DC link capacitor

Discussion in 'General Electronics Chat' started by cachapo, Feb 4, 2010.

  1. cachapo

    Thread Starter Member

    Aug 24, 2009
    Hi everybody:

    I´m fighting with an inverter circuit (three phase bridge, 48V, 2A) to drive a BLDC motor, but I don´t know how to choose the DC link capacitor value. I think it must be an electrolitic one, with an ESR as low as it can be, but no idea about the capacitance value.

    Does anybody know about how to size it? Any paper edxplaining it?

    Thanks in advance.
  2. bertus


    Apr 5, 2008

    Can you make a drawing of the intended setup?
    Capacitors are uses for AC coupling (series) or DC ripple suppression (parellel).

  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    On the UKs 50Hz supply (100Hz ripple when full wave rectified), the rough guide is that 10,000uF gives 1V ripple per amp of load current.

    Using three phase through a full wave bridge, you only need 1/3 the value to get the same ripple level. (Or rather less, as the three phase bridge output never drops to zero anyway).

    Based on that, a 3,300uF cap would leave a maximum 2V ripple on your supply at 2 Amps.

    The derivation is that a 1 Farad capacitor discharged at 1 Amp has a voltage drop of 1 volt per second.

    At 100Hz, a cap 1/100 the size (10,000 uF) will also drop about 1V at 1A before the next half cycle recharges it.

    (Ballpark figures as the recharge is not instantaneous & the rec supplies the load for part of each cycle etc., but OK for guideline calcs.)