LED driving circuit having a large operational range in voltage

An LED driving circuit includes a current selecting circuit. The current selecting circuit controls the current transmission path in the plurality of LEDs according to respective threshold voltages of corresponding LEDs and a plurality of current limits.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to an LED driving circuit, and more particularly, to an LED driving circuit having a large operational voltage range.

2. Description of the Prior Art

Compared to incandescent lamps, light emitting diodes (LEDs) are characterized in low power consumption, long lifetime, small size and fast optical response. LEDs can easily be manufactured as miniaturized or array devices, which are widely used in various electronic products. Common LED applications include outdoor stationary displays (such as billboards, signboards or traffic signs) and portable devices (such as mobile phones, notebook computers or PDAs).

Reference is made toFIG. 1for a voltage-current chart of an LED. When the forward-bias voltage of the LED is smaller than its threshold voltage Vb, the LED only conducts a negligible amount of current and the two ends of the LED are substantially open-circuited. When the forward-bias voltage of the LED is larger than its threshold voltage Vb, the current flowing through the LED exponentially increases with the forward-bias voltage and the two ends of the LED are substantially short-circuited. In an LED driving circuit, a current source is normally adopted for driving multiple LEDs so as to provide uniform luminescence.

Reference is made toFIG. 2for a diagram of a prior art LED driving circuit300. The LED driving circuit300, including a voltage source VS and a current source IS, is configured to drive a luminescent device10. The voltage source VS can provide a driving voltage Vf for turning on the luminescent device10, while the current source IS can stabilize a driving current If which flows through the luminescent device10so as to maintain uniform luminescence. Since the LED is a current-driven device whose luminescence is proportional to its driving current, the luminescent device10normally includes a plurality of serially-coupled light-emitting diodes LED1-LEDnin order to provide sufficient and uniform light in large-size applications. Assuming all the light-emitting diodes LED1-LEDnhave the ideal threshold voltage Vb, then a driving voltage Vf equal to n*Vb is required for turning on the luminescent device10. In the prior art LED driving circuit100, while more light-emitting diodes can provide higher light intensity, the forward-bias voltage of the luminescent device10also increases accordingly, thereby reducing the effective operational voltage range.

Reference is made toFIG. 3for a diagram of another prior art LED driving circuit400. The LED driving circuit400, including a power supply circuit110, a voltage detecting circuit410and a current-regulating circuit420, is configured to drive a luminescent device10. The power supply circuit110includes a voltage source VS and a bridge rectifier20. The voltage source VS can output an alternating current (AC) voltage which periodically switches between positive and negative phases, while the bridge rectifier20is configured to convert the AC voltage outputted in the negative phase. The power supply circuit110can thus provide a direct current (DC) voltage Vf for driving the luminescent device10, wherein the value of the driving voltage Vf periodically varies with time. The current-regulating circuit420includes a plurality of current sources IS1-ISnrespectively configured to control the light intensity of corresponding light-emitting diodes LED1-LEDnin the luminescent device10. The voltage detecting circuit410can detect the value of the driving voltage Vf, thereby turning on/off the current sources IS1-ISnof the current-regulating circuit420accordingly. Assuming all the light-emitting diodes LED1-LEDnhave the ideal threshold voltage Vb: when the driving voltage Vf reaches the threshold voltage (Vb) of the light-emitting diode LED1, the voltage detecting circuit410turns on the current source IS1and turns off the current sources IS2-ISn, thereby providing a current path which starts from the voltage source VS and sequentially passes through the light-emitting diode LED1and the current sources IS1; when the driving voltage Vf reaches the overall threshold voltage of the light-emitting diodes LED1and LED2(2Vb), the voltage detecting circuit410turns on the current source IS2and turns off the current sources IS1and IS3-ISn, thereby providing a current path which starts from the voltage source VS and sequentially passes through the light-emitting diode LED1, the light-emitting diode LED2and the current sources IS2; . . . ; similarly, when the driving voltage Vf reaches the overall threshold voltage of the light-emitting diodes LED1-LEDn(n*Vb), the voltage detecting circuit410turns on the current source ISnand turns off the current sources IS1-ISn−1, thereby providing a current path which starts from the voltage source VS and sequentially passes through the light-emitting diodes LED1-LEDnand the current sources ISn.

However, due to variations in material and manufacturing processes, the light-emitting diodes LED1-LEDnmay not have the ideal threshold voltage Vb. The prior art voltage detecting circuit410is unable to control each current source according to the actual threshold voltage of a corresponding light-emitting diode. For example, assuming the actual threshold voltage Vb1of the light-emitting diode LED1is larger than the ideal threshold voltage Vb. If the voltage detecting circuit410turns on the current source IS1when Vf=Vb, the light-emitting diode LED1cannot be turned on. Thus for non-ideal light-emitting diodes, the voltage detecting circuit410is normally configured to turn on the current source IS1when the detected driving voltage Vf reaches a switching voltage Vb′ larger than Vb. If the voltage detecting circuit410turns on the current source IS1until Vf=Vb′, the extra voltage (Vb′−Vb1) not only increases the power consumption of the current source IS1, but also reduces the effective operational voltage range of the LED driving circuit400.

