JP2019129080A - Light source device, projection type display device, and semiconductor device - Google Patents

Abstract

To improve linearity of the optical output from a semiconductor light source to the on-duty ratio of PWM signal by simple circuitry, without using a target value table based on the actual characteristic curve.SOLUTION: A light source device includes a semiconductor light source, a first switching element connected in series with the semiconductor light source, a drive circuit for turning the first switching element on when a PWM signal is activated, a power supply circuit including a second switching element and supplying a current to the semiconductor light source, a current detector for detecting a current flowing to the semiconductor light source and outputting a current detection signal, a compensation circuit generating a compensatory signal having a voltage which changes according to the on-duty ratio of the PWM signal, an adder circuit for adding the current control signal and the compensatory signal at a prescribed ratio, an error amplifier generating an error signal by amplifying the difference of the output signal from the adder circuit and the current control signal, and a power supply control circuit for controlling the power supply circuit on the basis of the error signal.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a light source device using a semiconductor light source such as a laser diode (LD) or a light emitting diode (LED), and a projection display device using such a light source device. Furthermore, the present invention relates to a semiconductor device and the like suitable for use in such a light source device.

  As a method for adjusting the brightness in a light source device using a semiconductor light source such as a laser diode or a light emitting diode, digital dimming and analog dimming are known. For example, digital dimming is realized by PWM (pulse width modulation) control of switching elements connected in series to a semiconductor light source to adjust the length of a period in which current flows in the semiconductor light source. On the other hand, analog dimming is realized by controlling the switching power supply circuit that supplies current or voltage to the semiconductor light source and adjusting the magnitude of the current flowing through the semiconductor light source.

  However, when current flows through the semiconductor light source, the temperature of the junction of the semiconductor light source rises and the forward voltage decreases. Since the light output (luminous flux) of the semiconductor light source is determined by the supplied current and the forward voltage, if the temperature of the junction of the semiconductor light source rises, the forward voltage decreases and a constant current is supplied to the semiconductor light source. Even if it is, the phenomenon that light output will fall will occur.

  As a related technique, Patent Document 1 describes that the temperature of the junction of the semiconductor laser diode changes according to the on time ratio of PWM control, and the forward voltage of the semiconductor laser diode changes. As a result, in digital dimming, there arises a problem that the on-time ratio of PWM control and the light output of the actual semiconductor light source are not proportional.

JP, 2006-225285, A (paragraph 0029-0032, FIG. 1, FIG. 3)

  In Patent Document 1, a target value table 108a for setting a target average current value using a characteristic curve at the time of actual semiconductor light source drive shown by a solid line in FIG. 3 by target value setting unit 108 (FIG. 1). Is set. If this target average current value is compared with the average current value of the semiconductor laser diode 102 and feedback control is performed on the switching power supply 101 using the comparison output, the average current value of the semiconductor laser diode 102 becomes the on time ratio of PWM control. Correspondingly, it becomes equal to the target average current value set by the target value setting unit 108.

  However, since there are variations in the characteristics of the semiconductor laser diode 102, it is necessary to adjust one current value for each product before shipping. In addition, when the target value table 108a is written in the ROM, the target value table 108a can not be rewritten when the semiconductor laser diode 102 is changed. In order to change the target value table 108a, it is necessary to replace an IC having a built-in ROM. On the other hand, even when the target value table 108a is written to a flash memory or the like, many man-hours are required to rewrite the target value table 108a.

  Accordingly, in view of the above points, the first object of the present invention is to provide a PWM signal with a relatively simple circuit configuration without using a target value table set using a characteristic curve when driving an actual semiconductor light source. The linearity of the light output of the semiconductor light source with respect to the on-duty ratio is improved. A second object of the present invention is to provide a projection display apparatus using such a light source device. Furthermore, the third object of the present invention is to provide a semiconductor device etc. suitable for being used in such a light source device.

  In order to solve at least a part of the above problems, a light source device according to a first aspect of the present invention includes a semiconductor light source, a first switching element connected in series to the semiconductor light source, and a PWM signal activated. A driving circuit for activating the first control signal to turn on the first switching element, and a second switching element for performing switching operation according to the second control signal; A switching power supply circuit for supplying current to the semiconductor light source when the switch is in the on state, a first current detection unit for detecting the current flowing through the semiconductor light source and outputting the first current detection signal, and on-duty of the PWM signal A compensation circuit that generates a compensation signal having a voltage that changes according to a ratio, an addition circuit that adds a current control signal and a compensation signal at a predetermined ratio, and an output signal of the addition circuit And a second current detection unit that detects a current flowing through a second switching element and outputs a second current detection signal. And a switching power supply control circuit that generates a second control signal based on the second current detection signal and the error signal.

