Driver circuit for supplying constant current to LED arrangement

Abstract

The invention describes a driver circuit for supplying a constant current to an LED arrangement (LED1... LEDN) formed by a buck converter comprising a current measurement resistor (Rsense) in series with a coil (Lbk) that measures the current through the coil (Lbk). A two-point control circuit turns on or off the first switching element (Qh) as a function of a measured current (Vsense) flowing through the coil (Lbk) in order to keep the average current flowing through the LED arrangement (LED1... LEDN) constant. A current amplifier circuit (A sense) is connected to a current measurement resistor (R sense) to detect a current (V sense) flowing through the current measurement resistor, and positive and negative currents can be measured by superimposing an offset voltage (V offset) applied to one of its inputs by an offset voltage source. An output terminal of the current amplifier circuit is connected to an input terminal of the two-point control circuit. The offset voltage (V offset) is also superimposed on its threshold voltage (V peak, V valley). Furthermore, a start-up pulse circuit (Vstart-up) is provided, which is connected to the control input of the first switching element (Qhs) via a diode (Dstart-up).

Description

[0001] The present invention relates to a driver circuit for supplying a constant current to an LED arrangement. The driver circuit is formed by a buck converter which is formed by a first switching element connected between a supply voltage and an output capacitor and a coil connected in series therewith, wherein the connection point of the first switching element and the coil and the reference potential point forming the connection point of the supply voltage and the output capacitor are connected via a rectifying element, and a current measuring resistor for measuring the current flowing through the coil is arranged in series with the coil. The driver circuit further comprises a two-point control circuit which switches the first switching element on or off according to the current flowing through the coil measured by means of the current measuring resistor in order to keep the average current flowing through the LED arrangement constant, wherein the two-point control circuit is formed by a window comparator, and a first threshold voltage source and a second threshold voltage source supply a high threshold voltage and a low threshold voltage to the window comparator, and these threshold voltages determine the values of the current flowing through the coil at which reaching them causes the first switching element to be switched on and off again, wherein a bistable trigger circuit is connected downstream of the window comparator, and a first output terminal of the bistable trigger circuit having a signal level indicating reaching the low threshold is connected to the control terminal of the first switching element.

[0002] Such a driver circuit is known from DE 10 2020 205 960 B4 and DE 10 2017 214 056 A1 and is shown in Figure 1 in a sketch.

[0003] LEDs (light-emitting diodes), usually in the form of an LED arrangement, for example as a series circuit of a plurality of light-emitting diodes, are also increasingly being used for lighting purposes in motor vehicles due to their efficiency, compact and robust design and their durability. LEDs are always operated by a constant current. For this purpose, a suitable power supply is usually implemented as a buck converter (step-down converter) with a current control loop.

[0004] Common concepts for the operation of an LED voltage converter are

[0005] · with fixed frequency control

[0006] · hysteresis control (two-point controller)

[0007] · with constant on / off time control.

[0008] Hysteresis controllers are particularly popular because they are easy and inexpensive to implement and have a high control bandwidth, which is advantageous in the case where the LEDs are operated with PWM (i.e., forced load jumps are generated) or the supply voltage varies greatly (as is customary in the automotive industry, for example).

[0009] In Figure 1schematically shows a typical LED driver in a buck topology. Transistors Qhs, Qls together with coil Lbk and capacitor C 入 and C 出 form a classic buck topology. The control block detects the current flowing through coil Lbk. This then generates PWM signals gh, gl such that the light-emitting diodes LED1...LEDN in the light-emitting diode arrangement are driven by a predetermined constant current.

[0010] In the Figure 1 circuit, the signal at current measurement resistor R 感测 is provided as control information to two comparators C 谷 and C 峰 . The comparators actuate trigger circuit RS 触发 , but these comparators require level shifters LSp and LSv which transfer the output signals of the comparators from the high voltage domain V 高 to the low voltage domain where the rest of the controller is located. The output of trigger circuit RS 触发 is actuated by AND gates A 高 , A 低 and dimming PWM generator V 调光 in such a way that these outputs allow switching of the two transistors Qhs, Qls according to the control concept during the dimming PWM on-phase and turning off of the two transistors during the dimming PWM off-phase.

