Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-07-13
2001-08-14
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S056060, C363S134000
Reexamination Certificate
active
06275391
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to power supplies utilizing transformer isolation and requiring very fast isolated output voltage turn-off, such as may be useful in grid-controlled x-ray tubes, for example.
U.S. Pat. No. 5,969,955 describes a power supply utilizing an isolation transformer including a crowbar circuit and control such that turn-off by discharging an output capacitor is made advantageously faster than conventional circuits. The forward converter has a push-pull arrangement of two switching devices. For operation in the output voltage-on mode, the forward converter transfers energy in a peak forward converter mode with the first switching device being turned on and off. For output voltage turn-off, the crowbar circuit is activated by turning on the second switching device, resulting in a reverse voltage at the secondary winding of the isolation transformer. The reverse voltage is superimposed on the converter output voltage, the sum of the converter output voltage and the reverse voltage being imposed across the crowbar circuit. If the sum of the converter output voltage and the reverse voltage exceeds the breakover voltage of the crowbar circuit, the output capacitor discharges rapidly through the crowbar circuit and the transformer secondary winding.
It is desirable to provide a push-pull circuit arrangement and control that minimizes the number of components in the crowbar circuit, hence providing a more compact and lower cost circuit design. It is furthermore desirable to provide such a circuit and control that maximizes efficiency and reliability.
BRIEF SUMMARY OF THE INVENTION
A push-pull converter, coupled through an isolation transformer to a crowbar circuit for controlling an isolated output voltage across an output capacitor, has a compact circuit arrangement with a minimal number of components. The push-pull converter comprises first and second switching devices having integrated, ultra-fast anti-parallel diodes. The crowbar circuit comprises a trigger device (i.e., a voltage-controlled bi-directional switch) coupled between the transformer and the output capacitor. The converter is controlled in an output-voltage-on mode by turning on and off the first switching device. In an output-voltage-off-mode, the control provides a gating signal to the second switching device sufficient to both activate the crowbar circuit and discharge the output capacitor for very fast isolated output voltage turn-off. In particular, the gating signal provides a pulse of sufficient length to (1) achieve the breakover voltage across the trigger device to activate the crowbar circuit, i.e., in a predetermined time equal to a fraction of the resonant period determined by the leakage inductance and winding capacitance of the transformer, and (2) discharge the output capacitor, i.e., in another predetermined time equal to a fraction of the resonant period determined by the leakage inductance and the output capacitance. In addition, a primary-side circuit comprising a switch and a resistance can be used to avoid excessive residual output voltage build-up after the initial output capacitor discharge.
REFERENCES:
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“Syllabus: A Categorical Course in Physics,” pp. 59-74—Generators and Tubes in Interventional Radiology, Ernst Ammann; Gunter Wiede, 81stScientific Assembly and Annual Meeting of the Radiological Society of North America, Nov. 26-Dec. 1, 1995.
“Compact Push-Pull Converter And Crowbar Circuit, and Control Therefor,” L. Laskai; MZ Ilic, filed concurrently herewith, Apr. 6, 2000.
Ilic Milan Zarko
Laskai Laszlo
Breedlove Jill M.
General Electric Company
Stoner Douglas E.
Vu Bao Q.
Wong Peter S.
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