Light source control driver and projector

Abstract

This light source control driver for controlling a laser diode that is a light source for a projector comprises: a DC-DC converter including a first transistor that controls a current input to the laser diode; and a second transistor that is connected in parallel to the laser diode and controls the current input to the laser diode. The light source control driver synchronizes the period of a pulse width modulation (PWM) signal for controlling the first transistor and the period of a PWM signal for controlling the second transistor.

Description

Light source control driver and projector

[0001] The present disclosure relates to a light source control driver and a projector.

[0002] Patent Document 1 discloses a projector including a light source device, a display panel irradiated with light from the light source device, and an optical system that projects an image displayed on the display panel.

[0003] Japanese Patent Application Laid-Open No. 2023-146051

[0004] When the brightness of the light irradiated by the light source device used in the projector is unstable, the quality of the projected image deteriorates.

[0005] An object of the present disclosure is to provide a technique for stabilizing the brightness of light irradiated by a light source device used in a projector.

[0006] One aspect of the present disclosure is a light source control driver that controls a laser diode which is a light source of a projector, including a DC / DC converter including a first transistor that controls a current input to the laser diode, and a second transistor connected in parallel with the laser diode and controlling a current input to the laser diode, and synchronizing a period of a Pulse Width Modulation (PWM) signal that controls the first transistor and a period of a PWM signal that controls the second transistor, and provides a light source control driver.

[0007] One aspect of the present disclosure provides a projector including the light source control driver described above.

[0008] These general or specific aspects may be implemented by a system, a device, a method, or an integrated circuit, or may be implemented by any combination of a system, a device, a method, and an integrated circuit.

[0009] According to the present disclosure, the brightness of the light irradiated by the light source device used in the projector can be stabilized.

[0010] Schematic diagram showing the optical configuration of the projection-type video display device according to the embodiment Schematic diagram showing the RGB light source unit in the embodiment Schematic diagram showing the excitation light source unit in the embodiment Schematic diagram showing the phosphor wheel in the embodiment Spectrum diagram of the color light in the projection-type video display device according to the embodiment Schematic diagram showing the subframe configuration of the embodiment Schematic diagram showing the video frame configuration of the embodiment Spectrum diagram of color segment R in the embodiment Spectrum diagram of color segment Ye in the embodiment Spectrum diagram of color segment G in the embodiment Spectrum diagram of color segment Cy in the embodiment Spectrum diagram of color segment B in the embodiment Diagram showing the configuration of the light source circuit to be compared Figures showing the time variation of the current value input to the LD of the light source circuit Figures showing an example configuration of the light source circuit according to the embodiment Figures showing the time variation of the current value input to the LD of the light source circuit according to the embodiment Figures showing an example of a color segment per subframe for a projection-type video display device equipped with a light source circuit to be compared Figures explaining the relationship between the light pulse of the LD and the ON time of the DMD's minute panel for a projection-type video display device equipped with a light source circuit to be compared Figures showing an example of a color segment per subframe for a projection-type video display device equipped with a light source circuit according to the embodiment Figures explaining the relationship between the light pulse of the LD and the ON time of the DMD's minute panel for a projection-type video display device equipped with a light source circuit according to the embodiment

[0011] Embodiments of the present disclosure will be described in detail below, with appropriate reference to the drawings. However, descriptions that are unnecessarily detailed may be omitted. For example, detailed descriptions of already well-known matters and redundant descriptions of substantially identical configurations may be omitted. This is to avoid the following description becoming unnecessarily verbose and to facilitate understanding for those skilled in the art. The accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure and are not intended to limit the subject matter of the claims. The functions of one configuration shown in this embodiment may be realized by two or more physical configurations, or the functions of two or more configurations may be realized by, for example, one physical configuration.

[0012] First, the configuration of the projection-type image display device 100 according to this embodiment will be described with reference to Figures 1 to 7E. Figure 1 is a schematic diagram showing the optical configuration of the projection-type image display device 100 according to this embodiment. Note that the projection-type image display device 100 may be read as a projector. In this embodiment, an example is given of the case in which red component light R, green component light G, blue component light B, and yellow component light Ye are used as the image light.

[0013] As shown in Figure 1, the projection-type image display device 100 includes a phosphor wheel 30, a dichroic mirror 40, a rod integrator 50, a DMD (Digital Mirror Device) 60, a projection unit 70, and a light source device 150. The light source device 150 includes an RGB light source unit 10, an excitation light source unit 20, and a control device 180. The control device 180 may be configured, for example, by a processor, an LSI (Large Scale Integration), an FPGA (Field Programmable Gate Array), or an ASIC (Application Specific Integrated Circuit).

