CN109362157B

CN109362157B - Constant current control circuit and television

Info

Publication number: CN109362157B
Application number: CN201811478251.5A
Authority: CN
Inventors: 陈建忠; 杨寄桃; 鲍晓杰; 沈思宽
Original assignee: Shenzhen Skyworth RGB Electronics Co Ltd
Current assignee: Shenzhen Skyworth RGB Electronics Co Ltd
Priority date: 2018-12-04
Filing date: 2018-12-04
Publication date: 2021-05-25
Anticipated expiration: 2038-12-04
Also published as: CN109362157A

Abstract

According to the technical scheme, the constant current control circuit is formed by arranging the single-stage PFC control circuit, the power supply circuit, the switch circuit and the overvoltage adjusting circuit. When receiving an enable signal, the power supply circuit starts to supply power to the single-stage PFC control circuit, and the single-stage PFC control circuit controls the switch circuit to convert an input power supply and then supply power to the load; and the control circuit is also used for controlling the switch circuit to output constant current according to the sampling signal fed back by the overvoltage adjusting circuit. According to the technical scheme, the power supply supplies power to the single-stage PFC control circuit only when the enable signal is received, and the single-stage PFC control circuit is powered off in a standby state, so that energy consumption is reduced. Due to the fact that the single-stage PFC control circuit is adopted, the constant-current control circuit can supply power to the backlight lamp bar without BOOST boosting, and conversion efficiency of electric energy is improved.

Description

Constant current control circuit and television

Technical Field

The invention relates to the technical field of televisions, in particular to a constant current control circuit and a television.

Background

In a traditional power supply, after Power Factor Correction (PFC) and AC-DC voltage conversion are performed, 24V voltage output by a constant voltage output power supply needs to be boosted by BOOST and then supplied to a backlight lamp bar, so that the power supply conversion efficiency is low, and the cost of a constant current board is increased.

Disclosure of Invention

The invention mainly aims to provide a constant current control circuit, aiming at improving the power supply conversion efficiency and reducing the cost of a constant current plate.

In order to achieve the purpose, the invention provides a constant current control circuit, which comprises a single-stage PFC control circuit, a power supply circuit, a switch circuit and an overvoltage adjusting circuit; the input end of the switching circuit is connected with a power supply, the output end of the switching circuit is connected with a load, the controlled end of the switching circuit is connected with the output end of the single-stage PFC control circuit, the output end of the power supply circuit is connected with the power supply end of the single-stage PFC control circuit, the sampling end of the overvoltage adjusting circuit is connected with the load, and the output end of the overvoltage adjusting circuit is connected with the feedback end of the single-stage PFC control circuit;

the power supply circuit is used for supplying power to the single-stage PFC control circuit when an enabling signal is received, and under the normal working state of the PFC control chip, the PWM directly controls the constant-current control circuit on the secondary side;

the single-stage PFC control circuit is used for controlling the switching circuit to convert an input power supply and then supply power to the load; and the control circuit is also used for controlling the switch circuit to output constant current according to the sampling signal fed back by the overvoltage adjusting circuit.

Preferably, the constant current control circuit further comprises a brightness smooth adjusting circuit, and the brightness smooth adjusting circuit is connected with the load; the brightness smooth regulating circuit samples the current of the load, converts the current into a voltage signal, compares the voltage signal with a reference voltage, and regulates the brightness of the load according to a comparison result.

Preferably, the brightness smoothing and adjusting circuit includes a reference voltage circuit and a comparison circuit, an input end of the reference voltage circuit receives an input control signal, an output end of the reference voltage circuit is connected with an input end of the comparison circuit, and an output end of the comparison circuit is connected with the load.

Preferably, the comparison circuit comprises a plurality of comparison branches, output ends of the comparison branches are connected with the load, and input ends of the comparison branches are connected with an output end of the reference voltage circuit.

Preferably, the comparison branch comprises an operational amplifier circuit, a sampling circuit and an amplifier tube circuit; the input end of the reference voltage circuit receives an input control signal, the output end of the reference voltage circuit is connected with the first input end of the operational amplification circuit, and the input end of the sampling circuit is connected with the output end of the amplification tube circuit; the output end of the sampling circuit is connected with the second input end of the operational amplification circuit, the output end of the operational amplification circuit is connected with the controlled end of the amplification tube circuit, and the input end of the amplification tube circuit is connected with the load.