[0011] The LED current exactly corresponds to the average value of the coil current and is defined by two reference voltage sources V 谷 and V 峰 . However, this again requires some level shifters which transfer the threshold voltage or reference voltage values of V 谷 and V 峰 from the low voltage domain to the high voltage domain V 高 .

[0012] However, there are two problems with using this known buck converter.

[0013] On the one hand, negative coil current is not possible. Due to the valley comparator C 谷It can only operate at an input voltage greater than or equal to zero, so in the best case, this comparator can recognize a coil current equal to zero but not a negative coil current. In the best case, the converter can thus operate in the so-called BCM (Boundary Conduction Mode). Considering the current ripple in the coil, the result is that the minimum average value of the LED current will no longer be undershot. Among other things, this depends on the coil and the value of the current measurement resistor, and in many cases, it results in an impractically high switching frequency. A high-inductance coil that extends the current range downward is also impractical for a high-current range because it can only tolerate a small saturation current at the same form factor. For clarity, Figure 2 CCM (Continuous Current Mode) is shown in the upper illustration, and BCM (Boundary Conduction Mode) is shown in the middle illustration.

[0014] On the other hand, the switching delay causes LED current faults. The driver circuits for the switching transistors Qhs and Qls have a specific response time measured from the moment the coil current reaches the desired peak or valley to the moment the switch actually conducts. Among other things, the response times of the comparator, logic gates, gate drivers, and level converters between the high-voltage domain and the low-voltage domain cause this delay. For example, the sum of these delays can be 40 to 80 ns, which is already 10% for a switching period of, for example, 800 ns. Since these delays usually also have different durations for peak and valley cases, the actual average value of the coil current (which is the same as the LED current) is offset relative to the target value. The switching frequency is also lower than expected, and the coil current ripple is greater. In the case of a lower LED current specification, this offset is more significant as a percentage. This is shown in Figure 3 which is presented.

[0015] Therefore, the object of the present invention is to solve these problems.

[0016] This object is achieved in the general driver circuit as described above: A current amplifier circuit is provided, which is connected to the current measurement resistor to detect the current flowing through the current measurement resistor, and the current amplifier circuit can measure both positive and negative currents by an offset voltage superimposed on one of its input terminals by an offset voltage source, and the output terminal of the current amplifier circuit is connected to the input terminal of the window comparator that is not supplied with a threshold; the offset voltage is also superimposed on the high threshold voltage and the low threshold voltage; and a start pulse circuit is provided, which is connected to the control input terminal of the first switching element via a diode.

[0017] This circuit design according to the present invention also enables, by means of Figure 2The negative valley current shown in the bottom illustration is controlled. In this so-called FCM (forced continuous mode) operation, the average LED current can be theoretically set to zero and even to a negative value, and thus this current is actively controlled.

[0018] In a development of the driver circuit, a rectifying element is formed by a second switching element, wherein the second output terminal of the bistable trigger circuit is connected to the control terminal of the second switching element, and the signal level of this second output terminal is used to indicate reaching the high threshold voltage.

[0019] This means that an active rectifier with particularly low losses is implemented.

[0020] In order to be able to influence the current flowing through the LED arrangement and thus dim the brightness, in a design of the driver circuit, the two output terminals of the bistable trigger circuit are each connected via an AND gate to the respective control terminals of the first switching element and the second switching element, wherein the respective other terminals of the AND gates are connected to a dimming circuit, which is arranged to emit a pulse width modulation signal.

[0021] In this case, the gate output can advantageously be connected via gate drivers, which drive the switching elements.

