Control circuit for led and corresponding operating method

Drive circuit for LED and associated operating method

Technical field

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

State of the art

When controlling light emitting diodes (LEDs) usually resistors are used for current limiting, for example, see US-A 5 907 569. A typical voltage drop across diodes (U) is a few volts (for example, is at Power TOPLED U_F = 2.1V). The known resistor R_v, In series with the LED (see Figure 1), produced especially a high power loss, when the battery voltage U_Batt high voltage fluctuations (such as in the automotive practice) subject. The voltage drop across the LED remains constant even when such voltage fluctuations, that is, the remaining voltage drops across the series resistor R. Thus R is_v alternately more or less heavily loaded. In practice, a number of LEDs are switched (phase) connected in series to achieve better drive efficiency (Figure 2). Depending on-board power supply (12 V or 42 V), many LEDs are combined into a strand accordingly. In 12V electrical system there is a lower limit of the battery voltage U_Batt> must be up to the legally required safety devices (eg hazard lights) function. She is 9 volts. This means that up to 4 Power TOPLEDs here are combined into a strand (4 x 2.1V = 8.4V).

The power loss in the series resistor is converted into heat, leading to additional heating - in addition to the self-heating of LEDs in the string - leads. The technical problem is to eliminate the additional heating (Ansteuerverlustleistung through the series resistors). There are several reasons. For the first huge losses in the resistor; this can in larger LED arrays lead to several watts power dissipation. For the second straight restricts these heating resistors by a the operating range of the LEDs. With an increased ambient temperature T_A has the maximum forward current l_F = F (T_A) Can be reduced, to protect the LEDs from being destroyed. That is the maximum forward current l_F can not be kept constant over the entire range of the ambient temperature of 0 to 100 ° C. In addition comes the operation of LEDs with series resistors still a problem, the fluctuating supply voltage added as (variation of 8 to 16V in 12V vehicle electrical system; fluctuation of 30 to 60V in future 42V vehicle electrical system) in automobiles is often the case. Fluctuating supply voltages lead to fluctuating Durchlaßströmen l_FWhat then different luminance levels and the associated brightness keitsschwankungen causes at the LEDs.

Series resistors have always been used to limit the forward current through the LEDs. In most cases, a common printed circuit board was used for all these series resistors and, if possible, mounted at a suitable distance to the LEDs. This distance was chosen so that the heating of the resistors R_v no temperature influence took on the LEDs.

Another problem is the choice of the maximum forward current l_F of LEDs. In the operation of LEDs with series resistors R_v can not the maximum forward current l_F be selected because at a higher ambient temperature T_A the forward current must be reduced. therefore One chooses a forward l_FWhich is smaller than the maximum allowable (Figure 3). In this way, although the temperature range is increased to drive the LEDs, but the forward l_F is not optimally utilized. Using the example of Figure 3 (Power TOPLED, Type LA E675 Fa. Siemens) shows the forward current l_F in dependence on the ambient temperature T_A, The maximum forward current l_F can be up to an ambient temperature of 70 ° C here 70 mA. From an ambient temperature of 70 ° C must be the forward l_F he will be linearly reduced until at the maximum permissible ambient temperature of 100 ° C is only 25 mA. For the optimal use of this mode of LEDs would have a variable resistor R_v be used.

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

Summary of the Invention

It is an object of the present invention to provide a control 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 configurations are given in the dependent claims.

To the series resistor R_v and thus to eliminate the large Ansteuerverlustleistung, is carried out with a pulsed LED drive. Figure 4a shows the principle of pulsed current regulation for LEDs. A semiconductor switch, such as a current-limiting circuit breaker, or preferably a transistor T (in particular of the pnp type, but also of the npn type is suitable when an additional charge pump is used for driving) has its emitter connected to the supply voltage U_ßatt (Especially battery voltage in the car) connected. When the transistor T conductive, a current flows i_LED through the LED strand (which here for example, four LEDs), namely so long until generator by a comparator, the transistor T is switched off again. The comparator is connected by its output to the base of the transistor. Of a (positive) input of the comparator is connected to a control voltage, the second (negative) input of the comparator to a frequency generator (preferably triangular waveform generator with pulse duration T_p and, accordingly, frequency 1 / T_PSince this beosnders beisitzt good electromagnetic compatibility, but also other pulse shapes, such as sawtooth are possible) connected. If the current amplitude of the delta voltage U_D the comparator is greater than the control voltage U_livelyi, the transistor T is turned on. It flows the current i_LED, If the actual amplitude of the delta voltage under the constant value of the control voltage U_reG_ei am comparator, the transistor T is again the off. This rhythm is repeated periodically with the frequency f, with the triangle generator operates.

