LED current-balance driving circuit

An LED current-balance driving circuit for driving a plurality of LED strings comprises a feedback transformer, a plurality of current-balance transformers, a plurality of full-wave rectifiers and a current control circuit. The feedback transformer has a detecting winding and an outputting winding for outputting a feedback signal. Each of the current-balance transformers has a primary winding and a secondary winding. The primary windings of the current-balance transformers and the detecting winding of the feedback transformer are coupled in series to compose a current-balance loop with an AC power being supplied thereto. Each of the full-wave rectifiers is coupled between a corresponding secondary winding of the current balance transformers and a corresponding LED string of the LED strings. The current control circuit receives the feedback signal and controls the AC power according to the feedback signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting diode (LED) driving circuit and, more particularly, to an LED current-balance driving circuit for driving a plurality of LED strings.

2. Description of the Prior Art

FIG. 1is a block diagram showing a conventional LED driving circuit for driving a plurality of LED strings. Referring toFIG. 1, an alternating-current (AC) power input101A, such as a household power, is supplied to a commutation circuit110and converted to a direct-current (DC) power. The DC power is then supplied to a power factor correction (PFC) circuit106and converted to a high-voltage DC voltage VDC1(approximately 80V˜400V). The high-voltage DC voltage VDC1is supplied to an isolated DC-to-DC converter107and converted to a low-voltage DC voltage VDC2(approximately 5V˜60V). The isolated DC-to-DC converter107divides the LED driving circuit in a primary side and a secondary side. Then, the low-voltage DC voltage VDC2is supplied to a conventional LED current-balance driving circuit108located in the secondary side.

FIG. 2is a circuit diagram showing the conventional LED current-balance driving circuit108inFIG. 1. Referring toFIG. 2, the low-voltage DC voltage VDC2is supplied to a current control circuit111A. The current control circuit111A outputs a driving signal to control the conductive time of the transistor Q1according to a current feedback signal generated by the resistor R1coupled to the transistor Q1so as to decide the time for the inductor L1to charge the capacitor C1through the diode D1. The output voltage VO across the capacitor C1as well as the output power is thus controlled. Each LED string105A,105B, . . . ,105M is series-coupled to a respective transistor QA,QB, . . . ,QM and a respective resistor RA,RB, . . . ,RM. The current control circuit111A also outputs driving signals to control the transistors QA,QB, . . . ,QM according to the feedback signals from the resistors RA,RB, . . . ,RM so as to adjust the currents IOA,IOB, . . . ,IOM flowing through the LED strings to achieve the object of current balance.

However, the above mentioned LED current-balance driving circuit needs a plurality of transistors to control the currents flowing through the LED strings. When the number of the LED strings is increased, the power dissipation on the transistors will become significant.

SUMMARY OF THE INVENTION

The present invention provides an LED current-balance driving circuit for driving a plurality of LED strings by using transformers instead of transistors to achieve current balance and simultaneously avoid the power dissipation on the transistors.

The LED current-balance driving circuit of the present invention for driving a plurality of LED strings comprises a feedback transformer, a plurality of current-balance transformers, a plurality of full-wave rectifiers and a current control circuit. The feedback transformer has a detecting winding and an outputting winding for outputting a feedback signal. Each of the current-balance transformers has a primary winding and a secondary winding. The primary windings of the current-balance transformers and the detecting winding of the feedback transformer are coupled in series to compose a current-balance loop with an alternating-current (AC) power being supplied thereto. Each of the full-wave rectifiers is coupled between a corresponding secondary winding of the current balance transformers and a corresponding LED string of the LED strings. The current control circuit receives the feedback signal and controls the AC power according to the feedback signal.

According to an embodiment of the present invention, the current-balance transformers employ isolated transformers, and the AC power is supplied by a non-isolated switching-mode power supply.

According to another embodiment of the present invention, the current-balance transformers employ non-isolated transformers, and the AC power is supplied by an isolated switching-mode power supply.

