Patent Application
20120169252
1. Technical Field
The present disclosure relates to a Light Emitting Diode (LED) driving circuit capable of adjusting a current flowing through LEDS.
2. Description of Related Art
A typical LED driving circuit includes a bridge rectifier circuit, a filter circuit, a buck converter (output voltage less than input voltage). The bridge rectifier circuit can convert alternating current voltage to direct current (DC) voltage. The buck converter outputs a low voltage to the LEDS. Then the LEDS are lit. However, the voltage and current supplied to the LEDS is not adjustable, and sometimes less or more than the normal range. The LEDS can be easily damaged in the case of over current or undercurrent.
Therefore, there is room for improvement within the art.
Many aspects of the embodiments can be better understood with references to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
The disclosure is illustrated by way of example and not by way of limitation. In the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one.
Referring to
The voltage comparing circuit 10 includes a comparator U1. The comparator U1 includes a positive input terminal connected to a reference voltage Vref, a negative input terminal connected to a first node P, and an output terminal connected to the switch control circuit 20.
The switch control circuit 20 includes an oscillator 22, a NAND gate U2, and an AND gate U3. The NAND gate U2 includes a first input terminal A1 connected to the oscillator 22, a second input terminal B1 connected to the comparator 10 output terminal, and a first output terminal T1. The AND gate U3 includes a third input terminal A2 connected to the first output terminal T1, a fourth input terminal B2 connected to the comparator 10 output terminal, and a second output terminal T2 connected to the power supply circuit 30.
The power supply circuit 30 includes a power supply 32, an inductor L, a Field Effect Transistor (FET) Q, a diode D, and a capacitor C. The power supply 32 can provide a direct current (DC) voltage of 5 volts, or 12 volts. A first terminal of the inductor L is connected to the power supply 32. A second terminal of the inductor L is connected to a second node M. The FET Q is an N-channel enhancement FET. The FET Q includes a gate terminal connected to the second output terminal T2, a drain terminal connected to the second node M, and a source terminal connected to ground. When a voltage at the gate terminal is at a high level (e.g., ≧5V), the FET Q is rendered conductive (switched on). When the voltage at the gate terminal is at a low level (e.g., 0V), the FET Q is rendered non-conductive (switched off). The diode D includes an anode connected to the second node M, and a cathode connected to a third node N. The capacitor C includes a first terminal connected to the third node N, and a second terminal connected to ground.
The LED circuit 40 includes LED1-LED3, which are connected in series, and a resistor R. LED1 includes a first anode connected to the third node N, and a first cathode connected to a second anode of the LED2. A second cathode of the LED2 is connected to a third anode of the LED3. A third cathode of the LED3 is connected to the first node P. The resistor R includes a first terminal connected to the first node P, and a second terminal connected to ground.
When the LED driving circuit starts, the power supply 32 is switched on. A current flowing through the inductor L increases. When the current flowing through the inductor L does not reach a predetermined value, a voltage at the first node P is less than the reference voltage Vref. Thus, the comparator U1 output terminal outputs a high level signal to the second input terminal B1 and the fourth input terminal B2. The oscillator 22 output a square wave signal to the first input terminal A1. Because a square wave signal repeats itself and will go, say, from a low level signal to a high level signal and vice versa, the first output terminal T1 goes from the high level to the low level and vice versa. The second output terminal T2 will follow the first output terminal T1 and go from the high level to the low level and vice versa. That is, a voltage level of each of the NAND gate U2 and the AND gate U3 is opposite to that of the square wave signal. The FET Q is switched on or off with a frequency equal to the frequency of the square wave signal. The inductor L stores electric power when the FET Q is switched on, and discharges the electric power to the capacitor C when the FET Q is switched off. As the electric power of the capacitor C increases gradually, the voltage at the third node N increases correspondingly. A current flowing through the LED circuit 40 increases gradually. When the current flowing through the LED circuit 40 exceeds a predetermined value, the voltage at the first node P exceeds the reference voltage Vref. Thus, the comparator U1 output terminal outputs a low level signal to the second input terminal B1 and the fourth input terminal B2. Because of the characteristics of the NAND gate U2, the first output terminal T1 will be maintained at a high level irrespective of the input at A1. And because of the characteristic of the AND gate U3, the second output terminal T2 will be maintained at a low level. The FET Q is switched off. A current flowing to the inductor L decreases. The current flowing to the LED circuit 40 also decreases. When the current flowing to the LED circuit 40 becomes less than the predetermined value, the voltage at the first node P becomes less than the reference voltage Vref. The comparator U1 output terminal returns to the high level. The second output terminal T2 returns output regular high/low signals to the FET Q. The FET Q returns to be switched on or off periodically.
In one embodiment, the LED driving circuit can automatically decrease the current flowing through the LED circuit 40 when a over-current is detected, and increase the current when a undercurrent is detected. Therefore, the current flowing through each of the LED1-LED3 can be maintained in a normal range.
While the present disclosure has been illustrated by the description of preferred embodiments thereof, and while the preferred embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such details. Additional advantages and modifications within the spirit and scope of the present disclosure will readily appear to those skilled in the art. Therefore, the present disclosure is not limited to the specific details and illustrative examples shown and described.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201010611376.8 | Dec 2010 | CN | national |
