Improved Valley-Fill Passive Current Shaper
K. Kit Sum Consultant
山水画简单P. O. Box 361110, Milpitas, California 95036-1110Telephone: (408) 263 7171 +++ Facsimile: (408) 263 7647Abstract
The original valley-fill current shaper permits input current conduction from 30° to 150°, and then from 210° to 330°. Due to the discontinuities from 0° to 30° and from 150° to 210°, substantial amount of harmonics were introduced into the input current waveform. This article prents an improved version of the valley-fill circuit which extends the conduction angle to near 360°, thus lowering unwanted harmonics as well as improving power line current waveform. Improvements are made with passive components.SPICE simulations compare original circuit with different improved versions of the circuit. 98% power factor is achievable with this new circuit.
Introduction
The valley-fill passive power factor correction circuit was communicated to the author by Spangler [1] in 1988. The idea evolved naturally from work previously performed by Spangler [2].Subquent work
on this idea was reported in 1991 [3] in which a number of circuits were compared. All the passive circuits came very clo to meeting the IEC specification limits. This obrvation led the author to further investigate the feasibility of providing a minor improvement to the valley-fill circuit to push it into the IEC limits.
Principle of Operation
The original valley-fill circuit is shown in Figure 1. The capacitors C 1 and C 2 are charged in ries,and discharged, via the diodes D 5 and D 7, in parallel. Current is drawn from the line from 30° to 150°, and then from 210° to 330°† . Discontinuities occur from 150° to 210° and from 330° to 360°,and then the cycle repeats itlf. Diode D 6 is inrted to prevent C 2 from discharging via C 1. The resistor R 1
is a very low value resistor inrted to monitor the input current waveform.
R L
A.C. in
Fig. 1. The Original Valley-Fill Circuit † Assume under steady state condition, the waveform starts from 0°, the sinusoid reaches half of the peak amplitude at 30°, the rest of the half peak locations follows accordingly.
The load R L is chon in such a way as to demand a light load for the associated component values, so that a higher harmonic input current wave can be obtained for better comparison. This is bad on the idea that a lighter load will have less demand on output current, and as a result, the charges on C1 and C2 are not greatly depleted quickly and the resulting charging spike will be narrower than the heavier load counterpart. This effect can easily be verified by experimentally testing or simulation with a heavier load.
Preliminary Obrvations
Figure 2 shows the input current waveform, from simulation, for the standard or original valley-fill circuit. Discontinuities are obrved from 30° to 150°, from 150° to 210° and from 330° to 360°, and t
hen the cycle repeats itlf. Much of the input current distortion is caud by the discontinuities which cross from positive to negative, and then from negative to positive, during each cycle. If this cross-over distortion can be lesned or eliminated, then the likelihood of using this circuit to meet the IEC specifications would be very high. The peak charging spike is also a major contributor of current harmonics, however, since the power content of this spike is not very high, one can conclude that it can be suppresd without too much sacrifice in efficiency.
Fig. 2. Standard Valley-Fill Input Current Waveform
For comparison purpos, Table 1 shows the simulated harmonic content of the standard circuit. The total harmonic distortion is en to be clo to 35%. Figure 3 shows the Fourier plot of the haromoni
cs.
HARMONIC FREQUENCY FOURIER NORMALIZED PHASE NORMALIZED
NO (HZ)COMPONENT COMPONENT (DEG) PHASE (DEG)
1 6.000E+01 7.425E-01 1.000E+00 1.547E+00 0.000E+00
2 1.200E+02 2.069E-0磅差
3 2.786E-03 -1.726E+01 -1.880E+01
3 1.800E+02 1.905E-01 2.566E-01 -1.655E+02 -1.671E+02
4 2.400E+02 1.189E-03 1.601E-03 2.445E+01 2.290E+01
5 3.000E+02 7.131E-02 9.604E-02 1.037E+02 1.021E+02
6 3.600E+02 8.807E-04 1.186E-03 -8.717E+00 -1.026E+01
圈r7 4.200E+02 1.303E-01 1.755E-01 -1.354E+02 -1.370E+02
8 4.800E+02 2.368E-03 3.189E-03 -1.709E+02 -1.725E+02
9 5.400E+02 9.409E-02 1.267E-01 7.783E+01 7.628E+01
TOTAL HARMONIC DISTORTION = 3.492362E+01 PERCENT
Table 1. Input Current Harmonic Content of Standard Valley-Fill Circuit.
Objectives and Circuit Improvement
To improve the standard circuit, two objectives must be accomplished: 1. Reduce cross-over distortion in the input current waveform; and 2. Suppress charging spike at the peak of the current wave.
To maintain continuous current drawn from the line, the voltage during the cross-over periods must not be less than half of the peak input voltage, at least for most of the cross-over periods. This is becau when the capacitors are discharging, they are being discharged in parallel. Only half of the peak line voltage appears across each capacitor.
Fig. 3. Standard Valley-Fill Input Current Harmonic Content
(The first number in parenthesis indicate the frequency, the cond number is the Fourier component)
成都周边游Since the bulk of the power is conveyed in the current waveform during conduction time, only a small amount of power will be required to supplement the missing currents during the discontinuities. In other words, the extra power required to fill the gaps is very small compared to the bulk of the power delivered by the valley-fill circuit to the load. Also, during the cross-over periods, the amplitude of the missing part of the waveforms is comparatively small, whereas the main conduction periods have current amplitudes of much higher level.
To maintain the flow of input current, a voltage doubler is inrted to feed the valley-fill circuit. This voltage doubler is configured in such a way as to contribute a very small amount of power to the main circuit, just enough to improve the current waveform at the cross-over points. This means that the capacitors ud for the voltage doubler can be orders of magnitude smaller than the values of C1 and C2. Under normal operating conditions, the energy from the voltage doubler is totally absorbed by the main circuit. But during the cross-over periods, the voltage doubler comes into play by continuing to draw current from the line, thus further extending the input current conduction angle.
R L
A.C. in Fig. 4. Valley-Fill Circuit with Voltage Doubler三分流水七分尘
Figures 5 shows the input current waveform with the improvements at the cross-over points,and Figure 6 is the Fourier plot of the harmonics. However, the peak charging current spike still persists.
Fig. 5. Valley-Fill with Voltage Doubler
该来的总会来
Fig. 6. Harmonic Content of Valley-Fill with Voltage Doubler
(The first number in parenthesis indicate the frequency, the cond number is the Fourier component)
我的修道岁月
To remedy this, a resistor R 11 is connected to the bottom electrode of C 2. This is shown in Figure 7. Note that the value of R 11 is, to some extend, inverly proportional to the output power,since for higher power output, the charging time would be longer, due to a faster rate of charge depletion from capacitors C 1 and C 2.
Table 2 shows that the total harmonic distortion has been reduced from the standard circuit value of 35% to 9.6% for the voltage doubler with R 11.
描写春天的优美句子The current respon in Figure 8 can further be improved by the inrtion of another resistor R 12 (shown only on the netlists). Inrtion of this resistor (one terminal of the resistor connected to the junction of D 3 and D 4, and the other terminal connected to the junction of C 3 and C 4) will remove the charging spike at the cross-over points, and further enhance the quality of the input current.
All component values can be found on the netlists provided in this article.
R L
A.C. in Fig. 7. Valley-Fill Circuit with Voltage Doubler and R 11.
Fig. 8. Input Current Waveform of Valley-Fill Circuit with Voltage Doubler (Fig. 7).