[0014] Figure 2 is a schematic diagram showing the RGB light source unit 10 in this embodiment. As shown in Figure 2, the RGB light source unit 10 is composed of a red light source 10R, a green light source 10G, a blue light source 10B, a mirror 14R, a dichroic mirror 14G, and a dichroic mirror 14B. The red light source 10R is composed of a red light source heat dissipation substrate 11R, a plurality of red light source emitters 12R, and a plurality of red light source collimating lenses 13R. The green light source 10G is composed of a green light source heat dissipation substrate 11G, a plurality of green light source emitters 12G, and a plurality of green light source collimating lenses 13G. The blue light source 10B is composed of a blue light source heat dissipation substrate 11B, a plurality of blue light source emitters 12B, and a plurality of blue light source collimating lenses 13B. The red light source emitter 12R, the green light source emitter 12G, and the blue light source emitter 12B are composed of, for example, laser diodes (LD) or light-emitting diodes (LED). In this embodiment, the red light source emitter 12R is composed of a red laser diode that emits light with a main wavelength of 642 nm (red component light R), the green light source emitter 12G is composed of a green laser diode that emits light with a main wavelength of 525 nm (green component light G), and the blue light source emitter 12B is composed of a blue laser diode that emits light with a main wavelength of 465 nm (blue component light B). However, these wavelengths are not limited; for example, the dominant wavelength of the red light source emitter 12R may be 630 nm or more and 650 nm or less, the dominant wavelength of the green light source emitter 12G may be 515 nm or more and 535 nm or less, and the dominant wavelength of the blue light source emitter 12B may be 440 nm or more and 470 nm or less.

[0015] Mirror 14R reflects the light emitted from the red light source 10R. Dichroic mirror 14G reflects the light emitted from the green light source 10G and transmits the light emitted from the red light source 10R that has been reflected by mirror 14R. Dichroic mirror 14B reflects the light emitted from the blue light source 10B, transmits the light emitted from the red light source 10R that has been reflected by mirror 14R and transmitted by dichroic mirror 14B, and transmits the light emitted from the green light source 10G that has been reflected by dichroic mirror 14G. Therefore, the light emitted from the RGB light source unit 10 is white light, which is a combination of light with a dominant wavelength of 642 nm (red component light R), light with a dominant wavelength of 525 nm (green component light G), and light with a dominant wavelength of 465 nm (blue component light B).

[0016] Figure 3 is a schematic diagram showing the excitation light source unit 20 in this embodiment. As shown in Figure 3, the excitation light source unit 20 consists of an excitation light source 20Ex and a mirror 24. The excitation light source 20Ex consists of an excitation light source heat dissipation substrate 21, a plurality of excitation light source emitters 22, and a plurality of excitation light source collimating lenses 23. The excitation light source emitters 22 are, for example, laser diodes (LD) or light-emitting diodes (LED).

[0017] In this embodiment, the excitation light source emitter 22 is composed of a blue laser diode that emits light with a main wavelength of 455 nm (excitation light Ex). However, this wavelength is not limited; for example, the main wavelength of the excitation light source emitter 22 may be 440 nm or more and 470 nm or less, or it may be the same main wavelength as the blue light source emitter 12B.

[0018] Figure 4 is a schematic diagram showing the phosphor wheel 30 in this embodiment. As shown in Figure 4, the phosphor wheel 30 is composed of a substrate 31, a reflective film 32 formed on the substrate 31, a phosphor film 33 coated in an annular shape on the reflective film 32, and a motor 34 for rotating the substrate 31. The phosphor wheel 30 is an example of a wavelength conversion element. Figure 4(a) is a view of the phosphor wheel 30 in the z direction of Figure 1, and Figure 4(b) is a view of the phosphor wheel 30 in the y direction of Figure 1.

[0019] The phosphor film 33 is composed of a yellow phosphor film that emits yellow light when irradiated with excitation light Ex. In Figure 4(a), the symbols in parentheses indicate that the components with symbols not in parentheses are located on the upper layer. That is, Figure 4(a) shows that the reflective film 32 is placed on the substrate 31, and the phosphor film 33 is placed on the reflective film 32.

[0020] The phosphor film 33 is composed of a yellow phosphor film that emits yellow light (light with a main wavelength of 560 nm to 590 nm) when irradiated with excitation light Ex. The yellow light mentioned above is an example of colored light output by a wavelength conversion element. The phosphor film 33 can be manufactured, for example, by mixing ceramic phosphor powder with an adhesive (silicone resin), coating it onto a substrate, and curing it at a high temperature. The ceramic phosphor used in the phosphor film 33 is, for example, a cerium-doped active garnet structure phosphor (a cerium-doped garnet structure phosphor). More specifically, the ceramic phosphor used in the phosphor film 33 may be a YAG phosphor or a LAG phosphor.

[0021] The phosphor wheel 30 causes the phosphor film 33 to emit light Ye while changing the position where the excitation light is irradiated in the phosphor film 33 in the circumferential direction by the rotation of the motor 34. In this embodiment, a phosphor wheel 30 is used, but a stationary, non-rotating fluorescent light source may also be used.

