A direct current output constant power control circuit

Abstract

The application provides a direct current output constant power control circuit, relates to the technical field of power supply control, and comprises an operational amplifier U1A, an operational amplifier U1B and a feedback photocoupling secondary light emitting diode OP1A; the anode of the feedback photocoupling secondary light emitting diode OP1A is connected with a line power supply VCC through a resistor R1, and the cathode of the feedback photocoupling secondary light emitting diode OP1A is connected with the output end of the operational amplifier U1B; the forward input end of the operational amplifier U1B is connected with the output end of the operational amplifier U1A; the forward input end of the operational amplifier U1A is connected with one end of a resistor R6 and one end of a resistor R8; the other end of the resistor R6 is connected with a reference voltage VRef; and the reverse input end of the operational amplifier U1A is connected with the positive end Vo+ of a switching power supply output through a resistor R5 and a resistor R4.The application realizes the constant power output characteristic, has simple circuit structure, is realized by using an analog operational amplifier circuit, has low circuit cost, high reliability, fast response speed and good compatibility.

Description

Technical Field

[0001] This invention relates to the field of power supply control technology, and more specifically, to a DC output constant power control circuit. Background Technology

[0002] In the field of switching power supply applications, it is necessary to meet the stringent limits of UL1310 and UL62368 standards for Class 2 power supplies. These standards clearly define that the output power of a Class 2 power supply must not exceed 100VA; for DC power supplies, 100W is typically considered a hard ceiling limit. Class 2 power supplies, due to their compliance with stricter output limits including voltage, current, and power, offer significant advantages in engineering applications, such as flexible wiring, easy installation, and high safety, and are therefore widely used in various low-voltage power supply scenarios. However, a technical challenge exists in practical engineering applications: for switching power supplies with adjustable output voltage, traditional overcurrent protection circuits typically use a fixed current protection threshold. For example, a 90W switching power supply with an adjustable output voltage of 24-28V has a rated output current of 3.75A. At an output voltage of 24V, the output power is 90W, which meets the power limit requirements of Class 2 power supplies. However, when the output voltage rises to 28V, if the output current capability is maintained at 3.75A, the output power will reach 105W, exceeding the 100W safety limit and failing to meet the certification requirements for Class 2 power supplies. Simply lowering the fixed current protection threshold, while ensuring that the power does not exceed 100W at 28V output, will unnecessarily limit the output current capability at 24V output, preventing it from reaching the rated 3.75A output capability and reducing the power supply's load-carrying capacity and practicality.

[0003] There are technical contradictions in existing adjustable output voltage switching power supplies when meeting Class 2 safety power limits. Current constant power control usually adopts digital control schemes, such as using a microcontroller with ADC sampling and PWM control. Although the control accuracy is high, the circuit is complex, the cost is high, and there is a delay problem in the digital feedback loop. Summary of the Invention

[0004] The present invention aims to solve the technical contradictions existing in adjustable output voltage switching power supplies when meeting Class 2 safety power limits.

[0005] To address the above problems, the present invention provides a DC output constant power control circuit, comprising:

[0006] A DC output constant power control circuit includes operational amplifier U1A, operational amplifier U1B, and a feedback optocoupler secondary light-emitting diode OP1A. The positive terminal of the feedback optocoupler secondary light-emitting diode OP1A is connected to one end of resistor R1, and the other end of resistor R1 is connected to the line power supply VCC. The negative terminal of the feedback optocoupler secondary light-emitting diode OP1A is connected to the output terminal of operational amplifier U1B. The non-inverting input terminal of operational amplifier U1B is connected to the output terminal of operational amplifier U1A. The inverting input terminal of operational amplifier U1B is connected to signal ground SGND. The ground terminal of operational amplifier U1B is connected to the ground signal VOGND. The non-inverting input terminal of operational amplifier U1A is simultaneously connected to one end of resistor R6 and one end of resistor R8. The other end of resistor R6... The reference voltage VRef is connected to the reference voltage; the other end of resistor R8 is connected to the ground signal VOGND; the inverting input of op-amp U1A is connected to one end of resistor R5; the other end of resistor R5 is connected to one end of resistor R4 and one end of resistor R7; the other end of resistor R4 is connected to the positive output terminal Vo+ of the switching power supply, and the other end of resistor R7 is connected to the ground signal VOGND; the positive output terminal Vo+ of the switching power supply is connected to one end of the output load; the negative output terminal Vo- of the switching power supply is connected to the ground signal VOGND; the negative output terminal Vo- of the switching power supply is connected to one end of resistor R2; the other end of resistor R2 is connected to the other end of the output load; the other end of the output load is connected to signal ground SGND.

