Patent Application
20120133299
The invention generally relates to driver circuitry, in particular, to circuitry configured to drive illumination devices based on light emitting diodes (LEDs).
As light emitting diodes (LEDs) are increasingly used for illumination purposes, in particular, as a substitute for light bulbs, adequate driver circuitry has been subject to research and development in recent times. Inter alia, one desired object of such development efforts is to increase efficiency, that is to reduce power dissipation in the driver circuitry. Other development goals include, an increased flexibility of use and low costs.
One LED based illumination device usually includes a series circuit of a plurality of LEDs, a so-called LED chain. As LEDs usually have to be driven by a defined current, each LED in a LED chain is supplied with a fixed (not necessarily the same for all the LED chains) current. The supply voltage, necessary for driving the LED chain depends on the number of LEDs present in the chains because the forward voltages of each of the single LEDs sum up to the required supply voltage of the LED chain. It is known that the forward voltages may heavily vary due to temperature variations, variances in the manufacturing process and other parameters. As a consequence, the supply voltage necessary to provide a desired load current may vary and the driver circuitry used to drive the LED chain should consider such variations.
In order to guarantee a defined brightness and color hue, the supply current of the LED chain is to be monitored and regulated so as to stay at a predefined reference level or at least stay within a small interval around the reference level. Linear current regulators are commonly used for the described purpose of supplying a defined current to the LEDs. However, the driver circuit has to be designed for the worst case, that is for the maximum possible supply voltage which might occur across the LED chain. Such a design entails undesirably high losses in the above-mentioned current regulators.
A driver circuit for driving at least two LED chains is described. In accordance with an embodiment of the invention the driver circuit includes a buck converter associated with each LED chain for supplying a load current thereto. The buck converter receives an input voltage and is configured to provide such a supply voltage to the associated LED chain that the resulting load current of the LED chain matches at least approximately a predefined reference current value. The driver circuit further comprises a switching converter that receives a driver supply voltage from a power supply and provides, as an output voltage, the input voltage for the buck converters. The switching converter is configured to provide an input voltage to the buck converters so that the maximum of the ratios between the input voltage and the supply voltages provided to the LED chains matches a predefined tolerance reference ratio.
Further, a method for driving at least two LED chains is described. In accordance with a further embodiment of the invention the method includes providing a driver input voltage to a switching converter. The driver input voltage is converted into a common input voltage in accordance with a switching converter duty cycle. For each LED chain, in accordance with a buck converter duty cycle the common input voltage is converted into a supply voltage for the respective LED chain using a buck converter such that a resulting load current supplied to the LED chain matches a desired reference value. The switching converter duty cycle is regulated dependent on the buck converter duty cycles such that a maximum duty cycle of the buck converter duty cycles matches a predefined reference duty cycle.
The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, instead emphasis being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts. In the drawings:
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
In order to provide a defined current, the buck converters 1 may receive a current feedback signal V1, V2, from the connected LED chains LD1, LD2. The current feed back signals V1, V2, may be the voltage drop across a shunt resistor RS1, RS2 included in or connected to the respective LED chain LD1, LD2. Of course any other current measuring device connected to or included in the LED chains LD1, LD2 may be readily applied to generate respective current feed back signals V1, V2, that are representative for the load currents flowing through the respective LED chains LD1, LD2. Various current measurement methods may be readily applied to measure the current in the LED chains (for example, measuring the current in the inductance or across the buck switches or using a sense-FET arrangement or a shunt resistor in series to the buck switches). The buck converters 1 are configured to provide a supply voltage VBUCK1, VBUCK2 to the respective LED chains LD1, LD2 such that the load current through the respective LED chains LD1, LD2 matches a given reference current level which may be represented by a reference voltage VREF.
