The present invention relates to a lightweight liquid-cooling-plate assembly having a plastic frame and a heat dissipation system using the same. The liquid-cooling-plate assembly includes a plastic frame and at least one coolant chamber unit. The plastic frame includes a plurality of lateral walls, at least one accommodation opening, and a plurality of fastening elements. The lateral walls are connected with each other to form and define the at least one accommodation opening. The fastening elements are disposed on a part of the lateral walls. The coolant chamber unit is connected with the plastic frame and embedded in the at least one accommodation opening, and includes at least one surface exposed.

An electric power converter includes a semiconductor module constituting a power conversion circuit, a capacitor electrically connected to the semiconductor module, and a cooling member for cooling the capacitor. The capacitor includes an element body provided with internal electrode, and a pair of end-face-electrodes provided on both end faces of the element body and connected to the internal electrode. The pair of end-face-electrodes are connected with a pair of bus bars, respectively, in a manner of surface contact. The capacitor is disposed in a state where one of the pair of end-face-electrodes is facing the cooling member. The end-face-electrode facing the cooling member is in contact with the cooling member via the bus bar.

A base plate (1) made of a metal has a through-hole (2). An insulating substrate (3) is provided on the base plate (1). A semiconductor chip (4) is provided on the insulating substrate (3). A case (8) has a screw.sup.-hole (9) communicating with the through-hole (2), covers the insulating substrate (3) and the semiconductor chip (4), and is disposed on the base plate (1). A screw (11) made of a metal is inserted into the through-hole (2) and the screw-hole (9) to fix the case (8) to the base plate (1). A flexible material (12) having flexibility is filled in a cavity between a bottom surface of the screw.sup.-hole (9) in the case (8) and a distal end of the screw (11).

A clamping assembly includes a configuration of mechanically clamped components disposed one on top of the other to form a stack. A clamping device generates a mechanical compressive force on the configuration of the components and a pressure element transmits the mechanical compressive force from the clamping device to the configuration. The pressure element contains a metal foam for a planar, homogeneous transmission of the compressive force. A sub module of a converter having at least one series circuit of power semiconductor switching units implemented as the clamping apparatus is also provided.