[0002] 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).
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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
[0007]
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.
[0008] The use of the same reference label in different drawings indicates the same or like
components.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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 |
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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).
[0040] 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,

[0041] 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.
[0042] 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%.
[0043] 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.
[0044] 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".
[0045] 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.
[0046] 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.
[0047] 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.
[0048] The following statements 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 claim AA, wherein the switch is configured to be periodically deactivated
in accordance to the output signal of the monitoring circuit.
AC. The circuit of claim AB, wherein the target circuit is a light emitting diode
(LED).
AD. The circuit of claim AC, wherein the LED is coupled in series with a plurality
of additional LEDs to form an LED string.
AE. The circuit of claim 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 claim AE, wherein the monitoring circuit comprises a comparator
for comparing the forward voltage of the LED to the reference voltage.
AG. The circuit of claim 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 claim 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 claim AH, wherein a clamping voltage of the Zener diode is higher
than a forward voltage of the LED.
AJ. The circuit of claim AI, wherein the monitoring circuit comprises a comparator
for comparing the forward voltage of the LED to a reference voltage.
AK. The circuit of claim 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 claim 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 claim AB, wherein the bypass switch is a MOSFET.
AN. The circuit of claim 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 claim 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 claim 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 claim BA, wherein the target circuit is a light emitting diode (LED)
in a string of LEDs.
BD The circuit of claim 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 claim 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 claim 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 claim 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 claim 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 claim 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 claim 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 claim 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 claim 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 claim 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 claim BO, wherein the target circuit is a LED in a string of LEDs.
BQ The method of claim BO, wherein an open status is detected when the capacitor voltage
is higher than a threshold voltage.
BR The method of claim BO, further comprising periodically turning off the switch
to check if the open status is eliminated.
BS The method of claim 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 claim BR, wherein the method of turning off the switch comprises
periodically forcing the switch off.