High efficiency LED driving method for odd number of LED strings
View Patent ↗An arrangement wherein a plurality of LED strings are driven with a balanced drive signal, i.e. a drive signal wherein the positive side and negative side are of equal energy over time, is provided. In a preferred embodiment, the drive signal is balanced responsive to a capacitor provided between a switching network and a driving transformer. Balance of current between various LED strings is provided by a balancing transformer.
1. A driving arrangement for light emitting diode (LED) based luminaire comprising:
a driving transformer comprising a first winding and a second winding, the second winding magnetically coupled to the first winding;
a switching control circuit;
a switching bridge comprising a pair of electronically controlled switches coupled to a common node, each of the pair of electronically controlled switches responsive to an output of the switching control circuit;
a direct current (DC) blocking capacitor coupled between the common node of said switching bridge and a first end of the primary winding of the driving transformer;
a balancing transformer comprising a first winding and a second winding, the second winding magnetically coupled to the first winding;
a first LED string;
a second LED string; and
a third LED string,
a first end of said first LED string coupled to a first end of the second winding of said driving transformer, and arranged to receive electrical energy there from;
a first end of said second LED string coupled to a second end of the second winding of said driving transformer, and arranged to receive electrical energy there from;
a second end of said first LED string coupled to a first end of the first winding of said balancing transformer;
a second end of said second LED string coupled to a second end of the first winding of said balancing transformer; and
a first end of said third LED string coupled to the second winding of said balancing transformer,
said switching control circuit arranged to provide a switching cycle comprising a first period wherein electrical energy is output from the second winding of said driving transformer with a first polarity, and a second period wherein electrical energy is output from the second winding of said driving transformer with a second polarity, the second polarity opposite the first polarity,
said DC blocking capacitor arranged such that the total electrical energy output from the second winding of said driving transformer during the first period of the switching cycle is equal to the total electrical energy output from the second winding of said driving transformer during the second period of the switching cycle, and
said balancing transformer arranged such that the average currents through said first LED string, said second LED string and said third LED string are equal.
2. The driving arrangement according to claim 1 , wherein the number of turns of the second winding of said balancing transformer is twice the number of turns of the first winding of said balancing transformer.
3. The driving arrangement according to claim 1 , further comprising a fail detection circuit,
wherein said balancing transformer further comprises a third winding magnetically coupled to the first winding and second winding of said balancing transformer,
said fail detection circuit coupled to the third winding of said balancing transformer and arranged to detect one of an open circuit and a short circuit in one of said first, second and third LED strings.
4. The driving arrangement according to claim 3 , wherein said fail detection circuit comprises a current sensor, and
wherein said detection of one of an open circuit and a short circuit is responsive to said current sensor sensing a current greater than a predetermined value.
5. The driving arrangement according to claim 1 , wherein:
a first end of the second winding of said balancing transformer is further coupled to the first end of the second winding of said driving transformer;
a second end of the second winding of said balancing transformer is coupled to the second end of the second winding of said driving transformer; and
said third LED string is coupled between a center tap of said second winding of said balancing transformer and a center tap of said second winding of said driving transformer.
6. The driving arrangement according to claim 5 , wherein said coupling between the first end of said second winding of said driving transformer and the first end of the second winding of said balancing transformer is via a respective unidirectional electronic valve; and said coupling between the second end of said second winding of said driving transformer and the second end of the second winding of said balancing transformer is via a respective unidirectional electronic valve.
7. A driving arrangement for light emitting diode (LED) based luminaire comprising:
a means for driving having a first winding and a second winding, the second winding magnetically coupled to the first winding;
a means for switching;
a switching bridge comprising a pair of electronically controlled switches coupled to a common node, each of the pair of electronically controlled switches responsive to an output of the means for switching;
a direct current (DC) blocking capacitor coupled between the common node of said switching bridge and a first end of the primary winding of the means for driving;
a balancing transformer comprising a first winding and a second winding, the second winding magnetically coupled to the first winding;
a first LED string;
a second LED string; and
a third LED string,
a first end of said first LED string coupled to a first end of the second winding of said means for driving, and arranged to receive electrical energy there from;
a first end of said second LED string coupled to a second end of the second winding of said means for driving, and arranged to receive electrical energy there from;
a second end of said first LED string coupled to a first end of the first winding of said balancing transformer;
a second end of said second LED string coupled to a second end of the first winding of said balancing transformer; and
a first end of said third LED string coupled to the second winding of said balancing transformer,
said means for switching arranged to provide a switching cycle comprising a first period wherein electrical energy is output from the second winding of said driving transformer with a first polarity, and a second period wherein electrical energy is output from the second winding of said means for driving with a second polarity, the second polarity opposite the first polarity,
said DC blocking capacitor arranged such that the total electrical energy output from the second winding of said means for driving during the first period of the switching cycle is equal to the total electrical energy output from the second winding of said means for driving during the second period of the switching cycle, and
said balancing transformer arranged such that the average currents through said first LED string, said second LED string and said third LED string are equal.
8. The driving arrangement according to claim 7 , wherein the number of turns of the second winding of said balancing transformer is twice the number of turns of the first winding of said balancing transformer.
9. The driving arrangement according to claim 7 , further comprising a means for detecting a failure,
wherein said balancing transformer further comprises a third winding magnetically coupled to the first winding and second winding of said balancing transformer,
said means for detecting a failure circuit coupled to the third winding of said balancing transformer and arranged to detect one of an open circuit and a short circuit in one of said first, second and third LED strings.
10. The driving arrangement according to claim 9 , wherein said means for detecting a failure comprises a means for sensing a current, and
wherein said detection of one of an open circuit and a short circuit is responsive to said means for sensing a current sensing a current greater than a predetermined value.
11. The driving arrangement according to claim 7 , further comprising:
a first end of the second winding of said balancing transformer is further coupled to the first end of the second winding of said driving transformer;
a second end of the second winding of said balancing transformer is coupled to the second end of the second winding of said driving transformer; and
said third LED string is coupled between a center tap of said second winding of said balancing transformer and a center tap of said second winding of said driving transformer.
12. The driving arrangement according to claim 11 , wherein said coupling between the first end of said second winding of said driving transformer and the first end of the second winding of said balancing transformer is via a respective unidirectional electronic valve; and said coupling between the second end of said second winding of said driving transformer and the second end of the second winding of said balancing transformer is via a respective unidirectional electronic valve.