety device (2) comprises a first transistor (T1) and a second transistor (T2), each of the transistors (T1, T2) comprising a current input terminal, a current output terminal and a control terminal (G), The input end and the output end of the first transistor (T1) and the second transistor (T2) are respectively connected to the input end (D1) and the output end (S1) of the safety device (2), and the input end (D1) of the safety device (2) is connected to the output end (BS) of the step-down chopper (9). The output terminal (S1) of the safety device (2) is connected to the main output terminal (LV), and the safety device (2) also includes a control input (G1) which is designed to receive an inhibition command and is designed to open at least one of the first transistor (T1) and the second transistor (T2) when the inhibition command is received; The DC / DC converter is characterized in that it also includes a monitoring device (8), which is designed to generate an inhibition command to the control device (2) of the given conversion unit (5), such as when the first transistor (T1) and the second transistor (T2) for a given conversion unit (5) are in their conductive state and the voltage difference between the main output terminal (LV) and the input terminal (D1) of the safety device (2) is lower than a detection threshold.

2. The DC / DC converter according to claim 1, wherein: At least one conversion unit (5) comprises a plurality of step-down choppers connected in parallel.

3. A DC / DC converter according to any one of the preceding claims, wherein: The monitoring device (8) is only active when the first transistor (T1) and the second transistor (T2) of each cell are in their conducting state.

4. A DC / DC converter according to any one of the preceding claims, wherein: The monitoring device (8) is also designed to generate a suppression command to the safety device (2, 2') of each conversion unit (5, 5').

5. A DC / DC converter according to any one of the preceding claims, wherein: The monitoring device (8) comprises: a. Input stage, which includes: i. The first voltage divider bridge and the second voltage divider bridge; ii. Current source or voltage source; iii. as many diodes (D2, D4) as the conversion units (5, 5'), the anodes of the diodes of the input stage being connected to each other and to the current source or the voltage source, Each of the conversion units (5, 5') uses the input terminals (D1, D2) of its safety device (2, 2') to D1') is connected to the cathode of one of the diodes (D2, D4); iv. a voltage shift diode (D3) having its cathode connected to the main output terminal (LV), Its anode is connected to the voltage source or the current source, b. A voltage comparator (12), whose non-inverting terminal is connected to the anode of the diodes (D2, D4) of the input stage by means of the first voltage-dividing bridge, and whose inverting terminal is connected to the anode of the voltage-shifting diode (D3) by means of the second voltage-dividing bridge.

6. The DC / DC converter according to claim 5, further comprising a third resistor (R4), wherein the third resistor (R4) is connected between the non-inverting input terminal and the output of the voltage comparator.

7. The DC / DC converter according to claim 5 or 6, wherein: The output end of the voltage comparator is also connected to the cathode of a trigger diode (D5, D6), the anode of which is connected to a control input (G1, G1') of a safety device (2, 2') of the conversion unit (5, 5'), and each of the control inputs (G1, G1') of the safety device (2, 2') is directly or via a resistor connected to a control end (G) of a first transistor (T1) and / or a second transistor (T2).

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