KR100911025B1 - Feedback Driving System and Driving Method for Maintaining Glow Discharge of a Flat Backlight - Google Patents

Abstract

A driving circuit for supplying driving power to the surface light source lamp; A sensing unit measuring a current flowing in the surface light source lamp; And analyzing the current value measured by the sensing unit and detecting the indication that the surface light source lamp transitions to the discharge contraction or partial discharge state, so that the current flowing through the surface light source lamp is maintained at a predetermined magnitude. Disclosed is a surface light source feedback driving system comprising a control unit for controlling the control. According to an embodiment of the present invention, by configuring a system for feedback control of the surface light source driving circuit by measuring the value of the current flowing through the surface light source, the value of the current flowing through the surface light source lamp can be maintained within a desired range, thereby discharging discharge or It is possible to construct a stable surface light source driving circuit which maintains glow discharge without transitioning to a partial discharge state.

Surface light source, feedback, drive, control

Description

Feedback Driving System and Driving Method for Maintaining Glow Discharge of Surface Light Source

1 is a graph showing a relationship between a driving circuit input voltage and a pulse width of a surface light source lamp.

2A is a graph showing the surface light source driving voltage and the lamp current before discharge occurs.

2B is a graph showing the surface light source driving voltage and the lamp current in the glow discharge state as the lamp current increases.

2C is a graph illustrating a driving voltage and a lamp current as the lamp current further increases.

FIG. 2D is a graph showing the lamp current in a state where the lamp current is increased and then shifted to the discharge contraction state.

FIG. 3A is a graph showing a relationship between a lamp current or a lamp voltage and a glow discharge holding time which are constantly maintained in the surface light source driving circuit according to the prior art.

3B is a graph illustrating a relationship between an increasing lamp current or a lamp voltage and a glow discharge holding time in the surface light source driving circuit according to the related art.

3C is a graph showing a relationship between a lamp current or a lamp voltage and a glow discharge holding time that decrease in a surface light source driving circuit according to the prior art.

4 is a block diagram showing the configuration of a surface light source feedback driving system according to an embodiment of the present invention.

5 is a flowchart showing each step of the method for driving the surface light source feedback according to the first embodiment of the present invention.

6 is a flowchart showing each step of the method for driving the surface light source feedback according to the second embodiment of the present invention.

FIG. 7A is a graph illustrating a relationship between a lamp voltage, a lamp current, and a glow discharge holding time that are reduced in the surface light source feedback driving system according to an exemplary embodiment of the present invention.

7B is a graph illustrating a relationship between a lamp voltage, a lamp current, and a glow discharge holding time that are constantly maintained in the surface light source feedback driving system according to the embodiment of the present invention.

8A is a photograph showing a front discharge state according to glow discharge of a surface light source lamp.

8B is a photograph showing a discharge contraction state of the surface light source lamp.

8C is a photograph showing a state in which the surface light source lamp has failed front discharge due to insufficient power supply.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface light source feedback driving system, and more particularly, to prevent a transition to a discharge contraction or partial discharge state or to light in a discharge contraction or partial discharge state as the operation time of the surface light source becomes longer. A feedback drive system for operating.

In the case of the recently developed flat panel type surface light source, the glow discharge is used for the front discharge, not the discharge shrinkage. The photo of FIG. 8A shows a surface light source lamp showing a stable front discharge state through a glow discharge mode. However, when the surface light source is operated for a long time, due to the change in the discharge state of the surface light source and the thermal change of the driving circuit, as shown in FIG. 8B, the operating point of the surface light source may pass to the discharge contraction region. In this case, there is a problem that the transition from the glow discharge to the discharge contraction state or the lighting is turned on at all in the discharge contraction state according to the operating point of the light source. Alternatively, when the power supply is insufficient due to the same environmental change and the change in its own driving characteristics, there may be a partial discharge instead of the front discharge as shown in FIG. 8C.

1 is a graph showing the relationship between the surface light source driving circuit input voltage and the pulse width. The X axis of the graph represents a pulse width and the Y axis represents a drive circuit input voltage. The lower graph 12 shows the value of the minimum input voltage for driving the surface light source lamp according to the pulse width. On the other hand, the upper graph 11 shows the maximum input voltage at which the surface light source lamp can maintain the glow discharge. When the surface light source lamp has a larger driving circuit input voltage than the upper graph 11, the graph 11 transitions to the discharge contraction state. Therefore, when the operating point is located in the area between the two graphs, the surface light source can maintain the glow discharge state.

