WO2011045715A2

WO2011045715A2 - Led driving method and circuit using linear and switched mode design

Info

Publication number: WO2011045715A2
Application number: PCT/IB2010/054551
Authority: WO
Inventors: Henricus M. J. M. Kahlman
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-10-13
Filing date: 2010-10-08
Publication date: 2011-04-21
Also published as: WO2011045715A3; WO2011045715A8

Abstract

A method and driver circuit for driving a LED lamp from a dimmer supplying a mains supply dimmer voltage between a minimum cut mains supply voltage and an uncut mains supply voltage. In the method and driver circuit, a DC supply voltage is generated from the mains supply dimmer voltage, and the DC supply voltage is supplied to the LED lamp. A current source is coupled in series with the LED lamp. An amount of cutting of the mains supply dimmer voltage is used to generate a pulse width modulation, PWM, signal having a duty cycle range between 0% and 100%. The current source is driven with the PWM signal to generate a predetermined pulse width modulated current through the LED lamp. The DC supply voltage is controlled to keep a voltage across the current source at a predetermined value.

Description

LED DRIVING METHOD AND CIRCUIT USING LINEAR AND SWITCHED MODE DESIGN

FIELD OF THE INVENTION

The invention relates to the field of driving light emitting diodes, LEDs, and more specifically to a LED driving method and circuit using linear and switched mode design. BACKGROUND OF THE INVENTION

In existing lighting applications, general lighting service, GLS, bulbs or halogen light sources are coupled to a mains power supply through a standard dimmer, such as a dimmer comprising a triac element or field effects transistors, FETs, to cut the alternating current, AC, mains supply every half period. The AC mains supply half period waveform can be cut at a leading edge and/or at a trailing edge with a suitable cutting circuit. The minimum and maximum cutting angles are not standardized, and will vary largely between the various dimmers and dimmer loads.

If it is desired to save energy by replacing a GLS bulb with a light emitting diode, LED, lamp, containing one or more LEDs, or one or more strings of LEDs, a problem occurs in that dimming of the LED lamp is not possible, or that the LED lamp starts to flicker. Both phenomena are unacceptable.

The standard dimmers are intended for GLS bulbs or halogen light sources to supply a minimum power level of e.g. 30 W. However, a LED lamp requires substantially less power, and consequently will be out of the specification range of the standard dimmer. This is the case already when the LED lamp would operate at full power, and in particular holds true when the LED lamp should be dimmed to lower power levels, such as 1 W.

Accordingly, a problem exists when a standard dimmer supplying a mains supply dimmer voltage between a minimum cut mains supply voltage and an uncut mains supply voltage is to be coupled to a LED lamp.

SUMMARY OF THE INVENTION

It would be desirable to provide a method for driving a LED lamp using a standard dimmer. It would also be desirable to provide a LED driving circuit which can be coupled to a standard dimmer to dim a LED lamp. It would further be desirable to provide a LED driving method and circuit having low dissipation.

To better address one or more of these concerns, in a first aspect of the invention a method of driving a LED lamp from a dimmer supplying a mains supply dimmer voltage between a minimum cut mains supply voltage and an uncut mains supply voltage is provided, the method comprising:

generating a DC supply voltage from the mains supply dimmer voltage, the DC supply voltage supplied to the LED lamp;

coupling a current source in series with the LED lamp;

generating a pulse width modulation, PWM, signal having a duty cycle range between 0% and 100% which is mapped to the amount of cutting of the mains supply dimmer voltage;

driving the current source with the PWM signal to generate a predetermined pulse width modulated current through the LED lamp;

controlling the DC supply voltage to keep a voltage across the current source at a predetermined value.

In a second aspect of the invention, a LED driver circuit configured to be coupled to a dimmer supplying a mains supply dimmer voltage between a minimum cut mains supply voltage and an uncut mains supply voltage is provided, the LED driver circuit comprising:

input terminals configured for receiving the mains supply dimmer voltage; output terminals configured for supplying a DC supply voltage to a LED lamp; a converter circuit configured for generating the DC supply voltage from the mains supply dimmer voltage;

a pulse width modulation, PWM, signal generator configured for generating a pulse width modulation, PWM, signal having a duty cycle range between 0%> and 100% which is mapped to the amount of cutting of the mains supply dimmer voltage;

a current source coupled in series with the output terminals, the current source being configured to be driven with the PWM signal;

a first control circuit configured for controlling the converter circuit to generate a DC supply voltage to keep a voltage across the current source at a predetermined value.

