KR20110048309A - Llc resonant converter and display device - Google Patents

Abstract

PURPOSE: An LLC resonant converter and a display device are provided to implement rapid response by detecting the change of a resonant switching current and feedbacking it. CONSTITUTION: In an LLC resonant converter and a display device, a switching unit(941) operates according to a switching control signal. An LCC resonant tank(942) transfer the energy of a primary winding to a secondary winding according to the switching of a switching unit. A rectifying smoothing unit converts the voltage induced from the secondary winding to DC current. A voltage compensation unit(944) maintains the output voltage of the rectifying smoothing unit.

Description

LLC resonant transducer and display device including same {LLC RESONANT CONVERTER AND DISPLAY DEVICE}

The present invention relates to an LLC resonant converter and a display device including the same.

Display devices used in monitors and TVs of computers include light emitting diodes (LEDs), electroluminescence (EL), vacuum fluorescent displays (VFDs), and electroluminescent devices (fields). emission display (FED), plasma display panel (PDP), and the like, a liquid crystal display (LCD) that does not emit light by itself and requires a light source.

The problem to be solved by the present invention is to design a stable voltage feedback controller based on the small signal analysis results of the LLC resonant converter, and further, to detect and feedback the change of the resonance switching current in the voltage feedback loop of the LLC series resonant converter By adding a feedback loop, we can achieve fast response.

According to a first embodiment of the present invention, an LLC resonant converter includes a switching unit for switching operation according to a switching control signal; An LLC resonant tank unit configured to transfer energy from the primary winding side to the secondary winding side according to a switching operation of the switching unit and perform LLC resonance; A rectifying smoother connected to the secondary winding side and converting the voltage induced on the secondary winding side into direct current; And a voltage compensator for performing feedback control to maintain a constant output voltage of the rectifying smoother, wherein the voltage compensator has a 3-pole-2 zero structure.

According to the second embodiment of the present invention, the LLC resonant converter is a switching unit for switching operation according to the switching control signal; according to the switching operation of the switching unit transfers the energy of the primary winding side to the secondary winding side, LLC resonance An LLC resonant tank unit comprising: a rectifying smoother connected to the secondary winding side and converting a voltage induced on the secondary winding side into a direct current; a voltage for controlling feedback to maintain a constant output voltage of the rectifying smoothing unit; Compensation unit; Frequency of the switching unit in accordance with the sum signal of the current detection unit for detecting and detecting the resonant current of the primary winding side and the first control signal output from the voltage compensation unit and the second control signal output from the current detection unit It includes a switching control unit for adjusting the.

This feature allows LLC resonant transducers to operate reliably in all operating areas, maintain sufficient phase margin of loop gain, and achieve fast response speeds.

A general liquid crystal display device includes two display panels provided with a field generating electrode and a liquid crystal layer having dielectric anisotropy interposed therebetween. A voltage is applied to the field generating electrode to generate an electric field in the liquid crystal layer, and the voltage is changed to adjust the intensity of the electric field, thereby adjusting the transmittance of light passing through the liquid crystal layer to obtain a desired image. In this case, the light may be a separate artificial light source or natural light.

As a light source for a liquid crystal display, that is, a backlight device, a plurality of fluorescent lamps or light emitting diodes (LEDs), such as a cold cathode fluorescent lamp (CCFL) or an external electrode fluorescent lamp (EEFL), are used as a light source. In the case of using a fluorescent lamp, power consumption is high and heat generation deteriorates device characteristics of the liquid crystal display. In addition, since fluorescent lamps are typically rod-shaped, they are susceptible to shock and have a high risk of damage.

On the other hand, since the light emitting diode is a semiconductor device, it has a long life, a fast lighting speed, and low power consumption. In addition, it is strong in impact and advantageous in miniaturization and thinning.

Therefore, a light emitting diode is used as a light source of a backlight device not only for a small liquid crystal display device such as a mobile phone but also for a medium and large liquid crystal display device such as a monitor or a television.

In general, a fluorescent lamp is driven by an alternating voltage (AC), while a light emitting diode is driven by a direct current voltage (DC). Accordingly, in order to use a light emitting diode as a light source of a liquid crystal display, a power supply device that converts an AC voltage into a DC voltage and generates a DC voltage having a desired level is required.

The LLC resonant converter is being used in the industry because it has a great advantage not only in reducing the price of the product but also in miniaturizing the product through the idea of using the parasitic component present in the transformer as the LLC component.

However, when the LLC resonant converter is applied to a liquid crystal display device showing a wide variation of input voltage and large output current, the steady state analysis alone cannot solve the instability that may cause unexpected malfunctions. There is a need to design a voltage feedback controller.

