Inrush current limiter

A two terminal inrush current limiter placed in series with 2 power lines and a load for providing a continuous controllable limitation to the peak current delivered to an AC or DC load independent of power source voltage comprising:

• a first circuit means for controlling the conduction of a Darlington power transistor in direct proportion to the peak current delivered to the load responsive to a surge condition of known duration,
• a second circuit means for sensing peak current to the load and effecting control of base current to said Darlington transistor utilizing a small, low voltage control transistor to effect control of currents in the hundreds of amperes range,
• a third circuit means for serving as a driver transistor for said Darlington transistor for use in combination with said control transistor and having a base drive resistor for providing minimal voltage drop across said inrush current limiter during times when no surges exist and, for providing adequate base drive to allow said Darlington transistor to provide current to the load in correspondence to said peak current limit setting, said circuit further providing compensation for decreasing values of said Darlington transistor gain values occurring with operation at high peak values of collector current,
• a fourth circuit means wherein said Darlington transistor, in an operating region beyond its continuous current rating at high peak values of collector current and wherein transistor ratings are defined by the safe-operating-area (SOA) curves for said transistor, said SOA curve utilized for combinational values of allowable peak current and surge duration such that the SOA curve for said transistor is coordinated with the decay characteristics of the surge condition,
• a fifth circuit means wherein said inrush current limiting circuitry is connected across the DC side of a single phase full wave rectifier diode bridge assembly for providing operation of AC circuits and non-polarity sensitive installation in DC circuits, said rectifier bridge having size and current rating reductions with current levels to match the low continuous current flow to the load, thus allowing matching of the surge current ratings of the diodes with the peak current limit value and the surge duration of the load,
• a sixth circuit means wherein varistors are connected across said two power line terminals of said inrush current limiter and having voltage levels equal to the maximum BVceo rating of said Darlington transistor for providing said limiter with capability of limiting the surge to load in the event of very high voltage transients, said varistors serve to divert the current to the load away from said Darlington transistor and diode assembly prior to a voltage breakdown of said transistor, said varistor network consisting of at least two varistors in series for distributing the power handling requirements for the high current values encountered and to minimize size and weight of the entire limiter circuit, said varistor breakover voltage values, Darlington transistor Vceo rating, peak current limit setting, and surge decay characteristics being collectively established for providing the maximum peak current limiting capability that is commensurate with the SOA ratings of the transistor,
• a seventh circuit means wherein the drive resistor for the Darlington transistor provides for adequate base drive for said transistor under high peak current conditions during times when said transistor is in the active operating region and the collector-base voltage is relatively high, and to maintain adequate base drive to said transistor when required to remain fully conductive (saturated) during times when current surges do not exist, said resistor having low values for providing transistor automation during times of no surge conditions for minimizing standby power dissipation, and having relatively high values during times of a surge condition for providing a controllable current value not to exceed capabilities of current control transistor,
• an eighth circuit means wherein adjustments of peak current limit settings and component selection for the limiter are such that the peak current limit value is approximately 2 to 10 times the normal current value drawn by the load to provide sufficient surge current to the load to allow proper load function, and
• a ninth circuit means wherein all power handling components have their power and current ratings selected based on the continuous current and average power requirement rather than on the amplitude and duration of the surge condition for minimizing size, weight, and heat dissipation requirements by a factor of up to 10 times since the peak-current limit value will be set up to 10 times higher than the normal continuous current.