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Posted on 22 February 2019

Mounting of Capsule Diodes and Thyristors (disc cells)

 

Disc cell housings possess two thermal contact surfaces. For maximum current capacity yields, double-sided cooling (DSC) is usually used. For this purpose, the cell is clamped between two heat sinks using a clamping fixture as described below. In lower load applications, single-sided cooling (SSC) is used as a standard practice as well.

The electrical contact between component and heat sink is established for disc cells, as well as components with screw-in housings via the thermal contact surfaces. This means that contact surfaces have to be able to fulfil both thermal and electrical requirements. They must be even and smooth (maximum permissible unevenness 10 μm, maximum surface roughness 6.3 μm). Individual nicks, notches, or scratches are acceptable, provided no flash protrudes. The surfaces must be free of impurities and dust.

Within just a few minutes of exposure to air, an oxide layer will form on aluminum. Due to this oxide layer, the electrical contact resistance will increase many times over, which may lead to contact corrosion after extended periods of current passage. For this reason, the contact surfaces of non-zinc-plated aluminum heat sink areas have to be mechanically cleaned directly before the disc cell is mounted. A fine wire brush immersed in thermal paste or an abrasive sponge will be suitable for this purpose (e.g. "Scotchbrite" from 3MCompany). Abrasive residues have to be removed thoroughly from the treated surfaces. Immediately afterwards, an ultrathin layer of thermal paste without (insulating) powder fillers, e.g. ELECTROLUBE GX, must be applied to the contact surfaces.

The clamping fixture must be designed such that one of the two heat sinks can move freely during mounting. For this purpose, one of the two mounting clamps forms a pivoting bearing (contact between spherical and flat surface). The second mounting clamp should be attached to the respective heat sink in such a way that it does not deform when the screws are tightened.

The heat sink which forms the pivoting bearing with the clamping fixture must be able to move freely. This means that no rigid mechanical connections to other heat sinks or support elements are mounted on this heat sink. The electrical connection between this heat sink and the corresponding busbar must be established with a flexible lead.

Figure 1 shows a diagram of a clamping fixture.

Capsule thyristor/diode mounted with "MC" clamping fixture

Figure 1. Capsule thyristor/diode mounted with "MC" clamping fixture

The following mounting instructions apply to "MC" clamping fixtures available at SEMIKRON as an accessory to some of their capsule diodes and thyristors.

The clamping fixture must be able to establish the contact pressure specified in the capsule datasheet. Firstly, the heat sink contact surfaces for the component and the power supply must undergo thorough pre-treatment, as described above. Thorough cleaning with a solvent will do for plated surfaces. Both surfaces are then coated with a thin layer of thermal paste, e.g. using a rubber roller. The contact surface of the capsule diode/thyristor should now be firmly pressed onto the mating surface of the respective heat sink by hand, turning it back and forth whilst doing so. The center pins (9) must be placed in position before. When the diode/thyristor is removed from the heat sink, the mating surfaces must be uniformly coated with thermal paste. If this is not the case, the surfaces must be recoated with a thin layer of thermal paste.

The clamping fixture is then positioned such that the cross piece (5) and pressure plate (4) with the tension bolts (10) is at the top. The cross piece (3) and pressure plate (4) is inserted from below, and the two tension bolts (10) are tightened alternately until a slight resistance is felt. Now check that both cross pieces (3) and (5) are parallel. To do so, it is sufficient to measure the lengths of the ends of the bolts protruding from the bottom of the two cross pieces (3). The bolt ends must be equal in length. The two tension bolts (10) should now be tightened alternately up to the point where the metal gauge (8) can be easily moved. Under no circumstances should the tension bolts be tightened beyond this point, as doing so would mean that the permissible mounting force has been exceeded. For the same reason, the pre-set nut (6) of the Belleville washer assembly (7) should never be adjusted. The metal gauge (8) is secured to prevent it from falling out.

As an alternative to such clamping fixtures, which may be installed by the customer, more simple mechanical clamping systems are used for the SEMISTACK product range, which must be mounted in presses using force measuring devices only. These clamping systems are not suitable for changing disc cells on the part of the customer.

 

For more information, please read:

Approaches to Mounting Power Semiconductor Devices

Device Failure due to Incorrect Mounting

Mounting Positions of Aluminum Electrolytic Capacitors with Screw Terminals

New Alternative Materials and Concepts for Power Electronics

 

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