According to still another embodiment of the present invention, the current-balance transformers employ non-isolated transformers, and the AC power is supplied by a non-isolated switching-mode power supply.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3is a block diagram showing an LED driving circuit for driving a plurality of LED strings in accordance with a preferred embodiment of the present invention. Referring toFIG. 3, the dashed line indicates the range of the primary side and the secondary side of the LED driving circuit, and an LED current-balance driving circuit109A is located in the primary side and the secondary side. An AC power input101A, such as a household power, is supplied to a commutation circuit110and converted to a DC power. The DC power is then supplied to a PFC circuit106and converted to a high-voltage DC voltage VDC1(approximately 80V to 400V). Then, the high-voltage DC voltage VDC1is supplied to the LED current-balance driving circuit109A.

FIG. 4is a circuit diagram showing the LED current-balance driving circuit109A inFIG. 3. Referring toFIGS. 3 and 4, the LED current-balance driving circuit109A for driving a plurality of LED strings105A,105B,105C, . . . ,105M includes a feedback transformer102, a plurality of current-balance transformers103A,103B,103C, . . . ,103M, a plurality of full-wave rectifiers104A,104B,104C, . . . ,104M and a current control circuit111B. The feedback transformer102has a detecting winding N1′ and an outputting winding N2′. Each of the current-balance transformers103A,103B,103C, . . . ,103M has a primary winding N1and a secondary winding N2. The primary windings N1of all the current-balance transformers103A,103B,103C, . . . ,103M are coupled with each other in series. The series-coupled primary windings N1are then coupled to a feedback transformer102and an AC power supply101B, so as to compose a current-balance loop with a balance current IFB flowing therethrough and an AC power (provided by the AC power supply101B) supplied thereto. The outputting winding N2′ of the feedback transformer102outputs a feedback signal according to the balance current IFB flowing through the current-balance loop. The current control circuit111B receives the feedback signal and controls the AC power provided by the AC power supply101B according to the feedback signal.

In addition, turns ratios of the current-balance transformers103A,103B,103C, . . . ,103M are the same. Thus, the output current IA,IB,IC, . . . ,IM of the secondary windings N2of the current-balance transformers103A,103B,103C, . . . ,103M would be the same because of the identical current IFB flowing through the primary winding N1of each of the current-balance transformers103A,103B,103C, . . . ,103M. Each of the full-wave rectifiers104A,104B,104C, . . . ,104M, such as a bridge rectifier, is coupled between a corresponding secondary winding of the current balance transformers103A,103B,103C, . . . ,103M and a corresponding LED string of the LED strings105A,105B,105C, . . . ,105M. For example, the full-wave rectifiers104B is coupled between the corresponding secondary winding N2of the current balance transformer103B and the corresponding LED string105B. The full-wave rectifiers104A,104B,104C, . . . ,104M convert the output currents IA,IB,IC, . . . ,IM of the secondary windings N2to DC currents IOA,IOB,IOC, . . . ,IOM to drive the LED strings105A,105B,105C, . . . ,105M. The LED strings105A,105B,105C, . . . ,105M would be operated under the same condition because of the same output current IA,IB,IC, . . . ,IM therefore, the object of balancing the current flowing through the LED strings105A,105B,105C, . . . ,105M can be achieved.

In the present embodiment, the current-balance transformers103A,103B,103C, . . . ,103M employ isolated transformers; therefore, the LED current-balance driving circuit109A is located in the primary side and the secondary side, and the AC power supply101B can employ a non-isolated switching-mode power supply. The non-isolated switching-mode power supply includes the commutation circuit110, the PFC circuit106and a DC-to-AC converter (not shown). The DC-to-AC converter, such as a half-bridge or full-bridge DC-to-AC converter, receives the high-voltage DC voltage VDC1and converted it to the AC power according a control signal, which is generated by the current control circuit111B according to the feedback signal.

