GB2105129A - Combined power line monitor and control regulator for a switching power supply - Google Patents

Abstract

A combined power line monitor and control regulator comprising a single transistor 62 and an integrated- circuit voltage regulator 50 and associated passive elements provides a valid and safe set of signal states for other operating circuits, including the control circuit of a pulse-width-modulated switching power supply (not shown), for any power-line voltage over a given range, regardless of rate of rise, drop- out duration, on-off cycle repetition rate, or previous signal state. A drop in ac supply causes C82 voltage to fall and T62 to turn off. QCL then turns on to disable output stages QO of the regulator. Regulation is achieved via feedback through R90, R86. <IMAGE>

Description

SPECIFICATION Combined power line monitor and control regulator for a switching power supply Background of the invention The present invention relates to voltage regulator circuits in general, and in particular to a combined power line monitor and control regulator for a pulse-width-modulated switching DC power supply.

Pulse-width-modulated switching is a well-known regulation technique employed in certain DC power supplies. A principal advantage drawn by this type of regulation is a substantially reduced power dissipation within the supply itself while maintaining a moderately high degree of regulation, resulting in the attendant use of smaller, lighter transformers and circuit components.

Traditional protection circuits for automatic shutdown in the event of power line aberrations are available, and such circuits include so-called crowbar circuits, silicon-controlled rectifiers, and programmable unijunction transistors. Traditional voltage regulators for providing regulated reference voltages are also available. However, the addition of circuits and components negates the advantage achieved by the pulse-width-modulated switching regulation technique. It was desired to incorporate into a proposed commercial pulse-width-modulated switching power supply specific protection and control functions, and yet maintain integrity of the design by keeping the parts count and cost low, and without unduly increasing power consumption.

Summary of the invention In accordance with the present invention, a simple combined power line monitor and control regulator for a switching power supply performs several protection and control functions with a relatively small number of components. Specifically, the proposed commercial embodiment, which comprises a single commercially-available integrated circuit voltage regulator, a transistor, and a small number of associated passive components, provides power line level detection, line-cycle drop-out detection, control voltage regulation, reference voltage generation, and the generation of certain transient (power up/down) state control signals.

It is therefore one object of the present invention to provide a combined power line monitor and control regulator for a switching power supply.

It is another object of the present invention to provide a novel and comparatively simple circuit which performs a large number of protective and control functions to insure proper operation of an electronic system.

It is a further object of the present to provide a valid and safe set of signal states to control circuit of a pulse-width-modulated switching power supply for any power-line voltage over a given range, regardless of rate of rise, drop-out duration, on-off cycle repetition rate, or previous signal state.

It is an additional object of the present invention to provide a combined power line monitor and control regulator for a switching power supply which is simple, uses relatively few parts for the large number of functions performed, and is inexpensive.

Other objects and advantages will become apparent to those having ordinary skill in the art upon a reading of the following description when taken in conjunction with the drawings.

Drawings Figure 1 is a block diagram of a switching power supply employing a combined power line monitor and control regulator in accordance with the present invention; and Figure 2 is a detailed circuit schematic of a preferred embodiment of the present invention.

Detailed description of the invention Referring to the block diagram of Figure 1, alternating current (AC) line voltage from a power source is applied via a transformer 10 to a rectifier and filter circuit 12. A rectified and filtered direc-current (DC) voltage produced thereby is applied to a switched output stage 14which develops one or more regulated DC voltage outputs. In a proposed commercial embodiment, the switched output stage 14 includes a pair of transistors which alternately conduct at a 20-kilohertz rate to drive the primary winding of an output transformer. One or more secondary windings may be provided, each incorporating an associated rectifier and filter circuit to produce the output voltages.Samples of the output voltages are fed back to an output regulator circuit 16, which compares the fed-back sample with a predetermined reference voltage and develops from this comparison a control logic signal indicative of the regulated DC voltage output being over or under the nominal output level. The control logic signal is applied to a control logic and driver circuit 18, which develops pulse width control signals two drive the switched output stage 14. In the proposed commercial embodiment, the control logic circuit is driven by a 40-kilohertz clock, which is divided down and modified by various control signals, including the control logic signal from output regulator 16, to develop appropriate 20-kilohertz pulse width control signals to drive the switched output stage 14.The drivers may suitably be grounded-emitter transistors responsive to the pulse width control signals to provide sufficient drive currentforthe output stage.

A combined power line monitor and control regulator 20 is connected to transformer 10 to provide power line level detection, line-cycle dropout detection, control voltage regulation, reference voltage generation, and the generation of valid and safe signal states to the driver circuit 18 and to one or more power indication circuits.

The details of the power line monitor and control regulator 20 are shown in Figure 2. Power for operation of the circuit is provided by a 16-volt AC secondary winding 40 of transformer 10, and is therefore derived directly from the power line AC voltage and is a representative sample thereof. The AC voltage is applied via a fuse 42 to a half-wave rectifier diode 44 and filter 46 to provide a nominal +20 volts DC. This voltage is applied to a regulator circuit 50, shown generally enclosed by a dashed line.

The regulator circuit 50 in the proposed commercial embodiment is a commercially-available A723 voltage regulator integrated circuit. Important internal details are shown and will be discussed, however, because they are germane to the circuit operation. Additionally, the voltage regulator could be fabricated of discrete components, although the advantages of space, cost, and perhaps power consumption savings would be lost.

