TWI698154B - LED dimming device - Google Patents

Abstract

An LED dimming device has: a step-down converter for converting a first voltage into a second voltage under the control of a first PWM signal, and generating a first voltage according to a ratio of the second voltage The second voltage is less than the first voltage; a linear dimming circuit has an LED module and a negative feedback operational amplifier circuit, the LED module is coupled to the second voltage and the negative feedback Between operational amplifier circuits, the negative feedback operational amplifier circuit generates an output current according to the control of a variable-level PWM signal, and generates a second feedback voltage according to the output current, wherein the variable-level PWM signal Generated by the combination of a variable voltage and a second PWM signal; and a microprocessor for executing an incremental PID according to a dimming command, the first feedback voltage and the second feedback voltage The control program generates the variable voltage, the first PWM signal and the second PWM signal.

Description

LED dimming device

The present invention relates to a light emitting diode (LED), particularly a light emitting diode (LED) dimming device.

Lighting is closely related to people’s lives and has become an indispensable part of life. With the continuous improvement of living standards, global lighting electricity consumption has accounted for more than 20% of the total annual electricity consumption, of which department store lighting equipment The amount is even more than 30% of the total electricity consumption.

The light emitting diode (LED) has high brightness, sturdy structure, no mercury, long life, low power consumption, high light source safety, and responsiveness compared with other lighting systems currently used. Fast speed, good color saturation, flat packaging into thin and short products, and many other advantages. It has been widely used in mobile phone/display backlights, electronic product indicators, car lights, indoor and outdoor lighting, etc., and has become a promising and sustainable development Lizhi high-tech products. Its power-saving and energy-saving features have also become the trend of future green lighting technology. Cooperating with solar photovoltaic energy to supply power and light, it can reduce the dependence on petrochemical energy and achieve the goal of energy-saving and carbon-reducing sustainable development.

Light emitting diodes (LEDs) have gradually replaced incandescent lamps in 2012 and officially entered the mainstream market for general lighting applications. According to the calculation of the annual electricity consumption of my country's lighting of about 26 billion kWh, if the light emitting diode (LED) is fully used, it is estimated that about 10.7 billion kWh of electricity can be saved every year, which is about 41%.

As energy conservation and environmental protection issues continue to ferment, the United States, Britain, Japan and the European Union have successively announced that they will completely ban and ban the production of incandescent lamps since 2014, and actively promote the use of light emitting diode (LED) lighting devices. The European Union plans to set energy efficiency standards to phase out incandescent lamps, and Australia has also completely banned incandescent lamps from 2010. These will accelerate the growth of the global solid-state lighting (SSL) industry.

Solid-state lighting (SSL) technology, including Light Emitting Diode (LED), Organic Light Emitting Diode (OLED), high score Sub-organic light emitting diodes (Polymer Light Emitting Diode, PLED), micro light emitting diodes (Micro Light Emitting Diode, Micro LED) have gradually replaced gas discharge lamps, becoming the future lighting technology and liquid crystal display (liquid-crystal display, LCD ) Is the mainstream backlight source, and the demand and business opportunities for driving circuits for driving various light emitting diode (LED) lighting in the future are bound to increase sharply.

In recent years, Solid-State Lighting (SSL) has become the main light source for many lighting applications. Compared with traditional incandescent lamps, fluorescent lamps, mercury lamps, etc., Light-Emitting Diode (LED) It has the advantages of energy saving, long life, less heat generation, fast switching speed, and no mercury. With the development and progress of science and technology, the luminous efficiency of LED is gradually increasing, the cost is gradually decreasing, and its applications are becoming more and more diversified, including street lights, screen displays, indoor lighting, car lights and so on.

According to the United States Department of Energy (DOE) in the "Energy Savings Forecast of Solid-State Lighting in General Illumination Applications" report, in 2015, the United States For road lighting equipment, LEDs account for 21%, while metal halide lamps and high-pressure sodium lamps account for 16% and 62%, respectively. The remaining fluorescent lamps and incandescent lamps account for 0.1%. It is predicted that by 2025, LEDs will reach 90% installation rate.

For the dimming control of LEDs, some documents propose to connect a linear current regulator to each group of LEDs, and use the feedforward voltage regulator feedback method to increase the efficiency of the circuit, but the effect is not significant; Some literature proposes to detect the voltage of the linear steady current circuit to adjust the output voltage so that the voltage drop of one of the linear steady current circuits approaches zero, reduces loss, and uses phase-shift pulse width modulation (Pulse Width Modulation, PWM) makes the output current change between the current of a group of LEDs and the maximum current, to solve the high ripple current generated by general pulse width modulation control; there are still documents that propose the use of analog circuits to detect gates- The voltage between the drains realizes adaptive adjustment of the output voltage to improve efficiency. Since the illuminance-current curve of most LEDs is non-linear, the smaller the forward current is, the greater the ratio of illuminance to current is, and it gradually decreases as the current increases. When using digital dimming, the length of the current on time is used to change the average current, and then dimming is carried out. However, through the LED The current will be converted from zero to the rated value, rather than the maximum value of the forward current, so that the illuminance cannot be fully utilized. Therefore, a novel LED dimming method is urgently needed in the art.

