Variable load circuit applied to flyback constant current driving and implementation method thereof

Abstract

The present application belongs to the technical field of flyback constant current driving, and relates to a variable load circuit applied to flyback constant current driving and an implementation method thereof. The present application aims to solve the technical contradiction that the existing fixed resistance dummy load cannot simultaneously consider the LED dimming function and high full-load efficiency and low no-load power consumption. The circuit comprises a flyback topology circuit unit and a variable load circuit unit. The variable load circuit automatically switches impedance according to the output voltage: when the output voltage is lower than the threshold value, low impedance is presented, the excess energy in the dimming state is efficiently consumed, and the LED is ensured to be stably extinguished; when the output voltage is higher than the threshold value, high impedance is presented, and the invalid power consumption is significantly reduced. Without an additional control circuit, adaptive adjustment is realized through pure hardware logic, the compatibility with the existing flyback topology is high, and the fixed resistance dummy load can be directly replaced, so that the LED dimming function is ensured, the driving full-load efficiency is improved, and the no-load power consumption is reduced.

Description

Technical Field

[0001] This invention belongs to the field of flyback constant current drive technology, and particularly relates to a variable load circuit and its implementation method for flyback constant current drive. Background Technology

[0002] The core of existing flyback constant current drive circuits is the flyback transformer, which typically consists of three windings: the primary winding T1A, the output winding T1B, and the demagnetization detection winding T1C. The primary winding T1A receives the external input voltage and stores energy; the output winding T1B provides a constant current output to the external LED load through rectifier diodes and filter capacitors; the demagnetization detection winding T1C has a dual function: firstly, it detects the demagnetization state of the primary winding T1A to achieve precise control of the flyback IC; secondly, after rectification and filtering, it provides the flyback IC with its operating power supply (VCC), and its voltage remains in phase with the voltage of the output winding T1B, satisfying a fixed turns ratio to ensure indirect monitoring and control of the output voltage by the flyback IC.

[0003] To address the risk of voltage spikes in "output no-load" scenarios (when no external LED load is connected to the output port), existing technologies typically connect a fixed-value resistor (denoted as R0) in parallel at the output port as a "dummy load." According to Ohm's law, the output current I of the output winding T1B under no-load conditions equals the output voltage Vout divided by the fixed resistance R0. The energy dissipation effect of the dummy load prevents the output voltage from rising excessively due to the lack of load discharge, thus ensuring the safety of the circuit components.

[0004] In specific applications requiring an "off" function for LED loads, the flyback IC regulates the output voltage by limiting the forward voltage of its FB (feedback) pin. Let the rated minimum output voltage be Vout_min, the rated maximum output voltage be Vout_max, and the output limiting voltage be Vout_CV.

[0005] When the driver is in the "non-off state", the flyback IC sets the limiting voltage of the FB pin to VFB_CV, and the limiting voltage of the corresponding output port is Vout_CV. The design ensures that the rated maximum value of the driver's output voltage Vout_max is not greater than the output limiting voltage Vout_CV.

[0006] When the driver is in the "off state", the limiting voltage of the FB pin is reduced to VFB_dim, corresponding to the output voltage Vout_dim, with the coefficient K = VFB_dim / VFB_CV < 1 (this coefficient is an inherent parameter of the flyback IC). Simultaneously, stable off-state operation of the LED load requires meeting the condition that "the output voltage is less than 0.667 times its minimum rated operating voltage", i.e., Vout_dim < Vout_min × 0.667; otherwise, the LED load will experience abnormal lighting.

[0007] Furthermore, in the actual design of flyback constant current drives, the significant difference between the rated output voltage range and the VCC supply voltage range results in the output winding T1B typically having more turns than the demagnetizing detection winding T1C. In addition, the widespread use of transformer sandwich structures leads to a much stronger coupling between the output winding T1B and the primary winding T1A compared to the coupling between the demagnetizing detection winding T1C and the primary winding T1A. Most of the energy stored in the primary winding T1A during the switching transistor's on-time is coupled to the output winding T1B during the switching transistor's off-time. When the drive is in the "off state," a sufficiently low-impedance "dummy load" is needed to dissipate this energy; otherwise, the output voltage will surge above the LED off threshold (Vout_min × 0.667), causing the LED to malfunction and light up.

