WO2006018208A1

WO2006018208A1 - Driver circuit for operating loads having a constant output, in particular a light-emitting diode driver circuit

Info

Publication number: WO2006018208A1
Application number: PCT/EP2005/008689
Authority: WO
Inventors: Klaus Lorscheider
Original assignee: Avaya-Tenovis Gmbh & Co. Kg
Priority date: 2004-08-11
Filing date: 2005-08-10
Publication date: 2006-02-23
Also published as: DE102004039050A1

Abstract

The invention relates to driver circuit (1; 1'; 1') for operating a load (3) having an average constant output, in particular a light-emitting diode driver circuit, provided with a circuit converter which is used to produce the constant output from the unstable input voltage by limiting a primary flow. The circuit converter comprises a clock pulse source (f, 7; 7'), an, in particular, bistable trigger circuit which is connected to the input side of the clock pulse, a circuit device which is functionally linked to the input-side of the trigger circuit (17), an inductivity(19; 19') which can be linked to the input voltage by means of a circuit device via the rectifier device (21) and a current sensor which is associated with the input side of the inductivity, which is connected, on the output side, to a control inlet of the trigger circuit such that said trigger circuit is reset when a primary flow threshold value is reached.

Description

description

Driver circuit for operating loads with constant power, in particular light-emitting diode driver circuit

The invention relates to a driver circuit according to the preamble of claim 1. An¬.

Light-emitting diodes (LED), also referred to as light-emitting diodes, have been used for years as electro-optical transducer elements in display devices of the most diverse types and in extremely diverse fields of use. With the development of blue and in particular white light-emitting diodes (white light LED) and the considerable simplification and thus cheapening of their production as well as the provision of particularly high-performance light-emitting diodes with different light colors, these have also been strengthened in recent years for effect lighting, but also for display background lighting as well as for general lighting purposes.

For low-cost applications, such as for general lighting and in particular also the backlighting of the display fields of portable electronic devices (mobile phones, PDAs, navigation devices, etc.), in addition to a favorable price of the actual converter elements, the provision also plays a role tion of powerful and nevertheless inexpensive driver circuits an essential role.

For operating light-emitting diodes (or other consumers for power supply), there are many circuit proposals on the market and integrated driver circuits which provide the necessary voltages and currents for a few light-emitting diodes. The special feature when operating light-emitting diodes is that they must be powered by a current. The level of the current regulates the brightness.

BESTATIGUNGSKOPiE Voltage regulators with a series resistor or even current sources can be used to generate power, but they have low efficiency at higher input voltages. Also, charge pump IC ^'s are found for such applications, sometimes with a separate output for each LED since paralleling is problematic. In Rei¬ henschaltung few ^LEDs can be powered. Another group of drivers consists of coil transducers which convert the input voltage into a voltage or a current. Here only converters up to an input voltage of 28V were found, which is too little for telecom applications.

The following is an overview of common modern circuits from different manufacturers:

Maxim

Max1759 charge pump principle, converts max. 4V input voltage to 5V at the output;

It can max. 4 white LEDs can be operated in parallel (individual series resistors required).

MAX684 charge pump principle, converts max. 4V input voltage to 5V at the output;

It can max. 3 white LEDs can be operated in parallel (individual series resistors required).

MAX683 / 682 charge pump principle, converts max. 4V input voltage to 5V at the output;

It can max. 5 or 16 white LEDs are operated in parallel (individual series resistors necessary). MAX1605 single-coil converter; Boost converter of max. 28V at the entrance to max. 28V at the output

MAX1522 coil boost converter of max. 5.5V at the entrance to max. 50V at the exit

LM3354 / 3355 charge pump principle, converts max. about 4V input voltage at 4V at the output;

Only a few white LEDs can be operated in parallel, (individual series resistors required)

LM2791 / 92 charge pump principle, has four power sources for e.g. four white LEDs

LM2794 / 95

LM2703 / 04 single-coil converter; It can max. 4 white LEDs are operated in series.

Texas Instruments

TPS61040 charge pump principle; converts max. about 6V input voltage to max. 27V at the exit; no brightness control available.

It can be seen that most ICs tolerate only small input voltages and output voltages. Only relatively few LEDs can be operated. In addition, a dimming or shutdown is not possible everywhere. In order to set the current via a microcontroller, two possibilities were found:

1.) switching a D / A converter zw. Microcontroller and driver, 2.) switching on and off the entire driver circuit with low Fre¬ frequency

Those converters that produce a fixed output voltage (no fixed current) operate at higher power dissipation. The output current is often measured to control the current and the output current is kept constant by way of a necessary error amplifier.

These solutions are therefore also disadvantageous, either with regard to the effi¬ cient utilization of the available (especially in portable devices limited by the battery power) and / or with respect to ei¬ nen increased circuit and thus cost.

