Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
  • H05B45/10—Controlling the intensity of the light
  • H05B45/14—Controlling the intensity of the light using electrical feedback from LEDs or from LED modules
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
  • H05B45/10—Controlling the intensity of the light
  • H05B45/18—Controlling the intensity of the light using temperature feedback

Definitions

• the invention is based on a control circuit for LEDs and the associated operating method according to the preamble of claim 1. It is in particular about reducing the control power loss in light-emitting diodes (LEDs) by means of a clocked LED control circuit.
• LEDs light-emitting diodes
• the power loss in the series resistor is converted into heat, which leads to additional heating - in addition to the self-heating of the LEDs in the string.
• the technical problem is to eliminate the additional heating (drive power loss due to the series resistors).
• drive power loss due to the series resistors There are mutliple reasons for this.
• this heating by series resistors limits the operating range of the LEDs.
• Another problem is the choice of the maximum forward current I F of LEDs.
• the maximum permissible forward current I F cannot be selected, since the forward current must be reduced at a higher ambient temperature T A.
• a forward current I F is therefore chosen which is smaller than the maximum permissible (FIG. 3). In this way, the temperature range for operating the LEDs is increased, but the forward current I F is not optimally used.
• FIG. 3 Power TOPLED, type LA E675 from Siemens
• the maximum forward current I F may be 70 mA up to an ambient temperature of 70 ° C.
• a clocked LED control is used to eliminate the series resistor R v and thus the large drive power loss.
• Figure 4a shows the principle of a clocked current control for LEDs.
• a semiconductor switch for example a current-limiting circuit breaker or preferably a transistor T (in particular of the pnp type, but also the npn type is suitable if a charging pump is also used for actuation), has its emitter on the supply voltage U ßatt (in particular battery voltage connected in the automobile). If the transistor T is conductive, a current i LED flows through the LED strand (which here consists, for example, of four LEDs) until the transistor T is switched off again by a comparator. The output of the comparator is connected to the base of the transistor.
• One (positive) input of the comparator is connected to a control voltage, the second (negative) input of the comparator to a frequency generator (preferably triangular generator with pulse duration T p and correspondingly frequency 1 / T P , since it has particularly good electromagnetic compatibility, but other pulse shapes such as sawtooth are also possible).
• a frequency generator preferably triangular generator with pulse duration T p and correspondingly frequency 1 / T P , since it has particularly good electromagnetic compatibility, but other pulse shapes such as sawtooth are also possible.
• the rectangular pulses have a pulse width that corresponds to a fraction of T p .
• the distance between the rising edges of two pulses corresponds to T p .
• the LEDs are in series with a means for measuring the current (in particular a measuring resistor Rs hunt between LEDs and ground (case 1) or between the semiconductor switch (transistor T) and terminal of the supply voltage U Ba tt (case 2)).
• the clocked current i ED is tapped at the measuring resistor Rs hUnt .
• the mean value of the current LED is then formed using an aid.
• the aid is, for example, an integration means (in case 1), preferably an RC low-pass filter, or a differential amplifier (in case 2).
• This mean value serves as the ACTUAL value for a current control, which is made available to a controller (for example a PI or PID controller) as an input value.
• a SET value, in the form of a reference voltage (U Ref ), for the current control is also made available to the controller as a second input value.
• the control voltage U Rege ⁇ at the output of the controller is set by the controller so that the ACTUAL value always corresponds as best as possible to the TARGET value (in terms of voltage). If the supply voltage U Batt changes in the event of fluctuations, the duty cycle of the transistor T and the length of the rectangular pulse (FIG. 4b) also adapt accordingly. This technique itself is known as PWM (pulse width modulation).
• the circuit according to the invention advantageously enables a detailed query of the operating states of individual LED strings. This enables simple error detection (query for short circuit, interruption) by sequential scanning (so-called LED SCANNING) of the individual LED strings.
• LED SCANNING sequential scanning
• the large series resistor R v previously required for setting the current for the LED strand is eliminated.
• the arrangement according to the invention results in a power loss in the shunt resistor R S below only about 5 mW (when the current is set with PWM), that is to say a reduction in the power loss by a factor of 50.
• a current-limiting circuit breaker can also serve as a switch, which automatically ensures that the clocked forward current I F does not exceed a maximum limit value, for example a limit value of 1 A.
• the circuit arrangement according to the invention is suitable for different requirements, for example for a 12V or 42V vehicle electrical system in a motor vehicle.
• FIG. 5 shows a snapshot of an oscillogram of the clocked current profile of the LED control circuit for a 12 V vehicle electrical system. It shows the peak current i LED through the LEDs (FIG. 5a), which is clocked and reaches about 229 mA. The pulse width is approximately 30 ⁇ s, the subsequent dead time 70 ⁇ s. This results in an average current i ED of 70 mA.
• the associated clock frequency on the triangular generator is shown in FIG. 5b, its frequency is approximately 9.5 kHz (corresponding to approximately 100 ⁇ s pulse width).
• the control voltage U Rege ⁇ is shown as a straight line ( Figure 5c), it has a value of 3.2 V.
• Forward current l F can easily be regulated constantly. Because if the value of the supply voltage changes, the control voltage U Rege ⁇ and also changes hence the on time of the transistor.
• This pulse width modulation in which an increase in the supply voltage causes a shortening of the transistor switch-on time (vice versa, the same applies), is always automatically regulated to a constant current, which is set in the form of a reference voltage U R ⁇ f on the controller (see FIG. 4a). Since the forward current I F in the LED string is constant, there can also be no brightness fluctuations with changing supply voltages.
