EP2427033B1

EP2427033B1 - Bypass circuitry for serially coupled light emitting diodes and associated methods of operation

Info

Publication number: EP2427033B1
Application number: EP11180344.1A
Authority: EP
Inventors: Eric Yang; Kaiwei Yao; Frank Xi; Zhengwei Zhang
Original assignee: Monolithic Power Systems Inc
Current assignee: Monolithic Power Systems Inc
Priority date: 2010-09-07
Filing date: 2011-09-07
Publication date: 2014-11-12
Anticipated expiration: 2031-09-07
Also published as: EP2427033A2; EP2427033A3

Description

This application claims the benefits of U.S. Provisional Application No. 61/380,646 filed on September 7, 2010 , U.S. Application No. US13/051,437 filed on March 18, 2011 , and CN Application No. 201010285957.7 filed on September 15, 2010 .
The present technology relates generally to bypass circuit, and particularly but not exclusively relates to bypass circuit configured to bypass an open circuited and/or otherwise defective light emitting diode (LED).
White LEDs (WLEDs) have gained significant applications in the display and general illumination market. One example is the WLED street lamp application. In another example, traditional cold cathode fluorescent lamp ("CCFL") backlighting is being replaced by LED backlight in the LCD TV market. In such applications, as shown in Figure 1, a large number of LEDs can be coupled in series as an LED string to provide a desired brightness. The LED string can be driven by a voltage supply as high as 200V. Multiple strings can be further configured to offer the desired backlight. The serially connected LEDs generally have a uniform current and less power consumption than other configurations. However, if any LED in a string is damaged and becomes open circuited, the whole string can be off.
A conventional solution is to bypass an open circuited LED by using a Zener diode. As shown in Figure 1, a Zener diode triggered snapback transistor ZD is placed in parallel with one of the serially coupled LEDs A. The Zener diode ZD can have a breakdown voltage higher than a normal forward voltage of the LEDs A. Thus, in normal operation, the Zener diodes ZD are open and do not consume any power. If an LED A in the string becomes open circuited, the supply voltage VSUP (a differential voltage between Sup+ and Sup-) builds up across the open LED A, and breaks down the corresponding Zener diode ZD to conduct. Once the Zener diode ZD conducts, it triggers a snapback and clamps the voltage VA across the open LED A at a clamping voltage of the Zener diode ZD.
However, the foregoing technique has several drawbacks. First, power consumption of Zener diodes is not low. For example, the snapback clamping voltage of Zener diodes is typically around 5V and has strong dependency on manufacturing processing, operating temperatures, and conduction current levels. Also when the failed LED is returned to normal operation and/or the corresponding Zener diode ZD has a temporary false trigger (e.g., by a spike in the power supply or a current spike during LED startup), the Zener diode ZD snapbacks and cannot recover unless the entire LED string is rebooted.
WO2009/138907 discloses a method comprising driving at least one lighting device and detecting a fail state. In order to use low voltage switches and to reduce oscillating, there is provided sensing a voltage parallel to the at least one lighting device, detecting a fail state in case of a sensed over-voltage, and driving a shunt switch to a closed state in case of a detected fail state.
US2009/0085489 discloses a light source driving circuit including a driving module, a plurality of first switches, a detecting unit, and a control unit. The plurality of first switches is respectively coupled to corresponding light sources and these light sources are driven by the driving module. The detecting unit is coupled to the driving module to detect a working parameter of each light source and transmit the working parameter to the control unit. When the detecting unit detects some of these light sources are broken down, the control unit sequentially turns on the first switches to find out the failed light source. Moreover, the first switch coupled to the failed light source is remained on, and the driving signal is regulated according to the number of the failed light source, such that the brightness of the panel does not decrease.
WO2010/034739 discloses a method for operating a series circuit of at least two LED's, wherein the method comprises the following steps in the order indicated: -supplying the series circuit with power from a current or voltage source of a power grid, -measuring the voltage drop across the entire series circuit, -turning off the power supply by a control unit when the measured voltage drop exceeds a previously established limit value, -recognizing the loss of at least one of the LED's by a control device of the power grid based on the reduced power draw, -recognizing and short-circuiting the lost LED by the control unit.
US2007/159750 discloses a fault detection mechanism for a LED string comprising a plurality of serially connected LEDs, the fault detection mechanism comprising: a control circuitry; and a voltage measuring means, in communication with the control circuitry, arranged to measure the voltage drop across at least one LED of the LED string, the control circuitry being operable to: measure the voltage drop, via the voltage measuring means, at a plurality of times, compare at least two of the measured voltage drops, and in the event the comparison of the at least two voltage drops is indicative of one of a short circuit LED and an open circuit LED, output a fault indicator.
The present invention provides a circuit as set out in claim 1 and a method as set out in claim 13.
In one embodiment, a circuit comprises a monitoring circuit and a switch. The monitoring circuit may be coupled to a target circuit and the monitoring circuit may be configured to monitor a differential voltage across the target circuit, to determine whether an open circuit condition exists based on the monitored differential voltage, and to generate an output signal selectively indicating the open circuit condition. And the switch may be coupled to the target circuit in parallel. The switch may have a control input coupled to the monitoring circuit to receive the output signal from the monitoring circuit, the switch may be configured to be selectively activated to bypass the target circuit in accordance with the output signal of the monitoring circuit.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows an LED string with a conventional open LED bypass circuit having parallel connected Zener diodes in accordance with the prior art.
Figure 2 is a schematic circuit diagram illustrating an open LED bypass circuit in accordance with an embodiment of the present technology.
Figure 3 is a schematic circuit diagram illustrating another open LED bypass circuit in accordance with an embodiment of the present technology.
Figure 4 illustrates waveforms of voltage versus time in the open LED bypass circuit of Figure 3 during one mode of operation.
Figure. 5 schematically illustrates an open LED bypass circuit further comprising a capacitor in accordance with an embodiment of the present invention.
Figure 6 shows simulated waveforms of the open LED bypass circuit of Figure 5 in accordance with an embodiment of the present invention.
Figure 7 schematically illustrates an open LED bypass circuit further comprising a latch, a pulse generator and a charge pump in accordance with an embodiment of the present invention.
Figure 8 shows example waveforms of the open LED bypass circuit of Figure 7 in accordance with an embodiment of the present invention.
Figure 9 is a block diagram illustrating a method of bypassing an open LED in a plurality of serially coupled LEDs in accordance with an embodiment of the present technology.