The invention is therefore based on the object of providing an improved driver circuit, which is simple and therefore kostengüns¬ tig can be realized and provides the provision of a suitable for an LED group, adjustable to a fixed value supply voltage with high energy efficiency, and in particular also work without interference and should be switched off.

This object is achieved by a driver circuit having the features of claim 1. Advantageous developments of the inventive concept are the subject of the dependent claims.

The core of the invention consists of a switching converter which generates an (largely) constant power (considered averaged) at the load (diodes) from an unstable voltage by limiting only the primary current. The circuit has a simple design, has a wide input and output voltage range, and you can adjust the output current via a clock source, which is another advantage.

With one edge of the clock source, a flip-flop is set, which turns on the switching transistor. The coil current increases up to a defined value at which the current measuring circuit resets the flip-flop, whereby the transistor switches off. This cycle is repeated continuously.

At a load, such as a light-emitting diode is thus generated a constant and independent of the input voltage power. Since the coil always charges to the same current value, the transmitted energy is not dependent on the input voltage. By limiting the primary switching current, therefore, the secondary current is also limited if the output voltage remains constant.

The clock source can be an oscillator or an already existing microcontroller. Changing the frequency also changes the current through the load. This works particularly easily with the microcontroller, in which the frequency output at the port is changed by software. In addition to the control and oscillator, a complete PWM controller with current feedback can also be used.

For the component-side realization of the proposed Treiberschal¬ device there is a variety of combination options. Thus, in an expedient embodiment, the switching device has a bipolar transistor or-if the gate voltage is sufficient-alternatively also a field-effect transistor or is designed as such. The coil or throttle zuge¬ arranged rectifier device is designed in a particularly simple manner as a semi-conductor diode. A differentiator at the input of the flip-flop can, in particular in its design as a flip-flop, deliver in a simple manner the needle pulses necessary for setting it.

For peak current cut-off, which resets the flip-flop when a threshold primary current is reached, in a preferred embodiment, in addition to the current sensor, a suitable interference voltage filter and a transistor which realizes the function of resetting the flip-flop are included.

If there is no load at the output, the output voltage can rise to a high value, which should be prevented to protect the components or against contact.

Only in these cases a surge protection is needed. This expediently generates an offset voltage when the output voltage at the switching device is too high and thus reduces the power.

The advantages and expediencies of the invention will become apparent from the explanation of exemplary embodiments with reference to the figures. From these show:

1 is a schematic circuit diagram of a driver circuit according to a first embodiment of the invention,

Fig. 2 is a schematic circuit diagram of a driver circuit according to a second embodiment of the invention and

3 shows a schematic circuit diagram of a driver circuit according to a third embodiment of the invention and FIG

4 shows a modification of the first to third embodiments. Fig. 1 shows - in the manner of a schematic diagram - a driver circuit 1 with constant power to supply a group of LEDs 3, which generates at a largely constant output voltage Uaus (on average) largely constant current laus. A voltage generator 5 provides a voltage VCC for a vibration generator 7 and a control unit 9, which comprises a flip-flop (not separately designated). At the oscillator 7 is on the input side via a control voltage U adjustable clock frequency f = F (U), and it outputs pulses with the frequency f to a differentiator 9a in the control unit 9 from. The generation of the oscillation frequency in accordance with f = F (U) can be realized by a voltage / frequency converter, for example by means of a PLL module in the manner of the known 74HC4046, wherein the variable voltage is supplied, for example, by a microcontroller with D / A converter. Converter output can come.

On the other hand, the frequency f of the vibrations generated can be varied via an adjustable resistance 1 1 in accordance with the relationship f = F (R). In the embodiment shown, the dashed lines around the oscillation generator 7 and the control unit 9 make it clear that they are implemented integrally as a PWM controller 13, for example of the Si91 14 type.

On the output side, the control unit 9 is connected via a series resistor 15 to the base of a bipolar transistor 17 as a switching device whose emitter is connected to a node between a coil 19 and a rectifier diode 21 and whose collector is grounded via a serving as a current sensor resistor 23. At the current sensor 23, a current Imess is measured and fed to a corresponding input of the control unit 9, where he - as stated above - upon reaching a threshold value resetting the there provided flip-flop be¬ acts. Via the circuit of the coil 19 and the diode 21, the output current laus is finally provided for the LEDs 3. On the output side of the Driver circuit 1 are otherwise still a filter capacitor 25 and (optio¬ nal) an overvoltage protection 27 is provided, which is also controlled by the control unit 9 and whose function is mentioned above.

FIG. 2 shows a driver circuit 1 'which is modified from the embodiment shown in FIG. 1 and described above. To avoid repetition, the elements matching the first embodiment are denoted by the same numerals as there and will not be mentioned again here.

