A VOLTAGE-FED PUSH-PULL LLC RESONANT LCD BACKLIGHTING INVERTER CIRCUIT

Abstract: An improved electronic LCD backlighting inverter circuit for high frequency operation under low frequency pulse width modulation (PWM) for dimming control. The improved electronic LCD backlighting inverter is based on a voltage-fed push-pull LLC resonant inverter circuit configuration including a resonant inductor (L), magnetizing inductance of an output transformer (L), and resonant capacitor (C). For large values of magnetizing inductance the LLC circuit effectively becomes an LC resonant circuit. By synchronizing the high frequency switching signal and the low frequency modulation frequency using logic control circuitry, a wide dimming range and higher efficiency are achieved under PWM control.

Description: A voltage-fed push-pull LLC resonant LCD backlighting inverter circuit The present invention relates generally to an electronic LCD backlighting inverter circuit suitable for LCD backlighting or the like, and more particularly, to an LCD backlighting inverter circuit which is highly efficient, has a low profile, and a wide dimming range. LCD backlighting applications demand efficient, low profile backlighting for information display. Narrow diameter cold-cathode fluorescent lamps (CCFL), such as the Tl type for example, are widely used in the industry for such applications. To drive these CCFLs, high frequency electronic LCD backlighting inverter circuits having high efficiency, low profile, and a wide dimming range are in demand. Presently, voltage-fed half bridge resonant converter circuits, as shown in FIG.l, and current-fed push-pull resonant converter circuits, as shown in FIG. 2 are used to drive CCFL and other fluorescent lamps. Despite their wide use, these circuits have shortcomings which make them less than optimum solutions for driving CCFL's and the like. For example, the efficiency of these circuits is optimum and their dimming range is limited. In particular, a disadvantage of the prior art circuit configuration of FIG. 1 is a high output transformer turns ratio, which translates to a higher primary side winding current which leads to higher conduction losses. A further disadvantage of the circuit of FIG. 1 is that the high turns ratio in the secondary winding requires a reduced wire size (e.g., to 44 AWG) which contributes to higher conduction losses in the winding. In addition, a smaller gauge wire may cause problems during manufacturing. Another disadvantage of using a high turns ratio transformer is a significant increase in parasitic capacitance which leads to low efficiency. The typical electrical efficiency of the circuit of FIG. 1 is about 84% (i.e., output power/input power). FIG. 2 is another prior art circuit configuration of a widely used electronic ballast for driving CCFLs. The backlight inverter of FIG. 2 has a smaller output transformer turns ratio than that described with reference to the circuit of FIG. 1, and is capable of current based lamp power dimming using a Buck regulator stage. While the smaller output transformer turns ratio will lead to smaller losses in the push-pull power stage, the total circuit efficiency is limited by the Buck regulator stage. Another disadvantage of the circuit of FIG. 2 is a narrow dimming range due to the thermometer effect in the LCD panel when the lamp current frequency is high. At higher frequencies, a parallel parasitic capacitance in the lamp shield draws more current from the lamp causing one end of the lamp to be bright and the other to be dim. In order to improve circuit efficiency and achieve a wide dimming range it has been proposed to use the push-pull resonant inverter stage operated in a low frequency pulse width modulation (PWM) dimming mode and using the push-pull converter switches, Ql and Q2, in FIG. 2 also as the low frequency switches for PWM dimming. However, the typically high inductance of LI limits the circuit start up performance and limits the dimming range. Accordingly, a need exists for an improved electronic LCD backlighting inverter circuit which is more efficient than a conventional electronic LCD backlighting inverter circuits having a wide dimming range, and low profile. In accordance with the present disclosure, an improved electronic LCD backlighting inverter circuit for use in LCD backlighting applications is provided which obviates the problems associated with the prior art. According to one aspect of the invention, there is provided an improved high frequency electronic LCD backlighting inverter circuit for powering a fluorescent lamp that is efficient, has a low profile, and a wide dimming range. It is a feature of the present invention that the LCD backlighting inverter circuit is optimally designed for high frequency switching, however, the invention provides capabilities for low frequency pulse-width modulated (PWM) switching using logic control circuitry to achieve a wider frequency range than can be realized in conventional LCD backlighting