TWI376984B - System and method for driving led with high efficiency in power consumption - Google Patents

Description

1376984 VIII. If there is a chemical formula in this case, please disclose the chemical formula that best shows the characteristics of the invention: • * 9. Description of the invention: [Technical field of the invention] The present invention relates to the driving of a light-emitting diode (LED), in particular A LED drive system and method for high power consumption performance. ^ [Prior Art] Light-emitting diodes (LEDs) are commonly used in various electronic devices and have various uses. For example, the LED can be used in a backlight module of a liquid crystal display (LCD) to provide a backlight or used in a charge coupled device (CCD) camera to provide flash illumination. The LED is actually a temperature-dependent component, in other words, the characteristics of the LED change as the temperature changes. The first figure shows an LED and its equivalent circuit. An LED is equivalent to a voltage source connected in series with a negative-temperature-coefficient (NTC) resistor. When the temperature rises through the LED, the resistance of the NTC resistor will decrease, and vice versa. Therefore, when the current through the LED is fixed, then the potential (or referred to as the forward voltage VF) across the anode and cathode of the LED decreases as the temperature rises, and vice versa. 1376984 There are two methods for traditional LED driving: constant-voltage (CV) driving and constant current • (constant-current, CC) driving. In the conventional constant voltage driving method, the anode of the LED receives a certain voltage source. As mentioned earlier, even if the anode receives a constant voltage, the current through the LED will still change. Therefore, the LED changes its brightness due to changes in the drive current. Furthermore, the conventional constant voltage driving method usually requires a current-limiting resistor in series, and φ causes a waste of power consumption. In the conventional constant current driving method, the current passing through the LED is controlled to a constant value. Although the conventional constant current drive LED current (and its brightness) is not

I will change with the change of the forward voltage VF. However, the LED needs to be connected in series with the current-sensing resistor, which also causes waste of power consumption. In view of the fact that the conventional constant voltage or constant current driving method causes waste of power consumption, it is urgent to propose a new driving mechanism that can not only increase the power consumption efficiency, but also maintain the stability of the driving current. [Invention] In view of the above, One of the objects of the present invention is to provide a <LED driver" 6 1376984 system and method for stabilizing LED drive current and brightness and increasing power consumption efficiency. In accordance with an embodiment of the invention, the drive system includes a current mode circuit for providing a fixed current to the light emitting diode; and a voltage mode circuit for providing a fixed voltage to the light emitting diode. A switch is used to switch between the constant current mode circuit and the constant voltage mode circuit to enter the constant current mode and the constant voltage mode, respectively. Thereby, the forward voltage of the light-emitting diode is maintained at a fixed value, and the power consumption efficiency is thereby increased. [Embodiment] The second figure shows a light emitting diode (LED) driving system 10 according to an embodiment of the present invention. Although the present embodiment has an LED as an illustration, it is common in the art to use a plurality of LEDs, and these LEDs may be in series or in parallel with each other. In the present embodiment, the regulator 12 sequentially turns on (off) and off (switches) the transistor Q2 sequentially, so that the supply voltage Vin can store energy in the inductor L1 when the transistor Q2 is turned on. The stored energy is transferred to the output voltage terminal Vout of the LED D1 when the transistor Q2 is off. The rectifying diode D2 is used to prevent the current of the output voltage terminal Vout from flowing improperly back to the supply voltage Vin. The switching duty cycle of the regulator 12 is varied according to the output of the error comparator 18. For example, as the output of the error comparator 18 increases (which represents a decrease in LED output voltage or current), the switching duty cycle of the regulator 12 will increase, and vice versa. According to this embodiment, the LED drive system 1A includes a current-sense resistor R3' which is connected in series between the LED di cathode and the ground. The LED drive system 1〇 also includes a voltage divider (R〇Rage divider) R1-R2 coupled between the anode (or voltage output terminal v〇ut) of the LED D1 and ground. The error comparator 18 is used to compare an (inverting input) input voltage with a - (non-inverting input) reference voltage. The input voltage and reference voltage will be different in different modes, which will be described in detail below. Controller 13 (details of which will be described in more detail below) is used to control and regulate the operation of lEd drive system 10. Controller 13 can be a hardware circuit, a software program, or a combination thereof. Furthermore, in practice, the controller 13 can also be divided into a plurality of connected or separated functional blocks. During operation, the LED drive system operates in two modes, namely a constant current (CC) mode and a constant voltage (cv) mode. In the drawings, the switching between the two modes is represented by a switch sw, which is controlled by the controller I3. When ai-a2 and bl-b2 in the switch SW are closed (cl〇se) 1376984, it is the constant current mode, and the equivalent system is shown in Figure 3A. Conversely, when a2-bl in the switch SW is closed and al and b2 are floating I * (floating), it is a constant voltage mode, and its equivalent system is as shown in the third B diagram. The duty cycle of the constant current (CC) mode and the constant voltage (CV) mode is illustrated in the timing chart of the fourth figure, in which the constant current (CC) period is much smaller than the constant voltage (CV) period. For example, the constant current (CC) period can be several milliseconds (ms), while the constant voltage (CV) period can last for a few minutes or longer. In the constant current mode, the controller 13 turns off the transistor Q1 (step 51) as in the system block shown in Fig. A and the flow shown in the fifth figure. Next, at blocks 14 and 52, a voltage divider VI of the voltage divider terminal d is obtained, which is provided by voltage dividers R1-R2 across the voltage output terminal Vout and ground. In this embodiment, the obtained voltage division VI can be converted into a digital form by an analog digital converter (ADC) and temporarily stored in the controller 13 for subsequent operations. The voltage at terminal c (or the potential across current-sense resistor R3) is maintained by the error comparator 18 to maintain a predetermined reference voltage Vref (block 16). According to the basic circuit principle:

