Circuit for bridging LED strings

Description

[0001] The present invention relates to a circuit according to the preamble of claim 1.

[0002] Such a circuit is already known from DE 10 2015 204526 A1.

[0003] US 2018 / 338361 A1 concerns a lighting module.

[0004] Another circuit for switching on and off at least one semiconductor light source arranged within a series circuit of first light-emitting diodes represents internal prior art. In the known circuit, a first end of the series circuit is connected to a constant current source, and a second end of the series circuit is connected to a ground terminal. A field-effect transistor is electrically connected in parallel with the semiconductor light source. A source terminal of the field-effect transistor is connected to a first terminal of the semiconductor light source. A drain terminal of the field-effect transistor is connected to a second terminal of the semiconductor light source. A gate terminal of the field-effect transistor is connected to a source terminal of the field-effect transistor via a capacitor.

[0005] Furthermore, a circuit arrangement for electrically controlling a motor vehicle headlight is known from DE 10 2006 031 679 A1.

[0006] In the internal state-of-the-art circuit, the at least one semiconductor light source is, for example, a single LED or a chain, i.e., a series connection of several LEDs. Such series circuits are used, for example, as light sources in automotive lighting systems. These light sources are generally operated with a pulse-width modulated signal having a frequency of, for example, 100 Hz to 200 Hz. The light sources are switched on and off at this frequency, so that current phases and current pauses alternate. The human sense of sight perceives this as a medium brightness, with the light sources appearing brighter the longer the current phases and the shorter the current pauses.

[0007] The rapid switching on and off leads to steep edges in the current curve over time. This can result in the emission of interference, which can lead to a violation of electromagnetic compatibility (EMC) regulations. The capacitor CGS2 in the known circuit can be used to reduce the edge steepness. A disadvantage of this solution is that the adjustment range for this measure, i.e., the possible extent of the reduction in the edge steepness, is very limited.

[0008] Against this background, the object of the present invention is to provide a circuit of the type mentioned at the outset, which is characterized by an enlarged adjustment range and thus by improved electromagnetic compatibility.

[0009] This problem is solved by the features of claim 1.

[0010] The control current source facilitates the adjustability of the slopes and charges capacitor CGD1 with a constant current. This causes the gate-source voltage of the field-effect transistor to rise constantly. The parallel connection of the Ddischarge diode with resistor R4 allows for different discharging times compared to charging the MOSFET capacitors and external capacitors.

[0011] The additional capacitor CGD1 makes it possible to flatten the critical edge when turning on the MOSFET, which is associated with a high current peak, and reduce the height of the current peak. The Ddischarge diode and resistor R6 now make it possible to accelerate the non-critical edge when turning off the MOSFET, increasing the drain-source voltage change, thus reducing the power dissipation in the field-effect transistor and reducing the delay required to switch to the desired high-impedance state.

[0012] By reducing the discharge currents, the invention enables the use of larger output capacitors of a DC / DC converter as a control current source, which facilitates compliance with EMC directives.

[0013] The invention enables smaller peak currents when bridging the LED chains, thus lower EMC interference, and fewer maximum current exceedances for the non-bridgeable LEDs.

[0014] The invention makes it possible to bridge several LEDs at the same time, as maximum current values ​​are maintained.

[0015] The invention makes it possible to use Ddischarge and R4 to design the drain-source voltage change between drain and source and thus for the output capacitor of the DC / DC converter differently for the turn-off moment of the MOSFET than for the turn-on moment via R6.

[0016] This allows losses in the field-effect transistor to be reduced while maintaining peak current suppression during slow turn-on. The field-effect transistor can transition to the desired high-impedance state more quickly.

[0017] The invention is characterized in that a capacitor is connected in parallel to the working constant current source.

[0018] It is preferred if the field effect transistor is a MOSFET, in particular an n-channel MOSFET.

[0019] According to the invention, the semiconductor light source comprises an interconnection of several second light-emitting diodes.

[0020] According to the invention, the interconnection comprises a series connection of several second light-emitting diodes.

[0021] It is preferred that the first light-emitting diodes and the second light-emitting diodes are the same.

