Multi-string LED load current sharing circuit and implementation method thereof
By using a driving circuit composed of transistors and operational amplifiers, the base current is adjusted to achieve uniform current distribution to multiple LED strings, solving the problems of inconsistent brightness and high cost, and realizing circuit stability and the function of shutting off all LEDs in case of failure.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SHANGHAI DEBANG INTELLIGENT CONTROL TECHNOLOGY DEVELOPMENT CO LTD
- Filing Date
- 2026-04-22
- Publication Date
- 2026-05-29
AI Technical Summary
In LED driver circuits, uneven current distribution among multiple LED strings leads to inconsistent brightness, and different constant current power supplies are required for different numbers of LED strings, increasing application costs.
The basic driving circuit, composed of transistors Q1, Q2 and Q3, achieves average distribution of current from a single constant current source by adjusting the base current of the transistors, and balances current among multiple LED strings. The additional driving circuit, composed of operational amplifier U1A and transistor Q8, further optimizes the current distribution.
It achieves uniform current distribution for multiple LED strings, reducing application costs, and enables all LED strings to turn off when one string experiences an open-circuit fault, thus improving circuit stability.
Smart Images
Figure CN122121006A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of LED driving technology, specifically relating to a current sharing circuit for multiple LED loads and its implementation method. Background Technology
[0002] In lighting applications, multiple LED strings are often connected in parallel. Each LED string consists of one or more LEDs connected in series. The luminous intensity of each LED is determined by the current flowing through it. Therefore, in practical applications, current regulation is used to control the brightness of the LED string. However, due to the inherent differences in characteristics of each LED string, the on-state voltage drop will vary when the same current flows through each string. Therefore, when using a single constant current source, uneven current distribution among the LEDs in each string can occur, leading to inconsistent brightness.
[0003] Therefore, in traditional LED driver circuits, different constant current power supplies are required for LED strings with different numbers of LEDs, which increases the application cost. Summary of the Invention
[0004] The purpose of this invention is to provide a current sharing circuit for multiple LED strings to solve the problems mentioned in the background art. The current sharing circuit for multiple LED strings provided by this invention has the characteristic of evenly distributing the current from a single constant current source to multiple LED strings with different numbers of LED beads.
[0005] Another objective of this invention is to provide a method for implementing a current sharing circuit for multiple LED loads.
[0006] To achieve the above objectives, the present invention provides the following technical solution: a multi-string LED load current sharing circuit, comprising a basic driving circuit, the basic driving circuit comprising transistors Q1, Q2, and Q3, wherein the base of transistor Q1 is connected to one end of resistor R3, the base of transistor Q2 is connected to one end of resistor R4, the other ends of resistors R3 and R4 are both connected to the emitter of transistor Q3, the emitter of transistor Q1 is connected to one end of resistor R1, the emitter of transistor Q2 is connected to one end of resistor R2, the other ends of resistors R1 and R2 are both connected to the output terminal of constant current source M+, the base of transistor Q3 is connected to one end of resistor R5, the other end of resistor R5 and the collector of transistor Q2 are both connected to the positive terminal of the first LED string, and the collectors of transistors Q1 and Q3 are both connected to the positive terminal of the second LED string.
[0007] To ensure circuit stability, the first LED string and the second LED string each include a number of LED beads connected in series. When the number of LED beads in the first LED string and the second LED string are different, the LED string with fewer LED beads is connected to the base of transistor Q3, and the LED string with more LED beads is connected to the collector of transistor Q3.
[0008] To ensure the accuracy of the current, the resistance of resistor R5 is further set to 100Ω-10 kΩ.