2A is a graph showing the driving voltage 21 and the lamp current 22 in the state before the glow discharge. However, as the surface light source lamp is driven in the glow discharge state, as shown in FIG. 2B, the lamp current 22 is increased due to the discharge change due to the plasma temperature rise and the panel temperature increase and the change of the surrounding environment as shown in FIG. 2B. To start. With the surface light source lamp being driven in the state shown, the lamp current 22 is further increased as shown in FIG. 2C, and eventually, with the sudden increase in the lamp current 22, FIG. 2D. As shown in FIG. 2, the state is transferred to the discharge contraction state.

3A and 3B are graphs showing the time taken for the glow discharge to transition to the discharge contraction state according to the lamp voltage or the lamp current. When driving of the surface light source is started, the operating point is located at the point indicated by the + symbol in FIG. 3. However, as time passes, the operating point moves along the time axis, which is the Y axis of the graph, as shown in FIG. 3A, or increases in the X-axis direction due to an increase in lamp voltage or current, as shown in FIG. 3B. It moves in the Y-axis direction, and as a result, it passes from the glow discharge to the discharge shrinkage region. As shown in the figure, as the lamp voltage or lamp current increases, the time for maintaining the glow discharge is shortened. On the contrary, as shown in FIG. 3C, the light may be transferred to the partial discharge region instead of the front discharge by the reduction of the lamp voltage or the lamp current.

Therefore, for stable operation of the surface light source, it is necessary to properly control the lamp voltage and the lamp current to be maintained within the desired range. An obvious control method is to prevent entry into the discharge contraction area by feeding back the operating state of the surface light source lamp. To this end, a sensing unit for a feedback operation and a control unit for adjusting an appropriate driving circuit are required, but there is currently no research on the surface light source feedback driving circuit. The simplest way to think about is to sense the brightness of the lamp directly through the illuminance sensor. However, due to constraints such as price, volume, and slow dynamics, there are a lot of difficulties in implementing the actual feedback.

Alternatively, a method of controlling using the correlation between brightness and lamp power is desired. Once discharge contraction occurs, the discharging current rapidly increases due to uneven charge particle distribution in the discharge space and local heating of the discharge space. There is a need for a method that can predict or post-measure such indications to allow the surface light source lamp to operate stably in the glow discharge region.

SUMMARY OF THE INVENTION The present invention provides a surface light source feedback driving circuit for determining and feeding back a state of a surface light source by measuring a lamp current so that the operating point of the surface light source stays in the glow discharge region. The purpose.

According to an aspect of the present invention, there is provided a surface light source feedback driving system including: a driving circuit configured to supply driving power to a surface light source lamp; A sensing unit measuring a current flowing in the surface light source lamp; And analyzing the current value measured by the sensing unit and detecting the indication that the surface light source lamp transitions to the discharge contraction or partial discharge state, so that the current flowing through the surface light source lamp is maintained at a predetermined magnitude. It may be configured to include a control unit for controlling the.

According to another aspect of the present invention, there is provided a method of driving a surface light source feedback, the method including: measuring a current flowing in a surface light source lamp; Normalizing the measured current; Analyzing the normalized current; And controlling the driving power supplied to the surface light source lamp so that the current is maintained at a predetermined size when detecting the indication that the surface light source lamp transitions to a discharge contraction or partial discharge state. Can be.

Hereinafter, with reference to the accompanying drawings looks at in detail a preferred embodiment of the present invention. 4 is a block diagram showing a surface light source feedback configuration system according to an embodiment of the present invention. As shown, the embodiment may include a driving circuit 41, the sensing unit 43 and the control unit 44, the intensity of the current flowing in the surface light source lamp 42 in the sensing unit 43 The current value measured by the sensing unit 43 is transmitted to the controller 44, and the controller 44 controls the driving circuit 41 to feedback control the driving power applied to the surface light source lamp 42. do. 5 and 6 are flowcharts illustrating each step of a feedback control method of a surface light source driving circuit according to an exemplary embodiment of the present invention.

Driving circuit

The driving circuit 41 included in one embodiment of the present invention is a circuit capable of supplying a driving voltage or driving current to the surface light source lamp 42 for the operation of the surface light source lamp 42. The drive circuit 41 may include an inductor, a capacitor, a diode, a switch, and a voltage source or a current source, or may be of other configuration known in the art. The driving circuit 41 generates power for driving the surface light source (for example, a voltage or current in the form of a pulse) and applies it to the surface light source lamp 42 (S501 and S601). An embodiment of the surface light source driving circuit is described in Patent Application No. 2006-52017 filed on June 9, 2006, entitled “Surface Light Source Driving Circuit of Liquid Crystal Display Device”, and thus, the detailed description thereof is omitted. do.

