In spacecraft intended for long lifetimes, electrical power is supplied by solar panels and stored in batteries. The electrical power is distributed by way of a power bus to electricity-using communications, attitude control, sensing and/or other systems as required from time to time. The solar panels may at any particular time produce more or less electrical power than is being consumed, with the difference being supplied to or from storage batteries. In order to accommodate the varying voltages and impedance characteristics of the solar panels and the batteries during use, to supply a controlled voltage on the spacecraft power bus, and to power the spacecraft during eclipse, bidirectional power conversion between the bus and the batteries is required. A power converter efficiently couples power among the various electrical devices. High efficiency is ordinarily accomplished by operation in one of the well-known switched modes, as opposed to a linear mode. In a switched operating mode, the power converter couples power by means of current pulses, which may be integrated at the end being supplied with power to produce a direct voltage. In order to achieve high reliability in a spacecraft context, and to reduce the total power which each power converter must handle, a number of power converters, such as ten converters, may be paralleled. All the paralleled converters should operate at the same switching frequency.
When power converters are paralleled, electromagnetic noise is reduced, and ripple of the integrated output voltage is also reduced, if the power converters are synchronized so that the current pulses which they produce occur sequentially over each switching period. For this to occur, the power converters must produce their current pulses evenly phased over each switching period in a sequential manner, i.e. they must be synchronized, and they must be synchronized in such a fashion that they produce their current pulses in a phase sequence of 360.degree./N, where N is the number of such converters. If there are ten paralleled converters, for example, they must produce their pulses at 36.degree. phase increments, corresponding to 0.degree., 36.degree., 72.degree., 108.degree., 144.degree., 180.degree., 216.degree., 252.degree., 288.degree., 324.degree., 0.degree.. Such a sequence may be generated by an oscillator driving a phase shifter array adjusted for the appropriate phase shifts at the oscillator frequency.
However, in a high-reliability context, it is desirable to be able to drop a malfunctioning converter from use, which reduces the number of operational converters. If the original phase sequence is followed, there will be a "hole" in the recurrent sequence of current pulses, which results in increased EMI and voltage ripple.