Circuit for driving a switching transistor

Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter

Reexamination Certificate

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Details

C363S133000

Reexamination Certificate

active

06262898

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is based on a circuit for driving a switching transistor with a driver stage. A circuit of this type is disclosed for example in DE-A-43 17 154 and is used in particular in horizontal line deflection circuits for television sets and computer monitors.
2. Description of the Related Art
Switching transistors used for this purpose must be driven with a defined current in order to keep the power demand and the losses of the driver stage low. Moreover, the switching transistor ought not to be operated at saturation since it then impairs the switching behaviour of the switching transistor as a result of high recombination times of electrons and electrons-holes in the transistor. Furthermore, the switching transistor ought, in each case in the shortest possible time, both to be turned off and to be turned on into a volume resistance which is as small as possible, in order to minimize the losses in the switching transistor. These partly conflicting requirements should be fulfilled as optimally as possible by a circuit for driving a switching transistor.
BRIEF SUMMARY OF THE INVENTION
The invention is based on the object of specifying a power-efficient circuit of this type for driving a switching transistor, which circuit is also cost-effective.
This object is achieved by means of the features of the invention as specified in claim
1
. Advantageous developments of the invention are specified in the sub-claims.
The circuit for driving a switching transistor according to the invention contains a comparator circuit, which monitors the voltage across the current input of the switching transistor and compares it with a reference voltage for the purpose of monitoring saturation of the switching transistor. Since this voltage is a measure of the saturation of the switching transistor and is likewise a measure of the losses of the said transistor, efficient regulation is made possible by the comparator circuit.
The voltage across the current input, the collector in the case of an npn switching transistor, can be tapped by means of a diode, for example. Since this voltage is very high at the switch-off instant of the switching transistor and is typically 1500 V in the case of deflection stages in television sets, in this case it is also possible for a plurality of diodes to be connected in series, as a result of which their voltage loading is reduced. The diode is advantageously connected via a resistor to an operating voltage, with the result that a defined current flows through this diode in the turned-on phase of the switching transistor and, as a result of this, there is a defined voltage drop across the said diode. This voltage is added to the collector voltage to form a voltage which can be utilized as a regulating signal. The comparator circuit is used to effect regulation to a constant collector voltage of the switching transistor when the latter is turned on.
The comparator circuit used may be, for example, a transistor which is connected by its terminals to the driver stage, the reference voltage and the voltage across the current input of the switching transistor. The reference voltage may be derived at an operating voltage, for example with the aid of a voltage divider.
Whereas individual properties of the switching transistor and loading through the output side are not taken into account by the circuit of DE 43 17 154, the circuit of the invention corrects variations in the transistor parameters or changes in the collector current due, for example, to varying picture brightnesses in a television set as well as temperature influences. Thus, for example, the base current demand may vary by as much as a factor of two as a result of the variation in the current gain of switching transistors. If the base current for the switching transistor is not currently adjusted, the losses in the switching transistor or in the driver stage will rise.
By virtue of the regulation by way of the saturation of the switching transistor, the base current is regulated to a value at which both the conduction losses in the switching transistor and the switch-on and -off losses due to transistor capacitances are low both in the event of fluctuating collector currents and in the event of variations in the current gain (h
FE
) and in the event of temperature effects.


REFERENCES:
patent: 4713561 (1987-12-01), Yamada
patent: 4755741 (1988-07-01), Nelson
patent: 4987362 (1991-01-01), Zwanziger
patent: 5703764 (1997-12-01), Hermann et al.
patent: 6166927 (2000-12-01), Farrington et al.
patent: 6191960 (2000-12-01), Fraidlin et al.
patent: 0114540 (1984-01-01), None
patent: 0504559 (1992-09-01), None
Search Report for PCT Appln. No. PCT/EP99/00360.

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