EP1118251B1 - Control circuit for led and corresponding operating method - Google Patents

Abstract

The drive circuit is suitable for an LED array, comprising a number of clusters of LEDs, with one cluster comprising a number of LEDs which are arranged in series and are connected to a supply voltage (UBatt). A semiconductor switch (transistor T) is arranged in series between the LED and the supply voltage and allows the LED current to be supplied in a pulsed manner. A measurement resistor (Rshunt) for measuring the LED current is arranged in series between the LED and ground, with a control loop controlling the semiconductor switch such that a constant mean value of the LED current is achieved.

Description

The power loss in the series resistor is converted into heat, resulting in additional heating - in addition to the self-heating of the LEDs in the strand - leads.

Technical area

The invention relates to a drive circuit for LED and associated operating method according to the preamble of claim 1. It is in particular the reduction of Ansteuerverlustleistung in light emitting diodes (LEDs) by means of a clocked LED drive circuit.

State of the art

From US Pat. No. 3,902,806 it is known to clock LEDs. In this case, a pulse width modulation is used.

In the control of light emitting diodes (LEDs) series resistors are used for current limiting in the rule, see for example US-A 5 907 569. A typical voltage drop at light emitting diodes (U _F ) is a few volts (for example, in Power TOPLED U _F = 2, 1V). The known series resistor R _v , in series with the LED (see FIG. 1), generates a high power loss particularly when the battery voltage U _{Batt is subject to} high voltage fluctuations (as is usual in a motor vehicle). The voltage drop across the LED remains constant even with such voltage fluctuations, ie the remaining voltage drops at the series resistor R _v . Thus, R _{v is} alternately loaded more or less strongly. In practice, several LEDs are usually connected in series (strand), in order to achieve better efficiency in the control (Figure 2). Depending on the vehicle electrical system (12 V or 42 V), many LEDs can be combined to form one line. In the 12V vehicle electrical system there is a lower limit of the battery voltage U _Batt , up to which legally prescribed safety devices (eg hazard warning lights) must be functional. It is 9 volts. This means that up to 4 Power TOPLEDs can be combined into one strand (4 x 2.1V = 8.4V).

The technical problem is to eliminate the additional heating (driving power loss through the series resistors). There are mutliple reasons for this. First, enormous losses occur in the series resistor; This can lead to several watts of power loss with larger LED arrays. On the other hand, this heating due to series resistors limits the operating range of the LEDs. At an increased ambient temperature T _A , the maximum forward current I _F = f (T _A ) must be reduced in order to protect the LEDs from destruction. That is, the maximum forward current I _F must not be kept constant over the entire range of ambient temperature from 0 to 100 ° C. In addition, when operating LEDs with series resistors, the fluctuating supply voltage is still a problem, as is often the case in automobiles (fluctuation of 8 to 16V in the 12V vehicle electrical system, fluctuation of 30 to 60V in the future 42V vehicle electrical system). Fluctuating supply voltages lead to fluctuating Durchlaßströmen I _F, then what different luminance levels and the associated causes brightness variations in the LEDs.

Previously, resistors were always used to limit the forward current through the LEDs. In most cases, a common board was used for all the series resistors and, if possible, mounted at an appropriate distance from the LEDs. This distance was selected so that the heating of the series resistors R _V did not influence the temperature of the LEDs.

Another problem is the choice of the maximum forward current I _F of LEDs. When operating LEDs with series resistors R _V , the maximum permissible forward current I _F can not be selected because at a higher ambient temperature T _A the forward current must be reduced. Therefore, one chooses a forward current I _F , which is smaller than the maximum allowable (Figure 3). In this way, although the temperature range for operating the LEDs is increased, but the forward current I _F is not optimally utilized. Using the example of FIG. 3 (Power TOPLED, type LA E675 from Siemens), one can see the forward current I _F as a function of the ambient temperature T _A. The maximum forward current I _F may be 70 mA up to an ambient temperature of 70 ° C. From an ambient temperature of 70 ° C then the forward current I _F must be linearly reduced until it is only 25 mA at the maximum permissible ambient temperature of 100 ° C. For the optimum utilization of this mode of operation of LEDs, a variable resistor R _{V would} have to be used.

