Constant-current and constant-voltage circuit for flyback switching power supply

Abstract

The invention relates to a constant-current and constant-voltage circuit for a flyback switching power supply, and relates to the field of switching power supply technology and charging. The circuit comprises a constant current control circuit, a constant voltage control circuit, an optical coupler isolation circuit and a main control chip control circuit. And the constant current control circuit samples the output current in real time, compares the output current with a preset value, and outputs a feedback signal to the main control chip for adjustment. And the constant voltage control circuit samples the output voltage in real time, compares the output voltage with a preset value, and outputs a feedback signal to the main control chip of the primary side for adjustment. And the optical coupler isolation circuit is used for isolating the primary side and the secondary side of the transformer and preventing the circuit on the other side from being damaged by faults of the primary side or the secondary side. And the main control chip control circuit is used for realizing constant-current and constant-voltage control by receiving a feedback signal and adjusting the duty ratio of a driving signal of a main MOS (Metal Oxide Semiconductor). Compared with an existing constant-current and constant-voltage control circuit, the current sampling circuit has the advantages that a differential proportion amplification mode is adopted, common-mode noise is well restrained, sampling precision is improved, and preset values of voltage and current can be changed according to needs.

Description

Technical Field

[0001] The invention relates to the technical field of switching power supplies, and in particular to a constant current and constant voltage circuit for a flyback switching power supply. Background Art

[0002] With the development of miniaturization, integration and high performance of electronic devices, the charging requirements of small devices are becoming more and more stringent. Even in a single constant current (CC) application, a voltage control loop always exists to prevent the output voltage from getting out of control under light load.

[0003] The existing constant current and constant voltage circuits have the problems of low precision, easy damage to the main control chip on the primary side by a fault on the secondary side, slow response to the output feedback signal, and large interference. Summary of the invention

[0004] In view of the existing problems in the above background technology, the present invention provides a constant current and constant voltage control circuit for a flyback power supply. It is characterized in that the circuit includes a constant current control circuit, a constant voltage control circuit, an optical coupling isolation circuit, and a main control chip control circuit. The circuit can monitor the state of the output voltage and current in real time, and isolate the primary side and the secondary side through the optical coupling to prevent the faults of the primary and secondary sides from affecting each other. After the feedback signal is transmitted to the main control chip, the main control chip realizes the control of constant current and constant voltage by adjusting the duty cycle of the driving signal of the main MOS.

[0005] The constant current control circuit monitors the output current in real time by differential proportional amplification and compares it with a preset value. If the current exceeds the preset value, a feedback signal is output through an optocoupler to the primary main control chip for regulation.

[0006] The constant voltage control circuit samples the output voltage in real time and compares it with the preset value by means of resistor voltage division. If the voltage exceeds the preset value, a feedback signal is output through an optocoupler to the primary main control chip for regulation.

[0007] The optical coupler isolation circuit is used to isolate the primary side and the secondary side of the transformer to prevent a fault on the secondary side from damaging the primary side.

[0008] The main control chip control circuit analyzes the feedback signal of the secondary side and adjusts the switching frequency of the main MOS in real time to achieve constant current and constant voltage control. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 The structure diagram of a constant current and constant voltage control circuit for a flyback power supply is shown in FIG.

[0010] Figure 2 The invention is an overall circuit diagram of a constant current and constant voltage control circuit for a flyback power supply. DETAILED DESCRIPTION

[0011] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0012] like Figure 1 As shown, the present invention provides a constant current and constant voltage control circuit for a flyback power supply, comprising a constant current control circuit (B1), a constant voltage control circuit (B2), an optical coupling isolation circuit (B3), and a main control chip control circuit (B4).

[0013] The constant current control circuit (B1) is used to sample the current change at the output end in real time and output a feedback signal to the main control chip. The output current converts the current signal into a voltage signal through the sampling resistor R22, and then outputs it to the reverse input end of the operational amplifier U6 for comparison after passing through the differential amplifier circuit. Its amplification factor is R14 / R15. The same-direction input end of the operational amplifier U6 is grounded through the resistor R19, and the preset value of the current is changed by adjusting the resistance value of R19.

[0014] When the sampling current at the output end is greater than the preset value, that is, When the output low-level signal of the constant current control circuit is pulled to the LED of the optical coupler through the diode D4, the main control chip adjusts the duty cycle of the output signal to the main MOS after receiving the feedback signal to realize the constant current control. When the sampling current at the output end is lower than the preset value, the operational amplifier U6 outputs a high level, and the diode D4 is reversely cut off. At this time, the constant current control circuit does not work, and the circuit is in the constant voltage control mode.

[0015] The constant voltage control circuit (B2) is used to detect the change of the output current in real time and output a feedback signal to the main control chip. The voltage at the output end is stabilized by the voltage stabilizer U3, that is, The reverse input terminal of the operational amplifier U7 is connected to the output voltage V+ of the power supply through the resistor R8, and the non-inverting input terminal of the operational amplifier U7 is connected to the reference voltage Vref through the resistor R9. The output terminal of the operational amplifier U7 is connected to the cathode of the diode D3. The preset value of the voltage is changed by changing the resistance value of the resistor R10.

