1.Relation between distorted THD and power factor PF of total harmonic wave

A lot on the market

Fig. 1

This circuit is only inputting the crest value adjacent place of the alternating current voltage, the kenotron presents turning on, so it turns on angle theta smaller, are probably about 60 degrees, causes the input current waveform to be the point shape pulse, the pulse width is about 3ms, and half a cycle 10ms 1/3. Input voltage and electric current waveform are shown as Fig. 2. Therefore, cause the basic reason of LED power input current distortion to result from using capacitive loads of filtering electrolytic capacitor of direct current.

Fig. 2

Drive the intersection of power and input current produce non-sinusoidal wave of distortion as to LED, must use leaf Fourier in the Fu Progression is described. Until leaf vary principle in the Fu, instant input current it expresses to be:

In the type, n harmonic order, the intersection of leaf and the intersection of coefficient and an and bn it parts tables to be in the Fu:

Every current harmonics, will usually last a piece of sine or cosine cycle, n effective value In of harmonic current once can be calculated with the following formula:

Input the mean value of the totalcurrent

In the aboving radical sign, I1 is a fundamental current mean value, other I2, 3, represent 2, 3 respectively, n harmonic current mean value once. The total harmonic content of electric current represented with the fundamental current percentage calls the total harmonic distortion THD ,The total harmonic content has reflected the corrugate distortion characteristic, so call total harmonic distortion rate too. Define as

According to the definition of the factor PF of the power, the factor PF of the power refers to the meritorious power P of ac input and inputs the ratio of the apparent power S, namely

Among them, in order to input the mains voltage; U cos 1 is called the phase shift factor, it reflects the phase relation between fundamental current i1 and voltage u, 1 is a phase angle of fundamental wave; It is K to input fundamental current effective value I1 and percentage of inputting the effective value Irms of the totalcurrent =I1 / Irms is called the distortion coefficient of input current. It shows that aboving, in LED drives power supply circuit of non linear switch such as the power, the factor PF of the power not only relates to phase place between the fundamental current i1 voltage u but also relate to distorted coefficient K of input current. Formula 6 Insert formula 7 ,Then there are the following relations in factor PF of the power and total harmonic wave distorted THD:

Aboving to prove, when the factor cos 1 of phase shift is constant, reduce the total harmonic wave distorted THD, all right up-rate factor PF; Can prove too on the contrary, the higher PF is, THD is the smaller. For example, through calculating, act as phase angle 1 =At 0 o’clock, THD =30% @ PF =0.9578; THD =10% @ PF =0.9950.

2.Harmonic measure and analysis

In order to well analyze LED as Fig. 1 shows drives the harmonic content of the power, introduce a method for using the ondoscope to measure the input current. First in the intersection of power and the intersection of input return circuit and the intersection of tandem connection and one 10-20W or large power resistor if R of the above =10 OHM, measure large the intersection of power resistor and voltage waveform of both ends after being energization, because voltage and electric current of both ends are the phase equality all the time on the pure power resistor, so namely pulse voltage waveform on the resistance has represented pulse shape of input current, but the number value is of different size. It is knowing to reveal by the waveform, its electric current it of pulse It is concerted with the electric current waveform of Fig. 2, see Fig. 3.

The pulse wave of this electric current is similar to the pulse wave of cosine, so can use the function table of cosine pulse:

In order to calculate convenient, fetch one cycle when the sine exchanges input voltage now T:-5msIt is less than or equal to 15ms to less than or equal to t, namely T =20 ms. therefore the intersection of input and expression of impulse current of 20ms as follows in one period of cycle:

In aboving, a value Im of impulse current of cosine can be calculated by ondoscope expicity voltage amplitude and proportion of resistance quantity, namely Im=Um/R,Have already known to examine Um =1.5V, then Im =1.5/10 =Impulse width in 0.15 A. pictures =3ms. As to the input current waveform illustrated in Fig. 2, it is about symmetrical wave once that the front and back half wave is symmetric and strange from head to foot, therefore only contain a1, a3, a5 ,etc. harmonic component once very,and weights direct-flow a0 and even harmonic weight a2, a4, a6 It is zero. Formula 10 Input current waveform carry on at Fu to if you can’t decompose leaf, have:

According to the integral formula:

And there is a = / , b =N, =2 / T, so as follows,

Act as n =T 1 o’clock =20ms, =3ms, Im =0.15A inserts abovingly, all right

Calculate fundamental current a value a1 =I1m=0.06* 0.6080.327 =0.056A .

The same, calculate a3 separately, a5, a7, a9 subharmonic amplitude, as Table 1 shows.

