CN113260116A - Self-adaptive current ripple filtering circuit, and mains supply solid-state LED lighting system and method - Google Patents
Self-adaptive current ripple filtering circuit, and mains supply solid-state LED lighting system and method Download PDFInfo
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Abstract
The invention provides a self-adaptive current ripple filtering circuit, a mains supply solid-state LED lighting system and a method, wherein the self-adaptive current ripple filtering circuit comprises: connect in the voltage detection unit of load LED lamp cluster negative pole end, the drain electrode connect in the power tube of load LED lamp cluster negative pole end, connect in the current sampling unit of power tube source electrode, connect in voltage detection unit with the error detection unit of current sampling unit, connect in the loop compensation unit of error detection unit, connect in the voltage reduction unit of loop compensation unit, connect in the voltage reduction unit the current sampling unit reaches the drive signal of power tube grid produces the unit. The invention solves the problem of stroboflash caused by the existence of ripple waves of frequency components which are twice the commercial power frequency and more than twice of the current which is input into the load LED lamp string in the existing commercial power solid-state LED lighting system.
Description
Technical Field
The invention relates to the field of integrated circuit design, in particular to a self-adaptive current ripple filtering circuit, a mains supply solid-state LED lighting system and a method.
Background
Commercial solid-state LED lighting systems, which use LEDs as light sources and are driven by constant-current LED driving circuits and are connected to an AC commercial network, have been widely used. In a commercial solid-state LED lighting system, a constant-current LED driving circuit is generally required to ensure that an ac input has a high power factor while providing a constant driving current for a load LED string, so as to reduce harmonic interference and pollution of the lighting system to a commercial power network. In order to simplify the structure and reduce the cost, such constant current LED driving circuits generally adopt a single-stage switching power supply architecture which can keep the average value of output current constant and has a Power Factor Correction (PFC) function.
The circuit of the existing commercial power solid-state LED lighting system is shown in fig. 1, commercial power alternating current is rectified by a full-wave rectifier bridge to generate pulse direct current with twice power frequency ripple, the pulse direct current is used as input of a constant current LED driving circuit, and current output by the constant current LED driving circuit passes through an output capacitor COUTThen flows through the load LED lamp string. Although the current output by the constant current LED drive circuit passes through the output capacitor COUTCan filter out a part of high-frequency switch ripple waves, but outputs a capacitor COUTThe filtering effect to two times power frequency ripple is limited, this also makes the electric current of input load LED lamp cluster have invariable steady direct current value, but wherein superpose great two times commercial power frequency and above frequency component ripple to the illumination light that leads to the LED light source to send contains the stroboscopic of two times power frequency, and then has great injury to people's eye.
Disclosure of Invention
In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide an adaptive current ripple filtering circuit, a solid-state LED lighting system and a method thereof, for solving the problem of stroboflash caused by the ripple of two times of the mains power frequency and more frequency components existing in the current of the input load LED string in the existing solid-state LED lighting system.
To achieve the above and other related objects, the present invention provides an adaptive current ripple filtering circuit, including:
the device comprises a voltage detection unit, a power tube, a current sampling unit, an error detection unit, a loop compensation unit, a voltage reduction unit and a driving signal generation unit; wherein the content of the first and second substances,
the voltage detection unit is connected to the negative end of the load LED lamp string and the error detection unit and is used for acquiring ripple voltage generated by the current input to the load LED lamp string at the negative end of the load LED lamp string;
the drain electrode of the power tube is connected to the negative electrode end of the load LED lamp string, the source electrode of the power tube is connected to the current sampling unit, and the grid electrode of the power tube is connected to the driving signal generating unit;
the current sampling unit is connected to the source electrode of the power tube, the driving signal generating unit and the error detecting unit, and is used for forming a power path between the negative electrode end of the load LED lamp string and the ground according to a current sampling resistor and the power tube, sampling the current flowing through the power path through the current sampling resistor to obtain a current sampling voltage, and amplifying the current sampling voltage to obtain a multiplied current sampling voltage;
the error detection unit is connected to the voltage detection unit, the current sampling unit and the loop compensation unit, and is used for performing error detection on the ripple voltage and the multiplied current sampling voltage to generate a compensation current;
the loop compensation unit is connected with the error detection unit and the voltage reduction unit and used for generating corresponding compensation voltage according to the compensation current;
the voltage reduction unit is connected with the loop compensation unit and the driving signal generation unit and is used for reducing the compensation voltage according to a preset reduction proportion to generate a reduced voltage;
the driving signal generating unit is connected to the voltage reducing unit, the current sampling unit and the grid electrode of the power tube, and is used for carrying out operational amplification processing on the reduced voltage and the current sampling voltage so as to generate a driving signal for driving the power tube.
Optionally, the voltage detection unit includes: and one end of the detection resistor is connected to the negative end of the load LED lamp string, and the other end of the detection resistor is used as the output end of the voltage detection unit and is connected to the positive phase input end of the error detection unit.
Optionally, the current sampling unit includes: the current sampling resistor, the transconductor and the multiplying resistor, wherein one end of the current sampling resistor is connected to the source electrode of the power tube and the positive phase input end of the transconductor, and simultaneously serves as the current sampling end of the current sampling unit is connected to the reverse phase input end of the driving signal generation unit, the other end of the current sampling resistor is grounded, the reverse phase input end of the transconductor is grounded, the output end of the transconductor is connected to one end of the multiplying resistor, and simultaneously serves as the multiplying output end of the current sampling unit is connected to the reverse phase input end of the error detection unit, and the other end of the multiplying resistor is grounded.
Optionally, the error detection unit includes: the first unidirectional output transconductor and the positive phase input end of the second unidirectional output transconductor are connected to serve as the positive phase input end of the error detection unit, the first unidirectional output transconductor and the negative phase input end of the second unidirectional output transconductor are connected to serve as the negative phase input end of the error detection unit, and the output ends of the first unidirectional output transconductor and the second unidirectional output transconductor are connected to serve as the output end of the error detection unit; the first unidirectional output transconductor and the second unidirectional output transconductor are a pair of asymmetric transconductors, and output current directions of the first unidirectional output transconductor and the second unidirectional output transconductor are opposite.
Optionally, the loop compensation unit includes: and one end of the loop compensation capacitor is connected to the output end of the error detection unit and the input end of the voltage reduction unit, and the other end of the loop compensation capacitor is grounded.
