Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2003-02-18
2003-12-16
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021090, C363S021170, C363S131000
Reexamination Certificate
active
06665197
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a circuit configuration for producing a switching signal for a current-controlled switch-mode power supply that has a transformer with at least one primary winding and with at least one secondary winding.
Prior art circuit configurations such as these, on which the invention is based, are known in a large number of modified forms from the prior art. In such a context, reference should be made, for example, to “U. Tietze and Ch. Schenk, Halbleiterschaltungstechnik 10th edition, Springer Verlag, Berlin Heidelberg, N.Y., 1993, pages 561 et seq.” These are based substantially on a controlled power switch that is connected in series with the primary winding to an input DC voltage. The circuit configuration also has a drive circuit for switching the power switch on and off clocked in time with a clock frequency, a current measurement device for measuring the current through the power switch and for producing a measurement signal that is a measure of the measured current, a reference signal source for producing a reference signal, with the reference signal being a constant reference signal that is independent of time, and a comparator circuit for comparing the measurement signal with the reference signal, with the comparator circuit being used to signal the drive circuit to switch off the power switch when the measurement signal is greater than the reference signal.
In principle, the method of operation of a switch-mode power supply such as this is as set forth in the following text.
The output voltage of the switch-mode power supply is regulated based upon evaluation of the current flowing through the power switch: first of all, a switching pulse from the drive circuit switches on the power switch. Due to the inductance of the primary winding of the transformer, which is connected in series with the control power switch, the current level of the current flowing through the power switch rises substantially linearly. When the current level reaches a specific value, then, the drive circuit switches the power switch off again.
The important factor in this case is that the current level of the current through the power switch does not exceed a maximum value. The maximum permissible value of the current level is dependent on the respective application, that is to say, on the applied input voltage and the load that is connected to the secondary side of the transformer. If the maximum value of the current level is exceed, then this leads to a high undesirable control error in the output voltage. In the worst case, if the maximum value of the current level through the power switch is exceeded, this can thermally overload and destroy the power switch. Furthermore, the excessively high output voltage on the second side of the transformer can lead to interference to the electrical supply to the load, or even to destruction of the load.
The signal for the drive circuit to switch off the power switch is generally produced by using a current measurement device to measure the current level of the current through the power switch. In such a case, a measurement signal is produced that is a measure of the measured current level of the current through the power switch. This instantaneous value is compared with a constant reference signal that is independent of time and is provided by the reference signal source mentioned above. If the measurement signal is higher than this reference signal provided, then the comparator circuit signals the drive circuit that it can switch off the power switch.
In current-controlled switch-mode power supplies such as these, the rate of rise of the current level of the current through the primary winding of the transformer and through the power switch is—as has already been indicated above—dependent on the respective operating state (operating voltage, load) of the application circuit.
In particular, this means that a higher input DC voltage results in the current level rising more rapidly. Furthermore, a greater load (higher output power, less load resistance), likewise, leads to the current level of the primary current through the primary winding and power switch rising more quickly.
Both the comparator circuit and the drive circuit with their integrated circuit blocks operate with delay times that are dependent on the circuitry. If the measurement signal, which is a measure of the measured current level of the measured primary-side current, reaches the value of the reference signal from the reference signal source, then the drive circuit switches off the power switch only after these signal delay times (gate delay times) resulting from these circuit blocks have elapsed. The current level on the primary side continues to rise linearly during these delay times.
The increase in the current level during these delay times is in this case dependent on the rate on which the current level rises, that is to say, on the respective operating state of the application circuit, and, hence, on the input voltage and output power. If the input voltage is low and the output power is low, the rate of rise of the current density on the primary side is low so that the increase in the current level during these delay times is also low. The control error, thus, remains low and there is no possibility of exceeding the maximum value, which could lead to thermal overloading and to destruction of the power switch and/or of the load. If the rate of rise is high, the change in the current level during the delay times caused by the circuitry is large. The control error is, thus, also large in a situation such as this. In this situation, the power switch and/or the load may very easily be thermally overloaded and destroyed if the maximum current level value is exceeded.
According to the prior art, the destruction of the power supply and/or of the load is prevented by selecting the components for the maximum power to be expected in the event of overloading.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a circuit configuration for producing a switching signal for a current-controlled switch-mode power supply that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that minimizes the control error, which is dependent on the rate at which the current rises on the primary side.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a circuit configuration for producing a switching signal for a current-controlled switch-mode power supply having transformer with at least one primary winding and with at least one secondary winding and an input DC voltage, including a controlled power switch to be connected to the input DC voltage in series with the at least one primary winding, a drive circuit connected to the power switch, the drive circuit switching the power switch on and off clocked in time with a signal having a clock frequency, a current measurement device connected to the power switch, the current measurement device measuring a current level through the power switch and producing a measurement signal being a measure of the measured current level, a reference signal source producing a reference signal, the reference signal being a constant reference signal independent of time, a comparator circuit connected to the drive circuit, to the current measurement device, and to the reference signal source, the comparator circuit comparing the measurement signal with the reference signal and signaling the drive circuit to switch off the power switch when the measurement signal is greater than the reference signal, and the reference signal source having an associated compensation signal source producing a compensation signal varying with time, the reference signal being obtained from a sum of the constant reference signal and the compensation signal, the reference signal at a time at which the power switch is switched on having a value less than at a time of the signal at which the power switch is swi
Fahlenkamp Marc
Gong Xiaowu
Jasberg Harmut
Greenberg Laurence A.
Han Jessica
Infineon - Technologies AG
Locher Ralph E.
Stemer Werner H.
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