Control circuit of switching power supply

Abstract

A control circuit of a switching power supply comprises a photoelectric coupler, a constant voltage controller and a constant current controller. And output control currents of the constant voltage controller and the constant current controller are connected in parallel and then are output to an input control end of the photoelectric coupler to control the magnitude of an optocoupler feedback signal. In order to prevent oscillation, constant-current control needs to be adopted, a constant-current control circuit and a constant-voltage control circuit are integrated together, constant-voltage output is adopted under normal conditions, and constant-current output is adopted during overcurrent. According to the utility model, the constant current control circuit and the constant voltage control circuit are integrated together, constant voltage output is adopted in normal conditions, constant current output is adopted in overcurrent, and through a reasonable constant current regulation method, the power supply can be ensured to keep stable current output when a load changes, and the oscillation problem is prevented, thereby improving the stability and reliability of the power supply.

Description

Technical Field

[0001] The utility model relates to the technical field of switching power supplies in power electronics, in particular to a control circuit of a switching power supply. Background Art

[0002] A switching power supply is a power supply that uses modern power electronics technology to control the on-time ratio of the switch tube to maintain a stable output. A switching power supply is generally composed of electronic devices such as MOSFET controlled by pulse width modulation (PWM).

[0003] A general switching power supply refers to a converter from high voltage AC to low voltage DC (AC-DC). The switching power supply is controlled by the control part, which includes the switch control of the switch tube, the output voltage and current control, various protection controls, etc. The control of the output part includes the output voltage and output current. The control of the output voltage is a constant voltage output, and the current is determined by the size of the load; the control of the output current is generally a constant current output, such as an LED driver power supply.

[0004] Defects and shortcomings of the existing technology:

[0005] The existing flyback switching power supply uses a secondary control circuit to control the switching action of the main switch tube to achieve constant voltage or constant current output of the power supply circuit. The output control can only be one: constant voltage output or constant current output. Under normal circumstances, the output is constant voltage control, and the current is determined by the size of the load, but when the output is overcurrent, it will cause oscillation. Utility Model Content

[0006] The purpose of the utility model is to provide a control circuit of a switching power supply to solve the above problems.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A control circuit of a switching power supply comprises a photocoupler, a constant voltage controller and a constant current controller; the output control currents of the constant voltage controller and the constant current controller are connected in parallel and then output to the input control terminal of the photocoupler to control the size of the photocoupler feedback signal.

[0009] Furthermore, the constant voltage controller includes a reference chip U3 and an output voltage-dividing resistor; the output voltage-dividing resistor includes a resistor R14 and a resistor R18, and the resistor R14 and the resistor R18 are connected in series; the anode of the reference chip U3 is connected to the lower end of the resistor R18, and the reference electrode of the reference chip U3 is connected to the upper end of the resistor R18; the upper end of the output voltage-dividing resistor R14 is connected to the output.

[0010] Furthermore, the constant current controller includes a reference chip U4 and a current sampling resistor Rs; the upper end of the current sampling resistor Rs is connected to the lower end of the output voltage divider resistor R18, the lower end of the current sampling resistor Rs is grounded SGND, the cathode of the reference chip U3 is connected to the cathode of the reference chip U4, and the anode of the reference chip U4 is grounded SGND.

[0011] Furthermore, the reference electrode of the reference chip U4 is connected to the upper end of the resistor R34 or the voltage stabilizing circuit, and the lower end of the resistor R34 or the voltage stabilizing circuit is connected to the upper end of the current sampling resistor Rs.

[0012] Furthermore, the upper end of the resistor R34 or the voltage stabilizing circuit is connected in series with the resistor R33, and the resistor R33 and the resistor R34 or the voltage stabilizing circuit are connected in parallel to one end of the capacitor C12, and the other end of the capacitor C12 is connected to the cathodes of the reference chip U4 and the reference chip U3.

[0013] Furthermore, the voltage stabilizing circuit is a voltage stabilizing tube D1.

[0014] Furthermore, the anode of the reference chip U4 is also connected to the lower end of the current sampling resistor Rs, and the anode of the reference chip U3 is also connected to the upper end of the current sampling resistor Rs.

