CN109803469B - Total harmonic distortion adjusting method and circuit in alternating-current direct-connection LED drive IC - Google Patents
Total harmonic distortion adjusting method and circuit in alternating-current direct-connection LED drive IC Download PDFInfo
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Abstract
The invention discloses a total harmonic distortion adjusting method and circuit in an alternating current direct connection LED driving IC. The method comprises a plurality of THD adjusting modules and corresponding external connectorsResistance RTDThe voltage output signals of each constant current module in the alternating current direct connection LED driving IC are converted into current signals respectively, the obtained current signals are superposed in a current source mode, the voltage values of the LED current control ends in different time periods are changed together, and therefore the current values of the current flowing through the LED lamp strings in different time periods are changed, the waveform of the current flowing through the LED lamp strings along with the change of time is closer to a sinusoidal signal, and the purpose of adjusting the total harmonic distortion of the alternating current direct connection LED driving IC is achieved. Moreover, the adjusted value can be adjusted by changing a plurality of externally connected RTDThe resistance value of the resistor is adjusted and can be selected according to different requirements.
Description
Technical Field
The invention relates to the technical field of alternating current direct connection LED driving, in particular to the field of adjusting total harmonic distortion of an alternating current direct connection LED driving IC.
Background
Compared with the traditional lighting lamp, the Light Emitting Diode (LED) serving as the fourth-generation green light source has the advantages of high brightness, high efficiency, good stability, long service life, safety, environmental friendliness and the like, and is widely applied to the field of lighting.
Conventional constant current type LED driving ICs generally employ a two-stage structure, Power Factor Correction (PFC) and DC/DC voltage conversion, wherein the power factor correction circuit is to adjust THD. In order to make the ripple of the driving current small, this type of driving IC inevitably requires the use of an electrolytic capacitor having a large capacity. The service life of the electrolytic capacitor is only about 5000 hours and is far less than that of the LED, so that the service life of the whole LED driving system is limited. In order to solve this problem, an ac-direct-connection LED driving IC that does not require the use of an electrolytic capacitor has been proposed and studied intensively.
For ac direct LED driver ICs, the THD problem needs to be considered as well. This is because THD is too large, which not only reduces the power factor of the line and affects the utilization rate of the ac power supply, but also pollutes the power grid and adversely affects other electric devices connected to the same ac power supply system.
For a conventional ac-direct LED driver IC driving scheme, the current flowing through the LED string is stepped during one power cycle. The magnitude of the current value of each current step, and the starting point and the ending point of each current step in the time domain all affect the THD of the ac-direct-connected LED driving IC. The starting point and the ending point of the current ladder in the time domain are determined by the voltage drop of each string of LED lamp strings, the amplitude and the frequency of the input power voltage and the like; the current value of each ladder is received by the input reference level V of the corresponding constant current moduleREFAnd (5) controlling. For a given ac power supply and LED string voltage drop distribution scheme, we need to input a specific reference voltage combination to achieve a smaller THD. However, the LED string voltage drop distribution scheme is different, and the combination of the input reference voltages is also different, and the reference voltages are generally generated from the inside of the chip and are fixed. Therefore, in order to obtain a smaller THD value under different conditions, it is necessary to study the THD adjustment method applied to the ac-direct LED driving IC.
Disclosure of Invention
In order to enable the alternating current direct connection LED driving IC to obtain a smaller THD value under different conditions, the invention provides a total harmonic distortion adjusting method and circuit in the alternating current direct connection LED driving IC.
A total harmonic distortion adjusting method in an alternating current direct connection LED driving IC comprises the following steps:
1) adopt a plurality of THD adjusting module and external resistance R who corresponds with itTDConverting voltage output signals of each constant current module in the alternating current direct connection LED driving IC into current signals;
2) superposing the obtained current signals in a current source mode;
3) the superposed current flows through the on-chip resistor to be reduced in voltage so as to change the voltage value of the LED current control end in different time periods, thereby changing the current value flowing through the LED lamp string in different time periods, so that the waveform of the current flowing through the LED lamp string along with the change of time is closer to a sinusoidal signal, and the purpose of adjusting the total harmonic distortion of the alternating current direct-connected LED drive IC is achieved.
The THD regulating module comprises:
1) by PMOS devices M1And M2Form a current mirror, wherein M2The method is a diode-connected method,
2) by NPN tube Q4And Q5Form a current mirror, wherein Q4The method is a diode-connected method,
3) diode connected NPN tube Q1，Q2，Q3Forming a voltage reduction circuit;
wherein Q is1As input to the THD module, Q1Emitter and Q2Are connected to the collector of, Q2Emitter and Q3Are connected to the collector of, Q3Emitter and Q4Are connected to the collector of, Q4And Q5All of their emitters are grounded and their bases are connected, Q5Collector and M2Is connected to the drain electrode of M1And M2Are connected with their sources to a supply voltage, M1As the output of the THD module.