[0022] Returning to Figure 1, the dichroic mirror 40 has a coating characteristic that transmits red component light R, green component light G, and blue component light B, and reflects yellow component light Ye1 of the emitted light Ye0.

[0023] Referring to Figure 1, the optical paths of the red component light R, the green component light G, and the blue component light B will first be described. The light emitted from the RGB light source unit 10 (red component light R, green component light G, and blue component light B) is guided to the rod integrator 50 via the mirror 111, diffuser plate 112, lens 113, mirror 114, diffuser plate 115, mirror 116, lens 117, dichroic mirror 40, lens 118, mirror 119, and lens 121 in this order.

[0024] Mirrors 111, 114, and 116 are reflective mirrors that reflect red component light R, green component light G, and blue component light B.

[0025] Mirror 119 is a reflective mirror that reflects red component light R, green component light G, blue component light B, and yellow component light Ye1.

[0026] Diffuser plates 112 and 115 are diffusers that adjust the divergence angles of red component light R, green component light G, and blue component light B.

[0027] Lens 113 is a lens that focuses the red component light R, the green component light G, and the blue component light B near the diffuser plate 115. The size and shape of the focused spot formed on the diffuser plate 115 are corresponding to the diffusion angle characteristics of the diffuser plate 112. Lens 117 is a lens that roughly parallelizes the divergent light emitted from the diffuser plate 115. Lenses 118 and 121 are lenses that focus the light that has been roughly parallelized by lens 117 onto the rod integrator 50. As a result, the focused spot formed on the diffuser plate 115 is roughly imaged onto the incident surface of the rod integrator 50 via lenses 117, 118, and 121 in that order. Therefore, the diffuser plate 115 and the incident surface of the rod integrator 50 are roughly conjugate.

[0028] Next, the optical paths of the excitation light Ex, emitted light Ye0, and yellow component light Ye1 will be described. The light emitted from the excitation light source unit 20 (excitation light Ex) is irradiated onto the phosphor wheel 30 via the diffuser plate 122, dichroic mirror 40, lens 123, and lens 124 in that order. Upon irradiation with excitation light Ex, emitted light Ye0 is emitted from the phosphor wheel 30. The emitted light Ye0 is incident on the dichroic mirror 40 via lenses 124 and 123 in that order. The dichroic mirror 40 reflects the yellow component light Ye1 of the emitted light Ye0 toward lens 118. The yellow component light Ye1 is guided to the rod integrator 50 via lens 118, mirror 119, and lens 121 in that order.

[0029] The diffuser plate 122 is a diffuser plate that adjusts the divergence angle of the excitation light Ex.

[0030] Lenses 123 and 124 are lenses that focus the excitation light Ex onto the surface of the phosphor wheel 30. The size and shape of the focused spot formed on the phosphor wheel 30 are determined according to the diffusion angle characteristics of the diffuser plate 122. Lenses 123 and 124 also make the emitted light Ye0 from the phosphor wheel 30 into approximately parallel light.

[0031] Lenses 118 and 121 are lenses that focus the light, which has been made into approximately parallel light by lenses 123 and 124, onto the rod integrator 50. As a result, the yellow component light Ye1 of the emitted light Ye0 emitted from the phosphor wheel 30 is substantially imaged onto the incident surface of the rod integrator 50 via lenses 124, 123, 118, and 121 in that order. Therefore, the exit surface of the phosphor wheel 30 and the incident surface of the rod integrator 50 are substantially conjugate.

[0032] Figure 5 is a spectral diagram of the color light in the projection-type image display device 100 of this embodiment. Figure 5 shows the spectra of each color light. The dichroic mirror 40 transmits the excitation light Ex, blue component light B, green component light G, and red component light R, and reflects the yellow component light Ye1 of the emitted light Ye0. Therefore, the wavelength band of the yellow component light Ye1 is located between the green component light (525 nm) and the red component light (642 nm). That is, the dominant wavelength of the yellow component light Ye1 is between 525 nm and 642 nm. In this embodiment, the dominant wavelength of the yellow component light Ye1 is 575 nm. However, the dominant wavelength of the yellow component light Ye1 may be any other dominant wavelength as long as it is within the yellow wavelength band (560 nm to 590 nm).

[0033] The rod integrator 50 is a solid rod made of a transparent material such as glass. The rod integrator 50 homogenizes the light emitted from the RGB light source unit 10 and the light emitted from the phosphor wheel 30. The rod integrator 50 may also be a hollow rod whose inner wall is made of a mirror surface. The rod integrator 50 is an example of a light homogenization element.

[0034] Light emitted from the rod integrator 50 passes through lenses 131, 132, and 133, enters a total internal reflection prism consisting of triangular prisms 141 and 142, and then enters the DMD 60.

