GB1578134A

GB1578134A - Power supply apparatus

Info

Publication number: GB1578134A
Application number: GB2322676A
Authority: GB
Original assignee: Gould Advance Ltd
Current assignee: Gould Advance Ltd
Priority date: 1977-06-08
Filing date: 1977-06-08
Publication date: 1980-11-05

Description

(54) IMPROVEMENTS IN AND RELATING TO POWER SUPPLY APPARATUS (71) We, GOULD ADVANCE LIMITED, a British Company of Roebuck Road, Hainault, Essex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to power supply apparatus of the converter type, in which output power is obtained by means of an oscillating circuit. The circuit is powered from the input power to be converted and an alternating output is obtained from the oscillatory current of the stage. Such units are convenient for use where it is desired to operate relatively low voltage equipment from an alternating mains supply. The invention has for its object to provide converters which are improved in various respects, as will appear.

The present invention provides a power supply apparatus including a direct-current source, an oscillator circuit comprising a feedback transformer having primary winding means and secondary winding means, and two semiconductor switching devices arranged to be turned on alternately to couple said source to a load circuit by way of said primary winding means of said feedback transformer, thereby to produce alternating current in said load and in said primary winding means, secondary winding means of said feedback transformer being coupled to provide control currents to said switching devices such as to ensure a constant relation between said load current and said control currents together with circuit means coupled between said load circuit and said secondary winding means of said feedback transformer to augment said control currents thereby to assist the commencement of oscillation in the oscillator circuit, said oscillator circuit further comprising pulse generating means arranged to generate a starter pulse and to apply said pulse to initiate oscillation in said oscillator circuit.

Features and advantages of the invention will appear from the following description of embodiments thereof, given by way of example, and the accompanying drawings, in which: Figure 1 shows a circuit diagram of a push-pull; converter; Figure 2 shows a circuit diagram of a half-bridge converter; Figure 3 shows a circuit diagram of a converter similar to that shown in Figure 2 but incorporating a low load protection circuit; Figure 4 shows a circuit diagram of apparatus similar to that shown in Figure 3 but incorporating a starter circuit, according to the present invention; Figure 5 shows a circuit diagram of apparatus similar to that shown in Figure 3 but incorporating an overcurrent protection circuit for use in the apparatus of the invention; Figure 6 shows a circuit diagram of a converter similar to that shown in Figure 3 but incorporating a regulating circuit for use in the apparatus of the invention; and Figure 7 shows a circuit diagram of a further regulator circuit for use in the apparatus of the invention.

Apparatus to which the present invention relates includes an oscillatory stage using alternately switched semi-conductor switching devices such as transistors, in which oscillation is maintained by means of a transformer which senses the transistor emitter or collector current in a primary winding and provides base current for the transistors from a secondary winding in such a manner as to ensure a constant proportionality between base and emitter currents, irrespective of the level of the currents. This type of operation is advantageous in that it provides a convenient method of driving the transistors and in particular is efficient in that only the current necessary for providing base current for the transistors is diverted to that purpose.

One known form of oscillatory stage of this kind is shown in Figure 1. Input alternating voltage is applied to terminals A C and rectified by diode brdige B I and applied to a capacitor Cl. A feedback transformer Tl has windings a and b which drive the base emitter circuits of two transistors Trl and Tr2. The collectors of the two transistors drive an output transformer T2 the secondary winding of which supplies the output terminals. The emitter current of the two transistors passes through two further windings c and d of feedback transformer Tl and maintain the circuit in oscillation by virtue of the positive feedback thus provided.

Transformer Tl is of the saturating core type. The circuit operates in push-pull fashion.

Figure 2 shows another known form of current feedback oscillatory stage. The mechanism by which the feedback operates is as follows. As before, the input is applied to terminals A C, rectified by bridge B I and applied to capacitor Cl. Transformer T3 has a primary winding T3a and secondary windings T3b and T3c which feed transistors Tr3 and Tr4 respectively.

Winding T3a of transformer T3 is connected in series with the primary winding T4a of output transformer T4 to the junction of two capacitors C2 and C3.

The transformer T4 has a secondary winding T4b which feeds the load.