Optionally, the adaptive current ripple filtering circuit further includes: and the over-temperature protection unit is connected with the loop compensation unit and the driving signal generation unit and used for performing over-temperature protection on the self-adaptive current ripple filtering circuit so as to increase the conduction degree of the power tube according to the continuously raised detection temperature and turn off the power tube when the detection temperature is greater than an over-temperature protection threshold value.
Optionally, the over-temperature protection unit includes:
the temperature detector is used for detecting the temperature of the working environment where the adaptive current ripple filtering circuit is located;
the protection current generator is connected with the temperature detector and the loop compensation unit and used for generating output current which is increased along with the increase of the detection temperature to the loop compensation unit when the detection temperature is higher than the power reduction initial temperature value, so that the driving signal generation unit is controlled to generate corresponding driving signals to control the conduction degree of the power tube, the conduction degree of the power tube is increased according to the continuously increased detection temperature, the power loss of the power tube is reduced, and meanwhile, the current ripple filtering function disappears slowly; when the detected temperature is greater than or equal to the final power-reducing temperature value, generating corresponding output current to the loop compensation unit, so as to control the driving signal generation unit to generate corresponding driving signals to control the power tube to be completely conducted, thereby reducing the power loss of the power tube to the minimum and simultaneously completely eliminating the current ripple filtering function;
the protection logic generator is connected with the temperature detector and the control end of the driving signal generating unit and is used for generating a turn-off signal to the driving signal generating unit when the detected temperature is greater than an over-temperature protection threshold value so as to control the power tube to be turned off; wherein the over-temperature protection threshold is greater than the final power-down temperature value.
The invention also provides a mains supply solid-state LED lighting system, which comprises:
the rectification circuit is used for rectifying the alternating current commercial power to generate rectified voltage;
the constant current LED driving circuit is connected to the rectifying circuit and used for processing the rectified voltage to generate constant current;
the load LED lamp string is connected with the constant current LED drive circuit and used for lighting the load LED lamp string under constant current drive to emit illumination light;
the adaptive current ripple filtering circuit of any one of claims 1 to 7, connected to the negative terminal of the load LED string, for performing current ripple filtering on the constant current input to the load LED string to eliminate ripple of frequency components of two times or more existing in the constant current.
Optionally, the mains solid state LED lighting system further comprises: and the high-frequency ripple filtering capacitor is connected in parallel with the output end of the constant current LED drive circuit.
The invention also provides a current ripple filtering method, which comprises the following steps:
obtaining ripple voltage generated at the negative electrode end of the current input to the load LED lamp string;
forming a power path between the negative end of the load LED lamp string and the ground based on a power tube and a current sampling resistor, sampling current flowing through the power path through the current sampling resistor to obtain current sampling voltage, and amplifying the current sampling voltage to obtain multiplied current sampling voltage;
carrying out error detection on the ripple voltage and the multiplied current sampling voltage to generate compensation current, and charging and discharging a loop compensation capacitor according to the compensation current to generate corresponding compensation voltage;
and reducing the compensation voltage according to a preset reduction proportion to generate a reduced voltage, and carrying out operational amplification processing on the reduced voltage and the current sampling voltage to generate a driving signal for driving the power tube, so that frequency component ripples of two times or more existing in the current flowing through the load LED lamp string are eliminated.
Optionally, the method of error detecting the ripple voltage and the multiplied current sample voltage to generate the compensation current comprises: error detection is carried out on the ripple voltage and the multiplied current sampling voltage through a pair of asymmetric unidirectional output transconductors with opposite output current directions, and when the difference between the ripple voltage and the multiplied current sampling voltage is larger than a first transconductance voltage and smaller than 0, negative compensation current I is generatedgm＝(VDET-VCSF)*Gm1(ii) a When the difference between the ripple voltage and the multiplied current sampling voltage is less than the first transconductance voltage, a negative compensation current I is generatedgm＝-IT1(ii) a At the ripple voltageWhen the difference between the current sampling voltage and the multiplied current sampling voltage is greater than 0 and less than the second transconductance voltage, a positive compensation current I is generatedgm＝(VDET-VCSF)*Gm2(ii) a When the difference between the ripple voltage and the multiplied current sampling voltage is greater than the second transconductance voltage, a forward compensation current Igm is generated as IT2(ii) a Wherein VDETIs a ripple voltage, VCSFSampling voltage for multiplied current, Gm1Is the negative output transconductance of the first unidirectional output transconductor, IT1Is the maximum value of the negative output current of the first unidirectional output transconductor, Gm2Is the forward output transconductance of the second unidirectional output transconductor, IT2Is the maximum value of the forward output current of the second unidirectional output transconductor.
Optionally, the current ripple filtering method further includes:
detecting the temperature of the working environment of the power tube to obtain a detected temperature;
comparing the detection temperature with the power reduction starting temperature value, and generating output current which is increased along with the increase of the detection temperature when the detection temperature is higher than the power reduction starting temperature so as to control the conduction degree of the power tube, so that the conduction degree of the power tube is increased according to the continuously increased detection temperature, the power loss of the power tube is reduced, and meanwhile, the current ripple filtering function is slowly disappeared; when the detected temperature is greater than or equal to the final power-reducing temperature value, generating corresponding output current so as to control the power tube to be completely conducted, so that the power loss of the power tube is reduced to the minimum, and meanwhile, the current ripple filtering function is completely eliminated;
comparing the detected temperature with an over-temperature protection threshold value, and generating a turn-off signal to control the power tube to turn off when the detected temperature is greater than the over-temperature protection threshold value; wherein the over-temperature protection threshold is greater than the final power-down temperature value.
As described above, according to the adaptive current ripple filtering circuit, the commercial power solid-state LED lighting system and the method of the present invention, through the design of the adaptive current ripple filtering circuit, the filtering of the ripple of twice power frequency and above of the output current of the preceding-stage constant-current LED driving circuit is achieved, and the current flowing through the load LED string is ensured to only include a steady dc component; meanwhile, the variation of the drain voltage ripple valley value of the power tube in the power path along with the source voltage of the power tube is realized, and the fluctuation of the drain-source voltage drop of the power tube in a reasonable range is ensured; and the complete over-temperature protection function including the functions of slowly reducing the power loss of the power tube along with the temperature rise and turning off the over-temperature is realized.