  According to the first aspect of the present invention, the compensation circuit generates a compensation signal according to the on-duty ratio of the PWM signal, and the adder circuit adds the current control signal and the compensation signal at a predetermined ratio. By controlling the switching power supply circuit based on the signal, the on-duty ratio of the PWM signal can be achieved with a relatively simple circuit configuration without using a target value table set using a characteristic curve when driving an actual semiconductor light source. Therefore, the linearity of the light output of the semiconductor light source can be improved.

  Here, the compensation circuit may include an integration circuit that generates a compensation signal having a voltage proportional to the on-duty ratio of the PWM signal. In that case, the compensation circuit is configured using an integration circuit, which is a relatively simple analog circuit. Therefore, by using a resistor or a capacitor included in the integration circuit as an external component, the light source device can be shipped before shipment. In addition, the compensation characteristics can be adjusted according to variations in the characteristics of the semiconductor light source.

  Further, the light source device may further include a voltage limiting circuit that limits the voltage of the output signal of the adding circuit not to exceed a predetermined voltage. As a result, even if the voltage of the current control signal or the compensation signal is increased, an overcurrent is prevented from flowing through the semiconductor light source, the current flowing through the semiconductor light source exceeds the rated current, or the light source device or the light source device is mounted. Further, it is possible to prevent the power consumption of the projection display device or the like from exceeding the rated power.

  In the above, the switching power supply circuit is connected to the inductor connected between the semiconductor light source and the second switching element, the capacitor connected between one end of the inductor and the power supply node, and the other end of the inductor. And a diode having a cathode connected to the anode and the power supply node, and when the first and second switching elements are in the on state, a current flows through the semiconductor light source and the inductor and energy is stored in the inductor, When the first switching element is in the ON state and the second switching element is in the OFF state, current may flow to the semiconductor light source and the diode by the energy stored in the inductor.

  In such a switching power supply circuit, by increasing the on-duty ratio of the second control signal in accordance with the on-duty ratio of the PWM signal, the period during which the second switching element is in the on state is lengthened. The current that flows through increases. Thereby, when the on-duty ratio of the PWM signal is large, even if the temperature of the junction of the semiconductor light source rises and the forward voltage decreases, the power supplied to the semiconductor light source can be made close to constant.

  A projection display apparatus according to a second aspect of the present invention includes any one of the light source devices described above. According to the second aspect of the present invention, a semiconductor with respect to the on-duty ratio of a PWM signal can be obtained with a relatively simple circuit configuration without using a target value table set by using a characteristic curve when driving an actual semiconductor light source. The brightness of the projected image can be accurately controlled by using a light source device with improved linearity of the light output of the light source.

  The semiconductor device according to the third aspect of the present invention activates the first control signal when the PWM signal is activated, and turns on the first switching element connected in series to the semiconductor light source. Switching power supply control that generates a second control signal that controls a driving circuit and a second switching element that performs switching operation in a switching power supply circuit that supplies current to a semiconductor light source when the first switching element is in an on state A circuit, a first current detection circuit that detects a current flowing through a first current detection element connected in series to a semiconductor light source and outputs a first current detection signal, and a voltage that changes according to an on-duty ratio of the PWM signal A compensation circuit for generating a compensation signal having the following equation, an addition circuit for summing the current control signal and the compensation signal at a predetermined ratio, and an output signal of the addition circuit and the first An error amplifier that amplifies the difference from the current detection signal to generate an error signal, and a current flowing through a second current detection element connected in series to a second switching element, and outputs a second current detection signal And a switching power supply control circuit generates a second control signal based on the second current detection signal and the error signal.

  According to the third aspect of the present invention, the switching power supply circuit is controlled based on the output signal of the adder circuit that adds the current control signal and the compensation signal according to the on-duty ratio of the PWM signal at a predetermined ratio. The linearity of the light output of the semiconductor light source with respect to the on-duty ratio of the PWM signal can be improved with a relatively simple circuit configuration without using the target value table set using the characteristic curve when driving the semiconductor light source.

