Power supply circuit of liquid crystal display screen

Abstract

The utility model relates to the technical field of switching power supplies, aims to solve the technical problem of single high-voltage protection mechanism of the conventional liquid crystal display screen power supply circuit, and discloses a liquid crystal display screen power supply circuit and a high-voltage protection circuit, which comprise an NPN-type triode Q8 and a PNP-type triode Q12, the base electrode of the triode Q8 is connected with the collector electrode of the triode Q12, and the base electrode of the triode Q8 is connected with the collector electrode of the triode Q12. The emitter of the triode Q8 is grounded, the collector of the triode Q8 is connected with the base of the triode Q12, and the base of the triode Q12 is connected with the reference voltage setting end of the PWM controller U6 through the diode D12; the base electrode of the triode Q8 is connected with the power supply end of the PWM controller U6 through a resistor R32 and a voltage-regulator tube D11 in sequence; the emitter of the triode Q12 is connected with the power supply end of the PWM controller U6 through a resistor R36, the collector of the triode Q8 is connected with the emitter of the triode Q12 through a resistor R35, and the collector of the triode Q12 is grounded through a resistor R34. And two mechanisms are adopted for high-voltage protection, so that the safety and the reliability are improved.

Description

Technical Field

[0001] The utility model relates to the technical field of switch power supplies, in particular to a liquid crystal display screen power supply circuit. Background Art

[0002] The main function of the LCD power supply circuit is to convert the AC power from the 220V mains into various stable DC power supplies required by the LCD. These DC power supplies provide the necessary working voltage for various control circuits, logic circuits and control panels in the LCD. The stability of the power supply circuit directly affects the performance and reliability of the LCD, ensuring its normal operation and long-term stable operation. Therefore, the power supply circuit plays a vital role in the entire LCD system. It is not just a simple power conversion, but also a key component to ensure the normal operation and long-term reliability of the equipment.

[0003] The power supply circuit of the LCD usually adopts a switching circuit. This design first converts the input AC 220V voltage into a DC voltage through a rectifier and filter circuit. Then, the voltage is converted into a high-frequency rectangular waveform through the chopping process controlled by the switch tube and the high-frequency transformer step-down. Finally, after another rectification and filtering process, a stable DC voltage is obtained to provide the working voltage required by each module of the LCD.

[0004] The stability of the power supply voltage is a basic guarantee. When the mains power fluctuates greatly, especially when the voltage is too high, effective protection is required to prevent high voltage from damaging the subsequent circuit. In the prior art, the output voltage is generally detected at the secondary end of the transformer. The secondary end is also connected to a large load. The dynamic voltage change of the load power consumption will affect the detection accuracy. In addition, the high-voltage protection circuit is relatively simple, and often only has one detection mechanism. After one detection mechanism fails, effective protection cannot be obtained. Utility Model Content

[0005] The utility model aims to provide a liquid crystal display screen power supply circuit to solve the technical problem that the existing liquid crystal display screen power supply circuit has a single high-voltage protection mechanism.

[0006] To achieve the above purpose, the specific technical solution of a liquid crystal display power supply circuit of the utility model is as follows:

[0007] A power supply circuit for a liquid crystal display screen comprises a rectifier circuit, a PWM controller, a high-frequency transformer and a switch tube, wherein the high-frequency transformer T1 is provided with a primary coil for connecting to an output end of the rectifier circuit, a first secondary coil for supplying power to the PWM controller and a second secondary coil for supplying power to a load, and is provided with a high-voltage protection circuit, comprising an NPN-type transistor Q8 and a PNP-type transistor Q12, wherein the base of the transistor Q8 is connected to the collector of the transistor Q12, the emitter of the transistor Q8 is connected to the ground, the collector of the transistor Q8 is connected to the base of the transistor Q12, the base of the transistor Q12 is connected to a reference voltage setting end of the PWM controller U6 through a diode D12, the anode of the diode D12 is connected to the reference voltage setting end, and the cathode of the diode D12 is connected to the base of the transistor Q12;

[0008] The base of the transistor Q8 is connected to the power supply terminal of the PWM controller U6 through the resistor R32 and the voltage regulator D11 in sequence, the cathode of the voltage regulator D11 is connected to the cathode of the diode D3, and the cathode of the voltage regulator D11 is connected to the resistor R32;

[0009] The emitter of transistor Q12 is connected to the power supply terminal of PWM controller U6 through resistor R36, the collector of transistor Q8 is connected to the emitter of transistor Q12 through resistor R35, and the collector of transistor Q12 is grounded through resistor R34.

