Patent Application
20220183123
This application claims priority to German Patent Application 10 2020 132 878.8 filed on Dec. 9, 2020, the contents of which are incorporated into the subject matter of the present application by reference.
The disclosure is directed to an LED light using light emitting diodes (LEDs) as illuminants, which minimizes the risks of IEEE 1789-2015 while providing good intensity adjustment and diagnostic capability.
IEEE 1789-2015, referenced here, reports health risks associated with the use of PWM-controlled LED drivers.
According to the current state of the art, LEDs are controlled by means of PWM-controlled constant current source. The advantage of this method is a minimal color shift of the color impression of the light of the luminaire emitted by the LED as illuminant, as the LED current is not modulated.
In the IEEE 1789-2015 standard, this modulation type is described and referred to as 100% modulation.
Depending on the technical possibilities of the LED drivers and PWM generators used, a PWM pulse instantaneous frequency above 100 Hz is used. The IEEE 1789-2015 standard recommends a PWM pulse instantaneous frequency greater than 2000 Hz for the modulation type used. According to the current state of the art, LEDs are driven by PWM-controlled constant current sources. A constant current source drives an illuminant with an electric current which is adjusted to a predetermined current value by design or by means of a control value signal. The illuminant thereby typically comprises an LED or an LED string comprising one or more LEDs connected in series, or a parallel connection of several LED strings. Preferably, the constant current source is switched on and off in a pulse-like repetitive manner using a PWM period. Thus, when the constant current source is on for a PWM pulse duration during a PWM period, the constant current source only feeds that preset current into the illuminant at on times for that PWM pulse duration. For the remainder of the PWM period outside of the PWM pulse duration, the constant current source therefore typically does not feed any current into the illuminant. When the constant current source injects the preset amount of electrical current into the illuminant during the PWM pulse period, the constant current source supplies electrical energy to the illuminant during the PWM pulse period, thereby causing the illuminant to emit light. The preset amount of electric current influences the color perception, i.e., the color temperature, of the emitted light by a human being. The advantage of this method is minimal color shift because the preset value of the LED current is not modulated. The illuminant is either on or off. Depending on the technical possibilities of the LED drivers and PWM generators used as illuminant drivers, a PWM pulse instantaneous frequency above 100 Hz is used to avoid unwanted flickering effects on the viewer. A PWM modulation of the control signal used by the illuminant driver to supply electrical energy to the illuminant, in this case the LEDs, therefore exhibits PWM pulses. For the purpose of this paper, for better clarity, each PWM pulse is associated with exactly one PWM period and, conversely, each PWM period is associated with exactly one PWM pulse in order to be able to precisely define the PWM pulse instantaneous frequency. The PWM modulation of the power supply to the illuminating means by said constant current source has, for the purposes of this writing, PWM periods respectively associated with the PWM pulses. Such a PWM period of a PWM pulse starts in terms of time with the rising edge of the PWM pulse and ends with the rising edge of the immediately following PWM pulse of the PWM modulation. For the purposes of this writing, the PWM pulse instantaneous frequency of a PWM pulse is then the reciprocal of the PWM period of that PWM pulse. Furthermore, for the purposes of this writing, the PWM duty cycle of a PWM pulse is understood to be the ratio of the value of the temporal duration of a PWM pulse in a PWM period of the PWM modulation of the control signal divided by the value of the temporal PWM period duration of this PWM period of the PWM modulation of the control signal.
For example, a PWM pulse instantaneous frequency of 300 Hz is typically used in the field of RGB interior lighting for automobiles at the time of this writing. PWM pulse instantaneous frequencies above 500 Hz are now being discussed. However, increasing the PWM pulse instantaneous frequency encounters technical limitations, since the minimum current source active time in which the respective illuminant driver of the respective LED supplies energy to the LED in question is in the nanosecond range at the typically required 16-bit resolution of the PWM duty cycle. However, an LED cannot be driven with this short PWM pulse duration due to parasitic capacitances. Furthermore, the error of the intensity setting increases due to the non-ideal edge shape.
At the same time, the lower active time makes diagnosis more difficult. This is understood to mean a measurement of the operating parameters of the LED or the LED string during operation in order to detect faults, such as already existing interruptions or indications of such developing interruptions or short circuits or developing short circuits, for example, by detecting the LED current and/or the LED voltage and their time characteristics.
