Overcurrent protection implementation method of high-precision isolation flyback switching power supply

Abstract

The invention relates to the technical field of switching power supplies, and discloses an overcurrent protection implementation method for a high-precision isolation flyback switching power supply, which comprises the following steps of: sampling output current at a direct-current output end of the switching power supply to generate sampling voltage; amplifying the sampling voltage to generate an amplified voltage; comparing the amplified voltage with a fixed internal reference voltage arranged at a secondary side; when the amplified voltage reaches or exceeds the internal reference voltage, an isolation unit is triggered to generate a shutdown instruction signal; and the primary side main control chip receives the shutdown instruction signal and executes a shutdown protection action. The sampling point and the comparison reference are arranged on the secondary side of the power supply, decoupling of the protection threshold and the primary side alternating current input voltage is achieved, and high-precision and high-stability over-current protection can be achieved within the wide voltage input range.

Description

Technical Field

[0001] This invention relates to the field of switching power supply technology, specifically to a method for implementing overcurrent protection in a high-precision isolated flyback switching power supply. Background Technology

[0002] Switching power supplies, especially isolated flyback switching power supplies, are widely used as indispensable power conversion units in modern electronic devices. In the design and operation of switching power supplies, overcurrent protection is a crucial element to ensure their safe and reliable operation, preventing component damage or even safety accidents caused by excessive output load or short circuits.

[0003] In existing technologies, a commonly used overcurrent protection scheme is implemented by sampling the current on the primary side of the power supply. This scheme typically involves connecting a current sampling resistor in series in the current path of the primary-side main power switch, and indirectly reflecting the magnitude of the primary-side peak current by monitoring the voltage across this resistor. When the detected primary-side peak current reaches a threshold set internally by the main control chip, the main control chip will take action, such as shutting down the drive pulse of the current cycle, to limit the current.

[0004] However, this protection mechanism based on primary-side peak current sampling has an inherent technical flaw. For switching power supplies required to operate over a wide AC input voltage range (e.g., 90VAC to 264VAC), the required primary-side peak input current to maintain a constant output power varies significantly with the input voltage. Specifically, when the AC input voltage is low, the power supply needs to draw a higher primary-side peak current to maintain power balance; conversely, when the input voltage is high, only a smaller primary-side peak current is required. Since the trigger threshold of the protection circuit is a fixed value set for the primary-side peak current, this causes the actual output overcurrent protection point to drift across the entire input voltage range. This drift greatly reduces the accuracy of overcurrent protection, causing premature protection at low input voltages and delayed protection at high input voltages, failing to provide a stable and consistent protection threshold across the entire voltage range, thus affecting the overall performance and reliability of the power supply. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for implementing overcurrent protection in a high-precision isolated flyback switching power supply. This method solves the problem that the overcurrent protection threshold based on primary-side current sampling in existing technologies drifts with changes in AC input voltage, resulting in low protection accuracy.

[0006] To address the aforementioned technical problems, this invention provides a method for overcurrent protection in a high-precision isolated flyback switching power supply, applicable to the switching power supply comprising a primary-side main control chip and primary / secondary-side isolation units, comprising the following steps:

[0007] At the DC output terminal of the switching power supply, the output current is sampled to generate a sampling voltage that is proportional to the output current.

[0008] The sampled voltage is amplified to generate an amplified voltage;

[0009] The amplified voltage is compared with a fixed internal reference voltage set on the secondary side;

[0010] When the amplified voltage reaches or exceeds the internal reference voltage, the isolation unit is triggered to generate a shutdown command signal that crosses the primary and secondary side isolation barriers.

[0011] The primary control chip receives the shutdown command signal and performs shutdown protection actions.

[0012] In one implementation, the step of sampling the output current is specifically achieved through a current sampling resistor connected in series at the DC output terminal. This is achieved by sampling voltage ( ). ) and the output current ( The relationship is:

[0013] ;

[0014] In another embodiment, the step of sampling the output current is specifically implemented using a current transformer located in the DC output circuit, and a signal processing circuit converts the induced current signal output by the current transformer into the sampled voltage. In one embodiment, the step of amplifying the sampled voltage is specifically performed using a non-inverting amplifier circuit composed of an operational amplifier. The output voltage of the operational amplifier ( (i.e., the amplified voltage) and the sampling voltage ( The relationship is:

[0015] ;

[0016] in, The input offset voltage of the operational amplifier; and These are the resistance values ​​of the feedback resistor and the gain resistor of the non-inverting amplifier circuit, respectively.