Another problem is voltage fluctuations. Until now, there are no drive circuits for LEDs, which are in practical use to prevent the voltage fluctuations and thus Durchlaßstromschwankungen (brightness variations). They must therefore necessarily be tolerated.

Presentation of the invention

It is an object of the present invention to provide a drive circuit for LED according to the preamble of claim 1, which generates as little waste heat and power loss.

This object is solved by the characterizing features of claim 1. Particularly advantageous embodiments can be found in the dependent claims.

In order to eliminate the series resistor R _v and thus the large Ansteuerverlustleistung, is working with a clocked LED drive. 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 pnp-type, but also the npn-type is suitable if a charge pump is additionally used for driving), with its emitter to the supply voltage U _Batt (in particular battery voltage in the automobile). If the transistor T is conductive, a current i _{LED flows} through the LED string (which here consists, for example, of four LEDs), specifically until the transistor T is switched off again by a comparator. The comparator has its output connected to the base of the transistor. The 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 accordingly frequency 1 / T _p , since this beosnders good electromagnetic compatibility, but also others Pulse shapes such as sawtooth are possible) connected. Is the current amplitude of the triangular voltage U _D at the comparator is greater than the control voltage U _control, the transistor T is turned on. The current i _{LED flows} . Decreases the current amplitude of the triangular voltage below the constant value of the control voltage U _control at the comparator, the transistor T is again switched off. This rhythm repeats itself regularly with the frequency f, with which the triangle generator works.

In this way, the current flowing through the LEDs is clocked (Figure 4b). The rectangular pulses have a pulse width which 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 R _shunt between LEDs and ground (case 1) or between semiconductor switch (transistor T) and terminal of the supply voltage U _Batt (case 2)). The clocked current i _LED is tapped at the measuring resistor R _shunt . Connecting-ßend is formed over an aid of the average value of current i _LED. 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 average 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 nominal value, in the form of a reference voltage (U _Ref ), for the current regulation is likewise made available to the controller as a second input value. The control voltage U _control the output of the controller is set by the controller so that the ACTUAL value always corresponds as well as possible the desired value (in terms of voltage). If the supply voltage U _Batt changes during fluctuations, the turn-on duration of the transistor T and the length of the rectangular pulse (FIG. 4b) also adapts accordingly. This technique in itself is known as PWM (Pulse Width Modulation).

The advantage of a clocked current control for LED strings is primarily in the fast compensation of supply fluctuations of U _Batt by means of PWM. Therefore, the mean value of the LED current ( i _LED ) remains constant. So there are no brightness changes of the LEDs with voltage fluctuations more. Another advantage is the protection against destruction against excessive temperature, as explained above (depending on the ambient temperature T _A ).

The circuit according to the invention advantageously enables a detailed query of the operating states of individual LED strings. This allows simple error detection (query for short circuit, interruption) by sequential scanning (so-called LED SCANNING) of the individual LED strands.

In addition, the previously necessary large series resistor R _{V is} omitted for the adjustment of the current for the LED string. As an example, a car battery with 12 V is called, to which a LED strand with four LEDs of the type Power TOPLED (U = 2.1V typ.) Is connected. This would result in a power setting resistor R _v of about 250 mW at a conventional current setting. In contrast, with the arrangement according to the invention results in a power loss in the shunt resistor R _shunt of only about 5 mW (at current setting with PWM), ie a reduction in power loss by a factor of 50.

Another advantage is the simple current limitation of an LED string using a current-limiting semiconductor switch (preferably a transistor). As a switch can also serve a current-limiting circuit breaker, which automatically ensures that the clocked forward current I _F does not exceed a maximum limit, for example, a limit 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 the vehicle.

FIG. 5 shows a snapshot of an oscillogram of the clocked current profile of the LED drive circuit for a 12 V electrical system. It shows the peak current i _LED through the LEDs (Figure 5a), which is clocked and reaches about 229 mA. The pulse width is about 30 μs, the subsequent dead time 70 μs. This results in a mean current i _LED of 70 mA.