[0007] The DC output constant power control circuit provided by this invention has, but is not limited to, the following beneficial effects compared to the prior art: This invention achieves constant power output characteristics. When the output voltage increases, the output current limit value is automatically reduced, and when the output voltage decreases, the output current limit value is automatically increased, so that the output power remains constant.

[0008] This invention meets safety requirements and is particularly suitable for adjustable output voltage switching power supplies that need to meet UL1310 and UL62368 Class 2 power supply limits, ensuring that the power does not exceed 100W across the entire output voltage range.

[0009] The circuit structure of this invention is simple and is implemented using an analog operational amplifier circuit. It does not require digital devices such as microcontrollers, ADCs, and DACs, resulting in low circuit cost and high reliability.

[0010] This invention features a fast response speed, with analog circuits responding in real time. It eliminates the sampling and computation delays associated with digital control and provides rapid overcurrent protection.

[0011] The parameters of this invention are flexible. By adjusting the resistance value of the external resistor, the constant power value can be flexibly set to adapt to the application requirements of different power levels.

[0012] This invention has good compatibility and can be directly connected to the feedback loop of existing switching power supplies. It is compatible with optocoupler isolation feedback circuits and is suitable for various topologies such as flyback and forward converters.

[0013] Furthermore, the positive input terminal of the operational amplifier U1A is connected to one end of the resistor R3, and the other end of the resistor R3 is connected to the output terminal of the operational amplifier U1A.

[0014] Furthermore, the operational amplifier U1A and resistors R3, R5, R4, R7, R6, and R8 constitute a proportional amplifier circuit with a negative output slope, and its output voltage Vos is linearly negatively correlated with the output voltage Vo+ of the switching power supply.

[0015] Furthermore, the resistor R2 is the output current sampling resistor, and the voltage of the signal ground SGND relative to the ground signal VOGND is proportional to the output current.

[0016] Furthermore, the operational amplifier U1B constitutes a comparator circuit.

[0017] Furthermore, the reference voltage VRef is provided by the line power supply VCC after being regulated. The line power supply VCC is generated by the auxiliary winding coupled to the secondary winding of the switching power supply transformer after rectification and filtering.

[0018] Furthermore, the secondary light-emitting diode OP1A of the feedback optocoupler, together with the primary light-receiving diode of the optocoupler, forms an isolated feedback path, which transmits the constant power control signal to the primary control circuit of the switching power supply.

[0019] Furthermore, a negative temperature coefficient thermistor is connected in parallel across the resistor R6.

[0020] Furthermore, the operational amplifiers U1A and U1B are integrated into the same dual operational amplifier chip, which is an LM358 or OP07 series operational amplifier.

[0021] Furthermore, the switching power supply is a flyback switching power supply or a forward switching power supply. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a DC output constant power control circuit according to an embodiment of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings showing multiple embodiments according to this application. It should be understood that the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments described in this application without creative effort will fall within the scope of protection of this application.

[0024] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in the specification of this application is for the purpose of describing specific embodiments only and is not intended to limit this application; the terms "comprising," "including," "having," "containing," etc., in the specification, claims, and accompanying drawings of this application are open-ended terms. Therefore, "comprising," "including," or "having" refers to, for example, a method or apparatus having one or more steps or elements, but is not limited to having only these one or more elements. The terms "first," "second," etc., in the specification, claims, or accompanying drawings of this application are used to distinguish different objects, not to describe a specific order or hierarchy. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0025] In the description of this invention, it should be understood that the terms "upper", "lower", "left", "right", "front", "rear", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, 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.

[0026] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "attachment" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0027] It should be emphasized that when the term "comprising / including" is used in this specification, it is used to explicitly indicate the presence of the stated feature, integer, step, or component, but does not exclude the presence or addition of one or more other features, integers, steps, parts, or groups of features, integers, steps, or parts.