[0035] The DMD 60 modulates the color component light (red component light R, green component light G, blue component light B, and yellow component light Ye) generated by the RGB light source unit 10 and the phosphor wheel 30 in a time-division manner. In detail, the DMD 60 is composed of multiple micro-mirrors, and these micro-mirrors are movable. Each micro-mirror basically corresponds to one pixel. The DMD 60 switches whether or not to reflect light towards the projection unit 70 by modulating the angle of each micro-mirror according to the video signal.

[0036] Figure 6A is a schematic diagram showing the subframe configuration of this embodiment. Figure 6B is a schematic diagram showing the video frame configuration of this embodiment. Figures 6A and 6B show the correspondence between the display period of each color in the DMD 60 and the emission period of each color light source. In this embodiment, in the operation of the DMD 60, one video frame (for example, 1 / 60 sec) is composed of multiple subframes (Figure 6B), and one subframe is composed of color segments corresponding to the display of R, Ye, G, Cy, and B colors (Figure 6A). The more subframes there are per video frame, the faster the color switching speed becomes, thus reducing the color breaking phenomenon. To achieve a switching speed at which the color breaking phenomenon is almost invisible, it is desirable that the number of subframes be 16 or more (16 times faster). The configuration of the color segments in one subframe may be R, G, B, or R, G, B, Ye, and configurations with different numbers of colors may be mixed in each subframe. The order of the color segments may also be regular or irregular.

[0037] The principle of image light generation in the DMD 60 will be explained with reference to Figures 6A, 6B, and 7A to 7E. Figure 7A is the spectral diagram of color segment R in this embodiment. Figure 7B is the spectral diagram of color segment Ye in this embodiment. Figure 7C is the spectral diagram of color segment G in this embodiment. Figure 7D is the spectral diagram of color segment Cy in this embodiment. Figure 7E is the spectral diagram of color segment B in this embodiment.

[0038] The DMD 60 performs gradation representation of each color corresponding to color segments R, Ye, G, Cy, and B. During the period of color segment R, the red light source 10R and the excitation light source 20Ex are lit, and yellow component light Ye1, as shown in Figure 7A, is generated in accordance with the red component light R and the excitation light Ex. Then, the yellow component light Ye1 generated in accordance with the red component light R and the excitation light Ex is modulated by the DMD 60. During the period of color segment Ye, the red light source 10R, the green light source 10G, and the excitation light source 20Ex are lit, and yellow component light Ye1, as shown in Figure 7B, is generated in accordance with the red component light R, the green component light G, and the excitation light Ex. Then, the yellow component light Ye1 generated in accordance with the red component light R, the green component light G, and the excitation light Ex is modulated by the DMD 60. During the period of color segment G, the green light source 10G and the excitation light source 20Ex are lit, and yellow component light Ye1, as shown in Figure 7C, is generated in response to the green component light G and the excitation light Ex. Then, the yellow component light Ye1 generated in response to the green component light G and the excitation light Ex is modulated by the DMD 60. During the period of color segment Cy, the green light source 10G, the blue light source 10B, and the excitation light source 20Ex are lit, and yellow component light Ye1, as shown in Figure 7D, is generated in response to the green component light G, the blue component light B, and the excitation light Ex. Then, the yellow component light Ye1 generated in response to the green component light G, the blue component light B, and the excitation light Ex is modulated by the DMD 60. During the period of color segment B, the blue light source 10B is lit, and the blue component light B, as shown in Figure 7E, is modulated by the DMD 60. The DMD 60 is an example of an optical modulation element. Thus, the red light source 10R, the green light source 10G, the blue light source 10B, and the excitation light source 20Ex are controlled by the control device 180 so that they are lit during corresponding segment periods and turned off during non-corresponding segment periods. In other words, the control device 180 controls each light source by pulse drive.

[0039] Returning to Figure 1, the image light generated by the DMD 60 is transmitted through the triangular prisms 141 and 142 and incident on the projection unit 70. The image light incident on the projection unit 70 is projected onto a screen (not shown) at an enlarged size.

[0040] Next, the light source circuit 300 included in the projection type video display device 100 (projector) of the present embodiment will be described.

[0041] FIG. 8 is a diagram showing the configuration of a light source circuit 200 for comparison. FIG. 9 is a diagram showing the time change of the current value of the current input to the LD (laser diode) 204 of the light source circuit 200 for comparison.

[0042] As shown in FIG. 8, the light source circuit 200 for comparison includes a first transistor 201A, a first transistor 201B, an inductor 202, a capacitor 203, an LD (laser diode) 204 which is an example of a semiconductor laser, a second transistor 205, and a resistor 206. The light source circuit 200 corresponds to the blue light source 10B, the green light source 10G, the red light source 10R, and the excitation light source 20Ex. The LD 204 corresponds to the blue light source emitter 12B, the green light source emitter 12G, the red light source emitter 12R, and the excitation light source emitter 22.

[0043] The first transistor 201A, the first transistor 201B, and the second transistor 205 are, for example, MOSFETs.

