Light-emitting diode lighting system with automatic bleeder current control
An LED lighting system includes a luminescent unit driven by a rectified AC voltage, a dimmer switch configured to adjusting a duty cycle of a system current, and a bleeder circuit. The bleeder circuit includes a first current source, a second current source, a third current source, a current-sensing element for providing a first feedback voltage associated with the system current, a capacitor, and a control unit. The control unit is configured to activate the first current source and deactivate the second current source for charging the capacitor when the system current exceeds a predetermined threshold, deactivate the first current source and activate the second current source for discharging the capacitor when the system current does not exceed the predetermined threshold, and deactivate the third current source to stop supplying the bleeder current according to a second feedback voltage established across the capacitor.
1. A light-emitting diode (LED) lighting system, comprising:
a luminescent unit driven by a rectified alternative-current (AC) voltage; and
a bleeder circuit comprising:
a first current source configured to provide a charging current;
a second current source configured to provide a discharging current;
a third current source configured to provide a bleeder current;
a current-sensing element for providing a first feedback voltage associated with a level of the system current;
a capacitor; and
a control unit configured to:
activate the first current source and deactivate the second current source for charging the capacitor when the system current exceeds a predetermined threshold according to the first feedback voltage;
deactivate the first current source and activate the second current source for discharging the capacitor when the system current does not exceed the predetermined threshold according to the first feedback voltage; and
deactivate the third current source to stop supplying the bleeder current according to a second feedback voltage established across the capacitor.
2. The LED lighting device of claim 1 , wherein the control unit is further configured to:
stop supplying the bleeder current when the second feedback voltage exceeds an upper threshold voltage; and
clamp the second feedback voltage at an upper limit voltage larger than the upper threshold voltage.
3. The LED lighting device of claim 1 , wherein the control unit is further configured to activate the third current source for supplying the bleeder current when the system current is lower than a minimum holding current of a dimmer switch.
4. The LED lighting device of claim 1 , wherein: a duty cycle of the system current is equal to a value D 1 when a dimmer switch is not in function; and the dimmer switch is further configured to adjust the duty cycle of the system current to a value D 2 according to a dimming input signal when in function; and D 2 is smaller than D 1 .
5. The LED lighting device of claim 4 , wherein:
the charging current is equal to a value I PD1 ;
the discharging current is equal to a value I PD2 ;
I PD1 *D 1 is larger than I PD2 *(1−D 1 ) when the dimmer switch is not in function;
I PD1 *D 2 is smaller than or equal to I PD2 *(1−D 2 ) when the dimmer switch is in function.
6. The LED lighting device of claim 1 , wherein a dimmer switch comprises a TRIAC (triode for alternative current) device configured to phase modulate the rectified AC voltage, thereby adjusting a duty cycle of the system current.
7. The LED lighting device of claim 1 , further comprising a dimmer switch configured to control an amount of light output by the luminescent unit by adjusting a duty cycle of a system current, wherein an operation of the dimmer switch is sustained by the bleeder current.