Furthermore, when only the voltage feedback control method is applied to the LLC resonant converter, there is a problem in that the response characteristic of the load change is relatively slow and the variation of the output voltage is large because the characteristic of the load variation is not excellent.

1 is an exploded perspective view of a liquid crystal display according to an embodiment of the present invention, FIG. 2 is a block diagram of a liquid crystal display according to an embodiment of the present invention, and FIG. 3 is a liquid crystal according to an embodiment of the present invention. It is an equivalent circuit diagram of one pixel of a display device. 4 is a detailed circuit diagram of a backlight unit according to an embodiment of the present invention.

As shown in FIG. 1, a liquid crystal display according to an exemplary embodiment of the present invention includes a liquid crystal module 350 including a display unit 330 and a backlight unit 900, and an upper portion and a lower portion accommodating the liquid crystal module 350. Chassis 361, 362, and mold frame 363.

The display unit 330 includes a liquid crystal panel assembly 300, a plurality of gate carrier package (TCP) 410 and data TCP 510 attached thereto, and a gate printed circuit board attached to the corresponding TCPs 410 and 510. A printed circuit board (PCB) 450 and a data PCB 550.

The liquid crystal panel assembly 300 includes a lower panel 100, an upper panel 200, and a liquid crystal layer 3 interposed therebetween.

The lower panel 100 includes a plurality of display signal lines G ₁ -G _n and D ₁ -D _m , and the lower and upper panels 100 and 200 have display signal lines G ₁ -G _n and D ₁ -D. _m ) and includes a plurality of pixels arranged in a substantially matrix form.

The display signal lines G _1- G _n and D ₁ -D _m are a plurality of gate lines G ₁ -G _n transmitting gate signals (also called "scanning signals") and data lines D transferring data signals. _1- D _m ). The gate lines G ₁ -G _n extend substantially in the row direction and are substantially parallel to each other, and the data lines D ₁ -D _m extend substantially in the column direction and are substantially parallel to each other.

Each pixel includes a switching element Q connected to a display signal line G ₁ -G _n , D ₁ -D _m , and a liquid crystal capacitor C _LC and a storage capacitor C _ST connected thereto. It includes. The holding capacitor C _ST can be omitted as necessary.

The switching element Q, such as a thin film transistor, is provided in the lower panel 100, and the control terminal and the input terminal thereof are three-terminal elements, respectively, with gate lines G ₁ -G _n and data lines D ₁ -D _m . The output terminal is connected to a liquid crystal capacitor (C _LC ) and a holding capacitor (C _ST ).

The liquid crystal capacitor C _LC has two terminals, the pixel electrode 190 of the lower panel 100 and the common electrode 270 of the upper panel 200, and the liquid crystal layer 3 between the two electrodes 190 and 270. It functions as a dielectric. The pixel electrode 190 is connected to the switching element Q, and the common electrode 270 is formed on the front surface of the upper panel 200 and receives a common voltage V _com . Unlike in FIG. 3, the common electrode 270 may be provided in the lower panel 100. In this case, at least one of the two electrodes 190 and 270 may be linear or rod-shaped.

The storage capacitor C _ST , which serves as an auxiliary part of the liquid crystal capacitor C _LC , is formed by overlapping a separate signal line (not shown) and the pixel electrode 190 provided on the lower panel 100 with an insulator interposed therebetween. A predetermined voltage such as the common voltage (V _com ) is applied to this separate signal line. However, the storage capacitor C _ST may be formed such that the pixel electrode 190 overlaps the front-end gate line directly above the insulator. An outer surface of at least one of the two display panels 100 and 200 of the liquid crystal panel assembly 300 is formed. The polarizer (not shown) which polarizes light is attached to this.

As shown in FIG. 1, the gate TCP 410 is attached to one edge of the lower panel 100 of the liquid crystal panel assembly 300, and a gate driving integrated circuit constituting the gate driver 400 is formed on the chip. Equipped with. The data TCP 510 is attached to the other edge of the lower panel 100 of the liquid crystal panel assembly 300, and a data driver integrated circuit constituting the data driver 500 is mounted in the form of a chip. The gate driver 400 and the data driver 500 may include the gate lines G ₁ -G _n and the data lines of the liquid crystal panel assembly 300 through signal lines (not shown) formed in the TCPs 410 and 510. D _1- D _m ) are each electrically connected.

The gate driver 400 applies a gate signal formed by a combination of the gate on voltage V _on and the gate off voltage V _off to the gate line G ₁ -G _n , and the data driver 500 supplies the data voltage. It is applied to the data lines D _1- D _m .

Alternatively, the integrated circuit chip that forms the gate driver 400 and the data driver 500 may be integrated on the display panel without using TCP (chip on glass, COG mounting method), and the gate driver 400 or the data driver 500 may be formed directly on the liquid crystal panel assembly 300 together with the switching element Q and the display signal lines G ₁ -G _n and D ₁ -D _m .

