GB2037033A - Switching regulator - Google Patents

Abstract

A switching regulator for a television display apparatus includes a controllable switch 16 coupled to a source of unregulated direct voltage (at 12). The switch is operated at the deflection rate to produce a pulsating direct voltage. A filter including a series choke 18 and a shunt capacitor 20 has its input (A) coupled to the switch 16 for filtering the pulsating direct voltage to form a direct voltage for operating the television display apparatus. An integrator 48, 50 has its input coupled to the input A of the filter for averaging the pulsating direct voltage. A control circuit 30 has an input (C) coupled to the integrator and an output (collector of 34) coupled to a control electrode of the controllable switch 16 for maintaining the average of the pulsating direct voltage constant. <IMAGE>

Description

SPECIFICATION Switching regulator This invention relates to switching regulators suitable for use in television receiver applications.

Self-oscillating switching regulators include a power switching device coupled to a source of unregulated direct voltage. The switching device is gated on and off by a control circuit to produce a pulsating direct voltage. A series inductor and shunt capacitor are coupled to the pulsating direct voltage for averaging the pulsations to produce a direct voltage for application to the load to be energized. The control circuit conventionaily compares a sample of the direct load voltage with a reference voltage to produce a switch control signal.

When the load voltage decreases, the switch is turned on in order to increase the load voltage, and when the load voltage is too high the switch is turned off so that the load voltage may decrease. Stability of such regulators may be assured by a hysteresis function of the control circuit. In such switching regulators, the output voltage level is proportional to the duty cycle of the switch. Thus, the duty cycle tends to vary with changes in the unregulated direct voltage to maintain a constant regulated voltage. With increasing load current, the filter capacitor tends to be discharged more quickly, and the repetition rate or duty cycle of the regulator tends to rise. A particular disadvantage of such prior art regulators is that the ripple of the output voltage is used to drive the comparator, and therefore the output ripple does not decrease at small output currents.

When used for television receiver applications, it is desirable to synchronize the switching of a switching type regulator with the horizontal deflection so as to eliminate apparent motion on the XXelevision screen of transients caused by switching of the regulator.

Consequently, the repetition rate of the regulato is held constant, and only duty cycle variations are available to control the voltage to be regulated. In normal operation, a television receiver can be expected to have considerable variations in its current demand, resulting primarily from changes in average kinescope beam current. These changes in current load the regulator, and because the synchronized repetition rate is fixed, result in changes in the waveshape of the voltage across the filter capacitor. These changes tend to unsynchronize the regulator by increasing the natural repetition rate or frequency to a value above the synchronizing frequency.

In accordance with a preferred embodiment of the invention, a switching regulator for synchronized operation with a load apparatus and adapted to be energized from a source of unregulated direct voltage includes a controllable switch having a control electrode and main current conducting path. The main current conducting path defines first and second terminals. The first terminal is coupled to a terminal of the source of unregulated voltage.

A filter including a choke is coupled to the second terminal of the switch and to a portion of the load apparatus for energizing the portion with a substantially constant voltage. A voltage control circuit is coupled to the control electrode of the switch and to the second terminal of the main current conducting path for controlling the switching time to maintain the average voltage at the second terminal at a substantially constant value.

In the drawing: Figure 1 is a schematic diagram of an embodiment of the invention; and Figure 2 illustrates amplitude-time graphs of voltages and currents occurring during operation of the apparatus of Fig. 1.

In Fig. 1 a switching regulator circuit designated generally as 10 includes terminal 1 2 and 14 adapted to be connected to a source of unregulated direct voltage (not shown) such as a rectifier and filter capacitor coupled to the alternating-current power mains. Regulator 10 includes a PNP power switching transistor 1 6 having its emitter coupled to terminal 1 2.

The collector of transistor 1 6 is connected at a circuit point A to a filter inductor or choke 1 8.

The end of inductor 1 8 remote from point A is connected at a circuit point E to one end of a filter capacitor 20, the other end of which is connected to terminal 14 (hereinafter referred to as "ground"). The direct output voltage to be regulated appears between point E and ground. A commutating or flywheel diode 22 has its cathode connected to point A and its anode connected to ground.

A control circuit designated generally as 30 includes a differential comparator including emitter-coupled NPN transistors 32 and 34, the emitters of which are joined and coupled to ground by means of a resistor 36. The collector of transistor 34 is connected to the base of transistor 1 6. The collector of transistor 32 is connected at a circuit point F to the base of a PNP transistor 38, the collectoremitter conducting path of which is connected between the base and emitter of transistor 1 6.