[0008] In existing technologies, a fixed-value resistor R0 is used as the dummy load. This design presents an irreconcilable technical contradiction: to meet the stable off-state requirements of the LED load, the fixed resistor R0 must be selected with a small resistance value—only a small-value resistor can provide a sufficiently large discharge current to dissipate the excess energy coupled in the output winding T1B during the off-state, ensuring that Vout_dim < Vout_min × 0.667. However, this design leads to the deterioration of two key performance indicators:

[0009] Reduced full-load efficiency: When the drive is in the non-off state and the output voltage reaches the rated maximum value Vout_max, i.e. full-load condition, according to the power calculation formula P=V² / R0, the power consumption of the fixed resistor R0 will increase sharply with the square of the output voltage. For example, if Vout_max is 42V and R0 is 5.16KΩ, its power consumption can reach 342mW. This part of the power consumption is ineffective energy consumption and will directly reduce the full-load efficiency of the drive.

[0010] Increased no-load power consumption: In no-load scenarios, the output voltage is limited to Vout_CV, and the fixed resistor R0 continues to consume energy, leading to a significant increase in no-load power consumption. Both ERP and EnergyStar have set clear limits for no-load power consumption. Full-load efficiency and no-load power consumption are core performance indicators for flyback constant current drive products, directly affecting customer acceptance and market competitiveness.

[0011] Therefore, the flyback constant current drive circuit that uses a fixed resistor as a dummy load in the existing technology cannot simultaneously meet the performance requirements of "effectiveness of the dimming function" and "high full-load efficiency and low no-load power consumption" in the specific scenario of "LED load that needs to be dimmed". It has significant technical defects and urgently needs to be solved through the optimization design of circuit topology. Summary of the Invention

[0012] The purpose of this invention is to provide a variable load circuit and its implementation method for flyback constant current drive, in order to solve the technical problem that the existing flyback constant current drive circuit, which uses a fixed resistor as a dummy load, cannot simultaneously meet the performance requirements of "effectiveness of the dimming function" and "high full-load efficiency and low no-load power consumption" in the specific scenario of "LED load that needs to be dimmed".

[0013] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0014] In a first aspect, a variable load circuit for flyback constant current drive is provided, including a flyback topology circuit unit and a variable load circuit unit.

[0015] The variable load circuit unit includes a first resistor, a second resistor, a third resistor, a fourth resistor, a first switching transistor, and a second switching transistor; wherein the first switching transistor is an enhancement-mode NMOS and the second switching transistor is an NPN transistor.

[0016] The positive output terminal of the flyback topology circuit unit is connected to one end of the first resistor, the third resistor, and the fourth resistor. The other end of the first resistor is connected to the gate of the first switching transistor and one end of the second resistor. The other end of the third resistor is connected to the drain of the first switching transistor and the base of the second switching transistor. The other end of the fourth resistor is connected to the collector of the second switching transistor.

[0017] The negative output terminal of the flyback topology circuit unit is connected to the other end of the second resistor, the source of the first switch, and the emitter of the second switch.

[0018] Preferably, the flyback topology circuit unit includes a flyback transformer, a flyback IC, a fifth resistor, a sixth resistor, a current sensing resistor, a first diode, a second diode, a first capacitor, a second capacitor, and a third switching transistor; wherein the third switching transistor is a field-effect transistor.

[0019] The flyback transformer includes a primary winding T1A, an output winding T1B, and a demagnetization detection winding T1C. The output winding T1B is isolated from T1A and T1C.

[0020] Preferably, the same-name terminal of the demagnetization detection winding T1C is connected to the anode of the first diode and one end of the fifth resistor, the other end of the fifth resistor is connected to one end of the sixth resistor and the demagnetization detection port FB of the flyback IC, and the cathode of the first diode is connected to the positive terminal of the first capacitor and the power supply port VCC of the flyback IC.

[0021] The opposite end of the demagnetization detection winding T1C is connected to the other end of the sixth resistor, the negative terminal of the first capacitor, the ground port GND of the flyback IC, and one end of the current sensing resistor. The other end of the current sensing resistor is connected to the source of the third switching transistor and the current sensing port CS of the flyback IC.

[0022] The drive port GATE of the flyback IC is connected to the gate of the third switching transistor, the drain of the third switching transistor is connected to the same-name terminal of the primary winding T1A, the opposite-name terminal of the primary winding T1A is connected to the positive IN+ of the input voltage, and the ground port GND is connected to the negative IN- of the input voltage.

[0023] The same-name terminal of the output winding T1B is connected to the anode of the second diode, and the cathode of the second diode is connected to the positive terminal of the second capacitor and the positive output terminal OUT+; the opposite-name terminal of the output winding T1B is connected to the negative terminal of the second capacitor and the negative output terminal OUT-.

[0024] Secondly, a method for implementing a variable load circuit for flyback constant current drive is provided, based on any one of the variable load circuits for flyback constant current drive, comprising the following steps:

[0025] S1: The circuit is assembled based on the variable load circuit structure applied to flyback constant current drive, including connecting the N-type field-effect transistor Q1: the source of Q1 is connected to the negative terminal OUT- of the output terminal, the drain D is connected to one end of the third resistor, and the other end of the third resistor is connected to the positive terminal OUT+ of the output terminal; the gate of the N-type field-effect transistor Q1 is connected to the common pin of the first resistor and the second resistor in series, the non-common pin of the first resistor is connected to OUT+, and the non-common pin of the second resistor is connected to OUT-.