[0017] In one implementation, the comparison step is specifically implemented using a precision reference voltage source (e.g., ME431) having the fixed internal reference voltage. The amplified voltage ( The amplified voltage () is applied to the reference pin of the precision reference voltage source. ) reaches or exceeds the internal reference voltage ( When the precision reference voltage source changes from cutoff to conduction, the state of the source changes from cutoff to conduction.

[0018] Accordingly, the step of triggering the isolation unit specifically includes: when the precision reference voltage source is turned on, driving the input light-emitting diode of an optocoupler to emit light, wherein the optocoupler serves as the isolation unit.

[0019] In another embodiment, the comparison step specifically involves generating the fixed internal reference voltage using a separate precision voltage reference source, and comparing the amplified voltage with the fixed internal reference voltage using a separate voltage comparator. When the amplified voltage reaches or exceeds the fixed internal reference voltage, the output level of the separate voltage comparator is inverted to drive the isolation unit.

[0020] In one embodiment, the step of the primary-side main control chip receiving the shutdown command signal specifically includes: turning on the output phototransistor of the optocoupler and pulling the level of an RT pin of the primary-side main control chip low to a preset shutdown threshold level. In another embodiment, the step of performing the shutdown protection action specifically includes: pulling the level of an enable pin or a fault pin of the primary-side main control chip low to its non-operating state or fault state through the isolation unit.

[0021] The technical solution provided by this invention has an overcurrent protection trigger condition of the amplified voltage ( To reach the fixed internal reference voltage At the critical trigger point, it can be considered that... Substituting the relationship between the sampling voltage and the amplified voltage into the equation, the overcurrent protection threshold current can be derived. The determining relation for ) is:

[0022] ;

[0023] in, This is the overcurrent protection threshold current; The resistance value of the current sampling resistor; and These are the resistance values ​​of the gain resistor and the feedback resistor; The fixed internal reference voltage; This is the input offset voltage of the operational amplifier. From this relationship, it can be seen that the overcurrent protection threshold current ( The value of ) is determined solely by the device parameters set on the secondary side of the power supply. ) and the inherent precision parameters of the device ( The decision was made by [the relevant authority].

[0024] This invention provides a method for implementing overcurrent protection in a high-precision isolated flyback switching power supply. It offers the following advantages:

[0025] 1. This invention samples the output current at the DC output terminal of the switching power supply and uses a fixed internal reference voltage located on the secondary side for threshold comparison. Since both the sampling signal source and the reference voltage are located on the secondary side, their determining factors ( and It is completely decoupled from the primary side AC input voltage, thus fundamentally solving the problem of overcurrent protection point drifting with input voltage changes caused by primary side sampling in the existing technology, and can achieve high stability and high consistency protection over a wide input voltage range.

[0026] 2. This invention adds a step to amplify the sampled voltage between the sampling and comparison steps. By amplifying the weak sampled voltage using an operational amplifier, the signal amplitude and signal-to-noise ratio can be effectively improved, allowing the subsequent comparison unit to make a decision based on a stronger and clearer signal. This enables the circuit to respond accurately to overcurrent conditions, reduces the risk of false triggering or non-triggering due to noise interference, and improves the sensitivity and reliability of the protection.

[0027] 3. In the method of this invention, the magnitude of the overcurrent protection threshold current is jointly determined by the resistance value of the current sampling resistor, the amplification factor of the signal amplification circuit (determined by its gain resistor and feedback resistor), and the fixed internal reference voltage. Those skilled in the art can conveniently and accurately set the required overcurrent protection point by selecting or adjusting the parameters of these resistor components located on the secondary side, offering high design freedom and easily meeting the protection requirements of power supplies of different specifications. Attached Figure Description

[0028] Figure 1 This is a block diagram of the overcurrent protection circuit structure of the present invention;

[0029] Figure 2 This is a schematic diagram of the overcurrent protection core circuit of the present invention;

[0030] Figure 3 This is a schematic diagram of the overall circuit of the isolated flyback switching power supply of the present invention;

[0031] Figure 4 This is a flowchart of the overcurrent protection method of the present invention. Detailed Implementation

[0032] The technical solutions in 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.

