Circuit and method for eliminating power-off flash for LED drivers

Abstract

An LED driver circuit and a method prevent LED turn-off flash when input power is lost to the driver circuit. The driver circuit includes a DC-DC converter that provides an LED drive voltage to an LED load. A voltage drop sensing circuit detects the loss of input power and discharges a filter capacitor that provides operating power to a controller in a DC-DC converter. The controller turns off to halt the operation of the DC-DC converter before the voltage provided to the LED load decreases to a turn-off threshold of the LED load. The DC-DC converter cannot recharge a load capacitor across the LED load. Thus, once the LEDs in the LED load turn off, the LEDs remain off until the input power is restored.

Claims

1. A drive circuit for providing a DC voltage to a plurality of light-emitting diodes (LEDs), comprising: a rectifier configured to convert an applied AC voltage to a rectified DC voltage; a passive voltage circuit configured to receive the rectified DC voltage and produce a first charging voltage; a power factor correction circuit having an input configured to receive the rectified DC voltage and having an output configured to provide a rail DC voltage; a switching DC-DC converter configured to receive the rail DC voltage and convert the rail DC voltage to an LED drive voltage and to a second charging voltage, the DC-DC converter including a controller, at least first and second semiconductor switches, and a resonant tank circuit, the semiconductor switches selectively switched by the controller to produce a switched DC voltage, the resonant tank circuit responsive to the switched DC voltage to produce the LED drive voltage, the controller having a power input terminal, the controller operable to switch the semiconductor switches only when a voltage on the power input terminal is at least as great as a controller threshold voltage; a filter capacitor coupled to provide a controller supply voltage to the power input terminal of the controller, the filter capacitor configured to receive the first charging voltage when the applied AC voltage is initially applied to the rectifier, the first charging voltage charging the capacitor to the controller threshold voltage, the capacitor receiving the second charging voltage when the controller is operable after the capacitor charges to the controller threshold voltage; and a voltage drop sensing circuit coupled to receive the first charging voltage, the voltage drop sensing circuit configured to sense when the first charging voltage decreases upon loss of the applied AC voltage, the voltage drop sensing circuit responsive to the decreasing first charging voltage to discharge the filter capacitor below the controller threshold voltage to halt the operation of the controller and thereby cease producing the LED drive voltage.

2. The circuit of claim 1, wherein the passive voltage circuit comprises a power input resistor.

3. The circuit of claim 2, wherein the power input resistor in the passive voltage circuit includes a first terminal and a second terminal, the first terminal connected to the rectifier, the second terminal coupled to the filter capacitor and coupled to the voltage drop sensing circuit.

4. The circuit of claim 2, wherein: the power input resistor in the passive voltage circuit includes a first terminal and a second terminal, the first terminal connected to the rectifier, the second terminal connected to the voltage drop sensing circuit; and the passive voltage circuit further includes a Zener diode and a forward-biased diode connected in series between the second terminal of the power input resistor and the filter capacitor.

5. The circuit of claim 2, wherein: the power input resistor in the passive voltage circuit includes a first terminal and a second terminal, the first terminal connected to the rectifier, the second terminal further connected to the voltage drop sensing circuit; and the passive voltage circuit further includes a forward-biased diode and resistor connected in series between the second terminal of the power input resistor and the filter capacitor.

6. The circuit of claim 1, wherein the voltage drop sensing circuit comprises a discharge resistor and a discharge transistor, the discharge resistor and the discharge transistor connected in series across the filter capacitor, the discharge transistor responsive to the decreasing first charging voltage to turn on the discharge transistor and to discharge the filter capacitor via the discharge resistor.

7. The circuit of claim 1, wherein the voltage drop sensing circuit further comprises a voltage sensing capacitor connected to the control terminal of the discharge transistor, the voltage sensing capacitor having a capacitance less than the capacitance of the filter capacitor, the voltage sensing capacitor discharging faster than the filter capacitor upon loss of the applied AC voltage to turn on the discharge transistor and increase the discharge rate of the filter capacitor.

