Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Superconductive device
Reexamination Certificate
1996-08-16
2001-01-09
Zweizig, Jeffrey (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Superconductive device
C327S110000
Reexamination Certificate
active
06172550
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to cryogenically-cooled high voltage, high power electronic switching circuitry.
Efforts in utilizing Metal Oxide Semiconductor Field Effect Transistor (MOSFET) switching devices in cryogenically-cooled high power conversion circuits are ongoing. It has been appreciated that these particular devices, when cryogenically cooled, have unique advantages not found in other conventional high power semiconductor switching devices, including IGBTs, MCTs, GTOs and bipolar transistors (see U.S. Pat. No. 5,126,830, Mueller et al. and U.S. Pat. No. 5,347,168, Russo). For example, power MOSFET's, when operated at a temperature of 77 K, exhibit a reduction of the on-resistance of the MOSFET's typically by a factor of 15 or more, and often by as much as a factor of 30, resulting in a significant decrease of the conduction losses within the devices.
One particularly advantageous use of cryogenically cooled electronics is in high power, switching power supplies. To understand the advantages in more detail, however, a brief discussion of the limitations of such power supplies operating at room temperature needs to be considered. Switch mode amplifiers, regulated power supplies, and frequency converters became a reality with the introduction of high speed power silicon devices. An important advantage of these switch mode applications is that, at least for ideal devices, the only losses involved are the saturation losses of the power devices in the forward direction known as the conduction losses. These conduction losses are very low compared to the losses sustained in linear regulation or amplification devices, thereby resulting in a considerable reduction in the physical size of regulated power supplies and an increase of operating efficiency. There are, however, other losses associated with switch mode converters including the commutation losses associated with the switching device, switching losses, and parasitic discharge losses. The commutation, parasitic discharge, and switching losses are all proportional to the switching frequency. Moreover, capacitive discharge losses may dominate, particularly at high operating voltages, because they are proportional to the square of the applied voltages.
The relationship between switching loss and frequency is important since the typical method used to obtain size reductions in switch mode power supplies is to operate at high switching frequencies which permits replacing conventional power frequency components with significantly smaller filter and active components. These smaller components operate at 300 to 30,000 times the frequency of the older power supplies. At these increased frequencies, the switching losses in the power devices often dominate the overall loss of the power supply.
A switch mode converter may include a commutating diode whose forward voltage drop generates additional losses, referred to as commutating diode losses. The commutating diodes provide a path for the inductive component of load current to flow when the load current is interrupted by turn-off of a switching device. With certain unidirectional high power devices (e.g., IGBTs, GTOs, thyristor diodes) the commutating diodes are added externally.
Soft-Switching techniques which generate so-called Zero-Voltage Switching (ZVS) or Zero Current Switching (ZCS) conditions have been used to reduce the switching losses of switching devices. The switching losses are reduced because the switching of the active device occurs at a point in time when, as the names imply, either the voltage or the current is zero, thus providing a zero power dissipation condition. These techniques are, in general, “waveshaping” circuits having a suitable inductor which resonates or “quasiresonates” with the output capacitances of the active devices in a manner that the ZVS or ZCS conditions are obtained. In the case of capacitive discharge losses, the resonant techniques assure that the capacitances are discharged, not through the active switching device, but through the supply or the load.
However, the resonant circuits used to provide conventional soft-switching generate large over-voltages and over-currents. The switching devices are subjected to these over-voltages and over-currents and, therefore, must be rated to withstand such stresses, a particular problem in high power applications.
SUMMARY OF THE INVENTION
The invention features reducing capacitive discharge and diode commutation losses in switch-mode electronic power conversion circuitry having cryogenically cooled MOSFET power switching devices. The losses are reduced by controlling the period between the switching-on of a MOSFET device and switching-off of another MOSFET device, the pair of MOSFET's connected in a series connected (half-bridge) switch-mode configuration to operate during different portions of a switching cycle. This period between switching one device off and switching the other on, often referred to as “deadtime,” is generally required to prevent the possibility of opposing MOSFET devices of the switch-mode circuitry being closed simultaneously. This condition, known as “shoot through,” can be fatal to the MOSFET devices, and even if not fatal constitutes an unacceptably large increase in power dissipation in high efficiency conversion circuitry. In accordance with the invention, when one of the MOSFET devices of the circuitry (e.g., a half bridge circuit) is switched off, the opposing MOSFET, after a short deadtime period, is switched-on so that current, normally flowing through the commutating intrinsic drain-to source diode of the opposing MOSFET, flows through the switched-on MOSFET. Thus, the opposing MOSFET serves as the commutating device rather than its intrinsic drain-source diode whose losses are substantially larger at cryogenic temperatures than those of the MOSFET itself.
Moreover, the deadtime period can be selected or dynamically controlled to allow the parasitic capacitances (i.e., drain-source capacitance) associated with the MOSFET devices to be discharged into the load (which generally includes a low pass filter) prior to the MOSFET device being switched on. In high voltage and high power circuitry, this has particular advantage because the MOSFETs are utilized in large parallel arrays exhibiting large values of parasitic capacitance. Thus, the MOSFET is effectively turned on at zero voltage, thereby reducing energy normally discharged through and dissipated by the MOSFET to near zero magnitude. It will furthermore be appreciated that the MOSFET serving as a commutating diode is also turned off at essentially zero voltage.
In one aspect of the invention, a cryogenically cooled switching circuit, having terminals coupled to a load which receives power from the circuit, includes first and second switching devices (arranged in a half-bridge switch mode configuration), each device intrinsically comprising a drain-to-source commutating diode capable of conveying an inductive current component from the load. The first switching device is turned-on during a commutating interval in response to a first control signal following turn-off of the second switching device to convey an inductive current component from the load through the first switching device rather than through the intrinsic drain-to-source commutating diode associated with the first switching device.
In a system utilizing a switch mode configuration as described above, the system may further include a refrigeration unit for cryogenically cooling the switching devices and a controller for both conveying the control signals to the switching devices and establishing the deadtime interval itself.
Particular embodiments of the invention may include one or more of the following features. The switch mode configuration may be a half-bridge circuit or combinations thereof (e.g., a full-bridge circuit). The first and second control signals are pulse width modulated control signals having a duty cycle between 1 percent and 99 percent. The controller may include means for regulating the deadtime
Gold Calman
Mueller Otward M.
American Superconducting Corporation
Fish & Richardson P.C.
Zweizig Jeffrey
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