Vl=(R2/(Rl+R2))*Vout or 9 1376984

Vout = (Vl / R2) * (Rl + R2) Thereby, the controller 13 can derive the forward voltage VF of the LED D1 as follows (step 53): VF = Vou t - Vref = (V1 / R2) * (R1 +R2) - Vref Next, the LED drive system 10 enters a constant voltage (CV) mode (the mode switching can be performed by the controller 13), as shown in the system block and the fifth figure shown in FIG. The process. The controller 13 turns on the electrical crystal Q1 (step 54) (for use as a bypass transistor), thereby connecting the cathode of the LED D1 to ground and bypassing or bypassing the resistor R3. In other words, no current will pass through the resistor R3, so in the constant voltage (cV) mode, the resistor R3 has no power consumption. Next, at step 55, the reference voltage 16 input to the error comparator 18 is changed to a new reference • voltage value Vl-VrePl^MRl+I^). In the present embodiment, controller 13 provides a new reference voltage value in analogy to error comparator 18 via a digital analog converter (DAC). The voltage at the end point d will tend to this new reference voltage value V Bu Vref * R2ARl + R2). According to the basic circuit principle, the following relationship can be obtained:

Vl-VrePR2/(Rl+R2)=(R2/(Rl+R2))*V〇ut or 1376984

Vout=(Vl-Vref*R2/(Rl+R2))*((Rl+R2)/R2)=(Vl/R2)*(R1+R2)-Vre f=VF t · Thereby, LED D1 The forward voltage VF can be maintained at a fixed voltage value VF. It is worth noting that resistor R3 is no longer used as a current limiting resistor in constant voltage mode, so resistor R3 will not have power consumption in this mode. By increasing the duty cycle of the constant voltage (CV) mode (figure 4) to make it as large as possible, the power consumption can be made higher than the conventional constant current driving method or the constant voltage driving method. The above description is only the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention; all other equivalent changes or modifications which are not departing from the spirit of the invention should be included in the following Within the scope of the patent application. [Simple description of the diagram] The first figure shows an LED and its equivalent circuit. The second figure shows a light emitting diode driving system according to an embodiment of the present invention. Figure 3A shows the equivalent system for the LED drive system of Figure 2 in constant current mode. Figure 3B shows the equivalent system of 11 1376984 when the LED drive system of Figure 2 is in constant voltage mode. The fourth graph shows the duty cycle of the constant current mode (third A picture) and the constant voltage mode (third B picture). The fifth figure shows a flow chart of the drive system shown in the second to third B.