8. The circuit of claim 1, further including a capacitor coupled to the output of the power factor correction circuit, the capacitor configured to maintain the DC rail voltage on the output of the power factor correction circuit at a slowly decreasing level for a selected time after the loss of the applied AC voltage to enable the DC-DC converter to continue generating the LED drive voltage, the LED drive voltage decreasing in response to the decreasing level of the DC rail voltage, the voltage drop sensing circuit operable to halt the operation of the controller before the LED drive voltage decreases to a threshold voltage for operating the plurality of LEDs.

9. A drive circuit for providing a DC voltage to a plurality of light-emitting diodes (LEDs) in response to an applied input voltage, comprising: a first charging voltage circuit responsive to the applied input voltage to generate a first charging voltage; a rail voltage circuit responsive to the applied input voltage to generate a rail voltage; a switching DC-DC converter responsive to the rail DC voltage to generate an LED drive voltage and a second charging voltage, the DC-DC converter including a controller having a power input terminal, the DC-DC converter operable only when a voltage on the power input terminal of the controller is at least as great as a controller threshold voltage; a filter capacitor coupled to provide a controller supply voltage to the power input terminal of the controller, the filter capacitor receiving the first charging voltage when the applied input voltage is active, the first charging voltage charging the filter capacitor to the controller threshold voltage, the filter capacitor receiving the second charging voltage when the controller is operable after the filter capacitor charges to the controller threshold voltage; and a voltage drop sensing circuit coupled to receive the first charging voltage, the voltage drop sensing circuit sensing when the first charging voltage decreases upon loss of the applied input voltage, the voltage drop sensing circuit responsive to the decreasing first charging voltage to discharge the filter capacitor below the controller threshold voltage to halt the operation of the controller and thereby cease producing the LED drive voltage.

10. The circuit of claim 9, further including a capacitor connected to the rail voltage circuit, the capacitor maintaining the DC rail voltage at a slowly decreasing level for a selected time after the loss of the applied input voltage to enable the DC-DC converter to continue generating the LED drive voltage, the LED drive voltage decreasing in response to the decreasing level of the DC rail voltage, the voltage drop sensing circuit operable to halt the operation of the DC-DC converter before the LED drive voltage decreases to a threshold voltage for operating the plurality of LEDs.

11. A method for preventing power-off flash in a light-emitting diode (LED) drive circuit, comprising: generating a switched DC voltage from an applied input voltage with a switching DC-DC converter, the switching DC-DC converter controlled by a switching controller having a power input terminal; generating an LED drive voltage from the switched DC voltage generating a first capacitor charging voltage responsive to the applied input voltage; generating a second capacitor charging voltage responsive to the switched DC voltage; applying the first capacitor charging voltage and the second capacitor charging voltage to a controller power input capacitor to charge the controller power input capacitor and provide a DC supply voltage to the switching controller; and sensing a loss of the applied input voltage and discharging the controller input capacitor to disable the switching controller before the LED drive voltage decreases to a voltage level below an operational threshold voltage of the plurality of LEDs.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 is a circuit diagram showing an electronic LED drive circuit as conventionally known in the art.

(2) FIG. 2 illustrates waveforms of a rail voltage V.sub.RAIL of FIG. 1, the voltage V.sub.LED across the LED load connected to the LED drive circuit of FIG. 1, and the current I.sub.LED through the LED load connected to the LED drive circuit of FIG. 1.

(3) FIG. 3 illustrates a circuit diagram showing an LED drive circuit with a voltage drop sensing circuit that prevents power-off flash of the LEDs driven by the drive circuit.

(4) FIG. 4 illustrates waveforms of the rail voltage V.sub.RAIL, the LED load voltage V.sub.LED, the LED load current I.sub.LED, the sensed voltage V.sub.SENSE, and the V.sub.CC voltage in the LED drive circuit of FIG. 3.

(5) FIG. 5 illustrates a circuit diagram showing an LED drive circuit similar to the LED drive circuit of FIG. 3 but with a modified voltage drop sensing circuit.

DETAILED DESCRIPTION OF THE INVENTION

(6) An exemplary solution to the problem disclosed in FIGS. 1 and 2 is illustrated by the improved LED driver circuit 300 in FIG. 3. In FIG. 3, elements corresponding to elements in FIG. 1 are identified with corresponding element numbers and are not described in detail below. The power tank circuit 190 and the charge pump circuit 260 are represented as blocks. The components within the two circuits are illustrated in FIG. 1 and are described above.