SUMMARY OF THE INVENTION

The present invention provides a driving circuit having a large operational voltage range and configured to drive a plurality of serially-coupled luminescent units. The driving circuit comprises a current-selecting circuit configured to control current paths in the plurality of luminescent units according to a plurality of current limits and respective threshold voltages of corresponding light emitting diodes in the plurality of luminescent units.

The present invention further provides a display device having a large operational voltage range and comprising a plurality of serially-coupled luminescent units; a power supply circuit coupled to plurality of serially-coupled luminescent units; and a driving circuit configured to drive the plurality of serially-coupled luminescent units. The driving circuit comprises a current-selecting circuit configured to control current paths in the plurality of luminescent units according to a plurality of current limits and respective threshold voltages of corresponding light emitting diodes in the plurality of luminescent units.

DETAILED DESCRIPTION

FIG. 4is a diagram illustrating an LED driving circuit100according to a first embodiment of the present invention.FIG. 5is a diagram illustrating an LED driving circuit200according to a second embodiment of the present invention. The LED driving circuit100having a current-selecting circuit120and the LED driving circuit200having a current-selecting circuit220are configured to drive a luminescent device10coupled in series with a power supply circuit110.

The power supply circuit110includes a voltage source VS and a bridge rectifier20. The voltage source VS can output an AC voltage which periodically switches between positive and negative phases, while the bridge rectifier20is configured to convert the AC voltage having negative phase. The power supply circuit110can thus provide a DC voltage Vf for driving the luminescent device10, wherein the value of the driving voltage Vf periodically varies with time. The luminescent device10may include a plurality of luminescent units D1-Dn+1each having a single LED or multiple LEDs. For illustrative purpose, each luminescent unit depicted inFIG. 4includes a single LED, but this structure does not limit the scope of the present invention. The voltages established between two adjacent luminescent units among the luminescent units D1-Dn+1are represented by V1-Vn, respectively.

In the LED driving circuit100according to the first embodiment of the present invention, the current-selecting circuit120includes a plurality of variable current sources IS1-ISnand a plurality of adjusting circuits CKT1-CKTn. The variable current sources IS1-ISnprovide adjustable current limits, based on which the currents flowing through the corresponding luminescent units D1-Dnare regulated at respective predetermined values, thereby providing brightness control and device protection. The adjusting circuits CKT1-CKTncan respectively detect the values of the voltages V1-Vn, thereby adjusting the current limits of the variable current sources IS1-ISnaccordingly.

As previously illustrated, the driving voltage Vf periodically varies with time. For illustration, assume that the driving voltage Vf gradually rises from 0 after initialization. When the voltage established across the luminescent unit D1exceeds the threshold voltage of the luminescent unit D1, the luminescent unit D1is turned on, thereby providing a current path which starts from the voltage source VS and sequentially passes through the luminescent unit D1and the current sources IS1. At this time, the current flowing through the luminescent unit D1is maintained at a constant value by the variable current source IS1. Next, as the voltage V1increases with the driving voltage Vf, the luminescent unit D2is turned on when the voltage established across the luminescent unit D2exceeds the threshold voltage of the luminescent unit D2. The adjusting circuit CKT1then detects the voltage V2or the current flowing through the luminescent unit D2, thereby gradually lowering the current limit of the variable current source IS1to zero as the current flowing through the luminescent unit D2increases. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1, the luminescent unit D2and the current sources IS2, while the currents passing through the luminescent units D1and D2are maintained at respective constant values by the variable current sources IS1and IS2, respectively. Similarly, as the driving voltage Vf gradually increases, the voltages V1-Vnalso increase accordingly, thereby sequentially turning on the luminescent units D1-Dn. On the other hand, the adjusting circuits CKT1-CKTnrespectively detect the voltages V2-Vn±1or respectively detect the currents flowing through the luminescent units D2-Dn+1, thereby sequentially lowering the current limits of the variable current sources IS1-ISnto zero.

Assuming that when the driving voltage Vf provided by the power supply circuit110has a maximum value, all of the luminescent units D1-Dnare turned on and the current limits of the variable current sources IS1-ISn−1are zero. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1-Dnand the current source ISn, while the current passing through the luminescent units D1-Dnis maintained at a constant value by the variable current source ISn. After the driving voltage Vf begins to decrease, the luminescent unit Dnis the first to be turned off due to insufficient applied voltage. The adjusting circuit CKTn−1then detects the voltage Vnor the current flowing through the luminescent unit Dn, thereby gradually raising the current limit of the variable current source ISn−1from zero. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent units D1-Dn−1and the current source ISn−1, while the current flowing through the luminescent units D1-Dn−1is maintained at a constant value by the variable current source ISn−1. Similarly, as the driving voltage Vf gradually decreases, the voltages Vn-V1also decrease accordingly, thereby turning off the luminescent units Dn-D1sequentially. On the other hand, the adjusting circuits CKTn−1-CKT1respectively detect the voltages Vn-V2or respectively detect the currents passing through the luminescent units Dn-D1, thereby sequentially increasing the current limits of the variable current sources ISn−-IS1.