Furthermore, the associated clock frequency at the triangular generator is shown in FIG. 5b, its frequency is approximately 9.5 kHz (corresponding to approximately 100 μs pulse width). The control voltage U _rule is shown as a straight line (Figure 5c), it has a value of 3.2 V.

The hitherto necessary large series resistor R _v for current adjustment is thus eliminated. This is replaced by a small measuring resistor in the order of R _shunt = 1Ω.

Fluctuations in the supply voltage U _Batt are now compensated and the forward current I _F can simply be controlled constantly. Because if the value of the supply voltage changes, also the control voltage U changes _rule and thus the turn-on time of the transistor. As a result of this pulse width modulation, in which an increase in the supply voltage causes the transistor turn-on time to be shortened (vice versa, the same applies), a constant current, which is set in the form of a reference voltage U _Ref at the regulator, is automatically regulated (see FIG. 4a). Since, therefore, the forward current I _F in the LED string is constant, no brightness fluctuations with variable supply voltages can adjust more.

The circuit arrangement according to the invention makes it possible to regulate the temperature. According to FIG. 3 (using the example of the power TOPLEDs), the maximum forward current I _F of 70 mA may not be kept constant over the entire permissible temperature range (up to T _A = 100 ° C. ambient temperature). From an ambient temperature of T _A = 70 ° C, the forward current I _F must be reduced and finally switched off at T _A = 100 ° C. To realize a temperature control, a temperature sensor (preferably in SMD design) is applied to the board in the LED array and that at the hottest point to be expected. If an ambient temperature of at least T _A = 70 ° C. is measured by the temperature sensor, the forward current I _F is reduced in accordance with the specification in the data sheet (FIG. 3). At an ambient temperature T _A = 100 ° C, the forward current I _{F is} turned off. This measure of temperature control is required to protect the light-emitting diodes against thermal destruction by overheating and thus not shorten their life.

The detection of malfunctions in the LED string is easy with this circuit arrangement. If an LED string fails in an LED array (consisting of several LED strings), it may be important to immediately report this failure to a service center. This is particularly important in safety equipment, e.g. at traffic lights. Also in the automotive sector (cars, trucks), it is desirable to be informed about the current state of the LEDs, for example, when the taillights are equipped with LEDs.

The most common types of errors are open circuit and short circuit. The type of fault short circuit can be practically excluded with LEDs. If LEDs fail, then most of the time by a break in the supply line. A break in an LED is mainly due to heat. The cause lies in the expansion of the resin (epoxy resin as part of the housing) under the action of heat, so that the embedded differently extending bonding wire (connecting line between the LED chip and outer pin) breaks off.

Another possibility of destruction is also caused by heat. Too much heat softens the resin (ie the material that makes up the housing) and becomes viscous. The chip can break loose and begin to migrate. As a result, the bonding wire can also break.

In general, therefore, mechanical defects (such as bonding wire crack) are to be expected due to strong heat. Through an interrupt detection circuit in an LED string, it is possible to signal the occurrence of a fault to an output (e.g., a status pin in a semiconductor device). Logical 1 (high) means, for example, the occurrence of an error, logic 0 (low) means proper status.

The drive circuit according to the invention can be realized as a compact LED drive module (IC), which is characterized by the possibility of constant current control of the forward current (I _F = const.) LEDs. Further advantages are the external and thus flexible Durchlaßstromeinstellung, the small power loss by switching operation (elimination of the large resistor R _V ), the interruption detection in the LED string and the temperature control to protect the LEDs. Added to this is the low self-current consumption of the LED drive circuit (economical standby mode).

In standby mode, the LED drive module remains connected to continuous plus (battery voltage in the vehicle) while it is turned off, i. there is no current flowing through the LEDs. In this state, the drive module may only absorb a small amount of internal current (self-current consumption approaches 0) in order not to load the battery in the vehicle. This is the case when the car is e.g. parked in the garage or parked. An additional power consumption would unnecessarily burden the battery here. The LED control module is switched on and off via a logic input (ENABLE input).