Current illuminant drivers use PWM generators based on configurable PWM clock generators and/or duty cycle timers. The basis of a PWM generator is usually a counter that is operated with a PWM clock and that restarts cyclically with the current PWM period. This restart is typically triggered by a reset logic of the PWM counter at a first count of the PWM counter. The exemplary PWM counter then jumps back to the restart value. For example, upon the PWM counter restarting and/or the PWM counter taking the PWM counter restart value, the PWM counter may turn on the constant current source to supply electrical power to the illuminant. At a second count value, which is typically taken between the restart value and the first count value by the PWM counter, the PWM counter typically switches the constant current source off again, so that until the first count value is taken by the PWM counter, the illuminant no longer emits light. The first number of counting steps of the PWM counter between taking the restart value of the PWM counter and taking the first counting state of the PWM counter divided by the PWM clock typically corresponds to the PWM period. The second number of counting steps of the PWM counter between taking the restart value of the PWM counter and taking the second counting state of the PWM counter divided by the PWM clock typically corresponds to the PWM pulse period. However, in addition to such a PWM generator based on a PWM counter, other PWM generators are conceivable, the use of which is claimed herein. The setting of the PWM frequency is typically only possible by means of integer dividers. If a PWM pulse instantaneous frequency is desired which cannot be generated as an integer from the PWM clock with which the PWM generator in the illuminant driver is operated, the PWM resolution of the PWM modulation of the control signal of the illuminant driver with which the illuminant driver supplies the illuminant with electrical energy from the illuminant driver is reduced.
A problem now arises from the fact that the respective illuminant cannot be switched on and off arbitrarily fast. Firstly, the electric current through the illuminant shows a typically exponential increase when switched on and a typically exponential decrease when switched off. Thus, at least the temporal course of the light intensity emitted by the illuminant exhibits a low-pass behavior compared to the ideal control signal that the respective illuminant driver PWM-modulates to provide power to the illuminant. This low-pass behavior and the typically non-linear characteristic of the illuminant lead to a non-linear distortion of the light intensity for small light intensities, since the short PWM pulses due to the low-pass characteristic and the non-linear characteristic of the illuminant then lead to a change in the light emission of the illuminant, the change in intensity of which is no longer proportional to the change in the temporal PWM pulse duration.
A fundamental increase of the PWM pulse instantaneous frequency to values above 500 Hz would thus cause a significant non-linearity of the rms current in the lower dimming range, i.e., in the range of low luminous intensities, which is unknown from the point of view of the application software, since the edge control by the illuminant drivers causes an increasing error at the short PWM pulses in this range, as described.
In current implementations, the use of a PWM frequency higher than 2000 Hz would mean a significant reduction of the PWM resolution or would require a very high clock frequency PWM clock of the PWM generator, e.g., a PWM counter.
The proposal is therefore based on the task of creating a solution which does not have the above disadvantages of the prior art and has further advantages. This task is solved by a device according to an independent claim for a luminaire and an independent claim for a method.
A method is proposed which allows the PWM frequency of PWM-controlled light sources to be increased while avoiding short pulse times in order to enable diagnosis and reduce the influence of edges on dimming, i.e., intensity adjustment by means of an externally transmitted default value.
In an example, the method allows a light intensity-dependent adaptive PWM pulse instantaneous frequency. This corresponds to an adaptation of the instantaneous PWM period to the instantaneous light intensity to be emitted.
Typically, the luminaire receives a preset value for the light intensity to be emitted from outside. This results in a duty cycle of the PWM modulation of the control signal with which the light source driver supplies the light source with electrical energy. The illuminant driver preferably comprises the PWM generator which generates the PWM modulation.
In an example, the illuminant driver comprises a first calculation device which determines the PWM period to be set from the preset value. This first determination can be made, for example, by calculation, for example in a microcomputer of the illuminant driver, or by using a correspondence table in a memory of the illuminant driver, or by a constructively predetermined logic.
In an example, the illuminant driver comprises a second calculation device which determines the PWM duty cycle to be set from the preset value. This second determination can be made, for example, by calculation, for example in a microcomputer of the illuminant driver, or by using a correspondence table in a memory of the illuminant driver, or by a constructively predetermined logic.
In a further example, at least either a first calculation device of the illuminant driver performs the first determination or a second calculation device of the illuminant driver performs the second determination, so that at least either the duty cycle of the PWM modulation of the control signal and/or the PWM period of the PWM modulation of the control signal is corrected according to the non-linear distortion of the light emission by the illuminant.