One objective of the present invention is to disclose an LED dimming device, which calculates the current value of the optimal control point by a variable current detection resistance control method, so as to achieve the dimming control closest to the original characteristic curve.

Another object of the present invention is to disclose an LED dimming device, which calculates the current value of the best control point by a variable dimming signal voltage level control method to achieve dimming control closest to the original characteristic curve.

Another object of the present invention is to disclose an LED dimming device, which uses the two dimming methods in this case to reduce the output power of the driving circuit by 17.08% compared with the conventional PWM dimming under the same illuminance. And 13.17%.

Another object of the present invention is to disclose an LED dimming device, which uses the two dimming methods in this case and the output power of the drive circuit is the same. Compared with the conventional PWM dimming technology, the illuminance is increased by an average of 13.66. % And 11.17%.

Another object of the present invention is to disclose an LED dimming device, which uses the two dimming methods in this case in the CIE1976 color space. The maximum difference of the u coordinate is 0.00397 and 0.00404, and the maximum difference of the v coordinate is 0.01029. And 0.01044.

Another object of the present invention is to disclose an LED dimming device, which uses the two dimming methods in this case in the CIE1931 color space. The maximum difference in x- coordinates is 0.01588 and 0.01609, and the maximum difference in y- coordinates is 0.01678. And 0.01703.

To achieve the foregoing objective, an LED dimming device is proposed, which has: a buck converter for converting a first voltage into a second voltage under the control of a first PWM signal, and according to the second voltage A proportion of the voltage generates a first feedback voltage, the second voltage is less than the first voltage; a linear dimming circuit having an LED module and a negative feedback operational amplifier circuit, the LED module is coupled to Between the second voltage and the negative feedback operational amplifier circuit, the negative feedback operational amplifier circuit generates an output current under the control of a variable-level PWM signal, and generates a second output current according to the output current. Feedback voltage, wherein the variable-level PWM signal is generated according to a combination of a variable voltage and a second PWM signal; and a microprocessor is used to generate a feedback voltage according to an input command voltage and the first feedback voltage The difference value executes a first incremental PID control program to generate the first PWM signal, performs a look-up operation on a dimming command voltage according to a voltage-voltage comparison table to generate the variable voltage, and according to the dimming The difference between the command voltage and the second feedback voltage executes a second incremental PID control program to generate the second PWM signal.

In one embodiment, the linear dimming circuit has a transistor circuit to generate the variable level PWM signal according to the variable voltage and the second PWM signal, and the transistor circuit has: a resistor, one end of which is connected to The variable voltage is coupled; an NMOS transistor having a drain, a gate and a source, the drain is coupled to the other end of the resistor to generate the variable level PWM signal, the source Is coupled with a reference ground; and an inverter having an input terminal and an output terminal, the input terminal is coupled with the second PWM signal, and the output terminal is coupled with the gate.

In one embodiment, the negative feedback operational amplifier circuit has a variable feedback resistance unit and a current sensor on the path of the output current, and the current sensor is used to generate the second feedback Voltage.

In one embodiment, the variable feedback resistance unit includes a plurality of disconnectable resistance units. Each of the disconnectable resistance units includes a switch and a resistor overlapped with the switch. The on-state relationship is affected by a switch. Signal control, the switch signal is generated by the microprocessor, and the microprocessor performs a look-up operation on the dimming command voltage according to a voltage-switch comparison table to generate the plurality of disconnectable resistance units A plurality of the switch signals.

In one embodiment, the microprocessor is a digital signal processor and realizes the first incremental PID control program and the second incremental PID control program by a firmware program.

In order to enable your reviewer to further understand the structure, features and purpose of the present invention, drawings and detailed descriptions of preferred specific embodiments are attached as follows.

100‧‧‧Step-down converter

200‧‧‧Linear dimming circuit

210‧‧‧LED Module

220‧‧‧Negative feedback operational amplifier circuit

221‧‧‧Variable feedback resistance unit

221a‧‧‧NMOS Transistor

221b‧‧‧Resistor

222‧‧‧Current Sensor

230‧‧‧Transistor circuit

231‧‧‧Resistor

232‧‧‧NMOS Transistor

233‧‧‧Inverter

300‧‧‧Microprocessor

Fig. 1 shows a block diagram of an embodiment of the LED dimming device in this case.