The use of the same reference label in different drawings indicates the same or like components.
Several embodiments of the present technology are described below with reference to bypass circuits for serially coupled LEDs and associated methods of operation. As used hereinafter, the term "LED" encompasses LEDs, laser diodes ("LDs"), polymer LEDs ("PLEDs"), and/or other suitable light emitting diodes. Many specific details that relate to certain embodiments are set forth in the following text to provide a thorough understanding of these embodiments. Several other embodiments can have configurations, components, and/or processes that are different from those described below. A person skilled in the relevant art, therefore, will appreciate that additional embodiments may be practiced without several of the details of the embodiments shown in Figures 2-9.
Figure 2 is a schematic circuit diagram illustrating an open LED bypass circuit 20 in accordance with an embodiment of the present technology. As shown in Figure 2, the bypass circuit 20 is coupled across an LED A to monitor the status of the LED A, and is configured to bypass the LED A when an open status of the LED A is detected. Even though only certain components are shown in Figure 2, in other embodiments, the bypass circuit 20 can also include switches, diodes, transistors, and/or other suitable components in addition to or in lieu of the components shown in Figure 2.
In certain embodiments, the LED A is serially connected to other LEDs (not shown) in a string of LEDs supplied by a power supply. Though only one LED A is shown in Figure 2 as a target circuit to be bypassed, in other embodiments, the target circuit may include any number of LEDs, electroluminescent devices, and/or other illumination devices configured as a single device, a string of devices, an array of devices, and/or other suitable arrangements. In other embodiments, the LED A may be connected to other LEDs in other suitable arrangements.
As shown in Figure 2, the bypass circuit 20 comprises a monitoring circuit 21 and a switch M. The monitoring circuit 21 monitors the status of LED A. In one embodiment, the monitoring circuit 21 monitors the status of LED A by monitoring the differential voltage V_LED+-V_LED- across the LED A. Thus input terminals of the monitoring circuit 21 are coupled to the anode LED+ of the LED A and to the cathode LED- of the LED A, respectively, to monitor the differential voltage V_LED+-V_LED- across the LED A. The term "couple" generally refers to multiple ways including a direct connection with an electrical conductor and an indirect connection through intermediate diodes, resistors, capacitors, and/or other intermediaries. Thus, the monitoring circuit is coupled to monitor the differential voltage generally and refers to monitoring the differential voltage across the target circuit either by a direct connection or by an indirect connection. In other embodiments, the monitoring circuit 21 can also monitor a current, a rate of change in voltage and/or current, and/or other suitable parameters for monitoring the status of LED A.
The bypass switch M is coupled to the LED A in parallel. The switch M has a control end coupled to the output of the monitoring circuit 21. Thus, when M is turned on by the monitoring circuit 21 once LED A is detected in an open circuit condition, LED A is bypassed with current flowing through the switch M, and the other LEDs (not shown) in a string continue to produce backlight. In one embodiment, the switch M is a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). The MOSFET can be either N type or P type. Other types of switches such as BJT (Bipolar Junction Transistor) or JFET (Junction Field Effect Transistor) can also be adopted as the bypass switch M. The on voltage drop V_ON of the switch M is substantially lower compared to the clamping voltage of a Zener diode, and thus power consumption accordingly is substantially lower. In one example, the switch M with a MOSFET can have an on voltage drop of about 50mV.
Continuing with Figure 2, when an LED A fails and/or is otherwise in an open status, the supply voltage supplying the entire LED string builds up on the open LED A, and its forward voltage V_A (V_LED+-V_LED-) rises. When this situation is detected by the monitoring circuit 21, the switch M is turned on to bypass the damaged LED A. In one example, the monitoring circuit 21 monitors and compares the forward voltage V_A to a threshold voltage. When V_A is higher than the threshold voltage, open circuit condition or open status of the LED A is indicated by the monitoring circuit 21 and the switch M is turned on. Thus, a current path forms through the bypass switch M, and the remaining LEDs in the LED string remain in normal operation.
During the open status of the LED A, the switch M is controlled by the output signal of the monitoring circuit 21 to be periodically deactivated (turned off) to check if the open LED heals back to its normal operation. If the LED A remains in open status, once the switch M is turned off, the forward voltage V_A rises again and exceeds the threshold voltage, and the switch M is turned on again and repeats this periodical function. When the LED A heals back to normal status, for example, the false triggering situation is eliminated or the failed LED is replaced with a new LED. Once the switch M is turned off, the forward voltage V_A is lower than the threshold voltage, the bypass switch M is kept off and the bypass circuit 20 will not interfere with the normal operation of the LED A.
Figure 3 shows an open LED bypass circuit 30 in accordance with an embodiment of the present technology. The bypass circuit 30 comprises a monitoring circuit 31, a bypass switch M, and a Zener diode ZD. The monitoring circuit 31 comprises a comparator U1 and a hold-on circuit 32. The non-inverting input of the comparator U1 is coupled to the anode of LED A, and the inverting input of the comparator U1 is coupled to a reference voltage V_REF. The reference voltage source of V_REF has its anode connected to the inverting input of the comparator U1 and has its cathode coupled to the cathode of the LED A. In this configuration, the comparator U1 is coupled across the two ends LED+ and LED- of the LED A to compare the forward voltage V_A to a reference voltage V_REF. In one embodiment, the reference voltage V_REF is generated by the bypass circuit 30. In another embodiment, V_REF is an external signal. Yet in another embodiment, the value of the reference voltage V_REF can be modulated.
Now referring to the hold-on circuit 32, the hold-on circuit 32 is coupled between the comparator U1 and the switch M. The input terminal of the hold-on circuit 32 is coupled to receive the output signal V_CMP of the comparator U1. The output terminal of the hold-on circuit 32 is coupled to the control end of the switch M with the output signal V_G. When V_A is higher than V_REF, the output signal V_CMP of the comparator U1 has a logic HIGH and the output signal V_G of the monitoring circuit 31 is triggered to a HIGH level, thus the switch M is turned on. The HIGH level of the V_G signal is maintained by the hold-on circuit 32 for a period of time. In another embodiment, the monitoring circuit 31 can keep the switch M on until the bypass circuit 30 restarts.
