Diode rectifiers place a large capacitance in parallel with the load. When the diodes conduct, they recharge the large filter capacitor. This recharging manifests itself as high current flow from the grid, with low voltage across that filter capacitor initially, until the capacitor voltage has built up. So this power supply, seen from the power grid, looks like a load with low power factor. The current flowing to it is out of phase with the voltage reflected back from it.

Using smaller filter capacitors improves the power factor, but obviously this also results in higher ripple from the power supply.

This phase difference between the current and the voltage, power factor, is problematic to a power grid. Why? Because the diode rectifier power supply wants to distort the power grid's nice, clean sinusoid. That inrush current every half cycle wants to draw down the voltage of the power grid. Other users of the power grid will therefore see a distorted sinusoid. The grid prefers to see a pure resistive load.

A non-regulated diode-based rectifier has a theoretical power factor of 0.95.

Regulation is achieved by replacing the diodes with SCRs. By controlling the phase delay angle power factor can be improved.

Diode rectifier causes distortion to the source. When distorted ,the waveform is non-sinusoidal and we talk about source powerfactor as ((fundamental)/Harmonic)*(cos(phi)). As can be seen from the expression, as harmonic magnitude increases source power factor deteriorates.