[0009] To achieve an even distribution of the constant current source current, when there are three or more LED strings, an additional driving circuit is also included. The additional driving circuit includes an operational amplifier U1A and a transistor Q8. Pin 1 of the operational amplifier U1A is connected to the positive terminal of diode D1, the negative terminal of diode D1 is connected to the base of transistor Q3, the emitter of transistor Q8 is connected to one end of resistor R7, the other end of resistor R7 and pin 4 of the operational amplifier U1A are connected to the output terminal of the constant current source M+, the base of transistor Q8 is connected to one end of resistor R8, the other end of resistor R8 and pin 2 of the operational amplifier U1A are connected to the emitter of transistor Q3, and the collector of transistor Q8 and pin 3 of the operational amplifier U1A are connected to the positive terminal of the third LED string.
[0010] To ensure even current distribution, transistors Q1, Q2, and Q3 are further designed to be PNP transistors of the same specifications.
[0011] To ensure that the voltage across the emitter of the transistors is the same, the resistance values of resistors R1, R2, and R7 are also the same.
[0012] To ensure that the collector voltages of transistors Q1, Q2, and Q8 are not significantly different, a dummy load RL is connected in series between the collector of transistor Q8, pin 3 of operational amplifier U1A, and the positive terminal of the third LED string. The dummy load RL is either a resistor or an LED chip.
[0013] Furthermore, in this invention, the method for implementing a multi-string LED load current sharing circuit includes the following steps:
[0014] (i) When the LED string has two paths, the different number of LEDs in each path results in different total voltages. The LED string with more LEDs has a higher total voltage and is connected to the collector of transistor Q3; while the LED string with fewer LEDs has a lower total voltage and is connected to the base of transistor Q3. Therefore, the collector voltage of transistor Q3 is higher than its base voltage. Since the emitter of transistor Q3 is closer to the power supply and has lower impedance, the collector voltage of transistor Q3 is higher than its base voltage. Since the emitter voltage of transistor Q3 is greater than its base voltage, the emitter and collector of Q3 are forward biased, operating in the saturation region. Resistor R5 limits the base current of transistor Q3, resulting in a smaller base current and emitter current. The bases of transistors Q1 and Q2 are connected to the emitter of transistor Q3, so their base voltages are approximately equal to the emitter voltage of Q3, and their base currents are also roughly the same. The values are all relatively small. Since the collector voltages of transistors Q1 and Q2 are connected to the collector and base of transistor Q3, transistors Q1 and Q2 are both in the amplification region. Because transistors Q1 and Q2 are of the same specification, their emitter voltages are also basically the same. Resistors R1 and R2 have the same resistance and voltage drop, so their emitter currents are also basically the same. When P... When the NP transistor is in the amplification region, Ie = Ic + Ib. The base currents of transistors Q1 and Q2 are very small and basically the same. Therefore, the collector currents of transistors Q1 and Q2 are also the same. At the same time, the number of LED beads in the LED string connected to the collector of transistor Q2 is minimized, so that transistor Q3 can work stably in the saturation region. At this time, the collector current of transistor Q3 does not change with the base current, ensuring that transistors Q1 and Q2 share the constant current source current equally.
[0015] (ii) When the LED string connected to transistor Q1 is open-circuited, the emitter voltage of transistor Q1 will suddenly increase, and all channels of the circuit will fail. When the LED string connected to transistor Q2 is open-circuited, there is no voltage at the emitter of transistor Q2 and no voltage at the base of transistor Q3, and all channels of the circuit will fail, thus achieving the function of turning off one and turning off all.
[0016] (III) When the LED string has three channels, since the bases of transistors Q1, Q2, and Q8 are connected to the emitter of transistor Q3, the base voltages of transistors Q1, Q2, and Q8 are basically equal to the emitter voltage of transistor Q3, and the base currents are also basically the same and relatively small. Since the collectors of transistors Q1 and Q2 are connected to the collector and base of transistor Q3, the transistors... Both Q1 and Q2 are in the amplification region. Since Q1 and Q2 are transistors of the same specification, their emitter voltages are also basically the same. Due to the dummy load RL, the collector voltages of transistors Q1, Q2 and Q8 are not much different. Therefore, the emitter voltages of transistors Q1, Q2 and Q8 are basically the same, so that the constant current source current is evenly distributed among the three LED strings.