[0026] Connecting NPN transistor Q2: Connect the base of Q2 to the drain of Q1, the emitter to OUT-, the collector to one end of the fourth resistor, and the other end of the fourth resistor to OUT+.

[0027] S2: Integrate the variable load circuit with the flyback topology: The flyback transformer includes a primary winding T1A, an output winding T1B, and a demagnetization detection winding T1C. The demagnetization detection winding T1C also supplies power to the flyback IC. Its voltage is in phase with the voltage of the output winding T1B and satisfies the turns ratio relationship. The variable load circuit and the external LED load are connected in parallel to the output ports OUT+ and OUT- of the flyback topology.

[0028] S3: Define the key electrical parameters for flyback constant current drive, including the minimum output voltage Vout_min, the maximum output voltage Vout_max, and the dimming output voltage Vout_dim, and satisfy Vout_dim < Vout_min × 0.667; define the turn-on voltage Vth of the N-type field-effect transistor Q1, set the output threshold voltage Vout_th of the variable load circuit, and Vout_th = Vout_min;

[0029] S4: Conduct parameter calibration and function verification;

[0030] S5: Conduct no-load and full-load performance tests.<000006- 3>

[0031] Preferably, the specific process of parameter calibration and function verification in step S4 is as follows:

[0032] S41: According to Calibrate the threshold voltage so that Vout_th = Vout_min;

[0033] S42: Calculate the current and power consumption of the variable load circuit in the dimming state and the non-dimming state:

[0034] The calculation formula in the dimming state is as follows:

[0035] Current ,

[0036] Get ;

[0037] Power consumption ​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Disconnect the external LED load to simulate no-load, and test the output voltage stability and no-load power consumption to ensure that there is no voltage surge.

[0045] The rated LED load is connected to simulate full load, and the full load efficiency of the drive is calculated under Vout=Vout_max to verify that the efficiency is improved compared with the fixed resistor dummy load scheme.

[0046] The system cycles through the on / off states to detect the timeliness and stability of the LED load's response when it is lit and turned off, ensuring that there are no abnormal lighting phenomena.

[0047] The beneficial effects of this invention include:

[0048] This invention, through an innovative design of "flyback topology circuit unit + variable load circuit unit," specifically addresses the core contradiction in existing technologies where a fixed resistive dummy load cannot simultaneously achieve LED dimming functionality and high full-load efficiency with low no-load power consumption. It also boasts advantages such as strong structural compatibility and high operational stability. Specific beneficial effects are as follows:

[0049] 1. It accurately balances LED dimming function and core performance indicators, significantly improving drive energy efficiency.

[0050] The variable load circuit of this invention can automatically switch impedance according to the output voltage, perfectly balancing "power-off requirements" and "energy efficiency requirements":

[0051] To ensure stable LED off state: When the output voltage is lower than the output threshold voltage Vout_th, the variable load circuit exhibits low impedance characteristics. At this time, the first switch is turned off and the second switch is turned on. Resistors R1, R2, R3, and R4 together form a low impedance loop, which can efficiently dissipate the excess energy coupled by the output winding T1B in the off state, ensuring that the output voltage Vout_dim < Vout_min × 0.667, thus meeting the stable off state requirement.

[0052] Significantly reduced ineffective power consumption: When the output voltage is higher than the output threshold voltage Vout_th, the variable load circuit exhibits high impedance characteristics. At this time, Q1 is turned on and Q2 is turned off, and only resistors R1, R2, and R3 form a loop, significantly reducing the ineffective power consumption of the variable load. High impedance can be maintained within the rated output voltage range [Vout_min, Vout_max] through proper parameter design.

[0053] 2. It achieves automatic load impedance matching without the need for additional control circuits, thus improving operational reliability.

[0054] The variable load circuit of this invention achieves automatic impedance switching through pure hardware logic, eliminating the need for additional signals from flyback ICs or external controllers. This simplifies the control chain and reduces the risk of failure. The impedance switching of the variable load circuit is entirely controlled by the output voltage Vout, the turn-on voltage Vth of the first switch Q1, the first resistor R1, and the second resistor R2. When Vout < Vout_th, the gate-source voltage Vgs of Q1 < Vth, Q1 is off and Q2 is on. When Vout > Vout_th, Vgs > Vth, Q1 is on and short-circuits the base-emitter junction of Q2, turning Q2 off. The entire process requires no software algorithm or additional drive signal, has a fast response speed (depending only on voltage changes and the turn-on / turn-off delay of the switch), and a stable hardware structure. It can adapt to the full range of output voltage [Vout_dim, Vout_CV] of the flyback constant current drive, avoiding issues such as adjustment failure or abnormal power consumption caused by control circuit malfunctions.