In the LED driving circuit200according to the second embodiment of the present invention, the current-selecting circuit220includes a plurality of constant current sources IS1-ISn, a plurality of switches SW1-SWnand a plurality of judging units CM1-CMn. The current sources IS1-ISnprovide constant current limits, based on which the currents flowing through the corresponding luminescent units D1-Dnare regulated at respective predetermined values, thereby providing brightness control and device protection. Each of the switches SW1-SWnincludes a first end coupled between two corresponding adjacent luminescent units among the luminescent units D1-Dn(respectively denoted by V1-Vn), and a second end coupled to a corresponding current source among the current sources IS1-ISn. The judging units CM1-CMncan respectively detect the values of the voltages V1-Vn, thereby turning on/off the corresponding switches SW1-SWnaccordingly.

As previously illustrated, the driving voltage Vf periodically varies with time. For illustration, assuming that at initialization, the driving voltage Vf is equal to 0 and all switches SW1-SWnare turned on (short-circuit). As the driving voltage Vf gradually increases, the luminescent unit D1is turned on when the voltage established across the luminescent unit D1exceeds the threshold voltage of the luminescent unit D1, while the luminescent unit D2remains off. At the time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1, the switch SW1and the current source IS1, while the current flowing through the luminescent unit D1is maintained at a constant value by the current source IS1. Next, as the voltage V1increases with the driving voltage Vf, the luminescent unit D2is turned on when the voltage established across the luminescent unit D2exceeds the threshold voltage of the luminescent unit D2, while the luminescent unit D3remains off. At the time, the voltage V2also increases with the driving voltage Vf. After having detected that the voltage V2has reached a predetermined value, the judging unit CM1turns off the switch SW1. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1, the luminescent unit D2, the switch SW2and the current source IS2, while the current flowing through the luminescent unit D1-D2is maintained at a constant value by the current source IS2. Similarly, as the driving voltage Vf gradually increases, the voltages V1-Vnalso increase accordingly, thereby sequentially turning on the luminescent units D1-Dn. On the other hand, the judging units CM1-CMnrespectively determine whether the voltages V2-Vn+1have reached respective predetermined values, thereby sequentially turning off the switches SW1-SWn.

Assuming that when the driving voltage Vf provided by the power supply circuit110has a maximum value, the luminescent units D1-Dnare turned on (short-circuit), the switches SW1-SWn−1are turned off (open-circuit), and the switch SWnis turned on. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1-Dn, the switch SWnand the current source ISn, while the current passing through the luminescent units D1-Dnis maintained at a constant value by the current source ISn. As the voltage Vndecreases with the driving voltage Vf and falls to a predetermined value, the judging unit CMn−1turns on the switch SWn−1and the luminescent unit Dnis turned off due to insufficient applied voltage. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1-Dn−1, the switch SWn−1and the current source ISn−1, while the current passing through the luminescent units D1-Dn−1is maintained at a constant value by the current source ISn−1. Similarly, as the driving voltage Vf gradually decreases, the voltages Vn-V1also decrease accordingly, thereby turning off the luminescent units Dn-D1sequentially. On the other hand, the judging units CMn−1-CM1respectively determine whether the voltages Vn-V2have reached respective predetermined values, and sequentially turn off the SWn−1-SW1. On the other hand, the luminescent units Dn-D1are also sequentially turned off as respective applied voltages gradually drop.

Reference is made toFIG. 6for a diagram illustrating the operation of the LED driving circuit100or200according to the present invention. Assuming that the LED driving circuit100or200includes five current sources IS1-IS5which provide identical current limit, and the luminescent device10includes five luminescent units D1-D5whose threshold voltages are respectively represented by Vb1-Vb5. InFIG. 6, Vf represents the DC voltage provided by the power supply circuit110, Vb represents the overall voltage established across all the turned-on luminescent units among the luminescent units D1-D5, and ID1represents the current flowing through the luminescent unit D1. As shown inFIG. 6, the present invention can provide a large operational voltage range (between t1and t2), as well as can reduce the power consumption of the current sources IS1-IS5(the differences between Vf and Vb, denoted by dotted regions inFIG. 6).

In conclusion, the present invention can control the current limit of each current source according to the actual threshold voltage of the corresponding luminescent unit, such as the digital adjustment provided by the current-selecting circuit120of the first embodiment or the analog adjustment provided by the current-selecting circuit220of the second embodiment. The current paths in the LED string can be controlled based on the threshold voltage of each LED without using filter capacitor or detecting the input voltage. Even the LEDS of each luminescent unit may have different threshold voltages, the present invention can still provide accurate current limits accordingly, thereby enlarging the effective operational voltage range and improving optical efficiency and power factor.