Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
2001-10-03
2003-07-22
Patel, Rajnikant B. (Department: 2838)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C363S127000
Reexamination Certificate
active
06597210
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to controlled rectifiers and, more specifically, to the efficient, economical and optimal driving and control of a bipolar junction transistor (BJT) used as a controlled rectifier (CR).
BACKGROUND OF THE INVENTION
To facilitate a better understanding of the present invention, the following information on the development of synchronous and controlled rectification and the operation of PN junction and bipolar semiconductor devices is presented.
DC to DC 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 teachings 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 conduction 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 semiconductor 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.
A controlled rectifier (CR) is typically achieved when a synchronous rectifier may be driven into an off state regardless of the direction of potential current flow across the rectifying transistor. Thus the “control” of a CR puts the rectifier into one of two states: a “normal” state where the device behaves as a synchronous (or conventional) rectifier, always conducting when current flow would be in a designated forward direction, and always blocking current flow in the reverse or opposite direction, and a second “forced off” state where the rectifier blocks current flow in either direction. Turning a CR off when a rectifier would normally conduct allows the average output voltage (or current) to be reduced, or controlled, by the CR. If the CR is allowed to conduct for only 50% of the time that a conventional or SR would conduct, e.g., the output voltage is reduced by half. The additional control achieved by replacing conventional or synchronous rectifiers with controlled rectifiers can be used to significant advantage in several applications.
In a DC to DC power converter with transformer isolation, control of output voltage is achieved by adjusting the switching times of the input side transistor switch or switches, which requires that the isolated output voltage information be sensed and fed back to the input side. Controlled rectification allows the input transistors to be switched at fixed times, while the conduction times of the CR on the output side are adjusted to regulate the output voltage, avoiding the need to send control information to the isolated input, which can reduce costs and/or improve performance.
Further advantages accrue with two or more output voltages from the same DC to DC converter. Conventional or SR allows only one output voltage to be fully regulated and current limited by controlling the input side transistor switching times; other outputs are at best “semi-regulated”, and often require post-regulation by additional circuitry which increases costs and reduces efficiency. Furthermore, a separate transformer winding is required for each distinct output voltage. Controlled rectification allows each output voltage to be completely and independently controlled with maximum efficiency, and a single transformer output winding can be used for several similar output voltages, reducing costs. For example, a single winding might be used to simultaneously provide 5V, 3.3V and 2.5V outputs with individual CR.
Hybrid approaches are also possible. In the previous example, the 5V output may be regulated by control of the input side transistors, while the 3.3V and 2.5V outputs are derived from the same transformer output winding by CR.
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 at present due to slow switching speeds in general, and a slow turn-off in particular. However, the BJT has the ability to block current flow in either direction when in an off state, and thus may also be used as a controlled rectifier transistor, also termed a “controlled rectifier” (CR). The conventional power MOSFET can only block current flow in one direction, and thus cannot be used as a true 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. Major technical costs of the lower v
Adamson Stephen J.
Patel Rajnikant B.
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