Bleeder circuit controller
A bleeder circuit controller ( 11 ) configured to control a bipolar junction transistor ( 10 ) having a collector ( 12 ) configured to be connected to an output of a phase cut dimmer ( 2 ) for receiving a bleed current, an emitter ( 13 ) for connecting to ground and a base ( 14 ), the bleeder circuit controller configured to generate a control signal for controlling the bleed current through the bipolar junction transistor ( 10 ) and measure a signal indicative of the current flow through the dimmer ( 2 ) and apply a current limit to a base-emitter current flow as a function of the measured signal.
1. A bleeder circuit controller configured to control a bipolar junction transistor having a collector configured to be connected to a rectified output of a phase cut dimmer to receive a bleed current, an emitter for connecting to ground, and a base, the bleeder circuit controller being configured to generate a control signal to control the bleed current through the bipolar junction transistor, to measure a signal indicative of the current flow through the dimmer, and to apply a current limit to a base-emitter current flow as a function of the measured signal,
wherein the function comprises a first region in which the base-emitter current limit increases with the measured signal between a first threshold and a second threshold.
2. The bleeder circuit controller of claim 1 , wherein the signal indicative of the current flow through the dimmer comprises at least one of:
a voltage at the base of the bipolar junction transistor;
a voltage at the emitter of the bipolar junction transistor;
a voltage at the collector of the bipolar junction transistor; and
a rectified phase cut mains voltage, the mains voltage comprising the voltage applied to an input of the phase cut dimmer.
3. The bleeder circuit controller of claim 1 , wherein the controller is configured to control the bipolar junction transistor by applying the control signal at the base for controlling the bleed current from the phase-cut dimmer through the bipolar junction transistor.
4. A bleeder circuit controller as defined in claim 3 , further comprising a voltage source that is configured to generate said control signal.
5. The bleeder circuit controller of claim 3 , wherein the controller is configured to control the current of the control signal as a function of the measured signal to provide said current limit.
6. The bleeder circuit controller of claim 5 , wherein the controller comprises a transistor configured to control the flow of current to the base of the bipolar junction transistor.
7. The bleeder circuit controller of claim 5 , wherein the controller is configured to provide a current limit control signal for controlling a controllable impedance connected to the emitter to control the flow of current from the controller to the base of the bipolar junction transistor.
8. The bleeder circuit controller of claim 1 , wherein the control signal generated by the controller is configured to control the bleed current to provide:
an off-state current during an off-state of the phase-cut dimmer;
a latching current required for the phase-cut dimmer to transition between an off-state and an on-state;
a holding current required to maintain the phase-cut dimmer in an on-state once it is in said on-state; and
a discharge current for backward phase cut dimmers to pull the dimmer output voltage low at the transition between an on-state and an off-state.
9. The bleeder circuit controller of claim 1 , wherein the controller comprises an error determination element configured to compare a voltage of the control signal with a measured voltage at the base and control the base-emitter current flow using said comparison.
10. The bleeder circuit controller of claim 1 , wherein the function further comprises a second region defining a maximum current in which the current limit is held constant when the measured signal exceeds the second threshold.
11. The bleeder circuit controller of claim 1 , wherein the function is configured to limit the base-emitter current such that the controller, when in use, drives the bipolar junction transistor in a linear operation mode substantially on an edge of a saturation mode.
12. The bleeder circuit controller of claim 1 , wherein the bipolar junction transistor is arranged to be cascaded with a metal oxide semiconductor (MOS) transistor, the collector of the bipolar junction transistor being connected to a source of MOS transistor, the drain of the MOS transistor being connected to a rectified mains signal, and the gate of the MOS transistor being connected to a fixed voltage source.