[0028] In this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, in this application, the character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0029] See Figure 1 An embodiment of the present invention provides a DC output constant power control circuit, including operational amplifier U1A, operational amplifier U1B, and feedback optocoupler secondary light-emitting diode OP1A. The positive terminal of the feedback optocoupler secondary light-emitting diode OP1A is connected to one end of resistor R1, and the other end of resistor R1 is connected to the line power supply VCC. The negative terminal of the feedback optocoupler secondary light-emitting diode OP1A is connected to the output terminal of operational amplifier U1B. The positive input terminal of operational amplifier U1B is connected to the output terminal of operational amplifier U1A. The inverting input terminal of operational amplifier U1B is connected to signal ground SGND. The ground terminal of operational amplifier U1B is connected to the ground signal VOGND. The positive input terminal of operational amplifier U1A is simultaneously connected to one end of resistor R6 and one end of resistor R8. Resistor R6... The other end of resistor R8 is connected to the reference voltage VRef; the other end of resistor R8 is connected to the ground signal VOGND; the inverting input of op-amp U1A is connected to one end of resistor R5; the other end of resistor R5 is connected to one end of resistor R4 and one end of resistor R7; the other end of resistor R4 is connected to the positive output terminal Vo+ of the switching power supply, and the other end of resistor R7 is connected to the ground signal VOGND; the positive output terminal Vo+ of the switching power supply is connected to one end of the output load; the negative output terminal Vo- of the switching power supply is connected to the ground signal VOGND; the negative output terminal Vo- of the switching power supply is connected to one end of resistor R2; the other end of resistor R2 is connected to the other end of the output load; the other end of the output load is connected to signal ground SGND.

[0030] In this invention, after the switching power supply is powered on, the output voltage Vo+, the power supply VCC, and the reference voltage VRef are established sequentially. The operational amplifier U1A and its peripheral resistors form a proportional amplifier circuit. Vos and the output voltage Vo+ have a linear negative correlation, that is, when the output voltage Vo+ increases, Vos decreases; when the output voltage Vo+ decreases, Vos increases.

[0031] At the same time, the output current generates a voltage drop across the sampling resistor R2, making the voltage of the signal ground SGND node negative relative to the ground signal VOGND, i.e., -Io×R2. This voltage value reflects the magnitude of the output current.

[0032] Operational amplifier U1B is used as a comparator. Its positive input voltage is Vos, and its inverting input voltage is the SGND node voltage, i.e., -Io×R2. The comparator's comparison condition is equivalent to: when Io×R2 > Vos, considering voltage polarity, operational amplifier U1B outputs a low level.

[0033] When the output current Io increases, causing the voltage at the SGND node (-Io×R2) to be lower than Vos, pin 7 of the operational amplifier U1B outputs a low level. The secondary LED OP1A of the feedback optocoupler is turned on, and the primary photodetector of the optocoupler is turned on, transmitting the feedback signal to the primary control circuit of the power supply. This causes the PWM controller of the power supply to reduce the duty cycle, thereby reducing the output voltage Vo+ and limiting the increase of the output current.

[0034] This invention achieves constant power output characteristics. When the output voltage increases, the output current limit value is automatically reduced, and when the output voltage decreases, the output current limit value is automatically increased, so that the output power remains constant.

[0035] This invention meets safety requirements and is particularly suitable for adjustable output voltage switching power supplies that need to meet UL1310 and UL62368 Class 2 power supply limits, ensuring that the power does not exceed 100W across the entire output voltage range.

[0036] The circuit structure of this invention is simple and is implemented using an analog operational amplifier circuit. It does not require digital devices such as microcontrollers, ADCs, and DACs, resulting in low circuit cost and high reliability.

[0037] This invention features a fast response speed, with analog circuits responding in real time. It eliminates the sampling and computation delays associated with digital control and provides rapid overcurrent protection.

[0038] The parameters of this invention are flexible. By adjusting the resistance value of the external resistor, the constant power value can be flexibly set to adapt to the application requirements of different power levels.

[0039] This invention has good compatibility and can be directly connected to the feedback loop of existing switching power supplies. It is compatible with optocoupler isolation feedback circuits and is suitable for various topologies such as flyback and forward converters.

