This invention relates to converters, and more particularly to a nondissipative circuit for equalized load sharing of power transistors.
The present approach for securing reliability of operation throughout the life of a spacecraft mission for the electrical power systems is to use small converter modules operating in parallel to achieve the required system rating. As is pointed out in a copending application Ser. No. 684,809 (now abandoned), titled METHOD AND APPARATUS FOR AUTOMATIC LOAD SHARING AMONG PARALLELED CONVERTERS, by Col. W. T. McLyman and G. W. Wester, new systems can be tailored to new requirements by adding or removing modules without the need for developing new circuit designs and incurring the attendant development costs. However, as also pointed out in that application, parallel operation often results in very large differences in load sharing among the modules and unbalanced operation of the power transistors of each individual module. This occurs because of the low impedance characteristics of each converter module, and the significant effect of slight parameter differences in components, such as the power transistors, power transformers, etc. Such inequalities and unbalanced operation can cause either module or component level overstresses which may produce premature failure.
The aforesaid copending application describes a method for equalizing load sharing in duty-ratio operated converter modules by appropriately adjusting the duty ratio of each module as a function of the differential load currents provided by each. The scheme described therein takes advantage of the fact that since the converter modules are driven by pulse width modulators, adjusting the pulse width of each individual converter modulator changes the duty cycle to effect the desired sharing of the load by all the converter modules. However, since inverter operation must always be with a fixed duty cycle of 50%, that technique cannot be used in systems using inverter modules. Also that technique does not achieve balanced operation of the push-pull transistors of the power output stage. The invention disclosed herein overcomes those limitations in a circuit for equalizing load sharing of power transistors in a converter operated as a DC-AC inverter or a DC-DC converter.
As will become apparent from the following description, a converter embodying the present invention may be operated to convert direct current into alternating current, i.e., operated as an inverter, or to convert direct current into direct current. Thus, although the generic term "converter" has come to be used almost exclusively for DC-DC converters, and a special term "inverter" has come into general use for DC-AC converters, the generic term "converter" will be employed hereinafter to encompass both DC-DC and DC-AC converters. However, because the embodiment disclosed to illustrate the concept of the invention in a specific embodiment is a DC-AC converter, it will be referred to by the special term "inverter."