The circuit can also perform verpolfest and secure against overvoltage. A polarity reversal protection diode ensures the case of a wrong Connection of the LED control module to the supply voltage (battery) before it is destroyed. A combination of a Zener diode and a normal diode additionally protects the LED drive module against destruction due to overvoltages at the supply voltage pin U _Batt .

In a particularly preferred embodiment, a microcontroller-compatible ENABLE input (logic input) is additionally provided, which enables the control with a microcontroller. Thus, it is possible to integrate the drive module (in particular an integrated circuit IC) for LEDs into a bus system (for example CAN bus in a motor vehicle, Insta bus for domestic installation technology).

characters

Figur 1

eine bekannte Ansteuerung für LEDs

Figur 2

ein weiteres Ausführungsbeispiel einer bekannte Ansteuerung für LEDs

Figur 3

die Abhängigkeit des Durchlaßstroms einer LED von der Umgebungstemperatur

Figur 4

das Grundprinzip einer getakteten Stromregelung für LED (Figur 4a) nebst einer Erläuterung des Spitzenstroms und Mittelwerts (Figur 4b)

Figur 5

den Stromverlauf einer getakteten Stromregelung für LED

Figur 6

eine getaktete Stromregelung mit Unterbrechererkennung

Figur 7

die Realisierung einer Unterbrechererkennung für einen LED-Strang

Figur 8

Blockschaltbild einer LED-Ansteuerschaltung

In the following the invention will be explained in more detail with reference to several embodiments. Show it:

FIG. 1

a known control for LEDs

FIG. 2

Another embodiment of a known control for LEDs

FIG. 3

the dependence of the forward current of an LED on the ambient temperature

FIG. 4

the basic principle of a pulsed current regulation for LED (FIG. 4a) together with an explanation of the peak current and average value (FIG. 4b)

FIG. 5

the current profile of a clocked current control for LED

FIG. 6

a clocked current control with breaker detection

FIG. 7

the realization of a breaker detection for an LED string

FIG. 8

Block diagram of an LED drive circuit

Description of the drawings

Figures 1 to 5 have already been described above.

An embodiment (entire block diagram) for the realization of an interruption detection is shown in FIG 6. The detection of an interruption in the LED string can via the direct monitoring of the control voltage U _rule by means of a Interrupt recognizer (see in detail Figure 7) done. In case of interruption is the rule zero voltage (U _control = 0). Via an evaluation circuit A (Figure 8), this error case can be displayed on an output (status pin).

It is advantageous to execute this output as an open-collector circuit (FIG. 8), because then the user of the circuit who later uses the LED drive module (IC) is independent of the output signal level. The circuit of the status output has as a final stage a transistor whose collector is open (ie has no pull-up resistor). The collector of the transistor leads directly to the status pin of the LED drive module (Figure 8). If an external pull-up resistor R _{P is} connected to the collector of the transistor T _OC , it can be connected to an arbitrary voltage V _cc . Accordingly, the output signal level depends on the voltage V _cc to which the pull-up resistor Rp is connected.

The technical realization of an interruption detection in the LED string is shown in FIG. The interruption detection in the LED string works according to the principle of scanning (scanning) a voltage (here: control voltage U _rule ). The control voltage U _rule has a minimum value which is as large as the smallest voltage U _{D_min of} the triangular generator. As is apparent from Figure 5, it is about 2 V. It is assumed that the scheme is active and there is no interruption in the LED string. In the case of an interruption in the LED cluster, the control voltage is 0 volts _(usually U = 0 V).

Figure 7 shows the complete block diagram of the interruption detection in the LED string according to the principle of sampling a voltage. From the internal oscillator (OSZ), which runs at a certain frequency (here: approx. 9.5 kHz), the clock (as a rectangular voltage U _R ) is applied to an n-bit binary counter (COUNTER). Depending on how many LED strings (and accordingly how many control voltages U _rule ) to be sampled, the interpretation of the binary counter must be made. By way of example, a 3-bit binary counter (for addresses from 0 to 7) is used. With it can be scanned so up to 8 control voltages U _rule .

The 3-bit binary pattern of the counter controls an analog multiplexer (MUX), which (depending on the applied binary word) scans each of the control _voltages U _{Regel1,2 ...} one after the other and provides them in turn at the output. The smallest control voltage U _{Regel_min} (regulation active and no interruption in the LED string) corresponds to the minimum value of the triangular voltage U _{D_min} .