Drawings
Fig. 1 shows a circuit diagram of a conventional commercial solid-state LED lighting system.
Fig. 2 is a circuit diagram of a solid-state LED lighting system with adaptive current ripple filtering according to the present invention.
Fig. 3 is a circuit diagram of an error detection unit in the adaptive current ripple filtering circuit according to the present invention.
Fig. 4 shows the input-output transfer characteristic of the first unidirectional output transconductor in the error detection unit according to the present invention.
Fig. 5 shows the input-output transfer characteristic of the second unidirectional output transconductor in the error detection unit according to the present invention.
FIG. 6 shows the input-output transfer characteristics of the error detection unit according to the present invention.
FIG. 7 shows the adaptive current ripple filtering circuit of the present invention at steady state operating condition VDET、VCSF、ICOMP、VCOMPA waveform diagram of (a).
Fig. 8 shows a characteristic curve of the output current of the overheat protection unit according to the present invention as a function of temperature.
Description of the element reference numerals
10 rectifier circuit
20 constant current LED drive circuit
30-load LED lamp string
40 self-adaptive current ripple filtering circuit
41 Voltage detection Unit
42 current sampling unit
43 error detection unit
44 loop compensation unit
45 voltage reduction unit
46 drive signal generating unit
47 over-temperature protection unit
50 high-frequency ripple filtering capacitor
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention.
Please refer to fig. 2 to 8. It should be noted that the drawings provided in the present embodiment are only schematic and illustrate the basic idea of the present invention, and although the drawings only show the components related to the present invention and are not drawn according to the number, shape and size of the components in actual implementation, the form, quantity and proportion of the components in actual implementation may be changed arbitrarily, and the layout of the components may be more complicated.
As shown in fig. 2, the present embodiment provides an adaptive current ripple filtering circuit, where the adaptive current ripple filtering circuit 40 includes:
a voltage detection unit 41, a power tube M1, a current sampling unit 42, an error detection unit 43, a loop compensation unit 44, a voltage reduction unit 45 and a driving signal generation unit 46; wherein the content of the first and second substances,
the voltage detection unit 41 is connected to the negative end LED of the load LED string 30 and the error detection unit 41, and is configured to obtain the current I input to the load LED string 30LEDLED-generated ripple power at its negative terminalPressure VDET；
The drain of the power tube M1 is connected to the negative terminal LED-of the load LED string 30, the source thereof is connected to the current sampling unit 42, and the gate thereof is connected to the driving signal generating unit 46;
the current sampling unit 42 is connected to the source of the power transistor M1, the driving signal generating unit 46 and the error detecting unit 43, and is configured to sample a resistor R according to a currentCSAnd the power tube M1A power path is formed between the negative end LED of the load LED string 30 and ground through the current sampling resistor RCSSampling the current flowing through the power path to obtain a current sample voltage VCSAnd by sampling said current with a voltage VCSAmplifying to obtain multiplied current sampling voltage VCSF；
The error detection unit 43 is connected to the voltage detection unit 41, the current sampling unit 42 and the loop compensation unit 44, and is configured to measure the ripple voltage VDETAnd the multiplied sampling voltage VCSFError detection is performed to generate a compensation current ICOMP；
The loop compensation unit 44 is connected to the error detection unit 43 and the voltage reduction unit 45, and is configured to compensate the current I according to the compensation currentCOMPGenerating a corresponding compensation voltage VCOMP；
The voltage reduction unit 45 is connected to the loop compensation unit 44 and the driving signal generation unit 46, and is configured to reduce the compensation voltage V by a predetermined reduction ratio of 1/KCOMPPerforming reduction to generate a reduced voltage VCOMP/K；
The driving signal generating unit 46 is connected to the voltage reducing unit 45, the current sampling unit 42 and the gate of the power transistor, and is used for reducing the voltage VCOMPK and the current sampling voltage VCSPerforming operational amplifier processing to generate and drive the power tube M1The drive signal of (1).
As an example, as shown in fig. 2, the voltage detection unit 41 includes: a detection resistor RDETThe detection resistance RDETIs connected to the negative end LED-of the load LED string 30, the detection resistor RDETAnd the other end thereof is connected as an output end of the voltage detection unit 41 to a non-inverting input end of the error detection unit 43.
As shown in fig. 2, the present example is implemented by the detection resistor RDETFor the current I flowing through the load LED lamp string 30LEDThe sampling is performed to obtain a sampling voltage, but since the non-inverting input terminal of the error detection unit 43 is a high-impedance input terminal, the input current thereof is zero, i.e., the detection resistor RDETThe voltage drop is zero, and at this time, the voltage waveform of the negative terminal of the load LED string 30 is completely the same as the voltage waveform of the positive input terminal of the error detection unit 43, so as to obtain the ripple voltage with the ac ripple component. It should be noted that the output current I of the preceding-stage constant-current LED driving circuitLEDThe LED lamp string with the load comprises a stable direct current component and an alternating current ripple component superposed on the stable direct current component, so that the ripple voltage with the alternating current ripple component can be obtained by sampling the voltage at the negative electrode end of the load LED lamp string.
As an example, as shown in fig. 2, the current sampling unit 42 includes: current sampling resistor RCSTransconductor GmAnd a multiplication resistor RCSFThe current sampling resistor RCSOne end of the second resistor is connected to the source electrode of the power tube M1 and the transconductor GmAnd also as the current sampling terminal of the current sampling unit 42, is connected to the inverting input terminal of the driving signal generating unit 46, and is used for outputting a current sampling voltage V to the inverting input terminal of the driving signal generating unit 46CSThe current sampling resistor RCSIs grounded, said transconductor GmIs grounded, said transconductor GmIs connected to the multiplier resistor RCSFAnd is connected to the inverting input terminal of the error detection unit 43 as the multiplication output terminal of the current sampling unit 42, for outputting the multiplied current sampling voltage V to the inverting input terminal of the error detection unit 43CSFSaid multiplication resistor RCSFAnd the other end of the same is grounded.