[0010] On the one hand, when the AC power voltage rises above a predetermined value, the output voltage of the first secondary coil also rises accordingly. When the maximum voltage is exceeded, the voltage regulator D11 is reversely broken down, and a voltage drop is generated on the resistor R34. When the turn-on voltage of the transistor Q8 is reached, the transistor Q8 is turned on, and then the base of the transistor Q12 becomes a low level, turning on the transistor Q12, and the diode D12 is also turned on at the same time, so that the voltage at the reference voltage setting end of the PWM controller U6 is pulled down. The loss of normal reference voltage causes the PWM controller U6 to shut down the pulse output, thereby achieving high-voltage protection.

[0011] On the other hand, after the transistor Q8 is turned on, the voltage at the power supply end of the PWM controller U6 is grounded through the resistor R36 and the transistor Q12, which also causes the power supply voltage of the PWM controller U6 to drop significantly. The PWM controller U6 stops working and cannot output pulse waves, and then the switch tube Q5 and the high-frequency transformer T1 stop working, thereby achieving high-voltage protection.

[0012] Furthermore, one end of the first secondary coil is connected to the power supply end of the PWM controller U6 through a rectifier diode D3, the cathode of the rectifier diode D3 is connected to the power supply end of the PWM controller U6, the anode of the rectifier diode D3 is connected to one end of the first secondary coil through a current limiting resistor R24, and the other end of the first secondary coil is grounded.

[0013] The rectifier diode D3 ensures that the voltage obtained from the first secondary coil is DC, and the current is controlled within a safe range through the current limiting resistor R24. The PWM controller U6 can obtain a stable power supply to ensure its normal operation.

[0014] The current limiting resistor R24 ​​limits the rectifier current, avoiding damage to the PWM controller U6 caused by excessive current shock, thereby extending the service life of the circuit components and improving the reliability of the system.

[0015] Furthermore, the switch tube Q5 is an NMOS tube, the driving end of the PWM controller U6 is connected to the gate of the switch tube Q5, the source of the switch tube Q5 is grounded through the current detection resistor R26, the source of the switch tube Q5 is connected to the current detection end of the PWM controller U6, the drain of the switch tube Q5 is connected to one end of the primary coil, and the other end of the primary coil is connected to the output end of the rectifier circuit.

[0016] The switch tube Q5 is an NMOS tube, and its conduction and cutoff are controlled by the driving end of the PWM controller U6, which effectively realizes the energy conversion in the power supply circuit. The PWM controller U6 can accurately control the switching frequency and duty cycle of the switch tube Q5 to reduce energy loss when converting voltage and improve the efficiency of the entire circuit.

[0017] The source of the switch tube Q5 is grounded through the current detection resistor R26, and the source is connected to the current detection terminal of the PWM controller U6. This design can achieve current protection and control by detecting the source current of the switch tube Q5, ensuring that the circuit works within a safe range and avoiding overcurrent damage to electronic components.

[0018] Furthermore, one end of the primary coil is connected to the other end of the primary coil via a diode D2 and a resistor R19 in sequence, and a capacitor C9 is connected in parallel to both ends of the resistor R19.

[0019] Resistor R19 can limit the surge current at initial startup to protect the circuit; diode D2 provides a voltage clamping function to prevent overvoltage from damaging the circuit; capacitor C9 is connected in parallel across resistor R19 to filter out high-frequency noise and spike voltage and stabilize the working state of the circuit.

[0020] Furthermore, the collector of the transistor Q12 is grounded via a capacitor C19, and a capacitor C18 is connected between the base and emitter of the transistor Q12.

[0021] Capacitor C18 forms a low-pass filter between the base and emitter of transistor Q12, which can filter out high-frequency noise and interference, making the control signal of transistor Q12 more stable, and capacitor C19 filters clutter.

