Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Reexamination Certificate
1999-08-26
2001-11-20
Huynh, Kim (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
C361S093700, C323S282000, C307S131000
Reexamination Certificate
active
06320733
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention concerns a pulse-width modulator for controlling a semiconductor circuit-breaker in a switched-mode power supply. The pulse-width modulator contains a comparator circuit for generating control pulses determined by an oscillator signal, whereby a duration of the individual control pulses depends on a first control signal and a second control signal. A measuring configuration is provided for generating a load-current signal dependent on the load current of the circuit-breaker. A feedback branch for feedback of the load-current signal to the comparator circuit is also provided. The feedback branch has a low-pass configuration with an input that receives the load-current signal and an output at which the second control signal can be picked off.
Pulse-width modulators of this kind serve for controlling the output voltage or the output power of a switched-mode power supply.
A pulse-width modulator with the above features is known, for example, from U.S. Pat. No. 5,390,101.
Furthermore, Published, European Patent Application EP 0 584 623 A1 discloses a converter for generating a constant output voltage with a current-mode regulator, which converter has a post-connected R-C element for achieving freedom from peaks whereby the peaks always arise at the beginning of the switch-on phase of the circuit-breaker.
The power input or output of the switched-mode power supply depends among other things on the duration of the periodically generated control pulses, which effect closure of the semiconductor circuit-breaker for the duration of the control pulse and therewith cause a load current to flow. Control of the duration of the control pulses, which are normally generated at the clock frequency of the oscillator signal, takes place in the circuit configuration in dependence on the first and a second control signals and whereby the first control signal depends among other things on the output voltage or output power of the switched-mode power supply.
Thus in the prior art pulse-width modulators for controlling the semiconductor circuit-breaker closure of the semiconductor circuit-breaker takes place at the clock frequency of the oscillator signal, whereby the semiconductor circuit-breaker is opened again in dependence on the sequence of the first and second control signals. Normally the control pulses are selected such that they end when the second control signal exceeds the first control signal, as a result of which the semiconductor circuit-breaker is opened.
In prior art pulse-width modulators of this kind the load-current signal is fed back to the comparator circuit either directly or through an amplifier. As a result of unavoidable parasitic capacitances, a short, high switch-on current pulse arises in the load current flowing in the switched-mode power supply after the semiconductor circuit-breaker is switched on, which if fed back directly into the comparator circuit as a second control signal would result in the semiconductor circuit-breaker being switched off immediately after being switched on if the switch-on pulse exceeds the first control signal. These kinds of pulse-width modulators for controlling the semiconductor circuit-breaker therefore have configurations in the feedback branch which blank out the load-current signal at the beginning for a period which is longer than the duration of the switch-on pulses.
A disadvantage of these kinds of pulse-width modulator is that it is not possible to generate any control pulses with a duration shorter than the period during which the load-current signal is blanked out. Because the power output of the switched-mode power supply depends on the duration of the control pulses, there is a lower limit to output power of a switched-mode power supply with a pulse-width modulator of this kind.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a pulse-width modulator that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which control pulses can be generated with as short a duration as required and independent of a switch-on current pulse, and which is realizable through simple construction. A further object is that the pulse-width modulator is highly reliable.
With the foregoing and other objects in view there is provided, a pulse-width modulator for controlling a semiconductor circuit-breaker in a switched-mode power supply. The pulse-width modulator, includes
a comparator circuit for receiving an oscillator signal, a first control signal, and a second control signal. The comparator circuit generating control pulses in dependence on the oscillator signal, the control pulses each having a duration being dependent on the first control signal and the second control signal. The pulse with modulator having a
measuring configuration for receiving a load current from a semiconductor circuit-breaker and generating a load-current signal being dependent on the load current of the semiconductor circuit-breaker; and
a feedback branch feeding back the load-current signal from the measuring configuration to the comparator circuit, the feedback branch having a low-pass configuration with an input receiving the load-current signal and an output supplying the second control signal, the low-pass configuration having a low-pass filter and a switching configuration providing a signal fraction of the second control signal, the signal fraction being independent of the load-current signal.
The object is achieved with the pulse-width modulator noted above. A key addition is that the low-pass configuration has a low-pass filter and a circuit configuration which provides a fraction of the signal of the second control signal. The fraction additionally being independent of the load-current signal.
The low-pass configuration exerts the effect that a useful fraction of the load-current signal, which generally increases more slowly and is thus of lower frequency, is integrated almost unfiltered into the second control signal whereas the short, and thus high frequency, interfering fraction of the load-current signal is suppressed through the low-pass configuration. In the presence of the low-pass configuration it is not necessary to blank out the load-current pulse for a specific length of time after opening the circuit-breaker so that, in particular, it is possible to generate control pulses as short as required.
The low-pass configuration has a low-pass filter and a switching configuration whereby the latter provides a fraction of the signal of the second control signal, which fraction is independent of the load-current signal. The switching configuration is chosen such that the signal fraction, which is independent of the load current, supplements the signal fraction of the second control signal resulting from the load current to a monotonically increasing signal as long as the semiconductor circuit-breaker is closed. The low-pass filter and the switching configuration are chosen such that even in the case of small load currents and resulting small load current signals a strictly monotonically increasing second control signal is available for the duration of the control pulses, as a result of which control pulses as small as required are possible.
The low-pass filter is preferably a first order filter which can therefore be realized simply and in a space-saving way.
To enable the first and second control signals to be mutually balanced the low-pass configuration preferably has an amplifier. This also serves, for example, to amplify before low-pass filtering the load-current signal or a signal dependent on this in order to reduce the influence on the second control signal of further disturbance caused through parasitic effects.
One embodiment of the pulse-width modulator according to the invention provides that the switching configuration consists of a square-wave signal source disposed before the low-pass filter. The square-wave pulses supplied by the square-wave signal source are chosen such that they begin at or just before the swi
Greenberg Laurence A.
Huynh Kim
Infineon - Technologies AG
Lerner Herbert L.
Stemer Werner H.
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