inverter circuits. By controlling the dimming range via logic control circuitry the need for a current driven front end Buck regulator stage, as used in conventional current driven push-pull circuits is removed. That is, the present invention removes the need for a Buck regulator stage switching transistor and associated diode which contribute to low circuit efficiency. Further, the output transformer turns ratio is greatly reduced leading to higher circuit efficiency. Higher circuit efficiency is further realized by selecting an inductance value for LI that is orders of magnitude lower than the value required in a conventional design. By choosing a smaller inductance value for LI, the inductor does not act as a current source, but instead is considered part of an LLC resonant circuit thereby providing the ability to switch the inventive circuit off at the zero-crossings of the inductance current. By selecting a small value for LI, the problems associated with limitations in circuit startup performance and limitations in achieving a wide dimming range are removed. An improved electronic LCD backlighting inverter circuit is provided for performing high frequency dimming with a low frequency modulation. The improved electronic LCD backlighting inverter circuit is preferably a voltage-fed push-pull LLC resonant circuit which includes: an LLC resonant circuit including a resonant inductor, a magnetizing inductor and a resonant capacitor; switching means for operating said LCD backlighting inverter circuit at a high frequency modulated by a low frequency signal; low frequency signal generator means for generating a low frequency signal, said low frequency signal having positive and negative going portions; logic means for controlling said switching means and being driven from said low frequency signal, said logic means for extinguishing the operation of said switching means during said negative portion of said low frequency signal thereby causing said electronic LCD backlighting inverter circuit to be frequency modulated by said low frequency signal. The foregoing features of the present invention will become more readily apparent and may be understood by referring to the following detailed description of an illustrative embodiment of the present invention, taken in conjunction with the accompanying drawings, in which: FIG. 1 is a circuit diagram illustrating an LCD backlighting inverter circuit of the prior art; FIG. 2 is a circuit diagram illustrating an LCD backlighting inverter circuit of the prior art; FIG. 3 is a circuit diagram illustrating an LCD backlighting inverter circuit in accordance with an embodiment of the present invention; FIGS. 4a and 4b illustrate representative waveforms present in the circuit of FIG. 3; and FIG. 5 illustrates timing diagrams of certain signals present in the circuit of FIG. 3. Turning now to the drawings, in which like reference numerals identify similar or identical elements throughout the several views, FIG. 3 illustrates an electronic LCD backlighting inverter circuit 10 according to the present invention. It is envisioned that the improved circuit according to the present invention will be used in LCD backlighting applications. The LCD backlighting inverter circuit 10 according to the present invention is a voltage-fed push-pull LLC resonant circuit for operating a load 35. The load 35 shown in FIG. 3 is shown to be resistive, however, the load can be, but is not limited to a fluorescent lamp of the cold cathode type (e.g., CCFL). The light from load 35 can be used to illuminate, for instance, a LCD flat panel display of a computer (not shown). The backlighting inverter circuitlO may be powered from a conventional AC power source which is then rectified and converted to provide the DC source voltage used by the backlighting inverter circuitlO. The LCD backlighting inverter circuitlO of the present invention provides two important advantages over LCD backlighting inverter circuits of the prior art. First, the LCD backlighting inverter circuitlO of the present invention is more efficient than LCD backlighting inverter circuits of the prior art. Second, the LCD backlighting inverter circuitlO of the present invention has a wider dimming range than backlighting inverter circuits of the prior art. Each advantage will be discussed below. The general circuit operation will first be described. General Circuit Operation The operation of the circuit arrangement shown in FIG. 3 is as follows. The backlighting inverter circuit 10 operates in two intervals, a first interval defined as [t_0, t_l], and a second interval [t_l, t_2] in each high frequency switching cycle. Assuming steady state, in the first interval [t_0, t_l], at time t_0, switching transistor Ql turns on and switching transistor Q2 turns off. The voltage across Q2 is equal to the voltage across the resonant capacitor Cr (See N cr in FIG. 4b, waveform 4f), which gradually becomes fully charged, as can be seen at point B in waveform 4f, via resonance