The gate PCB 450 is attached to the TCP 410 in the longitudinal direction in parallel to the lower panel 100, and a plurality of signal lines (not shown), electronic components, and the like for transmitting signals are formed thereon. The data PCB 550 is attached to the TCP 510 in the longitudinal direction in parallel to the lower panel 100, and a plurality of signal lines (not shown), electronic components, and the like for transmitting signals are formed thereon.

The gray voltage generator 800 generates two sets of gray voltages related to the transmittance of the pixel and provides them to the data driver 500 as data voltages. One of the two sets has a positive value for the common voltage (V _com ) and the other set has a negative value.

As shown in FIGS. 1, 2, and 4, the backlight unit 900 is accommodated in the lower chassis 362 and fixed, and the light source unit 960 and the assembly 300 are mounted below the liquid crystal panel assembly 300. And a plurality of optical instruments 908 disposed between the light source unit 960 and the light source unit 960, and a power supply unit 970 for supplying a power voltage necessary for the light source unit 960. At this time, the lower chassis 362 is fixed to the mold frame 363.

The light source unit 960 includes a plurality of PCBs 961 on which the plurality of light emitting diodes 962 are mounted. Each PCB 961 is arranged horizontally in the long axis direction of the liquid crystal panel assembly 300, and a plurality of red, green, and blue light emitting diodes 962 are alternately mounted. In this case, the number of green light emitting diodes mounted on each PCB may be greater than the number of red or blue light emitting diodes, for example, about twice as many. However, the number of these light emitting diodes may vary as needed.

On the other hand, to implement color display, each pixel uniquely displays one of the three primary colors (spatial division) or each pixel alternately displays the three primary colors over time (time division) so that the desired color can be selected by the spatial and temporal sum of these three primary colors. To be recognized. 3 illustrates that each pixel includes a red, green, or blue color filter 230 in an area of the upper panel 200 corresponding to the pixel electrode 190. Unlike FIG. 3, the color filter 230 may be formed above or below the pixel electrode 190 of the lower panel 100.

In FIG. 1, the optical apparatus 908 is positioned between the assembly 300 and the light source unit 960 and includes a diffuser plate 902 and a plurality of optical sheets that direct and diffuse light from the light source unit 960 to the assembly 300. 901).

Although not shown in FIG. 1, an upper case and a lower case are positioned at an upper portion of the upper chassis 361 and a lower portion of the lower chassis 362, respectively, to form a liquid crystal display device by combining them. The signal controller 600 controls operations of the gate driver 400 and the data driver 500.

The display operation of such a liquid crystal display device will now be described in detail.

The signal controller 600 controls the input image signals R, G, and B and their display from an external graphic controller (not shown), for example, a vertical synchronization signal Vsync and a horizontal synchronization signal. (Hsync), main clock (MCLK), data enable signal (DE) is provided. The signal controller 600 properly processes the image signals R, G, and B according to the operating conditions of the liquid crystal panel assembly 300 based on the input image signals R, G, and B and the input control signal, and controls the gate control signal. After generating the CONT1, the data control signal CONT2, the backlight control signal CONT3, and the like, the signal controller 600 then emits the gate control signal CONT1 to the gate driver 400 and the data control signal CONT2. The processed video signal DAT is sent to the data driver 500. The backlight control signal CONT3 is sent to the backlight unit 900.

The gate control signal (CONT1) is such as at least one clock signal for controlling the output time and the output voltage of the start scan to indicate the start of output of a gate-on voltage (V _on) signal (STV), a gate-on voltage (V _on) Include.

The data control signal CONT2 is applied to the horizontal synchronizing start signal indicating the start of the transmission of the image data DAT, the load signal LOAD for applying the data voltage to the data lines D _1- D _m , and the common voltage V _com . An inversion signal and a data clock signal for inverting the polarity of the data voltage with respect to the data voltage (hereinafter, referred to as the polarity of the data voltage by reducing the polarity of the data voltage for the common voltage).

The backlight control signal CONT3 may include a plurality of backlight signals such as a clock signal for controlling the operation of the current controller 950 and a light source control signal that is a pulse width modulation (PWM) signal for controlling the blinking of the light emitting diode of the light source 970. Control signal.

Unlike the exemplary embodiment of the present invention, the backlight control signal CONT3 may be generated by a control device other than the signal controller 600.

The data driver 500 sequentially receives and shifts the image data DAT for one row of pixels according to the data control signal CONT2 from the signal controller 600, and the gray voltage from the gray voltage generator 800. The image data DAT is converted into the corresponding data voltage by selecting the gray scale voltage corresponding to each of the image data DAT, and then applied to the corresponding data line D ₁ -D _m .