In the regulator circuit 50, series-pass output transistor QO is connected between power supply terminals V+ and Vc and a voltage output terminal VO. The output terminal V0 is connected to ground through a diode 52 and resistors 54 and 56 connected in series. A regulated + 10 volts DC is developed across resistors 54 and 56 and is available for the various switching power supply circuits. The junction of resistors 54 and 56 is connected to the inverting input (INV) of an internal differential amplifier DA to provide divided-down feedback voltage to establish the operational amplifier control of the output series-pass transistor QO. An internal reference voltage circuit (REF. CKT.) produces a +7.15 volts reference which is made available to the external circuits via terminal VREF.The noninverting input (NON INV) of the internal differential amplifier DA is connected to the +7.15V reference as well. The common emitters of the differential amplifier pair are connected through a current source to ground via the substrate voltage terminal Vss. An appropriate compensating capacitor 58 is connected across the INV and FREQ COMP terminals of regulator circuit 50.

Acurrent-limiting transistor QCL is provided within regulator 50 to disable the output series-pass transistor Q0 under certain circumstances to be discussed below. The base of transistors (1CL is connected via a terminal labeled CUR LIM and a current-limiting base resistor 60 to the collector of a switch transistor 62. The collector of switch transistor 62 is also connected through a load resistor 64 to the +20-volt DC nominal supply, while the emitter thereof is connected to ground. The emitter of current-limiting transistor QCL is connected to ground via the currentsense-terminal CUR SEN and a pair of low-resistance resistors 68 and 70.A pair of clamping diodes 72 and 74 ensures that the junction of resistors 68 and 70 is kept near ground under all operating conditions.

This point is also connected via the fuse 42 to one end of the secondary winding 40 to provide reference for the line monitor circuit, which includes the aforementioned switch transistor 62.

A diode 80 and capacitor 82 are connected in series across the secondary winding 40, and a resistor 84 connects the junction thereof to the base of transistor 62. A bias resistor 86 is connected from the base of transistor 62 to ground. Under normal operating conditions, capacitor 82 charges to the +20 volt nominal value, turning transistor 62 on. The collector of transistor 62 falls toward ground as the transistor saturates, cutting of transistor OCL inside the regulator circuit 50. With transistor (1CL cut off, the series-pass output transistor QO is released, allowing the regulated +10 volts output to come up to its steady state value as transistor QO rapidly turns on.The +10 volts is applied as positive feedback via resistor 90 to the base transistor 62, reinforcing the turn-on of transistor 62, resulting in a snap-action turn on of the +10 volt DC output.

The time constant of the above-described impedance network including capacitor 82 and resistor 84 is chosen to be relatively short -- for example, short enough to detect a two-cycle dropout of line voltage.

In such a situation, or if the line voltage level drops below a predetermined range, capacitor 82 discharges to a point where transistor 62 begins to shut off, the collector thereof rising as the transistor leaves saturation. This action in turn causes transistor QCLto turn on, cutting off the series-pass output transistor Oo. As the + 10-volt output drops and the regulator circuit 50 goes into current limiting, the voltage at the FREQ COMP terminal drops rapidly.

From the foregoing description of the circuit action, it can be seen that state signals indicating regulator condition are generated at the collector of transistor 62 and the FREQ COMP terminal. During proper operation of the regulator circuit 50, the collector of transistor 62 is low, and the FREQ COMP terminal is high. When a fault occurs as described hereinabove, the collector of transistor 62 goes high and the FREO COMP terminal goes low. Accordingly, the collector of transistor 62 is connected through a pair of resistors 94 and 96 to the bases of driver transistors 18-1 and 18-2 in the control logic and driver circuit 18 (see Figure 1) to block the pulse width control signal to the switched output stage during fault occurrence and to allow the pulse width control signals to pass during normal operation.The low at the FREQ COMP terminal of regulator 50 during fault occurrence is coupled via a diode 100 to certain power indicator circuits, to provide information indicative of voltage regulator, and hence, power supply, operating status.

From the foregoing description, it can be discerned that a combined power line monitor and control regulator provides a valid and safe set of signal states to the control circuit of a pulse-widthmodulated switching power supply for any powerline voltage over a given range, regardless of rate of rise, drop-out duration, on-off cycle repetition rate, or previous signal state. Also, the desired reference voltages are produced.

While I have shown and described herein the preferred embodiment of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects.

Therefore, the scope of the present invention should be determined only by the following claims.

Claims (8)

1. A combined power line monitor and control regulator, comprising: means for deriving a representative AC voltage sample of a power line voltage; first means for converting said AC voltage to a DC voltage; voltage regulating means coupled to said first converting means for providing a regulated DC voltage output; second means for converting said AC voltage to a DC voltage, said second converting means including means for detecting fault conditions of said power line voltage; and means coupled to said second converting means for generating control signals in response to said fault conditions.

2. A combined power line monitor and control regulator in accordance with claim 1 wherein said voltage regulating means comprises a series-pass element and an operational amplifier control circuit therefore, said voltage regulating means further comprising means connected to said control signal generating means for disabling said series-pass element in response to a detected fault condition.

3. A combined power line monitor and control regulator in accordance with claim 2 wherein said second converting means comprises a diode, and said fault condition detecting means comprises an impedance network having a predetermined time constant.

4. A combined power line monitor and control regulator in accordance with claim 3 wherein said control signal generating means comprises switch means coupled to said impedance network.

5. A combined power line monitor and control regulator in accordance with claim 4 wherein said switch means comprises a transistor, the base of which is connected to said impedance network.

6. A combined power line monitor and control regulator in accordance with claim 5 wherein the regulated DC output of said voltage regulating means is coupled to the base of said switch transistor.

7. A combined power line monitor and control regulator in accordance with claim 2 wherein said voltage regulating means further comprises means for generating state signals indicative of voltage regulator operating status.

8. A combined power line monitor and control regulator substantially as hereinbefore described and as illustrated in the accompanying drawings.