[0033] See attached document Figure 1 To be continued Figure 3 This embodiment provides a high-precision overcurrent protection method and circuit for use in an isolated flyback switching power supply. The isolated flyback switching power supply includes a power input and rectification module, a main power module, and a main control shutdown unit 50 (i.e., the main control chip IC1) located on the primary side. The overcurrent protection circuit is mainly located on the secondary side of the power supply and is connected to the main control shutdown unit 50 on the primary side through an isolation feedback unit 40.

[0034] The overcurrent protection circuit specifically includes: a current sampling unit 10, a signal amplification unit 20, a comparison and reference unit 30, an isolation feedback unit 40, and a main control shutdown unit 50.

[0035] The current sampling unit 10 is used to sample the output current of the switching power supply. In this embodiment, the current sampling unit 10 is specifically a current sampling resistor R11 connected in series in the DC output circuit of the switching power supply. When the output current... When current flows through the current sampling resistor R11, a sampling voltage is generated. The sampling voltage With output current The relationship is:

[0036] ;

[0037] in, This is the voltage across the current sampling resistor R11; This refers to the output current of the switching power supply. This is the resistance value of the current sampling resistor.

[0038] The signal amplification unit 20, whose input terminal is electrically connected to the current sampling unit 10, is used to sample the voltage. Enlarged. In this embodiment, as shown in the attached... Figure 2 As shown, the signal amplification unit 20 is specifically an operational amplifier and its peripheral circuitry. The non-inverting input of the operational amplifier is connected to one end of the current sampling resistor R11 to receive the sampled voltage. The operational amplifier forms a non-inverting amplifier circuit through feedback resistor R16 and gain resistor R18. This unit may also include resistors (e.g., R19, R223) and capacitors (e.g., C244, C22) for signal filtering and stabilization. Its output voltage... The calculation method is as follows:

[0039] ;

[0040] in, This refers to the output voltage of the operational amplifier. This is the input offset voltage of the operational amplifier; This is the resistance value of the feedback resistor; This is the resistance value of the gain resistor.

[0041] The comparison and reference unit 30 has its input terminal electrically connected to the output terminal of the signal amplification unit 20. In this embodiment, the comparison and reference unit 30 is specifically a precision reference voltage source U1 (model ME431). The reference pin (REF) of the precision reference voltage source U1 is connected to the output terminal of the operational amplifier to receive the output voltage. The precision reference voltage source U1 has a fixed internal reference voltage. 。

[0042] The isolation feedback unit 40 has its input side electrically connected to the comparison and reference unit 30, and its output side is located in the primary circuit of the switching power supply. In this embodiment, the isolation feedback unit 40 is specifically an optocoupler PC2. The input light-emitting diode of the optocoupler PC2 is connected in series in the circuit formed by the cathode and anode of the precision reference voltage source U1.

[0043] The main control shutdown unit 50 is located in the primary circuit of the switching power supply and is electrically connected to the output side of the isolation feedback unit 40. In this embodiment, as shown in the attached... Figure 3 As shown, the main control shutdown unit 50 is specifically the main control chip IC1 of the switching power supply. The output phototransistor of the optocoupler PC2 is connected to a control pin of the main control chip IC1, which is the RT pin. The on or off state of the phototransistor is used to change the voltage level of the RT pin, thereby controlling the operating state of the main control chip IC1. Specifically, when the voltage of the RT pin is pulled down below a preset shutdown threshold, the main control chip IC1 stops working. In this embodiment, the shutdown threshold is 1.05V.

[0044] See attached document Figure 2 and attached Figure 3 When the isolated flyback switching power supply is in normal operating condition, its output current... At the preset overcurrent protection threshold Below. In this state, the output current... A signal is generated on R11 through the current sampling unit 10 (i.e., the current sampling resistor R11) that is analogous to the current sampling resistor R11. Proportional sampling voltage .because At this time It is at a relatively low level.

[0045] The non-inverting input of the signal amplification unit 20 (i.e., the operational amplifier) ​​receives this The signal is amplified and output as a voltage through an amplification circuit composed of external resistors R16 and R18. According to the formula At this time It is also at a relatively low level.

