TABLE OF CONTENTS In direct-off-line switchmode supplies, where minimum size and cost are a major consideration, it is common practice to use direct-off-line semiconductor bridge rectification with capacitive input filters to produce the high-voltage DC supply for the converter section.
If the line input is switched directly to this type of rectifier capacitor arrangement, very large inrush currents will flow in the supply lines, input components, switches, rectifiers, and capacitors. This is not only very stressful on these components, it may also cause interference with other equipment sharing a common supply line impedance.
Various methods of inrush current control are used to reduce this stress. Normally these methods include some form of series limiting resistive device in one or more of the supply lines between the input point and the reservoir capacitors.
These limiting devices usually take one of the following three forms: series resistors, thermistor inrush limiting, and active limiting circuits.
For low-power applications, simple series resistors may be used, as shown in Fig. 1.7.1. However, a compromise must be made, as a high value of resistance, which will give a low inrush current, will also be very dissipative under normal operating conditions. Consequently, a compromise selection must be made between acceptable inrush current and acceptable operating losses.
The series resistors must be selected to withstand the initial high voltage and high current stress (which...
