Power Electronics

Rectifiers for filtering purposes

Why filter is used in rectifier

Rectifiers are used for creating the smooth DC output voltage, for this purpose there is usually filters are used in rectifiers. This systems can be divided as inductor-input DC filters and capacity-input DC filter. Inductor-input DC filter is used in high-power applications, capacitor-input DC filters are used in relatively-low power applications.

Inductive-input DC filters

The inductive-input DC filters are depicted on the figure below. If the induction ωLR, then rectifier output current is stable.

Why filter is used in rectifier

For simple inductor-input DC filter ripple can be reduced by the factor RR2+(2πfLLf)2 or v0vL, here v0 is the ripple voltage after filtering and vL is the ripple voltage before filtering.

Here below you can see the full-wave rectifier with inductor-input DC filter.

Why filter is used in rectifier

Inductance in the circuit plays important role, as if inductance is finite, the output voltage and current through the inductor have ripple component, if the inductance is infinite – output current and voltage are constant. The minimal inductance value, called critical inductance LC, required for maintaining continuous current.  For single-phase full-wave rectifier the critical inductance LC=R6πf, where f is an input frequency.

Current and voltage characteristics for for full-wave inductor-input DC rectifier for finite and infinite inductance are depicted below.


Usually the choice of inductance depends on the required output voltage ripple factor. Ripple voltage without filtering is vL=4Vmπ(n21), where n=2, 4, 8... The ripple factor RF=2(1n21)2n=2,4,8....

Total input distortion of the input current is THD=(ISIs1)21, here IS and Is1 are RMS and input current values. Input power factor is PF=Is1Iscosφ, here φ is an angle between input current and voltage.

Figure below shows the rectifier with an input AC filter.

Why filter is used in rectifier

RMS value of the n-th harmonic of current is IS=I11(2nπf)2LC, where I is RMS value of the n-th harmonic of the rectifier, here the total harmonic distortion THD=1n211(2πf)2LC2.

Here below you can see capacitive-input DC rectifier.

Here VS=VmsinωtRis resistor used to limit the input-surge current when the rectifier is connected to power supply.  When the voltage Vs is higher than voltage VL, diodes D1 and D2 conducts, capacitor C is charging. When the voltage of secondary winding Vs of transformer is lower than VL, diodes D1 and D2 reverse biased, capacitor  C is discharged. The average value of capacitor voltage VmVr(pp)<VL<VmVr(pp) is peak-to-peak ripple voltage Vr(pp)=VmfRC.

The average output voltage Vdc=Vm(1112fRC).

RMS output ripple voltage Vac=Vm22fRC.

Ripple factor RF=12(2fRC1).

For instance, Digi-Key Electronics offers great variety of rectifiers to order.