Current limit circuit for power supply regulators
Most power supply regulators either cause the output voltage to drop-off gradually, or caused output oscillation upon reaching or exceeding the limit current. Current limit feedback to an error amplifier within a switching regulator is used to control the output voltage, prevent oscillation, insure that current limiting does not occur until a well-defined threshold is achieved and then steeply limit the current. The circuit uses feedback to the switching regulator to maintain the output voltage within desired limits. The output current flows through the current limit resistor R.sub.c thus providing a voltage proportional to the current at the base-emitter junction of Q.sub.1 via CR.sub.1 and R.sub.2. During normal operation, the forward biased CR.sub.1 voltage maintains a reverse bias on the B-E junction of Q.sub.1. When the voltage across R.sub.c exceeds the forward bias on CR.sub.1, Q.sub.1 turns on, hence turning on Q.sub.2, thus providing its contribution to the current limit feedback circuitry for use by the switching regulator.
This invention pertains to a method and apparatus for limiting current in a power supply.
BACKGROUND OF THE INVENTIONMost prior art circuits either caused the output voltage to drop-off gradually, or caused output oscillation upon reaching or exceeding the limit current.
OBJECTIVES OF THE INVENTIONIt is therefore a primary objective of the present invention to provide a circuit for regulating voltage of a power supply to provide good regulation of voltage up to a desired current limit value.
It is another objective to drop the voltage sharply above the desired current limit.
Still another objective is to avoid oscillation of the output voltage.
A further objective is provide a highly reliable circuit.
SUMMARY OF THE INVENTIONCurrent limit feedback to an error amplifier within a switching regulator is used to control the output current, prevent oscillation, insure that current limiting does not occur until a well-defined threshold is achieved and then steeply limit the current.
These objects of the invention and other objects, features and advantages to become apparent as the specification progresses are accomplished by the invention according to which, briefly stated, current limit feedback to an error amplifier within a switching regulator is used to control the output voltage and prevent oscillation.
LIST OF ADVANTAGES OF THE INVENTIONAn important advantage of the present invention is that the circuit uses provides a square voltage-current curve.
Another advantage is that the circuit uses few components.
A further advantage is that most components of the circuit are fairly insensitive to component tolerances or parameters.
Still another advantage is that oscillation is prevented.
These and further objectives, constructional and operational characteristics, and advantages of the invention will no double be more evident to those skilled in the art from the detailed description given hereinafter with reference to the figures of the accompanying drawings which illustrate a preferred embodiment by way of non-limiting example.
BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 shows the circuit according to the invention.
GLOSSARYThe following is a glossary of circuit elements as referenced and employed in the present invention.
Q.sub.1 --2N2222A transistor
Q.sub.2 --2N2907A transistor
Q.sub.3 --2N3584 transistor
Q.sub.4 --2N3584 transistor
CR1,CR2--1N414B diodes
R.sub.c --0.249.OMEGA. resistor
R.sub.1 --1000.OMEGA. resistor
R.sub.2 --16.2.OMEGA. resistor
R.sub.3 --4.99.OMEGA. resistor
R.sub.4 --499.OMEGA. resistor
R.sub.5 --3.01k.OMEGA. resistor
R.sub.6 --499.OMEGA. resistor
R.sub.7 --90.9.OMEGA. resistor
R.sub.8 --10k.OMEGA. resistor
C.sub.1, C.sub.2 --0.01 mfd capacitors
SR--switching regulator IC, e.g. Harris FG1532
DESCRIPTION OF THE PREFERRED EMBODIMENTSReferring now to the drawings wherein like reference numerals are used to designate like or corresponding parts throughout the various figures thereof, there is shown in FIG. I the circuit according to the invention.
The invention uses feedback to the switching regulator to maintain the output voltage V.sub.out within desired limits. The output current flows through the current limit resistor R.sub.c thus providing a voltage proportional to the current at the base-emitter junction of Q.sub.1 via CR.sub.1 and R.sub.2. During normal operation, the forward biased CR.sub.1 voltage maintains a reverse bias on the B-E junction of Q.sub.1. When the voltage across R.sub.c exceeds the forward bias on CR.sub.1, Q.sub.1 turns on, hence turning on Q.sub.2, thus providing its contribution to the current limit feedback circuitry for use by the switching regulator SR.
R.sub.1 is a resistor that maintains the continuous forward bias current through CR.sub.1. R.sub.2 provides some current limiting for the base-emitter junction of Q.sub.1 during hard current limiting conditions and output current spikes. R.sub.3 prevents Q.sub.2 from being driven too hard by Q.sub.1. R.sub.4 provides a path for leakage current from the base-collector junction of Q.sub.2 thus helping to hold it off during normal operation. Capacitor C.sub.1 is simply a filter capacitor for the regulator output.
Within the feedback circuit, R.sub.5 and R.sub.6 scale the feedback signal to the power control circuit. C.sub.2 and CR2 are filters to reduce the noise to the power control circuit.
This invention is not limited to the preferred embodiment and alternatives heretofore described, to which variations and improvements may be made, without departing from the scope of protection of the present patent and true spirit of the invention, the characteristics of which are summarized in the following claims.
Claims
1. A circuit for regulating voltage in a power supply, comprising:
- a power control circuit means for controlling a voltage of a power source,
- a current limiting means for generating a feedback signal to said power control circuit means while limiting current to an output, and
- a feedback circuit means for scaling a signal from a signal generation means to be sent to said power control circuit means.
| 3078410 | February 1963 | Thomas |
| 3101441 | August 1963 | Curry |
| 3173078 | March 1965 | Farnsworth |
| 3201680 | August 1965 | Ross et al. |
| 3219912 | November 1965 | Harrison |
| 3234453 | February 1966 | Klees et al. |
| 3671852 | June 1972 | Ritzenthaler |
| 3959713 | May 25, 1976 | Davis et al. |
| 4180768 | December 25, 1979 | Ferraro |
| 4335345 | June 15, 1982 | Converse |
Type: Grant
Filed: Mar 19, 1991
Date of Patent: Jun 7, 1994
Inventor: Thomas C. Stroup (Port Orchard, WA)
Primary Examiner: Bernarr E. Gregory
Application Number: 7/671,594
International Classification: G05F 1575;