[0022] The utility model provides a liquid crystal display power supply circuit with the following advantages:

[0023] The power supply circuit of a liquid crystal display screen of the utility model is provided with a high-voltage protection circuit, and utilizes the interaction between the transistor Q8 and the transistor Q12. When the mains voltage rises and exceeds a predetermined value, the voltage regulator tube D11 is reversely broken down, triggering the transistor Q8 and the transistor Q12 to be turned on in sequence, and the diode D12 is turned on, so that the voltage at the reference voltage setting terminal of the PWM controller U6 is pulled down, and the PWM controller U6 turns off the pulse output. Further, through the conduction of the transistor Q8, the voltage at the power supply terminal of the PWM controller U6 is grounded through the resistor R36, the transistor Q12, and the resistor R34, so that the PWM controller U6 stops working, and the switch tube Q5 and the high-frequency transformer T1 stop working, thereby realizing high-voltage protection of the circuit. Compared with the prior art, the circuit adopts two mechanisms to effectively prevent the circuit from being damaged due to excessive mains voltage, thereby improving the safety and reliability of the power supply circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a functional block diagram of the front-stage input power supply provided by the utility model;

[0025] Figure 2 This is a circuit connection diagram of a PWM controller, a switch tube and a high-frequency transformer in a power supply circuit of a liquid crystal display provided by the utility model;

[0026] Figure 3 This is a high voltage protection circuit diagram provided by the utility model. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the utility model more clear, the utility model is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the utility model and are not used to limit the utility model.

[0028] See also Figure 1 and Figure 2 The utility model provides a power supply circuit for a liquid crystal display screen. The entire circuit includes an EMC circuit for improving electromagnetic compatibility, a rectifier circuit for converting AC power into high-voltage DC, a PMW controller U6, a switch tube Q5, a high-frequency transformer T1, an overcurrent protection circuit, and a high-voltage protection circuit.

[0029] The high-frequency transformer T1 includes a primary coil, a first secondary coil, a second secondary coil and a third secondary coil. The first secondary coil is used to supply power to circuits such as the PWM controller U6, and the second secondary coil and the third secondary coil are used to output low-voltage direct current DC2 and low-voltage direct current DC3.

[0030] The PWM controller U6 has a 7th terminal which is grounded; an 8th terminal which is a voltage feedback terminal at the load, which is used to measure the voltage at the inductive load through a photoelectric coupler; a 5th terminal which is a starting voltage input terminal, which is connected to the high voltage DC1 through two megohm resistors R23 and R22 in sequence; a 2nd terminal which is a PWM frequency setting terminal, which is grounded through a resistor R29; a 1st terminal which is an overvoltage protection terminal; a 4th terminal which is a current detection terminal; a 6th terminal which is a power supply terminal, and the first secondary coil of the high frequency transformer T1 is powered by the rectifier diode D3 after being rectified, and the anode of the rectifier diode D3 is connected to one end of the first secondary coil through a resistor R24, and the other end of the first secondary coil is grounded, and the cathode of the rectifier diode D3 is grounded through a capacitor C10; and a 3rd terminal which is a driving terminal.

[0031] A soft start circuit is provided, including a resistor R22 and a resistor R23 connected in series, and the start voltage input terminal is connected to the positive electrode of the output terminal of the rectifier circuit through two megohm-level resistors R23 and R22 in sequence. Since the resistance of the resistors R23 and R22 is large, the working current is very small. When the switching power supply is just started, the starting working current required by the PWM controller U6 is loaded to the start voltage input terminal of the PWM controller U6 after the high-voltage direct current DC1 is stepped down by the resistors R22 and R23 to achieve soft start.

[0032] The soft start circuit limits the starting working current required by the PWM controller U6 during startup by connecting two high-resistance resistors R22 and R23 in series. The two resistors are connected between the positive electrode of the output terminal of the rectifier circuit and the starting voltage input terminal of the PWM controller U6. Their high resistance ensures that only a very small working current passes through, so when the switching power supply is just started, the PWM controller U6 can gradually load the starting voltage to achieve the purpose of soft start. This design helps to reduce the current impact during the startup process, protect the circuit components and ensure stable startup of the system.

[0033] The driving end of the PWM controller U6 drives the switch tube Q5 to work, causing the primary coil of the high-frequency transformer T1 to oscillate, coupling out a high-frequency voltage in the first secondary coil, which is rectified and filtered by the rectifier diode D3 and the filter capacitor C10 to power the PWM controller U6.

[0034] The switch tube Q5 is an NMOS tube. The high-voltage direct current DC1 is connected to one end of the primary coil. The drain of the switch tube Q5 is connected to the other end of the primary coil. The source of the switch tube Q5 is grounded through the magnetic bead L1 and the current sensing resistor R26 in sequence. The source of the switch tube Q5 is connected to the current detection end of the PWM controller U6.

[0035] One end of the primary coil is connected to the other end of the primary coil via a resistor R19 and a diode D2 in sequence, and a capacitor C9 is connected in parallel to both ends of the resistor R19.

