Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2001-10-03
2003-07-22
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S089000
Reexamination Certificate
active
06597592
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to controlled and synchronous rectifiers and, more specifically, to the rapid, efficient and economical turn-off of a bipolar junction transistor (BJT) used as a controlled or synchronous rectifier.
BACKGROUND OF THE INVENTION
In the text that follows prior art related to synchronous and controlled rectifiers and related uses of BJTs is discussed. It should be recognized that BJTs have been seldom used as controlled rectifiers and for this reason a good part of the following prior art discussion is directed towards synchronous rectifiers and related uses of BJTs.
DC to DC switching mode power converters are typically used to stabilize or isolate a power supply signal from upstream irregularities (i.e., voltage/power surges, momentary power outages, etc.). Various transformer and non-transformer based power converters are known in the art. These power converters typically employ a rectifying device to convert either a transformed AC signal, a chopped DC or a similar signal (depending on the power converter arrangement) into a DC output signal. This output DC signal constitutes a relatively stable power supply signal. Depending on the range of voltage (and current) for which the power converter is designed, the power converter may be used, for example, in power supplies for personal electronic devices, laptop or personal computers, engineering workstations and Internet servers. While the present invention is particularly concerned with electronic/digital logic circuits, it should be recognized that the teaching of the present invention are applicable to rectifying device operation in any voltage/current range and for any purpose.
For many years the standard power supply voltage level for electronic logic circuits was 5V. Recently, this voltage level has dropped in many instances to 3.3V and 2.5V, and there are plans within the industry to further reduce this voltage level. As this voltage level drops, however, the forward voltage drop of the rectifying device becomes the dominant source of power loss and inefficiency. For example, a Schottky diode is typically used when a low voltage drop is desired, and a typical Schottky diode has a 500 mV forward voltage drop. This limits the theoretical efficiency of a DC to DC power converter to 80% at two volts output (before other power conversion losses are taken into account). This efficiency limit further drops to less than 67% at one volt output, and 50% at 500 mV output. These efficiency limits are deemed unacceptable.
In addition to concerns about forward voltage drop and other power inefficiencies, power converters and rectifying devices therein are expected to have high power densities. This mandates a higher switching frequency such that less energy is processed in each switching cycle, which in turn permits smaller component sizes. Switching frequencies have risen from 5 to 20 Khz thirty years ago (where the push was to get above the audible range) up to 100 KHz to 1 MHz at present. Thus, technology that does not support rapid switching is not preferred for most rectification applications.
With respect to known rectifying devices, these include rectifying diodes (PN and Schottky junction in Si, GaAs, etc.) and rectifying transistors (bipolar and field effect). The forward voltage drop of a rectifying diode can be reduced by design, but only to around 300 mV to 200 mV before a point of diminishing returns is reached where increasing reverse leakage current losses outweigh the decreasing conduction losses. This is due to an inherent physical limit of rectifying diodes and does not depend on semiconductor material or whether the construction is that of a conventional P-N junction diode or a Schottky junction diode. For this reason, amongst others, diodes are not desirable as rectifying devices for low voltage level applications.
Rectifying transistors in which transistor driving is in “synchronism” with the direction of current flow across the transistor have increased in popularity due to their favorable forward voltage drops relative to diodes. Typically, the synchronous rectifying transistor is driven “on” to provide a low forward voltage drop when current flow across the rectifying transistor is in a designated forward direction, and is driven “off” to block conduction when current flow across the rectifying transistor would be in the opposite direction.
Both the Bipolar Junction Transistor (BJT) and the Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) have been used as a synchronous rectifier transistor, also termed a “synchronous rectifier” (SR). Although the BJT has a longer history of use as an SR, the MOSFET is used almost exclusively at present due to its fast switching speed and perceived ease of driving. BJTs are little used as SRs at present due to slow switching speeds in general, and a slow turn-off in particular.
BJTs are even less used, as alluded to above, as controlled rectifiers (CRs). A difference between a SR and a CR is that a SR prevents conduction when a voltage of negative polarity is applied across the rectifier, whereas a CR can prevent conduction when a voltage of either polarity is applied across the rectifier. It should be recognized that a device configured to perform CR is capable of performing SR if it is turned off only during application of a negative polarity across the rectifier, but the converse is not true. Accordingly, when the term “controlled rectifier” or “CR” is used herein, it generally includes the function of a synchronous rectifier unless such function is implicitly or explicitly excluded.
An advantage of controlled rectification over synchronous rectification in switching mode power converters is that the average output voltage or current of an isolated power converter can be regulated from the output side by modulation of the CR conduction duty cycle. With multiple outputs, the voltage or current of each output can be independently controlled.
With respect to the use of MOSFETS as CRs, the construction of conventional power MOSFETs prevents their use as a true CR, and the technique of controlled rectification is little known in the field of switching mode power converters. While FETs are the dominant SR device at present, FETs cannot block a negative voltage and thus are not truly capable of CR, despite terminology to the contrary in some prior art patents discussed below. Two back-to-back FETs can block voltage in both directions and have been used occasionally where the ability to conduct current in either direction is useful, typically when the input or output is AC. Their double voltage drop, however, has prevented any attempted use as a CR.
The present invention recognizes that the BJT is a conductivity modulated device whereas the MOSFET is not. As a result of this distinction, the BJT can achieve a lower forward voltage drop for a given forward current density and reverse voltage blocking capability. A major technical cost of the lower voltage drop, however, is the presence of a conductivity modulating charge stored during the forward conduction which must be removed before the BJT can sustain a forward or reverse voltage without high leakage currents. Removal of this charge entails a turn-off “storage time” that results in an inherently slower turn-off in BJTs than is achievable with MOSFETs which do not have such a stored charge. The lower conduction voltage of the BJT could be used to advantage at lower output voltages, however, if the BJT turn-off speed could be improved (in a cost-effective manner) which is a purpose of the present invention.
Various prior art circuits for turning off a BJT are discussed below after the following definitions and notes. These prior art arrangements include those that turn-off a BJT used as a conventional transistor and those that turn-off a BJT used as a synchronous rectifier. There is also one arrangement of a BJT used as a controlled rectifier.
Definitions and Notes
In the following discussion, and for the remainder of this document, the following definitions and subsequen
Adamson Steven J.
Patel Rajnikant B.
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