Preferably, the reference voltage circuit includes a first resistor, a second resistor, a third resistor, a fourth resistor, a fifth resistor, a sixth resistor, a first triode, a second triode and a first voltage reference chip; a first end of the first resistor receives a control signal, a second end of the first resistor is connected with a base electrode of the first triode, a collector electrode of the first triode is connected with a reference power supply through the second resistor, and an emitting electrode of the first triode is grounded; the first end and the second end of the third resistor are connected between the base electrode and the emitting electrode of the first triode in parallel;

the base electrode of the second triode is connected with the collector electrode of the first triode, the collector electrode of the second triode is connected with the reference power supply through the fourth resistor, and the emitting electrode of the second triode is grounded; the output end of the first reference chip is connected with the first end of the fifth resistor, the input end of the first reference chip is grounded, and the reference end of the first reference chip is connected with the collector electrode of the second triode;

the second end of the fifth resistor is connected with the first input end of the operational amplification circuit; and the first end of the sixth resistor is connected with the second end of the fifth resistor, and the second end of the sixth resistor is grounded.

Preferably, the reference voltage circuit further includes a seventh resistor and a first capacitor; the first end of the seventh resistor is connected with the output end of the first voltage reference chip, the second end of the seventh resistor is connected with the first end of the fifth resistor, the first end of the first capacitor is connected with the first end of the seventh resistor, and the second end of the first capacitor is grounded.

Preferably, the reference voltage circuit further includes a first voltage regulator tube, an anode of the first voltage regulator tube is grounded, and a cathode of the first voltage regulator tube is connected to the first end of the first capacitor.

Preferably, the constant current control circuit further comprises an EMI filter circuit, an output end of the EMI filter circuit is connected with an input end of the switch circuit, and an input end of the EMI filter circuit is connected with a power supply.

In order to achieve the purpose, the television comprises the constant current control circuit. The constant current control circuit comprises a single-stage PFC control circuit, a power supply circuit, a switching circuit and an overvoltage adjusting circuit; the input end of the switching circuit is connected with a power supply, the output end of the switching circuit is connected with a load, the controlled end of the switching circuit is connected with the output end of the single-stage PFC control circuit, the output end of the power supply circuit is connected with the power supply end of the single-stage PFC control circuit, the sampling end of the overvoltage adjusting circuit is connected with the load, and the output end of the overvoltage adjusting circuit is connected with the feedback end of the single-stage PFC control circuit; the power supply circuit is used for supplying power to the single-stage PFC control circuit when an enabling signal is received; the single-stage PFC control circuit is used for controlling the switching circuit to convert an input power supply and then supply power to the load; and the control circuit is also used for controlling the switch circuit to output constant current according to the sampling signal fed back by the overvoltage adjusting circuit.

According to the technical scheme, the constant current control circuit is formed by arranging the single-stage PFC control circuit, the power supply circuit, the switch circuit and the overvoltage adjusting circuit. The power supply circuit supplies power to the single-stage PFC control circuit when receiving an enabling signal, and the single-stage PFC control circuit controls the switching circuit to convert an input power supply and then supply power to the load; and the control circuit is also used for controlling the switch circuit to output constant current according to the sampling signal fed back by the overvoltage adjusting circuit. According to the technical scheme, the power supply supplies power to the single-stage PFC control circuit only when the enable signal is received, and the single-stage PFC control circuit is powered off in a standby state, so that energy consumption is reduced. Due to the fact that the single-stage PFC control circuit is adopted, the constant-current control circuit can supply power to the backlight lamp bar without BOOST boosting, and conversion efficiency of electric energy is improved.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the structures shown in the drawings without creative efforts.

FIG. 1 is a functional block diagram of a constant current control circuit according to an embodiment of the present invention;

fig. 2 is a circuit structure diagram of a constant current control circuit according to an embodiment of the present invention.

The implementation, functional features and advantages of the objects of the present invention will be further explained with reference to the accompanying drawings.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

It should be noted that all the directional indicators (such as up, down, left, right, front, and rear … …) in the embodiment of the present invention are only used to explain the relative position relationship between the components, the movement situation, etc. in a specific posture (as shown in the drawing), and if the specific posture is changed, the directional indicator is changed accordingly.

In addition, the descriptions related to "first", "second", etc. in the present invention are for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In addition, the technical solutions in the embodiments may be combined with each other, but it must be based on the realization of those skilled in the art, and when the technical solutions are contradictory or cannot be realized, such a combination should be considered to be absent and not within the protection scope of the present invention.