In this way, the LEDs via the current flowing is clocked (Figure 4b). The square-wave pulses have a pulse width which is a fraction of T_p equivalent. The distance between the rising edges of two pulses corresponding to T_p,

The LEDs are connected in series with a means for measuring the current (in particular, a measuring resistor Rs_hunt between LEDs and ground (case 1) or between semiconductor switch (transistor T) and clamp the supply voltage U_Batt (case 2)). Clocked current i_ED is the sensing resistor Rs_Hunt tapped. Subsequently is a tool the average current_LED educated. The tool is for example an integrating means (in case 1), preferably an RC low-pass filter, or a differential amplifier (in case 2). This average is used as actual value for current control, which is provided to a controller (such as a PL or PID) as an input value. A desired value, in the form of a reference voltage (U_Ref), For the current control is also provided to the controller as a second input value available. The control voltage U_livelyι at the controller output is adjusted by the regulator that the actual value is always as good as possible to the desired value (in terms of voltage) corresponds. If in case of fluctuations of supply voltage U_Batt changed, also the duty of the transistor T and the length of the rectangular pulse (Figure 4b) adjusts accordingly. This technique is known per se as PWM (Pulse Width Modulation).

The advantage of pulsed current regulation for LED clusters is primarily the rapid compensation of fluctuations of supply U_Batt by PWM. Therefore, the average value of the LED current (/ stays_Le_D) constant. So there are no brightness variations of LEDs when voltage fluctuates more. Another advantage is the protection against destruction against excessive temperature, explained (depending on the ambient temperature above T_A).

The inventive circuit advantageously enables detailed monitoring of the operating states of individual LED strands. This enables simple error detection (search for short, interruption) by sequential scanning (so-called. LED SCANNING) of the individual LED strings. In addition, the previously required large series resistor R_v deleted for setting the current for the LED cluster. As an example, a car battery with 12 V may be mentioned, on the one LED strand with four LEDs of the type Power TOPLED (U = 2.1V typ.) Is connected. This would result in a conventional current setting in a power loss in the current setting resistor R_v of about 250 mW. However, it follows with the inventive arrangement, a power loss in the shunt resistor R_{S unt} of only about 5 mW (power adjustment with PWM), ie a reduction of the power loss by a factor of 50th

Another advantage is the simple current limiting of an LED strand using a Stromb-egrenzenden semiconductor switch (preferably a transistor). The switch can also serve as a current-limiting circuit breaker which automatically ensures that the clocked forward current l_F does not exceed a maximum limit, for example, a threshold of 1 A.

The inventive circuit arrangement is suitable for different requirements, such as a 12V or 42V vehicle electrical system in the vehicle.

Figure 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 by the LEDs (Figure 5a), which is clocked by approximately 229 mA. The pulse width is about 30 microseconds, the subsequent dead time 70 microseconds. This gives an average current i_ED of 70 mA.

Further shown in Figure 5b the associated clock frequency at the triangle generator, its frequency is about 9.5 kHz (corresponding to about 100 microseconds pulse width). The control voltage U_livelyι is shown as a straight (Figure 5c), it has a value of 3.2 V.

The previously required large series resistor R_v the current setting is thus omitted. This is replaced by a small measuring resistor in the order of 1Ω.

Fluctuations in the supply voltage U_Badd are now compensated and the

forward current l_F is easy to regulate constant. For if the change value of the supply voltage, also the control voltage U changes_livelyι and so that the ON time of the transistor. This pulse-width modulation in which an increase in the supply voltage to shorten the Transistoreinschaltzeit causes (the reverse is the same), is automatically always to a constant stream of U in the form of a reference voltage_Rβf is set on the regulator, regulated (see Figure 4a). Thus, since the forward current l_F in the LED cluster is constant, no brightness variations with varying supply voltages may choose more.

