Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2001-05-05
2003-04-08
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S056040, C363S098000, C363S132000
Reexamination Certificate
active
06545886
ABSTRACT:
BACKGROUND OF INVENTION
This invention pertains to the field of control systems for scale model railroad layouts, and specifically to improvements in power handling capacity of decoders used for control of elements around the layout.
Modern layout control systems allow the simultaneous control of many devices using decoder devices that are attached to or run on the tracks of model railroads. The ability to make smaller, less expensive and more reliable decoders is of great benefit, allowing the usage of the control technology in smaller railroad components and allowing greater flexibility in packaging design and installation.
The decoders derive power and control information via at least a two conductor electrical connection to the control system. This connection may be via wheel or slider pickups from the tracks, overhead catenary wires or any other conductive connection to the layout control system. The decoder analyses the encoded voltage waveform or signal conducted from this control system and, by using the information encoding rules defined for the control system, can detect and decode commands that are sent for execution or action required by the decoder.
Since the decoders may be connected in either orientation or polarity to the control system, they require an input rectifying full bridge arrangement to ensure a consistent and predictable voltage polarity can be extracted from the encoded voltage waveform when connected either way. This input rectifying full bridge carries the load current that the decoder then supplies in a switched or modulated manner, using additional power control elements, to a controlled load such as motors or lamps. The internal power control components of the decoder require an unvarying polarity inside to the decoder to operate correctly.
Additionally, the voltage encoding waveforms employed by some control systems may appear as a bipolar, or continuously alternating polarity, voltage waveform at the decoder. This then mandates the inclusion of a rectifying full bridge (also known as full wave bridge) at the input to condition the voltage waveform so it can be used by the decoder to power attached controlled loads such as motors, lamps or actuators.
Practical rectifying full bridge implementations typically include four semiconductor power diodes arranged in the full bridge rectifier configuration well known to those skilled in the art of electronic circuit design. In operation, the full bridge diode components experience a voltage drop in the forward voltage direction when conducting current. This forward voltage drop occurs while the diodes are conducting the full load current and so can represent a significant power loss. To minimize this power loss in a decoder, it is usual to use high quality and low forward voltage drop diodes, such as schottky barrier diodes. These devices represent the best conventional devices that can and have been used in prior art decoder designs.
The heat generation by the input rectifying full bridge imposes fundamental limits to the size and current control capacity of a decoder. As decoder designs strive for miniaturization the overall surface area decreases and consequently the heat dissipation capability also decreases. For a given amount of heat generation due to load current, decreased dissipation capabilities leads to increased internal temperatures and consequently lower long-term reliability. The fill bridge is the limiting device in the decoder design because the other power switching devices can take advantage of power switching devices, such as MOSFETs selected for very low losses and negligible voltage drops, at the current levels in use. Conventional rectifiers always have a minimum forward conduction voltage and losses.
To allow for a breakthrough in decoder miniaturization and increased current capacity a new and novel approach is required.
The key breakthrough is to discard prior art and to reconfigure the full bridge rectifier function with a new circuit topology hitherto unused in model railroad decoder design practices.
State of the art designs in high-energy switching power supplies, such as the design shown by Schwartz in U.S. Pat. No. 5,552,695, sometimes employ the unique conduction characteristics of metal-oxide-silicon field effect transistors, or MOSFETS, operating in their third-quadrant conduction mode. This is sometimes generically referred to as “synchronous rectification”. This mode takes advantage of the MOSFET's ability to conduct significant reverse current at a low voltage drop when the source to drain terminals are reverse biased while the gate to source terminals are forward biased or on. This technique is used to improve the efficiency of the power supply at high currents and low output voltages, since rectifier power losses are reduced and are a smaller percentage of the output voltage. Herein the term “third quadrant” is taken to mean the operation of a MOSFET with drain to source terminals in reverse bias whilst the gate to source terminals are biased on.
Synchronous rectifier designs are commonly limited to half wave, series-parallel or forward-flyback rectifier configurations at the power supply inductive energy storage node or output node, since all commercial design arrangements operate with a fixed power supply output connection polarity. Here, a full bridge configuration is redundant or impractical and would have twice the components and losses on the minimally sufficient half-wave design. The ac power line input to the power supply cannot practically use a synchronous full wave bridge because suitable device ratings are unavailable at these voltage levels, or are prohibitively expensive. Also the reduction of forward voltage from, for example 0.75 Volts to about 0.3 Volts represents a negligible efficiency saving on a switched voltage of several hundred volts or more. For these reasons wholly synchronous rectifier full wave bridges have not been practically required or used to date.
MOSFETs have a parasitic or intrinsic body diode between the source and drain terminals that is off, or reverse-biased, in normal first-quadrant operation. When the MOSFET source and drain terminals are reverse biased with a zero volt gate bias, this intrinsic body diode will conduct, but this body diode operates with similar voltage drops or losses to high speed non-schottky diodes, and is unsatisfactory for efficient operations. An example of this low efficiency usage of MOSFET body diodes for rectification is the Lenz Electronics LE077XF model locomotive decoder, a circa 2000 era design. Here two six pin devices, each containing two MOSFETs with zero gate-bias, provide four independent body diodes connected conventionally as a full rectifier bridge. This design is employed to provide a full rectifier bridge in a small space, but fails to obtain the advantage of the insight or innovation of using these same devices for high efficiency third quadrant MOSFET operation.
While in reversed drain to source bias and the body diode conducting, the application of a forward, or on, instead of zero bias voltage to the MOSFET gate will induce a current carrying mode with a significantly lower voltage drop than the body diode or even schottky diodes. In operation, a low Rds(on) or high current MOSFET used as a third-quadrant rectifier can typically have losses of 25%, or less, of even the best conventional schottky rectifiers. For example, the voltage losses at load may be in the range of 0.1 Volts to 0.2 Volts for an N-channel MOSFET as a third quadrant rectifier, where a schottky rectifier would be approximately in the range of 0.55 Volts to 0.7 Volts at the same device die size and ratings in forward conduction mode.
Conventional power switching designs stringently avoid the use of P-channel MOSFETs, since the manufacture of these devices necessarily yields performances of about half of the equivalent N-channel devices. Designers go to great lengths to arrange circuit topologies to allow for N-channel devices whenever possible. For this reason, there are no prior decoder full rectifier b
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