[0022] In a development of the driver circuit according to the invention, a compensator circuit is provided, which has a subtraction circuit, to the input terminals of which a current setpoint value and the current value measured by a current measurement circuit are supplied, and the output terminal of the subtraction circuit to which the fault current is applied is connected via a third controllable switching element to the input terminal of a filter circuit, the output terminal of which is connected to the first threshold voltage source and the second threshold voltage source, wherein the filtered fault current is added to the threshold voltage, and wherein the control terminal of the third switching element is connected to the dimming circuit.

[0023] If a fault current is mentioned here, this basically means a fault current signal, since the fault current can also be represented by a voltage, because this voltage usually drops across the current measurement resistor.

[0024] Thus, the compensator circuit is used to determine the deviation of the actual LED current from the target LED current and to minimize this deviation taking into account the control behavior of the converter.

[0025] In a development of the current amplifier circuit, the current amplifier circuit is formed by a non-inverting summing amplifier, the inverting terminal of which is connected to the terminal of the coil connected to the LED arrangement, and the non-inverting terminal of which is connected to the terminal of the coil connected to the first switching element and the offset voltage source.

[0026] The current amplifier circuit can also be designed differently, for example, by a discrete transistor current mirror.

[0027] The present invention will now be described in more detail based on exemplary embodiments with the aid of the drawings. In the drawings,

[0028] Figure 1 a driver circuit in the form of a buck converter for an LED arrangement according to the prior art is shown,

[0029] Figure 2 a graph of the coil current for various operating states of the driver circuit is shown,

[0030] Figure 3 a graph of the coil current of the driver circuit is shown to demonstrate overshoot and undershoot due to switching delays,

[0031] Figure 4 an exemplary embodiment of a driver circuit according to the invention in the form of a buck converter for an LED arrangement is shown,

[0032] Figure 5 a current graph of the coil current of the driver circuit in CCM or BCM operation is shown,

[0033] Figure 6 a current graph of the coil current of the driver circuit in FCM operation under the influence of a start pulse is shown.

[0034] Figure 1 and Figure 3 has already been discussed above when discussing the prior art and its problems.

[0035] Now, Figure 4 an exemplary embodiment of a driver circuit according to the invention is shown, in which the buck converter (step-down converter) has the same design as the prior art; it is understood here that other forms of buck converters can also be used here. The voltage source V 入 is connected to the input terminal of the buck converter, and the input capacitor C 入 is connected in parallel with this voltage source as a support capacitor. The rectifying element is designed as a second switching element Qls in the form of a MOSFET transistor. The first switching element Qhs is also designed as a MOSFET transistor.

[0036] The current amplifier circuit A 感测 is configured as a differential high-side amplifier, which is offset using resistors Rd1...Rd4. For this purpose, the non-inverting input terminal of this current amplifier circuit is connected to the offset voltage source V 偏移 . Due to its configuration, the amplifier can detect bidirectionally the current flowing through the current measuring resistor R 感测And thus the current flowing through the coil Lbk. The current amplifier circuit A 感测 causes the voltage drop at the current measuring resistor R 感测 to be transferred to the low voltage domain of Vs, and most of the rest of the circuit (except for the high side gate driver Vgh) is located in this low voltage domain. The current amplifier circuit A 感测 has an output voltage V 感测 which behaves as follows:

[0037] V 感测 = V 偏移 if the current flowing through the coil Lbk = 0 (1)

[0038] Vs >= V 感测 >= V 偏移 if the current flowing through the coil Lbk is positive (2)

[0039] V 偏移 >= V 感测 >= 0 if the current flowing through the coil Lbk is negative (3)

[0040] A two-point control circuit is formed by a window comparator which consists of two comparators C 谷 and C 峰 and a first threshold voltage source and a second threshold voltage source supply the two comparators with a high threshold voltage V 峰 and a low threshold voltage V 谷 , and these two threshold voltages determine the value of the current flowing through the coil V 感测 at which reaching it causes the first switching element Qhs to turn on and off again, and a bistable trigger circuit RS 触发 is connected downstream of the window comparator, and a first output terminal Q of the bistable trigger circuit having a signal level indicating reaching the low threshold V 谷 is connected to the control terminal of the first switching element.