The circuit diagram showing the example of composition of the light source device concerning one embodiment of the present invention. FIG. 2 is a circuit diagram illustrating a configuration example of a compensation circuit illustrated in FIG. 1. The figure which shows the relationship between the on-duty ratio of a PWM signal, and the temperature of a semiconductor light source. The figure which shows the relationship between the temperature of the junction part of a semiconductor light source, and a forward voltage. The figure which shows the relationship between the forward voltage of a semiconductor light source, and the electric current supplied to a semiconductor light source. BRIEF DESCRIPTION OF THE DRAWINGS The block diagram which shows the structural example of the projection type display apparatus which concerns on one Embodiment of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the same reference numerals are given to the same components, and redundant description will be omitted.
<Light source device>
FIG. 1 is a circuit diagram showing a configuration example of a light source device according to an embodiment of the present invention. As shown in FIG. 1, the light source device includes a semiconductor device (IC) 100, a semiconductor light source 110, N channel MOS transistors QN1 and QN2, diodes D1 to D3, a zener diode D4, an inductor L1, and a capacitor. C1-C6 and resistors R1-R4 are included.

  A power supply potential VDD (for example, 50 V) on the high potential side is supplied to the node N1 of the light source device, and a power supply potential VSS on the low potential side is supplied to the node N2. FIG. 1 shows the case where the power supply potential VSS is the ground potential (0 V). A resistor R1, a semiconductor light source 110, a transistor QN1, an inductor L1, a transistor QN2 and a resistor R2 are connected in series between the node N1 and the node N2.

  The resistor R1 is connected between the node N1 and one end of the semiconductor light source 110, and is a first current detection element for detecting a current flowing through the semiconductor light source 110. For example, the resistor R1 has a small resistance value of about 50 mΩ to 100 mΩ. Have. The semiconductor light source 110 includes, for example, at least one laser diode or light emitting diode, and emits light at a brightness according to the magnitude of the supplied current. The light output (light flux) of the semiconductor light source 110 is determined by the supplied current and the forward voltage.

  The transistor QN1 is a first switching element connected in series to the semiconductor light source 110 for digital dimming by PWM control, and is connected to the drain connected to the other end of the semiconductor light source 110 and one end of the inductor L1. It has a source and a gate to which a first control signal DDRV is applied via a capacitor C5. The transistor QN1 is turned on when the first control signal DDRV is activated to a high level, and is turned off when the first control signal DDRV is deactivated to a low level.

  When the first control signal DDRV is alternately activated and deactivated, the transistor QN1 performs a switching operation. Accordingly, by changing the duty ratio of the first control signal DDRV, it is possible to perform digital dimming by changing the period during which current flows through the semiconductor light source 110. Note that a P-channel MOS transistor may be used instead of the N-channel MOS transistor QN1, but in that case, a part of the circuit shown in FIG. 1 is changed.

  The transistor QN2 is a second switching element constituting a switching power supply circuit together with the diode D1, the inductor L1, the capacitor C4 and the like, and is connected to the node N2 via the drain connected to the other end of the inductor L1 and the resistor R2. It has a connected source and a gate to which a second control signal GATE is applied. The transistor QN2 is turned on when the second control signal GATE is activated to a high level, and is turned off when the second control signal GATE is deactivated to a low level.

  When the second control signal GATE is activated and deactivated alternately, the transistor QN2 performs a switching operation. The switching power supply circuit supplies a current to the semiconductor light source 110 when the transistor QN1 is in the on state by the transistor QN2 performing a switching operation according to the second control signal GATE.

  Therefore, by changing the duty ratio of the second control signal GATE, analog dimming can be performed by changing the magnitude of the current flowing through the semiconductor light source 110. In addition to the MOS transistor, a bipolar transistor, an IGBT (insulated gate bipolar transistor), a thyristor, or the like can be used as the switching element.

  Diode D1 has an anode connected to the other end of inductor L1, and a cathode connected to node N1. As the diode D1, for example, a Schottky barrier diode having a lower forward voltage and a higher switching speed than a PN junction diode is used.

  The resistor R2 is connected between the source of the transistor QN2 and the node N2 and is a second current detection element for detecting the current flowing to the transistor QN2, and has a small resistance value of, for example, about 50 mΩ to 100 mΩ. ing. The capacitor C1 is connected between the node N1 and the node N2, and smoothes the power supply voltage (VDD-VSS). The capacitor C4 is connected between one end of the inductor L1 and the node N1 and steps down the power supply voltage (VDD-VSS) to smooth the obtained step-down voltage.

<Semiconductor device>
The semiconductor device 100 is supplied with a PWM signal (digital dimming signal) PWM and a current control signal (analog dimming signal) IADJ from an external microcomputer or the like, and controls the transistors QN1 and QN2 of the light source device. Note that at least some of the components of the semiconductor device 100 shown in FIG. 1 may be discrete components or external ICs, and the diode D1, the resistor R1, the resistor R2, or the like is built in the semiconductor device 100. You may.