The invention provides a constant current control circuit.

Referring to fig. 1, in the embodiment of the present invention, the constant current control circuit includes a single-stage PFC control circuit 100, a power supply circuit 200, a switch circuit 300, and an overvoltage adjustment circuit 400; the input end of the switching circuit 200 is connected to a power supply, the output end of the switching circuit 200 is connected to a load, the controlled end of the switching circuit is connected to the output end of the single-stage PFC control circuit 100, the output end of the power supply circuit 200 is connected to the power supply end of the single-stage PFC control circuit 100, the sampling end of the overvoltage adjusting circuit 400 is connected to the load, and the output end of the overvoltage adjusting circuit 400 is connected to the feedback end of the single-stage PFC control circuit 00.

The power supply circuit 2000 is configured to supply power to the single-stage PFC control circuit 100 when receiving the enable signal. The single-stage PFC control circuit 100 is configured to control the switching circuit 200 to convert an input power and supply power to the load; and is also used for controlling the switching circuit to output a constant current according to the sampling signal fed back by the overvoltage regulating circuit 400.

In this embodiment, the load is a television LED backlight. The switching circuit 300 includes a flyback transformer T1, the flyback transformer T1 receives an ac power input by the EMI filter circuit 700, and performs voltage conversion on the ac power under the control of the single-stage PFC control circuit 100 to obtain an ac power with a preset voltage value. The alternating current output by the flyback transformer is processed by the rectifying and filtering circuit and then supplies power to the load.

The load comprises a preset number of LED lamp bars. The overvoltage adjusting circuit includes a plurality of overvoltage adjusting branches (not shown), that is, each LED light bar corresponds to one overvoltage adjusting branch. The overvoltage adjustment branch circuit collects the voltage of the corresponding LED light bar and feeds the voltage back to the single-stage PFC control circuit 100.

In this embodiment, the enable signal input to the power supply circuit is a backlight EN signal, wherein the backlight signal is isolated by an optical coupler and then output to the power supply circuit 200 as a control signal for turning on/off the power supply circuit.

According to the technical scheme, the single-stage PFC control circuit 100, the power supply circuit 200, the switch circuit 300 and the overvoltage adjusting circuit 400 are arranged to form a constant current control circuit. When receiving an enable signal EN, the power supply circuit 200 supplies power to the single-stage PFC control circuit 100, and the single-stage PFC control circuit 100 controls the switching circuit 300 to convert an input power supply and then supply power to the load; and is also used for controlling the switching circuit to output a constant current according to the sampling signal fed back by the overvoltage regulating circuit 400. In the technical scheme of the invention, the power supply supplies power to the single-stage PFC control circuit 100 only when the enable signal EN is received, and the single-stage PFC control circuit 100 is powered off in a standby state, so that the energy consumption is reduced. Due to the adoption of the single-stage PFC control circuit 100, the constant-current control circuit can supply power to the backlight lamp bar without BOOST boosting, and the conversion efficiency of electric energy is improved.

Furthermore, the constant current control circuit also comprises a brightness smooth adjusting circuit, and the brightness smooth adjusting circuit is connected with the load; the brightness smooth regulating circuit samples the current of the load, converts the current into a voltage signal, compares the voltage signal with a reference voltage, and regulates the brightness of the load according to a comparison result.

Through setting up the smooth regulating circuit of luminance, can realize that LED luminance is smooth adjustable when the user carries out PWM and adjusts luminance, and reduce the ripple current through LED lamp strip, reduced stand-by power consumption.

Specifically, the brightness smoothing and adjusting circuit includes a reference voltage circuit 600 and a comparison circuit 500, an input terminal of the reference voltage circuit 600 receives an input control signal, an output terminal of the reference voltage circuit 600 is connected to an input terminal of the comparison circuit 500, and an output terminal of the comparison circuit 500 is connected to the load. The comparison circuit 500 is configured to collect a voltage of the load, compare the voltage with a reference voltage provided by the reference voltage circuit 600, and adjust a current in the load according to a comparison result.

Further, in an embodiment, the comparing circuit 500 includes a plurality of comparing branches (not shown), wherein the output terminals of the comparing branches are connected to the load, and the input terminals of the comparing branches are connected to the output terminal of the reference voltage circuit. It is easy to understand that the number of the comparison branches is the same as the number of the LED light bars.