The illuminant driver then uses the PWM period and/or the PWM duty cycle determined in this way, if applicable, to generate the control signal PWM-modulated with these values to supply the illuminant with electrical energy.
IEEE 1789-2015 reports negative physiological effects whose impact depends on the light intensity. At high light intensities, the negative effects are larger than at low intensities. This property allows the PWM pulse instantaneous frequency to be selected very high at high intensities (=large duty cycle) and smaller at low intensities (=small duty cycle). Thus, when using DDS PWM generators (see also, for example, U.S. Pat. No. 7,284,025 B2), it is possible to use adaptive PWM pulse instantaneous frequency control as a function of the configured duty cycles, i.e. the duty cycle to be configured, in the range of, for example, 100 Hz (lower limit for avoiding perceptible flicker of the illuminant, i.e. the LED) and thus to minimize the health risks far beyond the 2 kHz mentioned in the IEEE. The minimum temporal PWM pulse durations can be extended by the reduced PWM pulse instantaneous frequency, so that the influence of the edge time is minimized, since the magnitude of the ratio of the magnitude of the temporal PWM pulse duration divided by the magnitude of the temporal PWM period duration remains constant by extending the temporal PWM period duration. However, the temporal ratio of the edges within a PWM period decreases.
It is proposed to establish a method that allows adaptive PWM pulse instantaneous frequency.
At high light intensities, the resulting temporal PWM pulse durations of the PWM pulses are sufficiently long in time so that the edge influence of the PWM pulses can be neglected, since the temporal duration of the edge rise or fall is small in total compared to the respective PWM pulse duration.
At low light intensities, the resulting temporal PWM pulse durations of the PWM pulses are so short in time that the edge influence can now no longer be neglected, since the temporal duration of the edge rise or fall relative to the respective PWM pulse duration is in sum large or at least relevant. However, at these low light intensities, the PWM pulse instantaneous frequency can now be selected lower, for example via said correspondence table or said logic of the first, so that the minimum PWM pulse durations in which the illuminant driver supplies energy to the illuminant at a temporal piece are extended by the low PWM pulse instantaneous frequency. Preferably, a minimum PWM pulse duration is not undercut. Alternatively or complementarily, at these low light intensities, the PWM period can be selected higher (i.e., longer), for example via said correspondence table or said logic, so that the minimum PWM pulse durations in which the illuminant driver delivers energy to the illuminant at a temporal piece are extended by the low PWM pulse instantaneous frequency. However, changing the PWM period duration is clearly preferable. By lowering the PWM pulse instantaneous frequency in this way, the proportional influence of the edge time can thus be minimized and the time for performing diagnostic functions, such as sensing the voltage drop across the illuminant during the PWM pulse period or sensing the electric current through the illuminant during the PWM pulse period, becomes longer. Thus, it is possible to achieve stabilization of the current through the illuminant or stabilization of the voltage drop across the illuminant before the illuminant driver, for example by means of an analog-to-digital converter of the illuminant driver, performs the measurement during the PWM pulse period outside the edge times in which the control signal changes its voltage or current state. For example, the illuminant driver may have a microcomputer. Preferably, the microcontroller acquires the diagnostic values for the magnitude of the illuminant current and/or the illuminant voltage determined by the analog-to-digital converter during the PWM pulse durations and outside the edge times and outputs them or transmits them, for example, via said data bus or holds them ready for interrogation by the higher-level system. Typically, a means, for example a shunt resistor, for current-to-voltage conversion is connected in series with the illuminant, through which the electrical illuminant current flows, so that the analog-to-digital converter can detect current and voltage in time-division multiplex at its input by means of a multiplexer, at least at times and with a time delay.
The method allows an adaptive PWM pulse instantaneous frequency to improve the diagnosis and dimming of the illuminant, in this case an LED or an LED string or several LED strings. An LED string is understood to be a series connection of several LEDs.
In the following, therefore, a method and a device are proposed which implement this idea. The proposed method is a method for controlling a light source with a control signal. The intensity of the light emitted by the illuminant depends on the value and the value progression of the control signal. In this method, the value progression of the control signal is PWM-modulated in time with a PWM modulation. This temporal PWM modulation of the control signal has PWM pulses and PWM periods associated with the PWM pulses, respectively, as a result of this PWM modulation. For purposes of this writing, a PWM period of a PWM pulse begins in time with the rising edge of the PWM pulse and ends in time with the rising edge of the immediately following PWM pulse of the PWM modulation. The value of the PWM pulse instantaneous frequency of a PWM pulse is, for the purposes of this document, the reciprocal of the value of the temporal duration of the PWM period of the PWM pulse.