[0015] Furthermore, the cathodes of the reference chip U4 and the reference chip U3 are connected to the input control terminal of the photocoupler, and the input terminal is connected in series with the resistor R16 and the resistor R13.

[0016] Furthermore, the model of the reference chip U4 and the reference chip U3 is TL431.

[0017] Furthermore, the model of the photocoupler is OPT-EL3H7A.

[0018] Compared with the prior art, the utility model has the following technical effects:

[0019] Under normal circumstances, the output is controlled by constant voltage, and the current is determined by the size of the load. However, when the output is overcurrent, it will cause oscillation. In order to prevent oscillation, constant current control is required. Therefore, it is necessary to integrate the constant current control and constant voltage control circuits together. Under normal circumstances, constant voltage output is used, and when overcurrent occurs, constant current output is used. Through a reasonable constant current regulation method, it can be ensured that the power supply maintains a stable current output when the load changes, preventing oscillation problems, thereby improving the stability and reliability of the power supply.

[0020] The working principle of the utility model: when the output is overcurrent, in order to prevent the oscillation problem, a constant current output is adopted, and the constant current controller works. The constant current controller comprises a reference chip U4 (TL431) and a current sampling resistor Rs. The current sampling resistor Rs is connected in series in the output circuit, and a voltage signal (Vi2) is obtained at the reference end of U4 (TL431). The internal reference voltage of U4 (TL431) is Vr2. When the output current has a small disturbance and increases, the voltage on the current sampling resistor Rs will increase. When Vi2>Vr2, the current passing through U4 (TL431) increases, thereby forming a current on the light-emitting diode side of the optocoupler U2. At the same time, the current on the primary side increases, and the COM pin voltage of the main control chip U1 is reduced, thereby reducing the system duty cycle, thereby reducing the output current, and finally keeping the output current constant. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a structural block diagram of the constant voltage and constant current controller of the utility model;

[0022] Figure 2 It is a circuit diagram of a constant voltage and constant current controller A of the utility model;

[0023] Figure 3 It is the circuit diagram of the improved version of the constant voltage and constant current controller A of the utility model;

[0024] Figure 4 This is the circuit diagram of a common flyback switching power supply;

[0025] Figure 5 It is a circuit diagram of a constant voltage and constant current controller B of the utility model; DETAILED DESCRIPTION

[0026] The utility model is described in detail below in conjunction with the accompanying drawings and specific implementation methods. Obviously, the described embodiments are only part of the embodiments of the utility model, not all of the embodiments. Based on these embodiments, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model.

[0027] Figure 4 This is a circuit diagram of a common flyback switching power supply. The system samples the output voltage, compares the voltage with the reference signal of U3 to obtain an error signal, and then feeds the error signal back to the input stage control chip U1 through the optocoupler U2. The control chip U1 controls the output of the system by changing the switch pulse to keep the output voltage constant.

[0028] Figure 1 It is a structural block diagram of a constant voltage and constant current controller of the utility model, wherein the constant voltage and constant current controller comprises a constant voltage controller and a constant current controller.

[0029] Figure 2 This is a circuit diagram of a constant voltage and constant current controller of the utility model, wherein the constant voltage and constant current controller comprises a constant voltage controller and a constant current controller. The constant voltage and constant current output controller is composed of four parts, namely, a reference chip U3 (TL431), an output voltage dividing resistor, a reference chip U4 (TL431), and a current sampling resistor Rs.

[0030] A control circuit of a switching power supply, characterized in that it includes a photocoupler, a constant voltage controller and a constant current controller; the output control currents of the constant voltage controller and the constant current controller are connected in parallel and then output to the input control terminal of the photocoupler to control the size of the photocoupler feedback signal.

[0031] The connection relationship of the four components of the constant voltage and constant current output controller is as follows:

[0032] (1) The output voltage divider resistors include two, namely R14 and R18, and R14 and R18 are connected in series; the anode of the reference chip U3 (TL431) is connected to the lower end of the output voltage divider resistor R18, and the reference electrode of the reference chip U3 (TL431) is connected to the upper end of the output voltage divider resistor R18. The upper end of the output voltage divider resistor R14 is connected to the output.