[Main component symbol description] 10 LED driver system 12 Regulator 13 Controller 14 Voltage divider VI acquisition 16 Reference voltage 18 Error comparator 51-55 Drive flow step CC Constant current drive CV Constant voltage drive NTC Negative temperature coefficient Resistance VF forward voltage D1 light-emitting diode 91 transistor 92 transistor Vin supply voltage Vout output voltage 12 1376984

Ll Inductor D2 Rectifier Diode R3 Current-Sense Resistor R1-R2 Voltage Divider c End Point d Dividing End Point VI Dividing SW Switch 13

Claims (1)

1376984 _ . |年伞月月/ &日修正本< X. Patent application scope: 1. A light-emitting diode driving system comprising: a certain current mode circuit for providing a fixed current to the light-emitting diode; a certain voltage a mode circuit for providing a fixed voltage to the light emitting diode; and a switch for switching the constant current mode circuit and the constant voltage mode circuit to enter a constant current mode and a constant electric mode respectively; The constant current mode circuit comprises: a current detecting resistor connected in series to the cathode of the light emitting diode; and an error comparator coupled to receive a predetermined reference voltage and a voltage across the current detecting resistor; wherein The constant voltage mode circuit comprises: a voltage divider connected between the anode of the light emitting diode and the ground; a device for obtaining a voltage division of the voltage divider; and a device for obtaining the light a forward voltage of the diode; a bypass transistor coupled between the cathode of the light emitting diode and the ground; and a device for using the error comparator according to the voltage divider Test voltage is changed to a new reference voltage value, thereby making the cis-emitting diode to maintain the weight of the cis to obtain the voltage to the electric. 14 1376984 2. The illuminating diode drive system of claim 1, wherein the constant current mode and the constant voltage mode are sequentially entered. 3. A light-emitting diode driving system comprising: a current mode circuit for providing a fixed current to the light-emitting diode; and a voltage mode circuit for providing a fixed voltage to the light-emitting diode; a switch for switching the constant current mode circuit and the constant voltage mode circuit to enter a constant current mode and a constant voltage mode, respectively; and a supply power source for supplying power to the light emitting diode; wherein The power supply comprises: a supply voltage; an inductor; a switching transistor connected between the inductor and the ground; and an end point between the inductor and the switching transistor for electrically coupling to the light emitting diode The anode of the body; wherein the switching transistor is sequentially turned on (on) and off (off), so that the supply voltage stores energy in the electrical 15 1376984 sense when the switching transistor is turned on, and the switching transistor The stored energy is transferred to the light emitting diode for shutdown. 4. The illuminating diode driving system of claim 3, further comprising a rectifying diode connected between the end point and the anode of the illuminating diode. Φ 5. A method for driving a light-emitting diode, comprising: intermittently storing energy of a supply voltage, and then transmitting the stored energy to the light-emitting diode to supply power to the light-emitting diode; Maintaining a fixed current through the light-emitting diode in a current mode; obtaining a partial voltage of an output voltage of the light-emitting diode; obtaining a forward voltage value of the light-emitting diode; and the light-emitting diode in a constant voltage mode The cathode is grounded; and • the forward voltage of the light emitting diode is maintained equal to the obtained forward voltage value. 6. The method of driving a light-emitting diode according to claim 5, wherein the light-emitting diode is passed through by comparing a voltage across the current-sense resistor with a predetermined reference voltage to maintain the constant current mode. Fixed current. The light emitting diode driving method of claim 5, wherein the output voltage of the light emitting diode is obtained by a voltage divider connected between the anode of the light emitting diode and the ground. Partial pressure. 8. The method of driving a light emitting diode according to claim 5, wherein the output voltage of the light emitting diode and the predetermined reference voltage are used to obtain a forward voltage value of the light emitting diode. 9. The method of driving a light-emitting diode according to claim 5, wherein a bypass transistor is used to ground the cathode of the light-emitting diode in a constant voltage mode. 10. The method of driving a light emitting diode according to claim 5, wherein the forward voltage of the light emitting diode is equal to the forward voltage obtained by comparing the divided voltage with a new reference voltage. value. 17