[0017] (iv) Pin 3 of operational amplifier U1A is connected to the third LED string, and pin 2 of operational amplifier U1A is connected between resistor R8 and transistor Q3. Since transistor Q3 is in saturation, when the voltage at pin 2 of operational amplifier U1A is higher than the voltage at pin 3, operational amplifier U1A outputs a low level, and diode D1 does not conduct. Therefore, when the third LED string connected to transistor Q8 is open-circuited, the emitter voltage of transistor Q8 will suddenly increase, operational amplifier U1A outputs a high level, and all channels of the circuit will fail. When the first LED string connected to transistor Q2 is open-circuited, there is no voltage at the emitter of transistor Q2 and no voltage at the base of transistor Q3, and all channels of the circuit fail. When the second LED string connected to transistor Q1 is open-circuited, the emitter voltage of transistor Q1 suddenly increases, and all channels fail, achieving the function of turning off one and turning off all.
[0018] Compared with the prior art, the beneficial effects of the present invention are:
[0019] 1. This invention achieves the average distribution of constant current source current by adjusting the base current of the transistor. Compared with the prior art, which requires a separate constant current source to drive each LED string in order to ensure that the brightness meets the requirements, this invention distributes the current of a single constant current source evenly to LED strings with different numbers of LED beads, effectively reducing the application cost.
[0020] 2. The LED string connected to the base of transistor Q3 and the collector of transistor Q2 in this invention has the lowest voltage, which effectively ensures the stability of the circuit.
[0021] 3. When one LED string in this invention experiences an open circuit fault, all circuit channels will also fail, thus achieving the function of turning off all LEDs when one is turned off. Attached Figure Description
[0022] Figure 1 This is a circuit diagram of the two-channel LED light string of the present invention.
[0023] Figure 2 This is a circuit diagram of the three-channel LED light string of the present invention.
[0024] Figure 3 This is a circuit diagram of the basic driving circuit of the present invention.
[0025] Figure 4 The circuit diagram shows the additional driving circuit for this invention.
[0026] In the diagram: 1. Basic drive circuit; 2. Additional drive circuit. Detailed Implementation
[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0028] In the description of this invention, unless otherwise explicitly specified and limited, the terms "connected," "linked," and "fixed" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.
[0029] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.
[0030] In the description of this invention, the terms "upper," "lower," "right," and "left," etc., refer to the orientation or positional relationship shown in the accompanying drawings. They are used only for ease of description and simplification of operation, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. Furthermore, the terms "first" and "second" are used merely for distinction in description and have no special meaning.
[0031] Example 1
[0032] Please see Figures 1-4 This embodiment provides the following technical solution: a multi-string LED load current sharing circuit, including a basic driving circuit, which includes transistors Q1, Q2, and Q3. The base of transistor Q1 is connected to one end of resistor R3, the base of transistor Q2 is connected to one end of resistor R4, and the other ends of resistors R3 and R4 are connected to the emitter of transistor Q3. The emitter of transistor Q1 is connected to one end of resistor R1, the emitter of transistor Q2 is connected to one end of resistor R2, and the other ends of resistors R1 and R2 are connected to the output terminal of constant current source M+. The base of transistor Q3 is connected to one end of resistor R5, and the other end of resistor R5 and the collector of transistor Q2 are connected to the positive terminal of the first LED string. The collectors of transistors Q1 and Q3 are connected to the positive terminal of the second LED string.
[0033] By adopting the above technical solution, the present invention achieves the average distribution of constant current source current by adjusting the base current of the transistor. Compared with the prior art, which requires a separate constant current source to drive each LED string in order to ensure that the brightness meets the requirements, the present invention distributes the current of a single constant current source evenly to LED strings with different numbers of LED beads in multiple channels, effectively reducing the application cost.
