LED based luminaires are rapidly replacing both incandescent and fluorescent luminaires for both general lighting and backlighting applications. In large liquid crystal based monitors, and in large solid state lighting applications, such as street lighting and signage, typically the LEDs are supplied in one or more strings of serially connected LEDs, which thus share a common current. A plurality of parallel strings may also be supplied.
The power supply which is to drive the LEDs preferably also supplies power to the operating circuitry of the device, thus reducing cost. Typically a single power supply comprising a power transformer with a plurality of secondary windings is utilized, the primary stage of which is controlled by a feedback circuit to provide a fixed direct current (DC) voltage for the operating circuitry of the device through a particular one of the secondary windings.
A resonant converter is a switching converter that comprises a tank circuit actively participating in determining input to output power flow. Power flow in a resonant converter is typically controlled either by changing the switching frequency, or the duty cycle, or both. In one embodiment two reactive elements, i.e. a capacitor and an inductor form the tank circuit, and such a resonant inverter is known as an LC inverter. A resonant converter comprises a resonant inverter, which has a switching network and a resonant tank circuit, and a rectifier circuit.
A resonant converter having two inductive elements, and a single capacitor in the tank circuit, where one of the inductive elements is arranged in parallel with the load, and another inductive element is in series with the load and the capacitor, is known as an LLC converter. Advantageously, an LLC converter exhibits a pair of resonant peaks, each associated with a particular one of the inductors. When properly designed, an LLC converter can be simply controlled by adjusting the frequency responsive to an output feedback, as long as the operating frequency is kept between the two resonant peak frequencies. Typically, a drop in output is compensated for by decreasing the operating frequency, and an increase in output is compensated for by increasing the operating frequency.
In a typical embodiment, the two inductors are implemented in an integrated transformer having a leakage inductance where the inductance of the primary winding acts as the parallel inductive element and the leakage inductance acts as the series inductive element. The transformer further enables scaling of the design output voltage based on the turns ratio of the primary and secondary windings of the integrated transformer.
In order to reduce cost, it is desired to have a single converter provide drive for both the LEDs and for the operating circuits of the device. Since the voltage for the operating circuits of the device must be well regulated, the LED drive voltage is not well regulated. One solution of a circuit 10 for driving at least one LED luminaire offered by the prior art is illustrated in FIG. 1. Circuit 10 comprises: a power source 20; a resonant mode controller 30, optionally comprising an LLC controller; a converter 40 comprising a bridge circuit 50, a primary side capacitance element CP and a transformer 60; a pair of unidirectional electronic valves D1; a unidirectional electronic valve D2; an inductance element 70; a capacitance element C1; a capacitance element C2; an electronically controlled switch SS; a plurality of LED luminaires, denoted respectively L1, L2 and L3; a plurality of electronically controlled switches SL; a plurality of sense resistive elements RS; an LED controller 80; a pair of unidirectional electronic valves D3; a voltage divider 110; and a reference voltage source 120. Bridge circuit 50 comprises a pair of electronically controlled switches, denoted respectively SB1 and SB2. Transformer 60 comprises: a primary winding 130; and a plurality of secondary windings, denoted respectively 140, 150 and 160.
In one embodiment, primary side capacitance element CP, capacitance element C1 and capacitance element C2 are each implemented as a capacitor, and are described herein as such. In another embodiment, unidirectional electronic valves D1, D2 and D3 are each implemented as a diode, and are described herein as such. In one embodiment, inductance element 70 is implemented as an inductor, and is described herein as such. In another embodiment, each sense resistive element RS is implemented as a resistor, and is described herein as such. In one embodiment, electronically controlled switches SB1, SB2, SS and SL are each implemented as an n-channel metal-oxide-semiconductor field-effect-transistor (NFET), and are described herein as such.
The output of power source 20 is coupled to the drain of NFET SB1 and the return of power source 20 is coupled to the source of NFET SB2. The gates of NFETs SB1, SB2 are each coupled to a respective output of resonant mode controller 30. The source of NFET SB1 is coupled to the drain of NFET SB2 and a first end of primary side capacitor CP. A second end of primary side capacitor CP is coupled to a first end of primary winding 130 of transformer 60 of converter 40. A second end of primary winding 130 is coupled to the return of power source 20. A first and second end of secondary winding 140 of transformer 60 are each coupled to the anode of a respective one of pair of diodes D1 and the center tap of secondary winding 140 is coupled to a common potential.