As shown in fig. 2, the power tube M1And the current sampling resistor RCSA power path is formed between the negative LED of the load LED string 30 and ground, and then through the current sampling resistor RCSSampling the current flowing through the power path to obtain a current sample voltage VCSThen through said transconductor GmSampling the current by a voltage VCSVoltage-current conversion is carried out to generate transconductance current, and the transconductance current passes through the multiplication resistor RCSFOperating the transconductance current to obtain a multiplied current sample voltage VCSF(ii) a However, since the inverting input terminal of the error detection unit 43 is a high-impedance input terminal, the voltage at the inverting input terminal of the error detection unit 43 is controlled by the transconductor GmOutput transconductance current and multiplication resistance RCSFDetermination, i.e. VCSF＝VCS*Gm*RCSF＝(ILED*RCS)*Gm*RCSFWherein G ismIs the transconductance of the transconductor. In this example, G is obtained by iterating a specific circuit in an actual simulationm*RCSFIs 4, when the positive input terminal voltage of the error detection unit 43 (i.e., the power tube M)1Drain voltage) waveform is always close to the bottom of the power tube M14 times of source voltage, thereby ensuring the power tube M1Reliable operation when different currents are flowing.
As an example, as shown in fig. 3, the error detection unit 43 includes: first one-way output transconductor Gm1And a second unidirectional output transconductor Gm2Said first unidirectional output transconductor Gm1And said second unidirectional output transconductor Gm2Is connected as a non-inverting input terminal of the error detection unit 43, the first unidirectional output transconductor Gm1And said second unidirectional output transconductor Gm2Is connected as an inverting input of the error detection unit 43, the first unidirectional output transconductor Gm1And said second unidirectional output transconductor Gm2As said error detection unit 43An output end; wherein the first unidirectional output transconductor Gm1And said second unidirectional output transconductor Gm2Is a pair of asymmetric transconductors and its output current direction is opposite.
Specifically, the first unidirectional output transconductor Gm1The input-output transfer characteristic of (a) is shown in fig. 4: when the first unidirectional output transconductor Gm1Is a positive signal, i.e. VDET-VCSF>When 0, the output current is zero; when the first unidirectional output transconductor Gm1Is a negative signal, i.e. VDET-VCSF<When 0, the output current flows into the first unidirectional output transconductor G from the COMP nodem1(ii) a In particular, when the first unidirectional output transconductor Gm1Is in the linear range, i.e. -Vdm1<VDET-VCSF<When 0, the output current flows into the first unidirectional output transconductor G from the COMP nodem1Size is Igm1＝(VDET-VCSF)*Gm1Wherein G ism1A transconductance of a first unidirectional output transconductor; when the first unidirectional output transconductor Gm1Out of the linear range, i.e. V, of the negative differential input signalDET-VCSF<-Vdm1When the current is not flowing into the first unidirectional output transconductor G, the output current flows into the first unidirectional output transconductor G from a COMP nodem1And hold-IT1And is not changed. The second unidirectional output transconductor Gm2The input-output transfer characteristic of (a) is shown in fig. 5: when the second unidirectional output transconductor Gm2Is a negative signal, i.e. VDET-VCSF<When 0, the output current is zero; when the second unidirectional output transconductor Gm2Is a positive signal, i.e. VDET-VCSF>At 0, its output current is output from the second unidirectional output transconductor Gm2Flowing out to a COMP node; in particular when said second unidirectional output transconductor G ism2Is in the linear range, i.e. 0<VDET-VCSF<Vdm2Its output current is passed from said second unidirectional output transconductor Gm2Outflow of the liquidTo COMP node, size Igm2＝(VDET-VCSF)*Gm2Wherein G ism2A transconductance of a second unidirectional output transconductor; when the second unidirectional output transconductor Gm2Out of the linear range, i.e. V, of the forward differential input signalDET-VCSF>Vdm2Then its output current is passed through said second unidirectional output transconductor Gm2Flows out to COMP node and keeps IT2And is not changed. Therefore, the first unidirectional output transconductor Gm1And said second unidirectional output transconductor Gm2The input-output transfer characteristic of the error detection unit 43 configured as described above is shown in fig. 6: when the differential input signal of the error detection unit 43 is in the negative linear range, i.e., -Vdm1<VDET-VCSF<When 0, the output current flows from COMP node and has a magnitude of Igm＝(VDET-VCSF)*Gm1(ii) a When the differential input signal of the error detection unit 43 exceeds the negative linear range, i.e., VDET-VCSF<-Vdm1When the current is zero, the output current flows into and keeps-I from the COMP nodeT1The change is not changed; when the differential input signal of the error detection unit 43 is in the forward linear range, i.e. 0<VDET-VCSF<Vdm2When the current is greater than the reference voltage, the output current flows out to the COMP node with the magnitude of Igm＝(VDET-VCSF)*Gm2(ii) a When the differential input signal of the error detection unit 43 exceeds the forward linear range, i.e., VDET-VCSF>Vdm2When the current is not flowing out, the output current flows out to the COMP node and keeps IT2And is not changed. Wherein the maximum value of the current I flows into the output terminal of the error detection unit 43T1Is its maximum value of the current flowing out IT2Multiple times while the negative output transconductance G of the error detection unit 43m1Also its forward output transconductance Gm2Multiple times to make the valley of the voltage waveform of the non-inverting input terminal of the error detection unit 43 close to the voltage value of the inverting input terminal thereof; in this example, the optimal value of the multiple is 7 by iterating the specific circuit in the actual simulation, i.e. the output of the error detection unit 43Maximum value of end inflow current IT1Is its maximum value of the current flowing out IT2While the negative output transconductance G of the error detection unit 43 ism1Also its forward output transconductance Gm27 times of the total weight of the powder.
As an example, as shown in fig. 2, the loop compensation unit 44 includes: a loop compensation capacitor C1Said loop compensating capacitor C1Is connected to the output of the error detection unit 43 and the input of the voltage reduction unit 45, and the loop compensation capacitor C1And the other end of the same is grounded.
As shown in fig. 2, the compensation current I generated by the error detection unit 43 in this examplegmCompensating the loop for a capacitance C1Charging and discharging to generate corresponding compensation voltage VCOMP(ii) a Specifically, when the output current of the error detection unit 43 flows from the COMP node, the loop compensation capacitor C1Performing a discharging operation; when the output current of the error detection unit 43 flows out to the COMP node, the loop compensation capacitor C1A charging operation is performed.