[0041] The set input terminal S of the trigger circuit RS 触发 is connected to the output terminal of the valley comparator C 谷 , and the reset input terminal R is connected to the output terminal of the peak comparator C 峰 . The inverted output terminal Q / is connected to the control input terminal of the second switching element Qls.

[0042] The output terminals Q, Q / of the trigger circuit RS 触发 are each connected to the control input terminal of the first switching element Qhs or the second switching element Qls via an AND gate. The second input terminal of the AND gate is connected to the dimming circuit V 调光 which outputs a pulse width modulation signal.

[0043] A start pulse circuit V 启动, the start pulse circuit is connected to the control input terminal of the first switching element Qhs via a diode D 启动 The start pulse circuit is connected to the control input terminal of the first switching element Qhs via a diode D

[0044] A compensator circuit is also provided, which has a subtraction circuit. A current setpoint value V 设定点 and a current value V measured by a current measurement circuit 感测 are supplied to the input terminal of the subtraction circuit, and the output terminal of the subtraction circuit to which a fault current V 误差 is applied is connected to the input terminal of a filter circuit Compensator via a third controllable switching element S 保持 . The output terminal of the filter circuit is connected to the first threshold voltage source and the second threshold voltage source. Among them, the filtered fault current V 补偿 is added to the threshold voltages V 谷 , V 峰 , and the control terminal of the third switching element S 保持 is connected to a dimming circuit V 调光 .

[0045] The voltages V 谷 , V 峰 , V 补偿 of these three voltage sources satisfy the following equations:

[0046] V 谷＝ (V 设定点 +V 补偿 )*(1 - ripple / 200) (4)

[0047] V 峰 =(V 设定点 +V 补偿 )*(1 + ripple / 200) (5)

[0048] V 补偿 =Hc(V 误差 )=Hc(V 设定点 -V 感测 ) (6)

[0049] Among them, the following parameters

[0050] ripple: Desired coil current ripple, valley to peak, in %

[0051] Hc: Transfer function of the compensator, adjustable

[0052] V 补偿 Output by the compensator, actuation control signal

[0053] V 感测 Output by the LED current measurement amplifier A 感测 Output

[0054] V 设定点 Target value specification of average LED current

[0055] V 峰 Specification of coil peak current

[0056] V 谷 Specification of coil valley current

[0057] Voltage reference V 谷 and V 补偿 must also be able to take negative values. However, and must

[0058] |V 谷 | + V 偏移 >= 0 (7),

[0059] such that comparator C 谷 、C 峰 always operates with an input signal greater than or equal to zero.

[0060] In fact, the threshold voltages V 谷 and V 峰 are most likely to be implemented by a DAC converter pre-biased with a voltage, such that equations (4), (5), (7) can already be converted into digital control signals of the DAC converter.

[0061] The task of the compensator unit (LED current error correction block) is to minimize the deviation of the actual LED current from the target. For this purpose, the error signal V 误差 is calculated as the difference between the set point V 设定点 and the actual current V 感测 , and this error signal is adjusted to the conditions of the converter using the compensator block Compensator. These include the open-loop transfer function of the power facility, and most importantly the delay caused by the entire digital logic system of the switching transistors, gate drivers, comparators, and control unit. To cancel the delay and the resulting overshoot / undershoot in the coil current V 感测 , the output signal V 补偿 of the compensator Compensator is fed to the threshold voltages V 谷 and V 峰 as a signed correction value and added to it.

[0062] Since the LED driver can be dimmed using the dimming circuit V 调光 , and the compensator Compensator usually also has an integrating element, the dimming circuit V 保持 is stopped by the third switching element S 调光Integration during the PWM off-phase. Thus, empty integration and the resulting increase in the error signal are avoided, and the operating point of the storage controller for the next PWM on-cycle - the output from the compensator Compensator is "frozen".