[0034] Specifically, the first LED string and the second LED string each include a number of LED beads connected in series. When the number of LED beads in the first LED string and the second LED string are different, the LED string with fewer LED beads is connected to the base of transistor Q3, and the LED string with more LED beads is connected to the collector of transistor Q3.
[0035] By adopting the above technical solution, the voltage of the LED string connected between the base of transistor Q3 and the collector of transistor Q2 is minimized, effectively ensuring the stability of the circuit.
[0036] Specifically, the resistance of resistor R5 is 100Ω-10 kΩ.
[0037] By adopting the above technical solution, the accuracy of the current is guaranteed.
[0038] Example 2
[0039] The difference between this embodiment and Embodiment 1 is that, specifically, when there are three or more LED light strings, an additional driving circuit is also included. The additional driving circuit includes an operational amplifier U1A and a transistor Q8. Pin 1 of the operational amplifier U1A is connected to the positive terminal of diode D1, the negative terminal of diode D1 is connected to the base of transistor Q3, the emitter of transistor Q8 is connected to one end of resistor R7, the other end of resistor R7 and pin 4 of the operational amplifier U1A are respectively connected to the output terminal of constant current source M+, the base of transistor Q8 is connected to one end of resistor R8, the other end of resistor R8 and pin 2 of the operational amplifier U1A are respectively connected to the emitter of transistor Q3, and the collector of transistor Q8 and pin 3 of the operational amplifier U1A are respectively connected to the positive terminal of the third LED light string.
[0040] By adopting the above technical solution, the average distribution of constant current source current is achieved.
[0041] Specifically, transistors Q1, Q2, and Q3 are all PNP type transistors with the same specifications.
[0042] By adopting the above technical solution, the current can be evenly distributed.
[0043] Specifically, resistors R1, R2, and R7 have the same resistance value.
[0044] By adopting the above technical solution, it is possible to ensure that the voltage of the transistor emitter is the same.
[0045] Example 3
[0046] The difference between this embodiment and Embodiment 1 is that, specifically, a dummy load RL is connected in series between the collector of transistor Q8, pin 3 of operational amplifier U1A, and the positive terminal of the third LED string. The dummy load RL is a resistor or an LED chip.
[0047] By adopting the above technical solution, the collector voltages of transistors Q1, Q2, and Q8 are not significantly different.
[0048] Example 4
[0049] Furthermore, the implementation method of the current sharing circuit for a multi-string LED load according to the present invention includes the following steps:
[0050] (i) When the LED string has two paths, the different number of LEDs in each path results in different total voltages. The LED string with more LEDs has a higher total voltage and is connected to the collector of transistor Q3; while the LED string with fewer LEDs has a lower total voltage and is connected to the base of transistor Q3. Therefore, the collector voltage of transistor Q3 is higher than its base voltage. Since the emitter of transistor Q3 is closer to the power supply and has lower impedance, the collector voltage of transistor Q3 is higher than its base voltage. Since the emitter voltage of transistor Q3 is greater than its base voltage, the emitter and collector of Q3 are forward biased, operating in the saturation region. Resistor R5 limits the base current of transistor Q3, resulting in a smaller base current and emitter current. The bases of transistors Q1 and Q2 are connected to the emitter of transistor Q3, so their base voltages are approximately equal to the emitter voltage of Q3, and their base currents are also roughly the same. The values are all relatively small. Since the collector voltages of transistors Q1 and Q2 are connected to the collector and base of transistor Q3, transistors Q1 and Q2 are both in the amplification region. Because transistors Q1 and Q2 are of the same specification, their emitter voltages are also basically the same. Resistors R1 and R2 have the same resistance and voltage drop, so their emitter currents are also basically the same. When P... When the NP transistor is in the amplification region, Ie = Ic + Ib. The base currents of transistors Q1 and Q2 are very small and basically the same. Therefore, the collector currents of transistors Q1 and Q2 are also the same. At the same time, the number of LED beads in the LED string connected to the collector of transistor Q2 is minimized, so that transistor Q3 can work stably in the saturation region. At this time, the collector current of transistor Q3 does not change with the base current, ensuring that transistors Q1 and Q2 share the constant current source current equally.
