An arrangement for cooling power semiconductor devices of a converter arranged in a closed casing with said semiconductor devices in a bottom of the casing on top of a heat sink having members, such as fins, for heat exchange extending externally of the casing out from the bottom, the arrangement comprising: • a channel for conducting a flow of air to pass said members for cooling thereof, in which a first portion of said channel has the casing bottom as a ceiling and receiving said members, • a fan located with a suction side connected to said first channel portion to create a flow of air from an inlet from the exterior into said channel while passing the heat sink members, and • a filter covering said inlet to the channel for removal of dust and debris from air entering the channel from the exterior, wherein the channel comprises a second portion connected to the first portion as seen in the direction of the air flow through the channel upstream of the first portion, wherein the second channel portion extends along and has as a delimiting wall a lateral wall of said casing connecting to said bottom to conduct air downwards along this lateral casing wall to enter the first channel portion, wherein the channel inlet is located in the second channel portion opposite to said lateral casing wall and has a longitudinal extension in the direction of the extension of this channel portion to the region of the start of the first channel portion, and wherein the channel inlet is designed to form a resistance to flow of air therethrough through the filter and into the second channel portion increasing in the direction towards said first channel portion so as to in this direction increase the air pressure difference between the exterior and the inner of the second channel portion and by that reduce the air flow into the second channel portion, characterised in that the channel inlet comprises an air-impermeable channel wall member having the filter internally thereof and provided with through-holes for air to pass, and that the total area of the through-holes in subsequent sections of the second channel portion decreases the closer the section is to the start of the first channel portion. • a channel for conducting a flow of air to pass said members for cooling thereof, in which a first portion of said channel has the casing bottom as a ceiling and receiving said members, • a fan located with a suction side connected to said first channel portion to create a flow of air from an inlet from the exterior into said channel while passing the heat sink members, and • a filter covering said inlet to the channel for removal of dust and debris from air entering the channel from the exterior, wherein the channel comprises a second portion connected to the first portion as seen in the direction of the air flow through the channel upstream of the first portion, wherein the second channel portion extends along and has as a delimiting wall a lateral wall of said casing connecting to said bottom to conduct air downwards along this lateral casing wall to enter the first channel portion, wherein the channel inlet is located in the second channel portion opposite to said lateral casing wall and has a longitudinal extension in the direction of the extension of this channel portion to the region of the start of the first channel portion, and wherein the channel inlet is designed to form a resistance to flow of air therethrough through the filter and into the second channel portion increasing in the direction towards said first channel portion so as to in this direction increase the air pressure difference between the exterior and the inner of the second channel portion and by that reduce the air flow into the second channel portion, characterised in that the channel inlet comprises an air-impermeable channel wall member having the filter internally thereof and provided with through-holes for air to pass, and that the total area of the through-holes in subsequent sections of the second channel portion decreases the closer the section is to the start of the first channel portion.