[0055] 3. It has strong compatibility with existing flyback topologies, reducing industrialization costs and promotion difficulties.

[0056] The variable load circuit of this invention can be used as a "modular replacement design," requiring only the replacement of the fixed resistor in the existing flyback constant current drive, without modifying the core components of the flyback topology, such as the flyback transformer, flyback IC, switching transistor Q3, rectifier filter components, etc.

[0057] Structural compatibility: The variable load circuit replaces the fixed resistor without redesigning the PCB layout; only minor adjustments are needed to the arrangement of the "dummy load" circuit.

[0058] Process compatibility: The components used in the variable load circuit are all common standard components in the electronics industry, with low procurement costs. Moreover, the welding and assembly processes are fully compatible with existing production lines, requiring no additional equipment or adjustments to the production process. This significantly reduces the cost of technology transfer and industrialization, facilitating rapid promotion and application.

[0059] 4. Ensure the reliability of energy discharge when the LED is off to avoid the risk of voltage surge.

[0060] The variable load circuit of the present invention consumes the energy coupled to the output winding T1B in the off state through multiple resistors. Due to the reasonable power distribution of multiple resistors, the temperature of each resistor in the variable load circuit is effectively controlled, which is significantly lower than the temperature of the single resistor design of the fixed resistor R0 in the existing flyback constant current drive, thereby improving reliability. Attached Figure Description

[0061] Figure 1 This is a schematic diagram of the variable load circuit applied to flyback constant current drive according to the present invention.

[0062] Figure 2This is a schematic diagram of the circuit structure of the variable load circuit unit of the present invention.

[0063] Figure 3 This is a schematic diagram of the simulation circuit constructed according to the present invention.

[0064] Figure 4 This is a schematic diagram of the simulation results of the present invention.

[0065] The attached diagram is labeled as follows: R1-R6 are the first to sixth resistors, respectively; D1 is the first diode, D2 is the second diode, Q1-Q3 are the first to third switching transistors; RS1 is the current sensing resistor, CE1 is the first capacitor, CE2 is the second capacitor, T1A is the primary winding, T1B is the output winding, and T1C is the demagnetization detection winding. Detailed Implementation

[0066] The following is in conjunction with the appendix Figures 1-4 The present invention will be further described in detail below:

[0067] Example 1

[0068] See appendix Figure 1 As shown, see Figure 1 and Figure 2 As shown, a variable load circuit for flyback constant current drive includes a flyback topology circuit unit and a variable load circuit unit. The variable load circuit unit includes a first resistor, a second resistor, a third resistor, a fourth resistor, a first switch transistor, and a second switch transistor, wherein the first switch transistor is an enhancement-mode NMOS and the second switch transistor is an NPN transistor;

[0069] The positive output terminal of the flyback topology circuit unit is connected to one end of the first resistor, the third resistor, and the fourth resistor. The other end of the first resistor is connected to the gate of the first switching transistor and one end of the second resistor. The other end of the third resistor is connected to the drain of the first switching transistor and the base of the second switching transistor. The other end of the fourth resistor is connected to the collector of the second switching transistor.

[0070] The negative output terminal of the flyback topology circuit unit is connected to the other end of the second resistor, the source of the first switch, and the emitter of the second switch.

[0071] Example 2

[0072] Based on Example 1, see Figure 1 The flyback topology circuit unit includes a flyback transformer, a flyback IC, a fifth resistor, a sixth resistor, a current sensing resistor, a first diode, a second diode, a first capacitor, a second capacitor, and a third switching transistor; wherein the third switching transistor is a field-effect transistor.

[0073] The flyback transformer includes a primary winding T1A, an output winding T1B, and a demagnetization detection winding T1C. The output winding T1B is isolated from T1A and T1C.

[0074] The same-name terminal of the demagnetization detection winding T1C is connected to the anode of the first diode and one end of the fifth resistor. The other end of the fifth resistor is connected to one end of the sixth resistor and the demagnetization detection port FB of the flyback IC. The cathode of the first diode is connected to the positive terminal of the first capacitor and the power supply port VCC of the flyback IC.

[0075] The opposite end of the demagnetization detection winding T1C is connected to the other end of the sixth resistor, the negative terminal of the first capacitor, the ground port GND of the flyback IC, and one end of the current sensing resistor. The other end of the current sensing resistor is connected to the source of the third switching transistor and the current sensing port CS of the flyback IC.

