Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Utility Patent
1999-10-27
2001-01-02
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S133000
Utility Patent
active
06169668
ABSTRACT:
BACKGROUND
The present invention relates generally to boost converters, and more particularly, to zero voltage switching isolated boost converters.
Hard switching Weinberg converters have achieved the nearest comparable performance to the present invention. These converters have a power stage, which is topologically identical to the present invention. The difference between the two is in the control methodology and the turns ratios in the transformer and inductor. Efforts to reduce power losses in the Weinberg converter have been centered on reducing conduction losses by using more and larger MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) as the switching devices, and increasing the size and interleaving of the transformer and coupled inductor. These measures have the effect of increasing the energy stored in the parasitic capacitances of the MOSFETs and transformer and inductor windings. This energy is dissipated during each switching transition, and the additional switching loss sets a lower limit on the power loss which can be obtained using the hard switching Weinberg converter and similar approaches.
There are also a number of other zero voltage switching converters currently in use. A typical zero voltage switching full bridge voltage fed buck converter is described in application note U136A published by Unitrode. The Unitrode circuit uses the magnetizing current of a transformer to produce resonant transitions and zero voltage switching.
The Unitrode circuit passes input current through two transistors and operates each transistor at less than 50 percent duty, compared to one transistor operating at greater than 50 percent duty in the zero voltage switching boost converter of the present invention. This causes the prior art full-bridge buck converter to have higher I
2
R losses than the zero voltage switching boost converter of the present invention.
The prior art circuit has losses due to transformer leakage inductance that are minimal compared to the present invention, however, because in a full bridge topology, leakage energy can be easily returned to the source. The present zero voltage switching boost converter requires tightly coupled magnetics to achieve high efficiency, because its leakage transients must be limited by a voltage clamp circuit.
Accordingly, it is an objective of the present invention to provide for improved zero voltage switching isolated boost converters.
SUMMARY OF THE INVENTION
To accomplish the above and other objectives. the present invention provides for improved zero voltage switching isolated boost converters. The zero voltage switching isolated boost converters provide high efficiency power conversion between input and output DC voltages. A first embodiment of the boost converter employs phase-locked loop control approach, while a second embodiment employs a time domain control approach.
The present boost converters use a resonant circuit to achieve soft (zero voltage) switching transitions in an isolated boost topology. The combination of zero voltage switching and the isolated boost topology minimizes both switching and I
2
R losses.
In general, the zero voltage switching boost converters comprise a voltage amplifying stage having first and second switching transistors, an input coupled inductor, and an output transformer. The voltage amplifying stage amplifies a DC input voltage to produce a higher DC output voltage higher than the product of the DC input voltage and the output transformer turns ratio. The resonant circuit comprises the input inductor, the output transformer and the parasitic capacitances of the transistors, inductor, transformer and output rectifiers. External capacitors may also be added at these positions.
A zero voltage control input circuit is coupled to gates and drains of the switching transistors. The input circuit generates synchronization signals that synchronize the rising edge of the gate drive signals applied to the switching transistors with the resonant circuit by observing the state of gate and drain voltages of the switching transistors and detecting the moment when both drains are low, and both gates are not high.
First and second pulsewidth modulators are coupled to the gates of the switching transistors and to current sensors which sense current flowing through the switching transistors and also to the output of the isolation circuit and the output of the zero voltage control circuit. The pulsewidth modulators generate gate drive signals that drive the gates of the switching transistors in response to the synchronization signals generated by the zero voltage control input circuit, current flowing in the switching device, and the pulsewidth command signal from the isolation circuit.
Alternatively, the pulsewidths of the gate drives to the switching transistors may be controlled solely based on the signal from the error amplifier and isolation circuit and the synchronization signals, eliminating the need for current sensors.
The output of the voltage amplifying stage is coupled to the input of an error amplifier, along with the output of a voltage reference. The error amplifier output is analogous to the difference between the reference voltage and the output voltage of the voltage amplifying stage. The output of the error amplifier is coupled to the input of an isolation circuit, for applications in which isolation between input and output returns is needed. If isolation of the returns is not needed, the isolation circuit may be omitted. In this case, the positive output terminal is isolated from the positive input terminal, unlike a conventional boost converter. The output of the isolation circuit is coupled to the first and second pulsewidth modulators.
Although MOSFETs are shown as the switching devices in the drawing figures, other types of transistors or thyristors may be used, including, but not limited to, IGBTs (Insulated Gate Bipolar Transistors), BJTs (Bipolar Junction Transistors) or gate turnoff thyristors (GTOs). These devices require an anti-parallel diode, unlike the MOSFETS shown, which incorporate this device as a parasitic element.
REFERENCES:
patent: 4893227 (1990-01-01), Gallios et al.
patent: 5510974 (1996-04-01), Gu et al.
patent: 5867379 (1999-02-01), Maksimovic et al.
Float Kenneth W.
Han Jessica
Space Systems Loral, Inc.
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