Overcurrent protection circuit and method for an LED driver
View Patent ↗A light fixture includes a ballast operable to drive a light source of the light fixture. The ballast includes a DC driver circuit and an output current limiting circuit. The output current limiting circuit is operable to minimize current overshoot in the event that an output of the ballast is shorted such as if the light source fails in a short circuit condition. The output current limiting circuit includes an output switch for limiting the current to the load (e.g., light source) and a control circuit. The control circuit senses the short circuit and clearing of the short circuit and controls the output switch accordingly to prevent excess output current to the shorted circuited load.
1. A current limiting circuit for a direct current (DC) driver circuit, wherein the DC driver circuit includes a positive lead and a ground lead and is operable to provide power to a load connected to first and second load terminals, the current limiting circuit comprising:
a positive input terminal configured to connect to the positive lead of the DC driver circuit;
a ground input terminal configured to connect to a ground lead of the DC driver circuit;
a positive output terminal configured to connect to the first load terminal;
a ground output terminal configured to connect to the second load terminal;
an output switch connected between the positive input terminal and the positive output terminal, the output switch comprising a control terminal and operable to provide a low resistance current path from the positive input terminal to the positive output terminal when closed and to substantially prevent current flow from the positive input terminal to the positive output terminal when open;
a control circuit connected to the control terminal of the output switch, wherein the control circuit is effective to
sense an output voltage between the positive output terminal and the ground output terminal;
close the output switch via the control terminal of the output switch when the output voltage exceeds a predetermined voltage limit; and
open the output switch via the control terminal of the output switch when the output voltage is less than or equal to the predetermined voltage limit.
2. The current limiting circuit of claim 1 , wherein the control circuit comprises a voltage divider configured to provide a test current from the positive input terminal to the positive output terminal, the voltage divider comprising:
a first resistor connected between the positive input terminal and the positive output terminal; and
a second resistor connected between the positive output terminal and the ground output terminal.
3. The current limiting circuit of claim 1 , wherein the ground input terminal and the ground output terminal are a single ground terminal configured to connect to the ground lead of the DC driver circuit and to the second load terminal.
4. The current limiting circuit of claim 1 , wherein the control circuit comprises a control switch connected between the control terminal of the output switch and the ground input terminal, the control switch having a control terminal connected to the positive output terminal.
5. The current limiting circuit of claim 1 , wherein:
the output switch comprises a PNP transistor having a base, a collector, and an emitter;
the control terminal of the output switch is the base of the PNP transistor;
the emitter of the PNP transistor is connected to the positive input terminal;
the collector of the PNP transistor is connected to the positive output terminal;
the control circuit comprises a control switch connected between the control terminal of the output switch and the ground input terminal, the control switch having a control terminal connected to the positive output terminal; and
the control circuit further comprises a current limiting resistor connected between the control switch and the base of the PNP transistor.
6. A ballast operable to provide power to a light source from a power source, the ballast comprising:
a direct current (DC) driver circuit operable to receive power from a power source and provide a predetermined amount of DC power to the light source, the DC driver circuit comprising
input terminals configured to connect to the power source,
a ground lead,
a positive lead configured to provide the predetermined amount of DC power in cooperation with the ground lead; and
an output current limiting circuit operable to limit an output current of the DC driver circuit, the output current limiting circuit comprising
a positive input terminal connected to the positive lead of the DC driver circuit,
a ground input terminal connected to the ground lead of the DC driver circuit,
a positive output terminal configured to connect to a first load terminal of the light source,
a ground output terminal configured to connect to a second load terminal of the light source,
an output switch connected between the positive input terminal and the positive output terminal, the output switch operable to provide a low resistance current path from the positive input terminal to the positive output terminal when closed and to substantially prevent current flow from the positive input terminal to the positive output terminal when open,
a control circuit connected to a control terminal of the output switch, wherein the control circuit is effective to
sense an output voltage between the positive output terminal and the ground output terminal;
close the output switch via the control terminal of the output switch when the output voltage exceeds a predetermined voltage limit; and
open the output switch via the control terminal of the output switch when the output voltage is less than or equal to the predetermined voltage limit.
7. The ballast of claim 6 , wherein the control circuit comprises a voltage divider configured to provide a test current from the positive input terminal to the positive output terminal, the voltage divider comprising
a first resistor connected between the positive input terminal and the positive output terminal of the output current limiting circuit, and
a second resistor connected between the positive output terminal and the ground output terminal of the output current limiting circuit.
8. The ballast of claim 6 , wherein the ground input terminal and the ground output terminal of the output current limiting circuit comprise a single ground terminal connected to the ground lead of the DC driver circuit.
9. The ballast of claim 6 , wherein the control circuit comprises a control switch connected between the control terminal of the output switch and the ground input terminal of the output current limiting circuit, the control switch having a control terminal connected to the positive output terminal of the output current limiting circuit.