Table 1. A property value table of harmonic wave

According to Table 1, LED drive leaf progression in the Fu of input current of power:

According to the relation between a value Inm and harmonic effective value In of harmonic wave, harmonic mean value:

By formula 16 ,Calculate every harmonic current mean value as follows unit A separately : I1 =0.040,I3 =0.033,I5 =0.023,I7 =0.012,I9 =0.003. According to formula 5 ,LED drives the input totalcurrent mean value of the power:

Will express A data insert formula 17 ,Input the effective value Irms of the totalcurrent =0.058A . In reality, this can be measured or read to input current value by the input current display screen of the dynamometer with the multimeter which measure the rms value. According to formula 6 Calculate the total harmonic distortion:

According to the harmonic amplitude data of Table 1, and with the fundamental wave first harmonic 100% of weight is the basic reference, make the amplitude spectrogram of the harmonic current the harmonic wave neglected and is higher than 9 or more times Refer to fig. 4.

Fig. 4

Pressing type 7 now Calculate the factor PF of the power, it is zero to act as phase angle 1 of fundamental wave, cos 1 =One is as follows,

Examine PF in fact =0.65,The two are basically unanimous. Real LED drives the input power of the power:

3.Harmonic danger

Harmonic danger is calculated that knowing by the above-mentioned analysis, this kind of LED drives the harmonic content of input current of the power high, use of drib as to the device such as power quite, harmfulness its might not show perhaps, but if hundreds and thousands of a large amount oves are used intensively, the total amount of harmonic current that it produces will pollute the whole power supply system and other electricity consumption users seriously, enable the Wellenform erzeugen distortion of the mains voltage at the same time. The theory and practice prove, the too big current harmonics will produce the following to endanger: A.Can make the core of the magnetic material such as electric voltage transformer and generator, perceptual loading equipment such as electric motor of electricity distribution facility decrease

1.Relation between distorted THD and power factor PF of total harmonic wave

A lot on the market

Fig. 1

This circuit is only inputting the crest value adjacent place of the alternating current voltage, the kenotron presents turning on, so it turns on angle theta smaller, are probably about 60 degrees, causes the input current waveform to be the point shape pulse, the pulse width is about 3ms, and half a cycle 10ms 1/3. Input voltage and electric current waveform are shown as Fig. 2. Therefore, cause the basic reason of LED power input current distortion to result from using capacitive loads of filtering electrolytic capacitor of direct current.

Fig. 2

Drive the intersection of power and input current produce non-sinusoidal wave of distortion as to LED, must use leaf Fourier in the Fu Progression is described. Until leaf vary principle in the Fu, instant input current it expresses to be:

In the type, n harmonic order, the intersection of leaf and the intersection of coefficient and an and bn it parts tables to be in the Fu:

Every current harmonics, will usually last a piece of sine or cosine cycle, n effective value In of harmonic current once can be calculated with the following formula:

Input the mean value of the totalcurrent

In the aboving radical sign, I1 is a fundamental current mean value, other I2, 3, represent 2, 3 respectively, n harmonic current mean value once. The total harmonic content of electric current represented with the fundamental current percentage calls the total harmonic distortion THD ,The total harmonic content has reflected the corrugate distortion characteristic, so call total harmonic distortion rate too. Define as

According to the definition of the factor PF of the power, the factor PF of the power refers to the meritorious power P of ac input and inputs the ratio of the apparent power S, namely

Among them, in order to input the mains voltage; U cos 1 is called the phase shift factor, it reflects the phase relation between fundamental current i1 and voltage u, 1 is a phase angle of fundamental wave; It is K to input fundamental current effective value I1 and percentage of inputting the effective value Irms of the totalcurrent =I1 / Irms is called the distortion coefficient of input current. It shows that aboving, in LED drives power supply circuit of non linear switch such as the power, the factor PF of the power not only relates to phase place between the fundamental current i1 voltage u but also relate to distorted coefficient K of input current. Formula 6 Insert formula 7 ,Then there are the following relations in factor PF of the power and total harmonic wave distorted THD:

Aboving to prove, when the factor cos 1 of phase shift is constant, reduce the total harmonic wave distorted THD, all right up-rate factor PF; Can prove too instead, the higher PF is, THD is the smaller. For example, through calculating, act as phase angle 1 =At 0 o’clock, THD =30% @ PF =0.9578; THD =10% @ PF =0.9950.