The essential difference from the first embodiment is that the oscillator 7 on the one hand and the control unit 9 with differentiating member on the other hand are provided here as separate assemblies, wherein the place of the vibrator 7 in Fig. 1, a microcontroller 7 'has entered, the one software-controlled set oscillation frequency f. From the oscillator frequency, it is possible to derive clocks via hardware or software timer with which the downstream flip-flop (or control unit 9) is driven. Another possibility is to use an existing PWM port of the microcontroller 7 ', where only the clock at this output is used. Furthermore, the possibility of setting the frequency via an adjustable resistance provided in FIG. 1 is omitted here.

FIG. 3 shows, as a further embodiment, a driver circuit 1 ", wherein, again, the parts corresponding to the embodiments according to FIGS. 1 and 2 are denoted by the same reference numerals as there and will not be explained further here.

First of all, it can be seen in this embodiment that as an (optional) overvoltage protection on the output side, here a circuit of one Zener diode 27a and a bipolar transistor 27b connected in parallel to the LEDs 3 ge. With dashed lines there is also symbolized that even a Zener diode can perform this function.

In addition, a control input 28 is provided here, via which the oscillator can be switched inactive with a logic LOW ("0"), ie. no longer delivers a beat. Without a clock, the flip-flop (in the controller 9 ') is no longer set, and the driver circuit is thereby completely switched off and any energy transport to the output side is prevented.

Furthermore, it is shown here that the current measurement signal Imess is fed from the current sensor (measuring resistor) 23 via an RC element 29 and a bipolar transistor 31 of the control unit serving as a peak current cut-off.

For generating the oscillations 7 ¹ and 9 'of the driver circuit, integrated circuits IC 1, for example of the type 74HC 132, are provided here, which are linked in the manner shown in the figure. A modified differentiating element 9a 'is formed by an RC circuit which connects the output of the oscillation generator 7' to the input of the controller 9 '.

4 shows a detail of a further modification of the driver circuit, namely by replacing the coil 19 present in the previously described embodiments by a transformer 19 'for energy transmission. This circuit has the advantage that the mass of the input voltage Uein can be equal to the mass of the output voltage Uout. Incidentally, in this embodiment, an overvoltage protection is provided only op¬ tionally, in the form of Zenerdio¬ de shown 27a dashed lines. The embodiment of the invention is not limited to these examples but is also possible in a large number of modifications which are within the scope of expert action.

LIST OF REFERENCE NUMBERS

1, r, 1 "driver circuit Uout output voltage

3 LED group VCC voltage

5 voltage generators

7 vibration generator

T microcontroller

9 control unit

9 'control

9a differentiator

9a 'differentiator

11 resistance

13 PWM controller / pulse control unit

15 ballast resistor

17 bipolar transistor

19, 19 'coil

21 Gleirichterdiode

23 current sensors

25 filter capacitor

27 overvoltage protection

27a zener diode

27b bipolar transistor

28 control input

29 RC element

31 bipolar transistor

f frequency

F = F (U) clock frequency lout output current

IC1 integrated circuits

U control voltage

Claims

claims

1. Driver circuit (1, 1 ', 1 ") for operating a load (3) with on average constant power, in particular light-emitting diode driver circuit, characterized by a switching converter for generating the constant output current from an unstable input voltage while limiting a primary current, wherein the switching converter comprises:

a clock source (f, 7; 7 '),

an input side connected to the clock source, in particular bistable, flip-flop,

a switching device (17) operatively connected on the input side with the flip-flop element,

- An inductance (19, 19 ') and connectable by the switching device via a rectifier device (21) with the input voltage

- One of the inductance on the input side associated current sensor, the output side is connected to a control input of the flip-flop such that it resets this upon reaching a primary current threshold.

2. Driver circuit according to claim 1, characterized in that the switching device has a bipolar or field effect transistor (17) auf¬.

3. Driver circuit according to claim 1 or 2, characterized in that the rectifier device comprises a semiconductor diode (21).

4. Driver circuit according to one of the preceding claims, characterized in that a vibration generator (7, 7 ') is provided in the driver circuit as the clock source.

5. Driver circuit according to one of the preceding claims, characterized in that the tipping member on the input side, a differentiating element (9a, 9a ') is assigned.

6. Driver circuit according to one of the preceding claims, characterized in that as a clock source (f, 7; 7 ') a pulse control unit (9; 9') for adjusting the output current of the driver circuit via an impulse size of the flip-flop provided or the flip-flop as a functional section ei¬ ner such acting pulse control unit (13) is executed.

7. Driver circuit according to one of the preceding claims, characterized in that the flip-flop and / or the optionally provided impulse Steuerein¬ unit (9; 9 '; 13) has an integrated circuit.

8. Driver circuit according to one of the preceding claims, characterized in that at its output the load (3) is associated with an overvoltage protection device (27; 27a, 27b).

9. Driver circuit according to claim 8, characterized in that the overvoltage protection device comprises a Zener diode (27a) and / or a bipolar transistor (27b).

10. Driver circuit according to one of the preceding claims, characterized in that at the output as an inductance, a transformer (19 ') for providing the kons¬ tanten output power to the load (3) is provided.