The THD regulating module has the following characteristics:
1) diode connected NPN tube Q1，Q2，Q3，Q4When the output voltage of the constant current module reaches a certain value, the THD regulating module is started;
2) the current signal obtained by the conversion of the THD regulating module is in direct proportion to the output voltage of the constant current module and is corresponding to the external resistor RTDIn inverse proportion;
3) the existence of the two pairs of current mirrors transmits the converted current signals to the constant current module so as to superpose the currents;
4) the more the current after superposition is, the smaller the voltage of the LED current control end is, the smaller the current flowing through the LED lamp string is, the smaller the current after superposition is, the larger the voltage of the LED current control end is, and the larger the current flowing through the LED lamp string is.
The voltage-current conversion proportion of each THD regulating module is changed by changing the corresponding external resistor RTDThe resistance values are changed, the proportion directly influences the value of total harmonic distortion adjustment, and the optimal resistance value combination can be selected according to different requirements, so that a smaller THD value can be obtained under different conditions.
The driving circuit adopting the method comprises the following steps:
1) an off-chip input AC voltage source;
2) an off-chip high power rectifier bridge;
3) n LED lamp strings connected in series;
4) a current limiting resistor for controlling the current of the LED lamp string;
5) n on-chip constant current modules;
6) n on-chip LDMOS transistors;
7) reference voltage sources generated in the N pieces of chips;
8) n on-chip THD adjusting modules;
9) n off-chip THDs adjust the resistance.
The input alternating voltage passes through the high-power rectifier bridge and then is used for driving the LED lamp strings connected in series by the N strings; the source electrodes of the N LDMOS tubes are connected with each other and are connected with an external current limiting resistor, the drain electrodes of the LDMOS tubes are connected with the tail end of the LED lamp string, and the grid electrodes of the LDMOS tubes are connected with the output end of the constant current module; the output end of the constant current module is connected with the THD regulating resistor and then connected to the input end of the THD regulating module; the output end of the THD regulating module is connected with the input end of the constant current module; the reference voltage source also serves as the input end of the constant current module.
The invention has the beneficial effects that: aiming at the conventional alternating current direct connection LED drive IC, the resistor R is externally connected by utilizing the output voltage of each constant current moduleTDAnd a total harmonic distortion adjusting module, which can adjust the THD under different conditionsTo smaller THD values.
Drawings
FIG. 1 is a schematic diagram of a conventional AC direct connection LED driver IC drive circuit;
fig. 2 is a constant current module circuit diagram;
FIG. 3 is a schematic diagram of sequential conduction of LED light strings in an AC direct connection LED driving power supply;
FIG. 4 is a waveform diagram of the constant current module voltage output;
FIG. 5 is a circuit diagram of a THD conditioning module;
FIG. 6 is a schematic diagram of an AC direct connection LED drive IC drive circuit with a THD adjustment module;
FIG. 7 is a current simulation waveform diagram of a conventional AC direct connection LED driver IC;
FIG. 8 is a waveform diagram of an AC direct-connected LED drive IC current simulation with a THD adjusting module.
Detailed Description
The invention will be further described with reference to the accompanying drawings and specific embodiments.
Fig. 1 is a schematic diagram of a conventional ac-direct-connection LED driving IC driving circuit, which takes driving 5 strings of LED strings as an example, and includes: off-chip input AC power supply VACOff-chip high-power rectifier bridge and off-chip 5 strings of LED lamp strings (LED)1~LED5) Current limiting resistor R for controlling LED lamp string current outside chipCS5 constant current modules in the chip and 5 LDMOS tubes in the chip1~LDMOS55 reference voltage sources V generated in-chipREF1~VREF5And V isREF1<VREF2<VREF3<VREF4<VREF5。
Fig. 2 is a circuit diagram of a constant current module, including: comparator CMP, resistance R1，R2Current source I1And NPN tube Q1And Q2. FIG. 3 is a schematic diagram of sequential conduction of LED light strings in an AC direct connection LED driving power supply. Fig. 4 is a voltage output waveform diagram of the constant current module.
With reference to fig. 1 to 4, a method of sequentially turning on LED strings in a conventional ac direct-connection LED driving power supply will be described in detail by taking 5 strings of LED strings as an example. Suppose VLED1~VLED5Respectively representing lamp string LED1~LED5Forward conduction voltage drop of VOUT1~VOUT5Respectively representing the output voltages of the constant current module (one) to the constant current module (five).