Further, the comparing branch comprises an operational amplifier circuit (not shown), a sampling circuit (not shown) and an amplifier tube circuit (not shown); the input end of the reference voltage circuit 600 receives an input control signal, the output end of the reference voltage circuit 600 is connected with the first input end of the operational amplification circuit, and the input end of the sampling circuit is connected with the output end of the amplification tube circuit; the output end of the sampling circuit is connected with the second input end of the operational amplification circuit, the output end of the operational amplification circuit is connected with the controlled end of the amplification tube circuit, and the input end of the amplification tube circuit is connected with the load.

In this embodiment, when the enable signal of the backlight EN is at a high level, the optical coupler controls the power supply circuit 200, so that the single-stage PFC control circuit 100 starts to operate and controls the flyback transformer T1 to oscillate.

Two LED light bars are taken as an example for explanation. The output is rectified and filtered and then connected with the LED1 and the LED 2. In the constant current control circuit, an LED1 is connected in series with a first amplifying tube circuit and a first sampling circuit, and an LED2 is connected in series with a second amplifying tube circuit and a second sampling circuit. The current passing through the light bar LED1 flows through the first sampling circuit, the micro voltage sampled by the first sampling circuit is fed back to the first operational amplification circuit, the first operational amplification circuit is output and controlled after the differential comparison operational amplification with the reference voltage, and the first amplification tube circuit works in a linear region, so that the current passing through the LED1 light bar is precise and constant, and the current ripple flowing through the LED1 light bar is reduced.

The current passing through the light bar LED2 flows through the second sampling circuit, the micro voltage sampled by the sampling circuit is fed back to the second operational amplification circuit, the second operational amplification tube current is output and controlled after the second operational amplification circuit is subjected to differential comparison and operational amplification with the reference voltage, and the second amplification tube circuit works in a linear region, so that the current passing through the LED2 light bar is precise and constant, and the current ripple flowing through the LED2 light bar is reduced.

Further, the reference voltage circuit comprises a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a fifth resistor R5, a sixth resistor R6, a first triode Q1, a second triode Q2 and a first voltage reference chip U1; a first end of the first resistor R1 receives a control signal, a second end of the first resistor R1 is connected with a base of the first triode Q1, a collector of the first triode Q1 is connected with a reference power supply through the second resistor R2, and an emitter of the first triode Q1 is grounded; the first end and the second end of the third resistor R3 are connected in parallel between the base electrode and the emitter electrode of the first triode Q1;

the base of the second triode Q2 is connected with the collector of the first triode Q1, the collector of the second triode Q2 is connected with the reference power supply through the fourth resistor R4, and the emitter of the second triode Q2 is grounded; an output end of the first reference chip U1 is connected with a first end of the fifth resistor R5, an input end of the first reference chip U1 is grounded, and a reference end of the first reference chip U1 is connected with a collector of the second triode Q2;

a second end of the fifth resistor R5 is connected to a first input end of the operational amplifier circuit; the first end of the sixth resistor R6 is connected with the second end of the fifth resistor R5, and the second end of the sixth resistor R6 is grounded.

In the reference voltage circuit, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a first triode Q1, a second triode Q2 and a first voltage reference chip U1 realize generation of reference voltage, and a fifth resistor R5 and a sixth resistor R6 are used for dividing the generated reference voltage.

Further, the reference voltage circuit further comprises a seventh resistor R7 and a first capacitor C1; a first end of the seventh resistor R7 is connected to an output end of the first voltage reference chip U1, a second end of the seventh resistor R7 is connected to a first end of the fifth resistor R5, a first end of the first capacitor C1 is connected to a first end of the seventh resistor R7, and a second end of the first capacitor C1 is grounded. The seventh resistor R7 and the first capacitor constitute an RC filter circuit for filtering the generated reference voltage.

Further, the reference voltage circuit further comprises a first regulator tube Z1, the anode of the first regulator tube Z1 is grounded, and the cathode of the first regulator tube Z1 is connected with the first end of the first capacitor C1. The first voltage-regulator tube Z1 is used for regulating the voltage of the output voltage, and the stability of the system is improved.

In the prior art, an EN signal controls a complex-side constant current control circuit to be turned on and off, a PWM controls a primary-side constant current management chip, a primary-side PFC control chip is turned on and off along with the PWM signal, and then the brightness of an output LED is indirectly adjusted, the brightness adjustment of a system is unsmooth, and the problem of lamp flicker exists in dimming. In the invention, when the EN signal is in a high level, the PWM directly controls the constant current control circuit on the secondary side under the normal working state of the PFC control chip, so that the brightness of the output LED is smooth and adjustable, the primary PFC control chip works stably, and the repeated on-off operation along with the PWM signal is not needed.