with the input inductor LI and the magnetizing inductance of T__l. The output transformer T_l primary current I p (See FIG. 4a, waveform 4a) is the sum of the resonant capacitor current I cr (See FIG. 4a, waveform 4b) and the resonant inductor current I (See FIG. 4a, waveform 4c). The current in the resonant capacitor L r is larger than the resonant inductor current Iu . The switching transistors Ql and Q2 only carry the resonant inductor current I II The resonant capacitor current I cr is sinked through load 35. When the resonant capacitor voltage N cr (See FIG. 4b, waveform 4f) reaches zero through a half resonance period at t_ l, switching transistor Ql is turned off and Q2 is turned on with zero voltage switching. The second half resonant period [t_l,t_2] is symmetric to the first half resonant period [t_0, t_l], as shown in (FIG. 4a, waveforms 4a and 4e), and (FIG. 4b, waveform 4f). The gate driving voltage at point V gsl is shown at point H in the inventive circuit of FIG. 3 is shown at waveform 4g of Fig. 4b. Voltage N gs ι represents a logic level associated with the output of AND gate AND1. Voltage V Q1 (Fig. 4b, waveform 4b) corresponds to the voltage at point I in Fig. 3; the same waveform would appear at point J. These voltages represent the voltage across the switching transistors Ql and Q2, respectively. Voltage V m (Fig. 4b, waveform 4i) corresponds to the voltage at point K of Fig. 3 and represents the voltage applied to the middle point of the primary winding of transformer T_l. It is also noted that the inductor current I (See FIG. 4a, waveform 4c) is almost a pure sinusoidal waveform. It is noted that resonant inductor LI is designed such that the resonant inductor current Iu reaches zero during each high frequency switching cycle, (see point C on FIG. 4a, waveform 4c). By reaching a zero level in each switching cycle it is therefore possible to synchronize a low frequency PWM signal with the I u zero points to simultaneously switch off switching transistors Ql and Q2, effectively shutting down the resonant inductor to facilitate low frequency PWM dimming, as will be described below. Hi her Efficiency As illustrated in FIG. 3, in one embodiment of the LCD backlighting inverter circuit 10, load 35 is connected to a secondary winding of a transformer T_l. A resonant LLC circuit is formed by resonant inductor LI, load 35, the magnetizing inductance of transformer T__l and the resonant capacitor C r . The inductance value selected for LI is typically on the order of 20-30 micro-henries. Such values are significantly lower than inductance values associated with prior art circuit configurations, as illustrated in FIG. 2. Typical inductance values for the circuit configuration of FIG. 2 are on the order of 150-300 micro-henries. It is well known that current driven push-pull configurations require higher inductance values, typically on the order of 150-300 microhenries, depending upon the circuit operating frequency, to ensure an almost constant current. The lower inductance value of the inductor LI of the present invention changes the circuit configuration from a current-fed parallel resonant circuit to voltage-fed LLC series resonant circuit which is a more efficient circuit configuration. The lower inductance value of LI is realizable because the push-pull LLC circuit of the present invention is voltage driven, in contrast with the prior art circuit, as illustrated in FIG. 2, which is current driven. Referring now to the prior art circuit of FIG. 1, it is noted that although this circuit is voltage driven, which is a more efficient circuit configuration, the inductance value cannot realize low values because a high inductance value of Lr is needed in order to convert the voltage source Vj n to a current source. Therefore, in the prior art circuit of FIG. 1, because of the large inductance value, the inductor is not a component of the resonant tank. By contrast, because of the circuit configuration of the inventive circuit of FIG. 3, the inductor LI is a component of the resonant tank. Accordingly, its value can be much smaller than the prior art circuit of FIG. 1. The inductance value of inductor LI in the present circuit configuration is small enough to be considered part of a resonant circuit formed by the inductor LI, load 35, and the magnetizing inductance of transformer Tl (not shown), and the resonant capacitor C r . Another desirable consequence of the inductor LI being one component of the resonant circuit is that the inductor current is substantially sinusoidal, with a certain DC bias, as shown in FIG. 4a waveform 4c. An AC current (e.g., a sinusoidal current) is required to synchronize a low frequency PWM signal (200 Hz) with the I zero points to simultaneously switch off switching transistors Ql and Q2, effectively shutting down the resonant inductor, to enable low frequency PWM dimming, as will be described below. Another feature of the present invention which contributes to higher circuit efficiency is