Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2002-03-15
2003-07-01
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S021010
Reexamination Certificate
active
06587361
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method for driving a switch in a switched-mode power supply, in particular in a free-running switched-mode power supply, and to a switched-mode power supply.
Switched-mode power supplies serve for providing an at least approximately constant, load-independent output voltage for a connected load. To that end, a primary coil of a transformer is connected in series with a switch to a supply voltage, and a secondary coil that is inductively coupled to the primary coil is connected, usually via a rectifier configuration, to the load for providing the output voltage. The primary coil takes up energy when the switch is closed and outputs the energy to the secondary side when the switch is subsequently opened. The switch is driven according to drive pulses generated by a drive circuit. The energy which is taken up by the primary coil per switch-on period of the switch and subsequently output to the secondary coil for supplying the load is larger, the longer the switch remains closed. The power consumed by the switched-mode power supply corresponds to the time average of this energy taken up per switch-on period of the switch.
The switch-on periods of the switch are prolonged with rising power consumption of the load, in order that the voltage present across the load can be kept at least approximately constant. In order to set the duration of the drive pulses, a control signal dependent on the output voltage is formed. If the output voltage changes on account of a load change, then the control signal also changes in order, in this way, to be able to adapt the duration of the drive pulses to the changed load conditions.
In order to define the switch-on instants of the switch, it is known to evaluate the voltage present across the primary coil or the switch and to close the switch again after opening when the primary coil is free of energy or demagnetized, that is to say when the primary coil has output its stored energy to the secondary side. In the case of small loads with a low power consumption, when the drive pulses are very short and the energy taken up by the primary coil per switch-on operation is correspondingly low, this results in a high switching frequency.
The switching losses of the power supply unit increase with a rising switching frequency of the switch. Therefore, Published, Non-Prosecuted German Patent Application DE 197 32 169 A1 proposes a switched-mode power supply in which, in the case of a small power output of the switched-mode power supply, the switch is not immediately switched on when the primary coil is free of energy or demagnetized. Rather, in the known switched-mode power supply, the switch-on instants are also set in a manner dependent on the control voltage, the time duration between two switch-on instants being longer, the smaller the power output by the switched-mode power supply. In order to set the switch-on instants, in the known apparatus, the control signal is used to form a pulse-width-modulated signal whose duration depends on the amplitude of the control signal and which concomitantly determines the time interval between two drive pulses.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for driving a switch in a switched-mode power supply and a switched-mode power supply that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, in which a rise in the circuit losses in the case of a low power output is avoided.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for driving a switched-mode power supply having a power switch connected in series with a primary coil of a transformer. The switched-mode power supply generates a control signal dependent on an output voltage of the switched-mode power supply. The method includes the step of detecting reference instants. The reference instants are first reference instants at which a power switch voltage present across the power switch corresponds to a first reference voltage value or second reference instants at which a coil voltage present across the primary coil corresponds to a second reference voltage value. The power switch is closed after the number of the reference instants detected has reached a comparison numerical value. The comparison numerical value is increased or decreased in dependence on whether the control signal exceeds an upper limit value or falls below a lower limit value.
The method according to the invention for driving the power switch in the switched-mode power supply, which is connected in series with the primary coil of the transformer, provides, after the opening of the switch, for reference instants at which a voltage present across the switch corresponds to a first reference voltage value or at which a voltage present across the switch corresponds to a second reference voltage value to be detected and for the switch to be closed only when the number of detected reference instants corresponds to a comparison numerical value. The comparison numerical value is increased or reduced depending on whether a control signal dependent on an output voltage exceeds an upper limit value or falls below a lower limit value.
The method according to the invention utilizes so-called free transformer oscillations, which are established if, after the demagnetization of the primary coil, the switch is not immediately closed again in order to supply the primary coil with energy again. The free transformer oscillations result from a resonant circuit that, in the case of free-running switched-mode power supplies, is formed by the primary coil and a snubber capacitor connected in parallel with the switch, or in series with the primary coil. The transformer oscillations bring about a periodic alternating voltage across the switch or the primary coil. In the method according to the invention, “zero crossings” of the periodic voltage present across the primary coil or across the switch are detected. In the text below, “zero crossing” designates an instant at which the amplitudes of the periodic alternating voltages across the switch, or across the primary coil, in each case intersect the amplitude value about which they periodically oscillate, or which corresponds to the average value of the respective periodic alternating voltage. The primary coil is at least approximately free of energy during the zero crossings. The interval between the zero crossings is constant and depends on the frequency of the resonant circuit.
The method according to the invention utilizes the uniformly spaced zero crossings as a temporal reference for switching on the switch again. The number of zero crossings that occur before the switch is switched on again is dependent on the comparison numerical value that is set in a manner dependent on the control signal. In this way, the switch-on frequency can be reduced as the output power decreases.
The comparison numerical value is increased or reduced depending on whether the control signal exceeds an upper limit value or falls below a lower limit value. In one embodiment, in which the control signal increases with a decreasing output voltage, the comparison numerical value is reduced if the control signal exceeds the upper limit value. The reduction of the comparison numerical value leads to an increase in the switching frequency or to a shortening of the time duration until the next switch-on of the switch and thus counteracts a further decrease in the output voltage. If the control signal drops below the lower limit value, then the comparison numerical value is increased in order to reduce the switching frequency and thus counteract a further rise in the output voltage.
The control signal is preferably evaluated before the counting up of the comparison numerical value during an evaluation time duration that is longer than the period duration of a periodically changing load, the comparison numerical value being counted up only when the control signal
Greenberg Laurence A.
Infineon - Technologies AG
Mayback Gregory L.
Stemer Werner H.
Sterrett Jeffrey
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