[0044] The source of the first transistor 201A is connected to the drain of the first transistor 201B and the first end of the inductor 202, and the drain of the first transistor 201A is connected to a power source (not shown).

[0045] The source of the first transistor 201B is connected to GND207, and the drain of the first transistor 201B is connected to the source of the first transistor 201A and the first end of the inductor 202.

[0046] The first end of the inductor 202 is connected to the source of the first transistor 201A and the drain of the first transistor 201B, and the second end of the inductor 202 is connected to the first end of the capacitor 203 and the anode of the LD204.

[0047] The first end of the capacitor 203 is connected to the second end of the inductor 202 and the anode of the LD204, and the second end of the capacitor 203 is connected to GND207.

[0048] The anode of LD204 is connected to the second end of inductor 202 and the first end of capacitor 203, and the cathode of LD204 is connected to the drain of the second transistor 205.

[0049] The drain of the second transistor 205 is connected to the cathode of LD204, and the source of the second transistor 205 is connected to the first end of resistor 206.

[0050] The first end of resistor 206 is connected to the source of the second transistor 205, and the second end of resistor 206 is connected to GND207.

[0051] The first transistor 201A, the first transistor 201B, the inductor 202, and the capacitor 203 constitute a DC / DC converter.

[0052] The control device 180 controls the ON and OFF of the current from the drain to the source of the first transistor 201A by inputting a control signal to the gate of the first transistor 201A. During the period when the control signal input to the gate of the first transistor 201A is ON, the first transistor 201A turns ON the current from the drain to the source, and during the period when the control signal input to the gate of the first transistor 201A is OFF, the first transistor 201A turns OFF the current from the drain to the source. Hereinafter, the period when the first transistor 201A turns ON the current from the drain to the source is referred to as the period when the first transistor 201A is ON, and the period when the first transistor 201A turns OFF the current from the drain to the source is referred to as the period when the first transistor 201A is OFF.

[0053] Similarly, the control device 180 controls the ON and OFF states of the current flowing from the drain to the source of the first transistor 201B by inputting a control signal to the gate of the first transistor 201B. During the period when the control signal input to the gate of the first transistor 201B is ON, the first transistor 201B turns on the current flowing from the drain to the source, and during the period when the control signal input to the gate of the first transistor 201B is OFF, the first transistor 201B turns off the current flowing from the drain to the source. The period during which the first transistor 201B turns on the current flowing from the drain to the source is referred to as the ON period of the first transistor 201B, and the period during which the first transistor 201B turns off the current flowing from the drain to the source is referred to as the OFF period of the first transistor 201B.

[0054] When the first transistor 201A is ON, the first transistor 201B is OFF, and when the first transistor 201A is OFF, the first transistor 201B is ON. In other words, the ON and OFF states of the first transistor 201A and the first transistor 201B are opposite. The control signals input to the gates of the first transistors 201A and 201B may be Pulse Width Modulation (PWM) signals.

[0055] The control device 180 controls the ON and OFF states of the current flowing from the drain to the source of the second transistor 205 by inputting a control signal to the gate of the second transistor 205. During the period when the control signal input to the gate of the second transistor 205 is ON, the second transistor 205 turns on the current flowing from the drain to the source, and during the period when the control signal input to the gate of the second transistor 205 is OFF, the second transistor 205 turns off the current flowing from the drain to the source. Hereinafter, the period during which the second transistor 205 turns on the current flowing from the drain to the source will be referred to as the ON period of the second transistor 205, and the period during which the second transistor 205 turns off the current flowing from the drain to the source will be referred to as the OFF period of the second transistor 205.

[0056] The control signal input to the gate of the second transistor 205 is a PWM signal. The sum of the ON and OFF periods of the PWM signal equals one cycle of the PWM signal. The ratio of the ON period to one cycle of the PWM signal is the duty cycle of the PWM signal. Hereinafter, the duty cycle of the PWM signal input to the gate of the second transistor 205 may be referred to as the PWM ratio of the second transistor 205.

[0057] Figure 9(a) shows the time variation of the current flowing through LD204 when the PWM ratio of the second transistor 205 is 100%. As shown in Figure 9(a), ripple occurs in the current flowing through LD204.

[0058] Figure 9(b) shows the time variation of the current flowing through the LD204 when the PWM ratio of the second transistor 205 is 5%. As shown in Figure 9(b), the current flowing through the LD204 when the PWM ratio is 5% fluctuates depending on the ripple that occurs when the PWM ratio is 100%. Therefore, as shown in Figure 9(b), the difference in ripple during the ON period of the second transistor 205 causes the current flowing through the LD204 to become unstable, resulting in a decrease in image quality, especially when displaying low-gradation images.

