Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-12-11
2001-08-07
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S056090
Reexamination Certificate
active
06272024
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an electronic circuit to be connected between a direct-current (d.c.) power supply and a load for converting one d.c. voltage into another preparatory to application to the load. More specifically the invention concerns improvements in such a d.c.-to-d.c. converter, particularly relating to a surge suppressor circuit, sometimes referred to as snubber circuit in the art, that is customarily incorporated with the converter.
Japanese Unexamined Utility Model Publication No. 1-127388 is hereby cited as describing and claiming a surge suppressor circuit in a d.c.-to-d.c. converter. A typical prior art d.c.-to-d.c. converter of this type (shown in
FIG. 1
of the drawings attached hereto) includes a step-up or step-down transformer having a primary winding to be connected to a d.c. power supply via an electronic switching device, and a secondary winding connected via a rectifying and smoothing circuit to a pair of converter output terminals between which is to be connected a load to be powered. A voltage detector circuit is also connected between the pair of converter output terminals for detecting the converter output voltage. The output from the voltage detector circuit is applied to a control circuit for the switching device. The control circuit responds to the voltage detector output by making on-off control of the switching device accordingly, keeping constant the converter output voltage.
As taught by the Japanese unexamined utility model application above, the d.c.-to-d.c. converter includes the surge suppressor circuit comprising a diode, a capacitor and a resistor. The capacitor is connected in parallel with the transformer primary via the diode, and the resistor is. connected in parallel with the capacitor.
An inconvenience with the d.c.-to-d.c. converter of the kind in question is that a voltage surge occurs across the transformer primary when the switching device is opened while the transformer primary is carrying a current. Consequently, were it not for the surge suppressor circuit, the sum of the surge voltage and the supply voltage would be applied to the switching device, resulting in its destruction in the worst case. Actually, however, the surge voltage is taken up by the capacitor of the surge suppressor circuit.
A problem left unsolved in this type of d.c.-to-d.c. converter arises from the fact that the transformer primary has a leakage inductance and parasitic or stray capacitance, and that the switching device also has a stray capacitance. The result has been the creation of a ringing circuit that, in cases where the switching device takes the form of a field-effect transistor for example, has caused oscillation of the drain-source voltage of that switching transistor. The ringing frequency is so high that high-frequency noise has been generated which has interfered with the operations of other circuits.
Generally, the d.c. power supply of the converter takes the form of a rectifying and smoothing circuit connected to an alternating current power supply. For elimination of the ringing noise, a relatively high. impedance noise filter has had to be connected to the a.c. power supply, at the costs of the lower efficiency, greater bulk, and higher cost of the converter.
SUMMARY OF THE INVENTION
The present invention seeks to eliminate or attenuate the ringing of a d.c.-to-d.c. converter of the kind defined.
Briefly, the invention may be summarized as a d.c.-to-d.c. converter for feeding a load from a d.c. power supply, comprising a transformer having at least one winding to be connected to a d.c. power via a switching device. A rectifying and smoothing circuit is connected to the output side of the transformer for applying a d.c. output voltage to a load. The switching device is connected to a switch control circuit thereby to be turned on and off at a rate depending upon the converter output voltage, in order to keep that voltage constant.
Also included is a surge suppressor circuit to which the invention particularly concerns. Having a serial connection of a capacitor, diode, and resistor, which is connected in parallel with the transformer winding, the surge suppressor circuit functions to absorb the surge voltage developing across the transformer winding each time the switching device is turned off. The diode has a predetermined reverse recovery time that is longer than half the cycle of a ringing voltage developing in the transformer winding owing to the leakage inductance and stray capacitance of the transformer winding and to the stray capacitance of the switching device and shorter than an expected minimum nonconducting period of the switching device.
Essentially, the present invention is based upon the discovery that the ringing of the prior art d.c.-to-d.c. converter is eliminable by, as set forth in the foregoing summary, making the reverse recovery time of the diode of the surge suppressor circuit longer than half the cycle of the ringing voltage and shorter than the expected minimum nonconducting period of the switching device. Thus, if the ringing cycle is 250 nanoseconds or so, as is usually the case, then the reverse recovery time of the diode according to the invention should be at least about 125 nanoseconds. The expected minimum nonconducting period of the switching device may be about seven microsecond. Thus the reverse recovery time of the diode according to the invention is from about 125 nanoseconds to about seven microseconds, compared to about 100 nanoseconds according to the invention, which is too short to prevent the ringing. More will be said later on this subject.
The surge suppressor circuit according to the invention, incorporating the diode having a longer reverse recovery time than heretofore, functions as follows to suppress the ringing: The capacitor of the surge suppressor circuit will absorb the voltage surging developing across the transformer when the switching device is turned off, as then a current flows into the capacitor through the diode. Reverse biased subsequently, the diode will nevertheless remain conductive throughout its extended reverse recovery time. The capacitor will then remain connected in parallel with the transformer winding for a longer period of time than in the prior art. The noted stray capacitances are in parallel with the capacitor during the reverse recovery, time of the diode, resulting in the suppression or elimination of the ringing due to the leakage inductance and the stray capacitances.
As an additional advantage, as the capacitor discharges through the transformer winding after taking up the surge, the power is recovered on the input or output side of the transformer for higher operating efficiency.
The above and other objects, features and advantages of the invention and the manner of realizing them will become more apparent, and the invention itself will best be understood, from the following description taken together with the attached drawings showing some preferred embodiments of the invention.
REFERENCES:
patent: 4271460 (1981-06-01), Baker
patent: 4334254 (1982-06-01), Baker et al.
patent: 4432032 (1984-02-01), Baker et al.
patent: 5260607 (1993-11-01), Kinbara
patent: 1-127388 (1989-08-01), None
Nguyen Matthew
Sanken Electric Co. Ltd.
Woodcock Washburn Kurtz Mackiewicz & Norris LLP
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