As an example, as shown in fig. 2, the voltage reduction unit 45 may couple the input compensation voltage V by a preset reduction ratio of 1/KCOMPPerforming reduction to generate a reduced voltage VCOMPand/K. In this example, the voltage finally input to the positive input terminal of the driving signal generating unit 46 is reduced to 1/K times of the original voltage by the design of the voltage reducing unit 45, so that the fluctuation range of the voltage input to the positive input terminal of the driving signal generating unit 46 is reduced by 1/K times in the same proportion, and by reasonably designing the value of K, the reduced fluctuation range of the voltage can be completely ignored, that is, the voltage input to the positive input terminal of the driving signal generating unit 46 is a flat and stable dc voltage; due to the current sampling resistor RCSThe fluctuation range of the voltage drop is the same as that of the voltage at the positive phase input terminal of the driving signal generating unit 46, so that the current sampling resistor R can be considered asCSVoltage drop V overCSIs a stable DC voltage, and can be considered to flow through the power tube M1The current of (A) is a steady direct current, i.e. flows through the loaded LEDThe current of the lamp string is a stable current, so that alternating current ripple components existing in the output current of the preceding stage constant current LED driving circuit are filtered. In this example, the optimal value of the preset reduction ratio 1/K is 1/10 by iterating the specific circuit in the actual simulation.
As an example, as shown in fig. 2, the driving signal generating unit 46 includes an operational amplifier OP, a non-inverting input terminal of the operational amplifier OP is connected to the output terminal of the voltage reduction unit, an inverting input terminal of the operational amplifier OP is connected to the current sampling terminal of the current sampling unit 42, and an output terminal of the operational amplifier OP is connected to the power transistor M1A gate electrode of (1).
As shown in fig. 2, the operational amplifier OP and the power tube M in this example1And the current sampling resistor RCSCurrent series negative feedback is formed to make the power tube M1Drain-source current I ofDSVoltage V at non-inverting input of the operational amplifier OPCOMPLDetermination of IDS＝VCOMPL/RCS。
Referring to fig. 7, the working principle of the adaptive current ripple filtering circuit according to the present embodiment will be described in detail with reference to fig. 2.
By the detection resistor RDETSampling current input to the load LED light string to obtain ripple voltage V with alternating current ripple componentDETAt this time, the voltage waveform of the positive phase input end of the error detection unit 43 is the same as the voltage waveform of the negative electrode end of the load LED lamp string, and both of them contain a double power frequency voltage ripple; while the current flowing through the power path is sampled by said current sampling unit 42 to obtain a current sample voltage V with only a steady dc componentCSAnd multiplied current sampling voltage VCSF(ii) a Ripple voltage V with AC ripple componentDETAnd a multiplied current sample voltage V with only a steady DC componentCSFAfter being respectively input to the error detection units 43, the error detection units 43 generate output currents IgmTo compensate the loop for a capacitance C1Charging and discharging are carried out, at the same time, the compensation current ICOMP＝Igm。
Under the steady-state working condition of the loop, compensating the current ICOMPThe integral in each two times power frequency period is equal to zero, and the loop compensating capacitor C1Voltage V onCOMPIs kept constant, compensating for the current ICOMPResulting VCOMPVoltage fluctuation range V ofPPOn the order of tens of millivolts. Compensation voltage VCOMPAfter passing through the voltage reduction unit 45, the voltage V at the non-inverting input of the operational amplifier OPCOMPLThe voltage fluctuation range of (a) is only in the order of a few millivolts, and the voltage fluctuation range is completely negligible. Due to the current sampling resistor RCSVoltage drop over voltage VCSVoltage fluctuation range of and the positive phase input terminal voltage V of the operational amplifier OPCOMPLThe voltage fluctuation range of (2) is the same, so that the current sampling resistor R can be considered asCSVoltage drop over voltage VCSIs a smooth direct current, so that the current flowing through the load LED string flows through the power tube M1The current of the LED driving circuit is a stable direct current, so that alternating current ripple components existing in the output current of the preceding-stage constant-current LED driving circuit are filtered.
And the voltage at the inverting input end of the error detection unit 43 is not a fixed reference voltage, but is applied to the power tube M through the current sampling unit 421The multiplied current sampling voltage V generated by the source sampling ofCSFTherefore, when the steady direct current component of the output current of the front-stage constant current LED drive circuit is different, the steady direct current component flowing through the load LED lamp string is also different, so that the power tube M is caused1Different source voltages; at the moment, the asymmetrical transconductance of the error detection unit and the loop compensation capacitor C are passed1The voltage of the negative terminal of the load LED lamp string, namely the power tube M1Will follow the power transistor M1I.e. the power transistor M1Will always be connected to the power transistor M1The source voltage of the power tube M is close to ensure that the power tube M is connected with the power tube1Drain-source voltage drop V ofDSFluctuating within a reasonable range.
While inputting a differential signal VDET-VCSFAt (-V)dm1，Vdm2) When the current is within the linear range, the error detection unit 43 has a linear transconductance characteristic, and the output current of the error detection unit linearly changes along with the input differential signal; when the input differential signal is close to zero, the output current of the error detection unit 43 is also close to zero; therefore, the error detection unit 43 can well reflect the part of the load LED string with the smaller ripple amplitude of the voltage ripple at the negative terminal to the voltage at the COMP node, so as to reduce the weak fluctuation of the compensation voltage, and especially when the ripple current included in the output current of the previous stage constant current LED driving circuit is small, the adaptive current ripple filtering circuit of this example does not introduce extra current ripple.
The embodiment also provides a current ripple filtering method, which includes:
obtaining ripple voltage generated at the negative electrode end of current input to the load LED lamp string;
forming a power path between the negative end of the load LED lamp string and the ground based on a power tube and a current sampling resistor, sampling current flowing through the power path through the current sampling resistor to obtain current sampling voltage, and amplifying the current sampling voltage to obtain multiplied current sampling voltage;
carrying out error detection on the ripple voltage and the multiplied current sampling voltage to generate compensation current, and charging and discharging a loop compensation capacitor according to the compensation current to generate corresponding compensation voltage;
and reducing the compensation voltage according to a preset reduction proportion to generate a reduced voltage, and carrying out operational amplification processing on the reduced voltage and the current sampling voltage to generate a driving signal for driving the power tube, so that frequency component ripples of two times or more existing in the current flowing through the load LED lamp string are eliminated.