[0059] Here, it is conceivable to reduce ripple by increasing the PWM frequency of the second transistor 205 (i.e., by shortening one period of PWM). However, in the light source circuit 200 used for comparison, current is stored in the capacitor 203 during the OFF period of the second transistor 205, and when the second transistor 205 is turned ON, the current stored in the capacitor 203 flows to the LD 204. Since charging and discharging of this capacitor 203 takes time, the voltage cannot change rapidly. Therefore, in the light source circuit 200 shown in Figure 8, it is difficult to increase the PWM frequency of the second transistor 205.

[0060] Therefore, in this embodiment, a light source circuit 300 capable of increasing the PWM frequency of the second transistor will be described.

[0061] Figure 10 shows an example of the configuration of the light source circuit 300 according to this embodiment. Figure 11 shows the time change of the current value of the current input to the LD of the light source circuit 300 according to this embodiment.

[0062] As shown in Figure 10, the light source circuit 300 according to this embodiment includes a first transistor 301A, a first transistor 301B, an inductor 302, a capacitor 303, an LD 304, a second transistor 305, and a resistor 306.

[0063] The first transistor 301A, the first transistor 301B, and the second transistor 305 are, for example, MOSFETs.

[0064] The source of the first transistor 301A is connected to the drain of the first transistor 301B and the first terminal of the inductor 302, and the drain of the first transistor 301A is connected to a power supply (not shown).

[0065] The source of the first transistor 301B is connected to GND 307, and the drain of the first transistor 301B is connected to the source of the first transistor 301A and the first terminal of the inductor 302.

[0066] The first terminal of inductor 302 is connected to the source of the first transistor 301A and the drain of the first transistor 301B, and the second terminal of inductor 302 is connected to the first terminal of capacitor 303, the drain of the second transistor 305 and the anode of LD 304.

[0067] The first terminal of capacitor 303 is connected to the second terminal of inductor 302, the drain of second transistor 305, and the anode of LD 304, while the second terminal of capacitor 303 is connected to the first terminal of resistor 306. Note that the capacitance of capacitor 303 may be smaller than the capacitance of capacitor 203 shown in Figure 8.

[0068] The drain of the second transistor 305 is connected to the second terminal of the inductor 302, the first terminal of the capacitor 303, and the anode of the LD 304, while the source of the second transistor 305 is connected to the first terminal of the resistor 306.

[0069] The anode of LD304 is connected to the second terminal of inductor 302, the first terminal of capacitor 303, and the drain of second transistor 305, while the cathode of LD304 is connected to the first terminal of resistor 306.

[0070] The first terminal of resistor 306 is connected to the second terminal of capacitor 303, the source of second transistor 305, and the cathode of LD 304, while the second terminal of resistor 306 is connected to GND 307.

[0071] As shown in Figure 10, the second transistor 305 and LD304 form a parallel circuit.

[0072] The DC / DC converter is composed of the first transistor 301A, the first transistor 301B, the inductor 302, and the capacitor 303.

[0073] The control device 180 controls the ON / OFF state of the first transistor 301A and the first transistor 301B by inputting control signals (PWM signals) to the gates of the first transistor 301A and the first transistor 301B, similar to the light source circuit 200 described in Figure 8.

[0074] The control device 180 controls the ON / OFF state of the second transistor 305 by inputting a control signal (PWM signal) to the gate of the second transistor 305, similar to the light source circuit 200 described in Figure 8.

[0075] When the second transistor 305 is ON, current flows through the second transistor 305 and almost none flows through the LD 304. When the second transistor 305 is OFF, current flows through the LD 304 and almost none flows through the second transistor 305. In the light source circuit 300 shown in Figure 10, there is almost no charging and discharging of the capacitor 203 as in the light source circuit 200 shown in Figure 8, so the voltage can change rapidly. Therefore, the PWM frequency of the second transistor 305 can be increased in speed.

[0076] Furthermore, at least the first transistors 301A, 301B, and second transistor 305, as well as the control device 180, included in the light source circuit 300 according to this embodiment may be referred to as a light source control driver.

[0077] Figure 11(a) shows the time variation of the current flowing through LD304 when the PWM ratio of the second transistor 305 is 100%. As shown in Figure 11(a), although ripple occurs in the current flowing through LD304, the ripple is smaller compared to Figure 9(a) because the PWM frequency of the second transistor 305 is relatively high.

[0078] Figure 11(b) shows the time variation of the current flowing through LD304 when the PWM ratio of the second transistor 305 is 5%. As shown in Figure 11(b), although the current flowing through LD304 fluctuates depending on the ripple that occurs when the PWM ratio is 100%, the effect of that ripple is small because the PWM frequency of the second transistor 305 is relatively high.