The bypass switch M is coupled in parallel to the LED A. In the embodiment shown in Figure 3, the switch M is an N type MOSFET. The drain of the switch M is coupled to the anode of the LED A, the source of M is coupled to the cathode of the LED A, and the gate of M is connected to the output terminal of the monitoring circuit 31. Thus, when signal V_G is HIGH, the switch M is turned on, and the LED A is bypassed with current flowing through the switch M, and the other LEDs in a string (not shown) continue to produce backlight. In one embodiment, the switch M is a LDMOS (Lateral Double-diffused MOSFET) integrated with the monitoring circuit 31 on a single semiconductor substrate. Though N type MOSFET is featured in this embodiment, P type MOSFET or other types of switches such as BJT (Bipolar Junction Transistor) can also be adopted as the bypass switch M.
In the illustrated embodiment in Figure 3, a Zener diode ZD is coupled in parallel with the target LED A, with its cathode coupled to the anode of LED A and its anode coupled to the cathode of LED A. The clamping voltage of ZD V_CP is higher than the normal forward voltage V_A0 of LED A. Thus during normal operation of the LED A, the Zener diode ZD does not interfere with the LED A. However, when the LED A fails, V_A will rise until the Zener diode ZD snapbacks and clamps the forward voltage V_A to its clamping voltage V_CP. The reference voltage V_REF is set higher than the normal operation forward voltage V_A0 of A, and is lower than the clamping voltage V_CP of the Zener diode ZD. In one example, the clamping voltage V_CP of the Zener diode ZD is about 7V, the forward voltage V_A0 of the LED A during normal operation is about 4V, and the reference voltage V_REF is about 5V. In other embodiments, the Zener diode ZD may be omitted.
The function of the bypass circuit 30 is described below with reference to Figure 4. As shown in Figure 4, the signal ST indicates the status of the LED A. LOW ST indicates that the LED A is in normal operation, and HIGH ST indicates the LED A has the open status or has false triggering. The second waveform shows the forward voltage V_A across the target LED A. The third waveform is the output signal V_CMP of the comparator U1. And the last waveform is the output signal V_G of the monitoring circuit 31 which drives the gate of the switch M.
Before time t0, the LED A operates in normal status (ST LOW) and the forward voltage V_A is at its normal level V_A0. The voltages of V_CMP and V_G remain in LOW level. The switch M is open. At time t0, the LED A fails and shifts from normal operation to open status (ST HIGH). The power supply voltage of the LED string builds up across the failed LED A, and the voltage V_A across the LED A rises up and is clamped by the Zener diode ZD at the voltage V_CP. After a short intrinsic delay time, the output signal V_CMP of the comparator U1 becomes HIGH and triggers the hold-on circuit 32 to produce a HIGH V_G signal at time t1. Thus the switch M is turned on. The delay time between t0 and t1 is an intrinsic parameter of the circuits, for example, because of the parasitic capacitance. Other conditions (e.g., a voltage spike) can also falsely trigger turning on the switch M.
Once the switch M is turned on, the forward voltage V_A drops to the low on voltage V_ON of the switch M. The hold-on circuit 32 holds the signal V_G in HIGH level for a predetermined time period of T. During this time, the voltage V_A is in low level of V_ON. After the holding on time period of T, at time t2, the hold-on circuit 32 puts out LOW V_G and the switch M is turned off. V_A rises up again and starts another cycle. In this way, the switch M is turned off periodically by the hold-on circuit 32 such that the open LED bypass circuit 30 periodically checks if the failed LED A is healed back to normal operation. If the LED A remains in open status, this operation will repeat by itself. At each cycle, switch M is turned off after a predetermined time of T, referring to time t2, t3, t4, t5 and t6.
During open status, the duty cycle of the signal V_G is determined by the intrinsic delay time (such as the time interval between t0 and t1) as LOW level and the predetermined pulse width of T as HIGH level. The intrinsic delay time can be very short. By setting the time period of T, the duty cycle of V_G signal during open status can be very high, which leads to a very low average voltage of V_A. The average voltage V_AVG of V_A during open status is: DV_ON+(1-D)V0, where D is the duty cycle of signal VG, V_ON is the on voltage of the switch M and V0 is the clamping voltage of the Zener diode ZD.
If healing condition is detected (ST LOW), the LED bypass circuit 30 turns off the bypass switch M to allow the healed LED A to operate normally. Referring to time t5, the LED A shifts to healing condition or false triggering situation is eliminated. Once the switch M is turned off at the falling edge of V_G at time t6, the forward voltage V_A rises up to its normal forward voltage V_A0. Since V_A0 is smaller than V_REF, the switch M stays in the off state. Thus, the normal operation of the LED A recovers and is not affected by the bypass circuit 30.
Figure 5 schematically illustrates an open LED bypass circuit 50 which adopts a capacitor C, in accordance with an embodiment of the present invention. Capacitor C has a first terminal 501 coupled to an anode (i.e., LED+) of the LED A and has a second terminal 502 coupled to a cathode (i.e., LED-) of the LED A. And monitoring circuit 51 monitors the status of LED A by sensing the voltage Vc across capacitor C. At the meantime, the output of monitoring circuit 51 is held on by capacitor C.
In the shown embodiment, a diode D is coupled between the anode of LED A and the first terminal 501 of capacitor C. If switch M is off and the forward voltage V_A of the LED A is higher than the voltage Vc across capacitor C, then capacitor C is charged by the forward voltage V_A. If switch M is turned on and forward voltage V_A is less than the capacitor voltage V_C, then capacitor C is discharged. In one embodiment, other devices having similar function as diode D may be adopted to replace diode D. The monitoring circuit 51 comprises a comparator U1 comparing the voltage Vc with a reference voltage VREF and configured to output a signal V_G indicating whether LED A is in open status. Comparator U1 may further comprise two power supply input terminals. The first power supply input P1 is coupled to the first terminal 501 of capacitor C and the second power supply input P2 is coupled to the second terminal 502 of capacitor C. In this configuration, capacitor C is discharged by a bias current between the first power supply input P1 and the second power supply input P2. And the monitoring circuit may be powered by the capacitor voltage Vc. When switch M is on, capacitor C is discharged through monitoring circuit 51 and Vc decreases gradually. Accordingly, Vc holds on a level higher than the reference voltage V_REF for a period of time and the output signal V_G indicating open status is held on also for the period of time.