[0022] It is further preferred that the first light-emitting diodes be of a first type and the second light-emitting diodes be of a second type, wherein the two types differ from each other. The difference is that the two types emit different luminous fluxes.

[0023] A further preferred embodiment is characterized in that the first light-emitting diodes are configured to emit light of a first light color and the second light-emitting diodes are configured to emit light of a second light color that differs from the first light color.

[0024] A preferred use of the circuit is for switching on and off light-emitting diodes in lighting systems of motor vehicles.

[0025] Further advantages will become apparent from the following description, the drawings, and the dependent claims. It is understood that the features mentioned above and those to be explained below can be used not only in the respective combinations specified, but also in other combinations or on their own, without departing from the scope of the present invention.

[0026] Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.

[0027] Showing, in schematic form: Figure 1a circuit diagram of the circuit, which represents the internal state of the art; Figure 2aLoops of the gate-source voltage over time in the circuit according to Figure 1 ; Figure 2b shows the current flow through LED D7 of the Figure 1 ; Figure 2cCurrent curves through the semiconductor light source; Figure 2dCurrent curves through the LED D13 of the Figure 1; Figure 2eCurves of the drain-source voltage of the in the circuit according to Figure 1 used MOSFETs; Figure 3 shows an embodiment of the circuit according to the invention; Figure 4 shows a current-voltage characteristic field of a MOSFET; Figure 5a shows the gate-source voltage curves in the circuit according to Figure 1 over time; Figure 5bCurves of the current through the LED D7 of the circuit according to Figure 1 ; Figure 5cCurves of the current through the semiconductor light source of the circuit according to Figure 1 ; Figure 5dCurves of the current through the LED D13 of the circuit according to Figure 1 ; Figure 5eCurves of the drain-source voltage of the in the circuit according to Figure 3 used MOSFETs; and Figure 6 shows known relationships between some field-effect transistor parameters when turning on a MOSFET.

[0028] In detail, the Figure 1a circuit 10 for switching on and off at least one semiconductor light source 12, which is arranged within a series circuit of light-emitting diodes. The semiconductor light source 12 here is a chain of LEDs D8, D9, D10, D11, D12, which in turn are connected in series. The chain forms the aforementioned series circuit with the outer LEDs D7 and D13. A first end 14 of the series circuit is connected to an operating constant current source 16, and a second end 18 of the series circuit is connected to a ground terminal 20. The operating constant current source 16 is preferably a DC / DC converter. An output capacitor C4, used for smoothing, is connected in parallel with the operating constant current source 16. A field-effect transistor 22 is electrically connected in parallel with the semiconductor light source 12.A source terminal Source 2 of the field-effect transistor 22 is connected to a first terminal 24 of the semiconductor light source 12, and a drain terminal Drain 2 of the field-effect transistor 22 is connected to a second terminal 26 of the semiconductor light source 12. A gate terminal Gate2 of the field-effect transistor 22 is connected via a capacitor CGS2 to the source terminal Source 2 of the field-effect transistor 22 and via a resistor R2 to a control voltage source V1. The control voltage source outputs a control voltage uin.

[0029] The switching speed with which the LEDs D8 to D12 of the semiconductor light source 12 are bridged depends on various field effect transistor-typical parameters, such as its total gate charge, its rise time and its full time.

[0030] Because, as in the example of Figure 1, several LEDs D8 to D12 have to be bridged at the same time, the ratio of the drain-source voltage change Δu to the change time Δt is very large, which in turn leads to a current peak in the non-switched LEDs D7 and D13 in Figure 1 This current spike can cause total LED failure or pre-damage.

[0031] In principle, the current peak can be reduced by selectively controlling the drain-source voltage of field-effect transistor 22, thereby reducing its switching speed. To reduce the switching speed of field-effect transistor 22, a capacitor CGS2 is installed between gate 2 and source 2, for example, in an n-channel field-effect transistor 22. A disadvantage of this solution is that the adjustment range is very limited.

[0032] The Figures 2a to 2eshow qualitative curves of various quantities as they occur during operation of the circuit 10, each for three different values ​​of the capacitor CG2. For each of the Figures 2a to 2e It is true that increasing the capacitance of the capacitor CGS2 reduces the steepness of the curves.