In this circuit the current load reflected into the primary winding a of transformer T4, caused by the conduction of transistor Tr3, flows in the primary winding a of feedback transformer T3 in such a sense that the winding T3b maintains the transistor Tr3 in conduction. Feedback is thus positive. Conduction in transistor Tr3 is maintained until the core of transformer T3 saturates, due to the voltage time integral impressed upon winding b of the transformer by the constant value V be of transistor Tr3. Transistor Tr3 then turns off and the consequent reduction in current flowing in winding a of transformer T3 causes a flux reversal in the core, which induces a current in winding T3c, causing the transistor Tr4 to turn on. This transistor is then maintained in conduction in a manner similar to Tr3 until it turns off due to saturation of the core of the transformer.

A practical difficulty that arises with a circuit of the type shown in Figure 1 or 2 is due to the fact that oscillation depends upon flow of current in the primary winding of the output transformer, representing the reflected secondary load current. For this reason, if the load is removed then the feedback mechanism fails, and the circuit ceases to operate. A circuit in which this can be overcome is shown in Figure 3.

In the known circuit of Figure 3, output transformer T4 is replaced by transformer T5 which has two further windings c and d.

Winding c of transformer T5 is connected in parallel with winding b of transformer T3, through resistor Rl, and in similar fashion winding d of transformer T5 is connected across winding c of transformer T3 through resistor R2. In this way, the voltage which appears across the winding a of transformer T5 is fed back to supply the base currents of transistors Tr3 and Tr4; the magnitude of the base current can be controlled by selection of the resistance of resistors RI and R2. These are accordingly selected to maintain the switching transistors Tr3 and Tr4 in conduction in conditions of zero or light load. At moderate or heavy load conditions, the operation becomes substantially that as described above.

A further practical difficulty which arises with the known circuits described above is that the oscillatory condition is not selfstarting and some means must be introduced of initiating oscillation.

According to the invention a means of initiating oscillation is therefore provided as indicated in the circuit of Figure 4.

Oscillation can be started by injecting an appropriate pulse into the base of one of the two transistors Tr3 and Tr4 but this is not easily accomplished since the baseemitter junction of transistor Tr4, for example, is shunted by the relatively low inductance presented by winding c of transformer T3. Consequently, the current pulse must have a very steep wavefront if the resultant base current is not reduced to a low value before transistor Tr3 has had time in which to become conductive. In Figure 4, components corresponding to those of Figure 3 bear like reference numerals and to this extent the circuit will not be further described. However, the circuit also includes a resistor R3 connected in series with a capacitor C4 across the capacitor C3. Alternatively R3 can be connected to the junction of C2 with the collector TR3. The junction A of R3 and C4 is connected through a silicon controlled rectifier, or thyristor, SCR1 to the base of transistor Tr4. A neon tube Vl is also connected between the anode and gate terminals of the thyristor and a resistor R4 is connected between the cathode and gate terminals of the thyristor. The junction A of R3 and C4 is connected also through resistor R5 and diode Dl to the common connection between the emitter of transistor Tr3 and the collector of transistor Tr4.

When power is applied to the circuit, capacitors C2 and C3 become charged. The junction of R3 and C4, point A, rises in potential at a rate determined by the time constant R3, C4. When the potential at point A reaches the striking potential of the neon tube Vl, the neon fires and the thyristor SCR1 is consequently triggered.

The charge in capacitor C4 is then discharged through the thyristor into the base of transistor Tr4. By appropriate component selection, this pulse can be made sufficient to cause the initiation of oscillation. When oscillation is established, point A will be held at a low potential by virtue of the rectifying effect of diode Dl; this causes the thyristor SCR1 and the neon VI to be held in the non-conductive condition. If at any time the circuit should cease to oscillate, diode D1 ceases to maintain the low potential at point A and the thyristor again fires, restarting oscillation. The thyristor is necessary since it is unlikely that the current through a neon tube would have a fast enough risetime to initiate oscillation due to its relatively high resistance. It is possible for the neon tube to be replaced by another switching device, such as a diac, although this may require a lower voltage feed point obtained by splitting C4 into two series components and feeding from the common connection.

Another difficulty that arises with apparatus according to the invention is that in the absence of means for controlling the output voltage it is difficult to provide means for limiting the output current, for protection purposes, under conditions of excessive load or short circuit on the output of the supply unit. Figure 5 shows a method by means of which overload protection can be obtained.

In Figure 5 (from which the starting means of Figure 4 are omitted) means are provided for sensing the load current and in the event of excessive load the oscillation transformer is damped to an extent which causes oscillation to cease.