A "low" signal of the control voltage U _usually detect (corresponding to 0 volts, interruption in the LED cluster) successfully and prepare it for subsequent storage in a storage medium, such as a flip-flop (FF) at the output of the analog multiplexer (MUX) a comparator (COMP) inserted. Its switching threshold U _SW must be smaller than the minimum value of the triangular voltage U _D , ie U _SW <U _{D_min} .

If a "low signal" is now detected for a sampled control voltage U _rule , a "high signal" is set at the comparator output. This high signal is then stored in the flip-flop (FF) until the error (interruption in the LED string) is corrected again.

High-Signal =

Unterbrechung in einem LED-Strang

Low-Signal =

keine Unterbrechung

The status output (status = output of the FF) has the following meaning:

High signal =

Open circuit in a LED string

Low signal =

no interruption

A reset of the flip-flop FF and thus the status output occurs only when the LED driver is turned off, i. if there is a bug in the LED string.

• Ausschalten des LED-Ansteuerbausteins (IC) über ENABLE Eingang. Der LED-Ansteuerbaustein (IC) ist über diesen Ausgang in einem System zusammen mit einem Microcontroller (µC) integriert (Figur 8). Im Kfz-Bereich kann die Ansteuerung z.B. über CAN-Bus erfolgen.
• Abklemmen der Versorgungsspannung am LED-Ansteuerbaustein (IC). Wird der ENABLE-Eingang nicht benötigt, ist dieser mit der Batteriespannung zu verbinden. In einfachen Systemen ohne Microcontroller-Ansteuerung ist diese Methode anzuwenden.

The reset of the status output can be done in 2 ways:

• Switching off the LED control module (IC) via ENABLE input. The LED drive module (IC) is integrated via this output in a system together with a microcontroller (μC) (FIG. 8). In the automotive sector, the control can take place, for example via CAN bus.
• Disconnect the supply voltage at the LED control module (IC). If the ENABLE input is not required, it must be connected to the battery voltage. In simple systems without microcontroller control this method is to be used.

The circuit arrangement for Verpoffestigkeit and overvoltage protection is also shown in Figure 8 (block diagram of the LED drive module). A polarity reversal protection diode between external (U _Batt ) and internal power supply ensures in the case of a wrong connection of the LED drive module to the supply voltage (battery) before its destruction. The overvoltage protection is realized with a Zener diode in combination with a reverse polarity diode.

The IC also includes a terminal pin for a temperature sensor (eg, an NTC) and a pin for connecting a current reference, and two pins for connecting the LED string.

An external and thus flexible adjustment (programming) of the forward current I _{F of} an LED string is realized by firstly connecting an internal pull-up resistor R _i to the internal voltage supply U _{V of} the IC and to an input for an LED current reference is such that an external resistor R _ext to ground with the internal pull-up resistor R _i forms a voltage divider and thus sets the desired forward current I _F , and that secondly at the input for the LED current reference, a DC voltage to the maximum forward current I _F can be adjusted is provided which serves as a measure of the forward current I _F.

A logic control of the device (IC) is realized in that via an input (ENABLE) a logic signal level (low or high) off or on the block.

An error message about a STATUS output is realized by the fact 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.

Claims (16)