  As shown in FIG. 1, the semiconductor device 100 includes internal regulators 10a and 10b, level shifters (L / S) 11 and 12, a drive circuit 20, a compensation circuit 30, an adder circuit 31, and a current detection circuit 40. An error amplifier 50, a switch circuit (SW) 51, a comparator 52, a slope compensation circuit 60, a clock signal generation circuit 70, a switching control circuit 80, and a drive circuit 90, and further includes a voltage limiter. A circuit 32 may be included.

  Internal regulators 10a and 10b each include, for example, a reference voltage generation circuit configured of a band gap reference circuit or the like, and internal power supply potentials VDA and VDR supplied to the internal circuit of semiconductor device 100 based on power supply potential VDD, respectively. Generate Capacitor C2a is connected between the output terminal of internal regulator 10a and node N2, and smoothes the internal power supply voltage (VDA-VSS). Capacitor C2b is connected between the output terminal of internal regulator 10b and node N2, and smoothes the internal power supply voltage (VDR-VSS).

  The level shifters 11 and 12 shift the high level potential of the PWM signal supplied from the outside of the semiconductor device 100 to a potential suitable for the internal circuit of the semiconductor device 100. The drive circuit 20 includes, for example, a driver amplifier and the like, and generates the first control signal DDRV based on the PWM signal supplied from the level shifter 11.

  For example, when the PWM signal is activated, the drive circuit 20 activates the first control signal DDRV to a high level to turn on the transistor QN1, and when the PWM signal is inactivated, Control signal DDRV to a low level to turn off transistor QN1.

  The first control signal DDRV transitions between a low level (for example, 0V) and a high level (for example, 7.5V). When the first control signal DDRV is activated to a high level, a current flows from the drive circuit 20 to the gate of the transistor QN1 via the capacitor C5, and the voltage between the gate and the source of the transistor QN1 rises, and the transistor QN1 Turns on. The Zener diode D4 clamps the transistor QN1 so that the gate-source voltage does not exceed a predetermined voltage (for example, 7.5 V).

  In a period in which the first control signal DDRV is maintained in the activated state, the third control signal GATE ′ transitions between the low level and the high level. As a result, the capacitor C6 and the diodes D2 and D3 perform a rectifying operation, so that the gate-source voltage of the transistor QN1 is maintained above the threshold voltage.

  When the second control signal GATE is maintained in the inactive state during the inactive period of the first control signal DDRV, the second control signal GATE is also used as the third control signal GATE ′. Can. On the other hand, when the second control signal GATE is activated and deactivated in the inactivation period of the first control signal DDRV, the PWM signal or the first control signal DDRV and the second control signal GATE The third control signal GATE ′ may be generated by providing the switching control circuit 80 with an AND circuit for obtaining the logical product of the two.

  When the first control signal DDRV is inactivated to a low level, the gate-source voltage of the transistor QN1 drops, and the transistor QN1 is turned off. The resistor R4 reduces the gate-source voltage of the transistor QN1 to maintain the transistor QN1 in the off state when the light emitting device stops light emission for a long time in the standby mode or the like.

  When the on-duty ratio of the PWM signal increases, the ratio of the period in which current flows through the semiconductor light source 110 in digital dimming increases, so the temperature of the junction of the semiconductor light source 110 rises and the forward voltage decreases. Therefore, the compensation circuit 30 generates a compensation signal VPWM having a voltage that changes in accordance with the on-duty ratio of the PWM signal supplied from the level shifter 12 in order to compensate for the decrease in the optical output of the semiconductor light source 110. The compensation signal VPWM desirably has a voltage that monotonously increases as the on-duty ratio of the PWM signal increases.

  For example, the compensation circuit 30 is configured by an integration circuit (low pass filter) that generates a compensation signal VPWM having a voltage proportional to the on-duty ratio of the PWM signal. In that case, since the compensation circuit 30 is configured using an integration circuit which is a relatively simple analog circuit, the light source device can be shipped by using a resistor or a capacitor included in the integration circuit as an external component. Before, the compensation characteristic can be adjusted in accordance with the variation in characteristics of the semiconductor light source 110. Since the resistance value of the resistor used in the integrating circuit has temperature dependence, the voltage of the compensation signal VPWM does not have to be completely proportional to the on-duty ratio of the PWM signal. Errors are acceptable.