[0063] The compensator block can be implemented analogously based on an operational amplifier using a subtractor and type I, II, III compensators or a PL or PID controller. However, the compensator block can also be implemented digitally by using an ADC and a digital error signal fed to a digital compensator V 感测 or V 误差 which processes this digital error signal in the digital domain by means of a transfer function of the above type (e.g., 2p2z, 3p3z, PI, PID) and generates a digital control signal V 补偿 . If the reference specifications of the comparators C 峰 and C 谷 are also performed digitally using a DAC converter as described above, then the entire control unit can be designed completely digitally except for the comparator and the current amplifier circuit A 感测 . The compensator does not need to be fast because the delays caused by gates, comparators, level converters, etc. are relatively static and mainly depend on temperature and the sample. Dynamic per-cycle readjustment is not necessary, and thus a simple integral compensator that keeps the error signal at a minimum is usually sufficient as well.

[0064] The voltage source V 启动 has a specific function. For this purpose, the start-up behavior of the proposed converter must be considered.

[0065] Case 1. CCM (Continuous Current Mode, coil current is always > 0)

[0066] In CCM mode, that is, if a larger LED current is required and thus the valley current in the coil can be greater than 0, the condition

[0067] V 谷 > V 偏移

[0068] is satisfied. In the first instance, V 感测 is approximately equal to V 偏移 because no current flows through the current measuring resistor R 感测 . Thus, V 谷 > V 感测 . Thus, the voltage V 谷 at the non-inverting input of the comparator C 谷 is higher than the voltage at its inverting input. The comparator output is set to "high", triggering the circuit RS 触发Set its in-phase output Q to correspond to "high", the first switching element Qhs becomes conducting and the coil current increases. This ensures smooth startup of the converter; no explicit startup pulse is required. The current curve is shown in Figure 5 as shown.

[0069] In FCM mode, that is, if a smaller LED current is required and thus the valley current in the coil must be below 0, the condition

[0070] V 谷 <V 偏移

[0071] is satisfied. In the first instance, V 感测 is approximately equal to V 偏移 , because no current flows through the current measuring resistor R 感测 . Thus, V 谷 <V 感测 . Therefore, the voltage V 谷 at the in-phase input of the comparator C 谷 is lower than the voltage at its anti-phase input. The comparator output is set to "low", the in-phase output Q of the flip-flop stage RS 触发 corresponds to "low", the first switching element Qhs is non-conducting and the LED converter cannot start.

[0072] Startup can only be carried out in the manner according to the invention using an additional startup pulse circuit V 启动 , which actuates the first switching element Qhs with a narrow startup pulse so that the current measuring resistor R 感测 is energized and the comparator C 谷 can change state. The converter can only be started using this pulse.

[0073] An advantage provided by the present invention is that the LED current setting range is very wide. Since control can be carried out in forced continuous mode (FCM), the described hysteresis converter allows a very wide current setting range from a few milliamperes to several amperes without the need to adapt the power components. Known hysteresis drivers cannot do this - these drivers require larger inductance and shunt values for smaller LED currents so that the coil ripple becomes smaller and thus the coil current remains positive even at the valley point.

[0074] The proposed converter also allows negative current flow and provides a small LED current even with small inductance and shunt values, which in turn is beneficial for high current operation.

[0075] Efficient compensation for switching delay (undershoot and overshoot compensation) is achieved. By directly evaluating the average LED current and using an integrated compensator that dynamically adjusts the peak and valley points of the coil current, the LED current error can be made almost zero.

[0076] This results in the following innovative features:

[0077] By using the offset for the peak comparator C 峰 and the valley comparator C 谷 as well as the measurement amplifier A 感测 , the function of the converter with negative coil current becomes possible. This enables the converter to operate in FCM (forced continuous mode).

[0078] The start pulse circuit V 启动 allows the converter to start in cases where negative coil current is required (i.e., at low LED current).

[0079] The compensator block significantly reduces the LED current error. This block can also be conveniently implemented digitally and is thus very flexible in terms of parameterization.

[0080] The output of the compensator can be easily integrated into the control loop through equations (4) and (5). These equations also apply to digital implementations.