[0076] The drive port GATE of the flyback IC is connected to the gate of the third switching transistor, the drain of the third switching transistor is connected to the same-name terminal of the primary winding T1A, the opposite-name terminal of the primary winding T1A is connected to the positive IN+ of the input voltage, and the ground port GND is connected to the negative IN- of the input voltage.

[0077] The same-name terminal of the output winding T1B is connected to the anode of the second diode, and the cathode of the second diode is connected to the positive terminal of the second capacitor and the positive output terminal OUT+; the opposite-name terminal of the output winding T1B is connected to the negative terminal of the second capacitor and the negative output terminal OUT-.

[0078] In practical applications of flyback constant current drives, there are situations where the output is not connected to an LED load (i.e., no-load). According to Ohm's law, the output current I of the output winding T1B = output voltage Vout / variable load resistance. The variable resistor has a lower impedance below the output threshold voltage Vout_th to meet the requirements of driving an adjustable LED load; the variable resistor has a higher impedance above the output threshold voltage Vout_th to meet the requirements of higher full-load efficiency and lower no-load power consumption.

[0079] The entire circuit consists of two parts: a flyback topology circuit unit and a variable load circuit unit, which work together.

[0080] The flyback topology circuit unit is responsible for the core functions of converting energy from input to output, providing constant current power to LED loads, and supplying operating voltage to variable load circuits. Its core components include a flyback transformer (containing primary winding T1A, output winding T1B, and demagnetization detection winding T1C), a flyback IC, a switching transistor Q3, rectifier diodes (D1, D2), filter capacitors (CE1, CE2), and resistors (R5, R6, RS1). The demagnetization detection winding T1C serves the dual purpose of "demagnetization state detection" and "flyback IC power supply," and its voltage is in phase with the voltage of the output winding T1B, satisfying a fixed turns ratio, ensuring that the flyback IC can indirectly control the output voltage through the FB pin.

[0081] Variable load circuit unit: It consists of a first resistor (R1), a second resistor (R2), a third resistor (R3), a fourth resistor (R4), a first switch (Q1), and a second switch (Q2), which are connected in parallel to the output port (between OUT+ and OUT-) to replace the fixed resistance dummy load R0 and form a parallel relationship with the external LED load; its core function is to automatically switch the impedance according to the change of output voltage (Vout) to match different energy dissipation requirements under different operating conditions.

[0082] In this embodiment, a method for implementing a variable load circuit for flyback constant current drive is described, based on a variable load circuit for flyback constant current drive, including the following steps:

[0083] S1: The circuit is assembled based on the variable load circuit structure applied to flyback constant current drive, including connecting the N-type field-effect transistor Q1: the source of Q1 is connected to the negative terminal OUT- of the output terminal, the drain D is connected to one end of the third resistor, and the other end of the third resistor is connected to the positive terminal OUT+ of the output terminal; the gate of the N-type field-effect transistor Q1 is connected to the common pin of the first resistor and the second resistor in series, the non-common pin of the first resistor is connected to OUT+, and the non-common pin of the second resistor is connected to OUT-.

[0084] Connecting NPN transistor Q2: Connect the base of Q2 to the drain of Q1, the emitter to OUT-, the collector to one end of the fourth resistor, and the other end of the fourth resistor to OUT+.

[0085] S2: Integrate the variable load circuit with the flyback topology: The flyback transformer includes a primary winding T1A, an output winding T1B, and a demagnetization detection winding T1C. The demagnetization detection winding T1C also supplies power to the flyback IC. Its voltage is in phase with the voltage of the output winding T1B and satisfies the turns ratio relationship. The variable load circuit and the external LED load are connected in parallel to the output ports OUT+ and OUT- of the flyback topology.

[0086] S3: Define the key electrical parameters for flyback constant-current drive, including the minimum output voltage Vout_min, the maximum output voltage Vout_max, and the dimming output voltage Vout_dim, and satisfy Vout_dim < Vout_min × 0.667; Define the turn-on voltage Vth of the N-type field-effect transistor Q1, set the output threshold voltage Vout_th of the variable load circuit, and Vout_th = Vout_min;

[0087] S4: Perform parameter calibration and function verification;

[0088] S5: Perform no-load and full-load performance tests.

[0089] The specific process of parameter calibration and function verification in step S4 is as follows:

[0090] S41: According to Calibrate the threshold voltage to make Vout_th = Vout_min;

[0091] S42: Calculate the current and power consumption of the variable load circuit in the dimming state and non-dimming state:

[0092] The calculation formula in the dimming state is as follows:

[0093] Current ,

[0094] Get ;

[0095] Power consumption ,

[0096] Get ;

[0097] The calculation formula in the non-dimming state is as follows:

[0098] Current , simplified to get

[0099] Power consumption ; simplified to get ;

[0100] S43: Verify that the flyback IC maintains the FB pin voltage not exceeding VFB_MIN in burst mode in the dimming state, the FB pin limiting voltage is VFB_CV in the non-dimming state, and the coefficient K = VFB_MIN / VFB_CV < 1.