The base of transistor 34 is coupled to a reference voltage source, illustrated as a battery 40, by way of a circuit point B and a resistor 42. Resistor 42 together with a resistor 44 connected between prints A and B provides a hysteresis characteristic to the comparator. The base of transistor 32 is coupled at a circuit point C to an integrating network designated generally as 46 and including a resistor 48 and a capacitor 50. The input of integrating circuit 46 is coupled to point A. A load represented as a resistor 52 is coupled across capacitor 20.

In operation immediately prior to a time TO, as illustrated in Fig. 2, transistors 32 and 38 are conductive and transistor 34 is noncon ductive. Transistor 16 is therefore nonconductive and diode 22 conducts to supply the current demand of inductor 1 8. Circuit point A is nominally at ground potential as illustrated by VA in Fig. 2a. The voltage at point B available as a reference for comparator 30 is less than (negative with respect to) the voltage of battery 40, as illustrated by VB in Fig. 2b.

Capacitor 50 is discharging through resistor 48 towards the nominal ground potential at circuit point A, and produces a decreasing ramp voltage VC at circuit point C as illustrated in Fig. 2c. Capacitor 20 is charged to the output voltage VE applied across load 52, and the current in inductor 1 8 is decreasing as illustrated by IL of Fig. 2d as a portion of the energy stored in its magnetic field is transferred to capacitor 20. The average current flow through choke or inductor 1 8 required to maintain the regulated voltage constant must equal the current drain of load 52.

The current drawn from capacitor 20 by load 52 exceeds that available from inductor 18, and the voltage VE at circuit point E is decreasing, as illustrated by VE in Fig. 2e. Fig.

2e illustrates only the alternating or ripple component of VE.

Near time TO, the decreasing ramp voltage at point C becomes equal to and then slightly less than the voltage at circuit point B, whereupon comparator 30 switches, making transistors 32 and 38 nonconductive and transistor 34 conductive. With transistor 34 conductive and transistor 38 nonconductive, base current is supplied to transistor 16, saturating transistor 1 6 and raising voltage VA at circuit point A to a value greater (more positive) than voltage VE. The increase in voltage VA raises the voltage at circuit point B to a value slightly greater than the voltage of battery 40 and also greater than the voltage then appearing at point C, thereby guaranteeing that transistor 34 remains conductive to the exclusion of transistor 32. Diode 22 becomes nonconductive, and the current IL in inductor 18 begins to increase, as illustrated in Fig. 2d.

After time TO, the increase in voltage VA causes a positive-going ramp portion of voltage VC to be generated at circuit point C as capacitor 50 begins to charge through resistor 48. However, in the interval between time TO and T1 the current IL in inductor 18, although increasing, continues to be less than the current drawn by load 52, and the difference is supplied by discharge of capacitor 20.

Consequently, voltage VE across capacitor 20 continues to decrease in the interval T0-T1, albeit the discharge is slow near time T1. At time T1, the current in inductor 1 8 equals the current drain of load circuit 52. At time T1, there is no net current flow in capacitor 20, and it therefore neither charges nor discharges.

In the interval T1-T2, the current flow in inductor 1 8 exceeds the average current demand of the load, and voltage VE consequently increases. At times near time T2, current IL significantly exceeds the load demand, and a relatively large current is available to charge capacitor 20 at a rapid rate.

Near time T2, the increasing ramp voltage VC at circuit point C becomes equal to and slightly exceeds the voltage then applied to the comparator at circuit point B. At time T2, comparator 30 switches, transistors 32 and 38 become conductive and transistor 34 becomes nonconductive. Residual collector current of transistor 34 is prevented from flowing through the base-emitter junction of transistor 1 6 by the conduction of transistor 38. Consequently, transistor 1 6 is cut off and the energy associated with the magnetic field of inductor 1 8 drives the voltage at point A sharply negative. Diode 22 becomes conductive, limiting the negative-going excursion at point A to 1 VBe below ground.With point A at nominal ground, the voltage at circuit point B decreases to a value less than that of battery 40, thereby guaranteeing that transistors 32 and 38 remain conducting and transistor 34 remains nonconducting. Capacitor 50 begins to form a negative-going ramp voltage at circuit point C as it discharges through resistor 48. Ramp voltage VC and output voltage VE continue to decrease until VC intersects VB, the comparator again switches and the cycle repeats.

