1. Field of the Invention
The present invention generally relates to power conversion and, more particularly, to a DC to AC power converter of the switching type with high efficiency and fast response time, i.e., broad band width.
2. Description of the Prior Art
If one were to enumerate the desired properties of the "ideal" or "perfect" DC to AC power converter, the list undoubtedly would be headed by high efficiency and fast response time, i.e., broad band width. Other properties that would be included are high power capacity, minimum maintenance, use of relatively simple, easily available components, compactness, and last but not least, inexpensiveness. None of the known converters have all or even most of the above-recited properties.
Converters of the linear type have very fast response time (broad band width) so that output signal fidelity is maintained for varying load conditions. However, linear type converters are quite inefficient. A considerable amount of power is wasted in the form of heat, which needs to be removed. Consequently, linear converters are relatively large and are limited to modest power levels, e.g., up to 50 KW.
Converters of the switching type, to which the present invention is related, also known and hereafter referred to as switching converters, are very efficient. However, their response time is usually much lower than desired. As is known by those familiar with the art, a switching converter includes a low pass filter at its output in order to filter out the high switching frequency current component, generally known as the ripple current. The filter, which typically consists of multiple L-C sections, is designed to inhibit most, if not all, of the switching energy from passing to the output load. As is known, the higher the reduction of the switching energy to the load, the slower is the response time to load requirements.
Theoretically, this problem may be alleviated by increasing the switching frequently significantly. However, with the present state of the art, an upper limit of such switching frequency is dictated. At present, the switching rate of reliable switching devices is typically on the order of less than 100 kHz. An upper switching frequency limit of 50-60 kHz is typically the present state of the art upper switching frequency limit. This is due to the fact that the types of solid state switching devices which are presently available and used in switching converters have relatively moderate rise time and even longer fall times, on the order of 200 nsec and more. Since in a switching converter one of these switchers has to be turned On only after the other is totally Off, due to the relatively long rise and fall times, for reliable operation, these switchers cannot be switched at anywhere near the optimal desired switching rates; i.e, several hundred kHz. Thus, their switching rate is, as herebefore stated, limited to about 60 kHz or less. Even if switchers with much shorter rise and fall times were available for possible use in switching converters, one has to consider the drop in efficiency that may occur due to the significantly higher switch rates. As is known, the loss of power in a switcher is directly proportional to its switching rate. Consequently, the higher the switching rate, the greater the power loss; i.e., lower efficiency.
It is for the above reasons that present-day switching converters incorporate switchers which are operated at only 50-60 kHz or less. Furthermore, to inhibit switch energy from passing to the load, typically, 80 dB or more of filtering is provided which results in relatively narrow band width, i.e., long response time, on the order of 500 usec or more, whereas one-tenth of such response time is often desired.