  FIG. 2 is a circuit diagram showing a configuration example of the compensation circuit shown in FIG. In the example shown in FIG. 2, the compensation circuit 30 is connected between the resistor R5 to which the PWM signal supplied from the level shifter 12 is supplied at one end, and the other end of the resistor R5 and the wiring of the power supply potential VSS. The integration circuit includes a capacitor C7 and generates an compensation signal VPWM at a connection point between the resistor R5 and the capacitor C7.

Referring again to FIG. 1, the adder circuit 31 adds the current control signal IADJ and the compensation signal VPWM at a predetermined ratio. The addition ratio in the addition circuit 31 may be adjusted by an external component. Assuming that the voltage in the activation period of the PWM signal is Von and the on-duty ratio is Duty, the voltage of the compensation signal VPWM to be added to the current control signal IADJ by the adding circuit 31 is expressed by the following equation.
VPWM = Von × Duty × K
Here, K is a conversion coefficient. For example, when Von is 5V and Duty is 50%, Von × Duty is 2.5V, which is too large. Therefore, for example, by setting K to 0.1, the voltage of the compensation signal VPWM can be set to 0.25V.

  The voltage limiting circuit 32 includes, for example, a clamp element such as a Zener diode connected between the output terminal of the adding circuit 31 and the wiring of the power supply potential VSS, and the voltage of the output signal of the adding circuit 31 has a predetermined voltage. Limit not to exceed. Accordingly, even if the voltage of the current control signal IADJ or the compensation signal VPWM is increased, an overcurrent is prevented from flowing through the semiconductor light source 110, the current flowing through the semiconductor light source 110 exceeds the rated current, the light source device or its light source It is possible to prevent the power consumption of a projection type display device or the like in which the device is mounted from exceeding the rated power.

  The current detection circuit 40 includes, for example, a current sense amplifier that amplifies the voltage between both ends of the current detection resistor R1 connected in series to the semiconductor light source 110, and detects the current flowing through the current detection resistor R1. This corresponds to a first current detection circuit that outputs one current detection signal. Here, the current detection circuit 40, together with the current detection resistor R1, constitutes a first current detection unit that detects a current flowing through the semiconductor light source 110 and outputs a first current detection signal. The output signal of the current detection circuit 40 is supplied to the inverting input terminal of the error amplifier 50.

  The error amplifier 50 amplifies the difference between the output signal of the addition circuit 31 and the first current detection signal to generate an error signal ERR, and supplies the error signal ERR to the switch circuit 51. The switch circuit 51, the comparator 52, the slope compensation circuit 60, the clock signal generation circuit 70, the switching control circuit 80, and the drive circuit 90 are connected to the second current detection signal DET obtained by detecting the current flowing through the transistor QN2. The switching power supply control circuit is configured to generate a second control signal GATE based on the error signal ERR.

  The switch circuit 51 is composed of, for example, an analog switch, and is turned on when the PWM signal supplied from the level shifter 12 is activated, and turned off when the PWM signal is deactivated. . Accordingly, the voltage of the error signal ERR generated when the transistor QN1 is in the on state is held in the capacitor C3 and supplied to the inverting input terminal of the comparator 52.

  The slope compensation circuit 60 includes, for example, a current sense amplifier that amplifies the voltage between both ends of the current detection resistor R2 connected in series with the transistor QN2, and detects the current flowing through the current detection resistor R2 to detect the second current. This corresponds to a second current detection circuit that outputs the current detection signal DET. Here, the slope compensation circuit 60, together with the current detection resistor R2, constitutes a second current detection unit that detects a current flowing through the transistor QN2 and outputs a second current detection signal DET.

  For example, the slope compensation circuit 60 generates the second current detection signal DET by amplifying the voltage between both ends of the current detection resistor R2 and adding the bias voltage, and the second current detection signal DET is compared with the comparator. 52 to the non-inverting input terminal. The comparator 52 compares the voltage of the second current detection signal DET with the voltage of the error signal ERR, so that when the voltage of the second current detection signal DET is higher than the voltage of the error signal ERR, the reset signal RST Activate.

  The clock signal generation circuit 70 includes, for example, a CR oscillation circuit or the like, and generates a clock signal CLK having a predetermined frequency by performing an oscillation operation. The oscillation frequency of the CR oscillation circuit is determined by the time constant which is the product of the capacitance value of the capacitor and the resistance value of the resistor. The resistor R3 is externally attached to the semiconductor device 100 in order to adjust the oscillation frequency of the CR oscillation circuit.