In a third aspect of the present invention, a lighting unit is provided, comprising the LED driver circuit of the present invention, and an LED lamp coupled between the output terminals thereof. These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWING

Figure 1 depicts a block diagram of an embodiment of an LED driver circuit according to the present invention driving a LED lamp comprising a string of LEDs. DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 depicts a standard dimmer 100, such as a dimmer comprising a triac element or field effects transistors, FETs, to cut a voltage supplied by an alternating current, AC, mains supply (not shown) every half period. The cutting angle, or phase angle, can be adjusted as desired in a range which is specific for the dimmer 100 used. The voltage may be cut at a leading edge and/or a trailing edge of a half period waveform of the mains supply. At an output, the dimmer 100 has an output 101 to provide a mains supply dimmer voltage between a minimum cut mains supply voltage and an uncut mains supply voltage, to an input of an LED driver circuit 102 indicated by a dashed line.

The LED driver circuit 102 is configured to supply a LED lamp comprising a plurality of LED elements 104 connected in series, i.e. an LED string. Other configurations of LED elements 104 may be used, e.g. one LED element 104, or a plurality of LED elements arranged in series and/or in parallel. The combination of the LED driver circuit 102 and the LED lamp may form a lighting unit.

The LED driver circuit 102 comprises a filter and rectifier circuit 106 having an AC input coupled to the output 101 of the dimmer 100. The filter and rectifier circuit 106 has a first DC output 107 coupled to an input of a DC/DC converter 110. An output 114 of the DC/DC converter 110 is coupled to a first output terminal 116 to be coupled with (an anode of) an LED element 104 of the LED string. The combination of the filter and rectifier circuit 106 and the DC/DC converter 110 forms a converter circuit which is configured for generating and outputting a DC supply voltage from the mains supply dimmer voltage.

The filter and rectifier circuit 106 comprises a second DC output 108 coupled to an input of a pulse width modulation, PWM, generator 112. An output 118 of the PWM generator 112 is coupled to a first input of a current reference, Iref, generator 120. A second input 119 of the current reference generator 120 receives a reference signal, such as a reference current signal Iref.

A second output terminal 122 of the LED driver circuit 102, to be coupled with (a cathode of) an LED element 104 of the LED string, is coupled to a switching element 124 arranged in series with a sense element 126. In Figure 1, the switching element 124 is embodied as a field effect transistor, FET, switching element 124 having a drain D coupled to the second output terminal 122, a source S coupled to the sense element 126, and a control terminal or gate G. In Figure 1, the sense element 126 is embodied as a resistor type sense element 126 for generating a voltage representative of a current flowing through the switching element 124 and the sense element 126.

The LED driver circuit 102 further comprises a first control circuit 130 having a first input 131 for sensing an LED voltage of (a cathode of) an LED element 104 of the LED string, which is a voltage at the drain D of the FET switching element 124 shown in Figure 1. The first control circuit 130 has a second input 132 supplied with a reference signal, such as a reference voltage signal Uref. A third input 133 of the first control circuit 130 receives a PWM signal from the output 118 of the PWM generator 112. An output 135 of the first control circuit 130 is coupled to the DC/DC converter 110 to supply a DC supply voltage control signal thereto for controlling the DC supply voltage supplied by the DC/DC converter 110.

The LED driver circuit 102 further comprises a second control circuit 140 having a first input 141 for sensing a current sense signal as a sensing element voltage generated by a current flowing in the sense element 126, and having a second input 142 for receiving a current reference signal from the current reference generator 120. An output 145 of the second control circuit 140 is coupled to the control terminal G of the switching element 124 to supply a control signal thereto.

The functional units 106, 110, 112, 120, 130 and 140 of the LED driver circuit 102 operate as follows.

The filter and rectifier circuit 106 receives a mains supply dimmer voltage, which is rectified and filtered in the filter and rectifier circuit 106 to be output as an at least partially stabilized DC voltage at output 107. A rectified and possibly downscaled mains supply dimmer voltage is supplied at output 108.

