1. Field of the Invention
The present invention relates generally to integrated circuits, and more specifically, the present invention relates to integrated circuits that switch high voltages in power converters.
2. Background Information
It is a common practice in the design of integrated circuits to include functions and modes of operation that are not used when the integrated circuit is in its intended application. These alternative modes of operation may be used instead for a variety of purposes in the production of the integrated circuits and in the maintenance of equipment that uses them. For example, it may be desirable to operate the integrated circuit at special frequencies, voltages or currents that reduce the time to test the device or that increase the accuracy and reliability of testing. The manufacturing process of some integrated circuits includes an adjustment or trimming of internal parameters after the integrated circuit is assembled as part of the final test. Final adjustment may be accomplished by selecting a special trimming mode of the integrated circuit. While in the trimming mode, parameters may be adjusted by appropriate manipulation of signals at the terminals of the integrated circuit. The trimming mode may then be disabled after the adjustments are completed.
An alternative mode of operation could also be an alternative normal mode of operation, as distinguished from a test mode or a trimming mode. A normal mode of operation is one in which the integrated circuit operates in its intended application. An alternative mode of operation that is also a normal mode of operation might relate to the frequency of a switching power supply. For example, one normal mode of operation could use a fixed frequency whereas another normal mode of operation could use a variable frequency. Some integrated circuits are field programmable, such that the user in each specific application may select the particular mode of operation.
The technology of most integrated circuits limits the voltage on the terminals to the maximum power supply voltage for the device, which is typically about 40 volts, but can vary depending on the manufacturing process and the feature size of the devices that comprise the integrated circuit. The voltage is measured with respect to a ground reference terminal that is usually a power supply terminal. The ground reference terminal is typically at the most negative electrical potential of the integrated circuit. In the subsequent discussion, such voltages that are usually less than about 40 volts are referred to as using ordinary low voltages. Terminals that can withstand a maximum voltage less than about 40 volts are referred to as low voltage terminals. Terminals that can withstand a maximum voltage substantially greater than about 40 volts are referred to as high voltage terminals.
Some terminals of the integrated circuit may be limited to a substantially lower voltage than the maximum power supply voltage. In the integrated circuits that are limited to ordinary low voltages, virtually any terminal of the device can be used to select alternative modes of operation without difficulty because only low voltage circuits are required to monitor the state of any terminal and to respond to any voltages that may be applied to the terminal.
Not all integrated circuits operate from low voltages. In the category of high voltage integrated circuits at least one terminal is exposed to high voltage in the intended application. These devices typically integrate ordinary low voltage circuitry with a high voltage power semiconductor. The high voltage terminal operates at voltages typically greater than 100 volts, which is substantially greater than the other terminals will tolerate without damage. A common application for high voltage integrated circuits is in switching power supplies. For applications in off-line alternating current (AC)-direct current (DC) power supplies, the voltage at the high voltage terminal can be in the neighborhood of 700 volts.
It will be recognized by one skilled in the art that the distinction between high voltage terminals and low voltage terminals is fundamentally one of relative voltage and not absolute voltage. For example, an integrated circuit technology may use materials or geometries such that ordinary low voltages are less than 1 volt, and any voltage greater than 10 volts is considered to be high voltage.
An objective in the design of integrated circuits is to realize the desired functions at the lowest cost. Two factors that influence the cost of the integrated circuit are the amount of semiconductor material and the number of terminals. Since high voltage devices are larger and therefore use more semiconductor material than low voltage devices, it is advantageous to minimize the number of circuits that are exposed to high voltage in the integrated circuit. It is also advantageous to use each terminal of the integrated circuit for as many functions as possible to minimize the number of terminals.