  The switching control circuit 80 includes an RS flip-flop, for example, and generates a second control signal GATE for controlling the transistor QN2 based on the clock signal CLK and the reset signal RST. The second control signal GATE is applied to the gate of the transistor QN2 via the drive circuit 90 configured with a driver amplifier or the like. For example, the switching control circuit 80 activates the second control signal GATE to a high level in synchronization with the rising of the clock signal CLK. Thereby, the transistor QN2 is turned on.

  When the transistors QN1 and QN2 are on, a current flows from the node N1 to the node N2 through the semiconductor light source 110, the inductor L1, and the like, and electric energy is converted into magnetic energy and stored in the inductor L1. Further, when the transistor QN1 is in the off state and the transistor QN2 is in the on state, current flows from the capacitor C4 to the node N2 through the inductor L1 and the like, and energy is accumulated in the inductor L1.

  The current flowing through the inductor L1 gradually increases with time. The voltage of the second current detection signal DET also increases as the current flowing to the current detection resistor R2 through the inductor L1 or the like increases. When the voltage of the second current detection signal DET exceeds the voltage of the error signal ERR, the reset signal RST is activated to a high level. The switching control circuit 80 deactivates the second control signal GATE to the low level in synchronization with the rising edge of the reset signal RST. Thereby, the transistor QN2 is turned off.

  When the transistor QN1 is in the on state and the transistor QN2 is in the off state, the magnetic energy stored in the inductor L1 is released as electric energy, and a current flows through the semiconductor light source 110, the diode D1, and the like. Further, when the transistors QN1 and QN2 are in the OFF state, the magnetic energy accumulated in the inductor L1 is released as electric energy, and a current flows through the capacitor C4, the diode D1, and the like.

  In such a switching operation, when the voltage of the current control signal IADJ increases, the on-duty ratio of the second control signal GATE increases, and the period during which the transistor QN2 is in the on state is lengthened. The flowing current increases. Therefore, by changing the current control signal IADJ, the current flowing through the semiconductor light source 110 can be changed to perform analog dimming.

  Alternatively, the switching control circuit 80 generates the second control signal GATE based on the PWM signal supplied from the level shifter 11 in addition to the clock signal CLK and the reset signal RST, so that the first control signal DDRV The second control signal GATE may be maintained in an inactive state during the inactive period.

  In that case, the transistor QN2 for analog light adjustment is turned off during a period in which the digital light adjustment transistor QN1 is turned off and current does not flow to the semiconductor light source 110. Thereby, it is possible to suppress the energy stored in the inductor L1 from being released without being used for light emission, and to reduce the power loss.

<Operation example>
FIG. 3 is a diagram showing the relationship between the on-duty ratio of the PWM signal and the temperature of the junction of the semiconductor light source. In FIG. 3, the horizontal axis represents the on-duty ratio of the PWM signal, and the vertical axis represents the temperature T at the junction of the semiconductor light source 110. As shown in FIG. 3, when the on-duty ratio of the PWM signal is increased, the temperature T of the junction of the semiconductor light source 110 is increased.

FIG. 4 is a diagram showing the relationship between the temperature of the junction of the semiconductor light source and the forward voltage. 4, the horizontal axis represents the temperature T of the junction of the semiconductor light source 110, the vertical axis represents the forward voltage V _F of the semiconductor light source 110. As shown in FIG. 4, when the temperature T of the junction of the semiconductor light source 110 rises, the forward voltage V _F of the semiconductor light source 110 decreases.

Therefore, the junction of the semiconductor light source 110 when the on-duty ratio of the PWM signal is a second value larger than the first value as compared to the case where the on-duty ratio of the PWM signal is a first value. The forward voltage V _F is reduced by ΔV _{F when} the temperature T of the voltage V T is increased by ΔT. Since the light output of the semiconductor light source 110 is determined by the supplied current and the forward voltage V _F , when the forward voltage V _F decreases, the light output decreases even if a constant current is supplied to the semiconductor light source 110. The phenomenon that happens.

  Therefore, in the present embodiment, the compensation circuit 30 generates the compensation signal VPWM according to the on-duty ratio of the PWM signal, and the addition circuit 31 adds the current control signal IADJ and the compensation signal VPWM at a predetermined ratio. When the on-duty ratio of the PWM signal is small, the voltage of the compensation signal VPWM becomes low, and the voltage of the output signal of the adding circuit 31 is mainly determined by the current control signal IADJ. Therefore, the current ILD supplied to the semiconductor light source 110 is mainly controlled by the current control signal IADJ.