Figure 2 shows the illuminance, current and temperature curves of the CREE CXB1304 used in this case.

Figure 3 shows a schematic diagram of the buck converter in this case.

FIG. 4 shows the waveforms of the inductor voltage and inductor current during a switching cycle of the buck converter in continuous conduction mode.

Figure 5 shows the waveform diagram of the capacitor current and capacitor voltage in continuous conduction mode operation.

Figure 6 shows the voltage, current and temperature curves of the CREE CXB1304 used in this case.

FIG. 7a shows the structure diagram of the linear steady current circuit.

Figure 7b shows the small signal equivalent circuit model of the linear steady current circuit.

Figure 7c is a block diagram of the feedback control system of the linear steady current circuit.

Figure 7d is an open-loop equivalent model of a linear steady current circuit.

Figure 8a shows the illuminance-current curve of CXB1304 LED under different current detection resistance values.

Figure 8b shows a schematic diagram of the LED characteristic curve and PWM dimming control in this case.

Fig. 8c shows a schematic diagram of the structure of the variable current detection resistor control in this case.

FIG. 9 is a schematic diagram of the structure of the variable dimming signal voltage level control in this case.

Figure 10 shows a schematic diagram of the photometric measurement environment of this case.

Figure 11a shows a flowchart of the firmware main program of the LED dimming device control system in this case.

Figure 11b shows the flow chart of the variable current detection resistance value control method in this case.

Figure 12a shows the illuminance-power curve of the variable current detection resistance control method for ten groups of CXB1304 LEDs in this case.

Figure 12b shows the illuminance-power curve of the variable dimming signal voltage level control method for ten groups of CXB1304 LEDs in this case.

FIG. 13 is a schematic diagram showing the improvement effect calculation method of the dimming method of this case and the PWM dimming method of the prior art.

Figure 14a shows the CIE 1976 color space diagram measured by the variable current detection resistance control method in this case.

Figure 14b shows the CIE 1976 color space diagram measured by the variable dimming signal voltage level control method in this case.

Figure 14c shows the CIE 1931 color space diagram measured by the variable current detection resistance control method in this case.

Fig. 14d shows the CIE 1931 color space diagram measured by the variable dimming signal voltage level control method of this case.

Please refer to FIG. 1, which shows a block diagram of an embodiment of the LED dimming device in this case.

As shown in the figure, the LED dimming device has a buck converter 100, a linear dimming circuit 200, and a microprocessor 300.

The step-down converter 100 is used to convert a first voltage V _I into a second voltage V _O under the control of a first PWM signal PWM _GD , and generate a first voltage according to a ratio of the second voltage V _O the feedback voltage ADC _V, the second voltage is less than the first voltage V _O V _I.

The linear dimming circuit 200 has an LED module 210 and a negative feedback operational amplifier circuit 220. The LED module 210 is coupled between the second voltage V _O and the negative feedback operational amplifier circuit 220. The negative feedback operational amplifier circuit 220 generates a signal under the control of a variable-level PWM signal V _r Output current I _O , and generate a second feedback voltage ADC _I according to the output current I _O , wherein the variable level PWM signal V _r is a combination of a variable voltage DAC _LED and a second PWM signal PWM _LED And produced.

The linear dimming circuit 200 further has a transistor circuit 230 to generate the variable level PWM signal V _r according to the variable voltage DAC _LED and the second PWM signal PWM _LED .

The transistor circuit 230 has a resistor R _ISO 231, an NMOS transistor 232, and an inverter 233.

One end of the resistor 231R _ISO is coupled to the variable voltage DAC _LED , the NMOS transistor 232 has a drain, a gate and a source, and the drain is coupled to the other end of the resistor R _ISO 231 To generate the variable-level PWM signal V _r , the source is coupled to a reference ground, the inverter 233 has an input terminal and an output terminal, and the input terminal is connected to the second PWM signal PWM _LED And the output terminal is coupled to the gate.

The negative feedback operational amplifier circuit 220 has a variable feedback resistance unit 221 and a current sensor 222 on the path of the output current I _O.

The current sensor 222 is, for example, but not limited to, used to generate the second feedback voltage ADC _I , the variable feedback resistance unit 221 includes a plurality of disconnectable resistance units, and each of the disconnectable resistance units includes A switch 221a and a resistor 221b overlapped with the switch, the switch 221a is controlled by a switch signal, and the switch signal is generated by the microprocessor 300.

