CN110868778A

CN110868778A - Constant current control method for low-voltage difference of LED driving input and output

Info

Publication number: CN110868778A
Application number: CN201911130033.7A
Authority: CN
Inventors: 方曙; 谢靖; 吴东生
Original assignee: Shenzhen Smart Semiconductor Technology Co Ltd
Current assignee: Shenzhen Smart Semiconductor Technology Co Ltd
Priority date: 2019-11-18
Filing date: 2019-11-18
Publication date: 2020-03-06
Anticipated expiration: 2039-11-18
Also published as: CN110868778B

Abstract

The invention relates to a constant current control method for LED driving input and output low-voltage difference, which comprises an LED lamp, a switching power supply driving control chip U1, an MOS (metal oxide semiconductor) starting time limiting module and a current detection comparator, wherein the LED lamp is connected with the switching power supply driving control chip U1; the switching power supply driving control chip U1 detects current by CS, the MOS opening time limiting module is provided with a TON limiting circuit, and the current detection comparator controls the turn-off of the MOS Q1 by detecting the current in real time and the output stage of the error amplifier EA; switching power supply can fix MOS Q1's on-time when input and output voltage are close, and error amplifier EA can also adjust MOS's off-time or adjust MOS Q1's duty cycle, realizes the constant current, has effectively solved the problem that the electric current that flows through the lamp pearl increases or the lamp dodges when output lamp pearl voltage drop and input voltage drop are close.

Description

Constant current control method for low-voltage difference of LED driving input and output

Technical Field

The invention relates to the technical field of LED lighting, in particular to a constant current control method for driving input and output low-voltage difference of an LED.

Background

The existing LED has the advantages of long service life, energy conservation, environmental protection, pure and thick color and the like, and is applied to the fields of stage lighting, building lightening, garden lighting and the like to obtain more and more extensive application. Because present LED DCDC's constant current step-down scheme is in order to reach the purpose that connects the lamp more, and the pressure differential between input voltage and the output lamp pressure can be very close, and DCDC can get into the on off state who is close to 100% duty cycle this moment, and output current can obviously increase, and chip internal current detection circuit is in abnormal operating condition moreover, causes the flashing light easily and burns out the lamp pearl even.

At present, a general average current type DCDC step-down constant current topology is shown in fig. 1, a chip U1 controls a switching duty ratio of a MOS Q1 according to the magnitude of a current flowing through Isense to realize a closed-loop constant current control, the current IOUT flows through an inductor L1, the MOS Q1 and a current detection resistor R1 to GND during the time when Q1 is turned on, the chip U1 detects the current value at this time, the Isense current presents a rising state of a sawtooth wave due to the relationship of the inductor, as shown in fig. 4, when the Isense current value reaches an internally limited peak current, the MOS is turned off, the current IOUT freewheels through a diode D1 after the MOS is turned off, and the process is repeated to realize the constant current of the system.

However, at present, the maximum turn-on time of the MOS is not limited by these chips, which results in that as shown in fig. 5, when the input voltage and the output LED lamp voltage are close to each other, due to a small voltage difference between both ends, the Isense current cannot reach the peak current limited internally, and the MOS Q1 cannot be turned off, so that the output current increases, which easily causes flashing and even burning of the lamp bead, resulting in a short service life of the LED and poor lighting quality.

Therefore, it is necessary to develop a constant current control method for driving the LED to input and output a low voltage difference.

Disclosure of Invention

The invention aims to solve the technical problem of providing a constant current control method for driving input and output low-voltage difference of an LED (light emitting diode), and solving the problem that the current flowing through a lamp bead is increased or the lamp flashes when the voltage drop of the output lamp bead is close to the voltage drop of the input lamp bead.

