Toward 99% efficiency for transistor inverters

Developments in power switching devices have been made toward reducing the conduction drop and improving the switching characteristics, in order to meet the demanding requirements for high efficiency. On the other hand, many efforts have been made to reduce the switching losses. Since bipolar junction transistors (BJTs) have the lowest on-state drops among those existing semiconductor switches with turn-off capability, BJTs are most suitable for realizing inverters of high efficiency at power levels under a few hundred kVA. However in the Darlington structure, there is higher on-state drop due to the driver transistors. It is therefore important to reduce the on-state drop in order to realize high efficiency with the Darlington transistors. In this article, two methods are proposed to greatly reduce their on-state drops. Firstly, a base drive circuit is presented that provides a direct drive to the output transistor of a Darlington transistor rather than through its driver transistors. To eliminate the disadvantage related to the direct base drive, an on-state drop cancellation method is then proposed, in which a DC voltage source is inserted in series with the driver transistor of a Darlington transistor to offset its on-state drop.