Circuit and method for driving LED lamp with a dimmer

1. A driver circuit for an LED (light-emitting diode) lamp, comprising:

a transformer having a primary winding and a secondary winding;

a dimmer circuit coupled to a power source and the primary winding for varying an input voltage to the primary winding;

a power switch coupled to the primary winding for controlling a current flow in the primary winding;

an output rectifying circuit coupled to the secondary winding and the LED lamp for providing an output current to the LED lamp;

a controller coupled to the dimmer circuit to receive an average input voltage signal from the dimmer circuit, the controller being coupled to a control terminal of the power switch and configured to vary the output current according to the average input voltage signal.

2. The driver circuit of claim 1 wherein the controller is configured to generate a control signal characterized by a duty cycle that is determined by a ratio of a charging current to a discharging current coupled to a capacitor in the controller, the charging current and the discharging current being related to the average input voltage signal from the dimmer circuit.

3. The driver circuit of claim 1 wherein the control circuit is configured to provide a pulsed control signal based, at least in part, on the input voltage.

4. The driver circuit of claim 3 wherein the control circuit is configured to turn on the pulsed control signal based on the feedback signal and the input signal.

5. The driver circuit of claim 3 wherein the control circuit is configured to turn off the pulsed control signal based on the current sense signal and the input signal.

6. The driver circuit of claim 3 wherein the control circuit is configured to turn on the pulsed control signal based on the feedback signal and the input signal, and the control circuit is configured to turn off the pulsed control signal based on the current sense signal and the input signal.

7. The driver circuit of claim 1 , wherein the controller is configured to provide an output current Io characterized by the following equations:

Ipk

=

Vcs

Rcs

;

Ipks

=

N

*

Ipk

;

Io

=

Tons

2

*

Tsw

*

Ipks

=

N

*

Tons

*

Vcs

2

*

Tsw

*

Rcs

;

wherein

Ipk is the peak primary current,

Ipks is the peak secondary current,

Vcs is a reference voltage in the controller;

Rcs is a resistance of the peak current detection resistor,

N is the turn ratio between the primary winding and secondary winding, and

Tons is the conduction time of the secondary current.

8. A driver circuit for an LED (light-emitting diode) lamp, comprising:

a transformer having a primary winding and a secondary winding;

a dimmer circuit coupled to a power source for varying an input voltage to the primary winding;

an output rectifying circuit coupled to the secondary winding for providing an output current to the LED lamp;

a controller coupled to the dimmer circuit to receive an average input voltage signal from the dimmer circuit, the controller being configured to vary the output current according to the average input voltage signal;

wherein the controller includes a current control module, the current control module comprising:

a voltage follower for receiving the DIM signal;

a substractor coupled to the voltage follower;

a first current mirror coupled to the substractor,

a current selector configured to receive a current from the first current mirror;

a second current mirror configured to receive a first reference voltage and to provide three reference currents, two of the reference currents being coupled to the current selector, wherein the current selector is configured to provide two output signals, a first output signal coupled to a peak current comparator, a second output signal coupled to a constant-current control circuit for adjusting a ratio of charging and discharging currents.

9. The driver circuit of claim 8 wherein the substractor comprises:

a first resistor,

a second resistor,

a third resistor, and

an operational amplifier, the substractor configured to produce a Vdim_in signal cauterized by:

V

⁢

⁢

dim_in

=

VREF

⁢

⁢

2

*

(

1

+

R

⁢

⁢

305

R

⁢

⁢

304

)

-

R

⁢

⁢

305

R

⁢

⁢

304

*

V

⁢

⁢

dim

wherein Vdim is the voltage at the DIM terminal.

10. The driver circuit of claim 8 wherein the maximum current selectorcomprises three current mirrors and is configured received three input currents I 1 , I 2 , and I 3 , with I 2 =I 3 , wherein:

If I 1 >I 2 , then the output current=I 2 , and

If I 1 <I 2 , then the output current=I 1 .

11. The driver circuit of claim 8 wherein, when the DIM signal is greater than a reference signal, the output current of the maximum current selector is set to zero.

12. A driver circuit for an LED (light-emitting diode) lamp, comprising:

a transformer having a primary winding and a secondary winding;

a dimmer circuit coupled to a power source for varying an input voltage to the primary winding;

an output rectifying circuit coupled to the secondary winding for providing an output current to the LED lamp;

a controller coupled to the dimmer circuit to receive an average input voltage signal from the dimmer circuit, the controller being configured to vary a duty cycle of a control signal to vary the output current according to the average input voltage signal.

13. The driver circuit of claim 12 wherein the controller is configured to vary the control signal characterized by a duty cycle that is determined by a ratio of a charging current to a discharging current coupled to a capacitor in the controller, the charging current and the discharging current being related to the average input voltage signal from the dimmer circuit.