Circuit 50 may comprise a Zener diode ZD coupled to LED A in parallel. In one embodiment, clamping voltage V_CP of the Zener diode ZD is substantially higher than normal forward voltage V_A0 of the LED A. Thus in normal status of the LED A, the Zener diode ZD does not interfere with the LED A. However, when the LED A fails, its forward voltage V_A rises until the Zener diode ZD snapbacks and clamps the forward voltage V_A to its clamping voltage V_CP. The threshold voltage V_REF is set higher than the normal forward voltage V_A0 of the LED A, and is set lower than the clamping voltage V_CP of the Zener diode ZD. In one example, the clamping voltage V_CP of the Zener diode ZD is about 7V, the normal status voltage V_A0 of the LED A is about 4V, and the threshold voltage V_REF is about 5V. In other embodiments without a Zener diode ZD, forward voltage V_A of the LED A rises to the supply voltage V_SUP of the LED string when the LED A fails in open circuit condition.
Switch M is coupled in parallel to LED A. The drain of switch M is coupled to the anode of the LED A, the source of switch M is coupled to the cathode of the LED A, and the gate of switch M is coupled to output 511 of monitoring circuit 51. Thus, when gate signal V_G is HIGH, switch M is turned on, the LED A is bypassed with current flowing through switch M, and the other LEDs in a string (not shown) continue to work and produce back light.
Figure 6 shows simulation waveforms of the open LED bypass circuit with reference to Figure 5 in accordance with an embodiment of the present invention. The first waveform signal ST indicates the status of LED A. LOW ST indicates that the LED A is in normal status, and HIGH ST indicates that the LED A is in open status or has false triggering. The second waveform shows capacitor voltage V_C. The third waveform is control signal V_G of switch M or the output signal of monitoring circuit 51. And the last waveform shows forward voltage V_A of the LED A. Average voltage V_AVG of the forward voltage V_A is also shown in the last waveform.
Before time T1, LED A operates in normal status (ST LOW) and forward voltage V_A of the LED A is at its normal level VA0 Capacitor voltage VC is VAO-VDROP, which is lower than threshold voltage V_REF. Comparator U1 compares capacitor voltage V_C with threshold voltage V_REF and outputs LOW CMP signal at output 511 indicating normal status of the LED A. Control signal V_G remains in LOW level and switch M kept off. At time T1, LED A fails and shifts to open status, (ST is HIGH). Power supply voltage of the LED string builds up across the failed LED A, then forward voltage V_A of the LED A rises and is clamped by the Zener diode ZD at clamping voltage V_CP. Capacitor voltage V_C is charged up to V_CP-V_DROP, which is higher than threshold voltage V_REF. Comparator U1 compares capacitor voltage V_C with threshold voltage V_REF and outputs HIGH CMP signal at output 511 indicating an open status after a short intrinsic delay time. Control signal V_G becomes HIGH accordingly and switch M is turned on to bypass the LED A. Other conditions such as a voltage spike can also falsely trigger turning on switch M.
Once switch M is turned on after time T1, forward voltage V_A of the LED A drops to the voltage drop V_ON of the switch M, e.g., 200mV. The diode D is under a reverse voltage and there is little or no current flowing from the first terminal 501 of capacitor C to the anode of the LED A. Capacitor C may be discharged by a bias current between the two power supply inputs of comparator U1. Capacitor voltage V_C is decreased slowly to hold control signal V_G HIGH for a period of time. At time T2, capacitor voltage V_C is decreased to be lower than the threshold voltage V_REF, then comparator U1 outputs LOW V_G signal and switch M is turned off. Once switch M is turned off, forward voltage V_A of LED A rises again and another cycle is started per the open status still exists. In this way, capacitor C is discharged and switch M is turned off periodically to check if the failed LED A is healed back to normal. If the LED A remains in open status, this operation will repeat by itself. Control signal V_G is periodically transition between HIGH and LOW, and forward voltage V_A of the LED A is periodically transite between the clamping voltage V_CP and voltage drop V_ON. The time period that positive control signal V_G lasts is increased when the capacitor C is discharged by a smaller current. As shown in Figure 6, when capacitor C is discharged far slower than it is charged, the duty cycle of control signal V_G is very high and the average bypass voltage V_AVG is very low. This may achieve a high efficiency for the whole bypass circuit 50.
If healing condition (normal status) is detected, i.e., ST is LOW, switch M is turned off to allow the healed LED A to operate normally. Referring to time T3, the LED A shifts to healing condition or false triggering situation is eliminated. When switch M is turned off at the falling edge of control signal V_G, forward voltage V_A of the LED A would rise to its normal forward voltage V_A0. Capacitor voltage V_C keeps less than threshold voltage V_REF and then switch M would remain in off state. Thus, the LED A recovers to normal status and is not affected by circuit 50.
Figure 7 schematically illustrates an open LED bypass circuit 70 further comprising a latch 721, a charge pump 722 and a pulse generator 723 in accordance with an embodiment of the present invention. Latch 721 comprises a set terminal (S), a reset terminal (R) and an output (Q). The set terminal of latch 721 is coupled to the output of the monitoring circuit 51 at node 701. The reset terminal of latch 721 is connected to the anode of LED A. The pulse generator 723 has input coupled to the output 702 of latch 721 and has an output 703 coupled to an input ENSW of charge pump 722. Charge pump 722 comprises the input ENSW, a first output VO1 connected to the control terminal of switch M at node 704 and a second output VO2 coupled to the first terminal 705 of capacitor C.
An activating signal (HIGH CMP) at the set terminal of latch 721 is used to produce HIGH output, i.e., Q= "1", and an activating signal at the reset terminal of latch 721 is used to produce LOW output, i.e., Q= "0". Output Q of latch 721 may change as soon as signal at the set terminal and/or at the reset terminal changes. The set terminal has higher priority than the reset terminal for latch 721, and the truth table is shown below.