The manner in which the drive circuit 41 drives the surface light source lamp 42 may be implemented in different ways depending on the embodiment. The surface light source lamp 42 generally includes a main electrode and an auxiliary electrode, and the driving circuit 41 controls the voltage between the main electrodes of the surface light source lamp 42 or the voltage between the auxiliary electrodes. Will be controlled. An embodiment of the surface light source lamp structure is filed on April 12, 2006, and is described in patent application No. 2006-33281 entitled "High efficiency mercury surface light source structure, surface light source device and its driving method". Detailed description will be omitted in the specification.

Sensing Part

When driving power is applied, a lamp current flows through the surface light source lamp 42, and the intensity of the lamp current should be measured for feedback. The position for measuring the lamp current may be implemented in various ways depending on the embodiment. In one embodiment, the lamp current may be measured by directly connecting a conductor to the surface light source lamp 42. In another embodiment, the lamp current may be connected to the surface light source lamp 42 by connecting a switch, an inductor, or a transformer according to the driving circuit. Indirect measurements can be made by measuring the lamp current in the circuit.

For example, when the driving circuit 41 including the transformer is used, the power supply unit may be connected to the transformer primary side and the surface light source to the transformer secondary side to supply power to the surface light source through the transformer. In this case, the sensing unit 43 may sense a current flowing in the surface light source lamp 42 directly from the surface light source lamp 42 side of the transformer. Alternatively, it is also possible to indirectly sense the current of the surface light source lamp 42 by sensing the current at the primary side of the transformer.

When the measurement position is determined as described above, the intensity of the lamp current is measured next (S502, S602). In one embodiment, by connecting a resistor to the wire, by measuring the voltage drop generated through the resistance it is possible to measure the strength of the current flowing through the wire. In another embodiment, the current transformer can be used to measure the intensity of the lamp current by flowing a current through the primary coil of the current transformer and measuring the strength of the current in the secondary coil. In another embodiment, the lamp current flowing through the conductive wire may be measured through the strength of the magnetic field by measuring the magnetic field generated by the conductive wire through the Hall sensor.

The above-described current measuring methods are described by way of example, and those skilled in the art can measure the intensity of the lamp current using any suitable method known in the art, which is included in the scope of the present invention.

The measured intensity of the lamp current is compared with a predetermined reference current value I _ref or a threshold current value I _threshold according to an embodiment for feedback, and analyzed to detect an indication of a transition to a discharge contraction state. 44). In this case, the driving power of the surface light source lamp 42 is generally implemented in the form of a pulse, and the lamp current flowing in the surface light source lamp 42 also takes the form of a pulse. Therefore, for comparison and analysis, a process of extracting an appropriate value such as an average value, a peak value, or a time width from a pulse type lamp current value is required. Hereinafter, the process of extracting the above-mentioned suitable value is referred to as a standardization process, and the sensing unit 43 performs a process of standardizing the measured current value by a method described below (S503 and S603).

The strength of the current to be measured can be standardized in various implementations depending on the embodiment. In an embodiment of the present invention, the intensity of the current may be measured using the overall average value of the lamp currents represented in the form of pulses. At this time, the lamp current may be passed through the low frequency filter to remove the AC component and obtain an average value of the DC component. In another embodiment, the intensity of the current can be measured using the average of the positive values of the drive current. Since the surface light source lamp current is a pulse having a positive value and a negative value repeatedly, after rectifying the current to a positive value through a rectifying circuit, the average value may be measured using a low frequency filter as described above.

Alternatively, the intensity of the current may be normalized by measuring the peak value of the lamp current or the time width of the pulse of the lamp current. Alternatively, the lamp current may be normalized to an integrated value obtained by integrating the pulses of the lamp current. In this case, however, since the integral value of the lamp current will continue to accumulate and increase over time, the integral value should be reset to zero every time the pulse period passes. It is also possible to sample the instantaneous current value from the measured current using a sample and hold circuit, and normalize the lamp current with the sampled current value.

The current can also be normalized using a negative current value that appears at the end of the ramp current pulse. As described above, the lamp current rapidly increases from the glow discharge to the discharge contraction state, and as shown in FIG. 2D, it can be seen that the negative current value is remarkable at this time. Therefore, with respect to the negative current appearing at the end of the lamp current pulse, it is also possible to measure the average value of the direct current component, the peak value of the current, or the time width in the same manner as described above, and normalize the lamp current as this value. Lamp current values normalized by these schemes are transmitted to the controller 44.