[0051] (ii) When the LED string connected to transistor Q1 is open-circuited, the emitter voltage of transistor Q1 will suddenly increase, and all channels of the circuit will fail. When the LED string connected to transistor Q2 is open-circuited, there is no voltage at the emitter of transistor Q2 and no voltage at the base of transistor Q3, and all channels of the circuit will fail, thus achieving the function of turning off one and turning off all.
[0052] (III) When the LED string has three channels, since the bases of transistors Q1, Q2, and Q8 are connected to the emitter of transistor Q3, the base voltages of transistors Q1, Q2, and Q8 are basically equal to the emitter voltage of transistor Q3, and the base currents are also basically the same and relatively small. Since the collectors of transistors Q1 and Q2 are connected to the collector and base of transistor Q3, the transistors... Both Q1 and Q2 are in the amplification region. Since Q1 and Q2 are transistors of the same specification, their emitter voltages are also basically the same. Due to the dummy load RL, the collector voltages of transistors Q1, Q2 and Q8 are not much different. Therefore, the emitter voltages of transistors Q1, Q2 and Q8 are basically the same, so that the constant current source current is evenly distributed among the three LED strings.
[0053] (iv) Pin 3 of operational amplifier U1A is connected to the third LED string, and pin 2 of operational amplifier U1A is connected between resistor R8 and transistor Q3. Since transistor Q3 is in saturation, when the voltage at pin 2 of operational amplifier U1A is higher than the voltage at pin 3, operational amplifier U1A outputs a low level, and diode D1 does not conduct. Therefore, when the third LED string connected to transistor Q8 is open-circuited, the emitter voltage of transistor Q8 will suddenly increase, operational amplifier U1A outputs a high level, and all channels of the circuit will fail. When the first LED string connected to transistor Q2 is open-circuited, there is no voltage at the emitter of transistor Q2 and no voltage at the base of transistor Q3, and all channels of the circuit fail. When the second LED string connected to transistor Q1 is open-circuited, the emitter voltage of transistor Q1 suddenly increases, and all channels fail, achieving the function of turning off one and turning off all.
[0054] Experimental Example 1
[0055] Simulation of the two-channel LED string driver circuit using LTSPICE software:
[0056] (1) Simulation conditions:
[0057] The constant current source has a current of 300mA. Transistors Q1, Q2, and Q3 are all PNP transistors. Resistors R1 and R2 are both 5 ohms, and resistors R3 and R4 are both 20 ohms. The LEDs are all the same, all LXHL-BW02.
[0058] (2) Simulation results:
[0059] The current drawn by LEDs 21, 22, and 23 is 150.4 mA, while the current drawn by LEDs 11 and 12 is 149.6 mA. Simulation results show that the current through transistor Q3 is very small. Transistors Q1 and Q2 have the same emitter voltage, specifications, and the same resistance values for resistors R1 and R2, therefore their emitter currents are the same. Transistor Q3 is in the saturation region with relatively small current values across all three stages. Transistors Q1 and Q2 are in the amplification region with relatively small base currents, and their emitter currents are approximately equal to their collector currents. Therefore, the current values of the two LED strings are essentially the same.
[0060] Experiment Example 2
[0061] Simulation of a three-channel LED string driver circuit using LTSPICE software:
[0062] (1) Simulation conditions:
[0063] The constant current source has a current of 450mA. Transistors Q1, Q2, Q3, and Q8 are all PNP transistors. Resistors R1, R2, and R8 are all 5Ω, while resistors R3, R4, and R6 are all 20Ω. The LEDs are all the same type, LXHL-BW02. The dummy load RL is a 20Ω resistor, ensuring that the collector voltages of transistors Q2 and Q8 are not significantly different.