10. The ballast of claim 6 , wherein:
the output switch of the output current limiting circuit is a PNP transistor having a base, a collector, and an emitter;
the control terminal of the output switch is the base of the PNP transistor;
the emitter of the PNP transistor is connected to the positive input terminal;
the collector of the PNP transistor is connected to the positive output terminal;
the control circuit of the output current limiting circuit comprises a control switch connected between the control terminal of the output switch and the ground input terminal, the control switch having a control terminal connected to the positive output terminal; and
the control circuit further comprises a current limiting resistor connected between the control switch and the base of the PNP transistor.
11. The ballast of claim 6 , wherein the power source is an AC power line and the DC driver circuit further comprises:
a rectifier operable to receive power from the AC power line via the input terminals of the DC driver circuit and provide DC power; and
a DC to DC converter connected to the rectifier, wherein the DC to DC converter is operable to receive the provided DC power from the rectifier and provide the predetermined amount of DC power to the output current limiting circuit via the positive lead and the ground lead of the DC driver circuit.
12. A light fixture operable to receive power from an alternating current (AC) power line and provide light, the light fixture comprising:
a housing;
a light source supported by the housing, the light source having a first load terminal and a second load terminal, wherein the light source is operable to provide light in response to receiving DC power;
a ballast supported by the housing, the ballast operable to provide power to the light source from the AC power line, wherein the ballast comprises
a direct current (DC) driver circuit operable to receive power from the AC power line and provide a predetermined amount of DC power to the light source, the DC driver circuit comprising
input terminals configured to connect to the AC power line;
a ground lead,
a positive lead configured to provide the predetermined amount of DC power in cooperation with the ground lead, and
an output current limiting circuit operable to limit an output current of the DC driver circuit, the output current limiting circuit comprising
a positive input terminal connected to the positive lead of the DC driver circuit,
a ground input terminal connected to the ground lead of the DC driver circuit,
a positive output terminal connected to the first load terminal of the light source,
a ground output terminal connected to the second load terminal of the light source,
an output switch connected between the positive input terminal and the positive output terminal, the output switch operable to provide a low resistance current path from the positive input terminal to the positive output terminal when closed and substantially prevent current flow from the positive input terminal to the positive output terminal when open,
a control circuit connected to a control terminal of the output switch, wherein the control circuit is operable to
sense an output voltage between the positive output terminal and the ground output terminal,
close the output switch via the control terminal of the output switch when the output voltage exceeds a predetermined voltage limit, and
open the output switch via the control terminal of the output switch when the output voltage is less than or equal to the predetermined voltage limit.
13. The light fixture of claim 12 , wherein the control circuit comprises a voltage divider configured to provide a test current from the positive input terminal to the positive output terminal, the voltage divider comprising:
a first resistor connected between the positive input terminal and the positive output terminal of the output current limiting circuit; and
a second resistor connected between the positive output terminal and the ground output terminal of the output current limiting circuit.
14. The light fixture of claim 12 , wherein the ground input terminal and the ground output terminal of the output current limiting circuit are a single ground terminal connected to the ground lead of the DC driver circuit.
15. The light fixture of claim 12 , wherein the control circuit comprises a control switch connected between the control terminal of the output switch and the ground input terminal of the output current limiting circuit, the control switch having a control terminal connected to the positive output terminal of the output current limiting circuit.
16. The light fixture of claim 12 , wherein:
the output switch of the output current limiting circuit is a PNP transistor having a base, a collector, and an emitter;
the control terminal of the output switch is the base of the PNP transistor;
the emitter of the PNP transistor is connected to the positive input terminal;
the collector of the PNP transistor is connected to the positive output terminal;
the control circuit of the output current limiting circuit comprises a control switch connected between the control terminal of the output switch and the ground input terminal, the control switch having a control terminal connected to the positive output terminal; and
the control circuit further comprises a current limiting resistor connected between the control switch and the base of the PNP transistor.
17. The light fixture of claim 12 , wherein the DC driver circuit further comprises:
a rectifier operable to receive power from the AC power line via the input terminals of the DC driver circuit and provide DC power; and
a DC to DC converter connected to the rectifier, wherein the DC to DC converter is operable to receive the provided DC power from the rectifier and provide the predetermined amount of DC power to the output current limiting circuit via the positive lead and the ground lead of the DC driver circuit.
18. The light fixture of claim 12 , wherein the light source comprises a plurality of series connected light emitting diodes.
19. The light fixture of claim 12 , wherein the output current limiting circuit further comprises a first resistor connected between the positive input terminal of the output current limiting circuit and the positive output of the output current limiting circuit, substantially in parallel with the output switch, wherein the first resistor is configured to provide a test current such that the control circuit can determine whether a short between the positive output terminal of the output current limiting circuit and the ground output terminal of the output current limiting circuit has been removed.