The inventive circuit arrangement makes it possible to regulate the temperature. After Figure 3 (the example of the Power TOPLEDs) may indeed the maximum throughput laßstrom l_F from here 70 mA not over the entire permissible temperature range (up to T_A = 100 ° C ambient temperature) are kept constant. From an ambient temperature of T_A = 70 ° C has the forward l_F be reduced and at T_A = 100 ° C will eventually shut down. In order to realize a temperature control, a temperature sensor (preferably in SMDs) with applied to the board in the LED array and indeed to the anticipated hottest spot. If the temperature sensor at an ambient temperature of at least T_A = 70 ° C measured, there is a reduction of the forward 1_F, According to the specification in the data sheet (Figure 3). At an ambient temperature T_A = 100 ° C, the forward current l_F off. This measure of the temperature control is required in order to protect the LEDs from thermal damage by overheating, and thus not to shorten their life span.

The detection of malfunctions in the LED cluster coincides with this circuit easily. If a LED cluster in an LED array (consisting of multiple LED clusters) from, it may be important to put this failure immediately to a maintenance report to. This is particularly important for safety equipment, eg 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 rear lights are equipped with LEDs.

The best-known types of errors are interruption and short circuit. The fault short circuit can be virtually eliminated with LEDs. If LEDs fail, it is usually due to an interruption of the supply line. An interruption in an LED is mainly due to the effect of heat. The cause is in the expansion of the resin (epoxy resin as part of the housing) under heat so that the embedded therein, differently expanding bonding wire (connecting line between the LED chip and Außenpin) breaks.

Another possibility for destruction is also pre-call due to heat produced. Due to excessive heat softens the resin (ie the material from which the housing) and is viscous. The chip can break off and begins to wander. This can also break the bonding wire.

In general, therefore are expected by extreme heat mechanical defects (such Bonddrahtriß). A circuit for interruption identification in the LED cluster, it is possible to signal the occurrence of an error to an output (eg status pin in a semiconductor device). Logic 1 (high) means, for example, an error occurs, logic 0 (low) means normal status.

The inventive control circuit can be as compact LED drive module (IC) realize that by the possibility of the constant current control of the forward (1_F = Const.) Is characterized in LEDs. Further advantages are the external and thus flexible Durchlaßstromeinstellung, the small power dissipation caused by switching operation (elimination of the large series resistor R_v), The interruption identification in the LED cluster and the temperature control to protect the LEDs. In addition, the low intrinsic power consumption of the LED drive circuit is (economical standby mode).

In standby mode, the LED drive to the permanent plus (battery voltage in the vehicle) is connected, while it is switched off, ie no current flows through the LEDs. In this state, the drive module may only small equity ström (Internal power consumption goes to 0), so as not to drain the battery in the vehicle. This is the case when the car is parked for example, or parked in the garage. An additional power consumption would unnecessarily burden the battery here. On and off the LED drive via a logic input (ENABLE input).

The circuitry can also reverse polarity protection run and secure it against overvoltage. A polarity reversal protection diode provides for the case of a wrong Terminal of the LED trigger component to the power supply (battery) before its destruction. A combination of a Zener diode and a normal diode protects the LED drive additionally from destruction by surges on supply voltage pin U_Batt-

In a particularly preferred embodiment, additionally a micro-controller-compatible ENABLE input is still provided (logic input), which enables the control of a microcontroller. Thus it is possible to drive module (in particular an integrated circuit IC) for LEDs in a bus system to integrate (eg CAN bus in the automotive, Insta bus for domestic installations).

characters

The invention is illustrated by reference to several embodiments. Show it:

Figure 1 shows a known drive for LEDs

2 shows a further embodiment of a known control for

LEDs Figure 3 shows the dependence of the forward current of an LED on the ambient temperature

4 shows the basic principle of pulsed current regulation for LED (Figure 4a) and an explanation of the peak current and the average value (Figure 4b)

Figure 5 shows the current profile of pulsed current regulation for LED 6, a pulsed current regulation with interrupter identification

Figure 7 shows the realization of a breaker recognition for an LED cluster

Figure 8 block diagram of an LED drive circuit

DESCRIPTION OF THE DRAWINGS

The figures 1 to 5 have already been described above.

An embodiment (entire block diagram) for the realization of a com- terbrechungserkennung shows figure 6. The detection of an interruption in the LED cluster can direct monitoring of the control voltage U_livelyι by a Unterbrechungserkenners (see in detail Figure 7) take place. In case of interruption, the control voltage is zero (U_livelyι = 0). About an evaluation circuit A (Figure 8) can be shown that an error occurs at an output (status pin).