The foregoing standardization methods are described by way of example, and those skilled in the art can standardize the strength of the current using any suitable method known in the art, which is included in the scope of the spirit of the present invention.

Control

The controller 44 receives the standardized lamp current value, analyzes it, and starts feedback control of the driving circuit 41 according to the analysis result. That is, the controller 44 controls the frequency or pulse width of the driving power supplied from the driving circuit 41 when the value of the lamp current is large enough to cause the discharge contraction or small enough to cause the partial discharge. Adjust the parameters appropriately to keep the lamp current in the desired area. The feedback scheme of the controller 44 may be implemented in various manners according to the exemplary embodiment, and there are representative examples thereof, a control scheme using a reference current and a control scheme using a limit current.

In the first embodiment of the present invention, the controller 44 controls the driving circuit 41 by using the reference current value. In the above embodiment, the controller 44 compares the measured lamp current I with the reference current value I _ref (S504). Reference Current I _ref Can be appropriately set to the value of the current which causes the surface light source lamp 42 to glow discharge, and the controller 44 feeds back the lamp current in such a manner as to keep the value of the lamp current close to the value of the reference current. . Whether the feedback is performed by the controller 44 is determined by whether the difference between the lamp current I and the reference current I _ref is within a tolerance K, which is represented by the following equations.

When the difference between the lamp current I and the reference current I _ref is within a predetermined tolerance K, that is, when the above Equation 1 is satisfied, there is no problem in maintaining the glow discharge because the error between the lamp current and the reference current value is not large. The controller 44 does not perform a control operation.

On the other hand, when the difference between the lamp current value I and the reference current I _ref is greater than or equal to the tolerance K, that is, when the equation 2 is satisfied, the controller 44 controls the driving circuit 41 to increase or decrease the value of the lamp current. do. That is, when the value of the lamp current is smaller than the reference current by a width larger than the tolerance, the controller 44 increases the driving power of the driving circuit 41 by a predetermined amount, and the value of the lamp current is larger than the tolerance. If greater than the reference current, the driving power is reduced (S505).

For example, when the reference current value is 250 and the tolerance K is 1, if the measured lamp current I is 248, the controller 44 controls the driving circuit 41 to increase the lamp current to 249. If the size is 252, the driving circuit 41 is controlled so that the lamp current is 251. In this manner, the controller 44 may adjust the lamp current I to be maintained near the reference current value.

As shown in Fig. 7B, when the reference current is sufficiently small, the surface light source lamp 42 can be prevented from transitioning to the discharge contraction state simply by maintaining the reference current. However, when the value of the reference current is large according to the operating point of the surface light source lamp 42, even if the value of the lamp current is kept constant as shown in FIG. 3A, the characteristics of the surface light source lamp 42 over time. There is a risk of transition to the discharge contraction state. Therefore, in this case, the surface light source should be adjusted to maintain the glow discharge by decreasing the value of the reference current itself. On the contrary, when the value of the lamp current decreases, it is necessary to increase the value of the reference current itself to prevent the operating point of the surface light source lamp 42 from crossing over to the partial discharge region shown by the dotted line in FIG. 7B.

Accordingly, the controller 44 adjusts the value of the lamp current to converge to the reference current value, and also analyzes the standardized current value measured by the sensing unit 43 (S506). As described above with reference to FIG. 2D, before the surface light source lamp 42 transitions to the discharge contracted state, a sudden change in the average value, peak value, pulse width, etc. of the lamp current appears. For example, a phenomenon in which the lamp current suddenly increases or the negative current value appearing at the end of the lamp current increases.

Therefore, by analyzing the lamp current and measuring the maximum value or increase in the average value of the sudden current, it is possible to detect the indication that the surface light source lamp 42 is about to transition to the discharge contraction state (S507). If it is determined that the surface light source lamp 42 intends to transition to the discharge contracted state, the controller 44 reduces the reference current value by a certain amount to maintain the glow discharge (S508). As shown in FIG. 7A, when the magnitude of the reference current decreases, since the controller 44 adjusts the lamp current to a value similar to the value of the reduced reference current, the operating point moves to the upper left corner of the graph, thereby preventing the glow discharge. I can keep it.