[0046] The reference pin (REF) of the comparison and reference unit 30 (i.e., the precision reference voltage source U1) receives this. Voltage. Under normal operating conditions, this The value is lower than the internal reference voltage of the precision reference voltage source U1. (For example, ME431) for .

[0047] Due to the application to the REF pin Not achieved At the threshold, the cathode and anode of the precision reference voltage source U1 are in a cutoff state, i.e., a high-resistance state. Because the input LED of the isolation feedback unit 40 (i.e., the optocoupler PC2) is connected in series with the conduction path of U1, insufficient current flows through the LED when U1 is in the cutoff state, preventing it from emitting light. Since the input of the optocoupler PC2 is not driven, the phototransistor at its output remains in the cutoff (non-conducting) state.

[0048] The RT pin (as a control pin) of the main control shutdown unit 50 (i.e., the main control chip IC1) is connected to the output electrode (e.g., collector or emitter) of the phototransistor. Since the phototransistor is in the off state, it cannot provide a low-impedance path to GND potential (ground potential) for the RT pin.

[0049] Therefore, the voltage at the RT pin is maintained at a preset normal operating level by the internal or external pull-up circuit of the main control chip IC1. When the main control chip IC1 detects that the RT pin is at a normal operating level, it determines that no overcurrent fault has occurred in the system and thus continues to execute its normal switching control logic (e.g., continuously outputting a PWM drive signal), ensuring stable energy transfer and voltage output for the isolated flyback switching power supply.

[0050] See attached document Figure 2 and attached Figure 3 When an overcurrent or short-circuit fault occurs at the output terminal of an isolated flyback switching power supply, its output current... The current increases rapidly until it reaches or exceeds the preset overcurrent protection threshold. In this state, the output current increases. The current flows through the current sampling unit 10 (i.e., the current sampling resistor R11), generating a correspondingly increased sampling voltage across R11. The non-inverting input of the signal amplification unit 20 (i.e., the operational amplifier) ​​receives this... Signal.

[0051] According to the formula The output voltage of the operational amplifier Also follow The voltage increases with the increase of the reference voltage. This is continuously monitored by comparing the reference pin (REF) of reference unit 30 (i.e., precision reference voltage source U1). Voltage. When Reaching or exceeding hour, The value accordingly reaches or exceeds the internal reference voltage of the precision reference voltage source U1. (For example, ME431) (2.5V).

[0052] once When the precision reference voltage source U1 is triggered, its internal state changes, causing the cathode (CATHODE) and anode (ANODE) ​​to change from the original high resistance state (cutoff) to the low resistance state (conduction).

[0053] The input light-emitting diode of the isolation feedback unit 40 (i.e., optocoupler PC2) is connected in series in the conduction path of the precision reference voltage source U1. When U1 is turned on, current flows through the light-emitting diode, causing it to emit light and generate an optical signal.

[0054] The optical signal crosses the electrical isolation barrier and is received by the phototransistor on the output side (located on the primary side of the power supply) of the optocoupler PC2. After receiving the optical signal, the phototransistor changes its state from cutoff to conduction (for example, a low-resistance path is formed between its collector and emitter).

[0055] The RT pin (as a control pin) of the main control shutdown unit 50 (i.e., the main control chip IC1) is electrically connected to the output electrode of the phototransistor. When the phototransistor is turned on, it provides a low-impedance path to the primary side GND potential (ground potential) for the RT pin.

[0056] Therefore, the voltage at the RT pin is quickly pulled down from the normal operating level to a low level close to the GND potential by the turned-on phototransistor.

[0057] The fault detection logic circuit inside the main control chip IC1 continuously monitors the level of the RT pin. When it detects that the RT pin is pulled low, it identifies it as an externally triggered overcurrent fault signal.

[0058] In response, once the voltage at the RT pin falls below its preset shutdown threshold (e.g., 1.05V), the main control chip IC1 immediately executes a shutdown protection action, for example, stopping the generation of pulse width modulation (PWM) signals to drive the primary-side switching transistor (such as Q1). Switching power transfer is thus interrupted, and the switching power supply stops supplying power to the output, thereby achieving overcurrent protection for the output.

