Electrical load driving circuit
Circuits and methods for driving electrical loads, where each is driven according to a desired current. A circuit comprising a switch mode converter comprising a transformer with primary and secondary windings, the primary connected to a voltage supply via one or more input control switches; output circuits, each comprising a switch connecting a load to an output of the secondary, each load series connected with a respective switch and in parallel with a capacitor; and a switching control circuit for control of each of the output circuit switches and for sensing a current through the loads. The switching control circuit operates the output circuit switches to maintain set current through the loads, the switching control circuit configured in successive output cycles of the switch mode converter to operate each output circuit switch in an order dependent on a forward voltage of each of the respective loads.
1. A circuit for driving a plurality of electrical loads, comprising:
a switch mode converter comprising a transformer with primary and secondary windings, the primary winding connected to a voltage supply via at least one input control switch;
a plurality of output circuits, each output circuit comprising an electrical load connected to an output of the secondary winding by a respective output circuit switch and connected in parallel with a capacitor; and
a switching control circuit connected for control of each of the output circuit switches and for sensing of a current through each of the electrical loads,
wherein the switching control circuit is configured to operate the output circuit switches to maintain a set current through each of the electrical loads, the switching control circuit configured in successive output cycles of the switch mode converter to operate each of the output circuit switches in an order dependent on a forward voltage of each of the respective electrical loads.
2. The circuit of claim 1 wherein the switching control circuit is configured to operate a first one of the switches connected to a first one of the electrical loads having a highest forward voltage before a second one of the switches connected to a second one of the electrical loads having a next highest forward voltage.
3. The circuit of claim 2 wherein the switching control circuit is configured to operate a third and any subsequent ones of the switches connected to a third and any subsequent ones of the electrical loads having successively lower forward voltages in successive order after operation of the second one of the switches.
4. The circuit of claim 1 wherein each electrical load comprises at least one light emitting diode.
5. The circuit of claim 4 wherein each electrical load comprises a plurality of series-connected light emitting diodes.
6. The circuit of claim 1 wherein each switch is connected to a respective output of the secondary winding by an inductor.
7. The circuit of claim 1 , wherein each electrical load is connected in series with a current sensing resistor, the switching control circuit being configured to measure a voltage across each current sensing resistor to determine a current through each respective electrical load.
8. The circuit of claim 1 wherein the switching control circuit is configured to determine the order of forward voltages of each of the respective electrical loads by measuring the current through each electrical load after closing the switches.
9. The circuit of claim 1 , wherein the switching control circuit is configured to store the order of forward voltages of each of the respective electrical loads.
10. The circuit of claim 1 wherein the plurality of output circuits is a first plurality of output circuits and the secondary winding is a first secondary winding, the circuit further comprising a second plurality of output circuits each comprising a switch connecting a further electrical load to an output of a second secondary winding of the transformer, each of the further electrical loads being connected in series with a respective switch and in parallel with a capacitor,
the switching control circuit being further connected for control of each of the second plurality of output circuit switches and for sensing of a current through each of the further electrical loads, wherein the switching control circuit is configured to operate the second plurality of output circuit switches to maintain a set current through each of the further electrical loads to operate each of the second plurality of output circuit switches in successive output cycles of the switch mode converter in an order dependent on a forward voltage of each of the further electrical loads.
11. The circuit of claim 10 wherein the first and second secondary windings are configured to provide outputs to the respective first and second plurality of output circuits over separate portions of each output cycle of the switch mode converter.
12. The circuit of claim 1 wherein each of the plurality of output circuits comprises a modulation switch connected to a respective electrical load, the switching control circuit being configured to operate the modulation switches according to a pulse width modulation scheme for control of the current through each respective electrical load.
13. The circuit of claim 12 wherein the switching control circuit is configured to sense current through each of the electrical loads only when a respective modulation switch is closed.
14. The circuit of claim 12 wherein each output circuit comprises a current mirror and a reference current source connected to the electrical load, the modulation switch connected to activate the current mirror to control current through the electrical load.
15. A method of driving a plurality of electrical loads, the method comprising:
providing a switch mode converter comprising a transformer with primary and secondary windings, the primary winding connected to a voltage supply via at least one input control switch;
providing a plurality of output circuits, each output circuit comprising a switch connecting one of the plurality of electrical loads to an output of the secondary winding, each electrical load connected in series with a respective switch and in parallel with a capacitor; and
providing a switching control circuit connected for control of each of the output circuit switches and for sensing of a current through each of the electrical loads,
wherein the switching control circuit:
operates the output circuit switches to maintain a set current through each of the electrical loads; and
in successive output cycles of the switch mode converter, operates each of the output circuit switches in an order dependent on a forward voltage of each of the respective electrical loads.