The PWM generator 112 receives the rectified and possibly downscaled mains supply dimmer voltage from the filter and rectifier circuit 106, and converts it into a PWM signal at the output 118 having a duty cycle within a range from 0 - 100%. The PWM signal may have a duty cycle of 0% or more when the mains supply has been cut by the dimmer 100 at maximum (in other words, when maximum dimming is performed by the dimmer 100), while the PWM signal will have a duty cycle of 100% or less when the mains supply is not cut by the dimmer 100 (in other words, when no dimming is performed by the dimmer 100). In terms of the duty cycle of the dimmer 100: depending on the duty cycle (or cutting-angle) of the dimmer 100 being between a minimum duty cycle (at maximum cutting-angle) and a maximum duty cycle (at minimum cutting-angle), the PWM signal will have a duty cycle in the range between x% en y% (x < y, 0 < x, y < 100). The PWM generator 112 thus maps the cutting angles provided by the dimmer 100 onto a duty cycle range of the PWM signal between 0% to 100%, and representative of the required dimming level of the LED string. The PWM signal generated by the PWM generator 112 has a frequency higher than the AC mains supply frequency, which usually is 50 Hz or 60 Hz. The PWM signal may have a frequency equal to or higher than 100 Hz or 1,000 Hz or 5,000 Hz, to avoid flicker of the LED elements 104, and can be selected freely irrespective of the mains AC frequency.

The current reference generator 120 receives a reference signal Iref at its second input 119. The reference signal Iref is adapted to determine a reference current in the LED string. In the current reference generator 120, the PWM signal is mixed with the reference signal Iref to produce an amplitude modulated, AM, PWM signal. The (active part of the) AM PWM signal has an amplitude corresponding to the reference signal Iref.

The second control circuit 140 receives the AM PWM signal through second input 142, and compares this signal with the sensing element voltage (representative of a current flowing through the switching element 124) at its first input 141 to output a corrected AM PWM control signal through output 145 to the control terminal G of the switching element 124 operating in a linear domain, i.e. switching element 124 conducts current substantially in proportion with the amplitude of the control signal. Thus, the sense element

126 in combination with the switching element 124 and the second control circuit 140 acts as a PWM current source, the amplitude of the current being controlled by the reference signal Iref.

The filtered and rectified mains voltage supplied by the filter and rectifier circuit 106 at output 107, is supplied to DC/DC converter 110. The DC/DC converter 110 may be embodied as an up-converter or a down-converter operating in discontinuous mode, such as a buck converter or a boost converter. The DC/DC converter may be operated at a fixed switching frequency, or at a variable frequency at a fixed on-time of the switching elements. The DC/DC converter may also be embodied as a flyback converter. By varying a duty cycle of the DC/DC converter 110, the output voltage thereof, supplied at the output 114, may be controlled. The DC/DC converter 110 is adapted to consume sufficient latch/hold current out of the mains supply during the active period of the dimmer 100 to avoid malfunctioning of the dimmer 100. Additionally, the DC/DC converter 110 acts as a power factor corrector, providing a high efficiency.

The first control circuit 130 receives the PWM signal output by the PWM generator 112, at the third input 133 thereof. The PWM signal is used to make the control circuit 130 operative only during an active period of the PWM signal, as indicated by a switch symbol in the first control circuit 130. When operative, the first control circuit 130 receives at its second input 132 a reference signal Uref which is adapted to determine a reference voltage across the LED string. At its first input 131, the first control circuit 130 receives a voltage at a terminal of the switching element 124 (a drain D of a FET switching element 124) at the side of the LED string. This voltage is representative of the voltage across the switching element 124, when conducting, and the sense element 126. Accordingly, the first control circuit 130 compares this voltage received at the first input 131 to the reference signal Uref received at the second input 132, and thereby provides a duty cycle control signal at the output 135 supplied to the DC/DC converter 110. Accordingly, the voltage provided by the DC/DC converter 110 at the output 114 thereof is controlled by the duty control signal provided by the first control circuit 130 depending on the voltage sensed at the switching element 124. In this way, a variation in forward voltages of the LED elements 104 in the LED string can be compensated to not affect a power dissipation in the switching element 124 and the sense element 126. In other words: despite a variation in forward voltages of the LED elements 104 in the LED string, a voltage across the switching element 124 and the sense element 126 can be kept at a predetermined value. Since the current flowing through the switching element 124 and the sense element 126 is kept constant by the second control circuit 140, the dissipation in the switching element 124 and the sense element 126 is kept at a predetermined value by the first control circuit 130.