[0059] See attached document Figure 1 To be continued Figure 3 The current sampling point is located at the source of the primary-side power switch (such as a MOSFET). When the AC input voltage of an isolated flyback switching power supply changes (e.g., over a wide range from 90VAC to 264VAC), even if the output power demand remains constant, the peak current required on the primary side to maintain power balance will change significantly. Because the sampling signal is directly derived from this primary-side current affected by the input voltage, the trigger point of its overcurrent protection drifts under different input voltages, making it impossible to maintain a consistent protection current value over a wide voltage range.

[0060] First, in this embodiment, the current sampling unit 10 (i.e., the current sampling resistor R11) is placed at the DC output terminal (i.e., the secondary side) of the isolated flyback switching power supply. In this configuration, the current sampling unit 10 monitors and samples the DC output current. .Should The value is determined by the load and is regulated and stabilized through the (secondary) voltage regulation feedback loop of the switching power supply (not shown in the figure, but inherent to the switching power supply). Therefore, the DC output current... The value of this value achieves electrical isolation and decoupling from fluctuations in the primary-side AC input voltage. This is achieved through sampling. The generated sampling voltage Its signal source itself is not directly related to the fluctuation of the primary side AC input voltage, thus removing the main interference source that causes the overcurrent point drift in the background technology.

[0061] Secondly, in this embodiment, the trigger threshold for overcurrent protection is determined by comparing it with the internal reference voltage of the reference unit 30 (i.e., the precision reference voltage source U1, such as ME431). Decision. (For example, ME431) (2.5V) is a highly stable reference voltage, whose value is fixed in the design and is not affected by changes in the primary or secondary operating voltage of the power supply.

[0062] The overcurrent protection is triggered by the output voltage of the signal amplification unit 20. Reaching or exceeding the internal reference voltage .Right now .

[0063] The output formula of signal amplification unit 20 and the sampling formula of current sampling unit 10 By combining these methods, the overcurrent protection threshold of this circuit can be derived. (i.e., when protection is triggered) The determining relation for ) is:

[0064] ;

[0065] From the above relationship, it can be seen that the overcurrent protection threshold... The value is determined solely by (U1's internal reference voltage) (Op-amp input offset voltage) (Sampling resistor value) (Feedback resistor value) and The gain resistor value is determined by the resistance value.

[0066] All these decisions Parameters ( These are all inherent precision parameters of the device, or ( ( ) represents the resistance value of the resistor assembly on the secondary side. These parameters are independent of the AC input voltage on the primary side.

[0067] Therefore, this technical solution sets the sampling point at the output terminal and combines it with a precision reference voltage source for threshold comparison, thus enabling overcurrent protection. It maintains high consistency and stability across the entire wide input voltage range (e.g., 90VAC to 264VAC), effectively solving the overcurrent protection point drift problem caused by input voltage variations in the prior art.

[0068] See attached document Figure 2 - Appendix Figure 4 Overcurrent protection threshold The setting is achieved by selecting and configuring the parameters of specific components in the circuit. The mathematical model derivation of this method is as follows:

[0069] The protection action is triggered by comparing the voltage on the reference pin (REF) of the reference unit 30 (i.e., the precision reference voltage source U1) with the voltage of the reference unit 30. Reaching or exceeding its internal reference voltage At the critical trigger point, it can be considered that... .

[0070] Substituting this condition into the output voltage formula of the signal amplification unit 20, we can obtain the critical sampling voltage at its input terminal at the moment of triggering. (recorded as) The following conditions must be met:

[0071] ;

[0072] By rearranging terms, the critical sampling voltage required to trigger the protection can be solved. :

[0073] ;

[0074] this The output current flowing through current sampling unit 10 (i.e., current sampling resistor R11) Generate. When Overcurrent protection threshold reached At that time, that is Substituting this relationship into the above formula, the overcurrent protection threshold can be derived. The calculation formula is as follows:

[0075] ;

[0076] in, This is the overcurrent protection threshold current; , and These are the resistance values ​​of the current sampling resistor, the feedback resistor, and the gain resistor, respectively. This is the internal reference voltage of the precision reference voltage source U1; This is the input offset voltage of the operational amplifier in the signal amplification unit 20.

[0077] Based on the above mathematical model, adjust the overcurrent protection threshold. The specific steps are as follows:

[0078] First, determine a desired overcurrent protection threshold current. .