Further, the constant current control circuit further comprises an EMI filter circuit 700, an output end of the EMI filter circuit 700 is connected with an input end of the switch circuit 300, and an input end of the EMI filter circuit 700 is connected with a power supply. The EMI filter circuit 700 is used to filter out electromagnetic interference input by the power grid, improving the anti-interference performance of the system.

In summary, the technical scheme of the present invention is further described with reference to specific circuit diagrams:

as shown in fig. 2, the power is rectified by an EMI filter circuit 700 and a rectifier bridge BD1 and then transmitted to a flyback transformer T1, a GATE pin of a PFC control chip U6 outputs a control switch M1, and further controls the transformer T1 to oscillate, so that the 7 pin and 8 pin of T1 output voltages, and the outputs are filtered by a rectifier D10 and a capacitor CE1 and then transmitted to an LED light bar LED1 … …, an LED1n and an LED2 … …, an LED2 n. The current passing through the LED passes through the constant current control circuit, so that the current ripple flowing through each channel of the LED is reduced, and the working stability of the lamp strip is improved. The single-stage PFC is adopted to control 100 the switch module and further control the switch converter, so that a high-voltage electrolytic capacitor is not needed in design, the area and cost of a PCB are saved, and the power factor of a power supply is improved.

Further, the LED lamp strip LED1 … … LED1n and the LED2 … … LED2n are fed back to the regulator U7 through a voltage regulating circuit, then the current passing through the optical coupler is controlled and fed back to the primary side, further the single-stage PFC controller U6 is controlled, the working frequency and the duty ratio of the first MOS transistor M1 are regulated and switched, and finally the output voltage is controlled. When the voltage of the lamp strip of the LED1 … …, the LED1n or the LED2 … …, the LED2n is too high, the higher voltage detected by the overvoltage adjusting circuit is fed back to the single-stage PFC control IC U6 as described above, so as to control the output voltage of the flyback transformer T to be reduced, so that the output current is generally constant.

As further described above, the first overvoltage adjusting circuit is composed of D1, R10, R11 and CE2, and detects the voltage of the LED1 … …, the LED1 n; the second overvoltage adjusting circuit is composed of D2, R12, R13 and CE3 and is used for detecting the voltage of the LED2 … …, the LED2 and the LED2 n.

Because the single-stage PFC control circuit is adopted, the high-voltage electrolytic capacitor is omitted, the low-frequency small ripple voltage output by the constant current is larger, the ripple voltage cannot be detected by the traditional constant current mode, and the output LED ripple current is also larger, so the operational amplifier is adopted in the invention, the small ripple voltage is amplified, and then the constant current source is controlled, and the output constant current ripple is lower.

Further, in the constant current control circuit, an LED1 … …, an LED1n, is connected in series with a triode Q3, a current sampling resistor RS1, an LED2 … …, an LED2n, is connected in series with a triode Q4 and a current sampling resistor RS 2.

Further, when the enable signal of the backlight EN is at a high level, the transistor Q7 controls the optocoupler U4 to further control the turn-on of the QP9 in the primary-side VCC power supply circuit, so that the VCC2 outputs VCC _ LED voltage through the diodes D407, R162, and Q5 to provide working voltage for the single-stage PFC control IC U6, so that the single-stage PFC control IC U6 starts to work, controls the flyback transformer T1 to oscillate, and outputs the rectified and filtered signals to the LED1 … …, the LED1n, and the LED2 … …, the LED2 n.

The current passing through the lamp strip LED1 … … LED1n flows through the current sampling resistor Rs1, the tiny voltage sampled by the current sampling resistor Rs1 is fed back to the operational amplifier U2a, the tiny voltage is subjected to differential comparison with the reference voltage of 0.5V, the tiny voltage is output through the operational amplifier to control the amplification triode Q3, the triode Q3 works in a linear region, and the voltage of a Q3 c-e electrode is adjusted, so that the current passing through the lamp strip of the LED1 is precise and constant.