When the transient value of the input power supply voltage is less than VLED1I.e. in the range of 0 to T1In the time period, no current flows in all the LED lamp strings, and R isCSThe voltage drop of the voltage-stabilizing circuit is zero, and the output voltages of the five constant-current modules are all higher level V1，LDMOS1~LDMOS5Are in a conducting but no current state.
When the transient value of the power supply voltage is VLED1And VLED1+VLED2In between, i.e. at T1~T2In time period, LED1Light string conducting LDMOS1In which a current flows, i.e. RCSIn which current flows due to R in the constant current module (one)1No current flows, so VCSApproximately equal to VREF1At this time, VOUT1Becomes V21，V21<V1So that the current in the lamp string is approximately VREF1/RCSDue to VREF1The median of the five reference voltage sources is minimum, so the output voltages of other constant current modules are all kept at V1The other LDMOS is still conducting but no current.
When the transient value of the power supply voltage is VLED1+VLED2And VLED1+VLED2+VLED3In between, i.e. at T2~T3In a time period when the LED is on1And an LED2Simultaneously on, i.e. LDMOS2There will be a current flow if the LDMOS1And LDMOS2At the same time, R is turned onCSOver current, i.e. VCSIs too large to result in LDMOS1，LDMOS2All are turned off, and to avoid this, we make VREF2Greater than VRFE1That is, during this time, VOUT1Approximately zero, LDMOS1Turn off, the whole current flows through LDMOS2When the current in the LED lamp string is approximately VREF2/RCS， VOUT2From V1Down to V22，VOUT3~VOUT5All maintain V1Invariable, LDMOS3~LDMOS5Still in a conducting but no current state.
When the transient value of the power supply voltage is VLED1+VLED2+VLED3And VLED1+VLED2+VLED3+VLED4In between, i.e. at T3~T4In time period, LED1~LED3Are simultaneously conducted, and make VREF3Greater than VREF2Then V isOUT1Maintaining a zero level, VOUT2Becomes zero level, LDMOS1~LDMOS2Turn off, the current flows through the LDMOS3The current in the lamp string is approximately VREF3/RCS，VOUT3From V1Becomes V23，VOUT4~VOUT5All maintain V1Invariable, LDMOS4~LDMOS5Still in a conducting but no current state.
When the transient value of the power supply voltage is VLED1+VLED2+VLED3+VLED4And VLED1+VLED2+VLED3+VLED4+VLED5In between, i.e. at T4~T5In time period, LED1~LED4Are simultaneously conducted, and make VREF4Greater than VREF3Then V isOUT1~ VOUT3Are all approximately zero level, and the current flows through the LDMOS4The current in the lamp string is approximately VREF4/RCS，VOUT4From V1Becomes V24，VOUT5Holding V1Invariable, LDMOS5Still in a conducting but no current state.
When the transient value of the power supply voltage is larger than VLED1+VLED2+VLED3+VLED4+VLED5I.e. at T5~T6In time period, LED1~LED5Are simultaneously conducted, and make VREF5Greater than VREF4Then V isOUT1~VOUT4Are all approximately zero level, and the current flows through the LDMOS5Lamp (lantern)The current in the string is approximately VREF5/RCS，VOUT5From V1Becomes V25. Similarly, when the transient value of the input voltage is gradually reduced, the LED5~LED1And sequentially extinguished, and the current in the lamp string is reduced in a stepped manner.
From the above analysis, it is known that the waveform of the LED current with time appears stepwise. The current value of any step is changed, and the THD value of the driving chip is changed accordingly. The current value of the ladder is proportional to the reference voltage inside the chip, and the THD value can be changed by changing the value of the reference voltage. However, the value of the reference voltage is generated from the inside of the chip, and is generally fixed and unchangeable.
We note that the condition that the current value of the ladder is proportional to the chip internal reference voltage is R in the constant current module1No current flows if the resistor R1When a current flows, the current flowing in the lamp string will be approximately equal to the reference voltage and the resistor R1Difference in pressure drop and RCSSo that we can change the resistance R1The current value in the lamp string is changed, so that the THD value of the alternating current direct connection LED driving IC is adjusted. Starting from the THD regulating module, the flow resistance R in the constant current module is changed1The effect of the current.
Since the current in the LED string changes in a step-like manner with time, the current value in different time periods needs to be changed by different values, so as to obtain a smaller THD as much as possible. Furthermore, in order to avoid increasing the chip area as much as possible, the input signal of the THD adjusting module is preferably selected from the existing signals inside the ac-direct-connection LED driving IC.