[0064] (2) Simulation results:
[0065] The current drawn by LEDs 21, 22, and 23 is 151.5mA, by LEDs 11 and 12 is 149.6mA, and by LEDs 31 and 32 is 148.2mA. Simulation results show that when the circuit is on, the base voltages of transistors Q1, Q2, and Q8 remain consistent, and all transistors operate in the amplification region with essentially the same emitter voltage. Due to the relatively small base current, the currents drawn by transistors Q1, Q2, and Q8 are essentially the same, resulting in an even distribution of the constant current source current.
[0066] In summary, this invention achieves an even distribution of the constant current source by adjusting the base current of the transistors. Compared to existing technologies where each LED string requires a separate constant current source to ensure sufficient brightness, this invention evenly distributes the current from a single constant current source to multiple LED strings with varying numbers of LEDs, effectively reducing application costs. In this invention, the voltage across the LED string connected to the base of transistor Q3 and the collector of transistor Q2 is minimized, effectively ensuring circuit stability. Furthermore, when one LED string experiences an open-circuit fault, all circuit channels fail, achieving a "one-to-all" effect.
[0067] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.
Claims
1. A current sharing circuit for a multi-string LED load, characterized in that: The system includes a basic driving circuit comprising transistors Q1, Q2, and Q3. The base of transistor Q1 is connected to one end of resistor R3, the base of transistor Q2 is connected to one end of resistor R4, and the other ends of resistors R3 and R4 are connected to the emitter of transistor Q3. The emitter of transistor Q1 is connected to one end of resistor R1, the emitter of transistor Q2 is connected to one end of resistor R2, and the other ends of resistors R1 and R2 are connected to the output terminal of the constant current source M+. The base of transistor Q3 is connected to one end of resistor R5, and the other end of resistor R5 and the collector of transistor Q2 are connected to the positive terminal of the first LED string. The collectors of transistors Q1 and Q3 are connected to the positive terminal of the second LED string.
2. The current sharing circuit for a multi-string LED load according to claim 1, characterized in that: The first LED string and the second LED string each include a number of LED beads connected in series. When the number of LED beads in the first LED string and the second LED string are different, the LED string with fewer LED beads is connected to the base of the transistor Q3, and the LED string with more LED beads is connected to the collector of the transistor Q3.
3. The current sharing circuit for a multi-string LED load according to claim 1, characterized in that: The resistance value of resistor R5 is 100Ω-10 kΩ.
4. The current sharing circuit for a multi-string LED load according to claim 1, characterized in that: When there are three or more LED strings, an additional driving circuit is also included. The additional driving circuit includes an operational amplifier U1A and a transistor Q8. Pin 1 of the operational amplifier U1A is connected to the positive terminal of diode D1, the negative terminal of diode D1 is connected to the base of transistor Q3, the emitter of transistor Q8 is connected to one end of resistor R7, the other end of resistor R7 and pin 4 of the operational amplifier U1A are connected to the output terminal of constant current source M+, the base of transistor Q8 is connected to one end of resistor R8, the other end of resistor R8 and pin 2 of the operational amplifier U1A are connected to the emitter of transistor Q3, and the collector of transistor Q8 and pin 3 of the operational amplifier U1A are connected to the positive terminal of the third LED string.
5. A current sharing circuit for a multi-string LED load according to claim 4, characterized in that: Transistors Q1, Q2, and Q3 are all PNP type transistors with the same specifications.
6. The current sharing circuit for a multi-string LED load according to claim 4, characterized in that: The resistors R1, R2, and R7 have the same resistance value.
7. A current sharing circuit for a multi-string LED load according to claim 4, characterized in that: A dummy load RL is connected in series between the collector of transistor Q8, pin 3 of operational amplifier U1A, and the positive terminal of the third LED string.
8. A current sharing circuit for a multi-string LED load according to claim 7, characterized in that: The dummy load RL is a resistor or an LED bead.