2.Harmonic measure and analysis

In order to well analyze LED as Fig. 1 shows drives the harmonic content of the power, introduce a method for using the ondoscope to measure the input current. First in the intersection of power and the intersection of input return circuit and the intersection of tandem connection and one 10-20W or large power resistor if R of the above =10 OHM, measure the voltage waveform of both ends on the large power resistor after being energization, because voltage and electric current of both ends are the phase equality all the time on the pure power resistor, so namely pulse voltage waveform on the resistance has represented pulse shape of input current, but the number value is of different size. It is knowing to reveal by the waveform, its electric current it of pulse It is concerted with the electric current waveform of Fig. 2, see Fig. 3.

The pulse wave of this electric current is similar to the pulse wave of cosine, so can use the function table of cosine pulse:

In order to calculate convenient, fetch one cycle when the sine exchanges input voltage now T:-5msIt is less than or equal to 15ms to less than or equal to t, namely T =20 ms. therefore the intersection of input and expression of impulse current of 20ms as follows in one period of cycle:

In aboving, a value Im of impulse current of cosine can be calculated by ondoscope expicity voltage amplitude and proportion of resistance quantity, namely Im=Um/R,Have already known to examine Um =1.5V, then Im =1.5/10 =Impulse width in 0.15 A. pictures =3ms. As to the input current waveform illustrated in Fig. 2, it is about symmetrical wave once that the front and back half wave is symmetric and strange from head to foot, therefore only contain a1, a3, a5 ,etc. harmonic component once very,and weights direct-flow a0 and even harmonic weight a2, a4, a6 It is zero. Formula 10 Input current waveform carry on at Fu to if you can’t decompose leaf, have:

According to the integral formula:

And there is a = / , b =N, =2 / T, so as follows,

Act as n =T 1 o’clock =20ms, =3ms, Im =0.15A inserts abovingly, all right

Calculate fundamental current a value a1 =I1m=0.06* 0.6080.327 =0.056A .

The same, calculate a3 separately, a5, a7, a9 subharmonic amplitude, as Table 1 shows.

Table 1. A property value table of harmonic wave

According to Table 1, LED drive leaf progression in the Fu of input current of power:

According to the relation between a value Inm and harmonic effective value In of harmonic wave, harmonic mean value:

By formula 16 ,Calculate every harmonic current mean value as follows unit A separately : I1 =0.040,I3 =0.033,I5 =0.023,I7 =0.012,I9 =0.003. According to formula 5 ,LED drives the input totalcurrent mean value of the power:

Will express A data insert formula 17 ,Input the effective value Irms of the totalcurrent =0.058A . In reality, this can be measured or read to input current value by the input current display screen of the dynamometer with the multimeter which measure the rms value. According to formula 6 Calculate the total harmonic distortion:

According to the harmonic amplitude data of Table 1, and with the fundamental wave first harmonic 100% of weight is the basic reference, make the amplitude spectrogram of the harmonic current the harmonic wave neglected and is higher than 9 or more times Refer to fig. 4.

Fig. 4

Pressing type 7 now Calculate the factor PF of the power, it is zero to act as phase angle 1 of fundamental wave, cos 1 =One is as follows,

Examine PF in fact =0.65,The two are basically unanimous. Real LED drives the input power of the power:

3.Harmonic danger

Harmonic danger is calculated that knowing by the above-mentioned analysis, this kind of LED drives the harmonic content of input current of the power high, use of drib as to the device such as power quite, harmfulness its might not show perhaps, but if hundreds and thousands of a large amount oves are used intensively, the total amount of harmonic current that it produces will pollute the whole power supply system and other electricity consumption users seriously, enable the Wellenform erzeugen distortion of the mains voltage at the same time. The theory and practice prove, the too big current harmonics will produce the following to endanger: A.Can make the core of the magnetic material such as electric voltage transformer and generator, perceptual loading equipment such as electric motor of electricity distribution facility loss Pkz and get extra increase, namely has increased hysteresis loss Ph weight and eddy current caused by harmonic current and lossed Pc weight, enable its excessive heating and damage, formula 21 ,Among them fn is every frequency of harmonic current.

B.The power capacitor of the compensation equipment of power of harmonic passing of current, Fig. 5 is the equivalent picture of the capacitor. Can be seen by Fig. 5, B. Form syntony when equal to inductive reactance that the parasitic inductance causes in capacitive reactance caused by harmonic current, produce the strong harmonic current, thus cause the excessive load or overvoltage of the power capacitor to be damaged.