[0033] Figure 2a shows the gate-source voltage over time. Figure 2b shows the current flow through LED D7. Figure 2c shows the current flow through the LED D8, i.e. through the semiconductor light source 12, and Figure 2d shows the current flow through LED D13. Figure 2e shows the drain-source voltage curves of the field-effect transistor 22.

[0034] As can be seen, the curves in each sub-figure are still relatively close to each other, which represents a relatively small adjustment range when changing the capacitance of the capacitor CGS2.

[0035] In principle, capacitor CGS2 has the following technical effect: At the moment of switching, in which LEDs D8 to D12 are bridged, the energy stored in the output capacitor C4 of the operating constant current source 16 is discharged. A change in the drain-source voltage of field-effect transistor 22 occurs at capacitor CGS2 within a specific time. The current is limited by the output resistance of field-effect transistor 22. The slower the output resistance decreases, i.e., the longer it remains high-ohmic, the smaller the current peak.

[0036] With the additional capacitance CGS2, the increase in the output resistance of MOSFET 22 can be slowed, as the gate-source voltage increases more slowly due to the external capacitor CGS2, and thus the LED peak current reaches its peak value later. The maximum value of the LED current is always lower with the external gate-source capacitance CGS2 than without it, which slows down the switching process as desired. As already mentioned, a disadvantage of this solution is that the adjustment range for this measure, i.e., the possible extent of the reduction in the edge steepness, is very limited depending on the capacitance of the capacitor CGS2.

[0037] Figure 3 shows an embodiment of the circuit 100 according to the invention with which the adjustment range of the switching speed of the MOSFET 22 is extended. In detail, the Figure 3 , as well as the Figure 1, a semiconductor light source 12 consisting of an LED chain arranged within a series circuit of light-emitting diodes. A first end SEG3 of the series circuit is connected to an operating constant current source 16, and a second end 18 of the series circuit is connected to a ground terminal 20. A field-effect transistor 22 is electrically connected in parallel to the semiconductor light source 12, with a source terminal Source1 of the field-effect transistor 22 connected to a first terminal SEG1 of the semiconductor light source 12, a drain terminal Drain1 of the field-effect transistor 22 connected to a second terminal SEG2 of the semiconductor light source 12, and a gate terminal Gate1 of the field-effect transistor 22 connected to a control current source I2 and, via a capacitor CGS1, to the source terminal Source1 of the field-effect transistor 22.

[0038] The circuit 100 after the Figure 3differs from circuit 10 according to the Figure 1 in that the drain terminal Drain1 of the field-effect transistor 22 in the circuit 100 is connected to the gate terminal Gate1 of the field-effect transistor 22 via a capacitor CGD1, and in that the control current source I2 is connected to the gate terminal Gate1 of the field-effect transistor 22 via a parallel circuit comprising a diode Ddischarge and a resistor R6, and is connected to the source terminal Source1 of the field-effect transistor 22 via a resistor R4. The control current source I2 is preferably a constant current source.

[0039] Fig. 4shows a current (Ids) - voltage (Vds) characteristic diagram of a MOSFET. The characteristic diagram has an ohmic region 30 and a saturation region 32. In an n-channel MOSFET as a field-effect transistor 22, the change in the voltage between gate and source is slowed by the additional capacitor CGD1 during the switching process. Thus, the field-effect transistor requires more time to transition from the ohmic region 30 of its current-voltage characteristics to the saturated region 32. The longer the field-effect transistor 22 remains in the ohmic region 30 of its current-voltage characteristics during the switching process, the more electrical energy stored in the output capacitor C4 is converted into heat in the field-effect transistor 22. This results in the desired effect that the current flowing through the field effect transistor 22 and the LEDs D0 and D6 does not reach quite as high values ​​as would be the case without the additional capacitor CGD1.

[0040] The additional capacitor CGD1 according to the invention is electrically connected between the gate Gate1 and the drain Drain1 of the field effect transistor 22.