As so far described, regulator circuit 10 is free-running. Synchronization of regulator 10 is accomplished by applying negative-going pulses to circuit point F. A negative-going pulse at circuit point F forward-biases transistor 38, thereby turning off transistor 1 6 and establishing time T2 as a reference. So long as the natural self-oscillation time of regulator 10 is greater than the time interval between synchronizing pulses, the regulator will remain synchronized. The increase in frequency due to synchronizing in the forced repetion rate or frequency over the natural repetition rate of the oscillator tends to reduce the duty cycle of switch 16, thereby tending to reduce the average output voltage. The tendency to a low average voltage at point A is coupled by integrator 46 to point C, and tends to maintain transistor 32 off for a longer portion of the available interval. Consequently, transistor 34 tends to turn on for a longer portion of the available interval, counteracting by feedback action the tendency to a lower output voltage.

The output voltage at circuit point E has substantially the same average voltage as circuit point A, subject only to the small IR drop in inductor 1 8. The duty cycle of the regulator switch after synchronization will be the same as the free-running duty cycle for the same output voltage.

The arrangement according to invention provides improved stability during synchronized operation, as compared with prior-art arrangements, by eliminating load-current dependent changes in the voltage sample applied to the comparator which tend to increase the natural repetition rate. The arrangement according to the invention has the additional advantage that the magnitude of the AC ripple voltage superimposed upon the regulator direct output voltage is reduced at low load currents, because the comparator is not driven from the output.

Other embodiments of the invention will be apparent to those skilled in the art. For example, reference voltage source 40 may be constituted as a zener diode, rather than as a battery. Any of a number of comparator arrangements may be used, and capacitor-20 may be referenced to terminal 1 2 rather than terminal 1 4. The polarities of the transistors and diode may be reversed for oppositely poled unregulated direct voltage.

Claims (7)

1. A switching regulator adapted for synchronized operation with a load apparatus and adapted to be energized from a source of unregulated direct voltage, comprising: controllable switch means including a control electrode and a main current conducting path defining first and second terminals, said first terminal being coupled to a terminal of the source; a filter including a choke coupled to said second terminal of said switch means and to a portion of the load apparatus for energizing said portion with a substantially constant voltage; and voltage control means coupled to said control electrode and to said second terminal for controlling the time of said switching to maintain the average voltage at said second terminal constant.

2. A regulator according to Claim 1 wherein said filter further comprises capacitor means coupled to said choke for further filtering said voltage energizing said portion of said load apparatus.

3. A regulator according to Claim 1 or 2 wherein said voltage control means further comprises: a reference source; a comparator including a first input coupled to said reference source and an output coupled to said control electrode, and also including a second input electrode; and an integrator coupled to said second terminal of said switch means for averaging the voltage at said second terminal and coupled to said second input electrode for establishing the switching time of said comparator to maintain the average voltage at said second terminal constant.

4. A switching regulator according to Claim 1 wherein said load apparatus comprises a television display apparatus, and wherein said voltage control means is coupled with said display apparatus for synchronization of the regulator with a deflection frequency of said display apparatus.

5. A switching regulator for a television display apparatus adapted to be energised from a source of unregulated direct voltage, comprising: controllable switch means including a control electrode and a main controlled current path coupled to the source, said switch means being operated at the horizontal deflection rate for producing a pulsating direct voltage; filter means including an input electrode coupled to said switch means for filtering said pulsating direct voltage to form an operating voltage for the apparatus; an integrator coupled to said input electrode of said filter means for generating a control signal related to said operating voltage; and feedback control means coupled to said integrator and to said control electrode for controlling the duty cycle of said switch means to maintain said control signal substantially constant.

6. A switching regulator for a television display apparatus adapted to be energized from a source of unregulated direct voltage, comprising: controllable switch means including a control electrode and a main controlled current path coupled to the source, said switch means being operated at the horizontal deflection rate for producing a pulsating direct voltage; series inductance means coupled to said main current path and to the apparatus for producing direct current for energizing said apparatus from said pulsating direct voltage; and; feedback voltage control means coupled to the juncture of said inductance means and said switch means and to said control electrode for maintaining the average value of said pulsating direct voltage substantially constant over a wide range of unregulated direct voltages.

7. A switching regulator substantially as hereinbefore described with reference to Fig 1.