As shown in Figure 5, in which components similar to those of the previous Figures bear similar reference numerals, transformer T3 is replaced by a transformer T6 which is similar but which is provided with a further winding d. Also, a further transformer T7 is used which has a winding a through which the output current flows and a secondary winding b which feeds a bridge rectifier B2. The current flowing in winding a of transformer T7 will thus be proportional to the load current and a corresponding voltage will be developed across the winding b of this transformer.

This is rectified and applied to capacitor C5, shunted by resistor R6. A fraction of the voltage which is developed across R6 is selected by means of the potential divider comprising resistors R7 and R8 and this is applied to the control electrode of a thyristor SCR2. The anode of the thyristor is connected through diode D2 and resistor R9 and the thyristor is shunted by capacitor C6.

A further capacitor C7 is connected in parallel with resistor R8.

When the voltage at the junction of resistors R7 and R8 reaches a value sufficient to cause the thyristor SCR2 to conduct, the thyristor is triggered into its conductive state and places a heavy load upon one diagonal diode bridge B3 of which the other diagonal is connected across the winding d of tranformer T6. The thyristor, in conduction, thus places a heavy load on the winding d of transformer T6 sufficient to cause the oscillatory condition to cease, stopping output from the supply unit.

If the circuit of Figure 5 is operated in conjunction with the starter circuit of Figure 4, the components of the Figure 4 circuit will then make periodic attempts to restart oscillation, at intervals determined by the time constant R1, C4. This will continue indefinitely, or until such time as the overload is removed after which the oscillatory condition will be reinstituted in the normal way.

It is frequently desirable that a supply unit of the type described should be provided with means for controlling the output voltage of the converter, to hold it constant against fluctuations in the input supply voltage or voltage variations due to variations of the load supplied by the converter. Figure 6 shows one means by which this can be achieved. This circuit operates by controlling the voltage which appears across the input reservoir capacitor of the unit, in response to a signal derived from the output voltage.

In Figure 6, from which the starting means of Figure 4 are omitted, components corresponding to those of the other Figures bear like reference numerals. The main reservoir capacitor C1 is shunted by capacitor C2 and C3 and oscillation is maintained by transformer T3, driving transistors Tr3 and Tr4. The voltage developed across capacitor Cl, from bridge rectifier B 1, is controlled by controlling the input alternating voltage to the bridge by means of a triac SCR3, in response to the output voltage of the converter.

An output transformer T8 of the converter has a centre-tapped secondary winding which, with diodes D4 and D5 produce a rectified output fed through a filter choke coil Ll to a capacitor C8. The voltage which appears across capacitor C8 is examined by a bridge circuit comprising resistors R10, Rll, R12 and zener diode D6; the differential voltage is applied to the input of an amplifier Al which feeds a light emitting diode Lid1. The light emission of this diode, which forms one part of an optocoupler OCI, will thus vary in accordance with the output voltage across capacitor C8.

The second part of the opto-coupler comprises a light sensitive diode PD1 in series with a resistor R13 and connected as shown to transistor Tr5, so that the collector emitter path of the transistor will be a function of the output voltage.

To control the triac SCR3, a resistor R14 is connected in series with a resistor R15 and capacitor C9 across the input alternating supply. Capacitor C9 and resistor R15 are shunted by two oppositely connected zener diodes D7 and D8 so that a limited voltage is established across the series connection of capacitor C9 and R15 in each half cycle of the applied alternating voltage. The voltage at the junction of C9: and R15 is applied through a diac SCR4 to the control electrode of the triac SCR3.

Capacitor C9 is also shunted by one diagonal of a diode bridge B4 of which the other diagonal is connected to the collector and emitter of transistor Tr5.

With this circuit, when an alternating current input is applied to the circuit, a limited alternating voltage is developed across the two zener diodes D7 and D8.

This causes capacitor C9 to charge, through resistor R15, in the appropriate direction during each half cycle of the alternating current input. The triac SCR3 will therefore fire at an appropriate point in the alternating point in the cycle when the voltage across capacitor C9 reaches the trigger voltage of the triac, through diac SCR4. Accordingly, the instant in the cycle at which the triac is caused to fire can be controlled by variation of an impedance presented across capacitor C9, and such an impedance is presented by the output of the bridge B4, which will be controlled by transistor Tr5 and in turn controlled by the opto-coupler OC1, in response to the output voltage of the converter.