[0033] (2) The upper end of the current sampling resistor Rs is connected to the lower end of the output voltage divider resistor R18, the lower end of the current sampling resistor Rs is grounded SGND, the cathode of the reference chip U3 (TL431) is connected to the cathode of the reference chip U4 (TL431), the anode of the reference chip U4 (TL431) is grounded SGND, the reference electrode of the reference chip U4 (TL431) is connected to the upper end of the resistor R34, and the lower end of the resistor R34 is connected to the upper end of the current sampling resistor Rs.

[0034] Working process: Under normal circumstances, the output is constant voltage control, the current is determined by the size of the load, and the constant voltage controller works;

[0035] The constant voltage controller comprises a reference chip U3 (TL431) and an output voltage divider resistor. The system samples from the output voltage divider resistor (R18 and R14) and obtains a voltage signal (Vi1) at the reference end of U3 (TL431). The internal reference voltage of U3 (TL431) is Vr1. When the output voltage has a small disturbance and increases, reaching Vi1>Vr1, the current through U3 (TL431) increases, thereby forming a current on the light-emitting diode side of the optocoupler U2. At the same time, the current on the primary side increases, and the COM pin voltage of the main control chip U1 decreases, thereby reducing the system duty cycle, thereby reducing the output voltage, and finally keeping the output voltage constant.

[0036] When the output is overcurrent, constant current output is adopted, and the constant current controller works. The constant current controller includes a reference chip U4 (TL431) and a current sampling resistor Rs. The current sampling resistor Rs is connected in series in the output circuit, and a voltage signal (Vi2) is obtained at the reference end of U4 (TL431). The internal reference voltage of U4 (TL431) is Vr2. When the output current has a small disturbance and increases, the voltage on the current sampling resistor Rs will increase. When Vi2>Vr2, the current through U4 (TL431) increases, thereby forming a current on the light-emitting diode side of the optocoupler U2. At the same time, the current on the primary side increases, and the COM pin voltage of the main control chip U1 is reduced, thereby reducing the system duty cycle, thereby reducing the output current, and finally keeping the output current constant.

[0037] Figure 3 This is the circuit diagram of the improved constant voltage and constant current controller of the utility model. Figure 2 The R34 in the circuit is replaced with a voltage regulator circuit (such as the voltage regulator D1 in the figure). The advantage of this circuit is that it can improve the sensitivity of current sampling. For the specific working process, refer to Figure 3 working process.

[0038] There are two cases for voltage sampling in the constant voltage controller: 1. The voltage sampling point does not include the current sampling resistor Rs, for example Figure 2 and Figure 3 ; 2. The voltage sampling point includes the current sampling resistor Rs, for example Figure 5 .

[0039] like Figure 2 and Figure 3 The voltage sampling point does not include the current sampling resistor Rs. The circuit structure is that the lower end of the current sampling resistor Rs is grounded, and the upper end of the current sampling resistor Rs is connected to the lower end of the output voltage dividing resistor R18; the reference electrode of TL431 is connected to the upper end of the output voltage dividing resistor R18, and the anode of TL431 is connected to the upper end of the current sampling resistor Rs.

[0040] Specifically: the voltage sampling in the constant voltage controller is divided into two cases: 1. The voltage sampling point does not include the current sampling resistor Rs; 2. The voltage sampling point includes the current sampling resistor Rs.

[0041] Case 1: The voltage sampling point does not include the current sampling resistor Rs. The circuit structure is that the lower end of the current sampling resistor Rs is grounded, and the upper end of the current sampling resistor Rs is connected to the lower end of the output voltage divider resistor R18; the reference electrode of TL431 is connected to the upper end of the output voltage divider resistor R18, and the anode of TL431 is connected to the upper end of the current sampling resistor Rs.

[0042] Case 2: The voltage sampling point includes the current sampling resistor Rs. The circuit structure is that the lower end of the current sampling resistor Rs is grounded, and the upper end of the current sampling resistor Rs is connected to the lower end of the output voltage divider resistor R18; the reference electrode of TL431 is connected to the upper end of the output voltage divider resistor R18, but the anode of TL431 is connected to the lower end of the current sampling resistor Rs.

[0043] Case 1: As the output load current continues to increase, when the overcurrent protection threshold is not reached, the output voltage hardly decreases, but when the overcurrent protection threshold is reached, the output voltage suddenly drops rapidly.