[0101] The specific process of no-load and full-load performance tests in step S5 is as follows:

[0102] Disconnect the external LED load to simulate no-load, and test the output voltage stability and no-load power consumption to ensure that there is no voltage surge.

[0103] The rated LED load is connected to simulate full load, and the full load efficiency of the drive is calculated under Vout=Vout_max to verify that the efficiency is improved compared with the fixed resistor dummy load scheme.

[0104] The system cycles through the on / off states to detect the timeliness and stability of the LED load's response when it is lit and turned off, ensuring that there are no abnormal lighting phenomena.

[0105] The energy conversion and voltage regulation logic of the flyback topology circuit is a prerequisite for the operation of the variable load circuit. The specific process is as follows:

[0106] Energy storage stage: The input voltage (IN+, IN- connected) is applied to the primary winding T1A and the third switch Q3. The flyback IC drive port GATE outputs a drive signal to turn on the third switch Q3, and the primary winding T1A stores energy. The current forms a loop along T1A, Q3, and current sensing resistor RS1.

[0107] Energy release phase: The flyback IC detects the current signal of RS1 through the CS port. When the current reaches the threshold, it controls the third switch Q3 to turn off. The energy stored in the primary winding T1A is transferred to the output winding T1B and the demagnetizing detection winding T1C through electromagnetic induction. The energy of the output winding T1B is rectified by the first diode D1 and filtered by the second capacitor CE2, and then converted into a DC voltage Vout to power the LED load and the variable load circuit. The energy of the demagnetizing detection winding T1C is rectified and filtered to provide VCC power to the flyback IC. At the same time, its voltage signal is divided by R5 and R6 and then input to the FB pin of the flyback IC to realize feedback regulation of the output voltage.

[0108] Voltage regulation rules: The flyback IC controls the output voltage by limiting the voltage of the FB pin. In the non-off state, the FB pin limiting voltage is VFB_CV, and the corresponding output voltage Vout operates in [Vout_min, Vout_max] (the normal operating range of the LED, and the design ensures that Vout_max <= Vout_CV). In the off state, the FB pin limiting voltage drops to VFB_dim, and the coefficient K = VFB_dim / VFB_CV < 1, and the corresponding output voltage Vout_dim < Vout_min × 0.667, ensuring that the LED is completely off.

[0109] The principle of adaptive impedance regulation in variable load circuits:

[0110] The core of the variable load circuit is to automatically switch the on / off states of switching transistors Q1 and Q2 according to the magnitude relationship between the output voltage Vout and the output threshold voltage Vout_th, and then change its own impedance to match the requirements of different operating states. Among them, the output threshold voltage Vout_th is determined by the turn-on voltage Vth of the first switching transistor Q1 and the voltage division relationship of R1 and R2. The derivation formula is: , when the gate-source voltage Vgs of the first switching transistor Q1 is Vgs = Vth, Vout reaches the threshold, and the first switching transistor Q1 starts to conduct, that is:

[0111] The output threshold voltage of the variable load circuit: .

[0112] When Vout < Vout_th (dim state, Vout = Vout_dim): In this state, the LED needs to be completely turned off, and since the output winding T1B is closely coupled with the primary winding T1A, a large amount of energy that needs to be dissipated will accumulate. The variable load needs to present a low impedance to quickly dissipate the energy:

[0113] Judgment of the switching transistor state:

[0114] When Vout < Vout_th, Vgs < Vth, and the first switching transistor Q1 (enhancement-mode NMOS) is turned off (because an enhancement-mode NMOS transistor needs Vgs > Vth to conduct). The b-e of the second switching transistor (NPN transistor) is not short-circuited, forming a base current loop OUT+ → R3 → Q2's b-e → OUT-. The second switching transistor conducts, forming a collector current loop OUT+ → R4 → Q2's c-e → OUT-. At this time, currents flow through R1, R2, R3, and R4. The design parameters ensure that the second switching transistor Q2 operates in deep saturation / quasi-saturation state, and the voltage drops across its be and ce can be ignored.

[0115] When Vout > Vout_th, Vgs > Vth, and the first switching transistor Q1 (enhancement-mode NMOS) is turned on (because an enhancement-mode NMOS transistor needs Vgs > Vth to conduct). The b-e of the second switching transistor (NPN transistor) is short-circuited, and the drain current loop OUT+ → R3 → Q1's DS → OUT- is formed. The base current loop fails, the second switching transistor is cut off, and the collector current loop fails. At this time, currents flow through R1, R2, and R3.