In order to solve the technical problem, the invention provides a constant current control method for LED driving input and output low-voltage difference, which comprises an LED lamp, a switching power supply driving control chip U1, an MOS (metal oxide semiconductor) opening time limiting module and a current detection comparator; the switching power supply driving control chip U1 detects current by CS, the MOS starting time limiting module is provided with a TON limiting circuit, and the current detection comparator controls the turn-off of the MOS Q1 by detecting the current in real time and the output stage of the error amplifier EA;

the constant current control method for driving the LED to input and output low differential pressure comprises the following steps:

a. when the MOS Q1 is in the starting stage, the switching power supply drives the control chip U1 to detect current from the CS;

b. the detected signal is filtered by a path of RC to obtain an average value, and the average value is sent to one side of an error amplifier EA for input;

c. during the period that MOS Q1 is turned off, the signal is kept in Cc, the error amplifier EA compares the obtained current detection signal with reference VREF, and then amplifies and outputs the error value to a current detection comparator and an OSC voltage-controlled oscillation generator;

d. the current detection comparator controls the turn-off of the Q1 by detecting the current in real time and the output stage of the error amplifier EA, and controls the frequency of the OSC to control the turn-on of the next period of the MOS Q1;

e. when the DRV level of the GATE driving signal of the MOS Q1 is high, the CTon starts to discharge through the RTon, after the level Vc of the CTon reaches the overturning level of the post-stage smit, the level of the TON signal is overturned, and the TON signal forces the MOSQ1 to be turned off;

f. and repeating the steps a to e, and adjusting the closing time of the MOS or adjusting the duty ratio of the MOS Q1 to realize constant current.

Further, the maximum on-time Ton signal generated by the MOS on-time limiting module and the Ipeak signal limited by the switching power supply driving control chip U1 control the turn-off of the MOS Q1 at the same time.

Further, the error amplifier EA adjusts the turn-off time of the MOS or adjusts the duty ratio of the MOS Q1 by adjusting the voltage controlled oscillator.

Compared with the prior art, the invention has the beneficial effects that: the invention effectively solves the problem that the current flowing through the lamp bead is increased or the lamp flashes when the voltage drop of the output lamp bead is close to the voltage drop of the input lamp bead, realizes constant current by the TON limiting circuit current detection comparator of the MOS starting time limiting module, and has the following main beneficial effects;

1. the problem that the current of the lamp beads is increased to burn the lamp beads when the input voltage and the output voltage are close is solved;

2. the problem of input and output voltage near lamp flashing is solved;

3. when the input voltage and the output voltage of the switching power supply are close to each other, the on time of the MOS Q1 is fixed, and at the moment, the EA can also adjust the off time of the MOS or adjust the duty ratio of the MOS Q1 to realize constant current.

4. The circuit of the invention is also applicable to whether the system works in a continuous mode or a discontinuous mode.

Drawings

FIG. 1 is a schematic diagram of a general DCDC buck-type constant current system of the present invention;

FIG. 2 is a schematic diagram of the operation of the present invention and the connection of the MOS turn-on time limiting module;

FIG. 3 is a schematic diagram of a TON limiting circuit of the MOS turn-on time limiting module according to the present invention;

FIG. 4 is a current pattern through MOS Q1 and current sensing resistor R1 when the input voltage is higher than the output lamp voltage;

FIG. 5 is a waveform of Isense current flowing through the CS pin without adding the MOS turn-on time limiting module;

FIG. 6 is an Isense waveform after the addition of a MOS turn-on time limit;

FIG. 7 shows the output current waveform of Iout without the MOS turn-on time limiting module;

fig. 8 shows the output current waveform after adding MOS turn-on time limit to Iout.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings.

As shown in fig. 1 to 8, in an embodiment of the present invention, a constant current control method for driving an input and an output low voltage difference of an LED includes an LED lamp, where the LED lamp is electrically connected to a switching power supply driving control chip U1, an MOS turn-on time limiting module, and a current detection comparator; the switching power supply driving control chip U1 detects current by CS, the MOS opening time limiting module is provided with a TON limiting circuit, and the current detection comparator controls the turn-off of the MOS Q1 by detecting the current in real time and the output stage of the error amplifier EA;

the constant current control method for driving input and output low differential pressure of the LED comprises the following steps:

the TON limiting circuit starts to discharge when the level of a GATE driving signal DRV of the MOS Q1 is high, the level Vc of the CTon reaches the inversion level of the post-stage smit, the level of a Tonmax signal is inverted, and the Tonmax signal forces the MOS Q1 to be turned off;

The invention provides a method, which solves the problem that when the voltage drop of an output lamp bead is close to the voltage drop of an input lamp bead, the current flowing through the lamp bead is increased or the lamp flashes.