[0040] Furthermore, the positive input terminal of the operational amplifier U1A is connected to one end of the resistor R3, and the other end of the resistor R3 is connected to the output terminal of the operational amplifier U1A.

[0041] In this invention, resistor R3 causes a sudden change in the circuit state when the output current approaches the overcurrent point. Positive feedback makes the output voltage Vos of U1A change faster and more decisively, avoiding the ambiguous state of the operational amplifier operating in the linear region. This allows for more decisive triggering of the optocoupler for protection actions, reducing oscillations near the protection point. Positive feedback introduces a certain degree of hysteresis. This means that the circuit has a small hysteresis between the protection entry and exit thresholds, preventing frequent switching between protection and normal operation due to small fluctuations in output current at the constant power critical point, thus improving system stability.

[0042] Furthermore, the operational amplifier U1A and resistors R3, R5, R4, R7, R6, and R8 constitute a proportional amplifier circuit with a negative output slope, and its output voltage Vos is linearly negatively correlated with the output voltage Vo+ of the switching power supply.

[0043] In this invention, when Vo+ increases, Vos decreases linearly according to the formula. Vos is the reference threshold for the next-stage comparator U1B to determine whether the current is overcurrent. A decrease in the threshold means that the maximum allowable current decreases. When Vo+ decreases, Vos increases linearly, and the maximum allowable current increases. By adjusting resistors R3, R5, R4, etc., the slope of the negative correlation can be changed. In this way, the same circuit board can be adapted to different power levels, such as 60W constant power or 100W constant power power supply designs, by changing a few resistors, increasing the reusability of the circuit.

[0044] Furthermore, the resistor R2 is the output current sampling resistor, and the voltage of the signal ground SGND relative to the ground signal VOGND is proportional to the output current.

[0045] In this invention, R2 converts current information into voltage information and cleverly uses this voltage value (SGND potential) as one of the common reference points of the circuit, thereby enabling the entire control circuit to sense changes in output current in real time and linearly, providing the most critical feedback signal for the subsequent realization of constant power control that "automatically lowers the current threshold when the output voltage is high".

[0046] Furthermore, the operational amplifier U1B constitutes a comparator circuit.

[0047] When the voltage at the signal ground SGND node is lower than the output voltage Vos of op-amp U1A, op-amp U1B outputs a low level, which turns on the secondary LED OP1A of the feedback optocoupler, thereby reducing the output voltage and current of the switching power supply through optocoupler feedback.

[0048] By adjusting the resistance values ​​of resistors R3, R5, R4, R7, R6, and R8, the target power value for constant power control is set, so that the output power Pout = Vo + × Io remains constant when the output voltage of the switching power supply changes.

[0049] Furthermore, the reference voltage VRef is provided by the line power supply VCC after being regulated. The line power supply VCC is generated by the auxiliary winding coupled to the secondary winding of the switching power supply transformer after rectification and filtering.

[0050] This invention achieves self-sufficiency, requiring no external power supply, making the constant power control circuit an independent module unaffected by external environmental interference. It ensures the accuracy and temperature stability of the reference voltage, achieves timing synchronization, prevents malfunctions, guarantees orderly circuit startup, and avoids false protection during power-on.

[0051] Furthermore, the secondary light-emitting diode OP1A of the feedback optocoupler, together with the primary light-receiving diode of the optocoupler, forms an isolated feedback path, which transmits the constant power control signal to the primary control circuit of the switching power supply.

[0052] The feedback optocoupler meets safety regulations, ensures personnel safety, and achieves electrical isolation—an essential function for any safety-compliant switching power supply. The internal structure of the optocoupler involves an "electrical-optical-electrical" conversion. The OP1A LED on the secondary side emits light, while the light receiver on the primary side receives the optical signal.

[0053] Furthermore, a negative temperature coefficient thermistor is connected in parallel across the resistor R6.

[0054] The negative temperature coefficient thermistor is used to compensate for the resistance drift of the output current sampling resistor R2 caused by temperature changes.

[0055] Furthermore, the operational amplifiers U1A and U1B are integrated into the same dual operational amplifier chip, which is an LM358 or OP07 series operational amplifier.