S "1" "0" "1" "0"

R "0" "1" "1" "0"

Q "1" "0" "1" No change
When S= "1", then Q= "1"; when S= "0" and R= "1" then Q= "0"; otherwise there is no change on Q. As a result, latch 721 produce HIGH output Q when the output of the monitoring circuit 51 is HIGH, i.e., signal at output CMP of comparator U1 is HIGH. Latch 721 produce LOW output Q, when signal at output CMP is LOW and forward voltage V_A of the LED A is HIGH. Normal forward voltage V_A0 of the LED A is logic HIGH. Latch 721 has a first power supply input P5 coupled to the first terminal 705 of capacitor C and has a second power supply input P6 coupled to the second terminal 706 of capacitor C. Thus latch 721 is powered by capacitor C and capacitor C is discharged partially by a bias current between power supply inputs P5 and P6. In other embodiments, latch 721 may be powered by other source such as external voltage source.
In the example of Figure 7, charge pump 722 is enabled to output power at output VO1 and switch M is turned on when with an activating signal at input ENSW. Charge pump 722 is disabled and switch M is turned off when with a deactivating signal at input ENSW. Charge pump 722 further comprises a second output VO2 coupled to the first terminal 705 of capacitor C. The second output VO2 may be configured to maintain capacitor voltage V_C above a minimum voltage V_C0 when charge pump 722 is enabled. In one embodiment, V_C0 is the voltage at output VO1 of charge pump 722. In one embodiment, the amplitude of voltage at output VO2 equals the amplitude of voltage at output VO1. Charge pump 722 has a first power supply input P3 coupled to the anode of the LED A, and has a second power supply input P4 coupled to the cathode of the LED A. In other embodiments, charge pump 722 may be powered by other source such as external voltage source. Charge pump 722 may be replaced by other circuit such as voltage regulator which could be enabled to generate power to turn on switch M.
Continuing with Figure 7, switch M may be forced off periodically by pulse generator 723 to check forward voltage V_A of the LED A and refresh the output Q of latch 721. Signal at output TOU is deactivated when signal at input TIN is deactivating. Signal at output TOU is activated when signal at input TIN become activating and is forced deactivated after a time period expires. In one embodiment, the maximum time period for signal at output TOU maintaining activating is determined by pulse generator 723. Thus, signal at output TOU is activated for a time period and is deactivated after the maximum time period expires. Charge pump 722 is enabled to output power (e.g., voltage) at output VO1 and VO2 when receives activating signal at node 703, and is disabled when receives deactivating signal at node 703. As a result, switch M is forced off periodically to check the forward voltage V_A and to judge if the LED A heals back to normal status. If the LED A remains in open status, when switch M is turned off, forward voltage V_A of the LED A rises and is clamped by the Zener diode ZD at the clamping voltage V_CP again, capacitor voltage V_C is charged up to V_CP-V_DROP, which is higher than threshold voltage V_REF, and then switch M is turn on again and repeats this periodical function. When the LED A heals back to normal status, when switch M is turned off, forward voltage V_A rises up to its normal operation voltage V_A0, and capacitor voltage V_C is charged to V_A0-V_DROP, which is less than threshold voltage V_REF. Latch 721 is reset to output LOW Q at node 702 and charge pump 722 is disabled, control signal V_G maintains LOW to keep switch M off and then circuit 70 will not interfere with the normal operation of LED A.
Figure 8 shows example waveforms of the open LED bypass circuit of Figure 7 in accordance an embodiment of the present invention. The first waveform shows forward voltage V_A of the LED A and capacitor voltage V_C. The second waveform shows the output signal of comparator U1 at output CMP. The third waveform is output signal of latch 721 at the Q output. The fourth waveform is input signal of charge pump 722 at input ENSW. And the last waveform is the control signal V_G of switch M. The signals at CMP, Q, ENSW and the control signal V_G only show a logic level, i.e., logic HIGH or logic LOW for simplicity and clarity. It is noted that the logics of "HIGH" or "LOW" for the logic signals may be in alternative levels since different logic levels may lead to the same result.
Before time T1, LED A operates in normal status, forward voltage V_A is at its normal level V_A0. Capacitor voltage V_C is V_A0 - V_DROP, which is lower than threshold voltage V_REF. Comparator U1 compares capacitor voltage V_C with threshold voltage V_REF and outputs LOW signal at output CMP. Signal at the Q output of latch 721, signal at input ENSW of charge pump 722, and control signal V_G of switch M remain LOW. Switch M kept off (i.e., open).