As shown in fig. 1, in a general DCDC buck constant current system, U1 is a switching power supply driving control chip, and U1 detects current through a CS pin, so that the control system realizes constant current output. Many existing DCDC drivers are faced with a situation that when the input voltage and the output lamp voltage are close to each other, the current Is flowing through the MOS Q1 (hereinafter referred to as Q1) and the current detection resistor R1 cannot be turned off because the current detection module detects that the current has not reached the peak current that needs to be turned off.

As shown in fig. 2, the operation principle and the MOS on-time limiting module are connected schematically, the present invention adds a MOS Q1 on-time limiting module on the existing DCDC buck constant current system, during the on stage of MOS Q1, the switching power supply driving control chip U1 detects current from CS, obtains an average value by one path of RC filtering of the detected signal, and sends the average value to one side of the error amplifier EA for input, the signal is kept in Cc during the off period of Q1, the error amplifier EA compares the obtained current detection signal with the reference VREF, and then amplifies and outputs the error value to the current detection comparator and the OSC voltage-controlled oscillator generator, and the current detection comparator controls the off of Q1 and controls the frequency of OSC by detecting the current in real time and the output stage of the error amplifier EA to control the on of the next period of Q1.

As shown in fig. 2, when the on-time of the MOS Q1 is less than the set time T, the current ISENSE is very close to the input voltage and the output lamp voltage, the on-time of the MOS Q1 is longer and longer, when the on-time of the Q1 reaches the maximum time limit, the on-time will not increase, the Q1 is forced to be turned off, the current flowing through the Q1 and the R1 can also be turned off without reaching Ipeak, and the switching power supply driving control chip U1 adjusts the output switching duty ratio through the error amplifier EA to control the system constant current.

Fig. 3 shows a schematic structural diagram of the TON limiting circuit of the MOS on-time limiting module, when the GATE driving signal DRV level of the MOS Q1 is high, the discharge of the CTon starts through RTon, after the level Vc of the CTon reaches the inversion level of the post-stage smit, the level of the Tonmax signal inverts, and the Tonmax signal forces the MOS Q1 to turn off.

The method comprises the steps that the TON limiting circuit can force the MOS Q1 to be closed after the set maximum opening time is over, meanwhile, the maximum opening time TON signal generated by the MOS opening time limiting module and the chip-limited Ipeak signal control the MOS Q1 to be closed at the same time, when the input voltage and the lamp voltage difference are much, the MOS opening time is short, before the TON signal arrives, Isense reaches an Ipeak threshold, Q1 is closed, the chip closed-loop working principle is the same as that of the current chip, when the input voltage and the lamp voltage are close, the Q1 opening time is prolonged, when the input lamp voltage and the output lamp voltage are close, the phenomenon that Isense cannot reach Ipeak is caused, the TON limiting circuit can force the Q1 to be closed, and meanwhile, the chip U1 is in a closed-loop constant current state.

According to the invention, the MOS starting time limiting module and the current detection comparator are added to realize the turn-off of the Q1, when a Q1 turn-off enabling signal output by the current detection comparator does not yet arrive, but the turn-off enabling signal of the MOS starting time limiting module is triggered, the Q1 is turned off, the starting time of the next period of the Q1 can be adjusted by the error amplifier EA in a mode of adjusting the voltage-controlled oscillator, and the switching power supply driving control chip U1 can also work in a constant current mode, so that the current cannot be increased.

As shown in fig. 4, a current mode flowing through the MOS Q1 and the current sensing resistor R1 Is shown when the input voltage Is higher than the output lamp voltage, and the output current Is constant, the system automatically adjusts the output current to keep the average value of the output current constant at Iavg, the current Is flowing through the MOS Q1 and the resistor R1 Is periodically turned off and turned on, and Is turned off when the peak value of the current reaches Ipeak and then turned on again after a certain period, and the operation Is repeated to realize constant current output, and the magnitude of the output current Is equal to Iavg.

