EP2001132A1

EP2001132A1 - Circuit and method for driving light emitting diodes

Info

Publication number: EP2001132A1
Application number: EP07010689A
Authority: EP
Inventors: Alessandro Scordino; Alessandro Brieda; Giovanni Scilla
Original assignee: Osram GmbH; Osram SpA
Current assignee: Osram GmbH; Osram SpA
Priority date: 2007-05-30
Filing date: 2007-05-30
Publication date: 2008-12-10

Abstract

A LED driver comprises a microcontroller that is adapted to perform at least two of the following functions: LED current regulation; dimming control; thermal management; input power absorption control; input current absorption control; alarms and fault conditions management; and LED light optical feedback.

Description

The invention relates to LED drivers and to a method to drive a LED system, in particular for use with high flux / high power LED systems.
In high flux / high power LED systems, often several functions / functionalities have to be performed to properly operate the LED system, e. g. to perform a LED current regulation, thermal management etc. Up to date, these functions are implemented using analog circuitry or integrated circuits dedicated to each functionality. The implementation requires a relatively large area on a respective printed circuit board (PCB), adds to the complexity of the system and reduces solution flexibility. Also, bill of materials (BOM) and manufacturing / quality costs are expensive.
It is thus the object of the present invention to overcome or improve at least some of the above stated problems.
The object is reached by the LED driver according to claim 1 and the method according to claim 8. Preferred embodiments are especially stated in the dependent claims.
The LED driver comprises a digital microcontroller that is adapted to perform at least two of the following functions:

LED current regulation;
dimming control;
thermal management;
input power absorption control;
input current absorption control;
alarms and fault conditions management; and
LED light optical feedback.

For this, the microcontroller is either programmed accordingly or hardwired, or both. The use of a single microcontroller gives the advantage of only needing a relatively small PCB area and only needing few components (e. g., omitting many auxiliary parts that are typical for an analog solution). Further, a programmable microcontroller achieves a higher flexibility of operation and reduced adaptation costs which is especially advantageously for the use in different platforms including different power ratings, LED source types, dimming requirements etc. Also, a smart management of auxiliary functions (e.g., optimized structure vs. RFI, active thermal management, alarms and fault condition management etc.) is easily implemented. Additionally, a simpler manufacturing and quality assurance can be achieved.
Advantageously, the LED driver further comprises a connection to a LED current sensor of the overall lighting system and comprises a current converter for converting an input current into an LED (source) current. To drive the converter, the microcontroller comprises a power stage controller. Thus, based on a reading of the LED current sensor, the power stage controller of the microcontroller is enabled to adjust the current converter to set the LED current. This is advantageously done by implementing the LED control components as a control loop. By using the microcontroller, current levels may be flexibly changed or adjusted on demand.
Advantageously, the dimming control includes a 3-wire dimming capability. This can be achieved by accessing a special function of the power stage controller of the microprocessor.
Advantageously, the LED driver further comprises a connection to at least one thermal sensor sensing a temperature related to the LED source(s). The microcontroller comprises a driver circuit for at least one fan. The microcontroller, based on a reading of the thermal sensor(s), is enabled to adjust the operating condition of the fan(s). The temperature sensing design advantageously comprises an analog digital converter device of the microcontroller which advantageously may only need one external resistor divider with a thermistor, for example. Further, only a simple driver circuit for the fan(s) may be implemented. This function may be implemented by dedicated software within the microcontroller.
Advantageously, the microcontroller, for input power absorption control, comprises at least one analog digital converter to sense an input voltage, an output current, and an output voltage, and the microcontroller setting an input power assuming a statistical efficiency value. The sensing may be performed by the ADC devices of the microcontroller.
Input current absorption from the power supply is important for a RFI point of view. To improve input current absorption, advantageously, the microcontroller is adapted to perform a sweep of the DC-DC converter switching frequency to spread noise along a wide range instead of a narrow distribution. This sweep may be implemented into the microcontroller by software.
Advantageously, the alarms and fault conditions management is achieved using interrupts and programming suitable operation logic components.
In a preferred embodiment, a LED driver further comprises a connection to an optical sensor; wherein the microcontroller in turn comprises a power stage controller such that, based on a reading of the optical sensor, the microcontroller is enabled to drive the power stage controller of the microcontroller in order to adjust an intensity of light emitted from at least one LED.
The method of driving a LED system is being performed by a microcontroller and comprises at least two of the following steps:

regulating a LED current;
controlling a dimming function;
performing a thermal management;
controlling an input power absorption;
controlling an input current absorption;
performing an alarms and fault conditions management; and
controlling the light emitted from the LED by means of optical feedback

The invention is schematically described in more detail in the following exemplary embodiment. Identical or similar components are denoted by the same reference numerals, where applicable.