For example, assuming that the rated current required by the LED lamp string is 100mA, when the ripple current peak value flowing through the lamp string is large, the current sampling resistor Rs1 is 5 ohms, and the sampled large voltage peak value is 0.55V, the LED passing current is 110mA, which is fed back to the operational amplifier U2a, and is compared with the reference voltage 0.5V in a differential manner, the deviated 0.05V voltage is output through the operational amplifier to control the amplifying triode Q3, so that the c-e voltage of the Q3 is increased, the ripple voltage peak value of the LED1 … … LED1n lamp string is reduced to 0.5V, and the LED lamp string current is adjusted to 100 mA; when the trough value of ripple current flowing through the lamp strip is small, the voltage value of the small trough sampled by the current sampling resistor Rs1 is 0.45V, the passing current of the LED is 90mA, the current is fed back to the operational amplifier U2a, the difference comparison is carried out between the current passing current and the reference voltage 0.5V, the deviated 0.05V voltage is output after passing through the operational amplifier to control the amplifying triode Q3, the c-e voltage of the Q3 is reduced, the trough value of the ripple voltage of the LED1 … … LED1n lamp string is increased and becomes 0.5V, and the current of the LED lamp strip is adjusted to 100 mA. In summary, the ripple voltage peak-to-peak value (i.e., the peak-to-valley value) at the two ends of the LED1 … … LED2n string is reduced, i.e., is constant at 0.5V, and then the current of the LED light bar is adjusted to be constant 100mA, so that the current ripple flowing through the LED1 light bar is reduced, i.e., constant 100 mA. Similarly, the working principle of the light bar LED2 … … LED2n is the same as that described above, so that the current ripple flowing through the LED2 … … LED2n light bar is reduced, the LED2 … … LED2n and the control circuit thereof are connected in parallel according to the channel number of the screen light bar, any LED channel number can be connected in parallel according to the same principle, and the precise constant current control circuit can be precisely controlled to enable the current ripple of each channel to be extremely small.

Further, when the PWM signal is at a high level, R1 is turned on, the base of Q2 is at a low level, Q2 is turned off, and SVCC provides a source voltage for the reference generating circuit U1 through the current limiting resistor R4, so that U1 generates a 2.5V voltage, which is filtered by the RC filter circuit, i.e., R7 and C1, and then is passed through the voltage dividing and sampling circuits R5 and R6 to obtain a high stable reference voltage of 0.5V, which is transmitted to the operational amplifier ICs U2a and U2b as a reference voltage. At this time, the voltage sampled by the current sampling resistor Rs1 takes the reference voltage of 0.5V as a reference, the operational amplifier U2a outputs, controls and adjusts the amplifying tube Q3, the current passing through the lamp strip is increased, the voltage sampled by the current sampling resistor is increased to the reference voltage, and finally the brightness of the lamp is lightened.

When the PWM signal is at a low level, Q1 is turned off, the base of Q2 is pulled up to a high level through resistor R3, Q2 is turned on, SVCC is pulled down through current limiting resistor R4, and is a passive voltage of reference generation circuit U1, then U1 generates a 0V voltage, which is filtered by RC filter circuit, and then is transmitted to operational amplifier ICs U2a and U2b as a reference voltage through voltage division sampling circuit R5 and R6 to obtain a lower stable reference voltage 0V. At the moment, the voltage sampled by the current sampling resistor Rs1 takes the reference voltage 0V as a reference, the operational amplifier U2a outputs, controls and adjusts the amplifying tube Q3, the current passing through the lamp bar is reduced, the voltage sampled by the current sampling resistor is reduced to the reference voltage, and finally the brightness of the lamp is darkened;

when linear dimming is adopted, when the duty ratio of a high level of a PWM signal is gradually increased, the on time of Q1 is prolonged, the base of Q2 is at a low level, Q2 is cut off, and the time of supplying a source voltage to a reference generating circuit U1 by SVCC through a current limiting resistor R3 is prolonged, so that U1 generates a reference voltage, a larger capacity is taken by C1 through an RC filter circuit to generate a gradually increased voltage, namely, after the voltage is filtered by R7 and C1, the gradually increased reference voltage is 0-0.5V after the voltage is filtered by voltage dividing and sampling circuits R5 and R6, and the reference voltage is transmitted to operational amplifier ICs U2a and U2b to serve as a reference voltage. At the moment, the voltage sampled by the current sampling resistor Rs1 takes the reference voltage as a reference, the operational amplifier U2a outputs, controls and adjusts the amplifying tube Q1, the current passing through the lamp bar is increased, the voltage sampled by the current sampling resistor is increased to the reference voltage, and finally the brightness of the lamp is gradually lightened; and similarly, when the duty ratio of the high level of the PWM signal is reduced, the brightness of the lamp is darkened.

