Analysis and practice of replacing Aluminum Electrolytic Capacitor with thin Film Capacitor(II)

     Three-phase bridge rectifier circuit or 12-phase rectifier circuit is not necessary to connect DC bus capacitor across the rectifier output because there is no current abrupt change in the application of load current no sudden change. Inductive potential produced by parasitic inductors of rectifiers and AC power sources will not affect the output voltage

     However, when the load is a switching power converter, the switching power converter will request the ripple power supply at the switching frequency from the DC bus, if the current flows into the parasitic inductance of the DC bus and the AC side, Ripple voltages will be generated at intolerable switching frequencies. From this point of view, the DC power supply is no longer just to provide DC current, but the need to provide a rich AC component of the pulse current, the DC power supply not only requires a low DC internal resistance, It is also necessary to have a good low impedance within a wide bandwidth. The low impedance of this wide band is not provided as the DC power supply of the rectifier. In order to obtain the good low impedance of the wide band, the capacitor with good performance must be applied. Taking advantage of the characteristics that capacitor voltage can not jump and capacitive reactance decreases with the increase of frequency, the AC impedance of DC busbar is reduced by capacitor.

     From this point of view, the capacitor connected by three-phase bridge rectifier or 12-phase rectifier output DC busbar is no longer a filter capacitor with smooth voltage, but a power supply bypass capacitor, or "DC support" or "DC-Link" capacitor.

     Direct current support "," DC-Link "capacitor can choose aluminum electrolytic capacitor, but also can choose thin film capacitor." Because the ripple current value of aluminum electrolytic capacitor itself is relatively low, it is necessary to satisfy the high amplitude ripple current in the application of "DC support" and "DC-Link". This requires the selection of aluminum electrolytic capacitor according to the size of ripple current, if the load produces 20A ripple current, the capacity of 1000F should be selected.

      The conclusion can be drawn from the above statement, The role of DC supporting capacitors is to provide a "non-inductive" DC "power supply" for the load when the load current fluctuates, eliminating the undesired amount of parasitic inductance that cannot be estimated between the switch and the power supply. The voltage spike of the inductive TV appears.

     While this solution may be optimal, the price can be very high and the average application will not be able to accept it. So, is there a better solution? The conclusion is that there are several low price and good performance solutions.