Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-04-19
2001-05-29
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
Reexamination Certificate
active
06239990
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a switched-mode power supply with a switch for applying a supply voltage to a primary coil of a transformer based on a drive signal fed to a pulse-width modulator. A first regulating signal that is dependent on an output voltage and a voltage signal that is dependent on a current through the primary coil is supplied to the pulse-width modulator for providing the drive signal.
Such switched-mode power supplies serve the purpose of supplying a consumer or load with a d.c. voltage, the load being connected to an output terminal of the switched-mode power supply at the secondary side. This d.c. voltage at the output side is to be kept substantially constant in a load-independent manner. For purposes of generating the output voltage, the primary coil absorbs energy via the supply voltage given a closed switch and outputs this energy via the secondary coil of the transformer and a rectifier arrangement to the load after the switch has been opened. Given switched-mode power supplies of fixed frequency, the switch is closed based on of the drive signal in periodical time intervals, whereby the periods of the individual drive input pulses vary in dependency on the output voltage and become longer with reduced output voltage or increasing load. What is referred to as “current-mode-regulation” is applied with respect to these switched-mode power supplies, which are particularly utilized in monitors and television apparatuses, for purposes of generating the drive input pulses in the pulse-width modulator. Thereby, the switch remains closed until the current through the primary coil or the current signal dependent on this current reaches the value of the regulating signal, which current increases subsequent to the switching-on.
Problems can arise with respect to a short-circuit at the output side when the transformer cannot output As energy to the load during the switch pauses. The current increases almost up to the value at which the switching-off has been carried out given the next closing of the switch. Therefore, the ON-time becomes extremely short given ideally functioning switched-mode power supplies. However, the signal propagation time determined by the signal processing in the pulse-width modulator—from reaching the switch-off value of the current signal until switching-off the switch—determines the minimal ON-time of the switch in practical operation. Therefore, the primary current further increases between the switch-off threshold is attained and the actual switching of the switch. When the transformer absorbs more energy during the ON-times of the switch than it can output to the secondary side in the switch pauses, the magnetization of the primary coil and therefore the primary current increases up to a point at which components are destroyed. Such known switched-mode power supplies provide a second operating point for purposes of preventing a destruction, whereby the switched-mode power supply is switched off by means of the current signal given exceeding of the operating point for the first time. Then, it can only be switched on again after a waiting period or after it has been disconnected from the network.
SUMMARY OF THE INVENTION
It is an object of the invention to solve the above-mentioned problem relative to switching on again after a waiting period or after it has been disconnected from the network.
In contrast thereto, the invention provides a second regulating signal for generating the drive signal, which is variable dependent on the current through the primary coil. The second regulating signal, as the first regulating signal, prescribes a switch-off threshold, whereby the switch is to be opened based on the criterion of the drive signal when the voltage signal representative of the current reaches the switch-off threshold. The determining factor for the switching-off is the regulating signal, the value of which is reached first by the voltage signal representative of the current. While the first regulating signal that is dependent on the output voltage assumes a large value in the case of a short-circuit in order to generate long drive input pulses and in order to maintain the output voltage, the second regulating signal can be reduced dependent on the current through the primary coil, in this case in order to achieve shorter ON-times. The second regulating signal is dimensioned such that it assumes a constant value given proper operation (no short-circuit) of the switched-mode power supply, which value lies above the value of the first regulating signal, so that it therefore does not influence the ON-times.
An embodiment of the invention provides a signal generation arrangement with an output terminal at which the second regulating signal can be picked off, and with an input terminal. A voltage signal that is dependent on the current through the primary coil can be supplied to the input. Preferably, the voltage singal representative of the current signal is the same voltage signal representative of the current, which is also supplied to the pulse-width modulator for purposes of generating the drive signal.
Advantageously, the signal generation arrangement has a series connection of a current source and a capacitance between a supply potential and a reference potential, whereby the second regulating signal can be picked off as a voltage across the capacitance. A second switch connected that is connected parallel to the capacitance is provided for purposes of changing the second regulating signal, this second switch being operated based on the criterion of a second drive signal that is dependent on the current through the primary coil or on the voltage signal dependent on the current in order to discharge the capacitance and thus reduce the second regulating signal.
For purposes of providing the second regulating signal and therefore for driving the switch, a comparing arrangement is preferably provided, which compares the voltage signal dependent on the current to a reference signal and which then closes the switch in order to discharge the capacitance when the voltage signal dependent on the current exceeds the reference signal. The switch is preferably designed such that a resistor in its load path can be adjusted dependent on the second drive signal in order to discharge the capacitance more quickly when the second drive signal is large and vice versa.
According to an embodiment of the invention, the comparing arrangement is designed as a trans-conductance amplifier to which the voltage signal dependent on the current and the reference signal can be supplied at input terminals, and the switch is designed as a bipolar transistor. A current that increases with increasing difference of the signals at the input can be picked off at the output of the trans-conductance amplifier. This current is supplied to the base of the bipolar transistor, which therefore conducts the better the higher the difference of the input signals is, in order to discharge the capacitance more quickly and thus lower the operating point prescribed by the second regulating signal more quickly.
Another embodiment of the invention utilizes a field-effect transistor as a switch and a difference amplifier as a comparing arrangement. The difference amplifier transforms a difference of the signals at the input into an output voltage with which the field-effect transistor is driven, so that the field-effect transistor conducts the better higher the output voltage of the difference amplifier is.
Also according to the invention, a method is provided for purposes of driving a switch in a switched-mode power supply, which switch is connected in series to a primary coil. The method provides the generation of drive signals for the switch based on the criterion of a first regulating signal that is dependent based on an output signal, on the criterion of a first voltage signal that is dependent on a current through the primary coil, and based on the criterion of a second regulating signal. The second regulating signal can be adjusted according to the curr
Infineon Technologies Aktiengesellschaft
Riley Shawn
Schiff & Hardin & Waite
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