The gate driver 400 applies the gate-on voltage V _on to the gate line G _1- G _n in response to the gate control signal CONT1 from the signal controller 600, thereby applying the gate line G _1- G _n. ) Turns on the switching element Q connected thereto, and accordingly, a data voltage applied to the data lines D ₁ -D _m is applied to the corresponding pixel through the turned on switching element Q.

The difference between the data voltage applied to the pixel and the common voltage V _com is shown as the charging voltage of the liquid crystal capacitor C _LC , that is, the pixel voltage. The liquid crystal molecules vary in arrangement depending on the magnitude of the pixel voltage.

The light source unit 960 operates according to the voltage from the backlight control signal CONT3 and the power supply unit 970 to perform the flashing operation of the red, green, and blue light emitting diodes 962. White light is obtained.

According to the operation of the backlight unit 900, the light emitted from the light source unit 960 passes through the liquid crystal layer 3, and its polarization changes according to the arrangement of the liquid crystal molecules. This change in polarization is represented by a change in the transmittance of light by the polarizer.

After one horizontal period (or 1H) (one period of the horizontal synchronization signal Hsync) passes, the data driver 500 and the gate driver 400 repeat the same operation for the pixels in the next row. In this manner, the gate-on voltage V _on is sequentially applied to all the gate lines G ₁ -G _n during one frame to apply the data voltage to all the pixels.

When one frame ends, the next frame starts and the state of the inversion signal RVS applied to the data driver 500 is controlled so that the polarity of the data voltage applied to each pixel is opposite to that of the previous frame (frame inversion). . In this case, the polarity of the data voltage flowing through one data line may be changed (row inversion or dot inversion) or the polarity of the data voltage applied to one pixel row may be different according to the characteristics of the inversion signal RVS within one frame. (Heat inversion, dot inversion).

Referring to FIG. 4, the power supply structure of the liquid crystal display will be described in detail. The power supply unit 970 of the liquid crystal display according to the exemplary embodiment of the present invention includes a bridge rectifier 910, a power factor correction unit 920, and an LLC resonant converter 930 as shown in FIG. 4.

The LLC resonant converter 930 includes a voltage compensator 944 for controlling feedback to maintain a constant output voltage of the converter, and detects and detects a resonance current of a primary winding side of a transformer included in the converter. A current sensing unit CSN 946 may be included.

The front end of the LLC resonant converter includes a boosted power factor correction circuit 920 in discontinous conduction mode, which is a voltage control mode, and receives input power from the circuit. Due to the characteristics of the voltage control method of the power factor correction unit 920, the adjustment response characteristic of the output voltage to the load change is slow, and the capacity of the output electrolytic capacitor of the power factor correction unit 920 is relatively small. Since the output voltage drop is large, the fluctuation range of the input voltage of the LLC resonant converter 930 connected to the rear stage also has a large circuit configuration. As a result, a voltage variation range of 340V to 390V occurs at the power factor correction unit 920.

In addition, the light source unit 960 is connected to a rear end of the output circuit of the LLC resonant converter 930. The light source unit 960 may be a light emitting diode (LED) or an EEFL lamp. When the light source unit is an EEFL lamp, a DC-to-DC AC full bridge inverter circuit for directly driving the EEFL lamp may be connected. Here, 5.5A output current is generated in the normal state and 1.5A output current is generated in the dimming mode.

FIG. 5 is a diagram illustrating the structure of an isolated LLC resonant converter having a voltage compensator 944 for voltage feedback control according to the first embodiment of the present invention.

The voltage compensator 944 according to the first embodiment of the present invention has a three-pole, two-zero structure. The isolated LLC resonant converter is electrically isolated through an opto-coupler and uses an output voltage feedback method to control the operating frequency.

As shown in FIG. 5, the LLC resonant converter includes a half bridge switching part 941, an LLC resonant tank 942, a center-tap rectifier part 943, A voltage compensator 944 for voltage feedback control and a switching controller 945 for controlling the switching operation of the switching part. The switching controller includes a voltage controlled oscillator 945.

The input voltage is considered a Vs voltage source with a variation of 340V-390V due to the influence of the Power Factor Correction (PFC) circuit.

The half bridge type switching unit 941 has a first switching element Q ₁ and a second switching element Q ₂ to switch the DC power supply Vs.

The first switching element Q ₁ and the second switching element Q ₂ are composed of MOSFETs having parasitic internal diodes.

LLC resonant tank portion 943 may be configured by using the ferrite (ferrite) and a transformer resonant capacitor (resonant capacitor, C _r) for. The transformer includes a parasitic leakage inductance (L _l ) and a parasitic magnetizing inductance (L _m ), and in particular, increases the leakage inductance by separating and winding the primary winding side and the secondary winding side. Made in a way.