As an example, a method of error detecting the ripple voltage and the multiplied current sample voltage to produce a compensation current includes: by a pair of inverters having output currents of opposite directionsThe symmetrical unidirectional output transconductor performs error detection on the ripple voltage and the multiplied current sampling voltage, and generates negative compensation current I when the difference between the ripple voltage and the multiplied current sampling voltage is greater than a first transconductance voltage and less than 0gm＝(VDET-VCSF)*Gm1(ii) a When the difference between the ripple voltage and the multiplied current sampling voltage is less than the first transconductance voltage, a negative compensation current I is generatedgm＝-IT1(ii) a When the difference between the ripple voltage and the multiplied current sampling voltage is greater than 0 and less than a second transconductance voltage, generating a forward compensation current Igm＝(VDET-VCSF)*Gm2(ii) a When the difference between the ripple voltage and the multiplied current sampling voltage is greater than the second transconductance voltage, a forward compensation current Igm is generated as IT2(ii) a Wherein VDETIs a ripple voltage, VCSFSampling voltage for multiplied current, Gm1Is the negative output transconductance of the first unidirectional output transconductor, IT1Is the maximum value of the negative output current of the first unidirectional output transconductor, Gm2Is the forward output transconductance of the second unidirectional output transconductor, IT2Is the maximum value of the forward output current of the second unidirectional output transconductor.
As shown in fig. 2, the present embodiment further provides a mains solid state LED lighting system, which includes:
a rectifier circuit 10 for rectifying ac commercial power to generate a rectified voltage;
the constant current LED driving circuit 20 is connected to the rectifying circuit 10 and used for processing the rectified voltage to generate a constant current;
the load LED lamp string 30 is connected to the constant current LED drive circuit 20 and used for lighting the load LED lamp string under constant current drive to emit illumination light;
the adaptive current ripple filtering circuit 40 is connected to the negative terminal of the load LED light string 30, and is configured to filter current ripples of the constant current input to the load LED light string, so as to eliminate ripples of frequency components twice or more existing in the constant current.
As an example, as shown in fig. 2, the mains solid state LED lighting system further comprises: and the high-frequency ripple filtering capacitor 50 is connected in parallel to the output end of the constant current LED driving circuit 20.
Example two
As shown in fig. 2, compared to the first embodiment, the difference of the adaptive current ripple filtering circuit 40 in the present embodiment is: the adaptive current ripple filtering circuit further comprises: an over-temperature protection unit 47 connected to the loop compensation unit 44 and the driving signal generation unit 46, for performing over-temperature protection on the adaptive current ripple filtering circuit 40 to increase the power tube M according to the continuously increased detection temperature1And when the detected temperature is greater than the over-temperature protection threshold value, the power tube M is switched off1. It should be noted that the adaptive current ripple filtering circuit 40 in this example removes the loop compensation capacitor C1And a current sampling resistor RCSWhen the other components are integrated in the same chip package, the over-temperature protection unit 47 in this example may be added to perform over-temperature protection on the chip.
As an example, the over-temperature protection unit includes:
the temperature detector is used for detecting the temperature of the working environment where the adaptive current ripple filtering circuit is located;
a protection current generator connected to the temperature detector and the loop compensation unit 44 for generating an output current increased with the increase of the detected temperature to the loop compensation unit 44 when the detected temperature is greater than the power-down initial temperature value, so as to control the driving signal generation unit 46 to generate a corresponding driving signal to control the power transistor M1The power tube M is increased according to the continuously increased detection temperature1Thereby reducing the conduction degree of the power tube M1The current ripple filtering function disappears slowly at the same time; and when the detected temperature is greater than or equal to the final power-down temperature value, a corresponding output current is generated to the loop compensation unit 44, so that the driving signal is controlled to generate a signalThe element 46 generates a corresponding driving signal to control the power tube M1 to be completely conducted, so that the power loss of the power tube M1 is minimized, and the current ripple filtering function is completely eliminated;
a protection logic generator connected to the temperature detector and the control end of the driving signal generating unit 46 for generating a turn-off signal to the driving signal generating unit 46 to control the power transistor M when the detected temperature is greater than the over-temperature protection threshold value1Turning off; wherein the over-temperature protection threshold is greater than the final power-down temperature value.
Wherein, a characteristic curve of the output current of the over-temperature protection unit 47 along with the temperature change is shown in fig. 8: when the detected temperature is less than the initial power-down temperature Th1The output current I of the over-temperature protection unit 47TPZero, when it has no effect on the loop compensation unit 44; when the detected temperature is greater than the initial power-down temperature Th1Then, the output end of the over-temperature protection unit 47 outputs a current I flowing to a COMP nodeTP>0 when the compensation current I is inputted to the loop compensation unit 44COMP＝Igm+ITP(specifically, when the detection temperature is higher than the power-down starting temperature Th1And is less than the final temperature T of the power reductionh2The output current I of the over-temperature protection unit 47TP>0 and gradually increases with increasing temperature; when the detected temperature rises to the final temperature T of the reduced powerh2The output current I of the over-temperature protection unit 47TPEqual to the maximum value I of the negative output current of the error detection unit 43T1(ii) a As the detected temperature continues to rise, the output current I of the over-temperature protection unit 47T1Will stop at higher than IT1At the point of (a). Because under steady state operating conditions, the compensation current I is still maintainedCOMPThe integral in each double power frequency period is equal to zero, so that the voltage V at the non-inverting input of the error detection unit 43 is equal to zeroDETWith respect to its inverting input terminal voltage VCSFWill be lowered.
With the rise of the detected temperature, the output current I of the over-temperature protection unit 47TPIs increased whenIn each two-times power frequency period, when the voltage V at the positive phase input end of the error detection unit 43 isDETI.e. the power tube M1Has a drain voltage waveform valley near the power transistor M1The operational amplifier OP will have to increase the power transistor M1To ensure the power transistor M1Can be conducted, and the power tube M in the time period1Only a switch with a small on-resistance. But this will also result in said power transistor M1The fluctuation of the drain-source voltage is reduced, so that alternating current ripple components in the output current of the preceding stage constant current LED drive circuit flow through the load LED lamp string, and at the moment, the current sampling resistor RCSVoltage drop V ofCSSignificant voltage fluctuations will also occur; and with the continuous rise of the detection temperature, the current ripple flowing through the load LED lamp string is larger and larger, and the voltage fluctuation of the negative terminal of the load LED lamp string is also gradually reduced. Since the power tube M1The average value of the drain-source voltage drop is reduced, so that the power tube M1The power loss will be reduced and thus contribute to a reduction in the chip temperature.