[0116] Formula derivation:

[0117] Define the output voltage Vout1, the variable load circuit current Iout1, and the variable load circuit power consumption Pout1 when the voltage is lower than the threshold voltage; at this time, the variable load circuit current:

[0118] , simplified to ;

[0119] At this point, the power consumption of the variable load circuit is:

[0120] Simple ;

[0121] Define the output voltage Vout2, the variable load circuit current Iout2, and the variable load circuit power consumption Pout2 when the voltage is above the threshold voltage; at this time, the variable load circuit current is:

[0122] Simple ;

[0123] At this point, the power consumption of the variable load circuit is:

[0124] Simple ;

[0125] Examples and simulations:

[0126] Given parameters Vout_min=21V, Vout_max=42V, and variable load circuit design parameters R1=130KΩ, R2=10KΩ, R3=130KΩ, R4=3KΩ, and selecting an enhancement-type NMOS with Vth=1.5V, based on the derived formula above, we can calculate Vout_th≈21V, Iout1≈4.178mA, and Pout1≈50mW, which can quickly dissipate excess energy in the off state and ensure Vout_dim<Vout_min×0.667 (21V×0.667≈14V). In the actual design, Vout_dim=12V, leaving a 2V margin to ensure that the LED load can be adjusted to off.

[0127] For ease of calculation, assume Vout_max = Vout_CV = 42V, that is, the no-load voltage and the maximum full-load voltage are the same. According to the above derivation formula, we can calculate that Iout2≈0.623mA and Pout2≈26mW. It can be seen that the power consumption of the variable load circuit is reduced under both no-load and maximum full-load output voltage.

[0128] If the existing fixed resistor dummy load scheme is adopted, it is first necessary to ensure the same 50mW power dissipation during switching off to ensure consistent switching off capability of the drive. The fixed resistor R0 = (12V)^2 / 50mW ≈ 2880Ω, and the power dissipation P0 = (42V)^2 / 2880Ω ≈ 612.5mW under no-load and full-load output voltage of 42V. Under the same power dissipation during switching off, the dummy load power dissipation of the variable load circuit and the fixed resistor R0 under no-load and full-load conditions is significantly different. Taking a flyback constant current drive with a rated input power of 20W as an example, using the variable load circuit, the full-load efficiency is improved by (612.5mW-26mW) / 20W ≈ 2.93%, which is a significant improvement in full-load efficiency.

[0129] Building simulation circuits, such as Figure 3 As shown, the simulation results can be found in [reference needed]. Figure 4 As shown, the green waveform is the Vout voltage, and the red waveform is the total power consumption of the variable load circuit. It can be seen that the total power consumption of the variable load circuit in the off state is 49.814mW, the total power consumption of the dummy load under the maximum output voltage is 26.169mW, and the output threshold voltage is about 21V, which is consistent with the theoretical calculation parameters.

[0130] In summary, the variable load circuit achieves precise matching of different operating states through a closed-loop logic of "output voltage threshold → switching transistor state switching → impedance adaptive adjustment": in the off state (Vout < Vout_th), low impedance ensures rapid energy dissipation and stable LED extinguishing; in the non-off state (Vout > Vout_th), high impedance reduces ineffective power consumption and improves energy efficiency; and the entire process requires no additional control signals, achieving self-adaptation solely through the natural characteristics of the hardware circuit, resulting in fast response and high stability, perfectly resolving the technical contradictions of existing fixed resistor dummy loads.

