Patent ID: 7430101

Claim:
An over-voltage protection system for an intermediate circuit of a frequency converter, said frequency converter including a voltage-controlled pulse-width modulation frequency converter comprising an uncontrolled mains bridge connected to an alternating-current source, a direct-voltage intermediate circuit and a controlled load bridge for producing a multi-phase alternating voltage of variable frequency, the direct-voltage intermediate circuit of the frequency converter including a low-capacitance direct-voltage capacitor as the only capacitor, the mains bridge being connected to the load bridge directly and the overvoltage protection system comprising a series connection of a varistor and a semiconductor switch, said series connection being connected in parallel with the direct-voltage capacitor, the semiconductor switch having a low voltage rating with a low conducting on-state resistance and low power loss; and the semi-conductor switch is configured to operate in an avalanche operation region when the overvoltage protection system is not limiting the intermediate circuit voltage.