Further as described above, when the enable signal of the backlight EN is low, the transistor Q7 controls the optocoupler U4 to be turned off, thereby controlling the Q5 in the primary VCC power supply circuit to be turned off, so that the VCC2 does not pass through the diodes D3, R14, and Q5, the output VCC _ LED voltage is 0, and the IC U6 is stopped providing the working voltage, so that the single-stage PFC controls the IC U6 to stop working, and controls the flyback transformer T1 to stop oscillating, the constant current control circuit stops working, the LED does not pass through current, the lamp is in a light-off state, and the single-stage PFC control circuit stops working, the output constant current control circuit also stops working, and the standby power consumption is reduced.

Further, the output of the transformer T1 passes through a ZCD circuit, namely R24 and R25, is sampled and then is connected with a ZCD pin of an IC U6, and a zero-crossing detection signal is provided for a PFC control management LED constant current module, so that the power factor of PFC with constant current output is improved.

Further, the backlight output overcurrent and overvoltage protection working principle is that the current sampling resistor R138 is connected to the ZS pin of the IC U6 after sampling, and when the output power is too large, the voltage sampled at R26 is larger than the internal reference voltage of the ZCD pin of the IC U6, so that the IC overload protection stops working. The sampling resistors R20 and R22 sample VCC voltage output by an auxiliary winding of the transformer, when the voltage output by the backlight transformer T1 is too high, the primary side auxiliary winding is increased in the same way, and the TSE pin of the IC U6 detects the voltage of VCC through the sampling circuit, so that the voltage of the auxiliary winding is limited in a safety range, and the condition that the lamp strip is damaged due to too large output voltage is avoided.

Further as described above, in another specific embodiment of a constant current control circuit with smooth and adjustable brightness and a television, when PWM dimming is to be adopted, C1 has a smaller capacity, and according to the principle described above, Q1 is turned on, the base of Q2 is at a low level, Q2 is turned off, SVCC provides a source voltage for the reference generating circuit U1 through the current limiting resistor R4, and then U1 generates a 2.5V voltage, which is filtered by the RC filter circuit, i.e., R7 and C1, and then is subjected to the voltage dividing sampling circuit R5 and R6 to obtain a higher stable reference voltage of 0.5V, which is transmitted to the operational amplifier ICs U2a and U2b as a reference voltage. At the moment, the voltage sampled by the current sampling resistor Rs1 takes the reference voltage of 0.5V as a reference, the operational amplifier U2a outputs, controls and adjusts the amplifying tube Q3, so that the current passing through the lamp strip is increased, the voltage sampled by the current sampling resistor is increased to the reference voltage, and finally the brightness of the lamp is lightened; when the PWM signal is at a low level, Q1 is turned off, the base of Q2 is pulled up to a high level through resistor R4, Q2 is turned on, SVCC is pulled down through current limiting resistor R4, and is a passive voltage of reference generation circuit U1, then U1 generates a 0V voltage, which is filtered by RC filter circuit, i.e., R7 and C1, and then is transmitted to operational amplifier ICs U2a and U2b through voltage division sampling circuit R5 and R6 to obtain a lower stable reference voltage 0V, which is transmitted as a reference voltage to operational amplifier ICs U2a and U2 b. At the moment, the voltage sampled by the current sampling resistor Rs1 takes the reference voltage 0V as a reference, and the 1 pin of the operational amplifier U1A outputs, controls and adjusts the amplifying tube Q3, so that the current passing through the lamp bar is reduced, the voltage sampled by the current sampling resistor is reduced to the reference voltage, and finally the brightness of the lamp is darkened; the frequency set by PWM is 100-200Hz, and the lamp works in a normally bright state due to the visual effect of human eyes.

According to the technical scheme, when the whole television works, the single-stage PFC is adopted to control output, a high-voltage electrolytic capacitor is omitted, power factors are improved, a precise constant-current control circuit is adopted, the ripple current of an LED is greatly reduced, the working stability of an LED lamp is improved, the efficiency is improved, the cost is reduced, the constant-current requirement of an LED backlight is met, the brightness of the LED can be adjusted smoothly when a user adjusts the light through PWM, the ripple current passing through an LED lamp strip is reduced, and the standby power consumption is reduced.

