Power-supply circuit supplied by at least two different input vo

Details Power-supply circuit supplied by at least two different input vo Power-supply circuit supplied by at least two different input vo

1999-08-27

2000-11-21

Wong, Peter S.

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

G05F 140

Patent

active

061507997

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND INFORMATION

A conventional power supply circuit is intended to be driven at a plurality of input-voltage sources which can have different voltages. The input-voltage sources cannot easily be interconnected. Therefore, a customary measure provides for the use of decoupling diodes, so that input-voltage source which has the higher voltage is connected to the further circuit, while the input-voltage source having the lower voltage is decoupled. The disadvantage of this known design approach is that the voltage difference at least occurring between one output of the power-supply circuit and the input-voltage source is increased by the forward voltage of one diode, in addition to the voltage difference occurring anyway in the power-supply circuit.
An object of the present invention is to specify a power-supply circuit which can be supplied from at least two different input-voltage sources, it is advantageous to provide as small a voltage difference as possible between the output of the power-supply circuit and the input-voltage source having the highest voltage.


SUMMARY OF THE INVENTION

A power-supply circuit according to the present invention includes a current mirror connected to each input-voltage source. The current mirror makes the current provided by one current source available at the control input of an output transistor. Using this measure, the control voltage of the output transistor can come near to the voltage of the input-voltage source. The voltage difference can be in the range of only 0.1 to 0.3 volts, which corresponds to the saturation voltage of a collector-emitter section of a transistor contained in the current mirror. The current mirrors are described in detail, e.g. in the reference book by U. Tietze and Ch. Schenk, "Halbleiter-Schaltungstechnik", (Semiconductor-Current Technology) 5th edition, 1980, pages 55-56. A current mirror makes a maximum current, which is preselected at one input-current terminal, available at one output-current terminal. The transformation ratio can be freely selectably predefined. An advantage of this feature is that there can be a difference in potential between the current flowing into the input-current terminal and the current flowing out of the output-current terminal. The current mirror must be connected to a power-supply terminal which must be able to make available the currents occurring in the current mirror, the voltage at the power-supply terminal determining the potential ratios.
According to a first embodiment of the circuit of the present invention, the at least two input-voltage sources are interconnected, via the conventional decoupling diodes, at an output transistor. Nevertheless, the voltage drop occurring is reduced due to the connection of the current mirror directly to the input-voltage source, so that, in addition to the voltage drop which occurs at the decoupling diodes, only a small voltage drop must be accepted at the output transistor. The voltage drop at the output transistor is determined by its saturation voltage of the emitter-collector section.
According to another embodiment of the power-supply circuit according to the present invention, an output transistor is allocated to each input-voltage source. The output transistors are interconnected at the output of the power-supply circuit. The output transistors take care of the necessary decoupling of the input-voltage sources. The power-supply circuit of the present invention according to this embodiment permits a particularly small voltage difference between the voltage present at the output of the power-supply circuit and the input-voltage source having the highest voltage. Due to the omission of the decoupling diodes, the minimal voltage difference falls to the forward voltage of the output transistors.
The power-supply circuit of the present invention provides a containment of a single current source which makes a preselected current available to the current mirrors. This measure permits cost-effective implementation. Furthermore, the maximum current which the power

REFERENCES:
patent: 4617473 (1986-10-01), Bingham
patent: 5103157 (1992-04-01), Wright
patent: 5430365 (1995-07-01), Taylor et al.
patent: 5929623 (1999-07-01), Hoshino
U. Tietze et al., "Halbleiter-Schaltungstechnik", Feb. 19580 Springer Verlag, Berlin*, pp. 55-56.

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