If the detected temperature continues to rise and reaches Th2At this time, the output current I of the over-temperature protection unit 47TPEqual to the maximum value I of the negative output current of the error detection unit 43T1Then the voltage at the COMP node will rise, causing the voltage V at the non-inverting input of the operational amplifier OPCOMPLCompletely higher than the voltage V at its inverting inputCSThe peak value of the fluctuation, at which the output of the operational amplifier is at a high level, drives the power tube M1And (3) the power tube M is completely conducted, the current ripple filtering function of the whole system is completely eliminated at the moment, and the power tube M is connected with the power tube M at the moment1Is a fully conducting switch with minimized power loss. Then if the detected temperature continues to rise, the output current I of the over-temperature protection unitTPWill stop at higher than IT2At the point (2).
If the detected temperature continues to rise and exceeds the over-temperature protection threshold TSDThen, the over-temperature protection unit 47 will generate a shutdown signal to be inputted to the control terminal of the operational amplifier OP, so as to enable the operationThe computing amplifier OP outputs a low level, i.e. the power transistor M1Is at a low level, thereby turning off the power transistor M1To protect the power path from being burned.
Accordingly, compared to the first embodiment, the current ripple filtering method of the present embodiment further includes: detecting the temperature of the working environment of the power tube to obtain a detected temperature; comparing the detection temperature with the power reduction starting temperature value, and generating output current which is increased along with the increase of the detection temperature when the detection temperature is higher than the power reduction starting temperature so as to control the conduction degree of the power tube, so that the conduction degree of the power tube is increased according to the continuously increased detection temperature, the power loss of the power tube is reduced, and meanwhile, the current ripple filtering function is slowly disappeared; when the detected temperature is greater than or equal to the final power-reducing temperature value, generating corresponding output current so as to control the power tube to be completely conducted, so that the power loss of the power tube is reduced to the minimum, and meanwhile, the current ripple filtering function is completely eliminated; comparing the detected temperature with an over-temperature protection threshold value, and generating a turn-off signal to control the power tube to turn off when the detected temperature is greater than the over-temperature protection threshold value; wherein the over-temperature protection threshold is greater than the final power-down temperature value.
In summary, according to the self-adaptive current ripple filtering circuit, the mains supply solid-state LED lighting system and the method, through the design of the self-adaptive current ripple filtering circuit, the filtering of the ripple waves with twice power frequency and above of the output current of the preceding-stage constant-current LED driving circuit is realized, and the current flowing through the load LED lamp string is ensured to only contain a steady dc component; meanwhile, the variation of the drain voltage ripple valley value of the power tube in the power path along with the source voltage of the power tube is realized, and the fluctuation of the drain-source voltage drop of the power tube in a reasonable range is ensured; and the complete over-temperature protection function including the functions of slowly reducing the power loss of the power tube along with the temperature rise and turning off the over-temperature is realized. Therefore, the invention effectively overcomes various defects in the prior art and has high industrial utilization value.
The foregoing embodiments are merely illustrative of the principles and utilities of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or change the above-mentioned embodiments without departing from the spirit and scope of the present invention. Accordingly, it is intended that all equivalent modifications or changes which can be made by those skilled in the art without departing from the spirit and technical spirit of the present invention be covered by the claims of the present invention.
Claims (12)
1. An adaptive current ripple filtering circuit, the adaptive current ripple filtering circuit comprising:
the device comprises a voltage detection unit, a power tube, a current sampling unit, an error detection unit, a loop compensation unit, a voltage reduction unit and a driving signal generation unit; wherein the content of the first and second substances,
the voltage detection unit is connected to the negative end of the load LED lamp string and the error detection unit and is used for acquiring ripple voltage generated by the current input to the load LED lamp string at the negative end of the load LED lamp string;
the drain electrode of the power tube is connected to the negative electrode end of the load LED lamp string, the source electrode of the power tube is connected to the current sampling unit, and the grid electrode of the power tube is connected to the driving signal generating unit;
the current sampling unit is connected to the source electrode of the power tube, the driving signal generating unit and the error detecting unit, and is used for forming a power path between the negative electrode end of the load LED lamp string and the ground according to a current sampling resistor and the power tube, sampling the current flowing through the power path through the current sampling resistor to obtain a current sampling voltage, and amplifying the current sampling voltage to obtain a multiplied current sampling voltage;
the error detection unit is connected to the voltage detection unit, the current sampling unit and the loop compensation unit, and is used for performing error detection on the ripple voltage and the multiplied current sampling voltage to generate a compensation current;
the loop compensation unit is connected with the error detection unit and the voltage reduction unit and used for generating corresponding compensation voltage according to the compensation current;
the voltage reduction unit is connected with the loop compensation unit and the driving signal generation unit and is used for reducing the compensation voltage according to a preset reduction proportion to generate a reduced voltage;
the driving signal generating unit is connected to the voltage reducing unit, the current sampling unit and the grid electrode of the power tube, and is used for carrying out operational amplification processing on the reduced voltage and the current sampling voltage so as to generate a driving signal for driving the power tube.
2. The adaptive current ripple filter circuit of claim 1, wherein the voltage detection unit comprises: and one end of the detection resistor is connected to the negative end of the load LED lamp string, and the other end of the detection resistor is used as the output end of the voltage detection unit and is connected to the positive phase input end of the error detection unit.
3. The adaptive current ripple filter circuit of claim 1, wherein the current sampling unit comprises: the current sampling resistor, the transconductor and the multiplying resistor, wherein one end of the current sampling resistor is connected to the source electrode of the power tube and the positive phase input end of the transconductor, and simultaneously serves as the current sampling end of the current sampling unit is connected to the reverse phase input end of the driving signal generation unit, the other end of the current sampling resistor is grounded, the reverse phase input end of the transconductor is grounded, the output end of the transconductor is connected to one end of the multiplying resistor, and simultaneously serves as the multiplying output end of the current sampling unit is connected to the reverse phase input end of the error detection unit, and the other end of the multiplying resistor is grounded.