As used herein, the PWM duty cycle of a PWM pulse is the ratio of the value of the temporal duration of a PWM pulse in a PWM period of the PWM modulation of the control signal divided by the value of the temporal PWM period duration of that PWM period of the PWM modulation of the control signal.
The method is characterized in that the PWM pulse instantaneous frequency of the PWM pulses of the PWM modulation depends on the PWM duty cycle of the PWM pulses of the PWM modulation of the control signal. That is, the PWM pulse instantaneous frequency of the PWM pulses of the PWM modulation is a first PWM pulse instantaneous frequency at a first PWM duty cycle of the PWM pulses of the PWM modulation of the control signal, and the PWM pulse instantaneous frequency of the PWM pulses of the PWM modulation is a second PWM duty cycle of the PWM pulses of the PWM modulation of the control signal, which is different from the first PWM duty cycle, is a second PWM pulse instantaneous frequency which is different from the second PWM pulse instantaneous frequency. In at least one operating condition of the proposed device performing the proposed method, the magnitude of the first PWM duty cycle is less than the magnitude of the second PWM duty cycle. Then, in this very operating condition, the magnitude of the first PWM pulse instantaneous frequency is smaller than the magnitude of the second PWM pulse instantaneous frequency. Preferably, the illuminant comprises at least one LED, LED string or is an LED or LED string.
Corresponding to this method is an associated luminaire comprising a sub-device performing the previously described method. Such a luminaire comprises at least one illuminant, an illuminant driver and a control signal. The illuminant driver supplies the illuminant with electrical energy at least secondarily via the control signal. The illuminant driver generates the control signal. The intensity of the light emitted by the illuminant depends on the value and the temporal value progression of the control signal. The illuminant driver may, for example, have said microcomputer, which may, for example, receive default values from a higher-level system via a data bus. The temporal value progression of the control signal generated by the illuminant driver, and thus the intensity of the light emitted by the illuminant, preferably depend on such a preset value. This preset value can possibly be generated by the illuminant driver or, for example, received from a higher-level computer system or the like via a data bus and/or, for example, held ready. Preferably, but not necessarily, the preset value is a preset value for the duty cycle. The illuminant driver PWM modulates the duty cycle of the control signal in time with a PWM modulation. The PWM modulation of the control signal of the illuminant driver has said PWM pulses in a temporal and typically non-overlapping sequence. The PWM modulation of the illuminant driver further comprises respective PWM periods, wherein a respective PWM period is associated with exactly one respective PWM pulse in the sense of this writing. A PWM period of a PWM pulse begins in terms of time with the rising edge of the PWM pulse and ends with the rising edge of the immediately following PWM pulse of the PWM modulation. For the purposes of this document, the PWM pulse instantaneous frequency of a PWM pulse is the reciprocal value of the PWM period of the PWM pulse.
As used herein, the PWM duty cycle of a PWM pulse is the ratio of the value of the temporal duration of a PWM pulse in a PWM period of the PWM modulation of the control signal divided by the value of the temporal PWM period duration of said PWM period of the PWM modulation of the control signal. As before, the PWM duty cycle and/or the PWM pulse instantaneous frequency preferably depend on said default value. For a first default value, the PWM pulse instantaneous frequency of the PWM pulses of the PWM modulation is a first PWM pulse instantaneous frequency for a first PWM duty cycle of the PWM pulses of the PWM modulation of the control signal. For a second default value different from the first default value, the PWM pulse instantaneous frequency of the PWM pulses of the PWM modulation at a second PWM duty cycle of the PWM pulses of the PWM modulation of the control signal different from the first PWM duty cycle is a second PWM pulse instantaneous frequency different from the first PWM pulse instantaneous frequency.
At least the magnitude of the first PWM duty cycle is thereby smaller than the magnitude of the second PWM duty cycle. In this operating state, the magnitude of the first PWM pulse instantaneous frequency is then smaller than the magnitude of the second PWM pulse instantaneous frequency. Preferably, again, the illuminant comprises at least one LED and/or LED string.
A higher PWM pulse torque approach frequency of the PWM control is made possible. However, the advantages are not limited to this.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2020 132 878.8 | Dec 2020 | DE | national |