[0041] The Figures 5a to 5e qualitatively show the curves of various quantities as they occur during operation of the circuit, each for three different values ​​of capacitor CG2. For each of the figures, increasing the capacitance of capacitor CGS2 reduces the steepness of the curves.

[0042] Figure 5a shows the gate-source voltage V(Gate1, SEG1) over time. Figure 5b shows the current flow through LED D0. Figure 5c shows the current flow through the LED D1, i.e. through the semiconductor light source 12 of the circuit 100, and Figure 2d shows the current flow through LED D6. Figure 2e shows the drain-source voltage V(Drain1, SEG1).

[0043] As can be seen, the curves are less close to each other in terms of time than the corresponding curves of the Figure 2 , which result without the capacitor CGD1. This stretching of the time courses of the Figure 5 compared to the temporal progression of the Figure 2 illustrates the desired enlargement of the adjustment range, which is achieved when using the circuit 100 according to the invention instead of the circuit 10 according to the Figure 1 results.

[0044] Figure 6 shows known relationships between some field-effect transistor parameters when turning on a field-effect transistor. V stands for voltage, I for current, Q for charge, D for drain, S for source, G for gate, pl for plateau, th for threshold, tot for total, and T for time.

[0045] During the time period T1 - T0, the gate-source capacitance of field-effect transistor 22 is charged to the breakdown voltage V GS(th). The drain-source voltage V DS and the drain current ID remain unchanged. During the time period T2 - T1, the gate voltage V GS continues to rise until it reaches the plateau. The drain current ID increases to its maximum value. During the time period T3 - T2, the gate-source voltage remains at the Miller plateau V GS(pl).

[0046] As in Figure 5a V(Gate1, SEG1) can be seen, the curve most to the right increases more slowly than the curve most to the left and the middle curve. The curve most to the right results from the largest CGD1. This measure ensures that not only the rise of the breakdown voltage V GS(th) to the plateau voltage V GS(pl) is delayed (see Figure 6), but rather that the lengthening of the Miller plateau (V GS(pl)) prevents the drain-source voltage (V DS) from dropping as quickly. This reduces the voltage between drain (Drain1) and source (Source1) over a defined time. As a result, the energy stored in capacitor C2, which is connected in parallel with the operating constant current source 16, is converted into heat in the field-effect transistor 22 over an extended period. The current rise is therefore reduced. The area under the current curve remains constant.

[0047] The use of a constant current source as the control current source I2 promotes the broadening of the adjustability and charges the capacitor CGD1 with a constant current. This causes the voltage V GS to rise constantly. The parallel circuit of the diode Ddischarge with the resistor R4 (see Figure 6) allows a different discharge time compared to the charging of the capacitances of the field effect transistor 22 and the external capacitances CGD1 and CGS1, which takes place via the ohmic resistor R6.

[0048] The additional capacitor CGD1 makes it possible to make the critical edge when turning on the field-effect transistor 22, which is associated with a high current peak, less steep and to reduce the height of the current peak. A less steep MOSFET turn-on edge means that the ratio Δu to Δt (drain-source voltage change to Δt) is reduced. As a result, the field-effect transistor 22 operates longer in the ohmic region 30 of its characteristic field, thereby reducing the height of the current peak as desired. The diode Ddischarge and the resistor R6 now make it possible to accelerate the non-critical edge when turning off the MOSFET and thus increase the drain-source voltage change. This reduces the power loss in the field-effect transistor 22 and decreases the delay with which the field-effect transistor 22 switches to the desired high-impedance state.

[0049] One element of the invention is the constant current source I3 for constantly charging the capacitances of the field-effect transistor 22 and the external capacitances. By reducing the discharge currents, the invention enables the use of larger output capacitors (C2, C4) of the DC / DC converter serving as the operating constant current source 16. This allows for better compliance with EMC regulations.

[0050] The invention also enables smaller peak currents at the moment the LED chains are bridged, thus lower EMC interference, and fewer maximum current exceedances for the non-bridgeable LEDs.

[0051] The invention makes it possible to bridge several LEDs at the same time, as maximum current values ​​are maintained.

