Active inrush current control for AC to DC converters

An AC to DC power converter having active inrush current control during operational and power disturbance conditions, said converter having two input terminals to which AC power is coupled and two output terminals where the output DC power is provided, comprising:

• a diode bridge rectifier for generating a rectified input voltage from a source of said AC power;
• a boost converter coupled to said rectifier for converting said input voltage to a DC voltage, said boost converter having a first switch, an inductor and a diode;
• an output capacitor connected to a first one of said DC output terminals;
• a resistor connected in series between said output capacitor and a second one of said DC output terminals;
• a second switch having a control input and being connected in parallel with said resistor;
• a control circuit for controlling inrush current, said control circuit being operatively connected to said control input of said second switch, said control circuit comprising:
• a first comparator for comparing the instantaneous AC input voltage to the DC output voltage of the converter, said first comparator having an output set high when the instantaneous AC voltage is lower than the DC output voltage;
• a second comparator for comparing the instantaneous AC input voltage to a predetermined voltage level, said second comparator having an output set high when the instantaneous AC input voltage is present and above said predetermined level;
• a third comparator coupled to the outputs of said first comparator and said second comparator;
• such that the output of said third comparator is high when the output of said first and second comparators are both high;
• wherein said output of said third comparator is connected to said control input of said second switch such that said second switch enters a conduction state when said third comparator output is high.

A method of controlling inrush current in a AC-DC converter when AC power is lost during power line disturbance conditions, wherein the AC to DC converter is coupled between two input terminals to which AC power is coupled and two output terminals where the DC output voltage is provided, the AC-to DC converter including a boost converter controlled by a first switch, the AC to DC converter having connected across the output terminals a capacitor connected in series with the combination of a limiting resistor connected in parallel with a second switch, comprising the steps of:

• a) maintaining said second switch in an off state to enable said limiting resistor to limit inrush current when said AC power is lost for a predetermined time interval;
• b) comparing the instantaneous AC input voltage to the DC output voltage of the converter;
• c) comparing the instantaneous AC input voltage to a predetermined voltage level to determine if the AC input voltage is present and non-zero;
• and
• d) triggering said second switch to the on state to shunt said limiting resistor when said AC power is restored to said predetermined level and the instantaneous input AC voltage is less than the DC voltage at the output of the AC-DC converter, such that inrush current is controlled and voltage surges at said DC output terminals are eliminated.