Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Patent
1996-03-15
1998-01-13
Wong, Peter S.
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
363 97, 363131, H02M 3335
Patent
active
057085722
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
This invention relates to a switched-mode power supply.
In switched-mode power supplies of this type, the mean value of the output current flowing through the load increases at a higher input voltage. This current rise amounts to about 50% with an increase in the input voltage from 100 V to 300 V and is primarily caused by the storage period of the first controllable semiconductor operating as a switching device in the saturated state, which storage period increases in effect in the presence of a higher input voltage, that is, it increases in proportion to the turn-on period.
In order to counteract this undesired current rise, current controls are known in which the current is measured through a voltage drop over a resistance and utilized as a controlled variable. Apart from requiring the arrangement of an additional component, these controls have the disadvantage that a voltage drop is produced in particular at relatively low output voltages by the insertion of the current sensing resistor.
SUMMARY
It is an object of the present invention to provide in a switched-mode power supply of the type initially referred to a simple and yet highly accurate and easily adjustable current control in which the need for an additional current sensing resistor is obviated.
According to the present invention, this object is accomplished in that the base of the first controllable semiconductor switching device is connected to an arrangement that limits the maximum peak current of the first controllable semiconductor switching device in dependence upon the output current and that detects the voltage drop caused by the output current through the secondary winding along the internal impedance of the secondary winding of the transformer.
The solution of the present invention enables the peak current of the controllable semiconductor switching device to be limited in dependence upon the output current in an accurately adjustable manner, utilizing for detection of the output current the internal impedance of the secondary winding as a current sensing resistance, thus eliminating the need for an additional current sensing resistor, omitting the voltage drop produced by such a current sensing resistor.
Further, by utilizing the internal impedance of the secondary winding as a current sensing resistance, the output current is reduced at elevated temperatures due to the positive temperature coefficient of the copper of the transformer winding, if no additional temperature compensation is performed. It is another advantage of the solution of the present invention that the winding temperature can be detected during operation of the switched-mode power supply by means of the temperature responsive internal impedance of the secondary winding of the transformer, enabling such temperature to be referred to for effecting protective and cut-off measures.
An advantageous feature of the solution of the present invention is characterized in that the arrangement limiting the maximum peak current of the first controllable semiconductor switching device performs such limitation in dependence upon the mean output current, comparing the voltage drop caused by the output current through the secondary winding along the internal impedance of the secondary winding of the transformer with a reference voltage that corresponds to the maximum output current.
The limiting of the peak current of the first controllable semiconductor switching device by detection of the mean output current considers the turn-on and turn-off relationships of the controllable semiconductor switching device, thus enabling a correct measure of the output current to be determined also during prolonged pauses of the controllable semiconductor switching device. The comparison of the voltage drop detected along the internal impedance of the secondary winding of the transformer with a reference voltage enables the permissible peak current to be easily adjusted in dependence upon the components employed.
A further advantageous feature of the solution of the present inventi
REFERENCES:
patent: 4523139 (1985-06-01), Schwarz et al.
patent: 4684871 (1987-08-01), Plagge
patent: 4969077 (1990-11-01), Plagge
patent: 5101334 (1992-03-01), Plagge et al.
Braun Aktiengesellschaft
Riley Shawn
Wong Peter S.
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