[0056] This invention ensures high synchronization of the operating states of the two operational amplifiers, avoiding constant power point drift caused by component differences. U1A is responsible for calculating the reference Vos, and U1B is responsible for comparing the current. If their characteristics are inconsistent, a fixed deviation may exist between the calculated and compared values. Using dual operational amplifiers can eliminate this deviation, ensuring that when U1A states "the current threshold is 3.75A," U1B can accurately trigger protection at 3.75A, guaranteeing the accuracy of the 100W constant power point. This simplifies circuit layout, saves PCB space, reduces the number of components, and shrinks the physical size of the power module.

[0057] Furthermore, the switching power supply is a flyback switching power supply or a forward switching power supply.

[0058] This circuit is connected to the secondary feedback loop of the switching power supply, enabling the switching power supply to meet the power limit requirements of Class 2 power supplies in UL1310 and UL62368 standards throughout the entire output voltage regulation range.

[0059] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A DC output constant power control circuit, characterized in that, This includes operational amplifiers U1A and U1B, and a feedback optocoupler secondary LED OP1A. The positive terminal of the feedback optocoupler secondary LED OP1A is connected to one end of resistor R1, and the other end of resistor R1 is connected to the line power supply VCC. The negative terminal of the feedback optocoupler secondary LED OP1A is connected to the output terminal of operational amplifier U1B. The non-inverting input terminal of operational amplifier U1B is connected to the output terminal of operational amplifier U1A. The inverting input terminal of operational amplifier U1B is connected to signal ground SGND. The ground terminal of operational amplifier U1B is connected to the ground signal VOGND. The non-inverting input terminal of operational amplifier U1A is simultaneously connected to one end of resistor R6 and one end of resistor R8. The other end of resistor R6 is connected to the reference voltage VR. ef connection; the other end of resistor R8 is connected to the ground signal VOGND; the inverting input terminal of op-amp U1A is connected to one end of resistor R5; the other end of resistor R5 is connected to one end of resistor R4 and one end of resistor R7; the other end of resistor R4 is connected to the positive output terminal Vo+ of the switching power supply, and the other end of resistor R7 is connected to the ground signal VOGND; the positive output terminal Vo+ of the switching power supply is connected to one end of the output load; the negative output terminal Vo- of the switching power supply is connected to the ground signal VOGND; the negative output terminal Vo- of the switching power supply is connected to one end of resistor R2; the other end of resistor R2 is connected to the other end of the output load; the other end of the output load is connected to signal ground SGND.

2. The DC output constant power control circuit according to claim 1, characterized in that, The positive input terminal of the operational amplifier U1A is connected to one end of the resistor R3, and the other end of the resistor R3 is connected to the output terminal of the operational amplifier U1A.

3. The DC output constant power control circuit according to claim 2, characterized in that, The operational amplifier U1A and resistors R3, R5, R4, R7, R6, and R8 constitute a proportional amplifier circuit with a negative output slope. Its output voltage Vos is linearly negatively correlated with the output voltage Vo+ of the switching power supply.

4. The DC output constant power control circuit according to claim 3, characterized in that, The resistor R2 is the output current sampling resistor, and the voltage of the signal ground SGND relative to the ground signal VOGND is proportional to the output current.

5. The DC output constant power control circuit according to claim 4, characterized in that, The operational amplifier U1B forms a comparator circuit.

6. The DC output constant power control circuit according to claim 1, characterized in that, The reference voltage VRef is provided by the line power supply VCC after voltage regulation. The line power supply VCC is generated by the auxiliary winding coupled to the secondary winding of the switching power supply transformer after rectification and filtering.

7. The DC output constant power control circuit according to claim 6, characterized in that, The secondary light-emitting diode OP1A of the feedback optocoupler, together with the primary light-receiving diode of the optocoupler, forms an isolated feedback path. The isolated feedback path transmits the constant power control signal to the primary control circuit of the switching power supply.

8. The DC output constant power control circuit according to claim 1, characterized in that, A negative temperature coefficient thermistor is connected in parallel across the resistor R6.

9. The DC output constant power control circuit according to any one of claims 1-8, characterized in that, The operational amplifiers U1A and U1B are integrated in the same dual operational amplifier chip, which is an LM358 or OP07 series operational amplifier.

10. The DC output constant power control circuit according to claim 9, characterized in that, The switching power supply is either a flyback switching power supply or a forward switching power supply.

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