Accordingly, irrespective of variations in the LED lamp voltage, the highest possible efficiency is reached.

If the first control circuit 130 were absent from the LED driver circuit 102, the voltage provided by the DC/DC converter 110 would be stable, which would lead to varying power dissipation in the switching element 124 and the sense element 126 at varying forward voltages of the LED elements 104. Since the forward voltages of the LED elements 104 tend to decrease during operation when the temperature of the LED elements 104 increases, the power dissipation in the switching element 124 and the sense element 126 would tend to increase during operation of the LED driver circuit 102. This undesired effect is cancelled by the first control circuit 130.

In accordance with the above, a method and driver circuit for driving a LED lamp from a dimmer supplying a mains supply dimmer voltage between a minimum cut mains supply voltage and an uncut mains supply voltage have been described. In the method and driver circuit, a DC supply voltage is generated from the mains supply dimmer voltage, and the DC supply voltage is supplied to the LED lamp. A current source is coupled in series with the LED lamp. An amount of cutting of the mains supply dimmer voltage is used to generate a pulse width modulation, PWM, signal having a duty cycle range between 0% and 100%. The current source is driven with the PWM signal to generate a predetermined pulse width modulated current through the LED lamp. The DC supply voltage is controlled to keep a voltage across the current source at a predetermined value.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.

A single processor or other unit may fulfill the functions of several items recited in the claims.

Claims

CLAIMS:

1. A method of driving a LED lamp from a dimmer supplying a mains supply dimmer voltage between a minimum cut mains supply voltage and an uncut mains supply voltage, the method comprising:

coupling a current source in series with the LED lamp;

generating a pulse width modulation, PWM, signal having a duty cycle range between 0% and 100% which is mapped to the amount of cutting of the mains supply dimmer voltage; driving the current source with the PWM signal to generate a predetermined pulse width modulated current through the LED lamp;

2. A LED driver circuit configured to be coupled to a dimmer supplying a mains supply dimmer voltage between a minimum cut mains supply voltage and an uncut mains supply voltage, the LED driver circuit comprising:

3. The LED driver circuit of claim 2, wherein the first control circuit comprises: a first input configured for receiving the voltage across the current source; a second input configured for receiving a voltage reference signal; and an output configured for supplying a DC supply voltage control signal to the converter circuit.

4. The LED driver circuit of claim 3, wherein the first control circuit further comprises:

a third input configured for receiving the PWM signal for turning the first control circuit on and off during the active and inactive period, respectively, of the PWM signal.

5. The LED driver circuit of claim 2, 3 or 4, wherein the current source comprises a field effect transistor, FET, operating in its linear domain;

a current sense element arranged in series with the FET; and

a second control circuit coupled to the current sense element for controlling a control signal supplied to a control terminal of the FET to keep the current in the FET at a predetermined value.

6. The LED driver circuit of claim 5, wherein the second control circuit comprises:

a first input configured for receiving a current sense signal from the current sense element;

a second input configured for receiving a current reference signal; and an output configured for supplying the control signal to the control terminal of the FET.

7. The LED driver circuit of any of claims 2 to 6, wherein the PWM signal has a frequency of at least 100 Hz, in particular a frequency of at least 1,000 Hz, in particular a frequency of at least 5,000 Hz.

8. The LED driver circuit of any of claims 2-7, wherein the converter circuit comprises a filter and rectifier circuit having an output coupled to an input of a DC/DC converter.

9. A lighting unit configured to be coupled to a dimmer supplying a mains supply dimmer voltage between a minimum cut mains supply voltage and an uncut mains supply voltage, the lighting unit comprising the LED driver circuit of any of claims 2 to 8, and an LED lamp coupled between the output terminals of the LED driver circuit.