the use of a smaller transformer turns ratio for transformer T_l which leads to lower conduction losses in the windings. In sum, the LCD backlighting inverter circuit 10 of the present invention achieves higher efficiency than LCD backlighting inverter circuits of the prior art in a number of ways including: using a voltage-fed push pull configuration obviating the need for a Buck regulator which is inherently inefficient; using a small inductance value for inductor LI which contributes to higher circuit efficiency; and using a smaller transformer turns ratio for transformer T_l. Low Frequency PWM Dimming In addition to providing higher efficiency over conventional LCD backlighting inverter circuits, the LCD backlighting inverter circuit 10 of the present invention achieves a wider dimming range than conventional LCD backlighting inverter circuits. It is a feature of the present invention that the backlighting inverter circuitlO is optimally designed for a fixed full output (i.e, high frequency switching as seen in FIG. 3, VSQ1= 50kHz); however, the backlighting inverter circuitlO is also capable of operating in a low frequency pulse-width modulated (PWM) switching mode) when desired. The combination of high frequency switching and low frequency PWM switching provides a wider dimming range than can be achieved in conventional LCD backlighting inverter circuits. Low frequency PWM switching is realized in the present invention using logic control with synchronization. This approach is in contrast with conventional approaches, such as the circuit of FIG. 2, which uses a switching transistor, Q0 to control the lamp dimming level. In the circuit of FIG. 2, the typical dimming range is 30% to 100%) of the full output value. By contrast, the dimming range of the present invention is approximately 3% to 100% of a full output value. Referring to FIG. 3, there is shown a first signal generator means (i.e., a low frequency PWM signal generator 30 ) which outputs a 200 Hz square wave at point F. The 200 Hz output is sourced to the D input of the D flip flop 32. Both inputs of the D flip flop 32 are leading edge triggered. The 200 Hz signal generated from the low frequency PWM signal generator 30 is also supplied to the SET input of an RS flip flop 34, which is also leading edge triggered. The Q output of the RS flip flop 34 is connected to a first input of respective AND gates, AND1 and AND2. Also shown in FIG. 3 is a resistor RSENSE from which a voltage is developed at point E ranging substantially from 0 to .5 volts. A zero voltage is developed at point E at the zero points of the resonant inductor current I . Low frequency PWM dimming is generally achieved by synchronizing the zero points (See point C in waveform diagram 4c of FIG. 4a) in the resonant inductor current Iu during each high frequency switching cycle with the negative going edge of the 200 Hz signal generated from the low frequency PWM signal generator 30. That is, the circuit configuration switches off switching transistors Ql and Q2 at the 200 Hz rate in synchronization with the zero points of inductor current Iu- Synchronization is required because turning off switching transistors Ql and Q2 at a point other than the zero point of inductor current Iu would not allow the energy stored in the resonant inductor LI to be smoothly dissipated. At the zero points of the inductor current I LI the stored energy is zero or near zero. Referring now to the waveform diagrams of FIGS. 3, 4a, 4b and 5. In operation, the 200 Hz signal generated from the low frequency PWM signal generator 30, shown in FIG. 5a, is simultaneously supplied to the D input of the D flip flop 32, and to the S input of the RS flip flop 34. Referring to waveform 5a in FIG. 5, the leading edge of one cycle of the 200 Hz waveform is indicated as reference numeral 501. The RS flip flop 34 follows waveform 5a and is therefore a logic high 503 at the leading edge 501 of the 200 Hz waveform. Accordingly, the first input of respective AND gates AND1 and AND2 are a logic high at the leading edge 501. The T input of the D flip flop 32 is connected to the output of op-amp 36 which outputs a 50 kHz output ranging from 0 to 0.5 volts as illustrated in FIG. 5b of FIG. 5 in response to a voltage developed at point E at resistor RSENSE. The T input of the D flip flop 32 is leading edge triggered and latches the 200 Hz waveform at the D input on each leading edge of the 50 kHz waveform which is received at the T input, as illustrated in FIG. 5b. Given the two inputs to the D flip flop 32 as described, the Q output of the D flip flop tracks the 200 Hz input at a 50 kHz latch rate. The Q output of the D flip flop 32 is connected to the RESET input of the RS flip flop 34 via a logic inverter 33. As stated above, the Q output of the D flip flop 32 tracks the 200 Hz input waveform at a 50 kHz latch rate. As a consequence of being negative edge triggered, the RS flip flop 34 is reset at each negative going edge (e.g., see point 505 of waveform 