The application of ceramic carbon resistor technology to soft starting the DC link capacitors in switch mode power supply units

Included in the problems faced by designers of switch mode power supply units (PSUs) is the ´soft´ starting of the DC link capacitor bank. This essentially involves charging the capacitor comprising several thousand microfarads from the rectified three phased AC supply. Without proper precautions, an excessive current could flow which could damage the capacitor and power supply. A method of reducing the excessive current flow is to introduce passive components into the DC line such as wire wound resistors, thick film resistors, NTCs and PTCs. However, all of these components tend to suffer from an inadequate surge energy capability. At switch on, capacitor uncharged, a current which is limited only by the source reactances etc., results in a current which can be ten or more times the normal. Such a current is unacceptable and could damage capacitors or other components. As a solution to the problem HVR Ceramic Resistors can be used. They are virtually 100% active in volume and consequently they offer a larger thermal capacity per unit volume than those described above. They are manufactured from a carefully blended mixture of clays, alumina and carbon. After blending, the material is pressed to the required shape. For the compact resistors considered, these are cylinders varying from 10 mm to 25 mm in diameter and from 5 mm to 50 mm in length. Aluminium is then name sprayed onto the end cross sections to achieve good electrical contact and then oversprayed with brass or copper to facilitate the soldering of the capped terminations