On the contrary, the lamp current may decrease as the surface light source lamp 42 operates. In this case, the controller 44 analyzes the lamp current, so that the lamp current value is excessively reduced, so that the surface light source lamp 42 does not maintain the glow discharge and detects the indication of the transition to partial discharge (S506, S507). . When a symptom of transition to partial discharge, such as a sudden decrease in lamp current, is detected, the controller 44 increases the reference current value by a certain amount to adjust the surface light source to maintain the glow discharge (S508). When the magnitude of the reference current increases, the lamp current converges to the reference current value under the control of the controller 44, so that the operating point is maintained in the glow discharge region.

As another feedback method, in the second embodiment of the present invention, the controller 44 may control the driving circuit 41 using the limit current value. In the above embodiment, the controller 44 performs a step of comparing the measured lamp current value with the threshold current value I _threshold (S604). As a result of the comparison, when the value I of the lamp current is smaller than the threshold current value (I <I _threshold ), the controller 44 does not perform a control process. On the other hand, when the lamp current value I is greater than or equal to the threshold current value (I ≥ I _threshold ), the controller 44 controls the driving circuit 41 to control the driving power applied to the surface light source lamp 42 in the driving circuit 41. The value is reduced by a predetermined value (S605).

The value of the limit current is a value of the current at which the discharge shrinkage may occur when the lamp current becomes more than the limit current value. That is, this is the maximum current value at which the surface light source lamp 42 can be kept in the glow discharge state. As shown in FIG. 3A, even when the lamp current is constant, a transition to discharge shrinkage may occur as the driving time becomes longer. Therefore, as in the case of the above-described reference current, if it is determined that the surface light source lamp 42 is to transition to the discharge contraction state by analyzing the standardized current (S606) (S607), the controller 44 decreases the value of the limit current. (S608).

Control of the driving circuit 41 by the controller 44 may be implemented in various ways, depending on the embodiment. When it is necessary to reduce the value of the lamp current, in one embodiment, the controller 44 may control the value of the lamp current by reducing the frequency of the driving power applied to the surface light source by a predetermined value. In another embodiment, the controller 44 may control the current of the driving circuit by reducing the pulse width of the driving power.

In another embodiment, the lamp current may be controlled by directly controlling the input voltage input to the driving circuit 41 so that the value of the driving power applied to the surface light source lamp 42 is reduced. In another embodiment, in the driving circuit 41 by a force current limiting method, a low frequency pulse width modulation (PWM) or a method of forcibly turning the driving circuit 41 on / off. You can limit the output current.

The aforementioned control methods are described by way of example, and those skilled in the art can control the magnitude of the driving power using any suitable method known in the art, which is included in the scope of the spirit of the present invention.

When the driving circuit 41 is controlled by the methods according to the various embodiments, the value of the driving power applied by the driving circuit 41 to the surface light source lamp 42 is maintained in a constant region, and as a result, the surface light source lamp The value of the lamp current flowing in (42) can be maintained in a desired range, thereby preventing the surface light source from transitioning to discharge contraction or partial discharge.

7A and 7B are graphs showing a relationship between a lamp voltage, a lamp current, and a glow discharge holding time according to an embodiment of the present invention. The operating point at the time when the operation of the surface light source lamp 42 is started is shown by a + sign, and as time passes, the operating point moves along the time axis, which is the Y axis of the graph. However, unlike the prior art shown in FIGS. 3A and 3B, the driving power is controlled by the surface light source feedback control circuit. Therefore, as time passes, the value of the lamp voltage or the lamp current remains constant as shown in FIG. 7B, or the value of the lamp current decreases as shown in FIG. Move to the upper left corner. Therefore, the transition from the glow discharge to the discharge contraction or partial discharge state does not occur, so that the surface light source can be adjusted to maintain the glow discharge state.

While specific embodiments of the present invention have been illustrated and described, the technical spirit of the present invention is not limited to the accompanying drawings and the above description, and various modifications can be made without departing from the spirit of the present invention. It will be apparent to those skilled in the art, and variations of this form will be regarded as belonging to the claims of the present invention without departing from the spirit of the present invention.

According to the present invention, by configuring a system for measuring the value of the current flowing in the surface light source and feedback-controlling the surface light source driving circuit, the value of the current flowing in the surface light source lamp can be maintained within a predetermined range, and thus the discharge contraction state or partial discharge It is possible to construct a stable surface light source driving circuit which sustains glow discharge without transitioning to a state.