At time T1, LED A fails and shifts from normal status to open status. Power supply voltage of the LED string builds up across the failed LED A, forward voltage V_A of LED A rises and is clamped by the Zener diode ZD at the clamping voltage V_CP, capacitor voltage V_C is charged up to V_CP-V_DROP, which is higher than threshold voltage V_REF. Comparator U1 compares capacitor voltage V_C with threshold voltage V_REF and outputs HIGH signal at output CMP indicating an open status. Latch 721 is set to generate HIGH Q output. Once receives the HIGH input signal at node 702, pulse generator 723 outputs HIGH at ENSW to enable charge pump 722. Charge pump 722 is enabled to generate outputs at both VO1 and VO2. As a result, the control signal V_G is HIGH and switch M is turned on to bypass the failed LED A. Once switch M is turned on, forward voltage V_A of the LED A decreased to voltage drop V_ON of switch M. Capacitor C is then discharged for example by the bias current of latch 721 and/or by the bias current of charge pump 722. The capacitor voltage V_C is decreased to V_C0 and is maintained at V_C0 which is the voltage at output VO1 of charge pump 722. In the example of FIG. 7, charge pump 722 is powered by the forward voltage V_A. The amplitude of voltage V_A equals the amplitude of voltage drop V_ON of switch M. And the amplitude of V_C0 may be determined by voltage drop V_ON of switch M and charge pump 722, $V_{C 0} = K * V_{ON}$
Wherein K is charge pump ratio from input voltage (i.e., V_ON) to output voltage (i.e., V_C0). In one embodiment, the charge pump ratio K is 6, i.e. V_C0= 6*V_ON. Capacitor C may have enough charge to power the monitoring circuit 51 or/and the latch 721, thus additional power may be not needed, and the power consumption of circuit 70 may be lower.
After time period (T2-T1) for signal at ENSW is HIGH, pulse generator 723 is configured to output LOW at ENSW. Control signal V_G is pulled down at time T2 to turn off switch M. If open status remain exists, once switch M is turned off, forward voltage V_A of LED A and the capacitor voltage V_C increases again. Once capacitor voltage V_C increases up to threshold voltage V_REF, comparator U1 output HIGH signal at CMP. Thereby switch M is turned ON again. During time period T1 to T4, LED A remains in open status, and the operation repeats by itself. At each cycle, switch M is turned off after a predetermined maximum time period for signal at ENSW is HIGH, referring t1, t2, t3 and t4. The duty cycle of switch M is determined by duty cycle of the signal at ENSW. In one embodiment, the duty cycle of the signal at ENSW is about 90%.
After time period (T4-T3) for HIGH signal at ENSW, pulse generator 723 is configured to output LOW at ENSW. Control signal V_G is pulled down at time T4 to turn off switch M. If LED A shifts to healing condition or in other words, the false triggering situation is eliminated, once switch M is turned off at the falling edge of control signal V_G at time T4, forward voltage V_A of the LED A rises up to its normal forward voltage V_A0, and capacitor voltage V_C is charged up to V_A0-V_DROP, which is lower than threshold voltage V_REF. Comparator U1 outputs LOW signal at CMP and Latch 721 is reset to output LOW Q. Signal at ENSW and control signal V_G is LOW. Switch M keeps off after time T4.
It is noted that the logics of "HIGH" or "LOW" for the logic signals can be in alternative levels since different logic levels can lead to the same result. For example, when V_A is higher than the reference voltage V_REF, the switch is turned on no matter the V_CMP or V_G signal is in logic "HIGH" or logic "LOW".
Figure 9 is a block diagram illustrating a method of bypassing an open LED in a plurality of serially coupled LEDs in accordance with an embodiment of the present technology. At stage 901, a switch is coupled in parallel to a target LED. At stage 902, a differential voltage across the LED is measured to determine whether the LED is in an open status. In one embodiment, the open status is monitored by comparing the forward voltage across the LED to a predetermined reference voltage. If the forward voltage is higher than the reference voltage, it indicates the LED is in open status.
When the LED fails and open status is detected, then in stage 903, the switch is turned on. Then, the failed LED is periodically checked to see if it is healed back to normal operation with cycles. Thus in stage 904, the switch is maintained on for a period of time. In one embodiment, the period of time is set by a hold on circuit. In one embodiment, the differential voltage across the LED is monitored through monitoring the voltage across a capacitor, the voltage across the capacitor is monitored and compared with a reference voltage to control the bypass switch, and the switch is maintained on by discharging the capacitor gradually. The period of time is determined by the capacitor, a discharging current and a reference voltage of a comparator of the monitoring circuit. Yet in another embodiment, the period of time is determined by a pulse generator to turn off the bypass switch periodically. And at stage 905, the switch is turned off at the end of the predetermined period of time. The process reverts to stage 902 to check if the target LED is healed. At stage 902, if healing condition is detected, the LED bypass circuit maintains the bypass switch at an off state at stage 906 to allow the healed LED to operate normally. If the LED is still in open status, the switch is turned on at stage 903 to start another cycle. Accordingly, the method may comprise periodically forcing off the switch during open status, for example, set by the periodical pulse signal.
From the foregoing, it will be appreciated that specific embodiments of the technology have been described herein for purposes of illustration, but that various modifications may be made without deviating from the disclosure. In addition, many of the elements of one embodiment may be combined with other embodiments in addition to or in lieu of the elements of the other embodiments. Accordingly, the disclosure is not limited except as by the appended claims.
The following statements, which are not claims, are incorporated as a part of the disclosure herein.