[0052] The invention provides the possibility of using Ddischarge and R4 to design the drain-source voltage change between drain and source and thus for the output capacitor C4 of the working constant current source 16 differently for the turn-off process of the field effect transistor than for the turn-on process in which a charging current flows through the ohmic resistor R6.

[0053] This allows the losses in field-effect transistor 22 to be reduced during slow turn-on while maintaining current peak suppression. Field-effect transistor 22 can transition to the desired high-impedance state more quickly.

[0054] During the turn-on process, the control constant current source I2 charges the capacitances of the field-effect transistor 22 and also the external capacitors CGS1 and CGD1 via the ohmic resistor R6. The larger the value of CGD1, the longer it takes until the necessary gate-source voltage for the conducting state is built up. See the Figure 5a the curve furthest to the right (largest CGD1) compared to the middle curve (medium CGD1) and the leftmost curve (smallest CGD1). Due to the slower rise of the gate-source voltage, the output resistance also increases at a slower rate and limits the current. The voltage from drain to source decreases at a slower rate. See the curve v(Drain-SEG1) in Figure 5e The discharge of capacitor C2 becomes increasingly slower with increasing CGD1 and increasing CGS2.

[0055] The field effect transistor 22 is switched off by discharging or recharging the internal capacitances through the diode Ddischarge and the resistor R4.

Claims

1. Circuit (100) for switching on and off at least one semiconductor light source (12) arranged within a series circuit of first light-emitting diodes (D0, D6), comprising a working constant current source (I2) a ground terminal (20), a field effect transistor (22), the at least one semiconductor light source (12), the series circuit of first light-emitting diodes (D0, D6), a capacitor (CGD1) a diode (Ddischarge), a resistor (R5), a control current source (I3) a further resistor (R4) a further capacitor (CGS1) a third capacitor (C2), a first end (SEG3) of the series circuit being connected to the working constant current source (I2) and a second end of the series circuit being connected to the ground terminal (20), and the field effect transistor (22) being electrically connected in parallel to the at least one semiconductor light source (12), a source terminal (Source1) of the field effect transistor (22) being connected to a first terminal (SEG1) of the at least one semiconductor light source (12), a drain terminal (Drain1) of the field effect transistor (22) being connected to a second terminal (SEG2) of the at least one semiconductor light source (12), the drain terminal (Drain1) of the field effect transistor (22) being connected via the capacitor (CGD1) to a gate terminal (Gate1) of the field effect transistor (22), the diode (Ddischarge) and the resistor (R5) being connected in parallel in a parallel circuit, and the control current source (13) being connected via the parallel circuit to the gate terminal (Gate1) of the field effect transistor (22) and via the further resistor (R4) to the source terminal (Source1) of the field effect transistor (22), the at least one semiconductor light source (12) comprising an interconnection of a plurality of second light-emitting diodes (D1, D2, D3, D4) connected in series, characterized in that the gate terminal (Gate1) of the field effect transistor (22) is connected to the source terminal (Source1) of the field effect transistor (22) via the further capacitor (CGS1), the third capacitor (C2) being connected in parallel to the working constant current source (16).

2. Circuit (100) according to claim 1, wherein the control current source (I2) is a constant current source.

3. Circuit (100) according to either of the preceding claims, wherein the field effect transistor (22) is a MOSFET.

4. Circuit (100) according to any of claims 1 to 3, wherein the first light-emitting diodes (D0, D6) and the second light-emitting diodes (D1, D2, D3, D4, D5) are the same.

5. Circuit (100) according to any of the preceding claims, wherein the first light-emitting diodes (D0, D6) are of a first type and the second light-emitting diodes (D1, D2, D3, D4, D5) are of a second type, wherein the two types differ from one another in that the two types emit different luminous fluxes.

6. Circuit (100) according to claim 5, wherein the first light-emitting diodes (D0, D6) are configured to emit light of a first light color and the second light-emitting diodes (D1, D2, D3, D4, D5) are configured to emit light of a second light color that is different from the first light color.

7. Use of a circuit (100) according to claim 1 for switching on and off light-emitting diodes in lighting devices of motor vehicles.