9. A method for implementing a multi-string LED load current sharing circuit according to any one of claims 1-9, characterized in that, Includes the following steps: (i) When the LED string has two paths, the different number of LEDs in each path results in different total voltages. The LED string with more LEDs has a higher total voltage and is connected to the collector of transistor Q3; while the LED string with fewer LEDs has a lower total voltage and is connected to the base of transistor Q3. Therefore, the collector voltage of transistor Q3 is higher than its base voltage. Since the emitter of transistor Q3 is closer to the power supply and has lower impedance, the collector voltage of transistor Q3 is higher than its base voltage. Since the emitter voltage of transistor Q3 is greater than its base voltage, the emitter and collector of Q3 are forward biased, operating in the saturation region. Resistor R5 limits the base current of transistor Q3, resulting in a smaller base current and emitter current. The bases of transistors Q1 and Q2 are connected to the emitter of transistor Q3, so their base voltages are approximately equal to the emitter voltage of Q3, and their base currents are also roughly the same. The values are all relatively small. Since the collector voltages of transistors Q1 and Q2 are connected to the collector and base of transistor Q3, transistors Q1 and Q2 are both in the amplification region. Because transistors Q1 and Q2 are of the same specification, their emitter voltages are also basically the same. Resistors R1 and R2 have the same resistance and voltage drop, so their emitter currents are also basically the same. When P... When the NP transistor is in the amplification region, Ie = Ic + Ib. The base currents of transistors Q1 and Q2 are very small and basically the same. Therefore, the collector currents of transistors Q1 and Q2 are also the same. At the same time, the number of LED beads in the LED string connected to the collector of transistor Q2 is minimized, so that transistor Q3 can work stably in the saturation region. At this time, the collector current of transistor Q3 does not change with the base current, ensuring that transistors Q1 and Q2 share the constant current source current equally. (ii) When the LED string connected to transistor Q1 is open-circuited, the emitter voltage of transistor Q1 will suddenly increase, and all channels of the circuit will fail. When the LED string connected to transistor Q2 is open-circuited, there is no voltage at the emitter of transistor Q2 and no voltage at the base of transistor Q3, and all channels of the circuit will fail, thus achieving the function of turning off one and turning off all. (III) When the LED string has three channels, since the bases of transistors Q1, Q2, and Q8 are connected to the emitter of transistor Q3, the base voltages of transistors Q1, Q2, and Q8 are basically equal to the emitter voltage of transistor Q3, and the base currents are also basically the same and relatively small. Since the collectors of transistors Q1 and Q2 are connected to the collector and base of transistor Q3, the transistors... Both Q1 and Q2 are in the amplification region. Since Q1 and Q2 are transistors of the same specification, their emitter voltages are also basically the same. Due to the dummy load RL, the collector voltages of transistors Q1, Q2 and Q8 are not much different. Therefore, the emitter voltages of transistors Q1, Q2 and Q8 are basically the same, so that the constant current source current is evenly distributed among the three LED strings. (iv) Pin 3 of operational amplifier U1A is connected to the third LED string, and pin 2 of operational amplifier U1A is connected between resistor R8 and transistor Q3. Since transistor Q3 is in saturation, when the voltage at pin 2 of operational amplifier U1A is higher than the voltage at pin 3, operational amplifier U1A outputs a low level, and diode D1 does not conduct. Therefore, when the third LED string connected to transistor Q8 is open-circuited, the emitter voltage of transistor Q8 will suddenly increase, operational amplifier U1A outputs a high level, and all channels of the circuit will fail. When the first LED string connected to transistor Q2 is open-circuited, there is no voltage at the emitter of transistor Q2 and no voltage at the base of transistor Q3, and all channels of the circuit fail. When the second LED string connected to transistor Q1 is open-circuited, the emitter voltage of transistor Q1 suddenly increases, and all channels fail, achieving the function of turning off one and turning off all.