5a of FIG. 5) of the 200 Hz waveform causing the Q output to be a logic low which in turn causes the respective first inputs to AND gates ANDl and AND2 to be a logic low at a 200 Hz rate. As a result, both Ql and Q2 are turned off at a point at which the current in inductor LI is substantially zero. Referring again to FIG. 3, it is noted that the respective second inputs to the AND gates are connected to a second signal generator means (i.e., a 50 kHz source, VSQ1) via the RS flip flop 31. It is noted that the output of AND gates AND 1 and AND2 are 50Khz waveforms (sourced from respective second inputs), modulated by the 200 kHz waveform (sourced from respective first inputs), where the 200 KHz modulating waveform is synchronized with the zero points of the inductor current I . It is also noted that the low frequency PWM signal generator 30 further includes dimming control knob 37 for controlling the duty ratio of the 200 Hz output signal from zero to 100%. A 0% duty ratio corresponds to a DC level zero voltage output, and a 100% duty ratio corresponds to a DC level 5V output. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and have been described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Claims: CLAIMS: 1. An electronic LCD backlighting inverter circuit (10) for performing high frequency dimming with a low frequency modulation, said improved electronic LCD backlighting inverter circuit (10) comprising: switching means for operating said LCD backlighting inverter circuit at a high frequency modulated by a low frequency signal; low frequency signal generator means (30) for generating said low frequency signal, said low frequency signal having positive and negative going portions; logic means for controlling said switching means and being driven from said low frequency signal, said logic means for extinguishing the operation of said switching means during said negative portion of said low frequency signal thereby causing said electronic LCD backlighting inverter circuit (10) to be frequency modulated by said low frequency signal. 2. The LCD backlighting inverter circuit of Claim 1, wherein said low frequency signal comprises a low-frequency pulse- width modulation signal. 3. The LCD backlighting inverter circuit of Claim 1 , wherein said improved backlighting inverter circuit (10) comprises a voltage-fed push-pull LLC resonant circuit including a resonant inductor (L), a magnetizing inductor (T_l) and a resonant capacitor (Cr). 4. The LCD backlighting inverter circuit of Claim 3, further comprising: synchronizing means for synchronizing a substantially minimum level of a substantially alternating inductor current associated with said resonant inductor (L) with said low frequency signal to enable said first and second switching transistors (Ql, Q2) to be switched off. 5. The LCD backlighting inverter circuit of Claim 1, wherein said improved backlighting inverter circuit (10) comprises a voltage-fed push-pull LC resonant circuit including a resonant inductor (L), and a resonant capacitor (Cr). 6. The LCD backlighting inverter circuit Claim 1, wherein said switching means comprises: a first switching transistor (Ql) and a second switching transistor (Q2); and a second signal generator for providing a second signal to said first and second switching transistors (Ql, Q2) to operate said LCD backlighting inverter circuit (10) in said first dimming mode. 7. The LCD backlighting inverter circuit of Claim 1, wherein said logic means comprises: a first AND gate (ANDl) connected to said first switching transistor (Ql) and a second AND gate (AND2) connected to said second switching transistor (Q2), said first and second AND gates having a first input connected to receive said low frequency signal from a low frequency signal source (30) and a second input connected to receive a high frequency signal from a high frequency signal source (VSQ1) , said first and second AND gates (ANDl, AND2) alternatively outputting a logic high and a logic low during said positive going portion of said low frequency signal, and outputting a logic low during said negative going portion of said low frequency signal. 8. The LCD backlighting inverter circuit of claim 1, comprising: (a switching stage having an output; and) - a circuit having a resonant frequency wherein the resonant frequency is formed from an resonant inductor (L), a load (35), and the magnetizing inductance of a transformer (T_l) and a resonant capacitor (Cr), said resonant inductor having an inductance value less than a predetermined threshold value. 9. The LCD backlighting inverter circuit of claim 6, wherein the switching stage includes switching transistors (Ql, Q2) controlled to switch under zero voltage switching turn-on conditions. 10. LCD equipment comprising a LCD screen, a fluorescent lamp and a LCD backlighting inverter as claimed in one or more of claims 1-9.

Inventors: CHANG CHIN, CHANG, CHIN
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H05B41
IPC codes: H02M7, H05B41