Claims

1. A variable load circuit for flyback constant current drive, characterized in that, It includes a flyback topology circuit unit and a variable load circuit unit; The variable load circuit unit includes a first resistor, a second resistor, a third resistor, a fourth resistor, a first switching tube, and a second switching tube; The positive output terminal of the flyback topology circuit unit is connected to one ends of the first resistor, the third resistor, and the fourth resistor. The other end of the first resistor is connected to the gate of the first switching tube and one end of the second resistor. The other end of the third resistor is connected to the drain of the first switching tube and the base of the second switching tube. The other end of the fourth resistor is connected to the collector of the second switching tube; The negative output terminal of the flyback topology circuit unit is connected to the other end of the second resistor, the source of the first switching tube, and the emitter of the second switching tube; A method for realizing a variable load circuit for flyback constant current drive, implemented based on the variable load circuit for flyback constant current drive, includes the following steps: S1: Perform circuit assembly based on the variable load circuit structure for flyback constant current drive, including connecting the N-channel MOSFET Q1: Connect the source of Q1 to the negative terminal OUT- of the output terminal, the drain D to one end of the third resistor, and the other end of the third resistor to the positive terminal OUT+ of the output terminal; Connect the gate of the N-channel MOSFET Q1 to the common pin after the first resistor and the second resistor are connected in series. The non-common pin of the first resistor is connected to OUT+, and the non-common pin of the second resistor is connected to OUT-; Connect the NPN-type triode Q2: Connect the base of Q2 to the drain of Q1, the emitter to OUT-, and the collector to one end of the fourth resistor, and the other end of the fourth resistor to OUT+; S2: Integrate the variable load circuit with the flyback topology: The flyback transformer includes a primary winding T1A, an output winding T1B, and a demagnetization detection winding T1C, and the demagnetization detection winding T1C supplies power to the flyback IC at the same time. Its voltage is in phase with the voltage of the output winding T1B and satisfies the turn ratio relationship. The variable load circuit and the external LED load are connected in parallel to the output ports OUT+ and OUT- of the flyback topology; S3: Define the key electrical parameters of the flyback constant current drive, including the minimum output voltage Vout_min, the maximum output voltage Vout_max, and the dimming output voltage Vout_dim, and satisfy Vout_dim < Vout_min × 0.667; Define the turn-on voltage Vth of the N-channel MOSFET Q1, and set the output threshold voltage Vout_th of the variable load circuit, and Vout_th = Vout_min; S4: Perform parameter calibration and function verification; S5: Perform no-load and full-load performance tests.

2. The variable load circuit for flyback constant current drive according to claim 1, characterized in that, The flyback topology circuit unit includes a flyback transformer, a flyback IC, a fifth resistor, a sixth resistor, a current sensing resistor, a first diode, a second diode, a first capacitor, a second capacitor, and a third switching tube; Among them, the third switching tube is a MOSFET; The flyback transformer includes a primary winding T1A, an output winding T1B, and a demagnetization detection winding T1C, and the output winding T1B is isolated from T1A and T1C.

3. A variable load circuit for flyback constant current drive according to claim 2, characterized in that, The same-name terminal of the demagnetization detection winding T1C is connected to the anode of the first diode and one end of the fifth resistor. The other end of the fifth resistor is connected to one end of the sixth resistor and the demagnetization detection port FB of the flyback IC. The cathode of the first diode is connected to the positive terminal of the first capacitor and the power supply port VCC of the flyback IC. The opposite end of the demagnetization detection winding T1C is connected to the other end of the sixth resistor, the negative terminal of the first capacitor, the ground port GND of the flyback IC, and one end of the current sensing resistor. The other end of the current sensing resistor is connected to the source of the third switching transistor and the current sensing port CS of the flyback IC. The drive port GATE of the flyback IC is connected to the gate of the third switching transistor, the drain of the third switching transistor is connected to the same-name terminal of the primary winding T1A, the opposite-name terminal of the primary winding T1A is connected to the positive IN+ of the input voltage, and the ground port GND is connected to the negative IN- of the input voltage. The same-name terminal of the output winding T1B is connected to the anode of the second diode, and the cathode of the second diode is connected to the positive terminal of the second capacitor and the positive output terminal OUT+; the opposite-name terminal of the output winding T1B is connected to the negative terminal of the second capacitor and the negative output terminal OUT-.

4. A variable load circuit for flyback constant current drive according to claim 1, characterized in that, The first switching transistor is an N-type field-effect transistor, and the second switching transistor is an NPN type transistor.

5. A method for implementing a variable load circuit for flyback constant current drive based on claim 1, characterized in that, The specific process for parameter calibration and functional verification in step S4 is as follows: S41: According to Calibrate the threshold voltage so that Vout_th = Vout_min; S42: Calculate the current and power consumption of the variable load circuit in both off-state and non-off-state conditions. The formula for calculating the extinguishing state is as follows: Current ; get ; Power consumption ; get ; The calculation formula for the non-extinguishing state is as follows: Current Simple ; Power consumption Simple ; S43: Verify that the flyback IC maintains the FB pin voltage no more than VFB_MIN in burst mode when in the off state, and the FB pin voltage limit is VFB_CV when in the non-off state, and the coefficient K = VFB_MIN / VFB_CV < 1.

6. A method for implementing a variable load circuit for flyback constant current drive as described in claim 1, characterized in that, The specific process for conducting no-load and full-load performance tests in step S5 is as follows: Disconnect the external LED load to simulate no-load, and test the output voltage stability and no-load power consumption to ensure that there is no voltage surge. The rated LED load is connected to simulate full load, and the full load efficiency of the drive is calculated under Vout=Vout_max to verify that the efficiency is improved compared with the fixed resistor dummy load scheme. The system cycles through the on / off states to detect the timeliness and stability of the LED load's response when it is lit and turned off, ensuring that there are no abnormal lighting phenomena.

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