The transformer includes a primary winding and a secondary winding, the secondary winding has a first winding and a second winding, and one end of the first winding and the second winding are connected to each other. Therefore, the transformer functions to transfer energy of the primary winding side to the secondary winding side according to the switching of the switching unit 941.

Resonant capacitor (C _r) is connected in a circuit including the primary winding of the half-bridge type switching unit 941 and the transformer loop. That is, one end is connected to one end of the primary winding of the transformer, the other end of the resonant capacitor (C _r) for the first switching element (Q ₁₎ and a second switching element (Q ₂₎ of the resonant capacitor (C _r) for It can be connected to the connection point of.

Since the resonant capacitor C _r and the transformer are connected in series, the LLC resonant converter 940 includes the resonant capacitor C _r and the leakage inductance included in the transformer connected to the primary winding side. L _l ) and magnetizing inductance (L _m ) constitute an LLC series resonant circuit.

Since the product of bandwidth and quality factor is a constant value, a large leakage inductor and a small capacitor consequently have a narrow bandwidth and a large quality factor, which is close to an ideal resonance. Thus, if the LLC resonant transducer 940 operates near the resonant frequency, it will be close to a near perfect sine wave and linear analysis techniques can be applied to facilitate the interpretation of the resonant transducer.

Thus, the LLC resonant tank portion 942 can be represented by a band pass filter characteristic, the operating frequency is only the basic component and other high harmonic components can be ignored. This first harmonic approach can be used to derive an equivalent circuit model for sine wave analysis of LLC resonant half-bridge converters.

The rectification smoothing unit 943 is connected to the secondary winding side of the transformer and converts the voltage induced on the secondary winding side into direct current. The rectifying smoother 943 has a first diode D ₁ , a second diode D ₂ , and a smoothing capacitor C, which are intermediate tap rectifiers.

As the _first and second diodes D ₁ and D ₂ , a schottky barrier diode having a short recovery time and a low conduction voltage is used.

The anode of the first diode D ₁ is connected to one end of the first winding on the secondary winding side, and the anode of the second diode D ₂ is connected to one end of the second winding on the secondary winding side. Connected. Cathodes of the _first and second diodes D ₁ and D ₂ are connected to each other and are connected to one end of the smoothing capacitor C.

Feedback control is required to maintain a constant voltage at all times with changes in load and input. LLC resonant transducers adjust the output voltage by adjusting the frequency, so a voltage controlled oscillator is used.

LLC resonant converter 942 according to an embodiment of the present invention applies a voltage controlled oscillator (VCO) having a linear characteristic. The voltage controlled oscillator (VCO) converts a frequency square wave whose frequency changes according to the converted control voltage to adjust the frequency of the switching elements Q ₁ and Q ₂ , and switches to this square wave square wave. The elements Q ₁ and Q ₂ are turned on and off. As a result, the output voltage is adjusted by the resonant tank network of the power stage where the gain value changes as the frequency changes.

The voltage controlled oscillator (VCO) is configured such that the current flowing by the input of the control voltage internally charges and operates through the external capacitor. As a result, the control voltage changes to the switching frequency, and as the control voltage decreases, the internal current rises, leading to fast charging and charging and switching at high frequencies.

The switching controller 945 controls the switching operation of the switching unit 941 by using two first and second switching control signals whose phases are opposite to each other according to the output signal of the voltage controlled oscillator VCO.

The first switching element Q ₁ operates by a first switching signal, and the second switching element Q ₂ operates by a second switching signal. In other words, when the first switching signal is high level and the second switching signal is low level, the first switching element Q _{1 of} the switching unit 941 is turned on and the switching unit 941 The second switching element Q ₂ is off.

Accordingly, sinusoidal resonant current flows through the first switching element Q ₁ through the leakage inductance of the transformer, the resonance capacitor C _r , and the primary winding, and the voltage of the primary winding of the transformer is induced to the secondary winding. Is converted according to the turns ratio. The converted voltage is output through the rectification smoothing unit 943.

On the contrary, when the first switching signal is low level and the second switching signal is high level, the second switching element of the switching unit is turned on and the first switching element Q1 of the switching unit 941 is turned off. (off) Accordingly, the second switching element capacitor resonant leakage inductance of the transformer, via a (Q ₂₎ (C _r) and the primary through the winding is sinusoidal resonance current flows, the first voltage of the primary winding of the transformer organic in the secondary winding Is converted according to the turns ratio. The converted voltage is output through the rectification smoothing unit 943.