[0036] One end of the second secondary coil outputs low-voltage direct current DC2 through two reverse-parallel diodes D7 and D8, and is also provided with a first RC filter circuit composed of a resistor R30 and a capacitor C14. The first RC filter circuit is connected in parallel at both ends of the diode D7 to absorb the surge voltage generated by the diodes D7 and D8; the low-voltage direct current DC2 is connected to ground two through capacitor C15 (ground two represents a ground that is not shared with the ground of the front stage of the high-frequency transformer, and is not the same ground plane).

[0037] One end of the third secondary coil is connected to the other end of the second secondary coil, and one end of the second secondary coil outputs a low-voltage direct current DC3 through two reverse-parallel diodes D9 and D10. A second RC filter circuit composed of a resistor R31 and a capacitor C16 is also provided. The second RC filter circuit is connected in parallel at both ends of the diode D9 to absorb the surge voltage generated by the diodes D9 and D10; the low-voltage direct current DC3 is connected to ground two through capacitor C17, and one end of the third secondary coil is connected to ground two.

[0038] The voltage of the low voltage direct current DC3 is half the voltage of the low voltage direct current DC2.

[0039] How it works

[0040] When the PWM controller U6 starts working, the driving end outputs a rectangular pulse wave to control the switch tube Q5 to switch at its operating frequency. When the switch tube Q5 is continuously turned on and off, self-excited oscillation is formed, and the high-frequency transformer T1 forms an oscillating voltage.

[0041] When the driving end outputs a high level, the switch tube Q5 is turned on, and current flows through the primary coil of the high-frequency transformer T1, generating a positive voltage at the top and a negative voltage at the bottom; at the same time, the three secondary coils of the high-frequency transformer T1 generate an induced electromotive force with a positive bottom and a negative top, and the diodes D7 and D8 on the second secondary coil are turned off. This stage is the energy storage stage; when the driving end outputs a low level, the switch tube Q5 is turned off, the current in the primary coil of the high-frequency transformer T1 is instantaneously zero, and the electromotive force of the primary coil is positive at the bottom and negative at the top, that is, a positive voltage at the bottom and a negative voltage at the top is generated; at the same time, the three secondary coils of the high-frequency transformer T1 generate an induced electromotive force with a positive top and a negative bottom, and the diodes D7 and D8 on the second secondary coil are turned on, and the voltage starts to be output.

[0042] The overcurrent protection circuit includes a current detection resistor R26. The source of the switch tube Q5 is grounded through the current detection resistor R26, and the source of the switch tube Q5 is connected to the current detection terminal.

[0043] After the switch tube Q5 is turned on, the current will flow from the drain of the switch tube Q5 to the source, and generate a voltage on the current sensing resistor R26. The voltage generated here is input to the current detection end of the PWM controller. When it exceeds the first predetermined voltage, the driving end stops outputting the pulse wave, causing the switch tube Q5 to stop working, and then the high-frequency transformer T1 stops working, thereby achieving overcurrent protection.

[0044] See also Figure 3 In the high-voltage protection circuit, an NPN transistor Q8 and a PNP transistor Q12 are included. The base of the transistor Q8 is connected to the collector of the transistor Q12, the emitter of the transistor Q8 is grounded, the collector of the transistor Q8 is connected to the base of the transistor Q12, the base of the transistor Q12 is connected to the reference voltage setting terminal of the PWM controller U6 through the diode D12, the anode of the diode D12 is connected to the reference voltage setting terminal, and the cathode of the diode D12 is connected to the base of the transistor Q12.

[0045] The base of the transistor Q8 is connected to the diode D3 via the resistor R32 and the voltage regulator D11 in sequence, the cathode of the voltage regulator D11 is connected to the cathode of the diode D3, and the cathode of the voltage regulator D11 is connected to the resistor R32.

[0046] The emitter of the transistor Q12 is connected to the cathode of the diode D3 through the resistor R36, the collector of the transistor Q8 is connected to the emitter of the transistor Q12 through the resistor R35, and the collector of the transistor Q12 is grounded through the resistor R34.

[0047] On the one hand, when the mains voltage rises above a predetermined value, the output voltage of the first secondary coil also rises accordingly. When it exceeds the second predetermined voltage, the voltage regulator tube D11 is reversely broken down, and a voltage drop is generated on the resistor R34. When the voltage drop reaches the conduction voltage of the transistor Q8, the transistor Q8 is turned on, and then the base of the transistor Q12 becomes a low level, so that the transistor Q12 is turned on. At the same time, the diode D12 is also turned on, so that the voltage of the reference voltage setting terminal of the PWM controller U6 is pulled down, and the normal reference voltage is lost, so that the PWM controller U6 turns off the pulse output.

