Retrofit lamp and a lighting system using the same

1. A retrofit lamp to be used with a ballast, comprising:

a pair of input terminals adapted to connect to said ballast;

an output load to be driven by said ballast and comprising at least a lighting element (LED);

a shunt switch (M 1 ) in parallel with said output load and adapted to shunt said input terminals using pulse width control so as to tune the current through the lighting element;

a detection circuit adapted to detect an abnormal drive condition of the retrofit lamp;

a control circuit adapted to override the pulse width control of the shunt switch and to hold the shunt switch at a stable state for a certain duration; and

a diode bridge rectifier (D 1 , D 2 , D 3 , D 4 ) between the input terminals and the output load, and the shunt switch (M 1 ) is placed between the diode bridge rectifier and the output load,

wherein said abnormal drive condition is a short circuit of at least one diode (D 1 , D 2 , D 3 , D 4 ) in said diode bridge rectifier, and

wherein said control circuit is adapted to hold the shunt switch at a low impedance state to override said pulse width control when the short circuit of at least one diode in said diode bridge rectifier is detected.

2. A retrofit lamp as claimed in claim 1 , said abnormal drive condition is an overcurrent condition, and the detection circuit comprises:

a current sensing element (R) adapted to sense the current through the shunt switch when said shunt switch is conductive; and

a comparing circuit (U 1 ) adapted to compare the sensed current with a threshold;

wherein the control circuit is adapted to hold the shunt switch at high impedance state to override said pulse width control when the sensed current exceeds the threshold.

3. A retrofit lamp as claimed in claim 2 , wherein the current sensing element comprises a current sense resistor (R) for sensing a current flowing to or from one of the input terminals, and said current sense resistor is in series between the diode bridge rectifier and an interconnection of the shunt switch (M 1 ) and the output load, and the control circuit is adapted to turn off said shunt switch.

4. A retrofit lamp as claimed in claim 3 , wherein the comparing circuit comprises a comparator (U 1 ) for comparing the voltage across the current sense resistor with a reference value (limit), and wherein the output of the comparator is used to control the shunt switch (M 1 ).

5. A retrofit lamp as claimed in claim 2 , wherein the control circuit comprises a control gate between the output of the comparing circuit and a control terminal of the shunt switch (M 1 ).

6. A retrofit lamp as claimed in claim 1 , wherein the output load further comprises a current limiting circuit in series with the lighting element.

7. A retrofit lamp as claimed in claim 6 , wherein the current limiting circuit is adapted to limit the current through the lighting element below a maximum value.

8. A retrofit lamp as claimed in claim 6 , wherein the current limiting circuit comprises a current source circuit (Q 1 , Q 2 , R 2 , R 3 ).

9. A retrofit lamp as claimed in claim 1 , wherein said detection circuit comprises a voltage detection circuit (R 3 , C 3 , Q 3 ) coupled at least to a first diode in said diode bridge rectifier and adapted to detect the short circuit according to a change of a voltage on said first diode, and said control circuit is adapted to turn on said shunt switch constantly in both positive and negative half cycles of an output of the ballast.

10. A retrofit lamp as claimed in claim 1 , wherein the lighting element comprises an LED array, and/or the retrofit lamp comprises a tubular LED lamp or a high bay lamp.

11. A retrofit lamp to be used with a ballast, comprising:

a pair of input terminals adapted to connect to said ballast;

an output load to be driven by said ballast and comprising at least a lighting element (LED);

a shunt switch (M 1 ) in parallel with said output load and adapted to shunt said input terminals using pulse width control so as to tune the current through the lighting element;

a detection circuit adapted to detect an abnormal drive condition of the retrofit lamp;

a control circuit adapted to override the pulse width control of the shunt switch and to hold the shunt switch at a stable state for a certain duration; and

A retrofit lamp as claimed in claim 1 , wherein said abnormal drive condition comprises an absence of a phase of an AC voltage input to the ballast, and

said control circuit is adapted to hold the shunt switch at high impedance state to override said pulse width control in synchronization with said absence of the phase of the AC voltage input into the ballast.

12. A retrofit lamp as claimed in claim 11 , wherein said control circuit is further adapted to:

control the shunt switch to turn on and off alternatively in at least one subsequent AC period after a recovery of the phase of the AC voltage input to the ballast so as to reduce the current amplitude provided to the output load; and

the control circuit is further adapted to:

switch a configuration of the output load to a high forward voltage state when holding the shunt switch at high impedance state in the abnormal drive condition of the retrofit lamp; and

switch the configuration of the output load to a low forward voltage state otherwise.

13. A retrofit lamp as claimed in claim 11 , wherein said control circuit comprises:

a current sensing component for sensing the current from the ballast;

a zero crossing detection component for detecting a zero crossing of the current from the ballast;

a dip detection component for detecting said absence of the AC voltage input to the ballast; and

a flip-flop component for outputting an logic to turn off said shunt switch upon a trigger of the detected zero crossing at the zero crossing detection component or the detected absence of the AC voltage input at the dip detection component.