# How to calculate the split DC-link capacitors used for making neutral point?

#### ronsimpson

Joined Oct 7, 2019
2,752
Resistors:
1) The reason there are three resistors in series is because resistors have a max voltage. It is hard to find a resistor that can handle the high voltage. In your case each resistor sees 1/3 the voltage.
2) There are 4 resistor strings in parallel. It is hard to find high wattage resistors. In your case each string sees 1/4 the wattage.
3) There are government safety regulations on storage capacitors. I don't know exactly the regulations, but I believe the Resistors must discharge the Capacitors before a person can take the case apart and sick their fingers in.
Capacitors: There are formulas on load and ripple voltage. The larger the caps the less the 60 (50)hz ripple voltage is. Capacitors also set the "hold up time", for your power supply.

#### Ian0

Joined Aug 7, 2020
8,407
b) is interesting, because it makes use of the fact that the maximum voltage between any two conductors is 230*√2*√3=563V whereas a) would require a DC link voltage of at least Vp-p = 230*2*√2=650V.
Microchip also has an application note on the subject. The individual waveforms get weird, but the difference between them produces a 3-phase sinusoid.