The present invention further provides a television, which includes the constant current control circuit described above, and the specific structure of the constant current control circuit refers to the above embodiments, and since the television employs all technical solutions of all the above embodiments, the television at least has all the beneficial effects brought by the technical solutions of the above embodiments, and details are not repeated herein.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention, and all modifications and equivalents of the present invention, which are made by the contents of the present specification and the accompanying drawings, or directly/indirectly applied to other related technical fields, are included in the scope of the present invention.

Claims

1. A constant current control circuit is characterized by comprising a single-stage PFC control circuit, a power supply circuit, a switching circuit, a brightness smooth regulation circuit and an overvoltage regulation circuit; the input end of the switching circuit is connected with a power supply, the output end of the switching circuit is connected with a load, the controlled end of the switching circuit is connected with the output end of the single-stage PFC control circuit, the output end of the power supply circuit is connected with the power supply end of the single-stage PFC control circuit, the sampling end of the overvoltage adjusting circuit is connected with the load, the output end of the overvoltage adjusting circuit is connected with the feedback end of the single-stage PFC control circuit, and the brightness smoothing adjusting circuit is connected with the load; wherein

the single-stage PFC control circuit is used for controlling the switching circuit to carry out voltage conversion on an input alternating current power supply to obtain alternating current with a preset voltage value, and the alternating current is processed by the rectifying and filtering circuit to supply power to the load; the overvoltage adjusting circuit is used for controlling the switching circuit to output a constant current according to a sampling signal fed back by the overvoltage adjusting circuit;

the brightness smooth regulating circuit is used for sampling the current of the load, converting the current into a voltage signal, comparing the voltage signal with a reference voltage, and regulating the brightness of the load according to a comparison result;

the brightness smooth adjusting circuit comprises a reference voltage circuit and a comparison circuit, wherein the input end of the reference voltage circuit receives an input control signal, the output end of the reference voltage circuit is connected with the input end of the comparison circuit, and the output end of the comparison circuit is connected with the load.

2. The constant current control circuit according to claim 1, wherein the comparison circuit includes a plurality of comparison branches, output terminals of the comparison branches are connected to the load, and input terminals of the comparison branches are connected to output terminals of the reference voltage circuit.

3. The constant current control circuit according to claim 2, wherein the comparison branch comprises an operational amplifier circuit, a sampling circuit and an amplifier tube circuit; the input end of the reference voltage circuit receives an input control signal, the output end of the reference voltage circuit is connected with the first input end of the operational amplification circuit, and the input end of the sampling circuit is connected with the output end of the amplification tube circuit; the output end of the sampling circuit is connected with the second input end of the operational amplification circuit, the output end of the operational amplification circuit is connected with the controlled end of the amplification tube circuit, and the input end of the amplification tube circuit is connected with the load.

4. The constant current control circuit according to claim 3, wherein the reference voltage circuit includes a first resistor, a second resistor, a third resistor, a fourth resistor, a fifth resistor, a sixth resistor, a first transistor, a second transistor, and a first voltage reference chip; a first end of the first resistor receives a control signal, a second end of the first resistor is connected with a base electrode of the first triode, a collector electrode of the first triode is connected with a reference power supply through the second resistor, and an emitting electrode of the first triode is grounded; the first end and the second end of the third resistor are connected between the base electrode and the emitting electrode of the first triode in parallel;

the base electrode of the second triode is connected with the collector electrode of the first triode, the collector electrode of the second triode is connected with the reference power supply through the fourth resistor, and the emitting electrode of the second triode is grounded; the output end of the first voltage reference chip is connected with the first end of the fifth resistor, the input end of the first voltage reference chip is grounded, and the reference end of the first voltage reference chip is connected with the collector of the second triode;

5. The constant current control circuit according to claim 4, wherein the reference voltage circuit further includes a seventh resistor and a first capacitor; the first end of the seventh resistor is connected with the output end of the first voltage reference chip, the second end of the seventh resistor is connected with the first end of the fifth resistor, the first end of the first capacitor is connected with the first end of the seventh resistor, and the second end of the first capacitor is grounded.

6. The constant current control circuit according to claim 5, wherein the reference voltage circuit further comprises a first regulator tube, an anode of the first regulator tube is grounded, and a cathode of the first regulator tube is connected with the first end of the first capacitor.

7. The constant current control circuit of claim 2, further comprising an EMI filter circuit, an output of the EMI filter circuit being connected to an input of the switch circuit, an input of the EMI filter circuit being connected to a power supply.

8. A television set, characterized in that the television set comprises the constant current control circuit according to any one of claims 1 to 7.