In accordance with one embodiment of the present invention, the current feed back signal (e.g., signal V1) received by a buck converter 1 is compared with a reference signal VREF that is representative of a desired current level. The difference between the actual load current (represented by current feedback signal V1) and the reference current (represented by reference signal VREF) may be seen as current error and be amplified by an error amplifier 40 that provides a corresponding error signal.
In addition to the error amplifier 40, the buck converter includes a buck converter control unit 30 that receives the (amplified) current error signal. The buck converter control unit 30 operates as a current regulator and is thus configured to derive a duty cycle D1 dependent on the error signal. The duty cycle D1 derived from the error signal is supplied to a modulator unit 20, which may be implemented as a pulse width modulator unit as illustrated in the example of
The modulator unit 20 is configured to provide a binary (on/off) switching signal SPWM having a duty cycle D1 as provided by the buck converter control unit 30. The switching signal SPWM may be provided to a driver circuit 10, which is configured to drive a corresponding switching unit 11 of the buck converter 1 in accordance with the switching signal SPWM. The switching unit 11 may be a MOSFET half-bridge as commonly used in buck converters. However other types of switching units may be applicable such as, for example, a switching half bridge including one MOSFET in the high side branch and a diode in the low side branch. Usually, an inductor L1 is connected between the output of the half bridge 11 and the load (LED chain) of the buck converter 1.
As explained above, each buck converter 1 includes a feedback loop for regulating the load current through the load (i.e., the respective LED chain). As the load current directly depends on the duty cycle of the switching signal SPWM, the buck converter control unit 30 is configured to regulate, dependent on the above-mentioned error signal, the duty cycle such that the actual load current provided by the respective switching converter matches a desired predefined reference value.
The actual duty cycle D1, D2, etc., of each buck converter 1 is supplied to the switching converter 5 which generates a common input voltage VBOOST supplied the buck converters 1. In the present example the switching converter 5 is a boost converter that converts a driver supply voltage VIN (e.g., from an automotive battery) into the common input voltage VBOOST supplied to the buck converters 1. Depending on the application, the switching converter 5 may also be a buck-boost converter. If, for whatever reason, the forward voltage drop of an LED chain LD1 rises, the corresponding buck converter 1 reacts by correspondingly increasing the duty cycle D1 and thus augmenting the buck converter output voltage VBUCK1 supplied to the LED chain LD1 so as to keep the load current through the LED chain LD1 at the desired level. Further, The switching converter 5 monitors the duty cycles D1, D2, etc. of the buck converters 1 connected downstream thereto and regulates its output voltage (which serves as common input voltage VBOOST for the buck converters) such that the duty cycle of the buck converter operating at the highest duty cycle matches a predefined desired value.
For the further explanation it is assumed that the first buck converter 1 is the buck converter operating at the highest duty cycle D1. If the duty cycle D1 increases such that it exceeds a predefined desired maximum duty cycle DREF then the switching converter will increase the input voltage VBOOST to the buck converters until the duty cycle D1 has dropped again to or below the maximum duty cycle DREF (for example, DREF=0.8 which means 80%). Such a duty cycle feedback to the switching converter 5 may be used for keeping the duty cycles D1, D2, etc., of the buck converters 1 in a limited range so as to provide sufficient margin (of 20% in the present example where DREF=0.8) for upwardly adjusting the buck converter output voltage VBUCK1.
As the buck converters 1, the switching transistor is driven by a gate driver 11, which receives a switching signal from a modulator unit (e.g., a PWM modulator) whose duty cycle is determined by a control unit 31. The control unit (in the example of
Although various exemplary embodiments of the invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. It will be obvious to those reasonably skilled in the art that other components performing the same functions may be suitably substituted. It should be mentioned that features explained with reference to a specific figure may be combined with features of other figures, even in those where not explicitly been mentioned. Further, the methods of the invention may be achieved in either all software implementations, using the appropriate processor instructions, or in hybrid implementations that utilize a combination of hardware logic and software logic to achieve the same results. Such modifications to the inventive concept are intended to be covered by the appended claims.