LLC resonant transducers applied to the system operate under a wide range of input voltage and output current variations. The LLC resonant converter does not have a controller design based on accurate small signal characteristics due to the strong dependence of the power conversion stage dynamics of the LLC resonant converter. Even in stable operation, under worst-case operating conditions, the phase margin (PM) may be insufficient, resulting in unstable oscillation of the output, causing system failure.

Therefore, after analyzing the small signal dynamic characteristics of the power conversion stage of the LLC resonant converter under the worst operating conditions, the circuit structure of the voltage feedback compensator is designed based on this, so that the LLC resonant converter is not only the worst operating condition, It can be confirmed that stable operation under given operating conditions.

6 shows the voltage gain curve of the output Vo versus the input voltage Vs according to the frequency of the resonant circuit, and defines four operating points for the change of the input and output conditions. That is, FIG. 6 illustrates a voltage gain curve on the frequency axis after deriving an equivalent circuit model for sine wave analysis of the LLC resonant half-bridge converter 940 using the first harmonic approach (FHA) technique.

The voltage gain ratio decreases as the switching frequency increases or decreases around the resonance point (resonant frequency fo = 56 KHz) having the maximum voltage gain ratio. Therefore, the output voltage can be kept constant by controlling the frequency.

A power factor correction unit is positioned in front of the LLC resonant converter 940, and a voltage varying from 340V to 390V is input to the LLC resonant converter 940 from a condition such as an AC input instantaneous power failure. In addition, the output of the LLC resonant converter 940 is a backlight system in which a plurality of CCFL lamps are connected in parallel with the system signal processing circuit. This backlight system will vary from a maximum of 6A to a minimum of 1A. The controller design is performed at the operating point showing the worst small signal characteristic among these four operating points, and the stability and dynamic characteristics analysis is evaluated at all four operating points.

The small signal characteristic of the LLC resonant converter 940 varies greatly depending on the operating conditions by the operating frequency control characteristic of the converter.

7 shows the characteristics of the frequency versus output transfer function when the operating point changes from A to B. FIG. Referring to FIG. 7, in the LLC resonant converter 940, the pole position changes according to the operating frequency.

At operating point A, the transfer function yields the double poles ω _bt & ω ^* _bt in the high frequency region at the frequency equal to the difference between the resonant frequency and the operating frequency, the pole ω _pl located in the low frequency region by the output filter stage, and the output at a fixed frequency. It has ω _esr due to filter ESR (Equivalent Series Resistance).

As the operating point moves to B, the double poles ω _bt & ω ^* _bt split into two poles, ω _sp1 & ω _sp2 , and the pole ω _pl located at the low frequency moves to ω ' _pl . As ω _spl and ω ' _{pl add up} , they exhibit a quadratic transfer function and the phase decreases drastically. Therefore, in order to select the operating point B as the worst operating point and compensate for such a sudden decrease in phase, a voltage compensation circuit design is performed at the operating point B.

An opto-coupler functions to electrically insulate the LLC resonant converter. Opto-couplers have a large parasitic capacitance component in the opto-coupler by increasing the collector-base junction area for the purpose of collecting a large number of photoelectrons to improve the current transfer ratio. The capacitor forms a pole.

Table 1 shows the opto coupler characteristics according to operating points. As shown in Table 1, when the converter's operating conditions change, the bias condition of the opto coupler also changes, so that not only the parasitic capacitance of the opto coupler but also the current transfer ratio change simultaneously. Bring.

Operating point Current carrying ratio Parasitic capacitance A: Vs = 390V / Io = 6A 24% 7.3 nF B: Vs = 340V / Io = 6A 15% 9.4 nF C: Vs = 340V / Io = 1A 18% 8.4 nF D: Vs = 390V / Io = 1A 42% 4.7 nF

FIG. 8 shows curves (| T _m |) of positions of poles and zeros of the voltage compensator and total loop gain for compensating for the small signal characteristics of the power conversion stages of the operating points A and B, respectively. In order to compensate the secondary transfer function characteristic shown at the operating point B, a voltage compensator 944 having a three-pole two-zero structure is used.

Two zeros are placed to compensate for the two poles of the low frequency region causing a sharp decrease in phase at operating point B, and poles are added to compensate for the ESR zero, and the second pole is located in the high frequency region.

From this three-pole, two-point voltage compensator design, the loop gain maintains a slope of -20 dB / dec to the crossover frequency, with sufficient phase margin of 45 ° or more at operating point B as well as all other operating points. It can be confirmed through the experiment and simulation in Figure 9 to secure.

As can be seen from the comparison of the loop gain waveforms of the respective operating points, the small signal characteristics of the operating points B and C whose operating frequencies operate below the resonance frequency are compared with those of A and D which operate in the region where the operating frequency is higher than the resonance frequency. It can be seen that there is little change in the small signal characteristic, which is also closely related to the difference in the transient response characteristic.