(7) Unlike the driver circuit 100 in FIG. 1, the driver circuit 300 in FIG. 3 does not connect the power input resistor 280 directly to the V.sub.CC node 272 and the power input terminal of the controller 180. Instead, the power input resistor is connected to a voltage sensing node 310, which has a sense voltage (V.sub.SENSE) thereon.

(8) The voltage sensing node 310 is also connected to the cathode of a Zener diode 312. In the illustrated embodiment, the Zener diode has a Zener voltage of approximately 10 volts. The anode of the Zener diode is connected to the anode of an isolation diode 314. The cathode of the isolation diode is connected to the V.sub.CC node 272. Thus, the passive voltage source in FIG. 3 includes three passive components: the power input resistor; the Zener diode; and the isolation diode. The V.sub.CC node is connected to the power input terminal (V.sub.CC) of the controller. The V.sub.CC node is also connected to the first terminal of the V.sub.CC filter capacitor 270 and to the output of the charge pump 260 as described above in connection with FIG. 1.

(9) Unlike the previously described driver circuit 100, the LED driver circuit 300 in FIG. 3 further includes a voltage drop sensing circuit 320. The voltage drop sensing circuit has an input terminal 322 connected to the voltage sensing (V.sub.SENSE) node 310 and has an output terminal connected to the V.sub.CC node 272. The structure and the operation of the voltage drop sensing circuit are described in more detail below.

(10) During power up and during normal operation, the LED driver circuit 300 in FIG. 3 operates in a similar manner to the LED driver circuit 100 in FIG. 1. When power from the AC source 110 is initially applied to the driver circuit in FIG. 3, the voltage V.sub.BRIDGE on the V.sub.BRIDGE bus 144 is applied to the V.sub.CC node 272 via the power input resistor 280, the Zener diode 312 and the isolation diode 314. The Zener diode and the isolation diode cause the voltage at the V.sub.CC node provided from the V.sub.SENSE node 310 to be approximately 10.7 volts below the voltage V.sub.BRIDGE. The initial voltage on the V.sub.CC node is sufficient to charge the V.sub.CC filter capacitor 270 to a sufficient voltage level to cause the controller 180 to begin operating. Thus, the two switching elements 182, 184 begin switching to supply the AC voltage to the power tank circuit 190 and to the charge pump circuit 260 as described above. The charge pump provides current to further charge the V.sub.CC filter capacitor. When the V.sub.CC filter capacitor is fully charged, the voltage provided by the charge pump circuit is greater than the voltage provided by the power input resistor via the Zener diode and the isolation diode. Thus, the isolation diode is reverse-biased when the controller is operating. While the AC source is connected, the LED driver circuit of FIG. 3 operates to provide power to the LED load via the power tank circuit 190 as described above.

(11) The voltage drop sensing circuit 320 operates to prevent the LED flash problem described above. The voltage drop sensing circuit includes a discharge resistor 340, which has a first terminal connected to the V.sub.CC node 272 via the output terminal 324. The discharge resistor has second terminal connected to the emitter terminal of a discharge transistor 342, which is a PNP bipolar transistor in the illustrated embodiment. The collector of the discharge transistor is connected to the ground reference 146.

(12) The anode of a base clamping diode 344 is connected to the base of the discharge transistor 342. The cathode of the base clamping diode is connected to the emitter of the discharge transistor. The base clamping diode prevents the voltage on the base of the discharge transistor from exceeding the voltage on the emitter of the discharge transistor by more than one forward diode drop.

(13) The anode of a base diode 346 is also connected to the base of the discharge transistor 342. The cathode of the base diode is connected to a first terminal of a voltage sensing capacitor 350 and to the first terminal of a bleeder resistor 352. The commonly connected cathode and first terminals are connected to the input terminal 322 of the voltage drop sensing circuit 320 are thus connected to the V.sub.SENSE node 310. The respective second terminals of the voltage sensing capacitor and the bleeder resistor are connected to the ground reference.

