Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
1999-09-20
2002-04-09
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S017000
Reexamination Certificate
active
06370050
ABSTRACT:
TECHNICAL FIELD
This invention relates to electric power converters and, more particularly, relates to an isolated and soft-switched power converter having two resonant tank circuits coupled back-to-back through an isolation transformer. The invention also relates to electric power storage and generation systems and electric vehicles using the converter.
BACKGROUND OF THE INVENTION
Soft switching techniques have been used in power converters to reduce switching losses and alleviate electromagnetic interference (EMI). For example, soft switching techniques can be particularly important in electric power applications involving DC/AC and DC/DC power conversion and relatively large power requirements, such as electric vehicle, hybrid electric vehicle, and electric power storage and generation systems. In electric vehicle applications, for example, the power controller for the electric motor uses frequency control, phase control, pulse control, and other types of power supply manipulation to smoothly control the power output of the electric motor. This type of power supply manipulation requires a high rate of switching in the power controller to generate the precisely-controlled power supply waveforms to drive the electric motor.
If this switching occurs in the power controller when current is flowing through or voltage is applied across the switching element (i.e., “hard switching”), the resulting switching losses in the power controller decrease the efficiency of the vehicle's power plant, the high voltage rise rate (dV/dt) may damage the motor, and the resulting voltage change rate can cause additional losses and overheating in the electric motor, as well as interfering with the operation of other electrical systems. Switching with zero current flowing through or zero voltage across the switching element, which is known as “soft switching,” alleviates these problems. Obtaining soft-switching can be difficult, however, because it requires repeatedly driving the voltage across or forcing the current through the switching element to zero, and then holding the voltage or current at this zero level long enough for the switching element to physically switch. Because an AC voltage wave periodically passes through a zero-voltage state, but does not remain at the zero-voltage state for an extended period, the switching element may not have enough time to physically switch while the voltage transitions through the zero-voltage state.
The need to switch during zero-voltage and/or zero-current states or transitions sets the stage for the basic soft switching design objectives. The voltage across each switching element should repeatedly obtain zero-voltage and/or zero-current periods long enough for the element to soft switch, or looked at from the other direction, the switching elements must be capable of switching fast enough to soft switch during the zero-voltage and/or zero-current transitions. As another design concern, the cost of the switching elements increase with increasing power transmission capability and switching speeds.
Electric vehicle or hybrid electric vehicle designs also have other important objectives, including physical and electrical isolation of high-voltage circuits and components (e.g., the electric drive system) from the low-voltage system. Another important objective is bidirectional operation of the power converter so that the automotive battery, which is used to start the vehicle, can be recharged during vehicle operation. Electric or hybrid electric vehicles typically use a relatively low-voltage DC automotive electrical system (e.g., about 12 Volts or 36 Volts), and a relatively high-voltage DC electrical system for the electric motor that drives the vehicle (e.g., about 300 Volts). Thus, the typical electric or hybrid electric vehicle application calls for a bidirectional DC/DC power converter with high-voltage isolation. Certain load-side electric generation applications, such as battery storage peak shaving, also require bidirectional DC/DC power conversion.
Because DC voltage cannot be increased without intermediate conversion to an alternating voltage, and the underlying automotive battery operates at a relatively low DC voltage, the DC/DC power converter for an electric or hybrid electric vehicle implements the steps of DC/AC conversion at low voltage, AC/AC voltage boost through an AC transformer, and AC/DC conversion at high-voltage. Accordingly, the DC/AC and AC/DC conversion steps involve switching in the inversion and rectification processes which, if uncontrolled, can generate large switching losses and EMI. In addition, subsequent inversion of the high-voltage DC output, typically in the power controller or inverter for the electric motor, can generate large switching losses and EMI.
Conventional bidirectional DC/DC power converters developed for hybrid electric vehicle and other applications have a number of disadvantages. Specifically, two separate full-bridge converters are needed to utilize the full DC voltage in the power conversion. That is, a first full-bridge converter is needed for the DC/AC conversion, and a second full-bridge converter is needed for the subsequent AC/DC conversion. This requires an excessive number of components in the duplicate full-bridge converters. In addition, transformer leakage can result in voltage surges, power losses, and control difficulties.
Furthermore, some conventional DC/DC power converter designs require a clamping or snubber circuit to provide a DC current path through an inductor for limiting voltage surge during switching. These circuit designs also experience hard-switching related problems, such as high EMI and high voltage rise rates, which require sophisticated filters and shielding. Moreover, for traditional DC/DC/AC power conversion, these circuit designs require two separate stages (DC/DC and DC/AC power conversion), which further duplicate parts without synergy.
In general, conventional DC/DC and DC/DC/AC power converters typically provide an output voltage that is equal to or less than the voltage available from the internal DC power supply. For safety considerations, manufacturers of electric drive systems in products for general distribution, such as electric vehicles, would prefer to have power distribution busses operate at no more than 50 Volts. But lower voltage motors are inherently larger and heavier than higher voltage motors delivering equivalent power, because lower voltage motors must be made with copper wire that is large enough to safely handle the higher current required for equivalent power delivery at a lower voltage.
These conflicting design objectives create serious design constraints and unresolved problems for the designers of power converters. Thus, there is a need in the art for improved DC/DC and DC/DC/AC converters for many applications, including hybrid electric vehicles and electric utility applications, such as remote load-side electric power generation. In particular, there is a need for a soft switched power converter that isolates high-voltage components, uses conventional switching devices, exhibits stable voltage control, alleviates EMI production, and avoids unnecessary duplication of components.
SUMMARY OF THE INVENTION
The present invention meets the needs described above in an isolated and soft-switched power converter for DC/DC and DC/DC/AC power conversion. The power converter utilizes the full DC voltage of internal voltage sources in both the positive and negative polarities for intermediate AC/AC transformation, with a minimum number of switching devices and other electric components. The power converter also repeatedly maintains zero-voltage periods across its switching elements to allow low-stress soft switching by conventional switching elements. That is, the power converter provides extended zero-voltage switching periods to allow soft switching by conventional switching devices, rather than relying on very fast switching during very short or transient zero-voltage periods. These attributes provide the advantages of economic construction an
Adams Donald Joe
Peng Fang Zheng
Kilpatrick & Stockton
Patel Rajnikant B.
UT-Batelle, LLC
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