Valley fill circuit
The valley fill circuit - ' German valley fill circuit ' - is a form of passive power factor correction filters for filtering undesired harmonics in electronic power supplies . In contrast to conventional passive power factor correction filters, this circuit does not consist of chokes , which have a high weight at higher powers, but the reduction of the harmonic content is achieved by a special electrical circuit in the area of the smoothing capacitors .
General
The valley fill circuit consists of a bridge rectifier , which first rectifies the supplied AC voltage . The valley fill circuit shown on the right is used to reduce the current peaks due to a smoothing capacitor connected directly to the DC voltage. The current peaks are reduced, and as a result there is a lower proportion of harmonics on the AC voltage side. First, the two smoothing capacitors C 1 and C 2 are slowly charged via the diode D 3 and the series resistor R to half the value of the peak value of the direct voltage. The series resistance is chosen so that, on the one hand, the current peaks are as minimal as possible and, on the other hand, the capacitors are charged to half the peak voltage after a few periods.
At the times when the DC voltage at the bridge rectifier falls below half the peak value, the two diodes D 1 and D 2 begin to conduct, D 3 blocks, and the smoothing capacitors are discharged via the load connected to the terminals on the right.
The valley fill circuit reduces the harmonic component on the AC voltage side compared to a simple smoothing capacitor, the disadvantage is the comparatively high ripple voltage of 50% on the DC voltage side, for which the connected loads must be designed.
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- K. Kit Sum: Improved Valley-Fill Passive Current Shaper . PowerSystems World, 1997 ( Online [PDF; 34 kB ]).