LED driver circuit, and LED arrangement and a driving method
The invention provides a driver circuit for driving an LED arrangement which uses a switch mode power converter, for example a flyback ringing choke converter, which comprises a main switch (e.g. bipolar transistor) and a sub-circuit for generating a current for the control terminal of the main switch. The sub-circuit in some examples makes use of an auxiliary winding as a voltage supply, and further comprises a ramp circuit for generating a ramp voltage from the voltage supply and a voltage follower, such as a control transistor, connected between the voltage supply and the control input of the main switch. By ramping up the current of the main switch, the losses arising as a result of the current flowing to the control input of the main switch are reduced. One set of examples makes use of a flyback ringing choke converter, which enables low cost implementation and good efficiency. The driver is able to receive a wide range of input voltages, by ensuring that the power loss is kept low. In particular, by ramping up the control current of the main switch, the losses arising as a result of the current flowing are reduced.
1. A driver circuit for driving an LED arrangement, said driver circuit comprising a switch mode power converter, the switch mode power converter being with a current-driven main switch and a sub-circuit for driving the main switch, the sub-circuit comprising:
a supply adapted for providing a supply voltage;
a ramp circuit connected to the supply, wherein said ramp circuit comprises a first capacitor adapted to be charged by said supply voltage so as to be with a linearly increasing voltage;
a voltage follower connected to said first capacitor and adapted for providing a linearly increasing driving voltage that follows a linearly increasing voltage on said first capacitor as said first capacitor is being charged by said supply voltage; and
a voltage to current conversion unit between said voltage follower and a control terminal of the main switch, for providing the control terminal of the main switch with a linearly increasing driving current that depends on the linearly increasing driving voltage as said capacitor is being charged by said supply voltage.
2. A driver circuit for driving an LED arrangement according to claim 1 , wherein the driver circuit comprises a flyback ringing choke converter switch mode power converter, which switch mode power converter comprises:
the main switch which comprises a main transistor;
a primary winding in series with a collector and an emitter of the main transistor;
a secondary winding which forms an inductor of a flyback output stage;
and wherein the sub-circuit for driving the main switch is adapted to generate a base current for the main transistor.
3. A driver circuit for driving an LED arrangement according to claim 2 , wherein the base drive circuit comprises:
the ramp circuit for generating a ramp voltage from the auxiliary winding voltage; and
the voltage follower, which comprises a control transistor connected with its collector and emitter between the auxiliary winding and the base of the main transistor, with the ramp voltage applied to the base of the control transistor.
4. A driver circuit as claimed in claim 3 , wherein the ramp circuit comprises a series first resistor and the first capacitor, with the first resistor connected at a first terminal to the auxiliary winding and at a second terminal to a first terminal of the first capacitor, wherein the second terminal of the first resistor is connected to the base of the control transistor.
5. A driver circuit as claimed in claim 4 , wherein the second terminal of the first capacitor is connected through a second resistor to ground.
6. A driver circuit as claimed in claim 4 , wherein the second terminal of the first capacitor is connected through a first diode to ground.
7. A driver circuit as claimed in claim 3 , wherein the voltage to current conversion unit comprises a base resistor in series between the emitter of the control transistor and the base of the main transistor.
8. A circuit as claimed in claim 7 , further comprising a second diode in parallel with the series combination of the control transistor and the base resistor.
9. A driver circuit as claimed in claim 3 , further comprising a start-up resistor connected to the base of the main transistor.
10. A driver circuit as claimed in claim 3 , further comprising a base diode in series in forward bias direction from the emitter of the control transistor to the base of the main transistor.
11. A driver circuit as claimed in claim 10 , further comprising a base capacitor (C 1 ) in parallel with the base diode (D 5 ).
12. A driver circuit as claimed in claim 3 , wherein the first capacitor is in parallel with a Zener diode.
13. A LED circuit comprising:
a driver circuit as claimed in claim 1 ; and
an LED arrangement connected to the driver circuit.
14. A method of driving an LED arrangement, comprising:
operating a current-driven main switch of a switch mode power converter, using a start-up resistor between a DC power line and the control input of the main switch and using a sub-circuit to generate a current for the control input of the main switch,
wherein the method comprises, within the sub-circuit, generating a ramp voltage from a supply voltage using a ramp circuit which comprises a first capacitor adapted to be charged by the supply voltage so as to be with a linearly increasing voltage;
applying the ramp voltage to a voltage follower to provide a linearly increasing driving voltage that follows a linearly increasing voltage on said first capacitor as said capacitor is being charged by the supply voltage; and
converting the voltage to current thereby providing the control terminal of the main switch with a linearly increasing driving current that depends on the linearly increasing driving voltage as said capacitor is being charged by said supply voltage.
15. A method as claimed in claim 14 , comprising operating a flyback ringing choke converter switch mode power converter, wherein the supply voltage is provided by an auxiliary winding.