From the above analysis we find that the output signals V of the five constant current modules are at different time periodsOUT1~VOUT5The sum is different, that is, the five voltage signals are respectively converted into current signals, and then are superposed, so that the superposed current flows through the resistor R in the constant current module1The purpose of changing different values of the current value flowing through the lamp string in different time periods can be realized。
For convenient calculation of the required RTDThe resistance value is such that the values of 5 reference voltage sources generated inside the chip are approximately equal, but V is still satisfiedREF1<VREF2<VREF3<VREF4<VREF5. Thus, V is enabled21~V25Are all approximately equal to V2。
Then, the THD regulating module and the external resistor R are usedTDThese five voltage signals are converted into current signals. The circuit diagram of the THD regulating module is shown in fig. 5, and includes: by PMOS devices M1And M2A current mirror; by NPN tube Q4And Q5A current mirror; diode connected NPN tube Q1，Q2，Q3The voltage reduction circuit is formed. Wherein Q is1As input to the THD module, Q1Emitter and Q2Are connected to the collector of, Q2Emitter and Q3Are connected to the collector of, Q3Emitter and Q4Are connected to the collector of, Q4And Q5All of their emitters are grounded and their bases are connected, Q5Collector and M2Is connected to the drain electrode of M1And M2Are connected with their sources to a supply voltage, M1As the output of the THD module. The voltage-current conversion ratio of the THD regulating module is not only related to internal devices of the THD regulating module, but also related to an external resistor RTDIt is related.
From fig. 4, it can be seen that, in the whole period, the value of the output voltage signal of the constant current module has three stable values, V1，V2And 0, for simplifying calculation, when the value of the output voltage signal of the constant current module is V2When the THD regulating module does not work, namely a threshold value V is set for the THD regulating moduleTH，VTH>V2This is an NPN transistor Q connected by four diodes1，Q2，Q3，Q4To be realized. When the input voltage of the constant current module is V1When the output current of the constant current module is (V)1-VTH)/RTDSo that I can change the external resistor RTDTo change the magnitude of the converted current.
After the voltage signal is converted into the current signal by the THD adjusting module, we also need to superimpose the obtained five current signals. The current source superposition mode is adopted, as shown in fig. 6, five output ends of the THD regulating module must be connected together, otherwise, the current signal superposition purpose cannot be achieved.
This is because if the output terminals of the five THD regulating modules are not connected to one block, the voltage at the output terminal of one constant current module cannot reach VTHWhen the current does not flow in the THD regulating module, the constant current module forces VCSThe voltage of the constant current module is equal to the input reference voltage of the constant current module, so that other THD regulating modules cannot work normally, the THD regulating method is invalid, and the current on the LED lamp string in different time periods cannot be changed normally.
After current superposition, the current can be easily pushed out, and the alternating current direct connection LED driving IC with the THD adjusting module is 0-T1In the time period, the current on the LED lamp string is approximately zero; at T1~T2During the time period, the current on the LED lamp string is approximately (V)REF1-(R1/5)*(V1-VTH)*((1/RTD2)+ (1/RTD3)+ (1/RTD4)+ (1/RTD5))）/RCSAt T2~T3During the time period, the current on the LED lamp string is approximately (V)REF2-(R1/5)*(V1-VTH)*( (1/RTD3)+ (1/RTD4)+ (1/RTD5))）/RCSAt T3~T4During the time period, the current on the LED lamp string is approximately (V)REF3-(R1/5)*(V1-VTH)*( (1/RTD4)+ (1/RTD5))）/RCSAt T4~T5During the time period, the current on the LED lamp string is approximately (V)REF4-(R1/5)*(V1-VTH)*(1/RTD5))/RCSAt T5~T6In time period, on the LED lamp stringHas a current of approximately VREF5/RCS. Due to VREF1~VREF5，R1，V1，VTHAnd RCSHas determined that we can adjust RTD1~RTD5To obtain the current value we want, and thus to obtain a smaller THD value.
We have found that the current value of each stage of the AC direct-connected LED driving IC with the THD regulating module is equal to RTD1This is irrelevant because, for computational convenience, we make V when thresholding the THD adjustment moduleTH>V2. If no such setting is made, then at T5~T6During the time period, the current on the LED lamp string will be equal to RTD1It is related.