In the figure, the variable feedback resistance unit 221 includes three disconnectable resistance units, but it is not limited thereto. As shown, the variable feedback resistor unit comprises three switches 221a and three resistors 221b, 221a, respectively, the three switching by the switching signal GPIO _R1, GPIO _R2 GPIO _R3 and control, the switching signal GPIO _R1, Both GPIO _R2 and GPIO _R3 are generated by the microprocessor 300.

The microprocessor 300 is used for executing a first incremental PID control program according to the difference between an input command voltage and the first feedback voltage ADC _V to generate the first PWM signal PWM _GD , according to a voltage-voltage comparison The table performs a look-up operation on a dimming command voltage to generate the variable voltage DAC _LED , and executes a second incremental PID control program according to the difference between the dimming command voltage and the second feedback voltage ADC ₁ To generate the second PWM signal PWM _LED . In addition, the microprocessor 300 performs a table look-up operation on the dimming command voltage according to a voltage-switch comparison table to generate the plurality of switch signals (GPIO _R1 , GPIO _R2 ) of the plurality of disconnectable resistance units. And GPIO _R3 ).

In other words, in this case, the output voltage V _O of the buck converter 100 is sampled and sent to the microprocessor 300 for conversion by an analog-to-digital converter (ADC), and then the value is input Calculate the incremental PID control method and the target value to obtain the duty cycle of the power switch to achieve the effect of voltage stabilization. The linear current-regulating circuit part uses the current sensor 222 to sense the output current I _O , and returns a corresponding voltage value to the ADC module of the microprocessor 300, and uses this value to determine the three switches SW _R1 and SW _R2 and SW _R3 are turned on or off.

Please refer to Figure 2, which shows the illuminance, current and temperature curves of the CREE CXB1304 LED used in this case.

Since the illuminance-current curve of most LEDs is not linear, as shown in the figure, the smaller the forward current is, the greater the ratio of illuminance to current will be, and it will gradually decrease as the current rises.

The digital dimming system of the prior art uses the length of the current conduction time to change the average current to perform dimming. Taking the characteristic curve at 25°C as an example, when the current is 500mA, the The corresponding relative illuminance is about 230%, but if you use digital dimming to control the average current to 250mA, you will get 115% of the relative illuminance instead of the original characteristic of 138%, so the illuminance cannot be fully utilized.

Please refer to Figure 3, which shows a schematic diagram of the step-down converter in this case.

In this case, a Buck Converter is used as the structure of the LED drive circuit, and it is operated in Continuous Conduction Mode (CCM). As shown in the figure, the buck converter consists of power switches SW, Diode D, inductor L and capacitor C are formed.

Among them, the inductor L and the capacitor C form a low-pass filter to reduce harmonics. The working state of the power switch SW can be divided into on and off, and the output voltage V _O is changed through pulse width modulation to make it equal to or less than the input The voltage value V _i . The circuit specifications of the step-down converter in this case are shown in Table 1.

For a buck converter operating in continuous conduction mode, its inductance must comply with Volt-Second Balance in order to achieve a steady state, otherwise the flux linkage will continue to increase, so its magnetic flux will continue to increase during a switching period T _s The net change of the chain is zero, as shown in equation (1).

Among them, D _on is the switch responsibility cycle.

After finishing equation (1), the voltage conversion rate M (D _on ) can be obtained, as shown in equation (2).

Please refer to FIG. 4, which shows a waveform diagram of the inductor voltage and inductor current of the buck converter operating in continuous conduction mode for a switching period.

The change in inductor current Δi _{L in} a switching cycle is shown in equation (3).

Derive from equation (3) to get the sense value L, as shown in equation (4).

The DC current I _L on the inductor must all flow to the load, and the AC component is shunted by the capacitor. However, the capacitor will be designed to have a large enough value so that its impedance at the switching frequency is much smaller than the load, thereby causing ripples on the inductor. All waves flow to the capacitor.

Please refer to FIG. 5, which shows the waveform diagram of the capacitor current and the capacitor voltage under continuous conduction mode operation.

As shown in the figure, when the current on the capacitor is positive, the voltage rises. Within the time between the two zero points of the current waveform, the change in capacitor voltage is the output voltage ripple △V _o , and the current waveform is symmetrical , So the intersection is at the midpoint between D _on T _s and (1-D _on )T _s . Using the relationship between capacitance C, charge Q and voltage, the amount of charge change △Q within this time can be calculated, as in equation (5 ) Shown.

△ Q = C △ V _o (5)

It is also possible to use integral calculations, as shown in equation (6).

Combining formula (3), formula (5) and formula (6), the capacitance relationship can be derived as shown in formula (7).

In the choice of power switch, the withstand voltage must be greater than the input voltage, and the withstand current must be greater than the maximum input current; in terms of diodes, the withstand voltage must be greater than the input voltage, and the withstand current must be greater than the maximum output current.