On the other hand, when the on-duty ratio of the PWM signal is large, the voltage of the compensation signal VPWM increases, and the voltage of the output signal of the adder circuit 31 increases. In the switching power supply circuit, the on-duty ratio of the second control signal GATE is increased according to the on-duty ratio of the PWM signal, so that the period in which the transistor QN2 is in the on state is extended. To increase. Thereby, even when the temperature T of the junction of the semiconductor light source 110 rises and the forward voltage V _F decreases when the on-duty ratio of the PWM signal is large, the power supplied to the semiconductor light source 110 approaches a constant be able to.

FIG. 5 is a diagram showing the relationship between the forward voltage of the semiconductor light source and the current supplied to the semiconductor light source. 5, the horizontal axis represents the forward voltage V _F of the semiconductor light source 110, the vertical axis represents the current ILD to the current control signal IADJ is supplied to the semiconductor light source 110 when the constant. Operating points at 0 ° C., 25 ° C., and 50 ° C. are shown in FIG. 5 as an example.

When the on-duty ratio of the PWM signal is large, the temperature of the junction of the semiconductor light source 110 is increased and the forward voltage V _F is decreased, but by controlling the switching power supply circuit based on the output signal of the adding circuit 31, The current ILD supplied to the semiconductor light source 110 can be increased to make the power supplied to the semiconductor light source 110 approach a constant. As a result, the linearity of the light output of the semiconductor light source with respect to the on-duty ratio of the PWM signal is improved with a relatively simple circuit configuration without using the target value table set using the characteristic curve at the time of actual semiconductor light source drive. can do.

<Projection display device>
Next, a projection type display (video projector) according to an embodiment of the present invention will be described.
FIG. 6 is a block diagram showing an example of the configuration of a projection display apparatus according to an embodiment of the present invention. The projection display device 200 is supplied with a power supply voltage from the outside and is supplied with image data from an image data supply device such as a personal computer or a video player, and displays an image on a screen (projection surface) 300 based on the image data. Project.

  As shown in FIG. 6, the projection display apparatus 200 includes a power supply circuit 210, an image data processing unit 220, a control unit 230, a light source device 240, a panel 250, and a projection optical system 260. Here, the light source device 240 is a light source device according to an embodiment of the present invention, and includes the semiconductor device 100 and the semiconductor light source 110.

  The power supply circuit 210 generates a DC logic power supply voltage based on a power supply voltage of AC 100 V supplied from the outside, for example, and supplies it to the image data processing unit 220, the control unit 230, etc. A DC power supply voltage is generated and supplied to the semiconductor device 100 or the like of the light source device 240.

  The image data processing unit 220 and the control unit 230 are configured by, for example, at least one microcomputer. The image data processing unit 220 processes the image data supplied from the outside to generate a display image signal and a synchronization signal, and supplies the image signal and the synchronization signal to the panel 250 to drive the panel 250. Draw.

  Control part 230 controls each part of projection type display 200 according to operation which an operator performs using a remote control or an operation panel (not shown). When the operator instructs dimming, the control unit 230 generates a PWM signal and a current control signal IADJ for realizing the dimming instructed by the operator, and supplies the PWM signal and the current control signal IADJ to the semiconductor device 100 of the light source device 240. Do. Thus, the semiconductor device 100 controls the light emitting operation of the semiconductor light source 110.

  The light source device 240 emits light with brightness according to the PWM signal and the current control signal IADJ supplied from the control unit 230 and irradiates the panel 250 with light. For example, when the semiconductor light source 110 includes a plurality of laser diodes that generate blue light, the light source device 240 receives a blue light generated by a part of the laser diodes and generates a yellow light; The light emitting device may further include a light separating unit that separates red light and green light from light. In that case, the light source device 240 can generate light of three colors of R (red), G (green), and B (blue).

  The panel 250 modulates the light emitted from the light source device 240 according to the image signal and the synchronization signal supplied from the image data processing unit 220. For example, the panel 250 may include three liquid crystal panels corresponding to three colors of RGB. Each liquid crystal panel forms an image by changing the transmittance of light in a plurality of pixels arranged in a matrix. Modulated light modulated by the panel 250 is guided to the projection optical system 260.

  The projection optical system 260 includes at least one lens. For example, the projection lens, which is a lens group for projecting and forming modulated light modulated by the panel 250 on the screen 300, and the state of the aperture of the projection lens, the state of zoom, or the shift position etc. are adjusted. Various adjustment mechanisms are provided in the projection optical system 260. The adjustment mechanism is controlled by the control unit 230. The projection optical system 260 projects the modulated light onto the screen 300 to display an image on the screen 300.