(14) The voltage drop sensing circuit 320 does not affect the operation of the LED driver circuit 300 when the AC power is initially applied and while the LED driver circuit continues to operate with the AC power connected. When AC power is initially applied to the LED driver circuit, the voltage on the V.sub.BRIDGE bus 144 is applied to the voltage sensing node 310 via the power input resistor 280. Accordingly, the voltage is applied to the respective first terminals of the bleeder resistor 352 and the voltage sensing capacitor 350 via the input 332 of the voltage drop sensing circuit. The resistance of the bleeder resistor is substantially greater than the resistance of the power input resistor. Thus, substantially all of the V.sub.BRIDGE voltage is applied across the voltage sensing capacitor as the V.sub.SENSE voltage. The capacitance of the voltage sensing capacitor is relatively small compared to the capacitance of the V.sub.CC filter capacitor 270. Thus, the voltage sensing capacitor charges very quickly while the V.sub.CC filter capacitor charges slowly on initial power up such that the voltage on the voltage sensing capacitor is initially greater than the voltage on the V.sub.CC filter capacitor. The higher voltage on the voltage sensing capacitor prevents the emitter-base junction of the discharge transistor 342 from being forward biased. Thus, the discharge transistor remains off during initial power on of the LED driver circuit.

(15) After the LED driver circuit 300 is powered up, the voltage sensing capacitor 350 remains charged to the V.sub.SENSE voltage determined by the voltage divider formed by the power input resistor 280 and the bleeder resistor 352. The voltage is slightly less than the V.sub.BRIDGE voltage. The V.sub.CC filter capacitor 270 is charged to a voltage less than the V.sub.RAIL voltage, which is less than the V.sub.BRIDGE voltage. Accordingly, the emitter-base junction of the discharge transistor 342 remains reverse biased during normal operation.

(16) When the AC source 110 is disabled or is no longer connected to the inputs 122, 124 of the LED driver circuit 300, the voltage drop sensing circuit 320 operates to prevent the LED driver circuit from causing the LED flash described above. The operation of the voltage drop sensing circuit is illustrated by waveforms in FIG. 4. An upper waveform in FIG. 4 represents a timing diagram illustrating the reduction in a rail voltage V.sub.RAIL after the AC source to the LED drive circuit of FIG. 3 is disconnected. A second waveform in FIG. 4 represents a voltage V.sub.LED across the LED load connected to the LED drive circuit of FIG. 3. A third waveform in FIG. 4 represents a current I.sub.LED through the LED load connected to the LED drive circuit of FIG. 3. A fourth waveform in FIG. 4 represents the V.sub.SENSE voltage across the voltage sensing capacitor 350 and thus represents the voltage on the voltage sensing node 310. A fifth waveform in FIG. 4 represents the voltage on the V.sub.CC node 272 corresponding to the voltage across the V.sub.CC filter capacitor 270.

(17) The five waveforms in FIG. 4 represent the normal operation of the LED driver circuit 300 from a time t.sub.0 to a time t.sub.1 when the AC power from the AC source 110 continues to be applied to the inputs 122, 124 of the full-wave bridge rectifier 120. At the time t1, the AC power is disconnected or otherwise disabled such that the V.sub.RAIL voltage begins to decrease as the V.sub.RAIL filter capacitor 168 discharges. The decreasing V.sub.RAIL voltage causes corresponding decreases in the V.sub.LED voltage and the I.sub.LED current. As further shown in FIG. 4, the V.sub.CC voltage on the V.sub.CC node 272 initially remains substantially constant because the charge pump 260 continues to provide charging current to the V.sub.CC filter capacitor 270 as the controller 180 continues to operate despite the decreasing V.sub.RAIL voltage.

(18) If the V.sub.CC voltage across the V.sub.CC filter capacitor 270 were allowed to remain at the initial level as the V.sub.RAIL voltage decreases, the controller 180 would continue to switch the two switching elements 182, 184, and the LED flash would occur as before; however, in the embodiment of FIG. 3, the voltage drop sensing circuit 320 prevents the LED flash. When the AC source 110 is disconnected or otherwise disabled, the V.sub.BRIDGE voltage on the V.sub.BRIDGE voltage bus 144 decreases rapidly and no longer provides current through the power input resistor 280 to maintain the charge across the voltage sensing capacitor 350. The voltage sensing capacitor begins to discharge through the bleeder resistor 352, the power input resistor 280 and the bridge load resistor 150. The capacitance of the voltage sensing capacitor is much less than the capacitance of the V.sub.CC filter capacitor. Thus, the discharge rate of the voltage sensing capacitor is much greater than the discharge rate of the V.sub.CC filter capacitor as illustrated by the steep decrease in the V.sub.SENSE voltage in FIG. 4 between the time t.sub.1 and a time t.sub.a. As discussed above, the decreasing straight line actually represents a first portion of an exponential discharge.