• the circuit arrangement according to the invention makes it possible to regulate the temperature.
• the most common types of faults are interruption and short circuit.
• the short circuit fault type can practically be ruled out for LEDs. If LEDs fail, it is usually due to an interruption in the supply line.
• An interruption in an LED is mainly due to the effects of heat. The cause is in the expansion of the resin (epoxy resin as part of the housing) under the action of heat, so that the embedded differently expanding bond wire (connecting line between LED chip and outer pin) breaks off.
• a circuit for interrupt detection in an LED string makes it possible to signal the occurrence of an error at an output (e.g. status pin for a semiconductor device).
• Logical 1 high means, for example, that an error has occurred
• Logical 0 low means that the condition is correct.
• the LED control module In standby mode, the LED control module remains connected to permanent plus (battery voltage in the vehicle) while it is switched off, i.e. no current flows through the LEDs. In this state, the control module may only absorb a small amount of own current (own current consumption approaches 0) so as not to strain the battery in the vehicle. This is the case if the car e.g. is parked or parked in the garage. Additional power consumption would put unnecessary strain on the battery.
• the LED control module is switched on and off via a logic input (ENABLE input).
• the circuit arrangement can also be polarized and secured against overvoltage.
• a reverse polarity protection diode ensures the case of a wrong one Connection of the LED control module to the supply voltage (battery) before its destruction.
• a combination of a Zener diode and a normal diode additionally protects the LED control module from being destroyed by overvoltages on the supply voltage pin U Batt -
• a microcontroller-compatible ENABLE input (logic input) is additionally provided, which enables control with a microcontroller. It is thus possible to integrate the control module (in particular an integrated circuit IC) for LEDs in a bus system (for example CAN bus in a motor vehicle, Insta bus for house installation technology).
• a bus system for example CAN bus in a motor vehicle, Insta bus for house installation technology.
• Figure 1 shows a known control for LEDs
• Figure 2 shows another embodiment of a known control for
• FIG. 4 shows the basic principle of clocked current regulation for LEDs (FIG. 4a) along with an explanation of the peak current and mean value (FIG. 4b)
• Figure 5 shows the current profile of a clocked current control for LED
• Figure 6 shows a clocked current control with breaker detection
• FIG. 8 block diagram of an LED control circuit
• this output is an open collector circuit (FIG. 8), since then the user of the circuit, who will later use the LED control module (IC), is independent of the output signal level.
• the circuit of the status output has a transistor as the output stage, the collector of which is open (ie has no pull-up resistor).
• the collector of the transistor leads directly to the status pin of the LED drive module (FIG. 8). If an external pull-up resistor R P is connected to the collector of the transistor T 0 c, this can be connected to any voltage V cc .
• the output signal level therefore depends on the voltage V cc to which the pull-up resistor R P is connected.
• the technical implementation of an interruption detection in the LED string is shown in FIG. 7.
• the interruption detection in the LED strand works on the principle of scanning a voltage (here: control voltage U Rege ⁇ ).
• FIG. 7 shows the complete block diagram of the interruption detection in the LED strand according to the principle of scanning a voltage.
• OSZ internal oscillator
• U R clock
• COUNTER n-bit binary counter
• a 3-bit binary counter (for addresses from 0 to 7) is used as an example. It can be used to sample up to 8 control voltages U Rege ⁇ .
• the 3-bit binary pattern of the counter controls an analog multiplexer (MUX), which (depending on the binary word present) samples all control voltages U R ⁇ ge ⁇ , 2 ... one after the other and makes them available in sequence at the output.
• MUX analog multiplexer
• the A comparator (COMP) is inserted in the output of the analog multiplexer (MUX), and its switching threshold U S w must be less than the minimum value of the delta voltage U D , that is, U S w ⁇ U _ ⁇ n.
• the FF flip-flop and thus the status output are only reset when the LED control module is switched off, i.e. when troubleshooting in the LED string takes place.
• the status output can be reset in two ways:
• FIG. 8 block diagram of the LED control module.
• a reverse polarity Protective diode between external (U Batt ) and internal voltage supply ensures that the LED control module is incorrectly connected to the supply voltage (battery) before it is destroyed.
• the overvoltage protection is implemented with a Zener diode in combination with a reverse polarity diode.
• the IC also contains a connection pin for a temperature sensor (for example an NTC) and a pin for connecting a current reference as well as two pins for connecting the LED string.
• a temperature sensor for example an NTC
• a pin for connecting a current reference as well as two pins for connecting the LED string.
• An external and thus flexible setting (programming) of the forward current I F of an LED string is realized in that firstly an internal pull-up resistor Rj is connected to the internal voltage supply U v of the IC and to an input for an LED current reference , so that an external resistance R ext to ground forms a voltage divider with the internal pull-up resistor Rj and so the desired forward current I F is set, and that secondly at the input for the LED current reference a DC voltage that reaches the maximum forward current l F can be set, is made available, which serves as a measure of the forward current strength l F.
• Logic control of the module is realized in that a logic signal level (low or high) switches the module on or off via an input (ENABLE).
• An error message via a STATUS output is realized in that this output has an open collector ("Open Collector” for bipolar integration) or an open drain (Open Drain for CMOS integration) and by connecting an external pull-up resistor R P the Output signal level for the error signal level (high signal) can be freely defined.

