Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
1999-09-22
2001-01-23
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S075000, C363S082000, C363S090000
Reexamination Certificate
active
06178098
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power conversion and, more specifically, to a phase-shifted post-regulator, a method of operating the post-regulator and a power converter employing the system or the method.
BACKGROUND OF THE INVENTION
The development of more efficient and lower noise power converters is a continuing goal in the field of power electronics. Power converters are typically employed in applications that require conversion of an input DC voltage to various other DC voltages, higher or lower than the input DC voltage. Examples include telecommunication and computer systems wherein high voltages are converted to lower voltages to operate the systems. Power converters often suffer from problems such as switching losses, switching noise and common-mode noise originating in the power transformer. Switching losses reduce system efficiency, resulting in greater input power requirements for the same output power. Switching and common-mode transformer noise, both conducted and radiated, require filtering to prevent or reduce interference with other sensitive electronic equipment.
Current power converter designs often implement one of two full bridge control strategies, namely, the conventional full bridge or the phase-shifted full bridge. Both control strategies employ a full bridge inverter topology having four controllable switches (e.g., power metal-oxide semiconductor field-effect transistors), an isolation transformer, an output rectifier and an output filter. A controller is included and employed to control the controllable switches.
The conventional full bridge generally operates as follows. The controllable switches are arranged in two diagonal pairs that are alternately turned on for a portion of a switching period to apply opposite polarities of the input DC voltage across a primary winding of the isolation transformer. The controllable switches thus operate to convert the input DC voltage into an AC voltage to operate the isolation transformer. Between conduction intervals of the diagonal pairs, all of the controllable switches are turned off for a fraction of the switching period. Ideally, this should force a voltage across the primary winding of the isolation transformer to zero. The output rectifier then rectifies the AC voltage from the isolation transformer. A rectified voltage of the isolation transformer should, therefore, ideally be a square wave with an average value proportional to a duty ratio of the diagonal pairs of controllable switches.
The output filter smooths and filters the rectified voltage to provide a substantially constant output voltage at the output of the power converter. The controller monitors the output voltage and adjusts the duty ratio of the diagonal pairs of controllable switches to maintain the output voltage at a constant level as the input DC voltage and the load current vary.
In practice, the rectified voltage is not a perfect square wave, however, because turning off all of the controllable switches induces a ring between a leakage inductance of the isolation transformer and a parasitic capacitance of the controllable switches. The ringing dissipates energy, thereby reducing the efficiency of the power converter. The ringing also gives rise to significant noise, such as conducted and radiated electromagnetic interference.
The phase-shifted full bridge was developed to alleviate the switching loss and switching noise problems of the conventional full bridge. The construction of the phase-shifted full bridge is essentially identical to that of the conventional full bridge. Its advantages result, however, from the operation of the controllable switches to produce a zero voltage across the controllable switches before the controllable switches are turned on. The phase-shifted full bridge operates by turning off only one controllable switch of a diagonal pair to begin the zero voltage period, instead of turning off both of the controllable switches. A controllable switch from the alternate pair is then turned on, allowing the current in the primary circuit to circulate through the two controllable switches with substantially zero volts across the isolation transformer.
The two controllable switches thus clamp the voltage across the isolation transformer at about zero, thereby substantially eliminating the ringing behavior suffered by the conventional full bridge when the controllable switches are off. By clamping both ends of the primary winding of the isolation transformer to one rail and then, to the other rail, however, the phase-shifted full bridge induces a current flow through an intrinsic primary-to-secondary capacitance of the isolation transformer. As a capacitor potential is alternately charged from rail to rail, a common-mode noise may be generated.
Furthermore, alternately circulating the primary current through the two top or bottom controllable switches may result in additional conduction losses. During the circulation intervals of the primary current, both the input current to the bridge and the output voltage applied to the output filter are substantially zero, requiring both input and output filtering.
Accordingly, what is needed in the art is a power converter that retains the efficiency advantages of the full-bridge topology while alleviating the deficiencies associated with the conventional design.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides, for use in a power converter having an inverter coupled to an output rectifier, a post-regulator, a method of operating the same and a power converter incorporating the post-regulator or the method. In one embodiment, the post-regulator includes: (1) a switching circuit, having at least one controllable switch and coupled to an output of the power converter, that receives an output voltage from the power converter and produces a phase-shifted waveform therefrom and (2) a transformer, having a primary winding coupled to the switching circuit and a secondary winding coupled to the rectifier, that delivers the phase-shifted waveform to the rectifier to regulate the power converter, thereby allowing the inverter to be unregulated.
The present invention therefore introduces the broad concept of feeding back the phase-shifted waveform to regulate the power converter. This advantageously allows the inverter to operate in a more efficient mode.
In one embodiment of the present invention, the inverter is unregulated. In a related embodiment, the inverter is operated at a full duty cycle. Those skilled in the pertinent art realize the efficiency advantages associated with operating the inverter at full duty cycle.
In one embodiment of the present invention, the switching circuit phase shifts the waveform to increase and decrease the output voltage. The capability to increase and decrease the output voltage allows a power rating of the post-regulator to be substantially reduced (e.g., by a factor of two).
In one embodiment of the present invention, the inverter is selected from the group consisting of: (1) a full-bridge inverter and (2) a half-bridge inverter. Those skilled in the pertinent art will understand, however, that any conventional or later-discovered inverter topology can be employed with the post-regulator of the present invention.
In one embodiment of the present invention, the switching circuit is selected from the group consisting of: (1) a full-bridge topology and (2) a half-bridge topology. Those skilled in the pertinent art will understand, however, that any conventional or later-discovered topology can be employed in the post-regulator of the present invention.
In one embodiment of the present invention, the secondary winding is coupled to a first rectifying diode of the rectifier and the transformer has a second secondary winding coupled to a second rectifying diode of the rectifier. In an embodiment to be illustrated and described, the secondary windings are coupled in series with the first and second rectifying diodes.
In one embodiment of
He Jin
Jacobs Mark E.
Lucent Technologies - Inc.
Riley Shawn
LandOfFree
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