Claims (35)

A driving circuit for supplying driving power to the surface light source lamp; A sensing unit measuring a current flowing through the surface light source lamp and calculating a standardized value by standardizing the measured current; And And a controller configured to control the driving circuit to maintain the standardized value within the preset range when the standardized value increases above the preset range or decreases below the preset range. The control unit, If the normalized value is greater than or equal to a threshold current value, the driving circuit is controlled to decrease the standardized value by a predetermined size, And decrease the threshold current value by a predetermined magnitude when the normalized value increases above the preset range. delete delete The method of claim 1, The sensing unit may connect a circuit including a switch, an inductor, or a transformer to the surface light source lamp to measure a current flowing through the circuit. The method of claim 1, The sensing unit, the surface light source feedback drive system, characterized in that for measuring the current by measuring the voltage drop generated as the current passes through the resistance. The method of claim 1, The sensing unit, the surface light source feedback drive system, characterized in that for measuring the current using a current transformer. The method of claim 1, The sensing unit measures the current by measuring a magnetic field generated by the current using a Hall sensor. The method of claim 1, And the sensing unit calculates an average value obtained by passing the current through the low frequency filter as the standardized value. The method of claim 1, And the sensing unit rectifies the current by the rectifying circuit and calculates the average value of the amount obtained by passing the rectified current through the low frequency filter as the standardized value. The method of claim 1, The sensing unit calculates a peak value or a time width of the pulse of the current as the normalized value. The method of claim 1, And the sensing unit calculates a value obtained by integrating the current as the normalized value for a time equal to the pulse period of the current. The method of claim 1, And the sensing unit calculates an average value of the negative current appearing at the pulse terminal of the current as the normalized value using a low frequency filter. The method of claim 1, And the sensing unit calculates a peak value or a time width of a negative current appearing at the pulse terminal of the current as the normalized value. The method of claim 1, And the sensing unit samples a current value of a predetermined time using a sample and hold circuit, and calculates a sampled current value as the standardized value. The method of claim 1, The control unit, the surface light source feedback drive system, characterized in that for controlling the frequency or pulse width of the voltage or current supplied from the drive circuit. The method of claim 1, The control unit, the surface light source feedback drive system, characterized in that for controlling the magnitude of the voltage or current input to the drive circuit. The method of claim 1, And the control unit limits a pulse width of a current supplied from the driving circuit by low frequency pulse width modulation. The method of claim 1, And the control part controls the drive circuit by forcibly switching the drive circuit to an on or off state. Measuring a current flowing in the surface light source lamp; Normalizing the measured current to yield a normalized value; And Controlling driving power supplied to the surface light source lamp so that the standardized value is maintained within the preset range when the standardized value increases above the preset range or decreases below the preset range. , The controlling of the driving power may include: Reducing the driving power supplied to the surface light source lamp by a predetermined magnitude when the normalized value is greater than or equal to a threshold current value; And And decreasing the limit current value by a predetermined magnitude when the normalized value increases above the preset range. delete delete The method of claim 19, The measuring of the current may include measuring a voltage drop generated while the current passes through a resistance. The method of claim 19, The measuring of the current may include measuring the current using a current transformer. The method of claim 19, The measuring of the current may include measuring a magnetic field generated by the current using a hall sensor. The method of claim 19, The calculating of the normalized value includes passing the current through a low frequency filter to calculate an average value. The method of claim 19, The calculating of the normalized value may include rectifying the current and passing the rectified current through a low frequency filter to calculate a positive mean value. The method of claim 19, The calculating of the normalized value may include calculating a peak value or a time width of the pulse of the current. The method of claim 19, The calculating of the normalized value may include calculating a value obtained by integrating the current for a time equal to the pulse period of the current. The method of claim 19, The calculating of the normalized value includes passing the current through a low frequency filter to calculate an average value of the negative current appearing at the pulse terminal of the current. The method of claim 19, The calculating of the normalized value includes calculating a peak value or a time width of a negative current appearing at the pulse end of the current. The method of claim 19, The calculating of the normalized value may include: calculating a sampled current value by sampling a current value of a preset time. The method of claim 19, The controlling of the driving power may include controlling a pulse width or a frequency of a voltage or a current applied to the surface light source lamp. The method of claim 19, The controlling of the driving power may include controlling a magnitude of a voltage or a current applied to the surface light source lamp. The method of claim 19, The controlling of the driving power may include controlling a pulse width of a current applied to the surface light source lamp by low frequency pulse width modulation. The method of claim 19, The controlling of the driving power may include forcibly switching the power applied to the surface light source lamp into an on or off state.

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