[0016] When the output voltage is higher than the preset value, When the output voltage is lower than the preset value, the operational amplifier U7 outputs a high level, and the diode D3 is reversely cut off. At this time, the constant voltage control circuit does not work, and the circuit is in constant current control mode.

[0017] The optical coupler isolation circuit (B3) is used to isolate the primary and secondary sides of the switching power supply to prevent the failure of the primary or secondary side from damaging the circuit on the other side. Pin 2 of the optical coupler is connected to the anode of the diode, pin 1 is connected to VCC through resistor R20, pin 3 is grounded, and pin 4 is connected to the FB pin of the main control chip through resistor R25. When the constant current control circuit or the constant voltage control circuit outputs a low-level signal, the low-level signal enters pin 2 of the optical coupler through diode D4. At this time, the voltage drop across the light-emitting diode inside the optical coupler is greater than the conduction voltage drop. At this time, the light-emitting diode inside the optical coupler is turned on, and the phototransistor is turned on after receiving the signal generated by the light-emitting diode.

[0018] The main control chip control circuit (B4) adjusts the duty cycle of the driving signal of the main MOS according to the received feedback signal to increase or reduce the energy transmitted to the secondary side to achieve constant current and constant voltage control. The GATE pin of the main control chip is shunted into the gate of the main MOS through the resistor R2. When the drive is at a high level, the parasitic capacitance on the main MOS is charged, and the voltage across the parasitic capacitance gradually increases. When the voltage of the parasitic capacitance reaches the gate voltage, the MOS starts to turn on. When the drive signal is at a low level, to discharge the parasitic capacitance, a resistor R1 and a diode D1 need to be connected in parallel at both ends of the R2 resistor. When the voltage of the parasitic capacitance drops to the gate voltage, the main MOS tube can be turned off.

[0019] Regardless of the current loop or the voltage loop, since the outputs of the two operational amplifiers are simultaneously connected in series with diodes to form a logical "AND" relationship, the signal that causes the larger diode current in the two outputs takes effect, so only one control loop is working at any time.

[0020] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the protection scope of the present invention.

Claims

1. A constant current and constant voltage control circuit for a flyback power supply, wherein the circuit comprises a constant current control circuit (B1), a constant voltage control circuit (B2), an optical coupling isolation circuit (B3), and a main control chip control circuit (B4). The constant current control circuit (B1) monitors the output current in real time by differential proportional amplification and compares it with a preset value. If the current exceeds the preset value, a feedback signal is output through an optical coupler to the primary main control chip for regulation. The constant voltage control circuit (B2) samples the output voltage in real time and compares it with the preset value by means of resistor voltage division. If the voltage exceeds the preset value, a feedback signal is output through an optical coupler to the primary main control chip for regulation. The optical coupling isolation circuit (B3) is used to isolate the primary side and the secondary side of the transformer to prevent a fault on the primary side or the secondary side from damaging the circuit on the other side. The main control chip control circuit (B4) analyzes the feedback signal of the secondary side and adjusts the duty cycle of the driving signal of the main MOS in real time to achieve constant current and constant voltage control.

2. The constant current control circuit (B1) according to claim 1, characterized in that: The constant current control circuit includes a sampling resistor R22, operational amplifiers U5 and U6, and a diode D4. The output current converts the current signal into a voltage signal through the sampling resistor R22, and is connected to the same-direction input terminal of the operational amplifier U5 through the resistor R15. The reverse input terminal of the operational amplifier U5 is connected to the signal ground through the resistor R16. The output terminal of the operational amplifier U5 is connected to the reverse input terminal of the operational amplifier U6, and the same-direction input terminal of U6 is connected to the reference voltage Vref through the resistor R18. The output terminal of the operational amplifier U6 is connected to the cathode of the diode D4.

3. The constant voltage control circuit (B2) according to claim 1, characterized in that: The constant voltage control circuit includes a voltage stabilizer U3, an operational amplifier U7, and a diode D3. The voltage at the output end is stabilized by the voltage stabilizer U3, the reverse input end of the operational amplifier U7 is connected to the output voltage V+ of the power supply through a resistor R8, and the non-inverting input end of the operational amplifier U7 is connected to the reference voltage Vref through a resistor R9. The output end of the operational amplifier U7 is connected to the cathode of the diode D3.

4. The optocoupler isolation circuit (B3) according to claim 1, characterized in that: The circuit includes an optocoupler U1 and an optocoupler U2. Pin 1 of the optocoupler U2 is connected to VCC through a resistor R20, pin 2 is connected to the anode of D4, pin 3 is connected to GND, and pin 4 is connected to the FB pin of the main control chip. Pin 1 of the optocoupler U1 is connected to VCC through a resistor R3, pin 2 is connected to the anode of D3, pin 3 is connected to GND, and pin 4 is connected to the FB pin of the main control chip.

5. The main control chip control circuit (B4) according to claim 1, characterized in that: The circuit includes a main control chip U8, a main MOS tube Q1, a resistor R1, a resistor R2, and a diode D1. The FB pin of the main control chip is connected to the 4 pins of U1 and U2 respectively, the GATE pin of the main control chip is connected to the gate of the main MOS through the resistor R2, the diode D1 and the resistor R1 are connected in series and then in parallel at both ends of R2, the drain of the main MOS tube Q1 is connected to the flyback converter, and the source is grounded.

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