Regulator to Control Current Sharing among Parallel-Connected Power Transistors

A current regulator is described that controls the peak current of a power transistor driving an inductive load, such as magnet coils. Presently, the current pulse has a 0.1- to I-ms adjustable duration with duty cycle to 5%. Ultimately, the pulse length must be ~ 7 ms with 50% duty cycle. The peak current nominally is 200 A, as set by a reference pulse. Several transistors, each controlled by an individual regulator, are connected in parallel to provide the total peak load current. The regulators compensate for differences between the power transistors and assure equal current sharing among the collectors. The regulator comprises an 8-bit multiplying digital-to-analog (D/A) converter that drives the base of the power transistor, an 8-bit up-down counter, an analog differencing circuit, and a comparator. For control purposes, a current-sensing resistor in the transistor's collector produces the second input to the differencing circuit. Regulation is accomplished over a collector current range of 200 ± 50 A per stage.