Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
1999-11-01
2001-05-15
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S019000, C363S132000
Reexamination Certificate
active
06233164
ABSTRACT:
BACKGROUND
The present invention relates to a protection circuit for a switched-mode power supply comprising a transformer with a primary winding and secondary windings, a switching transistor in series with said primary winding and a driver stage. Instead of the driver stage the power supply may have a control capacitor for the regulation. A switched-mode power supply of this kind is disclosed for example in EP-A-0 808 015.
In case of a short circuit the switching transistor of the power supply may remain in an unstable state not being completely closed during the off-interval. In this case the switching transistor may dissipate a power being to high which will damage it. This is especially the case for a self-oscillating switched mode power supply which has no driver circuit providing defined switching voltages and which may continue to oscillate in an undefined manner during said short circuit.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to specify a protection circuit for a switched-mode power supply as stated above. The protection circuit of the invention comprises a storage capacitor being coupled with one end to a terminal of one of the secondary windings providing a charging voltage for the storage capacitor during the conducting interval of the switching transistor. The charge of this capacitor is fed to a driver stage or a control capacitor during the off-interval of the switching transistor to provide a voltage for switching off and/or keeping closed the switching transistor in case of a short circuit at one of the secondary windings. This additional charge of the charging capacitor guarantees that the switching transistor is completely off during the whole off-interval to avoid damaging of the switching transistor. In case of a short circuit at one of the outputs of the switched mode power supply no voltage is present at the secondary windings as long as the diode of said voltage output is conducting. But very often an output voltage at a secondary winding of the primary side is required for closing the switching transistor. This is especially the case for self oscillating switch mode power supplies working in flyback mode.
The storage capacitor may be coupled advantageously to a control capacitor which may be part of the driver stage and which is charged by a regulating current provided during operation of the power supply. With the other end the storage capacitor can be connected to a terminal of a secondary winding which provides a negative voltage during the on-interval of the switching transistor by which the storage capacitor is charged. When the switching transistor switches off, the terminal of said winding switches from negative to 0 V which transfers the charge of the charging capacitor to the control capacitor and provides herewith a positive voltage or current for keeping the switching transistor closed.
The invention can be used, in particular, for television sets and video recorders.
REFERENCES:
patent: 3911352 (1975-10-01), Slack
patent: 4283759 (1981-08-01), Koiki et al.
patent: 4504775 (1985-03-01), Becker
patent: 5867373 (1999-02-01), Lohrer et al.
patent: 4032629 (1992-04-01), None
patent: 0336392 (1989-10-01), None
patent: 0808015 (1997-11-01), None
Koh Kian Meng
NG Seng Huat
Zee Kum Yoong
Kolodka J. J.
Laks J. J.
Patel Rajnikant B.
Thomson Multimedia S.A.
Tripoli J. S.
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