AA A circuit, comprising:
- a monitoring circuit coupled to a target circuit, the monitoring circuit being configured to monitor a differential voltage across the target circuit, to determine whether an open circuit condition exists based on the monitored differential voltage, and to generate an output signal indicating the open circuit condition; and
- a bypass switch coupled to the target circuit in parallel, the bypass circuit having a control input coupled to the monitoring circuit to receive the output signal from the monitoring circuit, the switch being configured to be selectively activated to bypass the target circuit in accordance with the output signal indicating the open circuit condition.
AB. The circuit of statement AA, wherein the switch is configured to be periodically deactivated in accordance to the output signal of the monitoring circuit.
AC. The circuit of statement AB, wherein the target circuit is a light emitting diode (LED).
AD. The circuit of statement AC, wherein the LED is coupled in series with a plurality of additional LEDs to form an LED string.
AE. The circuit of statement AC, wherein when a forward voltage of the LED is higher than a reference voltage, the switch is configured to be activated.
AF. The circuit of statement AE, wherein the monitoring circuit comprises a comparator for comparing the forward voltage of the LED to the reference voltage.
AG. The circuit of statement AF, wherein the comparator further comprises:
- a non-inverting input terminal coupled to an anode of the LED;
- an inverting input terminal coupled to the reference voltage; and
- an output terminal coupled to a gate of the bypass switch.
AH. The circuit of statement AC, further comprising a Zener diode having a cathode and an anode, the cathode of the Zener diode being coupled to an anode of the LED and the anode of the Zener diode being coupled to a cathode of the LED.
AI. The circuit of statement AH, wherein a clamping voltage of the Zener diode is higher than a forward voltage of the LED.
AJ. The circuit of statement AI, wherein the monitoring circuit comprises a comparator for comparing the forward voltage of the LED to a reference voltage.
AK. The circuit of statement AJ, wherein the monitoring circuit further comprises a hold-on circuit having an input and an output, and wherein the input of the hold-on circuit is coupled to an output terminal of the comparator, and wherein the output of the hold-on circuit is coupled to the control input of the bypass switch.
AL. The circuit of statement AK, wherein the hold-on circuit is configured to activate the bypass switch for a predetermined period of time and to deactivate the bypass switch after the predetermined period of time expires.
AM. The circuit of statement AB, wherein the bypass switch is a MOSFET.
AN. The circuit of statement AM, wherein the MOSFET is a LDMOS device integrated with the monitoring circuit on a single semiconductor substrate.
AO. A method for bypassing an open LED in a string of LEDs, comprising:
monitoring a differential voltage across the LED;
determining an open status for the LED based on the monitored differential voltage across the LED; and
if the open status of the LED is detected, activating a switch coupled to the LED in parallel.
AP The method of statement AO, further comprising periodically deactivating the switch and repeating the monitoring and determining operations.
BA A circuit, comprising:
- a sample circuit, coupled to a target circuit, the sample circuit comprising a capacitor, wherein the capacitor has a first terminal coupled to an anode of the target circuit and wherein the capacitor has a second terminal coupled to a cathode of the target circuit;
- a monitoring circuit, coupled to the capacitor and the monitoring circuit having an output configured to generate an output signal selectively indicating an open status of the target circuit;
- a bypass circuit, comprising a switch, wherein the switch comprises a control terminal coupled to the output of the monitoring circuit, and wherein the switch is configured to be selectively activated to bypass the target circuit in accordance with the output of the monitoring circuit.
BB The circuit of statement BA, wherein the sample circuit further comprises a diode having an anode connected to the anode of the target circuit and having a cathode connected to the first terminal of the capacitor.
BC The circuit of statement BA, wherein the target circuit is a light emitting diode (LED) in a string of LEDs.
BD The circuit of statement AA or BA, wherein the monitoring circuit comprises a comparator which is configured to compare a capacitor voltage with a threshold voltage, and wherein the monitoring circuit is configured to generate an output signal indicating the open status when the capacitor voltage is higher than the threshold voltage.
BE The circuit of statement BA, further comprising a Zener diode, the Zener diode having a cathode coupled to the anode of the target circuit and having an anode coupled to the cathode of the target circuit, wherein a normal forward voltage of the target circuit is less than a clamping voltage of the Zener diode.
BF. The circuit of statement BA, wherein the capacitor is configured to be discharged with a speed such that a capacitor voltage holds the switch on for a period of time when the target circuit is bypassed.
BG The circuit of statement BF, wherein the monitoring circuit further comprises:
- a first power supply input coupled to the first terminal of the capacitor; and
- a second power supply input coupled to the second terminal of the capacitor;
- wherein the capacitor is configured to be discharged by a bias current between the first power supply input and the second power supply input.
BH The circuit of statement BA, wherein the bypass circuit further comprises:
- a latch, comprising a set terminal, a reset terminal and an output, wherein the set terminal is coupled to the output of the monitoring circuit, and wherein the reset terminal is coupled to the anode of the target circuit; and
- a charge pump, comprising:
  - an input, coupled to the output of the latch;
  - a first power supply input, coupled to the anode of the target circuit;
  - a second power supply input, coupled to the cathode of the target circuit;
  - a first output, coupled to the control terminal of the switch; and
  - a second output, coupled to the first terminal of the capacitor.
- BI The circuit of statement BH, wherein the bypass circuit further comprises a pulse generator coupled between the latch and the charge pump, the pulse generator comprising:
  - an input, connected to the output of the latch; and
  - an output, connected to the input of the charge pump;
  - wherein the pulse generator is configured to periodically turn off the switch.
- BJ A circuit, comprising:
  - a sample circuit, coupled to a target circuit, the sample circuit comprising a capacitor, wherein the capacitor has a first terminal coupled to an anode of the target circuit and wherein the capacitor has a second terminal coupled to a cathode of the target circuit;
  - a monitoring circuit, coupled to the capacitor and the monitoring circuit having an output configured to generate an output signal selectively indicating an open status of the target circuit; a latch, comprising a set terminal, a reset terminal and an output, wherein the set terminal is coupled to the output of the monitoring circuit, and wherein the reset terminal is coupled to the anode of the target circuit;
  - a charge pump, comprising an enable terminal coupled to the output of the latch and further comprising a first output; and
  - a switch, comprising a control terminal, a first terminal and a second terminal, wherein the control terminal is coupled to the first output of the charge pump, wherein the first terminal is coupled to the anode of the target circuit, and wherein the second terminal is coupled to the cathode of the circuit.
- BK The circuit of statement BJ, wherein the charge pump further comprises a second output, wherein the second output is coupled to the first terminal of the capacitor, and wherein the second output is configured to maintain a capacitor voltage above a minimum voltage for a period of time.
- BL The circuit of statement BJ, wherein the latch comprises a first power supply input and a second power supply input, wherein the first power supply input is coupled to the first terminal of the capacitor, and wherein the second power supply input is coupled to the second terminal of the capacitor.
- BM The circuit of statement BJ, wherein the charge pump comprises a first power supply input and a second power supply input, wherein the first power supply input is coupled to the anode of the target circuit, and wherein the second power supply input is coupled to the cathode of the target circuit.
- BN The circuit of statement BJ, wherein the bypass circuit further comprises a pulse generator, wherein the pulse generator is connected between the output of the latch and the input of the charge pump, and wherein the pulse generator is configured to periodically turn off the switch.
- BO A method for bypassing a target circuit, comprising:
  - coupling a switch in parallel to a target circuit;
  - sampling a forward voltage across the target circuit through a capacitor coupled to the target circuit;
  - monitoring the status of the target circuit based on the forward voltage;
  - if an open status is detected, turning on the switch to bypass the target circuit, and holding the switch on for a period of time per the capacitor holding a capacitor voltage; and
  - if a normal status is detected, keeping the switch off.
- BP The method of statement BO, wherein the target circuit is a LED in a string of LEDs.
- BQ The method of statement BO, wherein an open status is detected when the capacitor voltage is higher than a threshold voltage.
- BR The method of statement BO, further comprising periodically turning off the switch to check if the open status is eliminated.
- BS The method of statement BR, wherein the method of turning off the switch comprises:
  - discharging the capacitor and maintaining the capacitor voltage larger than the threshold voltage for a period of time; and
  - turning off the switch if the capacitor voltage is decreased to lower than the threshold voltage.
- BT The method of statement BR, wherein the method of turning off the switch comprises periodically forcing the switch off.