[0079] Secondly, a current sampling resistor R11 is selected for the current sampling unit 10. The selection of the R11 resistance value requires a trade-off between two factors: one is the power loss of the resistor itself (…). On the other hand, there is the sampling voltage it generates. The amplitude. Larger. Amplitude is beneficial for improving the signal-to-noise ratio and reducing the impact of circuit noise on measurement accuracy;

[0080] Then, according to the established and Calculate the critical sampling voltage required at the trigger point. .

[0081] Next, based on the known... (Inherent parameters of the device) (Inherent parameters of the device) and the calculated Determine the voltage amplification factor required by the signal amplification unit 20;

[0082] Finally, the amplification factor of the signal amplification unit 20 is configured by selecting or adjusting the values ​​of the feedback resistor R16 and the gain resistor R18 to meet the requirements. The relationship.

[0083] By performing the above steps, those skilled in the art can precisely set and adjust the overcurrent protection threshold of the switching power supply according to specific application requirements. Since all parameters in the formula are independent of the primary AC input voltage of the power supply, the set... The value remains stable across the entire operating voltage range.

[0084] See attached document Figure 2 and attached Figure 3 In this embodiment, the current sampling unit 10 is implemented using a current sampling resistor R11. In another alternative embodiment, the current sampling unit 10 can also be implemented using a current transformer (CT) to achieve the same overcurrent protection purpose.

[0085] In this alternative implementation, a current transformer is placed in the DC output circuit of the isolated flyback switching power supply. Its primary winding is connected in series in the output circuit to sense the actual output current flowing through it. .

[0086] The secondary winding of a current transformer is used to output a current that is the same as the primary winding current (i.e., ...). The circuit generates an induced current signal in a specific ratio. The circuit also includes a signal processing circuit connected to the secondary winding of the current transformer. This signal processing circuit (e.g., may include a load resistor, a rectifier circuit, and a filter circuit) converts the induced current signal output from the secondary winding into a DC sampling voltage, which is proportional to the output current. The value is directly proportional to the value.

[0087] This DC sampling voltage (its function is equivalent to the sampling voltage in the aforementioned embodiments) The signal is transmitted to the input terminal (similar to the inverting input terminal) of the signal amplification unit 20 (i.e., the operational amplifier).

[0088] The structure, connection relationship, and subsequent working principle (including signal amplification, comparison triggering, isolation feedback, and shutdown protection) of the signal amplification unit 20, comparison and reference unit 30, isolation feedback unit 40, and main control shutdown unit 50 are the same as those described in detail in the aforementioned embodiment using current sampling resistor R11.

[0089] See attached document Figure 2 and attached Figure 3 In this embodiment, the comparison and reference unit 30 is implemented using a precision reference voltage source U1 (model ME431). This device U1 integrates a fixed internal reference voltage. It utilizes the characteristics of its REF pin, cathode, and anode to simultaneously provide voltage comparison and reference functions.

[0090] In another alternative implementation, the comparator and reference unit 30 can be replaced by a combination of a separate voltage comparator IC and a separate precision voltage reference source IC. In this alternative configuration, the separate precision voltage reference source IC is used to generate a fixed, highly stable alternative reference voltage. An independent voltage comparator IC has a first input (e.g., a non-inverting input), a second input (e.g., an inverting input), and an output.

[0091] Output voltage of signal amplification unit 20 Connected to the first input of the voltage comparator IC. An alternative reference voltage generated by a separate precision voltage reference IC. Connect to the second input terminal of the voltage comparator IC.

[0092] Output voltage of signal amplification unit 20 Connected to the first input of the voltage comparator IC. An alternative reference voltage generated by a separate precision voltage reference IC. Connect to the second input terminal of the voltage comparator IC.

[0093] The output of the voltage comparator IC is electrically connected to the circuit containing the input LED of the isolation feedback unit 40 (i.e., the optocoupler PC2). When The voltage value exceeds When the voltage value is reached, the output level of the voltage comparator IC is reversed (for example, from high level to low level, or from high impedance state to low impedance state), thereby driving the light-emitting diode of the optocoupler PC2 to conduct.