Landscapes

• Led Devices (AREA)
• Control Of El Displays (AREA)
• Control Of Indicators Other Than Cathode Ray Tubes (AREA)
• Lighting Device Outwards From Vehicle And Optical Signal (AREA)
• Circuit Arrangement For Electric Light Sources In General (AREA)

Priority Applications (5)

Applications Claiming Priority (2)

Publications (1)

Family

ID=7913192

Family Applications (1)

Country Status (7)

Cited By (11)

Families Citing this family (177)

Citations (5)

Family Cites Families (6)

• 1999
  • 1999-06-30 DE DE19930174A patent/DE19930174A1/de not_active Withdrawn
• 2000
  • 2000-04-01 CA CA002341657A patent/CA2341657A1/en not_active Abandoned
  • 2000-04-01 DE DE50013044T patent/DE50013044D1/de not_active Expired - Lifetime
  • 2000-04-01 US US09/762,685 patent/US6400101B1/en not_active Expired - Lifetime
  • 2000-04-01 EP EP00926699A patent/EP1118251B1/de not_active Expired - Lifetime
  • 2000-04-01 AT AT00926699T patent/ATE331422T1/de not_active IP Right Cessation
  • 2000-04-01 JP JP2001508200A patent/JP2003504797A/ja active Pending
  • 2000-04-01 WO PCT/DE2000/000989 patent/WO2001003474A1/de not_active Ceased

Patent Citations (5)

Non-Patent Citations (1)

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