Please refer to Figure 6, which shows the voltage, current and temperature curves of the CREE CXB1304 LED used in this case.

As shown in the figure, when the temperature is 25℃ and the current is 500mA, the voltage across is about 19.5V, plus the resistance of the linear current-regulating circuit and the voltage drop between the drain and source of the power switch, so the output voltage Set it to 23V, and take 2% as the peak-to-peak output voltage ripple. The European Union stipulates that electronic equipment products with an input power of more than 75W must pass the EN61000-3-2 current harmonic quality standard. Therefore, if it is used with UCC29910AEVM-730 step-down power factor correction circuit, the input voltage is set to 84V. In this case, ten groups of CXB1304 LEDs are connected in parallel, with a maximum output current of 5A, and 20% of it is taken as the peak-to-peak value of the inductor current ripple. Among them, the power switch uses IPP030N10N3G, with a withstand voltage of 100V and a current withstand of 100A; the diode uses STPS20150C, with a withstand voltage of 150V and a current of 20A.

LED dimming methods can be basically divided into analog dimming and digital dimming (ie PWM dimming). Among them, the analog dimming system changes the brightness by changing the forward current of the LED, but its relative color temperature (CCT) and spectrum will also change, thereby affecting the luminous quality. Digital dimming (ie PWM dimming) changes its average value according to the difference in current conduction time per unit time to achieve the effect of dimming, because the forward current of the LED always only changes between the rated current and zero. , This dimming method has no chromatographic shift. In terms of operating frequency, if the frequency of change is too low, the human eye will feel flicker. The conventional technology sets the frequency between 100 and 400 Hz. In this case, 300 Hz is used as the dimming frequency.

Linear steady current circuit architecture and circuit analysis:

The linear steady current circuit architecture will drive the LED in a constant current mode, which not only allows the LED to reach a stable current, but also improves the constant voltage drive because the temperature rise of the LED causes the internal resistance to change, thereby affecting the forward current The size of the situation.

Please refer to Figures 7a to 7d together, in which Figure 7a shows the structure of the linear steady current circuit, Figure 7b shows the small signal equivalent circuit model of the linear steady current circuit, and Figure 7c shows the linear steady current circuit. The block diagram of the feedback control system, Fig. 7d shows the open-loop equivalent model of the linear steady current circuit.

As shown in Figure 7a, the linear constant current circuit architecture is a constant current circuit composed of an operational amplifier (OPA) and a power switch (MOSFET), and the power switch is operated in the linear region, where the variable-level PWM signal V _r is the dimming signal. When the voltage at the non-inverting input terminal of the operational amplifier is high, the power switch is turned on, which is equivalent to a variable resistor, and the output current is fed back to the operational amplifier through the current detection resistor R _S for comparison; When the input terminal voltage is low, the power switch is turned off.

The equivalent model obtained from the small signal analysis of the operational amplifier and the power switch in Figure 7a is shown in Figure 7b.

Among them, r _i is the input impedance of the operational amplifier, _ro1 is the output impedance, A _OL is the open-loop gain, V _d is the voltage difference between the non-inverting and inverting input terminals, g _m is the transconductance of the power switch, and _ro2 is Drain-source equivalent resistance, V _S is the feedback signal, V _o is the output voltage of the drive circuit, and R _LED is the internal resistance when the LED is turned on.

As shown in Figures 7c and 7d, to obtain the closed-loop gain, the open-loop gain value A needs to be calculated first. The system uses voltage input and uses current feedback to determine that it is a series of feedback amplifiers. Therefore, when the input and output are disconnected, the current detection resistors are connected in series to the circuits of both.

From the models in Figures 7b to 7d, the relevant circuit parameters as shown in equations (8) to (12) can be derived.

Among them, if the output impedance r _{o1 is} ignored, and equations (9) to (11) are substituted into equation (12), the open loop gain A is calculated, as shown in equation (13).

Assuming that r _{i is} much smaller than R _s and r _{o2 is} close to infinity, equation (13) can be simplified as shown in equation (14).

Substituting equation (8) and equation (14) into the closed-loop transfer function, the closed-loop gain of the linear steady current circuit can be obtained, as shown in equation (15).

1+R _S ．g _m ，可將方程式(15)簡化如方程式(16)所示。 Because of A _OL . R _S. g _m

1+ R _S. g _m , equation (15) can be simplified as shown in equation (16).

It can be known from the above equation that the forward current of the LED can be controlled by the voltage level of the dimming signal or the current detection resistor.

The variable current detection resistance value control method in this case:

Please refer to Figures 8a to 8c together. Figure 8a shows the illuminance-current curve of the CXB1304 LED with different resistance values of the current detection resistor. Figure 8b shows the LED characteristic curve and PWM dimming control schematic diagram of this case. 8c shows the structure diagram of the variable current detection resistor control in this case.

