Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-01-25
2001-05-08
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S097000
Reexamination Certificate
active
06229716
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an apparatus for supplying a DC voltage to a load that can be connected to output terminals. The apparatus has a semiconductor switching element for the clocked application of a supply voltage to a primary of a transformer on the basis of a sequence of driving pulses applied to a control input of the semiconductor switching element. A driving circuit for producing the driving pulses is also provided. The driving circuit has a first input terminal for applying an analog load-dependent control signal which is produced by a measuring configuration and governs the duration of the individual driving pulses, and a second input terminal for applying a starting signal defining the turn-on instants of the driving pulses.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an apparatus for supplying a DC voltage which overcomes the above-mentioned disadvantages of the prior art devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus having output terminals for supplying a DC voltage to a load connected the output terminals of the apparatus, containing:
a measuring configuration for generating and outputting an analog load-dependent control signal;
a pulse generator having an input terminal receiving the analog load-dependent control signal and an output terminal, the pulse generator generating and outputting at the output terminal a pulsed starting signal on a basis of the analog load-dependent control signal, a temporal spacing of individual pulses of the pulsed starting signal being influenced by the analog load-dependent control signal;
a driving circuit for generating and outputting driving pulses, the driving circuit having a first input terminal receiving the analog load-dependent control signal from the measuring configuration, the analog load-dependent control signal governing a duration of the driving pulses, and a second input terminal connected to the output terminal of the pulse generator and receiving the pulsed starting signal for defining turn-on instants of the driving pulses;
a supply voltage;
a transformer having a primary side connected to the supply voltage; and
a semiconductor switching element having a control input receiving the driving pulses from the driving circuit, the driving pulses controlling a clocked application of the supply voltage to the primary side of the transformer on a basis of a sequence of the driving pulses applied to the control input.
The object of such apparatuses used in free-running switchedmode power supplies is to provide an at least approximately load-independent output voltage at the output terminals. In this case, the output terminals are usually connected to terminals of a secondary of the transformer via a rectifier configuration. The output voltage depends not only on the load but also on the power transmitted from the primary to the secondary. If the load is reduced, the transmitted power needs to be reduced, and, if the load is increased, the transmitted power needs to be increased in order to maintain a particular output voltage in each case. The transmitted power derives from the temporal average of the energy consumed by the primary when the semiconductor switching element is on and the energy transmitted to the secondary when the semiconductor switching element is off, the energy consumed being greater the longer the semiconductor switching element is on in each case.
The durations for which the semiconductor switching element is turned on are determined by driving pulses produced by the driving circuit on the basis of the load-dependent control signal. The variable duration of the driving pulses determines the charging time of the primary, that is to say the period of time for which the primary is connected to the supply voltage for consuming energy. In this case, the following applies: the smaller the load, the lower the power to be transmitted and, usually, the shorter the driving pulses, and vice versa.
The discharging time, that is to say the period of time during which the primary transmits energy to the secondary, depends on the energy stored during the charging time and thus on the duration of the driving pulses. To prevent the semiconductor switching element from turning on during the discharging time, known apparatuses are provided with a primary signal in order to minimize switching losses, and this primary signal is used to derive the instants at which the primary is free of energy or voltage, and it is taken into account for producing the driving pulses.
Known apparatuses are usually configured such that the starting signal is derived directly from the primary signal and is selected to be such that the driving pulses start, at the end of the discharging time, at the next instant at which the primary is free of voltage. Note that the instants at which the primary is free of voltage are called zero-voltage instants below.
Turning on at the end of the discharging time calls for the switching frequency, determined by the temporal spacing of the individual driving pulses, to increase for decreasing loads. With a decreasing load, the duration of the driving pulses (which is the same as the charging time for the primary) is reduced, which results in that the energy consumption is also reduced and the discharging time and the temporal spacing between the individual driving pulses are reduced. Since switching losses are significantly increased as the switching frequency rises, the efficiency of the apparatus is significantly reduced as the load becomes smaller.
The object of the present invention is to provide an apparatus for supplying a DC voltage to a load, in which the switching frequency does not exceed a predeterminable value with a decreasing load, so that, in particular, the aforementioned disadvantages do not arise.
The object is achieved by the apparatus mentioned in the introduction, which additionally has the feature of the pulse generator having the output terminal. The output terminal is connected to the second input terminal of the driving circuit, for providing the pulsed starting signal on the basis of the control signal, which is also supplied to a first input terminal of the pulse generator, in which configuration the temporal spacing of the individual pulses of the starting signal can be influenced by the control signal. Besides the duration of the driving pulses, the temporal spacing of the driving pulses is also dependent on the load-dependent control signal in the apparatus according to the invention. The turn-on instants of the driving pulses, and thus the switching frequency, are determined by the pulsed starting signal produced by the pulse generator, whose frequency is influenced by the load-dependent control signal.
The pulse generator preferably has a voltage/time converter unit that produces a pulse-width-modulated signal on the basis of a modified control signal that is applied to an input terminal and is dependent on the control signal. The duration of the individual pulses of the pulse-width-modulated signal depends on the amplitude of the modified control signal; this modified control signal can, as proposed in one embodiment of the invention, be the same as the control signal which is supplied by the measuring configuration and is applied to the first input terminal of the pulse generator. The pulse-width-modulated signal releases the starting signal, i.e. the pulse-width-modulated signal determines the instants after which the pulses of the starting signal, and thus the driving pulses, are to be produced as soon as possible. Without any restriction of the general nature, it is assumed below that a small load results in a large control signal, with the durations of the pulses of the pulse-width-modulated signal in each case being longer the larger the control signal, and vice versa.
To reduce switching losses, it is desirable to turn on the semiconductor switching element using the driving pu
Greenberg Laurence A.
Infineon - Technologies AG
Laxton Gary L.
Lerner Herbert L.
Stemer Werner H.
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