1um 700V BCD technology is subjected to simulation verification: when 220V AC is input and 5 strings of LED lamp strings are respectively provided with conduction voltage drops of 110V, 60V, 50V, 40V and 30V, and THD adjustment is not performed, a simulation waveform diagram of the change of LED current along with time is shown in FIG. 7, and the corresponding THD value is 86.08%; carrying out THD regulation and externally connected RTDWhen the resistance values of the resistors are all 100k, the simulation oscillogram of the LED current changing along with the time is shown in FIG. 8, the THD value is 11.55%, and the THD value is greatly reduced; under the condition that the input 220V AC and the 5 strings of LED lamp strings have conduction voltage drops of 60V, 60V, 60V, 60V and 50V respectively, the adopted reference voltage combination is that under the condition that the input 220V AC and the 5 strings of LED lamp strings have conduction voltage drops of 110V, 60V, 50V, 40V and 30V respectively, the reference voltage combination with the minimum THD is enabled to obtain the THD value of 21%, and after the THD value is adjusted by the THD adjusting module, the optimal value of the THD is 15.6% and is reduced by 25.7%. For different LED string voltage distribution schemes and input power supply voltages, THD adjustment can be carried out through the THD adjusting method, so that a smaller THD value is obtained.
The proposed method for adjusting total harmonic distortion of an ac-direct-connection LED driving IC is only described here with 5 strings of LED strings as an example, but is not limited to 5 strings of LED strings. This method is applicable to N strings of LED strings (N =1,2,3,4 … …). For the N-string LED lamp string, N THD regulating modules, N RTD resistors, N constant current modules, N LDMOS tubes and N reference voltages are used.
Claims (2)
1. A total harmonic distortion adjusting method in an AC direct connection LED driving IC is characterized in that,
1) adopt a plurality of THD adjusting module and external resistance R who corresponds with itTDConverting voltage output signals of each constant current module in the alternating current direct connection LED driving IC into current signals;
2) superposing the obtained current signals in a current source mode;
3) the superposed current flows through the on-chip resistor to be reduced in voltage so as to change the voltage value of the LED current control end in different time periods, thereby changing the current value flowing through the LED lamp string in different time periods, so that the waveform of the current flowing through the LED lamp string along with the change of time is closer to a sinusoidal signal, and the purpose of adjusting the total harmonic distortion of the alternating current direct-connected LED drive IC is achieved;
the THD regulating module comprises:
1) by PMOS devices M1And M2Form a current mirror, wherein M2The method is a diode-connected method,
2) by NPN tube Q4And Q5Form a current mirror, wherein Q4The method is a diode-connected method,
3) diode connected NPN tube Q1，Q2，Q3Forming a voltage reduction circuit;
wherein Q is1As input to the THD module, Q1Emitter and Q2Are connected to the collector of, Q2Emitter and Q3Are connected to the collector of, Q3Emitter and Q4Are connected to the collector of, Q4And Q5All of their emitters are grounded and their bases are connected, Q5Collector and M2Is connected to the drain electrode of M1And M2Are connected with their sources to a supply voltage, M1The drain of the THD module is used as the output of the THD module;
the THD regulating module:
1) diode connected NPN tube Q1，Q2，Q3，Q4When the output voltage of the constant current module reaches a certain value, the THD regulating module is started;
2) the current signal obtained by the conversion of the THD regulating module is in direct proportion to the output voltage of the constant current module and is connected with a corresponding external resistor RTDIn inverse proportion;
3) the existence of the two pairs of current mirrors transmits the converted current signals to the constant current module so as to superpose the currents;
4) the more the current after superposition is, the smaller the voltage of the LED current control end is, the smaller the current flowing through the LED lamp string is, the smaller the current after superposition is, the larger the voltage of the LED current control end is, and the larger the current flowing through the LED lamp string is.
2. A driver circuit using the method of claim 1, comprising:
1) an off-chip input AC voltage source;
2) an off-chip high power rectifier bridge;
3) n LED lamp strings connected in series;
4) a current limiting resistor for controlling the current of the LED lamp string;
5) n on-chip constant current modules;
6) n on-chip LDMOS transistors;
7) reference voltage sources generated in the N pieces of chips;
8) n on-chip THD adjusting modules;
9) n off-chip THD regulating resistors;
the input alternating voltage passes through the high-power rectifier bridge and then is used for driving the LED lamp strings connected in series by the N strings; the source electrodes of the N LDMOS tubes are connected with each other and are connected with an external current-limiting resistor, the drain electrodes of the LDMOS tubes are connected with the tail end of the LED lamp string, and the grid electrodes of the LDMOS tubes are connected with the output end of the constant current module; the output end of the constant current module is connected with the THD regulating resistor and then connected to the input end of the THD regulating module; the output end of the THD regulating module is connected with the input end of the constant current module; the reference voltage source also serves as the input end of the constant current module.
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| CN109803469A (en) | 2019-05-24 |
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