According to the LED illuminance and current curve diagram in Figure 2, the following experiment was carried out with a set of CXB1304 LEDs in this case. First, the resistance value of the current detection resistor on the linear steady current circuit was changed to control the LED dimming. The dimming signal is voltage-based. 5V, 300Hz pulse signal.

As shown in Figure 8a, when the illuminance value is the same, the larger the resistance value, the smaller the output current obtained by the drive circuit. Therefore, when more different resistance values are used for multi-stage control, the power saving effect will be more obvious.

Using 25℃ as the experimental environment temperature, use the computer-aided design software AutoCAD to draw the figure 2 proportionally and divide it into 100 equal parts to calculate the corresponding value of current and illuminance, and then use the mathematical software MATLAB to curve the data points Fitting, the curve function equation can be obtained.

Because this experiment adopts the linear control dimming method of PWM dimming, it is necessary to find the best control point to achieve the best effect. In order to prevent the control from becoming too complicated, this case uses three-stage control as an example. When the current can be controlled in the full range, one stage must be rated current, so the other two control point currents must be found.

As shown in Figure 8b, PWM1 to PWM3 represent the three-segment control curve, I ₄ , I ₆ and I ₇ represent the maximum current value of each segment. The area of the gray area is calculated by integration, that is, the PWM1 line segment is integrated by 0 To I ₄ ; the line segment of PWM2 is integrated from I ₄ to I ₆ ; the line segment of PWM3 is integrated from I ₆ to I ₇ , and the sum is compared with the area of the original curve. The smaller the difference, the closer to the control point of the original curve. For the CXB1304 LED's illuminance-current curve, the current values of the best control points found by this method are 259mA, 392mA and 500mA respectively. Because the experimental architecture is ten sets of CXB1304 LEDs in parallel, this current value must be multiplied by 10 Times.

If two resistors are used to form a three-stage control, I ₄ and I ₆ determine the two resistance values, but the two resistance values in parallel cannot control the current at I ₇ , so this case uses three resistors in parallel. For control.

As shown in Figure 8c, the dimming signal voltage level is set to 1.3V, the frequency is 300Hz, and SW _R1 , SW _R2 and SW _R3 are three sets of power switches. In order to control the forward current of the LED to the above-mentioned current value, R ₁ , R ₂ and R ₃ are respectively 0.5Ω, 1Ω and 1.2Ω through the calculation of equation (16), and their conditions can be satisfied by connecting them in parallel. Because each resistance value is different, in fact, up to seven stages of control can be achieved, and I ₁ , I ₂ , I ₃ and I ₅ are the control current points of the other four stages.

The variable dimming signal voltage level control method in this case:

Since the current detection resistor value change method is used to achieve multi-stage control, the structure will be more complicated. Equation (16) knows that when the forward current of the LED is the same, the voltage of the dimming signal is The current detection resistance value is proportional to the relationship, so this case proposes a method to control by varying the voltage level of the dimming signal.

Please refer to FIG. 9, which illustrates the structure diagram of the variable dimming signal voltage level control in this case.

As shown in the figure, this method is simpler and easier to achieve multi-stage control. In addition, in order to reduce the power loss generated by the current sense resistor, this case chose to connect the three resistors in the previously proposed architecture in parallel to minimize the resistance value, and the power switch used to select the current sense resistor can be removed. In order to make the LED forward current value of this method the same as the LED forward current value of the variable current detection resistance value control method of this case, the voltage value of the dimming signal is added by seven resistance values generated by three resistors in parallel. Calculate the corresponding value.

Please refer to Figure 10, which shows a schematic diagram of the photometric measurement environment of this case.

When measuring the illuminance value, in order to prevent the influence of other light sources in the environment, a wooden box with a length of 55 cm, a width of 35 cm and a height of 30 cm was made as a test space, and a square hole of 4 cm was opened on the side to facilitate wiring. , And seal it with tape during measurement to make it opaque. Due to the high illuminance of 100W LED, the illuminance meter is placed on the side during the test to prevent the illuminance value from exceeding the energy measurement range of the illuminance meter. The illuminance meter used is TES-1339R. The overall structure of the wooden box, the placement of the illuminance meter, the LED and the driving circuit are shown in the figure.

Please also refer to FIGS. 11a to 11b, where FIG. 11a shows a flowchart of the firmware main program of the LED dimming device control system in this case, and FIG. 11b shows a program flow chart of the variable current detection resistance value control method in this case.