10 shows the response characteristics of the output voltage and the resonant circuit current under the output load maximum variation condition. When the same amount of output current Io is changed, the response characteristic when operating at operating points B and C in Fig. (A) is the response characteristic when operating at operating points A and D in Fig. (B) You can see that it is faster than Vo.

Since the dynamic characteristics of the power conversion stage show a sharp decrease in phase due to the secondary transfer function characteristic at the operating point B, it can be confirmed that it is the worst operating condition among all operating conditions. The first embodiment of the present invention compensates for such a sharp decrease in phase, and thus includes an LLC resonant converter including a voltage compensator 944 having a three-pole, two-zero structure at the operating point B so that the converter operates stably under the worst operating conditions. 940 was designed.

It can be seen that the LLC series resonant converter 940 has a large difference in its small signal characteristic according to the variable position of the operating frequency according to the change of the input / output operating conditions with respect to the fixed resonant frequency. In addition, the LLC series resonant converter 940 having a controller designed under the worst operating conditions can be confirmed through simulation and measurement that has a satisfactory phase margin at all operating points.

11A and 11B relate to an LLC resonant converter according to a second embodiment of the present invention, in which a voltage feedback control method and a current feedback control method are simultaneously applied.

In general, a switching power supply is a device that regulates an output voltage by changing a conduction ratio by switching a switch. As one of the methods of controlling the conduction ratio, the control of the output voltage in the resonant converter, unlike the conventional PWM control method, conducts the switch as a clock signal of a constant frequency, and the switch current or the inductor current causes the error of the output voltage. There is a method of controlling the conduction rate by turning off the switch at the moment when the set value corresponding to the control value is reached. This is called current mode control or current feedback control.

In order to add the current feedback control scheme, the LLC resonant converter according to the second embodiment of the present invention additionally requires a current sensing unit (CSN) 946 and an adder 947 as well as the circuit shown in FIG. 4. .

The current detector 946 detects a change in the primary side resonance current envelope of the transformer included in the LLC resonant tank.

12, unlike the current waveform of the PWM converter (FIG. 12B), in the case of the LLC resonant converter, the waveform of the primary side resonant current of the LLC resonant tank for transferring energy (see FIG. 12A-FIG. 11A). Measured at point A) is an AC sine wave. Therefore, in order to implement current feedback control (Current Mode Control), it is important to effectively detect the AC sinusoidal current flowing in the alternating polarity.

The adder 947 adds the first control signal output from the voltage compensator 944 and a second control signal sensed by the current sensing unit CSN 946 from inside the LLC resonant tank to add a sum signal. Supply. The sum signal added by the adder 947 is input to the switching control unit 945 including the voltage controlled oscillator VCO, thereby forming an overall control circuit.

It is a feature of the second embodiment of the present invention to provide a scheme for converting AC current to DC current. To this end, the current detector 946 includes a low pass filter composed of a rectifier D ₃ , a resistor R ₆ , and a capacitor C ₄ . In addition, a resistor may be included to detect a voltage corresponding to the rectified current. The current detector 946 rectifies the primary side resonant current of the LLC resonant tank through the rectifier D3, and the rectified DC current component includes a low pass filter including a resistor R ₆ and a capacitor C ₄ . By passing through the Low Pass filter, a waveform with sufficiently averaging information (measured at point B of Fig. 13-Fig. 11B) is detected.

The rectified and smoothed current is detected as a voltage signal, that is, a second control signal by a resistor R ₇ connected in parallel with a low pass filter, and summed with the first control signal by an adder 947. Supplied to a control oscillator (VCO).

By adding the current feedback control method to the voltage feedback control method as compared to the voltage feedback control method which receives the control signal from the secondary winding side where the storage speed time of the smoothing capacitor C on the secondary winding side is delayed and controls the feedback. There is an advantage to implement the transient response characteristics.

Although the above has been described with reference to a preferred embodiment of the present invention, those skilled in the art to which the present invention pertains without departing from the spirit and scope of the present invention as set forth in the claims below It will be readily understood that modifications and variations are possible.

1 is an exploded perspective view of a liquid crystal display according to an embodiment of the present invention, FIG. 2 is a block diagram of a liquid crystal display according to an embodiment of the present invention, and FIG. 3 is a liquid crystal according to an embodiment of the present invention. It is an equivalent circuit diagram of one pixel of a display device.

4 is a detailed circuit diagram of a backlight unit according to an embodiment of the present invention.

5 is a diagram illustrating a structure of an insulated LLC resonant converter having a voltage compensator for voltage feedback control according to a first embodiment of the present invention.

