Swing back diode
A resonance diode is a diode connected in anti-parallel to the charging capacitor ( smoothing capacitor ) of the DC voltage intermediate circuit of a converter . In normal operation, this diode is switched in reverse direction, the intermediate circuit voltage is applied to it and no current flows through it.
In the event of a short circuit in the intermediate circuit, it prevents an inverse voltage on the capacitor and on the converter - the capacitor forms an oscillating circuit with the short-circuit impedance; otherwise, this oscillates through until a negative voltage occurs in the intermediate circuit.
The swingback diode relieves the load on the freewheeling diodes that are already present in converters .
If there is an intermediate circuit short circuit, the short circuit path represents a very small inductance, the intermediate circuit capacitor is a large, parallel capacitance. This creates an oscillating circuit in which current and voltage - excited by the switching process - oscillate against each other. The full swing through (voltage after zero crossing) is limited to its forward voltage by the free-wheeling diodes in the converter without a swingback diode.
However, the freewheeling diodes are not designed for such a high i 2 t load, since they normally only have to carry the operating current. For this reason, back-swing diodes are installed in parallel to the entire intermediate circuit, and thus also to the free-wheeling diodes, which absorb a large part of the short-circuit current.