(19) At the time t.sub.a, the V.sub.SENSE voltage on the voltage sensing capacitor 350 drops below the V.sub.CC voltage (e.g., by the total of a forward emitter-base drop and a forward diode drop) such that the discharge transistor 342 starts conducting and the emitter of the discharge transistor is pulled down to a voltage near the zero volts on the ground reference 146. The V.sub.SENSE voltage on the voltage sensing capacitor continues to exponentially discharge through the bleeder resistor 352, the power input resistor 280 and the bridge load resistor 150 as represented by a second straight line segment.

(20) When the discharge transistor conducts, the V.sub.CC filter capacitor 270 is discharged rapidly through the discharge resistor 340 as illustrated by a steep discharge portion of the V.sub.CC waveform in FIG. 4 between the time t.sub.a and a time t.sub.b. When the V.sub.CC filter capacitor discharges below the operating voltage threshold of the controller 180, the controller will no longer switch the two switching elements 182, 184 to produce an AC voltage on the common node 186. Thus, the power tank circuit 190 no longer provides a DC voltage to maintain the charge on the load capacitor 240. The V.sub.LED voltage on the load capacitor will continue to decrease as the voltage is discharged through the LED load 172.

(21) When the V.sub.LED voltage on the load capacitor 240 reaches the threshold voltage for the series-connected LEDs in the LED load 172 at a time t.sub.2, the LEDs will discontinue conducting, which causes the I.sub.LED current to quickly drop to zero. Although the load on the output of the power tank circuit 190 is reduced, the reduction in the load does not cause the voltage across the load capacitor to temporarily increase because the controller and the two switching elements are no longer operating drive to produce an AC voltage at the input to the power tank circuit. Accordingly, the charge pump circuit 260 is not able to replenish the charge on the load capacitor. As a result the V.sub.LED voltage continues to slowly discharge without producing a voltage spike to cause the LED flash described above.

(22) As described above, the LED drive voltage (V.sub.LED) drifts downward as the V.sub.RAIL filter capacitor 168 and the load capacitor 250 discharge in the embodiment of FIG. 3. The capacitance of the voltage sensing capacitor 350 and the resistance of the bleeder resistor 352 are selected such that the discharge transistor 342 is turned on well before the V.sub.LED voltage decreases to the threshold voltage of the LED load 172. The V.sub.CC filter capacitor 270 is discharged rapidly via the discharge resistor 340 such that the controller 180 is disabled while the V.sub.LED voltage is still above the threshold voltage of the LED load. Thus, when the V.sub.LED voltage reaches the threshold voltage and the LEDs in the LED load turn off, the disabled controller cannot cause switching of the two switching elements 182, 184 to increase the V.sub.LED voltage regardless of the voltage remaining on the V.sub.RAIL filter capacitor.

(23) FIG. 5 illustrates a second embodiment of an LED drive circuit 500, which is similar to the LED drive circuit 300 of FIG. 3. The LED drive circuit of FIG. 5 includes a modified voltage drop sensing circuit 510 having fewer components than the voltage drop sensing circuit 320 of FIG. 3. The LED drive circuit of FIG. 5 also has fewer components providing power to the controller 180. Other than as described below, the elements in FIG. 5 correspond to the elements in FIG. 3 and are numbered accordingly.

(24) The voltage drop sensing circuit 510 includes an input terminal 512 and an output terminal 514. The output terminal is connected to the V.sub.CC node 272 and thus is connected to the first terminal of the V.sub.CC filter capacitor 270 as previously described. The input terminal of the voltage drop sensing circuit of FIG. 5 is connected directly to the second terminal of the power input resistor 280 via the V.sub.SENSE node 310 as in FIG. 3.