FIG. 5is a block diagram showing an LED driving circuit for driving a plurality of LED strings in accordance with another preferred embodiment of the present invention. Referring toFIG. 5, the dashed line indicates the range of the primary side and the secondary side of the LED driving circuit, and the LED current-balance driving circuit109B is totally located in the secondary side. An AC power input101A is supplied to a commutation circuit110and converted to a DC power. The DC power is then supplied to a PFC circuit106and converted to a high-voltage DC voltage VDC1. The high-voltage DC voltage VDC1is supplied to an isolated DC-to-DC converter107and converted to a low-voltage DC voltage VDC2. The isolated DC-to-DC converter107divides the LED driving circuit in a primary side and a secondary side. Then, the low-voltage DC voltage VDC2is supplied to the LED current-balance driving circuit109B.

FIG. 6is a circuit diagram showing the LED current-balance driving circuit109B inFIG. 5. Referring toFIGS. 5 and 6, the major difference between the LED current-balance driving circuit109B in the present embodiment and the LED current-balance driving circuit109A inFIG. 4is that the LED current-balance driving circuit109B is totally located in the secondary side. In the present embodiment, the current-balance transformers103A,103B,103C, . . . ,103M employ non-isolated transformers, and the AC power supply101B can employ an isolated switching-mode power supply. The isolated switching-mode power supply includes the commutation circuit110, the PFC circuit106, the isolated DC-to-DC converter107and a DC-to-AC converter (not shown). The DC-to-AC converter, such as a half-bridge or full-bridge DC-to-AC converter, receives the low-voltage DC voltage VDC2and converted it to the AC power according a control signal, which is generated by the current control circuit111B according to the feedback signal.

FIG. 7is a block diagram showing an LED driving circuit for driving a plurality of LED strings in accordance with still another preferred embodiment of the present invention. Referring toFIG. 7, the LED current-balance driving circuit109C is totally located in the primary side. An AC power input101A is supplied to a commutation circuit110and converted to a DC power. The DC power is then supplied to a PFC circuit106and converted to a high-voltage DC voltage VDC1. Then, the high-voltage DC voltage VDC1is supplied to the LED current-balance driving circuit109C.

FIG. 8is a circuit diagram showing the LED current-balance driving circuit109C inFIG. 7. Referring toFIGS. 7 and 8, the major difference between the LED current-balance driving circuit109C in the present embodiment and the LED current-balance driving circuit109A inFIG. 4is that the LED current-balance driving circuit109C is totally located in the primary side. In the present embodiment, the current-balance transformers103A,103B,103C, . . . ,103M employ non-isolated transformers, and the AC power supply101B employ a non-isolated switching-mode power supply. The non-isolated switching-mode power supply includes the commutation circuit110, the PFC circuit106and a DC-to-AC converter (not shown). The DC-to-AC converter, such as a half-bridge or full-bridge DC-to-AC converter, receives the high-voltage DC voltage VDC1and converted it to the AC power according a control signal, which is generated by the current control circuit111B according to the feedback signal.

In conclusion, the present invention has the flowing advantages:1. In order to control the amount of current flowing through each of the LED strings, the LED current-balance driving circuit of the present invention only needs one feedback transformer, which generates a feedback signal for the current control circuit to achieve the object of current balance.2. Referring toFIG. 3, the LED current-balance driving circuit located in the primary side and the secondary side can be driven by using the high-voltage DC voltage VDC1from the PFC circuit106. Thus, the DC-to-DC converter107can be skipped to reduce the cost.3. Referring toFIG. 5, the LED current-balance driving circuit located in the secondary side can be easily applied to the existed conventional LED driving circuit shown inFIG. 1.4. Referring toFIG. 7, the LED current-balance driving circuit located in the primary side can be driven by using the high-voltage DC voltage VDC1from the PFC circuit106. Thus, the DC-to-DC converter107can be skipped to reduce the cost.