The cathodes of diodes D1 are each coupled to a first end of inductor 70 and to a first end of capacitor C1. The second end of capacitor C1 is coupled to the common potential. The second end of inductor 70 is coupled to the drain of NFET SS and the anode of diode D2. The source of NFET SS is coupled to the common potential and the gate of NFET SS is coupled to an output of LED controller 80. The cathode of diode D2 is coupled to a first end of capacitor C2 and the anode end of each of LED luminaires L1, L2, L3. The second end of capacitor C2 is coupled to the common potential. The cathode end of each of LED luminaires is coupled to the drain of a respective NFET SL and to a respective input of LED controller 80. The source of each NFET SL is coupled to a first end of a respective sense resistor RS and a respective input of LED controller 80. The second end of each sense resistor RS is coupled to the common potential and the gate of each NFET SL is coupled to a respective output of LED controller 80.
Secondary winding 150 is coupled to a respective load (not shown). Each end of secondary winding 160 is coupled to the anode of a respective diode D3 and the center tap of second winding 160 is coupled to the common potential. The cathode of each diode D3 is coupled to a load (not shown) and a first end of voltage divider 110. A second end of voltage divider 110 is coupled to the common potential and a division junction of voltage divider 110 is coupled to a respective input of resonant mode controller 30. The output of reference voltage source 120 is coupled to a respective input of resonant mode controller 30 and the return of reference voltage source 120 is coupled to the common potential.
In operation, resonant mode controller 30 is arranged to alternately open and close NFETs SB1 and SB2 such that primary winding 130 is charged when NFET SB1 is closed and discharged when NFET SB2 is closed. Resonant mode controller 30 typically operates at a fixed duty cycle of near 50%, with a variable frequency, as will be described further. The power supplied to transformer 60 is controlled via secondary winding 160 and voltage divider 110. Particularly, when NFET SB1 is closed and primary winding 130 is charging, power is output from secondary winding 160 via a first diode D3. When NFET SB2 is closed and primary winding 130 is discharging, power is output from secondary winding 160 via the second diode D3. The rectified voltage at the cathodes of diodes D3 is supplied to the load and is additionally divided by voltage divider 110. The divided voltage is compared to the reference voltage output by reference voltage source 120. In the event that the divided voltage is higher than the output of reference voltage source 120, resonant mode controller 30 is arranged to increase the switching frequency of bridge circuit 50 thereby reducing the amount of power supplied to secondary winding 160. In the event that the divided voltage is lower than the output of voltage source 120, resonant mode controller 30 is arranged to reduce the switching frequency of bridge circuit 50 thereby increasing the amount of power supplied to secondary winding 160.
Secondary winding 140 is similarly influenced by the control of resonant mode controller 30. Since the voltage across secondary winding 140 is not independently controlled, the voltage appearing across capacitor C2 needs to be controlled so as to provide an appropriate operating voltage for LED luminaires L1, L2, L3. The operation of inductor 70, NFET SS and diode D2 act as a boost converter to increase the output voltage of secondary winding 140, stored across capacitor C1. Particularly, when NFET SS is closed, inductor 70 is charged by secondary winding 140. When NFET SS is open, capacitor C2 is charged and LED luminaires L1, L2, L3 are powered by the combination of the power supplied by secondary winding 140 and the power stored on inductor 70. LED luminaires L1, L2, L3 are thus powered at a voltage greater than the voltage provided by secondary winding 140. LED controller 80 is arranged to detect the voltage at the cathode end of each LED luminaire L1, L2, L3 and compare the detected voltages to a predetermined reference voltage. In the event that one or more of the detected voltages are lower than the predetermined reference voltage, i.e. the voltage across capacitor C2 is less than the optimal operating voltages of at least one of LED luminaires L1, L2, L3, LED controller 80 is arranged to increase the duty cycle of the boost converter, i.e. increase the percentage of time that NFET SS is closed. The voltage across capacitor C2 thus increases.
The current flowing through each of LED luminaires L1, L2, L3 generates a voltage across the respective sense resistor RS, which is detected by LED controller 80. In one embodiment, LED controller 80 is arranged to adjust the pulse width modulation (PWM) duty cycle of each NFET SL to control the current flowing through each LED luminaire L1, L2, L3. In another embodiment, LED controller 80 is arranged to adjust the gate voltage of each NFET SL to thereby adjust the current flowing through the respective one of LED luminaires L1, L2, L3, by increasing the effective voltage drop across the respective NFET SL. Any excess power is dissipated across the NFET SL. LED controller 80 may be a single unit controlling both NFET SS and the respective NFET SLs, or may be separate control units without exceeding the scope.
Another solution of a circuit 200 for driving at least one LED luminaire offered by the prior art is illustrated in FIG. 2. Circuit 200 comprises: power source 20; resonant mode controller 30; converter 40 comprising bridge circuit 50, primary side capacitor CP and transformer 60; capacitor C1; diode D2; inductor 70; capacitor C2; LED luminaire L1; NFET SL; sense resistor RS; LED controller 80; voltage divider 110; reference voltage source 120; and a pair of rectifier bridges, denoted respectively 210 and 220. A single LED luminaire is illustrated, however this is not meant to be limiting in any way and any number of LED luminaires may be provided. Bridge circuit 50 comprises NFETs SB1 and SB2. Transformer 60 comprises: primary winding 130; and plurality of secondary windings 140, 150 and 160. In one embodiment, rectifier bridges 210 and 220 are each implemented as a diode bridge, and are described herein as such.