As shown in Figure 11a, the modules are initialized and enabled at the beginning, and then enter an infinite loop to execute the subroutine and wait for an interrupt to occur. When the ADC interrupt is triggered, the output voltage will be sampled, and through incremental PID calculation, this calculation result is used as the basis for changing the duty cycle of the power switch of the buck converter, so that the buck converter can achieve the effect of voltage regulation. The interruption will also transmit the current and its corresponding voltage value, which is used in the control of the dimming sub-program.

The dimming sub-program of the variable current detection resistance value control method in this case:

The PWM module is used to generate a pulse signal with a frequency of 300 Hz to control the duty cycle of the dimming signal. The voltage level value is generated by using a Digital-to-Analog Converter (DAC). The seven-segment control current composed of three current detection resistors is defined as I ₁ to I ₇ from small to large. The lower limit of each current is the upper limit of the previous period, and it corresponds to the duty cycle The lower limit values are defined as D _dim2 to D _{dim7 respectively} , and then the reading value _sent back to the ADC module by the current sensor is used to determine the current size to determine whether SW _R1 , SW _R2, and SW _R3 are turned on or off. When the value exceeds the upper limit of the segment, it will switch to the next segment.

The dimming subroutine of the variable dimming signal voltage level control method in this case:

This method is the same as the above-mentioned variable current detection resistance control method in the generation and control of the dimming signal. The voltage level is changed through the DAC module to achieve multi-stage control. This case uses six levels of control, that is, seven different dimming signal voltage levels (0.33V, 0.39V, 0.72V, 0.78V, 1.1V, 1.2V).

Experimental results:

Please refer to Figures 12a to 12b. Figure 12a shows the illuminance-power curve of the variable current detection resistance control method for ten groups of CXB1304 LEDs in this case, and Figure 12b shows the variable dimming of ten groups of CXB1304 LEDs in this case The illuminance-power curve of the signal voltage level control method.

As shown in the figure, the illuminance is the normalized value, and the power is the output power of the LED drive circuit. The legends are the current detection resistor and dimming signal voltage level used in the curve. R ₁ , R ₂ and R ₃ are respectively It is 0.5Ω, 1Ω and 1.2Ω.

In the variable current detection resistance value control method in this case, the dimming signal voltage level (V _r ) is 1.5V; in the variable dimming signal voltage level control method, the current detection resistors are R ₁ , R ₂ and R _{3 in} parallel The value of V _r =0.33V, 0.39V, 0.72V, 0.78V, 1.1V, 1.2V.

Please refer to FIG. 13, which shows a schematic diagram of the improvement effect calculation method of the dimming method in this case and the PWM dimming method of the prior art.

As shown in the figure, it is the dimming curve of the full control section. When the power reaches the upper limit of this section, it will switch to the next section, and the control range of the next section is from the upper limit of the previous section to the upper limit of this section. Value, and so on. At the same illuminance, the power difference between point A and point B divided by the power value of point B is the percentage of the drive circuit output power reduction; at the same output power of the drive circuit, the illuminance difference between point C and point D is divided by the illuminance at point D The value is the percentage increase in illuminance.

The improvement effect of the variable current detection resistance value control method in this case in each control section is shown in Table 2.

The improvement effect of the variable dimming signal voltage level control method in this case in each control stage is shown in Table 3.

The improvement effects of the two dimming methods in this case in the full control section are shown in Table 4.

In order to verify the effectiveness of the two dimming methods in this case, the Probe4Light handheld integrating sphere was also used to measure the color coordinates of the LED forward current at each stage.

Please refer to Figures 14a to 14d. Figure 14a shows the CIE 1976 color space diagram measured by the variable current detection resistance control method in this case, and Figure 14b shows the variable dimming signal voltage level control method in this case The measured CIE 1976 color space diagram, Figure 14c shows the CIE 1931 color space diagram measured by the variable current detection resistance control method of this case, and Figure 14d shows the variable dimming signal voltage level control method of this case The measured CIE 1931 color space diagram.

As shown in the figure, the two dimming methods in this case can stabilize the color in the entire dimming range.

The color coordinates of the variable current detection resistance value control method in this case are shown in Table 5.

table 5

The color coordinates of the variable dimming signal voltage level control method in this case are shown in Table 6.

Table 7 shows the maximum variation of the coordinate axes of the two dimming methods in this case in each color space.

In summary, this case will use a 115W LED drive circuit as the architecture, where the drive circuit uses a buck converter as a power stage circuit, and digital control is achieved through the dsPIC33FJ16GS502 microprocessor. The part of the dimming circuit is a constant current circuit composed of an operational amplifier (OPA) and a power switch (MOSFET). Its working mode operates in the linear region and the cut-off region, and uses current detection with three different resistance values. Resistance and different dimming signal voltage levels to realize the two dimming methods in this case.