[0044] Case 2: As the output load current continues to increase, the voltage across the current sampling resistor also gradually increases, causing the output voltage to gradually decrease. When the overcurrent protection threshold is reached, the output voltage will suddenly drop rapidly.

[0045] As the output load current continues to increase, when the overcurrent protection threshold is not reached, the output voltage hardly decreases, but when the overcurrent protection threshold is reached, the output voltage will suddenly drop rapidly.

[0046] like Figure 5 The voltage sampling point includes the current sampling resistor Rs. The circuit structure is that the lower end of the current sampling resistor Rs is grounded, and the upper end of the current sampling resistor Rs is connected to the lower end of the output voltage divider resistor R18; the reference electrode of TL431 is connected to the upper end of the output voltage divider resistor R18, but the anode of TL431 is connected to the lower end of the current sampling resistor Rs.

[0047] As the output load current continues to increase, the voltage across the current sampling resistor also gradually increases, causing the output voltage to gradually decrease. When the overcurrent protection threshold is reached, the output voltage will suddenly drop rapidly.

[0048] The above embodiments are only used to illustrate the present invention, rather than to limit the present invention. Technicians in the relevant technical field can make various modifications without departing from the scope of the present invention. Therefore, all equivalent technical solutions should also belong to the scope of the present invention and should be defined by the claims.

Claims

1. A control circuit of a switching power supply, characterized in that: It includes a photocoupler, a constant voltage controller and a constant current controller; the output control currents of the constant voltage controller and the constant current controller are connected in parallel and then output to the input control terminal of the photocoupler to control the size of the photocoupler feedback signal.

2. A control circuit for a switching power supply according to claim 1, characterized in that: The constant voltage controller includes a reference chip U3 and an output voltage-dividing resistor; the output voltage-dividing resistor includes a resistor R14 and a resistor R18, and the resistor R14 and the resistor R18 are connected in series; the anode of the reference chip U3 is connected to the lower end of the resistor R18, and the reference electrode of the reference chip U3 is connected to the upper end of the resistor R18; the upper end of the output voltage-dividing resistor R14 is connected to the output.

3. A control circuit for a switching power supply according to claim 2, characterized in that: The constant current controller includes a reference chip U4 and a current sampling resistor Rs; the upper end of the current sampling resistor Rs is connected to the lower end of the output voltage divider resistor R18, the lower end of the current sampling resistor Rs is grounded SGND, the cathode of the reference chip U3 is connected to the cathode of the reference chip U4, and the anode of the reference chip U4 is grounded SGND.

4. A control circuit for a switching power supply according to claim 3, characterized in that: The reference electrode of the reference chip U4 is connected to the upper end of the resistor R34 or the voltage stabilizing circuit, and the lower end of the resistor R34 or the voltage stabilizing circuit is connected to the upper end of the current sampling resistor Rs.

5. A control circuit for a switching power supply according to claim 4, characterized in that: The upper end of the resistor R34 or the voltage stabilizing circuit is connected in series with the resistor R33, and the resistor R33 and the resistor R34 or the voltage stabilizing circuit are connected in parallel to one end of the capacitor C12, and the other end of the capacitor C12 is connected to the cathodes of the reference chip U4 and the reference chip U3.

6. A control circuit for a switching power supply according to claim 5, characterized in that: The voltage stabilizing circuit is the voltage stabilizing tube D1.

7. A control circuit for a switching power supply according to claim 3, characterized in that: The anode of the reference chip U4 is also connected to the lower end of the current sampling resistor Rs, and the anode of the reference chip U3 is also connected to the upper end of the current sampling resistor Rs.

8. The control circuit of a switching power supply according to claim 3, characterized in that: The cathodes of the reference chip U4 and the reference chip U3 are connected to the input control terminal of the photocoupler, and the input terminal is connected in series with the resistor R16 and the resistor R13.

9. A control circuit for a switching power supply according to claim 2, characterized in that: The model of the reference chip U4 and the reference chip U3 is TL431.

10. A control circuit for a switching power supply according to claim 2, characterized in that: The model of the photocoupler is OPT-EL3H7A.

Images