The output of power source 20 is coupled to the drain of NFET SB1 and the return of power source 20 is coupled to the source of NFET SB2. The gates of NFETs SB1, SB2 are each coupled to a respective output of resonant mode controller 30. The source of NFET SB1 is coupled to the drain of NFET SB2 and a first end of primary side capacitor CP. A second end of primary side capacitor CP is coupled to a first end of primary winding 130 of transformer 60 of converter 40. A second end of primary winding 130 is coupled to the return of power source 20.
Each end of secondary winding 140 of transformer 60 is coupled to a respective input of diode bridge 210 and the return of diode bridge 210 is coupled to a common potential. The output of diode bridge 210 is coupled to a first end of capacitor C1, the cathode of diode D2 and a first end of inductor 70. The second end of capacitor C1 is coupled to the common potential. The second end of inductor 70 is coupled to a first end of capacitor C2 and the anode end of LED luminaire L1. The second end of capacitor C2 is coupled to the anode of diode D2, the cathode end of LED luminaire L1 and the drain of NFET SL. The source of NFET SL is coupled to a first end of sense resistor RS and an input of LED controller 80. The second end of sense resistor RS is coupled to the common potential and the gate of NFET SL is coupled to an output of LED controller 80.
Secondary winding 150 is coupled to a respective load (not shown), or alternatively not provided. Each end of second winding 160 is coupled to a respective input of diode bridge 220 and the return of diode bridge 220 is coupled to the common potential. The output of diode bridge 220 is coupled to a load (not shown) and a first end of voltage divider 110. A second end of voltage divider 110 is coupled to the common potential and a division junction of voltage divider 110 is coupled to a respective input of resonant mode controller 30. The output of reference voltage source 120 is coupled to a respective input of resonant mode controller 30 and the return of reference voltage source 120 is coupled to the common potential.
In operation, resonant mode controller 30 is arranged to alternately open and close NFETs SB1 and SB2, typically at a predetermined duty cycle near 50%. Primary winding 130 is charged when NFET SB1 is closed and discharged when NFET SB2 is closed. The voltage output across diode bridge 200 is controlled by resonant mode controller 30, as described above. Particularly, the voltage across secondary winding 160, rectified by diode bridge 220, is supplied to the load, typically across an output capacitor (not shown) and is additionally divided by voltage divider 110. The divided voltage is compared to the voltage output by reference voltage source 120. In the event that the divided voltage is higher than the output of reference voltage source 120, resonant mode controller 30 is arranged to increase the switching frequency of bridge circuit 50 thereby reducing the amount of power supplied to secondary winding 160. In the event that the divided voltage is lower than the output of voltage source 120, resonant mode controller 30 is arranged to reduce the switching frequency of bridge circuit 50 thereby increasing the amount of power supplied to secondary winding 160.
The output by secondary winding 140 is similarly impacted by the operation of resonance mode controller 30, and thus the voltage across capacitor C1 changes responsive to changes in the load of secondary winding 160. The operation of inductor 70, diode D2 and NFET SL act as a buck converter to reduce the output voltage of secondary winding 140, stored across capacitor C1 to the appropriate voltage for LED luminaire L1. Particularly, when NFET SL is closed, power is provided to LED luminaire L1 by secondary winding 140 and inductor 70 is charged by secondary winding 140. When NFET SL is open, LED luminaire L1 is powered by the power stored on inductor 70. LED luminaire L1 is thus powered at a voltage less than the voltage provided by secondary winding 140 responsive to the duty cycle of NFET SL as controlled by LED controller 80.
The current flowing through LED luminaire L1 generates a voltage across the respective sense resistor RS, which is detected by LED controller 80. LED controller 80 is arranged to adjust the pulse width modulation (PWM) duty cycle of NFET SL to control the current flowing through LED luminaire L1. Additionally, the PWM adjustment of NFET SL adjusts the duty cycle of the buck converter of inductor 70, diode D2 and NFET SL, thus adjusting the voltage provided to LED luminaire L1 accordingly.
Advantageously, a resonant LED luminaire driving circuit, such as the above described LLC converter circuits 10 and 200, automatically provides for zero voltage switching for the LLC switching elements. However, the above converter circuits 10, 200 require an additional inductor 70. Additionally, NFETs SS and SL are operated in hard switching mode. What is desired, and not provided by the prior art, is an integrated converter which provides both a regulated voltage for use by operating circuits, and a regulated LED drive voltage, without requiring the additional inductors, dissipative regulators, or hard switching of the prior art.