  According to the present embodiment, the light of the semiconductor light source 110 with respect to the on-duty ratio of the PWM signal can be obtained with a relatively simple circuit configuration without using the target value table set using the characteristic curve at the time of actual semiconductor light source driving. Using the light source device 240 with improved output linearity, the brightness of the projected image can be accurately controlled.

  When setting the target value table for each product or model, the cost increases because it takes many man-hours, but according to the present embodiment, it is possible to adjust the compensation characteristic only with a relatively simple analog circuit. Therefore, the light source device 240 or the projection display device 200 can be manufactured at low cost.

  The present invention is not limited to the embodiments described above, and many modifications can be made within the technical idea of the present invention by those having ordinary knowledge in the technical field.

  DESCRIPTION OF SYMBOLS 10a, 10b ... Internal regulator, 11, 12 ... Level shifter, 20 ... Drive circuit, 30 ... Compensation circuit, 31 ... Adder circuit, 32 ... Voltage limiting circuit, 40 ... Current detection circuit, 50 ... Error amplifier, 51 ... Switch circuit , 52: Comparator, 60 ... Slope compensation circuit, 70 ... Clock signal generation circuit, 80 ... Switching control circuit, 90 ... Drive circuit, 100 ... Semiconductor device, 110 ... Semiconductor light source, 200 ... Projection display device, 210 ... Power supply Circuit, 220 ... Image data processing unit, 230 ... Control unit, 240 ... Light source device, 250 ... Panel, 260 ... Projection optical system, 300 ... Screen, QN1, QN2 ... N-channel MOS transistor, D1-D3 ... Diode, D4 ... Zener diode, L1 ... inductor, C1 to C7 ... capacitor, R1 to R5 ... resistor

Claims (6)

Semiconductor light source,
A first switching element connected in series to the semiconductor light source;
A drive circuit that activates a first control signal to turn on the first switching element when the PWM signal is activated;
A switching power supply circuit that includes a second switching element that performs a switching operation according to a second control signal, and that supplies current to the semiconductor light source when the first switching element is in an on state;
A first current detection unit that detects a current flowing through the semiconductor light source and outputs a first current detection signal;
A compensation circuit that generates a compensation signal having a voltage that varies in accordance with the on-duty ratio of the PWM signal;
An addition circuit for adding a current control signal and the compensation signal at a predetermined ratio;
An error amplifier for amplifying a difference between an output signal of the addition circuit and the first current detection signal to generate an error signal;
A second current detection unit that detects a current flowing through the second switching element and outputs a second current detection signal;
A switching power supply control circuit that generates the second control signal based on the second current detection signal and the error signal;
A light source device comprising:   The light source device according to claim 1, wherein the compensation circuit includes an integration circuit that generates a compensation signal having a voltage proportional to an on-duty ratio of the PWM signal.   The light source device according to claim 1, further comprising a voltage limiting circuit that limits a voltage of an output signal of the adding circuit not to exceed a predetermined voltage. The switching power supply circuit is
An inductor connected between the semiconductor light source and the second switching element;
A capacitor connected between one end of the inductor and a power supply node;
A diode having an anode connected to the other end of the inductor and a cathode connected to the power supply node;
Further, when the first and second switching elements are in the on state, current flows through the semiconductor light source and the inductor, energy is stored in the inductor, and the first switching element is in the on state. 4. The light source device according to claim 1, wherein when the second switching element is in an off state, a current flows through the semiconductor light source and the diode by energy accumulated in the inductor.   A projection display device comprising the light source device according to claim 1. A drive circuit that activates the first control signal when the PWM signal is activated to turn on the first switching element connected in series to the semiconductor light source;
A switching power supply control circuit that generates a second control signal for controlling a second switching element that performs switching operation in a switching power supply circuit that supplies current to the semiconductor light source when the first switching element is in an on state ,
A first current detection circuit that detects a current flowing through a first current detection element connected in series to the semiconductor light source and outputs a first current detection signal;
A compensation circuit that generates a compensation signal having a voltage that varies according to an on-duty ratio of the PWM signal;
An addition circuit for adding a current control signal and the compensation signal at a predetermined ratio;
An error amplifier for amplifying a difference between an output signal of the addition circuit and the first current detection signal to generate an error signal;
A second current detection circuit that detects a current flowing through a second current detection element connected in series to the second switching element and outputs a second current detection signal;
And the switching power supply control circuit generates the second control signal based on the second current detection signal and the error signal.

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