Claims

A circuit, comprising:
a target circuit (A), a monitoring circuit (21) coupled to the target circuit (A), wherein the monitoring circuit (21) is configured to monitor a differential voltage across the target circuit (A), and wherein the monitoring circuit (21) is configured to generate an output signal (CMP) selectively indicating an open circuit condition of the target circuit (A); and

a switch (M) coupled to the target circuit (A) in parallel, the switch (M) having a control input coupled to the monitoring circuit (21) to receive the output signal (CMP) from the monitoring circuit (21), the switch (M) being configured to be selectively activated to bypass the target circuit (A) in accordance with the output signal (CMP) of the monitoring circuit (51), wherein the target circuit (A) includes an illumination device;

characterised in that, during the open circuit condition of the target circuit (A), the switch (M) is configured to be periodically deactivated.
The circuit of (50) claim 1, wherein the switch (M) is configured to be periodically deactivated in accordance with the output signal (CMP) of the monitoring circuit (51).
The circuit (50) of claim 2, wherein the target circuit (A) is a light emitting diode (LED) of an LED string.
The circuit (50) of claim 1, further comprises a capacitor (C), wherein the capacitor (C) has a first terminal coupled to an anode (LED+) of the target circuit (A) and wherein the capacitor (C) has a second terminal coupled to a cathode (LED-) of the target circuit (A), and wherein the monitoring circuit (51) monitors the differential voltage across the target circuit (A) by monitoring the voltage across the capacitor (C).
The circuit (50) of claim 4, further comprising a diode (D) and a Zener diode (ZD), wherein the diode (D) has an anode connected to the anode (LED+) of the target circuit (A) and a cathode connected to the first terminal of the capacitor (C), and wherein the Zener diode (ZD) has a cathode coupled to the anode (LED+) of the target circuit (A) and an anode coupled to the cathode (LED-) of the target circuit (A).
The circuit (50) of claim 4, wherein the monitoring circuit (51) further comprises:
a first power supply input (P1) coupled to the first terminal of the capacitor (C); and

a second power supply input (P2) coupled to the second terminal of the capacitor (C);

wherein the capacitor (C) is configured to be discharged by a bias current between the first power supply input (P1) and the second power supply input (P2).
The circuit (50) of claim 1, wherein the monitoring circuit (51) comprises a comparator (U1) comparing the differential voltage across the target circuit (A) to a reference voltage (REF), and wherein when the differential voltage across the target circuit (A) is higher than the reference voltage, the switch (M) is configured to be activated.
The circuit (30) of claim 7, wherein the monitoring circuit (31) further comprises a hold-on circuit (32) configured to activate the switch (M) for a predetermined period of time and to deactivate the switch (M) when the predetermined period of time expires, wherein the hold-on circuit (32) has an input and an output, the input of the hold-on circuit (32) is coupled to an output terminal of the comparator (U1) and wherein the output of the hold-on circuit (32) is coupled to the control input of the switch (M).
The circuit (70) of claim 1, further comprises:
a latch (721), comprising a set terminal (S), a reset terminal (R) and an output (Q), wherein the set terminal (S) is coupled to the output of the monitoring circuit (51), and wherein the reset terminal (R) is coupled to an anode (LED+) of the target circuit (A);

a pulse generator (723), comprising an input (TIN) and an output (TOU), wherein the input (TIN) is coupled to the output (Q) of the latch (721); and

a charge pump (722), comprising an input (ENSW) and an output (VO1), wherein the input (ENSW) is coupled to the output (TOU) of the pulse generator (723) and wherein the output (VO1) is coupled to the control input of the switch (M).
The circuit (70) of claim 9, wherein the charge pump (722) further comprises a second output (VO2), wherein the second output (VO2) is coupled to the first terminal (705) of the capacitor (C), and wherein the second output (VO2) is configured to maintain a voltage across the capacitor (C) above a minimum voltage for a period of time.
The circuit (70) of claim 9, wherein:
the latch (721) further comprises a first power supply input (P5) and a second power supply input (P6), wherein the first power supply input (P5) is coupled to the first terminal (705) of the capacitor (C), and wherein the second power supply input (P6) is coupled to the second terminal (706) of the capacitor (C); and

the charge pump (722) further comprises a third power supply input and a fourth power supply input, wherein the third power supply input is coupled to the anode (LED+) of the target circuit (A), and wherein the fourth power supply input is coupled to the cathode (LED-) of the target circuit (A).
The circuit (30) of claim 2, wherein the switch (M) is a metal oxide semiconductor field effect transistor (MOSFET).
A method for bypassing an open LED in a string of LEDs, comprising:
monitoring a differential voltage across the LED;

determining an open status for the LED based on the monitored differential voltage across the LED; and

if the open status of the LED is detected, activating a switch coupled to the LED in parallel and holding the switch on for a period of time;

characterised in that, during the open status of the LED, the switch is periodically deactivated.
The method of claim 13, wherein the differential voltage across the LED is monitored through a capacitor, and the switch is held on for a period of time by discharging the capacitor gradually.
The method of claim 13, further comprising periodically forcing the switch off.