Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2000-03-16
2001-04-24
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S131000
Reexamination Certificate
active
06222748
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
A circuit arrangement is provided for determining the output power of switched-mode power supplies so that the output power can be determined independent of the frequency and the input voltage.
2. Description of the Related Art
Switched-mode power supplies are characterized by the fact that they deliver a constantly corrected d.c. voltage that is optimally independent of the load. The power output can encompass a broad range. For example, for television devices, the output power range of a switched-mode power supply can range from 5 W to 150 W. For a lower delivered power, a low switching frequency is often set in order to minimize switching losses, or circuit components of the television device are switched in or out. To minimize these losses, the instantaneous power of the switched-mode power supply must be measured at the primary side and appropriately conditioned for controlling the frequency or controlling circuit components. A simple way to determine the power is to measure the primary current in the switching transistor using a measuring resistor. The power is proportional to the square of the current. Since the level of this current also depends heavily on the frequency and the input voltage, this manner of power determination is highly inaccurate.
German Patent Document DE 3 129 021 A1 teaches a circuit arrangement for measuring the through power in d.c. transformers, however, this arrangement is only suitable for transformers that are timed with a constant frequency.
SUMMARY OF THE INVENTION
It is the object of the present invention to set forth a circuit arrangement with which it is possible to determine a measure of the output power of a switched-mode power supply in a simple fashion.
This object is inventively achieved using a circuit arrangement for determining a measure of the output power of a switched-mode power supply, comprising a field effect transistor, a transformer having a primary side supplied with energy via the field effect transistor, the transformer delivering the energy to a secondary side of the transformer during a discharging time period, an integration element a constant current source; an energy storage element being charged with a current by the constant current source during a period in which a drain voltage of the field effect transistor exceeds a specific threshold, and is discharged in a remaining time via a switch, and the integration element being connected to the energy storage element, the integration element integrating a voltage across the energy storage element and feeds it to an output as a measure of output power of the switched-mode power supply.
The inventive circuit arrangement has the advantage that a measure of the power output of a switched-mode power supply can be determined with this arrangement independent of the frequency and the input voltage. Furthermore, the small wiring expenditure of the circuit arrangement is also advantageous.
The circuit arrangement delivers a voltage that is proportional to the power output of a switched-mode power supply. This voltage can be given to one or more comparators, which serve to switch based on certain parameter values for a desired function (e.g., the frequency) in a simple manner.
The energy storage element of the inventive circuit arrangement is preferably realized as a capacitor. The integration element integrates the voltage that is present at a terminal of the energy storage element. It preferably consists of an RC element. Its time characteristic can be adjusted via the magnitude of the resistance and the capacitance of the RC element.
The inventive circuit arrangement for power measurement can be applied in multiple ways. It is preferably used when the frequency is to be switched over in a power-dependent manner and with only slight hysteresis, such as when the power is measured in standby burst mode. A further application for the inventive circuit arrangement is in what is known as temporary high load mode. In this application, within the power range of a switched-mode power supply, a threshold for the measured power output is declared, which may be exceeded only for a short time. When this threshold is exceeded for a longer time, a protective circuit is activated, which, for example, switches out the switched-mode power supply. Enhanced safety is achieved in this way, since the cutoff eliminates the risk of fire for a sustained overload. A further advantage derives from the cost savings, since, with respect to the resulting heat dissipation, the switched-mode power supply need not be designed to accommodate a temporary peak power, but rather for a significantly lower continuous power. This in turn permits the use of smaller transformers and smaller cooling plates.
Further advantageous developments are characterized by a circuit arrangement for determining the output power of a switched-mode power supply where the energy storage element is a charge capacitor, where an integration element has an RC element with a resistor and an integration capacitor, and where a constant current source is connected to the charge capacitor and a resistor, and the constant current source is connected to the switch such that, when the switch is closed, the current of the constant current source is discharged to a ground.
REFERENCES:
patent: 3629677 (1971-12-01), Means
patent: 4338554 (1982-07-01), Fukushima et al.
patent: 4783621 (1988-11-01), Drefahl
patent: 4996638 (1991-02-01), Orr
patent: 5719575 (1998-02-01), Hashimoto
patent: 25 27 392 B2 (1977-12-01), None
patent: 24 08 780 B2 (1978-06-01), None
patent: 31 29 021 A1 (1983-02-01), None
patent: 38 38 430 A1 (1990-05-01), None
patent: 42 12 472 A1 (1993-10-01), None
patent: 0 744 818 A1 (1996-11-01), None
JP-63299778 Dec. 1988, abstract only, Katsumi.
Niederreiter Hans
Preller Peter
Infineon Technologies Aktiengesellschaft
Riley Shawn
Schiff & Hardin & Waite
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