Claims

1. A driver circuit for supplying a constant current to an LED arrangement (LED1 ... LEDN), the driver circuit comprising: The buck converter consists of a voltage regulator connected to the supply voltage (V 入 ) and the output capacitor (C 出 ) between a first switching element (Qhs) and a coil (Lbk) connected in series therewith, wherein the connection point between the first switching element (Qhs) and the coil (Lbk) and the power supply voltage (V 入 ) and the output capacitor (C 出 ) is connected via a rectifying element (QLs), and a current measuring resistor (R) for measuring the current flowing through the coil (Lbk) is connected. 感测 ) is arranged in series with the coil (Lbk), A two-point control circuit, which is based on the current measuring resistor (R 感测 ) measured the current (V 感测 ) to turn on or off the first switching element (Qh) so as to keep the average current flowing through the LED arrangement (LED1 ... LEDN) constant, wherein the two-point control circuit is composed of a window comparator (C 峰 , C 谷 ) is formed, the first threshold voltage source and the second threshold voltage source supply a high threshold voltage (V 峰 ) and low threshold voltage (V 谷 ), these threshold voltages determine the current (V ) flowing through the coil (Lbk) that causes the first switching element (Qhs) to be turned on and off again when reached. 感测 ) value, where the window comparator (C 峰 , C 谷 ) is connected downstream with a bistable trigger circuit (RS 触发 ), the bistable trigger circuit has an indication that the low threshold (V 谷 ) is connected to a control terminal of the first switching element (Qhs), wherein: Provides a current amplifier circuit (A 感测 ), the current amplifier circuit is connected to the current measuring resistor (R 感测 ) to detect the current (V 感测 ), the current amplifier circuit is superimposed by an offset voltage (V 偏移 ) is capable of measuring both positive and negative currents, and the output terminal of the current amplifier circuit is connected to the window comparator (C 峰 , C 谷 ) of the unsupplied threshold input terminal, The offset voltage (V 偏移 ) is also superimposed on the high threshold voltage (V 峰 ) and low threshold voltage (V 谷 ) and Provides a start pulse circuit (V 启动 ), the start pulse circuit passes through a diode (D 启动 ) is connected to the control input terminal of the first switching element (Qhs).

2. The driver circuit according to claim 1, wherein: The rectifying element is formed by the second switching element (Qls), wherein the bistable trigger circuit (RS 触发 ) is connected to the control terminal of the second switch element (Qls), and the signal level of the second output terminal is used to indicate that the high threshold voltage (V 峰 ).

3. The driver circuit according to claim 1 or 2, wherein: The bistable trigger circuit (RS 触发 The two output terminals (Q, Q / ) of the first switching element and the second switching element (Qhs, Qls) are each connected to the corresponding control terminal of the first switching element and the second switching element (Qhs, Qls) via an AND gate, wherein the other corresponding terminal of the AND gate is connected to the dimming circuit (V 调光 ), the dimming circuit is configured to transmit a pulse width modulated signal.

4. The driver circuit according to claim 3, wherein: A compensator circuit is provided, the compensator circuit having a subtraction circuit, a current set point value (V 设定点 ) and by the current measurement circuit (A 感测 )Measured current value (V 感测 ), and the subtraction circuit is applied with fault current (V 误差 ) is connected to the output terminal of the third controllable switch element (S 保持 ) is connected to the input terminal of a filter circuit (compensator), the output terminal of which is connected to the first threshold voltage source and the second threshold voltage source, wherein the filtered fault current (V 补偿 ) and these threshold voltages (V 谷 , V 峰 ) are added, and wherein the third switching element (S 保持 ) is connected to the dimming circuit (V 调光 ).

5. A driver circuit as claimed in any one of the preceding claims, wherein: The current amplifier circuit (A 感测 ) is formed by a common-phase adding amplifier having an inverting terminal connected to a terminal of the coil (Lbk) connected to the LED arrangement (LED1 . . . LEDN), and having a common-phase terminal connected to a terminal of the coil (Lbk) connected to the first switching element (Qhs) and the offset voltage source.

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