It is favorable to perform this output as an open collector circuit (Figure 8), since the user of the circuit of the LED drive module (IC) used later by the output signal level is then independent. The circuit of the output status has as a final stage a transistor whose collector is open (ie no pull-up resistor has). The collector of transistor leads directly to the status pin of the LED trigger component (Figure 8). Is connected to the collector of the transistor T₀c an external pull-up resistor R_P connected, it can with any voltage V_cc get connected. The output signal level therefore depends on the voltage V_cc from to which the pull-up resistor R_P connected.

The technical implementation of an interruption identification in the LED cluster is shown in FIG. 7 The interruption identification in the LED cluster operates on the principle of scanning (scanning) a voltage (here: control voltage U_livelyι). The control voltage U_livelyι has a minimum value which is as large as the smallest voltage U_D_ i_n the triangle generator. As shown in Figure 5, it is about 2 V. It is assumed that the control is active and no interruption in the LED cluster prevails. In case of interruption in the LED cluster, the control voltage is 0 volts (U_livelyι = 0 V).

Figure 7 shows the complete block diagram of the interrupt detection in the LED cluster on the principle of sampling a voltage. From the internal oscillator (OSZ), which runs at a particular frequency (here, approximately 9.5 kHz), the clock (as rectangular voltage U_R) Was added to a n-bit binary counter (COUNTER). Depending on how many LED clusters (and, accordingly, how many control voltages U_livelyi) to be scanned must be made to the interpretation of the binary counter. By way of example a 3-bit binary counter (for addresses 0 to 7) is used. With him, therefore, up to 8 regulation voltages U_livelyι are scanned.

The 3-bit binary pattern of the counter controls an analog multiplexer (MUX), which (depending on the applied binary word) all control voltages U_Rβgeιι,₂... Sequentially scans and they in turn provides the output. The smallest re- gelspannung U_regβL_mι_n (Regulation active and no interruption in the LED cluster) represents the minimum value of the triangular voltage U __mιn-

To a "low signal" the control voltage U_Regegg (corresponding to 0 volts, interruption in the LED cluster) to detect successfully and prepare it for subsequent storage in a storage medium, such as a flip-flop (FF), is at the output of the analog multiplexer (MUX), a comparator (COMP) inserted. Meanwhile, switching threshold U_Sw must be less than the minimum value of the triangular voltage U_DSo U_Sw <U _ ιn.

If now a "low signal" at a sampled control voltage U_livelyι detected, a "high" signal is set at the comparator output. This high signal level remains stored in the flip-flop (FF) until the fault (interruption in the LED cluster) is fixed again.

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

High signal = interruption in an LED cluster

Low signal = no interruption

A reset of the flip-flop FF and thus the status output is carried out only when the LED drive is switched off, ie when a troubleshooting LED strand takes place.

Resetting (reset) the status output can be done in 2 ways:

• Turning off the LED trigger component (IC) via ENABLE input. The LED drive module (IC) is integrated via this output in a system together with a microcontroller (.mu.C) (Figure 8). In the automotive sector, the control can be set for example via the CAN bus.

• disconnecting the power supply to the LED drive module (IC). If the ENABLE input is not required, this must be connected to the battery voltage. In simple systems without microcontroller control, this method is applied.

The circuitry for polarity reversal and surge protection is also shown in Figure 8 (block diagram of the LED drive). A reverse polarity protection diode between external (U_Batt) And internal power supply provides for the case of a wrong connection of the LED trigger component to the power supply (battery) before its destruction. The overvoltage protection is provided with a zener diode in combination with a gegengepolten diode.

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

An external and thus flexible adjustment (programming) of the forward current l_F one LED strand is realized in that firstly, an internal pull-up resistance Rj to the internal voltage supply U_v of the IC and connected to an input of an LED current reference, such that an external resistor R_ext to ground with the internal pull-up resistor Rj forms a voltage divider and thus the desired Durchlaßstromstärke l_F sets, and secondly, that the input for the LED current reference, a DC voltage to the maximum Durchlaßstromstärke l_F can be adjusted, is provided, which as a measure of the Durchlaßstromstärke l_F serves.

A logic control the block (IC) is realized by an input (ENABLE) a logic signal level (low or high) Removing the block or turns.

An error message about a STATUS output is realized by the output open circuit ( "Open Collector" for bipolar integration) or an open drain has (open drain for CMOS integration) and R by connecting an external pull-up resistor_P the output signal level of the error signal level (high signal) can be freely defined.