4. The adaptive current ripple filter circuit according to claim 1, wherein the error detection unit includes: the first unidirectional output transconductor and the positive phase input end of the second unidirectional output transconductor are connected to serve as the positive phase input end of the error detection unit, the first unidirectional output transconductor and the negative phase input end of the second unidirectional output transconductor are connected to serve as the negative phase input end of the error detection unit, and the output ends of the first unidirectional output transconductor and the second unidirectional output transconductor are connected to serve as the output end of the error detection unit; the first unidirectional output transconductor and the second unidirectional output transconductor are a pair of asymmetric transconductors, and output current directions of the first unidirectional output transconductor and the second unidirectional output transconductor are opposite.
5. The adaptive current ripple filter circuit of claim 1, wherein the loop compensation unit comprises: and one end of the loop compensation capacitor is connected to the output end of the error detection unit and the input end of the voltage reduction unit, and the other end of the loop compensation capacitor is grounded.
6. The adaptive current ripple filtering circuit of any one of claims 1 to 5, further comprising: and the over-temperature protection unit is connected with the loop compensation unit and the driving signal generation unit and used for performing over-temperature protection on the self-adaptive current ripple filtering circuit so as to increase the conduction degree of the power tube according to the continuously raised detection temperature and turn off the power tube when the detection temperature is greater than an over-temperature protection threshold value.
7. The adaptive current ripple filter circuit of claim 6, wherein the over-temperature protection unit comprises:
the temperature detector is used for detecting the temperature of the working environment where the adaptive current ripple filtering circuit is located;
the protection current generator is connected with the temperature detector and the loop compensation unit and used for generating output current which is increased along with the increase of the detection temperature to the loop compensation unit when the detection temperature is higher than the power reduction initial temperature value, so that the driving signal generation unit is controlled to generate corresponding driving signals to control the conduction degree of the power tube, the conduction degree of the power tube is increased according to the continuously increased detection temperature, the power loss of the power tube is reduced, and meanwhile, the current ripple filtering function disappears slowly; when the detected temperature is greater than or equal to the final power-reducing temperature value, generating corresponding output current to the loop compensation unit, so as to control the driving signal generation unit to generate corresponding driving signals to control the power tube to be completely conducted, thereby reducing the power loss of the power tube to the minimum and simultaneously completely eliminating the current ripple filtering function;
the protection logic generator is connected with the temperature detector and the control end of the driving signal generating unit and is used for generating a turn-off signal to the driving signal generating unit when the detected temperature is greater than an over-temperature protection threshold value so as to control the power tube to be turned off; wherein the over-temperature protection threshold is greater than the final power-down temperature value.
8. A mains solid state LED lighting system, characterized in that the mains solid state LED lighting system comprises:
the rectification circuit is used for rectifying the alternating current commercial power to generate rectified voltage;
the constant current LED driving circuit is connected to the rectifying circuit and used for processing the rectified voltage to generate constant current;
the load LED lamp string is connected with the constant current LED drive circuit and used for lighting the load LED lamp string under constant current drive to emit illumination light;
the adaptive current ripple filtering circuit of any one of claims 1 to 7, connected to the negative terminal of the load LED string, for performing current ripple filtering on the constant current input to the load LED string to eliminate ripple of frequency components of two times or more existing in the constant current.
9. The mains solid state LED lighting system according to claim 8, further comprising: and the high-frequency ripple filtering capacitor is connected in parallel with the output end of the constant current LED drive circuit.
10. A current ripple filtering method, comprising:
obtaining ripple voltage generated at the negative electrode end of the current input to the load LED lamp string;
forming a power path between the negative end of the load LED lamp string and the ground based on a power tube and a current sampling resistor, sampling current flowing through the power path through the current sampling resistor to obtain current sampling voltage, and amplifying the current sampling voltage to obtain multiplied current sampling voltage;
carrying out error detection on the ripple voltage and the multiplied current sampling voltage to generate compensation current, and charging and discharging a loop compensation capacitor according to the compensation current to generate corresponding compensation voltage;
and reducing the compensation voltage according to a preset reduction proportion to generate a reduced voltage, and carrying out operational amplification processing on the reduced voltage and the current sampling voltage to generate a driving signal for driving the power tube, so that frequency component ripples of two times or more existing in the current flowing through the load LED lamp string are eliminated.
11. The method for filtering current ripples according to claim 10, wherein the method of error detection of the ripple voltage and the multiplied current sample voltage to produce a compensation current comprises: error detection is carried out on the ripple voltage and the multiplied current sampling voltage through a pair of asymmetric unidirectional output transconductors with opposite output current directions, and when the difference between the ripple voltage and the multiplied current sampling voltage is larger than a first transconductance voltage and smaller than 0, negative compensation current I is generatedgm＝(VDET-VCSF)*Gm1(ii) a When the difference between the ripple voltage and the multiplied current sampling voltage is less than the first transconductance voltage, negative compensation electricity is generatedStream Igm＝-IT1(ii) a When the difference between the ripple voltage and the multiplied current sampling voltage is greater than 0 and less than a second transconductance voltage, generating a forward compensation current Igm＝(VDET-VCSF)*Gm2(ii) a When the difference between the ripple voltage and the multiplied current sampling voltage is greater than the second transconductance voltage, a forward compensation current Igm is generated as IT2(ii) a Wherein VDETIs a ripple voltage, VCSFSampling voltage for multiplied current, Gm1Is the negative output transconductance of the first unidirectional output transconductor, IT1Is the maximum value of the negative output current of the first unidirectional output transconductor, Gm2Is the forward output transconductance of the second unidirectional output transconductor, IT2Is the maximum value of the forward output current of the second unidirectional output transconductor.
12. The method for current ripple filtering according to claim 10, further comprising:
detecting the temperature of the working environment of the power tube to obtain a detected temperature;
comparing the detection temperature with the power reduction starting temperature value, and generating output current which is increased along with the increase of the detection temperature when the detection temperature is higher than the power reduction starting temperature so as to control the conduction degree of the power tube, so that the conduction degree of the power tube is increased according to the continuously increased detection temperature, the power loss of the power tube is reduced, and meanwhile, the current ripple filtering function is slowly disappeared; when the detected temperature is greater than or equal to the final power-reducing temperature value, generating corresponding output current so as to control the power tube to be completely conducted, so that the power loss of the power tube is reduced to the minimum, and meanwhile, the current ripple filtering function is completely eliminated;
comparing the detected temperature with an over-temperature protection threshold value, and generating a turn-off signal to control the power tube to turn off when the detected temperature is greater than the over-temperature protection threshold value; wherein the over-temperature protection threshold is greater than the final power-down temperature value.
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