1. Drive circuit for LEDs, in particular for an LED array, which comprises one or more clusters of LEDs with one cluster comprising a number of LEDs which are arranged in series, with the LEDs or the LED array being connected to a supply voltage (U_Batt), characterized in that a semiconductor switch (T) with two branches is arranged in series between the LED or the LED array and the supply voltage as part of the drive circuit, and makes it possible to supply the forward current (i_LED) in a pulsed manner for the LED or the LED array in the first branch which leads to the LED or the LED array, and in that a means for measurement of the forward current (i_LED), in particular a measurement resistor (R_Shunt), is arranged in series with the LED or LED array as part of the drive circuit in this first branch for the forward current (i_LED), in particular between LEDs and earth, with a control loop, which is connected to the second branch of the semiconductor switch, controlling the semiconductor switch (T) as part of the drive circuit in such a way as to achieve a constant mean value of the forward current, in that the control loop comprises an integration element, which supplies the ACT value of the mean value of the forward current, as well as a regulator which is connected to it and compares the ACT value of the mean value of the forward current with an external nominal value, with the regulator producing an output value of the control voltage, and the control loop furthermore having a comparator, which compares the signal from a frequency generator, in particular a triangle-waveform generator (OSZ), with the output value of the control voltage (U_Control), with the control process being carried out by pulse-width modulation.
2. Drive circuit according to Claim 1, characterized in that the semiconductor switch is a transistor (T).
3. Drive circuit according to Claim 1, characterized in that the regulation voltage (U_Reg) is monitored by a means for interruption identification.
4. Drive circuit according to Claim 3, characterized in that an LED array of a number of LED clusters is monitored by a frequency generator (OSZ) passing its clock to a binary counter which controls an analog multiplexer (MUX) which samples the regulation voltages (U_{Control1,2...}) of all the LED clusters of the array.
5. Drive circuit according to Claim 4, characterized in that the output signal from the multiplexer is passed via a comparator (COMP) to a memory medium (FF).
6. Drive circuit according to one of the preceding claims, characterized in that said drive circuit is in the form of an integrated module (IC) which is connected to the LEDs or the LED array and the supply voltage.
7. Drive circuit according to Claim 6, characterized in that in the module (IC) external, and thus flexible, adjustment (programming) of the forward current (i_LED) in an LED cluster is provided in that, firstly, an internal pull-up resistor R_i is connected to the internal voltage supply (U_v) of the module (IC) and to one input of an LED current reference, such that an external resistor (R_ext) connected to earth forms a voltage divider together with the internal pull-up resistor (R_i) and thus sets the desired forward current level (i_LED), and such that, secondly, a DC voltage which can be adjusted as far as the maximum forward current level (i_LED) is provided at the input for the LED current reference and is used as a measure of the forward current level (i_LED).
8. Drive circuit according to Claim 6, characterized in that a logic drive for the module (IC) is provided in that a logic signal level (low or high) for the module is switched off or on via an input (ENABLE).
9. Drive circuit according to Claim 6, characterized in that in the module (IC) fault signalling is provided via a STATUS output of the module (IC), in that this output has an open collector (for bipolar integration) or an open drain (for CMOS integration), and the output signal level for the fault signal level (high signal) can be freely defined by connection of an external pull-up resistor Rp.
10. Drive circuit according to Claim 6, characterized in that in the module (IC) protection against polarity reversal when the module (IC) is connected to a supply voltage (for example a motor vehicle battery) is provided in that a polarity reversal protection diode protects the internal circuits of the module.
11. Drive circuit as claimed in Claim 6, characterized in that in the module (IC) protection against any overvoltages which occur at the input of the module for the supply voltage is provided by a combination of a zener diode and a diode in the opposite polarity acts at the input pin for the supply voltage (U_Batt) .
12. Method for operation of an LED, in particular of an LED array, characterized in that the forward current (i_LED) of the LED is pulsed by means of a fast semiconductor switch (transistor T), and in that the actual value of the mean value of the forward current is compared with an external nominal value via a regulating means, with the regulation being carried out by pulse-width modulation.
13. Method according to Claim 12, characterized in that the output signal of the regulating means is compared with the signal from a frequency generator (OSZ), in particular from a triangle-waveform generator.
14. Method according to Claim 12, characterized in that the signal from the regulating means is monitored by a means for interruption identification, in particular a flipflop (FF), or by means of LED scanning.
15. Method according to Claim 12, characterized in that, in addition, temperature-dependent control of the forward current of the LEDs is provided in that a temperature-sensing element (in particular an NTC) can be connected via a sensor input, and the forward current (i_LED) is regulated back in accordance with a predetermined characteristic if the ambient temperature T_A exceeds a specific threshold value.
16. Method according to Claim 12, characterized in that the circuit can be operated with different supply voltages, in that the internal voltage supply produces a stable internal supply voltage from each input voltage (U_Batt).

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