[0048] On the other hand, after transistor Q8 is turned on, the reference voltage at the power supply end of the PWM controller U6 is grounded through resistor R36 and transistor Q12, which also causes the power supply voltage of the PWM controller U6 to drop significantly. The PWM controller U6 stops working and cannot output pulse waves, and then the switch tube Q5 and the high-frequency transformer T1 stop working, thereby achieving high-voltage protection.

[0049] The beneficial effects of implementing the liquid crystal display power supply circuit provided by the utility model are as follows:

[0050] The power supply circuit of a liquid crystal display screen of the utility model is provided with a high-voltage protection circuit, and utilizes the interaction between the transistor Q8 and the transistor Q12. When the mains voltage rises and exceeds a predetermined value, the voltage regulator tube D11 is reversely broken down, triggering the transistor Q8 and the transistor Q12 to be turned on in sequence, and the diode D12 is turned on, so that the voltage at the reference voltage setting terminal of the PWM controller U6 is pulled down, and the PWM controller U6 turns off the pulse output. Further, through the conduction of the transistor Q8, the voltage at the power supply terminal of the PWM controller U6 is grounded through the resistor R36, the transistor Q12, and the resistor R34, so that the PWM controller U6 stops working, and the switch tube Q5 and the high-frequency transformer T1 stop working, thereby realizing high-voltage protection of the circuit. Compared with the prior art, the circuit adopts two mechanisms to effectively prevent the circuit from being damaged due to excessive mains voltage, thereby improving the safety and reliability of the power supply circuit.

[0051] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A liquid crystal display power supply circuit, comprising a rectifier circuit, a PWM controller U6, a high-frequency transformer T1 and a switch tube Q5, wherein the high-frequency transformer T1 is provided with a primary coil for connecting to the output end of the rectifier circuit, a first secondary coil for supplying power to the PWM controller U6 and a second secondary coil for supplying power to a load, characterized in that: A high-voltage protection circuit is provided, including an NPN-type transistor Q8 and a PNP-type transistor Q12, the base of the transistor Q8 is connected to the collector of the transistor Q12, the emitter of the transistor Q8 is connected to the ground, the collector of the transistor Q8 is connected to the base of the transistor Q12, the base of the transistor Q12 is connected to the reference voltage setting terminal of the PWM controller U6 through a diode D12, the anode of the diode D12 is connected to the reference voltage setting terminal, and the cathode of the diode D12 is connected to the base of the transistor Q12; The base of the transistor Q8 is connected to the power supply terminal of the PWM controller U6 through the resistor R32 and the voltage regulator D11 in sequence, the cathode of the voltage regulator D11 is connected to the cathode of the diode D3, and the cathode of the voltage regulator D11 is connected to the resistor R32; The emitter of transistor Q12 is connected to the power supply terminal of PWM controller U6 through resistor R36, the collector of transistor Q8 is connected to the emitter of transistor Q12 through resistor R35, and the collector of transistor Q12 is grounded through resistor R34.

2. The liquid crystal display power supply circuit according to claim 1, characterized in that: One end of the first secondary coil is connected to the power supply end of the PWM controller U6 through a rectifier diode D3, the cathode of the rectifier diode D3 is connected to the power supply end of the PWM controller U6, the anode of the rectifier diode D3 is connected to one end of the first secondary coil through a current limiting resistor R24, and the other end of the first secondary coil is grounded.

3. The liquid crystal display power supply circuit according to claim 2, characterized in that: The switch tube Q5 is an NMOS tube, the driving end of the PWM controller U6 is connected to the gate of the switch tube Q5, the source of the switch tube Q5 is grounded through the current detection resistor R26, the source of the switch tube Q5 is connected to the current detection end of the PWM controller U6, the drain of the switch tube Q5 is connected to one end of the primary coil, and the other end of the primary coil is connected to the output end of the rectifier circuit.

4. The liquid crystal display power supply circuit according to claim 3, characterized in that: One end of the primary coil is connected to the other end of the primary coil via a diode D2 and a resistor R19 in sequence, and a capacitor C9 is connected in parallel to both ends of the resistor R19.

5. The liquid crystal display power supply circuit according to claim 1, characterized in that: The collector of the transistor Q12 is grounded via a capacitor C19, and a capacitor C18 is connected between the base and emitter of the transistor Q12.

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