FIG 1: shows a block scheme diagram of a lighting system comprising a LED driver according to the invention;
FIG 2: shows a schematic diagram of the driver according to the invention;
FIG 3: shows a layout drawing of the driver according to the invention;
FIG 4: shows a plot of RFI results in a first frequency range;
FIG 5: shows a plot of RFI results in a second frequency range; and
FIGs 6-10: show a software embodiment of a LED driver comprising several functions.

FIG 1 shows a block scheme diagram of components of a lighting system comprising at least parts of a LED driver 1 for three LED sources. At the beginning of the operation of the lighting system (activation), a system input voltage U = 24 V is applied. to a filter stage 2 after that the input voltage is input into a DC/DC converter 3 that transforms the input voltage of 24 V into a LED current suitable to operate the multiple LED sources 4. The LED current of about 600 mA is sensed by a LED current sense circuitry 5. Subsequently, the sensed current values are transmitted to a comparator 6 of a microcontroller 7. The comparator 6 is part of a current control section 8 of the microcontroller 7.
The comparator 6 compares the LED current with a reference current value I_REF. The result of this comparison, e. g. a value for the current difference, is fed back to a power stage controller section 9 of the microcontroller 7 that uses this value to set / drive the DC/DC converter 3 to control the LED current, thus being part of a LED current control loop.
The LED driver 1 further comprises an input voltage sense circuit 10 fed by the filter stage 2, and an output voltage sense circuit 11 fed by the LED 4. The voltage sense circuits 10,11 are input into a current and power absorption control section 12.
The microprocessor 7 further, via a thermal control section 13, senses temperature values from the LEDs 4by means of a temperature sensor 14. The thermal control section 13 further controls a fan driver 14 that in turn drives a fan 15 (e.g., a fan speed) to cool the LED sources 4 to - in turn - adjust their temperature. This can be regared as a LED source temperature control loop.
The microcontroller 7 also comprises a dimming control section 17 fed by a dimming input 18 and a alarms / fault conditions management section 19.
Additionally, a start-up control may be included (not shown).
FIG 2 shows a schematic diagram of the LED driver 1 with a relatively small number number of components and a small occupied space. Additionally to a combined filter stage 2 and input voltage sense circuit 10 there is shown the DC/DC converter 3, the LED current sense circuitry 5, the microcontroller 7, the output voltage sense circuit 11, the thermal sense circuitry 14, the fan driver 15, the dimming input 18, LED connections 20, and external connections 21.
FIG 3 shows a layout drawing of the LED driver 1. In addition to the filter stage 2, the DC/DC converter 3, the LED current sense circuitry 5, the microcontroller 7, the input voltage sense circuit 10 the output voltage sense circuit 11, the thermal sense circuitry 14, the fan driver 15, and the dimming input 18, there are shown receiving areas 22 for the LEDs. This layout is very compact due to the relatively small number of components needed to drive the lighting system.
FIG 4 shows RFI results of the lighting system in a plot of dB in µV over f in MHz in a first frequency range of 9 to 30 MHz with attached testing parameters.
FIG 5 shows RFI results of the lighting system in a plot of dB in µV over f in MHz in a second frequency range of 30 to 300 MHz.
FIGs 6 to 10 show control functions of a LED driver in a software embodiment. FIG 6 shows the respective flow diagram on a high level, including a query S1 for a dimming / no dimming routine, followed by a power control segment S2 and a frequency sweep segment S3. FIG 7 shows a more detailed view on the no-dim routine S4 that is followed by a power control routine S5 and, on FIG 8, a frequency sweep routine S6. FIG 9 shows the dim routine S7 in greater detail while FIG 10 shows a current control routine S8, followed by a sweep routine S9.
The invention is not limited to the shown exemplary embodiment. For example, the number of light sources could be othewr than three, e. g., one, two, four, or more. Also, the type of light sources is not limited LEDs but may, for example, also comprise laser diodes. Further, the microcontroller may comprise fewer functions than shown, or more, like an optical intensity control.

