Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2001-10-03
2003-05-13
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S089000, C361S101000
Reexamination Certificate
active
06563725
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to synchronous rectifiers and, more specifically, to the efficient, economical and optimal driving and control of a bipolar junction transistor (BJT) used as a synchronous rectifier (SR).
BACKGROUND OF THE INVENTION
To facilitate a better understanding of the present invention, the following information on the development of synchronous rectification and the operation of PN junction, field effect 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 decreases to less than 67% at one volt output, and 5% 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.
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.
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 voltage drop, however, are associated with the requirement to inject, maintain and remove the conductivity modulating stored charge. Nonetheless, the lower conduction voltage of the SR BJT could be used to advantage at lower output voltages, if the BJT switching speed (e.g., turn-off and turn-on) could be improved in a cost-effective and efficient manner (which as discussed below is a purpose, amongst others, of the present invention).
Though MOSFETs and BJTs have certain similarities in design and construction, they also have substantial differences that impact their behavior and the type of circuits that are suitable for driving them. This should be better understood after review of the following discussion of construction and operation of PN-junction, field effect and bipolar devices.
P-N Diode Construction and Operation
Semiconductor materials are nearly insulating in the pure state, but they may be doped with impurities to create mobile electric charges and improve their conductivity markedly. These impurities may be either “N” type (for “negative”) which produces free electrons in the semiconductor material or “P” type (for “positive”) which produces holes in the semiconductor material. The negatively charged free electrons are mobile, and will flow towards a positive charge and away from a negative charge. In P-type semiconductors, the “positive” charge carrier is the local deficiency of an electron, often referred to as a “hole,” and holes are also considered to be mobile (such as an air bubble in water). Holes will flow in a direction opposite to that of electrons in the presence of an applied electric field. When no electric field is applied, mobile electrons and holes diffuse with no net flow in any direction.
FIG. 1A
illustrates a representative P-N junction diode
1
formed by adjacent regions of equally doped P-type semiconductor
2
and N-type semiconductor
3
. Conductive contacts for an anode (A)
4
and a cathode (K)
5
are made to regions
2
and
3
, respectively. Diffusion of electrons and holes near the junction causes some to meet, recombine and neutralize each other; conceptually, the electrons fall into the holes. Fixed oppositely charged atoms are left behind, which produce a voltage field opposing further electron and hole flow towards the junction. A small region near the junction, often termed the “space charge layer,” is left depleted of mobile charge carriers and the diode will carry essentially no current if small voltages are applied across the device.
FIG. 1B
illustrates the P-N junction of
FIG. 1A
to which is applied a reverse biased voltage by voltage source
6
. The negative terminal of the voltage source is connected to anode
4
and the positive terminal to cathode
5
. This causes holes and electrons to move towards the oppositely charged electrodes and away from the junction, creating a larger depletion region. Only a small “leakage” current flows across this region and it is due to thermal (or radiation) generated
Adamson Steven J.
Patel Rajnikant B.
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