(25) The LED drive circuit 500 does not include the Zener diode 312 and the isolation diode 314 shown in FIG. 3 to provide power to the V.sub.CC node 272 from the power input resistor 280. Rather, power from the power input resistor is provided to the V.sub.CC node via the base clamping diode 344 and the discharge resistor 340 in the voltage drop sensing circuit. Thus, the passive voltage source in FIG. 5 comprises the power input resistor, the base clamping diode and the discharge resistor.

(26) The discharge resistor 340 in the voltage drop sensing circuit 510 of FIG. 5 has the first terminal connected to the output terminal 514 and has the second terminal connected to the emitter of the discharge transistor 342 as previously described. The collector of the discharge transistor is connected to the ground reference 146. Unlike the discharge transistor in the previously described embodiment, the base of the discharge transistor in FIG. 5 is connected directly to the input terminal 512 and to the first terminal of the voltage sensing capacitor 350. The second terminal of the voltage sensing capacitor is connected to the ground reference. The bleeder resistor 352 (FIG. 3) is not included in the embodiment of FIG. 5 to reduce parts count and to reduce power dissipation.

(27) In the voltage drop sensing circuit 510 of FIG. 5, the base clamping diode 344 has the anode connected to the base of the discharge transistor 342 and has the cathode connected to the emitter of the discharge transistor as in the embodiment of FIG. 3. The anode of the base clamping diode in FIG. 5 is connected directly to the input terminal 512 of the voltage drop sensing circuit.

(28) In the embodiment of FIG. 5, the base clamping diode 344 is in the supply path to the V.sub.CC filter capacitor 270 when the LED drive circuit 500 is initially powered on. Current flows from the V.sub.BRIDGE bus 144 through the power input resistor 280 to the V.sub.SENSE node 310. Current is conducted from the V.sub.SENSE node through the base clamping diode and through the discharge resistor 340 to the V.sub.CC node 272 to charge the V.sub.CC filter capacitor 270. The V.sub.CC filter capacitor is charged via the charging path until the voltage on the V.sub.CC node reaches the threshold voltage for operation of the controller 180. After the controller starts to operate to switch the two switching elements 182, 184, power is provided to the V.sub.CC node via the charge pump circuit 260 to maintain the charge on the V.sub.CC filter capacitor as described above.

(29) The voltage drop sensing circuit 510 of FIG. 5 operates in a similar manner to the previously described voltage drop sensing circuit 310 of FIG. 3. The voltage sensing capacitor 350 remains charged while the V.sub.BRIDGE voltage on the V.sub.BRIDGE bus 144 is maintained at a high voltage level by the rectified output of the full-wave bridge rectifier 120. When the AC source 110 is disconnected or otherwise disabled, the V.sub.BRIDGE voltage drops rapidly as described above. The V.sub.SENSE voltage on the V.sub.SENSE node 310 is initially maintained by the voltage sensing capacitor 350; however, the voltage sensing capacitor begins to discharge via the power input resistor 280 and the bridge load resistor 150. The base clamping diode 344 is reverse-biased, which precludes the V.sub.CC node 272 from providing current to maintain the charge on the voltage sensing capacitor. Accordingly, the voltage on the base of the discharge transistor 244 drops to cause the emitter-base junction of the discharge transistor to become forward biased. The discharge transistor conducts to start discharging the V.sub.CC filter capacitor 270 via the discharge resistor 340. Thus, the voltage on the V.sub.CC node drops rapidly while the voltage on the V.sub.RAIL bus 166 remains at a relatively higher voltage and decreases at a slower rate.

(30) When the voltage on the V.sub.CC node 272 drops below the operational threshold voltage of the controller 180, the controller ceases operation and no longer switches the two switching elements 182, 184 to produce the AC voltage on the common node 186. The power tank circuit 190 ceases operation, and the V.sub.LED voltage on the rectifier output node 236 continues to drop as the load capacitor 240 discharges through the LED load 172. Since the controller remains off, the V.sub.LED voltage does not spike when the LEDs within the LED load no longer conduct.

(31) The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of the present invention of a new and useful “Circuit and Method for Eliminating Power-Off Flash for LED Drivers,” it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.