List of reference numbers

1: LED driver
2: Filter stage
3: DC/DC converter
4: LEDs
5: LED current sense circuitry
6: Comparator
7: Microcontroller
8: Current control section
9: Power stage controller section
10: Input voltage sense circuit
11: Output voltage sense circuit
12: Power absorption control section
13: Thermal control section
14: Temperature sensor
15: Fan driver
16: Fan
17: Dimming control section
18: Dimming input
19: Alarms / fault conditions management section
20: LED connections
21: External connections
I_REF: Reference current
S1: query for dimming / no dimming routine
S2: power control segment
S3: frequency sweep segment
S4: no-dim routine
S5: power control routine
S6: frequency sweep routine
S7: dim routine
S8: current control routine
S9: sweep routine

Claims

A LED driver (1), comprising a microcontroller (7) that is adapted to perform at least two of the following functions:
- LED current regulation;

- dimming control;

- thermal management;

- input power absorption control;

- input current absorption control;

- alarms and fault conditions management; and

- LED light optical feedback.
The LED driver (1) according to claim 1,
- further comprising a connection to a LED current sensor; and

- further comprising a current converter (3) for converting an input voltage into an LED current; and

- the microcontroller (7) comprising a power stage controller (9),

- such that, based on a reading of the LED current sensor, the power stage controller (9) of the microcontroller (6) is enabled to adjust the current converter (3) to set the LED current.
LED driver (1) according to claim 1 or 2, wherein the dimming control includes a 3-wire dimming capability.
The LED driver (1) according to any of the preceding claims,
- further comprising a connection to a thermal sensor; and

- the microcontroller (7) comprising a driver circuit (15) for a fan (16),

- such that, based on a reading of the thermal sensor, the microcontroller (7) is enabled to adjust the operating condition of the fan (16).
The LED driver (1) according to any of the preceding claims, wherein the microcontroller (7), for input power absorption control, comprises at least one analog digital converter (10, 11) to sense an input voltage, an output current, and an output voltage, and the microcontroller (7) setting an input power assuming a statistical efficiency value.
The LED driver (1) according to any of the preceding claims, wherein the microcontroller (7), for a input current absorption control providing reduced RFI, is adapted to perform a sweep of the DC-DC converter switching frequency.
The LED driver (1) according to any of the preceding claims:
- further comprising a connection to an optical sensor; and

- the microcontroller (7) comprising a power stage controller (9)

- such that, based on a reading of the optical sensor, the microcontroller (7) is enabled to drive the power stage controller (9) in order to adjust an intensity of light emitted from at least one LED (4).
A method of driving a LED system, the method being performed by a microcontroller (6) and comprising at least two of the following steps:
- regulating a LED current;

- controlling a dimming function;

- performing a thermal management;

- controlling an input power absorption;

- controlling an input current absorption;

- performing an alarms and fault conditions management; and

- controlling the LED emitted light by means of optical feedback
The method according to claim 8, wherein the step of regulating a LED current comprises:
- reading the actual LED current;

- converting an input voltage into a LED current based on the reading of the actual LED current.
The method according to claim 8 or 9, wherein the step of performing a thermal management comprises:
- reading a temperature; and,

- based on the reading of the temperature, adjusting the operating condition of a fan.
The method according to any of the claims 8 to 10, wherein the step of controlling the dimming function comprises:
- receiving an external dimming signal; and

- halting the power stage controller (9) for a time defined by the dimming signal.
The method according to any of the claims 8 to 11, wherein the step of controlling an input power absorption comprises:
- sensing an input voltage, an output current, and an output voltage; and

- setting an input power assuming a statistical efficiency value.
The method according to any of the claims 8 to 12, wherein the step of controlling an input current absorption providing reduced RFI comprises performing a sweep of the DC-DC converter switching frequency.