As shown in fig. 5, because most of the current step-down constant-current DCDC driving adopts an average current mode, the system needs to be constant-current, the closed peak current Is higher than the average current, at this time, the output current Is equal to the current flowing through the MOS Q1 and the resistor R1 and Is higher than the average current Iavg, the current Is flowing through the lamp bead Is also higher than Iavg, the current Is higher than the current Iavg in the normal operating state, at this time, if the current exceeds the rated current of the lamp bead, the lamp bead Is easily burned out, and when the values of the current Is close to the peak current Ipeak when the MOS Q1 Is turned off, the current at the CS end exceeds Ipeak, the Q1 Is turned off, otherwise, the Q1 Is kept on, the MOS Q1 Is in the critical state of on and off, if the system Is interfered by external factors, the MOS 1 Is easily irregularly switched at this time, thereby causing a phenomenon of lamp flash.

Fig. 5 is a waveform of current Isense flowing through the CS pin without adding the MOS turn-on time limit module, and fig. 6 is a waveform of Isense after adding the MOS turn-on time limit. Fig. 7 shows an output current waveform of Iout without adding a MOS on-time limiting module, the current value is always higher than the current average value Iavg of the reference limit, the output current is larger than the set current, and at this time, a problem of lamp flash still exists. Fig. 8 is a waveform of an output current after MOS turn-on time limitation is added, the average value of the current is equal to Iavg, and the output current does not increase when the system operates in a constant current output state.

The problems and the beneficial effects solved by the invention are as follows:

1. the invention adds the MOS turn-on time limiting module which is used for limiting the maximum turn-on time of the MOS Q1 of the switching power supply, thereby avoiding the problem that the lamp beads are burnt out due to the increase of the output current and solving the problem of dimming lamp flash.

2. FIG. 3 shows a TON limiting circuit of a MOS ON time limiting block, which is functional to generate a pulse with a fixed length of high duration, during which time MOS Q1 is turned off independent of the block, and MOS Q1 is turned off when MOS Q1 is turned on for the time duration

3. When input and output lamp voltage are close, because MOS closes in fixed opening time T, the time that MOS opened next time can also be adjusted to the inside error amplifier EA of chip so, adjusts the duty cycle, lets the system keep constant current output, and output current can not increase, can not burn the lamp pearl.

4. When input and output lamp voltage are close, because MOS closes in fixed opening time T, in time T, if the system rationally selects the inductance, then Is can keep the sawtooth wave shape of at least slightly taking the radian, can not become the shape like figure 4, just also can not be disturbed by external and lead to the flashing light, has avoided the condition of flashing light.

5. When the input voltage and the output voltage of the switching power supply are close to each other, the on time of the MOS Q1 is fixed, and at the moment, the EA can also adjust the off time of the MOS or adjust the duty ratio of the MOS Q1 to realize constant current.

6. The circuit of the invention is also suitable for the system to work in a continuous mode or a discontinuous mode;

7. the invention is equally effective for switching power supplies operating in continuous mode and discontinuous mode.

The above description is only a preferred embodiment of the present patent, and not intended to limit the scope of the present patent, and all modifications of equivalent structures and equivalent processes, which are made by using the contents of the specification and the drawings, and which are directly or indirectly applied to other related technical fields, belong to the scope of the present patent protection.

Claims

1. A constant current control method for LED drive input and output low differential pressure is characterized in that: the LED lamp comprises an LED lamp, a switching power supply driving control chip U1, an MOS (metal oxide semiconductor) opening time limiting module and a current detection comparator; the switching power supply driving control chip U1 detects current by CS, the MOS starting time limiting module is provided with a TON limiting circuit, and the current detection comparator controls the turn-off of the MOS Q1 by detecting the current in real time and the output stage of the error amplifier EA;

e. when the DRV level of the GATE driving signal of the MOS Q1 is high, the CTon starts to discharge through RTon, after the level Vc of the CTon reaches the turnover level of the post-stage smit, the level of the TON signal is reversed, and the TON signal forces the MOS Q1 to be turned off;

2. The LED drive input and output low dropout constant current control method according to claim 1, wherein: the maximum on-time Ton signal generated by the MOS on-time limiting module and the Ipeak signal limited by the switching power supply driving control chip U1 control the turn-off of the MOS Q1 at the same time.

3. The LED drive input and output low dropout constant current control method according to claim 1, wherein: the error amplifier EA adjusts the closing time of the MOS or adjusts the duty ratio of the MOS Q1 by adjusting a voltage-controlled oscillator.