Dimming controlling circuit for LED

1. A dimming controlling circuit for LED, comprising:

a switch configured to connect a main circuit and a power grid;

a switch-on duration controlling circuit configured to output a switch-on duration controlling signal according to action times of the switch;

a switch-off controlling circuit configured to compare a voltage of the switch-on duration controlling signal and a voltage of a signal from a terminal of a switch transistor in the main circuit to obtain a comparison result and to output a switch-off controlling signal according to the comparison result;

a switch-on controlling circuit configured to generate a switch-on controlling signal after a first preset duration since receiving the switch-off controlling signal; and

a logic unit configured to output a switch transistor controlling signal according to the switch-off controlling signal and the switch-on controlling signal,

wherein the switch-on duration controlling circuit comprises:

a power generating circuit configured to generate a first voltage according to an input voltage;

a detecting circuit configured to detect the input voltage, to output an invalid shift controlling signal when determining the input voltage reaches a first threshold voltage and to output a valid shift controlling signal when determining the input voltage descends below a second threshold voltage and above the third threshold voltage;

a shift register configured to output a shift result according to the invalid or valid shift controlling signal;

a resetting circuit configured to reset the shift register to zero;

a decoding circuit configured to decode according to the shift result to obtain a decoding result; and

a selecting circuit configured to select a voltage corresponding to the switch-on duration controlling signal according to the decoding result.

2. The dimming controlling circuit for LED according to claim 1 , wherein the first threshold voltage is higher than the second threshold voltage and the second threshold voltage is higher than the third threshold voltage.

3. The dimming controlling circuit for LED according to claim 1 , wherein the switch-off controlling circuit comprises:

a comparator, wherein a first input terminal of the comparator is connected with the voltage of the signal from the terminal of the switch transistor in the main circuit and a second input terminal of the comparator is connected with the voltage selected by the selecting circuit.

4. The dimming controlling circuit for LED according to claim 3 , wherein the first input terminal of the comparator is a positive input terminal and the second input terminal of the comparator is a negative input terminal.

5. The dimming controlling circuit for LED according to claim 1 , wherein the logic unit comprises a R-S trigger, a R terminal of the R-S trigger is configured to receive the output switch-off controlling signal sent by the switch-off controlling circuit, a S terminal of the R-S trigger is configured to receive the switch-on controlling signal sent by the switch-on controlling circuit and an output terminal of the R-S trigger is configured to output the switch transistor controlling signal.

6. The dimming controlling circuit for LED according to claim 1 , wherein the main circuit comprises a second inductor, a LED group, a flywheel diode and a switch transistor, in which the second inductor, the LED group and the flywheel diode are connected in series to form a loop and the flywheel diode has a negative terminal connected with the power grid and a positive terminal connected with the switch transistor.

7. The dimming controlling circuit for LED according to claim 6 , wherein the switch transistor is a NMOS transistor.

8. The dimming controlling circuit for LED according to claim 6 , further comprising a driving circuit, which is set at a controlling terminal of the switch transistor to drive the switch transistor.

9. The dimming controlling circuit for LED according to claim 1 , wherein the main circuit is also connected with a rectifying circuit.

10. The dimming controlling circuit for LED according to claim 9 , wherein the rectifying circuit is a rectifying bridge formed by four diodes.

11. The dimming controlling circuit for LED according to claim 9 , further comprising a filtering circuit arranged between the main circuit and the rectifying circuit.

12. The dimming controlling circuit for LED according to claim 11 , wherein the filtering circuit comprises a first inductor, a first capacitor and a second capacitor, in which the first capacitor and the second capacitor are connected in series, the first inductor is connected with the first capacitor and the second capacitor in parallel, and two terminals of the first inductor are connected with the rectifying circuit and the main circuit respectively.