[0079] Furthermore, in this embodiment, the drive cycles of the first transistor 301A and the first transistor 301B are synchronized with the PWM cycle of the second transistor 305. In other words, the phases of the drive cycles of the first transistor 301A and the first transistor 301B and the PWM cycle of the second transistor 305 are aligned. Specifically, the total period of the ON period (or OFF period) of the first transistor 301A and the OFF period (or ON period) of the first transistor 301B are synchronized with one cycle of the PWM of the second transistor 305 (i.e., the period of one ripple when the PWM ratio is 100%, as shown in Figure 11(a)). As a result, as shown in Figure 11(b), the ripple of the current value for each cycle of the PWM of the second transistor 305 becomes almost common. In other words, the current value of the current flowing through the LD 304 becomes stable, and as a result, the image quality is improved, especially when displaying low-gradation images.

[0080] <Application of high-speed PWM drive to projectors> Next, we will explain the application of the light source circuit 300, in which the PWM frequency of the second transistor 305 described above has been increased, to a projector.

[0081] While the PWM period of the second transistor 205 in the comparative light source circuit 200 is, for example, 5.88 μs (PWM frequency of 170 kHz), the PWM period of the second transistor 305 in the light source circuit 300 of this embodiment can be increased to, for example, 2 μs (PWM frequency of 500 kHz). However, "PWM period of 2 μs (PWM frequency of 500 kHz)" is just one example, and the value of the PWM period (PWM frequency) of the second transistor 305 in the light source circuit 300 of this embodiment is not particularly limited.

[0082] Figure 12 shows an example of a color segment per subframe in a projection-type image display device (projector) equipped with the light source circuit 200 for comparison. One subframe is, for example, 4170 μs (240 Hz). Figure 14 is a diagram illustrating the relationship between the light pulse of the LD 204 and the ON time of the DMD's minute panel in a projection-type image display device (projector) equipped with the light source circuit 200 for comparison.

[0083] As shown in Figure 12, in the comparison model, the ON time of the minute panel of the DMD60 (i.e., the number of times the minute panel is turned ON) is controlled for each of the R, G, B, and Ye color segments in one subframe, thereby representing the minimum gradation LSB (0 bits) to the maximum gradation MSB for each color.

[0084] As shown in Figure 13, the DMD60's micro-panel operates for approximately 10 μs, for example. Therefore, in order to ensure LSB (0-bit) gradation, the light pulse from the LD204 must be irradiated onto the DMD60's micro-panel during its minimum ON time (10 μs).

[0085] Here, in the comparison light source circuit 200, if the PWM period of the second transistor 205 is set to 5.88 μs (PWM frequency of 170 kHz) and the PWM ratio is set to 8%, then, as shown in Figure 13, the irradiation interval of the light pulses will be 5.88 μs, and the irradiation time of the light pulses (light pulse width) will be 0.47 μs (= PWM period of 5.88 μs × 8%).

[0086] In this case, as shown in Figure 13, the number of light pulses emitted from the LD204 during the minimum ON time (10 μs) of the DMD60's micropanel is either two or three, exhibiting randomness. Here, there is a significant difference in brightness depending on whether the DMD60's micropanel is irradiated with light pulses two or three times. In other words, the variation in brightness at low gradations becomes large, resulting in unstable image quality at low gradations (e.g., from LSB to a predetermined height).

[0087] Figure 14 shows an example of a color segment per subframe in a projection-type image display device 100 (projector) equipped with the light source circuit 300 of this embodiment. One subframe is, for example, 4170 μs (240 Hz). Figure 15 is a diagram illustrating the relationship between the light pulse of the LD 304 and the ON time of the DMD's minute panel in the projection-type image display device 100 (projector) equipped with the light source circuit 300 of this embodiment.

[0088] As shown in Figure 14, in this embodiment, the R, G, B, and Ye color segments are arranged in a time-distributed manner within a single subframe. The total of the distributed color segments within a single subframe controls the ON time of the minute panels of the DMD 60 (i.e., the number of times the minute panels are turned ON), thereby representing the minimum gradation LSB (0 bits) to the maximum gradation MSB for each color.

[0089] In this embodiment, if the PWM period of the second transistor 305 in the light source circuit 300 is set to 2 μs (PWM frequency of 500 kHz) and the PWM ratio is set to 5%, then, as shown in Figure 15, the irradiation interval of the light pulses will be 2 μs, and the irradiation time of the light pulses (light pulse width) will be 0.1 μs (= PWM period of 2 μs × 5%).

[0090] In this case, as shown in Figure 15, the number of light pulses emitted from the LD304 during the minimum ON time (10 μs) of the DMD60's micropanel is either 5 or 6. Here, the difference in brightness is smaller whether the DMD60's micropanel is irradiated with light pulses 5 or 6 times compared to whether it is irradiated 2 or 3 times, as in the comparison case. In other words, the variation in brightness at low gradations is reduced, and as a result, the image quality at low gradations (e.g., from LSB to a predetermined height) is stabilized.

