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Pre-charge circuit utilizing non-linear firing angle control

A new capacitor pre-charge circuit is described that limits both peak DC link and peak AC input line current to any desired magnitude, thereby providing for optimum pre charge control and robust operation. Pre-charge control is optimum since the link capacitor is charged without voltage overshoot in the shortest possible time with maximum allowable power converter current. The new control is robust in applications containing partially precharged bus capacitors, ground and bus faults, input AC line transients or momentary AC line loss. The precharge circuit employs a microprocessor which implements a novel nonlinear discontinuous current control scheme without the need for current sensors. The pre-charge circuit is well suited for use with an AC to DC power converter having a link inductor and a DC bus capacitor which requires pre-charging before full power is applied to the power converter. The paper describes the problem with existing pre-charge circuits while emphasizing the advantages of the new control. Governing equations and a control block diagram of the new nonlinear control algorithm is given along with implementation suggestions.