[0094] Furthermore, in this embodiment, the output phototransistor of the isolation feedback unit 40 (i.e., optocoupler PC2) is connected to the RT pin of the main control shutdown unit 50 (i.e., main control chip IC1).

[0095] In other alternative implementations, the output phototransistor of optocoupler PC2 can also be connected to other types of control pins of the main control chip IC1 for controlling chip start-up or triggering protection.

[0096] For example, the control pin can be an enable (EN) pin or a disable (Disable) pin. In this configuration, when the output phototransistor of optocoupler PC2 is turned on, it pulls the level of the EN pin or DIS pin to its inactive state (e.g., pulls a high-active EN pin to GND), thereby stopping the main control chip IC1 from operating.

[0097] For example, the control pin can be a dedicated fault (FAULT) pin. In this configuration, when the output phototransistor of the optocoupler PC2 is turned on, it pulls the level of the FAULT pin to its fault state (e.g., to GND ground potential). After detecting the fault state level of the FAULT pin, the logic circuit inside the main control chip IC1 immediately performs a shutdown protection action and stops generating PWM drive signals.

Claims

1. A method for implementing overcurrent protection in a high-precision isolated flyback switching power supply, characterized in that, Includes the following steps: At the DC output terminal of the switching power supply, the output current is sampled, and a sampling voltage proportional to the output current is generated. The sampled voltage is amplified to generate an amplified voltage; The amplified voltage is compared with a fixed internal reference voltage set on the secondary side; When the amplified voltage reaches or exceeds the internal reference voltage, the isolation unit is triggered, and a shutdown command signal that crosses the primary and secondary isolation barriers is generated. The primary control chip receives the shutdown command signal and executes a shutdown protection action.

2. The overcurrent protection implementation method for a high-precision isolated flyback switching power supply according to claim 1, characterized in that, The step of sampling the output current specifically includes: The output current is converted into the sampled voltage through a current sampling resistor connected in series at the DC output terminal.

3. The overcurrent protection implementation method for a high-precision isolated flyback switching power supply according to claim 1, characterized in that, The step of amplifying the sampled voltage specifically includes: The sampled voltage is amplified in phase using an operational amplifier to generate the amplified voltage.

4. The overcurrent protection implementation method for a high-precision isolated flyback switching power supply according to claim 1, characterized in that, The comparison steps specifically include: The amplified voltage is applied to the reference pin of a precision reference voltage source; When the amplified voltage reaches or exceeds the internal reference voltage of the precision reference voltage source, the state of the precision reference voltage source changes from cutoff to conduction.

5. The overcurrent protection implementation method for a high-precision isolated flyback switching power supply according to claim 4, characterized in that, The step of triggering the isolation unit specifically includes: When the precision reference voltage source is turned on, it drives the input light-emitting diode of an optocoupler to emit light, and the optocoupler serves as the isolation unit.

6. The overcurrent protection implementation method for a high-precision isolated flyback switching power supply according to claim 5, characterized in that, The specific steps for the primary-side main control chip to receive the shutdown command signal include: The output phototransistor of the optocoupler is turned on, and the level of one RT pin of the primary-side main control chip is pulled down to a preset shutdown threshold level.

7. The overcurrent protection implementation method for a high-precision isolated flyback switching power supply according to claim 1, characterized in that, The step of sampling the output current specifically includes: The output current is sensed by a current transformer installed in the DC output circuit; The induced current signal output by the current transformer is converted into the sampling voltage through a signal processing circuit.

8. The overcurrent protection implementation method for a high-precision isolated flyback switching power supply according to claim 1, characterized in that, The comparison steps specifically include: The fixed internal reference voltage is generated by an independent precision voltage reference source; The amplified voltage is compared with the fixed internal reference voltage using an independent voltage comparator.

9. The overcurrent protection implementation method for a high-precision isolated flyback switching power supply according to claim 8, characterized in that, The step of triggering the isolation unit specifically includes: When the amplified voltage reaches or exceeds the fixed internal reference voltage, the output level of the independent voltage comparator is reversed, thereby driving the isolation unit.

10. The overcurrent protection implementation method for a high-precision isolated flyback switching power supply according to claim 1, characterized in that, The specific steps for performing the shutdown protection action include: The isolation unit pulls the level of one enable pin or one fault pin of the primary-side main control chip low to its non-operating or fault state.

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