FIG. 6 illustrates a voltage gain curve of an output Vo versus an input voltage Vs according to a frequency of a resonant circuit, and defines four operating points with respect to a change in input and output conditions.

7 is a diagram showing the characteristics of the frequency-to-power transfer function when the operating point changes from A to B. FIG.

FIG. 8 is a diagram illustrating curves (| T _m |) of positions of poles and zeros of the voltage compensating unit and the total loop gain for compensating for the small signal characteristics of the power conversion stages of the operating points A and B, respectively.

9 is a view showing the results of experiments and simulations can be confirmed that having a sufficient phase margin of more than 45 ° at all operating points to ensure the stability.

10 shows the response characteristics of the output voltage and the resonant circuit current under the output load maximum variation condition.

11A and 11B illustrate an LLC resonant converter according to a second embodiment of the present invention, in which a current feedback control method is added to a first embodiment to which a voltage feedback control method is applied.

12A is a waveform diagram of a primary side resonance current of an LLC resonant tank measured at point A of FIG. 11A, and FIG. 12B is a current waveform diagram of a PWM converter.

FIG. 13 is a waveform diagram with smoothing information measured at point B of FIG. 11B.

Claims (25)

A switching unit configured to switch according to a switching control signal; An LLC resonant tank unit configured to transfer energy from the primary winding side to the secondary winding side according to a switching operation of the switching unit and perform LLC resonance; A rectifying smoother connected to the secondary winding side and converting the voltage induced on the secondary winding side into direct current; And a voltage compensator for feedback control to maintain the output voltage of the rectified smoother constant, wherein the voltage compensator has a 3-pole-2 zero structure. The method of claim 1, And the voltage compensator comprises an opto-coupler. The method of claim 2, The LLC resonant converter of claim 1, wherein the opto-coupler forms one pole. A switching unit configured to switch according to a switching control signal; An LLC resonant tank unit configured to transfer energy from the primary winding side to the secondary winding side according to a switching operation of the switching unit and perform LLC resonance; A rectifying smoother connected to the secondary winding side and converting the voltage induced on the secondary winding side into direct current; A voltage compensator for performing feedback control to maintain a constant output voltage of the rectifying smoother; A current detector for detecting and detecting a resonance current of the primary winding side; And a switching controller for adjusting the frequency of the switching unit according to a sum signal of the first control signal output from the voltage compensator and the second control signal output from the current detector. The method of claim 4, wherein And an adder for summing the first control signal output from the voltage compensator and the second control signal output from the current detector. The method of claim 4, wherein The current detector detects a change in the resonance current (envelope) of the primary winding side LLC resonant converter. The method of claim 4, wherein The current resonator includes a rectifier element and a low pass filter (Low Pass Filter) characterized in that the LLC resonant converter. The method of claim 7, wherein The low pass filter (Low Pass Filter) LLC resonant converter, characterized in that to smooth the current rectified by the rectifying element. The method of claim 7, wherein The current detection unit further comprises a resistor for detecting a voltage corresponding to the rectified rectified current LLC resonant converter. 10. The method of claim 9, And the resistor is connected in parallel with the low pass filter. The method of claim 4, wherein The switching control unit includes a voltage controlled oscillator (VCO) characterized in that the LLC resonant converter. The method according to any one of claims 1 and 4, The switching control unit is a LLC resonant converter for controlling the switching operation of the switching unit using the first and second switching control signals of the opposite phase. The method according to any one of claims 1 and 4, And the LLC resonance is made by a leakage inductance, a magnetization inductance, and a resonance capacitor. The method of claim 4, wherein And the waveform of the primary winding side resonant current is a sine wave. The method of claim 4, wherein LLC resonant converter, characterized in that the voltage compensation unit has a three-pole, two-point structure. The method of claim 4, wherein And the voltage compensator comprises an opto-coupler. The method of claim 16, And the opto-coupler electrically insulates the LLC resonant transducer. The method of claim 4, wherein The smooth rectifier is LLC resonant transducer, characterized in that the center-tap rectifier part (Center-tap rectifier part). The method of claim 4, wherein And the switching unit is a half bridge switching part including two switching elements driving alternately. The method of claim 21, LLC resonant converter, characterized in that the switching element is a MOSFET. Multiple pixels A light source unit supplying light to the pixel, A display device including a power supply for applying a driving voltage to the light source. The display device of claim 1, wherein the power supply unit includes an LLC resonant converter having the features of claim 1. The method of claim 21, The power supply unit further comprises a power factor correction unit. The method of claim 21, The light source unit includes a light emitting diode. The method of claim 21, The light source unit is a fluorescent lamp, The power supply unit further comprises an inverter for driving a fluorescent lamp. The method of claim 24, And the inverter is a DC-AC full bridge inverter.

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