[0091] Furthermore, by providing two light source circuits 300 as shown in Figure 10, and shifting the phase of the PWM period of the second transistor 305 of the first light source circuit 300 by 180 degrees, light pulses are alternately irradiated from the LD 304 of the first light source circuit 300 and the LD 304 of the second light source circuit 300, as shown in Figure 15. As a result, approximately 10 to 12 light pulses can be irradiated during the minimum ON time (10 μs) of the DMD 60's micro-panel. This allows more light pulses from the LD 304 to irradiate the DMD 60's micro-panel, further reducing brightness variations due to differences in the number of light pulses irradiated, and stabilizing the image quality at low gradation levels (e.g., from LSB to a predetermined gradation height).

[0092] <Modification> In the light source circuit 300 shown in Figure 10, the control device 180 may adjust the current value of the current flowing through the LD 304 by controlling the ON / OFF periods of the first transistor 301A and the first transistor 301B. For example, with the PWM ratio of the second transistor 305 set to 100%, the ON duty cycle of the first transistors 301A and 301B (the ratio of the ON period of the first transistor 301A to the total period of the ON and OFF periods) can be reduced to reduce the current value of the current flowing through the LD 304 and achieve low gradation.

[0093] Furthermore, for example, if the ON duty cycle (duty cycle of the PWM signal) of the first transistors 301A and 301B is greater than 50%, the control device 180 may drive the second transistor 305 at a PWM ratio of 100%. If the ON duty cycle (duty cycle of the PWM signal) of the first transistors 301A and 301B is 50% or less, the control device 180 may drive the second transistor 305 at a reduced PWM ratio. This makes it possible to achieve low-gradation stabilization as described above.

[0094] Based on the above description of embodiments, the following technologies are disclosed.

[0095] <Technology 1> The light source control driver for the laser diode (LD304), which is the light source of the projector (projection-type image display device 100), comprises a DC / DC converter equipped with first transistors (301A, 301B) that control the current input to the laser diode, and a second transistor (305) connected in parallel with the laser diode and controlling the current input to the laser diode. The period of the Pulse Width Modulation (PWM) signal controlling the first transistor is synchronized with the period of the PWM signal controlling the second transistor. This allows the PWM frequency of the second transistor to be increased, and the ripple of the current value flowing through the second transistor at each PWM period becomes almost common. As a result, the current value flowing through the LD is stabilized, and the image quality of low gradation is improved.

[0096] <Technology 2> In the light source control driver described in Technology 1, the second transistor is driven when the duty cycle of the PWM signal of the first transistor is 50% or less. This improves the image quality of low-gradation images.

[0097] <Technology 3> The projector is equipped with the light source control driver described in Technology 1 or 2. This allows the projector to project images with improved low-gradation quality.

[0098] While embodiments have been described above with reference to the attached drawings, this disclosure is not limited to such examples. It is clear to those skilled in the art that various modifications, alterations, substitutions, additions, deletions, and equivalents can be conceived within the scope of the claims, and these are also understood to fall within the technical scope of this disclosure. Furthermore, the components of the embodiments described above can be combined in any way without departing from the spirit of the invention.

[0099] The technology disclosed herein is useful for light source devices and projectors.

[0100] 10 RGB light source unit 10B Blue light source 10G Green light source 10R Red light source 11B Blue light source heat dissipation substrate 11G Green light source heat dissipation substrate 11R Red light source heat dissipation substrate 12B Blue light source emitter 12G Green light source emitter 12R Red light source emitter 13B Blue light source collimating lens 13G Green light source collimating lens 13R Red light source collimating lens 14B, 14G, 40 Dichroic mirrors 14R, 24, 111, 114, 116, 119 Mirrors 20 Excitation light source unit 20Ex Excitation light source 21 Excitation light source heat dissipation substrate 22 Excitation light source emitter 23 Excitation light source collimating lens 30 Phosphor wheel 31 Substrate 32 Reflective film 33 Phosphor film 34 Motor 50 Rod integrator 60 DMD 70 Projection unit 100 Projection-type image display device 112, 115, 122 Diffuser plate 113, 117, 118, 121, 123, 124, 131, 132, 133 Lens 141, 142 Triangular prism 150 Light source device 180 Control device 200 Light source circuit 201A, 201B First transistor 202 Inductor 203 Capacitor 204 LD 205 Second transistor 206 Resistor 207 GND 300 Light source circuit 301A, 301B First transistor 302 Inductor 303 Capacitor 304 LD 305 Second transistor 306 Resistor 307 GND

Claims

1. A light source control driver for controlling a laser diode, which is the light source of a projector, comprising: a DC / DC converter having a first transistor for controlling the current input to the laser diode; and a second transistor connected in parallel with the laser diode for controlling the current input to the laser diode, wherein the period of the Pulse Width Modulation (PWM) signal controlling the first transistor and the period of the PWM signal controlling the second transistor are synchronized.

2. The light source control driver according to claim 1, wherein the second transistor is driven when the duty cycle of the PWM signal of the first transistor is 50% or less.

3. A projector comprising the light source control driver according to claim 1 or claim 2.

Images