With the design disclosed above, the present invention has the following advantages:

1. The present invention discloses an LED dimming device, which calculates the current value of the optimal control point by a variable current detection resistance value control method to achieve the dimming control closest to the original characteristic curve.

2. The present invention discloses an LED dimming device, which calculates the current value of the optimal control point by a variable dimming signal voltage level control method to achieve the dimming control closest to the original characteristic curve.

3. The present invention discloses an LED dimming device. By using the two dimming methods in this case, the output power of the driving circuit is reduced by 17.08% and 13.17%, respectively, compared with the conventional PWM dimming under the same illuminance. .

4. The present invention discloses an LED dimming device. By using the two dimming methods of the present application and the output power of the drive circuit is the same, compared with the conventional PWM dimming technology, the illuminance is increased by 13.66% and 11.17 respectively. %.

5. The present invention discloses an LED dimming device, which uses the two dimming methods of this case in the CIE1976 color space. The maximum difference of u'coordinates is 0.00397 and 0.00404, and the maximum difference of v'coordinates is 0.01029 and 0.01029 respectively. 0.01044.

6. The present invention discloses an LED dimming device. By using the two dimming methods of this case, in the CIE1931 color space, the maximum difference in x-coordinate is 0.01588 and 0.01609, and the maximum difference in y-coordinate is 0.01678 and 0.01703, respectively.

The disclosure of the present invention is a preferred embodiment, and any partial changes or modifications that are derived from the technical idea of the present invention and can be easily inferred by those familiar with the art will not depart from the scope of the patent right of the present invention.

In summary, the present invention is showing that it is quite different from The technical characteristics of the known, and the first invention is suitable for practicality, are also in compliance with the patent requirements of the invention. I sincerely ask your examiner to observe it carefully and pray that the patent will be granted as soon as possible.

100‧‧‧Step-down converter

200‧‧‧Linear dimming circuit

210‧‧‧LED Module

220‧‧‧Negative feedback operational amplifier circuit

221‧‧‧Variable feedback resistance unit

221a‧‧‧NMOS Transistor

221b‧‧‧Resistor

222‧‧‧Current Sensor

230‧‧‧Transistor circuit

231‧‧‧Resistor

232‧‧‧NMOS Transistor

233‧‧‧Inverter

300‧‧‧Microprocessor

Claims (5)

An LED dimming device has: a step-down converter for converting a first voltage into a second voltage under the control of a first PWM signal, and generating a first time according to a ratio of the second voltage The second voltage is less than the first voltage; a linear dimming circuit has an LED module and a negative feedback operational amplifier circuit, the LED module is coupled to the second voltage and the negative feedback Between operational amplifier circuits, the negative feedback operational amplifier circuit generates an output current according to the control of a variable-level PWM signal, and generates a second feedback voltage according to the output current, wherein the variable-level PWM signal It is generated by the combination of a variable voltage and a second PWM signal; and a microprocessor for executing a first incremental PID control program based on the difference between an input command voltage and the first feedback voltage To generate the first PWM signal, perform a look-up operation on a dimming command voltage according to a voltage-voltage comparison table to generate the variable voltage, and according to the difference between the dimming command voltage and the second feedback voltage A second incremental PID control program is executed to generate the second PWM signal. For the LED dimming device described in claim 1, wherein the linear dimming circuit has a transistor circuit to generate the variable-level PWM signal according to the variable voltage and the second PWM signal, and the electrical The crystal circuit has: a resistor, one end of which is coupled to the variable voltage; an NMOS transistor, having a drain, a gate and a source, and the drain is coupled to the other end of the resistor to generate For the variable-level PWM signal, the source is coupled to a reference ground; and an inverter has an input terminal and an output terminal, the input terminal is coupled to the second PWM signal, and the output The terminal is coupled to the gate. According to the LED dimming device described in claim 1, wherein the negative feedback operational amplifier circuit has a variable feedback resistance unit and a current sensor on the path of the output current, and the current sensor The detector is used to generate the second feedback voltage. For the LED dimming device described in item 3 of the scope of patent application, the variable feedback resistance unit includes a plurality of disconnectable resistance units, and each of the disconnectable resistance units includes a switch and and The switch is overlapped with a resistor, and the switch-on relationship is controlled by a switch signal, the switch signal is generated by the microprocessor, and the microprocessor performs a check on the dimming command voltage according to a voltage-switch comparison table The watch operates to generate a plurality of the switching signals of the plurality of disconnectable resistance units. The LED dimming device described in the first item of the scope of patent application, wherein the microprocessor is a digital signal processor and realizes the first incremental PID control program and the second incremental through a firmware program Type PID control program.

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