Fig. 5

C.Can be to relay protection, instrument, automaticallies control, the telecommunications, satellite navigation and computer department C on the circuit. The interconnected system produces the strong interference, thus cause abnormal phenomenas such as the faulty action, presenting the noise,etc.. D.In power supply system of the three phase four wire system, the three-phase alternating current is in a basic balance when the circuit is normal, every phase current D in the central line. Offset each other, the Neutral Current approaches zero in theory, so the central line of the Chinese electric system is generally more detailed than the phase line. But three three-phases time and high order harmonic current when it is too big,it will make phase current of electric wire netting unable in in in line to offset each other, the electric current in line is produced and superposed and the excessive load is damaged while causing, the routing map such as Fig. 6. In addition, the Neutral Current over causes three phases to be uneven, namely the three-phase electric potential is deflected, cause large quantities of LED lamps and lanterns to burn when being serious, even cause the fire!

Fig. 6

E.When a large amount of power supplies of heavy high harmonic content use, its even harmonic a2, a4, a6 Can’t ignore, it makes the plus-minus half cycle of the loop current of supplying power asymmetric. Especially the second harmonics of bigger content, its DC component makes the electric voltage transformer core produce the local magnetization, loss and increase, will jeopardize voltage transformer and electricity and run safely when being serious. So, no matter from protecting the electric system safely or according to protecting with the electrical equipment and personal security, and rigid control limits the electric current harmonic content and already becomes people’s common understanding in order to reduce the harmonic wave to pollute the danger caused.

4.Reduce the measure of THD

Popularize with the application of the switching power supply electronic product, International Electrotechnical Commission has made IEC61000-3-2, European Union and made regulations such as GB17625.1-2003,etc. after making EN60555-2 and China, to making concrete restriction and regulation with voltage of the electrical equipment, electric current wave distortion. These regulations are applicable too to drive the power in LED lamps and lanterns and LED at present. For inputting the big LED lighting device of the active power in 25W, the harmonic current should not exceed the limit of Table 2.

2. C Limit of one kind of apparatuses

For not inputting the active power quite in LED lighting device of 25W, stipulate the following one of these of coincidence:

a.The harmonic current should not exceed the limit correlated to power in the second column in Table 3;

Restriction of the 3 D apparatus of the table

b. Should not exceed 86% with fractional harmonic current 3 times of fundamental current one hundred, 5 subharmonics do not exceed 61%; Moreover, suppose it is 0 degrees that the fundamental wave voltage crosses the zero, input current waveform should be 60 degrees or begin to circulate before this, 65 degrees or there is end before this A crest value if there are several crest values within half a cycle ,Should not stop circulating before 90 degrees.

The input power that LED that Fig. 1 shows drives the power is 8.8W, according to the limit of the second column of Table 3, THD obviously exceeds standard. A good LED drives the power, not only need heavy-duty factor PF but also realize low THD, specified value of making very harmonic content once not above standard.

But some power artificers, reduce filtering capacitance value in order to put undue emphasis on high PF, its result is conduction angle increase of a bridge rectifier, PF increases, but the pulsation direct-current volts that the bridge rectifier exports leads to the fact the crest current of the circuit is extremely high, make power tube of supply convertor,etc. loss, increase severely, very apt to damage power tube, high-frequency voltage transformer, high frequency, export the component of the rectifier tube.

At present, performance comparison fine LED drives the power, adopts active power factor correction Advantage Power Factor Correetion APFC circuit, Fig. 7 is a kind of commonly used critical feed-through mode TCM Single grades of PFC is anti more exciting than supply convertor for synoptic diagram.

Fig. 7

The input current namely the wave form of the inductive current refer to fig. that this kind of circuit can make 8The pulsation voltage waveform exported with the kenotron keeps the concerted characteristic, exist kenotron turn on angular influence, so input current and input voltage have the same phase places, as Fig. 9 shows.

Fig. 8

Fig. 9

The power factor PF of this kind of circuit and relation of total harmonic wave distorted THD are as follows:

This circuit can usually make sure PF is greater than or equal to 0.96, THD and less than or equal to 30%, even can make PF value close in 1, distorted coefficient K of input current =I1 / Irms is less than or equal to 3, THD is less than or equal to 10%. The incoming circuit of Fig. 10 is the passive power factor control PPFC of a kind of general valley filling type Circuit, drive the power to adopt this circuit, advantage of low cost, circuit are simpling etc. as to LED minor in input power. Its power factor can be in 0.85-0.9, but the harmonic content will often exceed accords with the regulation.

Fig. 10

Its voltage and wave of the input current are similar to pursuing 11

Fig. 11

Fig. 12 It is its test result, the result indicates the harmonic content exceeds standard.

Fig. 12

Fig. 13

To this defect of the circuit of Fig. 10, can propose a kind of improved scheme. In having source PFC circuit, increases the resistance of one 2-5 OHM/2W and tube D3 of the dipole to connect in series, sees Fig. 13. So can reduce the harmonic content effectively, can also further improve PF at the same time, drive the power as to LED of this kind of structure, it is an effectual improvement method.