Figure 7, from which the starting means of Figure 4 are again omitted, shows another arrangement for controlling the input voltage developed across the main input reservoir capacitor of the system. In Figure 7 a power transistor is used in a high frequency DC to DC chopper. Such circuits can take various forms but that of Figure 7 is an "on-demand" chopper regulator. In Figure 7 the input alternating current is applied to bridge B I and the main reservoir capacitor Cl. Connected across capacitor Cl is a zener diode D9 in series with a resistor R16. The voltage appearing at the zener diode is applied through resistor R17 to the base of transistor Tr6, the collector current of which passes through resistor R18. The voltage developed across this last resistance controls the base emitter voltage of transistor Tr7, the emitter collector path of transistor Tr7 being included in series with the output of bridge B1 and a filter inductor L2 to an output capacitor C10.

The voltage across capacitor C10 is the voltage which is then applied to the input of the converter proper. The output voltage is also applied to the emitter of transistor Tr6 which thus acts as a voltage comparator.

When the output voltage is low compared with t'ne reference voltage developed across the zener diode D9 the main control transistor Tr7 is driven to a conductive condition by operation of transistor Tr6 in a manner to restore the output voltage across capacitor C10. Similarly, if the output voltage is high, transistor Tr7 is turned off.

The circuit will thus oscillate with Tr7 being turned on and off in accordance with the demand made by the load. A small proportion of the collector switching waveform of transistor Tr7 is fed back to the reference voltage source by a potential divider presented by resistor R17 and a further resistor R18. By this means positive feedback is provided to ensure that the circuit operates in an oscillatory manner. A diode D10 is used as a flywheel diode. The DC to DC chopper described can be used to regulate either the input or the output of a converter.

It will be seen that the invention thus provides improvements applicable to current feedback inverters having a low load protection circuit and involving improved starting means and/or means for controlling the output both with respect to load conditions and to voltage variations.

WHAT WE CLAIM IS: 1. Power supply apparatus including a direct-current source, an oscillator circuit comprising a feedback transformer having primary winding means and secondary winding means, and two semiconductor switching devices arranged to be turned on alternately to couple said source to a load circuit by way of said primary winding means of said feedback transformer, thereby to produce alternating current in said load and in said primary winding means, said secondary winding means of said feedback transformer being coupled to provide control currents to said switching devices such as to ensure a constant

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. winding which, with diodes D4 and D5 produce a rectified output fed through a filter choke coil Ll to a capacitor C8. The voltage which appears across capacitor C8 is examined by a bridge circuit comprising resistors R10, Rll, R12 and zener diode D6; the differential voltage is applied to the input of an amplifier Al which feeds a light emitting diode Lid1. The light emission of this diode, which forms one part of an optocoupler OCI, will thus vary in accordance with the output voltage across capacitor C8. The second part of the opto-coupler comprises a light sensitive diode PD1 in series with a resistor R13 and connected as shown to transistor Tr5, so that the collector emitter path of the transistor will be a function of the output voltage. To control the triac SCR3, a resistor R14 is connected in series with a resistor R15 and capacitor C9 across the input alternating supply. Capacitor C9 and resistor R15 are shunted by two oppositely connected zener diodes D7 and D8 so that a limited voltage is established across the series connection of capacitor C9 and R15 in each half cycle of the applied alternating voltage. The voltage at the junction of C9: and R15 is applied through a diac SCR4 to the control electrode of the triac SCR3. Capacitor C9 is also shunted by one diagonal of a diode bridge B4 of which the other diagonal is connected to the collector and emitter of transistor Tr5. With this circuit, when an alternating current input is applied to the circuit, a limited alternating voltage is developed across the two zener diodes D7 and D8. This causes capacitor C9 to charge, through resistor R15, in the appropriate direction during each half cycle of the alternating current input. The triac SCR3 will therefore fire at an appropriate point in the alternating point in the cycle when the voltage across capacitor C9 reaches the trigger voltage of the triac, through diac SCR4. Accordingly, the instant in the cycle at which the triac is caused to fire can be controlled by variation of an impedance presented across capacitor C9, and such an impedance is presented by the output of the bridge B4, which will be controlled by transistor Tr5 and in turn controlled by the opto-coupler OC1, in response to the output voltage of the converter. Figure 7, from which the starting means of Figure 4 are again omitted, shows another arrangement for controlling the input voltage developed across the main input reservoir capacitor of the system. In Figure 7 a power transistor is used in a high frequency DC to DC chopper. Such circuits can take various forms but that of Figure 7 is an "on-demand" chopper regulator. In Figure 7 the input alternating current is applied to bridge B I and the main reservoir capacitor Cl. Connected across capacitor Cl is a zener diode D9 in series with a resistor R16. The voltage appearing at the zener diode is applied through resistor R17 to the base of transistor Tr6, the collector current of which passes through resistor R18. The voltage developed across this last resistance controls the base emitter voltage of transistor Tr7, the emitter collector path of transistor Tr7 being included in series with the output of bridge B1 and a filter inductor L2 to an output capacitor C10. The voltage across capacitor C10 is the voltage which is then applied to the input of the converter proper. The output voltage is also applied to the emitter of transistor Tr6 which thus acts as a voltage comparator. When the output voltage is low compared with t'ne reference voltage developed across the zener diode D9 the main control transistor Tr7 is driven to a conductive condition by operation of transistor Tr6 in a manner to restore the output voltage across capacitor C10. Similarly, if the output voltage is high, transistor Tr7 is turned off. The circuit will thus oscillate with Tr7 being turned on and off in accordance with the demand made by the load. A small proportion of the collector switching waveform of transistor Tr7 is fed back to the reference voltage source by a potential divider presented by resistor R17 and a further resistor R18. By this means positive feedback is provided to ensure that the circuit operates in an oscillatory manner. A diode D10 is used as a flywheel diode. The DC to DC chopper described can be used to regulate either the input or the output of a converter. It will be seen that the invention thus provides improvements applicable to current feedback inverters having a low load protection circuit and involving improved starting means and/or means for controlling the output both with respect to load conditions and to voltage variations. WHAT WE CLAIM IS:

1. Power supply apparatus including a direct-current source, an oscillator circuit comprising a feedback transformer having primary winding means and secondary winding means, and two semiconductor switching devices arranged to be turned on alternately to couple said source to a load circuit by way of said primary winding means of said feedback transformer, thereby to produce alternating current in said load and in said primary winding means, said secondary winding means of said feedback transformer being coupled to provide control currents to said switching devices such as to ensure a constant

relation between said load current and said control currents, together with circuit means coupled between said load circuit and said secondary winding means of said feedback transformer to augment said control currents thereby to assist the commencement of oscillation in the oscillator circuit, said oscillator circuit further comprising pulse generating means arranged to generate a starter pulse and to apply said pulse to initiate oscillation in said oscillator circuit.

2. Apparatus according to claim 1, wherein said circuit means comprises a further transformer, the primary winding of which is in series with the primary winding of the feedback transformer, and the secondary windings of which are in parallel with and connected via respective resistive paths to the secondary windings of the feedback transformer.

3. Apparatus according to claim 1 or 2, wherein the starter circuit comprises means for accumulating charge and means for triggering release of the charge to provide a current pulse to a control terminal of one of the switching devices.

4. Apparatus according to claim 3, wherein said triggering means comprises a controlled switching device and a gating device for controlling operation of the controlled switching device at a predetermined voltage.

5. Apparatus according to claim 4, wherein the controlled switching device is a semi-conductor controlled rectifier.

6. Apparatus according to claim 5, wherein the gating device is a neon or diac in the gate circuit of the semi-conductor controlled rectifier.

7. Apparatus according to any one of claims 3 to 6, and comprising means for detecting cessation of oscillation of said oscillator circuit and for controlling operation of said charge accumulating means.

8. Apparatus according to claim 7, wherein the means for detecting cessation of oscillation comprises a diode connected to the primary winding of the transformer and poled to bypass the triggering means.

9. Apparatus according to any one of the preceding claims, and comprising means for sensing output current and for loading the transformer to control oscillation of the oscillator when the output current exceeds a predetermined level.

10. Apparatus according to any one of the preceding claims, and comprising means for regulating the input to the oscillator, the regulating means being arranged to be connected to an a.c. supply.

11. Apparatus according to claim 10, and comprising means for sensing the output voltage, and sensing means being coupled to the regulating means for controlling operation of the regulating means to maintain the output voltage constant.

12. Apparatus according to claim 10, wherein the regulating means comprises means sensitive to charges in the supply voltage for chopping the supply to the oscillator circuit to maintain the input voltage constant.

13. Power supply apparatus substantially as hereinbefore described with reference to Figure 4 or with reference to Figure 4 as modified in accordance with Figure 5, 6 or 7 of the accompanying drawings.