Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2002-04-11
2003-02-04
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021130
Reexamination Certificate
active
06515874
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a monolithically integratable circuit configuration for driving a semiconductor switch in a switched-mode power supply, having the following features:
the circuit configuration has a first and a second supply potential terminal;
a control unit for producing control pulses for the semiconductor switch according to a variable oscillator signal, whereby the duration of the individual control pulses depends on a first control signal and on a second control signal;
a measurement system for producing the second control signal, which is dependent on the load current of the semiconductor switch.
Circuit configurations of this type are used to regulate the output voltage, or the output power, of a switched-mode power supply.
The power consumed or output by a switched-mode power supply is determined by, among other factors, the duration of the periodically produced control pulses that cause a closing of the semiconductor switch for the duration of the control pulses, thus causing a flow of the load current. The regulation of the duration of the control pulses, which are usually generated in sync with the oscillator signal, takes place in the circuit configuration dependent on a first and on a second control signal. The first control signal depends on, among other factors, the output voltage, or the output power, of the switched-mode power supply.
In the prior art circuit configurations, there thus takes place a closing of the semiconductor switch in time with the oscillator signal, whereby the semiconductor switch is opened again dependent on the course of the first and second control signal. The control pulses are usually selected such that they terminate when the first control signal is exceeded by the second control signal, through which the semiconductor switch is opened.
Switched-mode power supplies are used, among other things, for supplying power to monitors or television sets. So that electrical and magnetic scatter fields of the switched-mode power supply will not be able to cause disturbances of the picture, the switched-mode power supply is usually synchronized with the line frequency of the monitor. There exist a multiplicity of different standards and different screen resolutions. For this reason, a monitor must be able to be adapted to different line frequencies over a broad frequency range. As a lower limit, almost all monitors operate with a line frequency of 31.5 kHz, in order to ensure compatibility with the VGA standard in DOS mode.
In order to achieve a higher display screen resolution, as well as a higher vertical scanning frequency of the electron beam, higher line deflection frequencies must be used. With higher line deflection frequencies, the flickering of the monitor can be reduced. Currently, for 17″ monitors, the upper limit of the line frequency is 85 kHz. For 21″ monitors, this limit is 108 kHz. In the future, a further increase of the line deflection frequency is planned.
In order to be able to meet the requirements of the synchronization of the line frequency of the monitor with the oscillator frequency of the switched-mode power supply, the switched-mode power supply must operate over a broad frequency range, from 31.5 kHz up to approximately 120 kHz.
The power requirement of a monitor varies according to the size of the picture tube, between approximately 70 watts and 140 watts. For reasons of cost, as a rule a flyback converter is used. Due to the lower radiation of electromagnetic interference fields, the flyback converter is preferably operated in delta current operation. Here, the primary winding of the transformer is periodically connected with the rectified input voltage between the first and second supply potential terminal until the flow of current through the primary winding, starting from the value zero, has reached a value that depends on a control signal. Subsequently, the flow of current in the primary winding is interrupted, and the overall magnetically stored energy flows off to the load at the secondary side via rectifying diodes. The power emission of a synchronized flyback converter in delta current operation is dependent on the maximum current in the primary winding and on the frequency of the power supply, i.e., the line deflection frequency.
This has the consequence that at the highest line frequency, the switched-mode power supply in a monitor can emit a significantly higher power than at the lowest line deflection frequency. However, the actual power requirement of a monitor in fact hardly depends on the selected line deflection frequency.
If, in operation with a high line deflection frequency, a fault occurs in which there arises a considerable additional power loss, without a lowering of the output voltage of the switched-mode power supply and the possibility of detecting the error in this way, the monitor can cause a fire.
It is therefore known from the prior art to limit the emitted power of the switched-mode power supply by monitoring the secondary current in the secondary transformer winding and providing a feedback loop in order to control the semiconductor switch. In addition, it is known to control the primary current dependent on the line frequency. However, none of these solutions can be realized in monolithically integrated form. In addition, the power limitation operates in a relatively imprecise fashion.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a clocked current supply, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a circuit configuration that can be monolithically integrated that prevents an excessive emission of power of the switched-mode power supply in all operating states, in order to prevent damage to the power supply, as well as to the additional components supplied by it. In addition, the power regulation is to take place with a high degree of precision.
With the foregoing and other objects in view there is provided, in accordance with the invention, a monolithically integratable control circuit for driving a semiconductor switch in a switched-mode power supply with a first and a second supply potential terminal, which comprises:
a control unit for generating control pulses for the semiconductor switch based on an oscillator signal having a variable oscillator frequency, wherein a duration of individual drive pulses depends on a first control signal that is dependent on an output voltage of the switched-mode power supply, and on a second control signal;
a measurement system connected to said control unit for generating the second control signal in dependence on a load current of the semiconductor switch;
a power regulation system connected to said control unit, said power regulation system receiving the oscillator signal, generating a third control signal inversely proportional to a square root of the oscillator frequency, and supplying the third control signal to said control unit.
The invention is based on the knowledge that the power emission in delta current operation in a flyback converter increases as the square of the maximum current in the primary winding, and proportional to the frequency, i.e., to the line deflection frequency. The power regulation system provided in the switched-mode power supply has the effect that the maximum peak current in the primary winding is taken back in a manner inversely proportional to the square root of the frequency. The power regulation system therefore provides the control unit with a third control signal, which, dependent on the already-selected frequency, controls the semiconductor switch in such a way that the maximum peak current (load current) in the primary winding is influenced in a manner corresponding to the above-named relationship.
For this purpose, and in accordance with an added feature of the invention, the power regulation system has the following features:
a pulse generator that receives the oscillator signal;
a series circuit of a s
Berhane Adolf Deneke
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
Stemer Werner H.
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