GB2124804A - Electrical power supply arrangements - Google Patents

Abstract

In an electrical power supply arrangement for a television receiver in which the a.c. mains at input terminals (1,2) is rectified (D1,D2,D5,D6) to produce unidirectional half sine wave pulses which are applied to a series voltage regulator circuit (R1 ,T1) to produce a low voltage d.c. output across a smoothing capacitor (C2) at output terminals (5, 6), the output voltage is determined by a reference voltage diode (D7) in a potential divider (R2,D8,D7). In order to reduce dissipation in the regulator circuit, the regulator transistor (T1) is rendered non-conducting for a substantial period around the peak of each pulse. This is achieved by a control circuit (R3,R4,D9,R5,T2) which places an effective short circuit across diodes (D7,D8) when the pulse voltage rises to a given value. The series transistor (T1) conducts only during periods adjacent the beginning and end of each unidirectional pulse. <IMAGE>

Description

SPECIFICATION Electrical power supply arrangements The present invention relates to an electrical power supply arrangement for providing a low voltage d.c. supply from a high voltage a.c. supply and which comprises rectifying means for producing unidirectional pulses of half sine-wave shape from an applied a.c. supply of sinusoidal shape, connecting means for applying said unidirectional pulses to the input of a series voltage regulator circuit from whose output the low voltage d.c. supply is derived.

Such power supply arrangements are used to produce low voltage d.c. supplies from a domestic a.c. mains supply in domestic electrical and electronic equipment. It is normal to include a step-down transformer between the mains supply and the rectifying means so that the unidirectional pulses so produced are low voltage and thus the power dissipated in the series regulator circuit is kept to a minimum. Without such a transformer the power that would have to be dissipated by the series regulator circuit would be excessive. Such transformers are however expensive and occupy a substantial amount of space.

It is an object of the invention to provide such a power supply arrangement which does not require a transformer and where the power dissipated in the series regulator circuit is not excessive.

The invention provides an electrical power supply arrangement for providing a low voltage d.c. supply from a high voltage a.c. supply, said arrangement comprising rectifying means for producing unidirectional pulses of half sine wave shape from an applied a.c. supply of sinusoidal shape, connecting means for applying said unidirectional pulses to the input of a series voltage regulator circuit from whose output the low voltage d.c. supply is derived, characterised in that said unidirectional pulses are high voltage pulses, the arrangement additionally comprising control means for rendering said regulator circuit non-conducting for a substantial period around and including the peak of each unidirectional pulse such that said regulator circuit only conducts during corresponding periods adjacent the beginning and the end of each unidirectional pulse.

The expression "high voltage a.c. supply" is used to include the United Kingdom domestic mains supply of nominally 240 volts and the corresponding supply in Continental Europe of nominally 220 volts, both voltages being r.m.s. By the expression "low voltage d.c. supply" we mean a d.c. supply of a value which is of the order of 20% or less of the r.m.s. value of the high voltage a.c. supply.

The regulator circuit may comprise a regulator transistor whose main current path is connected between the circuit's input and output, the control means being connected to the base of said regulator transistor to periodically render the transistor conducting and nonconducting at the frequency of the a.c. supply.

The control means may comprise a potential divider connected to the rectifying means, the series arrangement of a voltage reference element and the base-emitter junction of a second transistor being connected across part of the potential divider, the collector of the second transistor being connected to the base of the regulator transistor and arranged such that the regulator transistor is rendered non-conducting when the instantaneous voltage across the series arrangement exceeds the reference voltage of the voltage reference device. The collector of the second transistor and the base of the regulator transistor may be connected to a point or points on a second potential divider of substantially like potential which potential determines the voltage of the d.c. supply from the output of the series regulator circuit.

The base of the regulator transistor may be connected to the junction of a third transistor and a second voltage reference element present in a second potential divider also connected to the rectifying means, the collector of the second transistor being connected to the base of the regulator transistor through the base-emitter junction of the third transistor.

The portion of the first mentioned potential divider across which the series arrangement is connected may be formed by two resistive components connected in series across one of which is connected a diode and capacitor in series so as to delay the time at which the instantaneous voltage across the series arrangement reaches the reference voltage of the first mentioned voltage reference device for a finite period after the supply arrangement is energised.

A protection circuit may be connected in parallel with the emitter-collector path of the second transistor to short circuit the path of that transistor in the event of an increase of the mean voltage at the collector of the second transistor.

The above and other features of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of a power supply arrangement according to the invention, Figure 2 shows graphs of voltages and currents present at various points in the arrangement of Fig. 1, Figure 3 is a modification of part of the arrangement shown in Fig. 1, Figure 4 shows a further modification of part of the arrangement of Fig. 1, and Figure 5 is a modification of the circuit shown in Fig. 3.

Fig. 1 shows an electrical power supply arrangement in which the terminals 1 and 2 are for connection to a domestic a.c. mains supply which in the U.K. has a nominal voltage of 240 volts r.m.s, whilst that in Continental Europe is nominally 220 volts r.m.s. The terminals 1 and 2 are respectively connected through mechanically operated on/off switches 51 a and 51 b which switches are linked to the input of a full-wave bridge rectifier circuit comprising diodes D1, D2, D3 and D4 poled in the manner shown. The junction of diodes D3 and D4 is taken through a switch S2a to a terminal 3 whilst the junction of diodes D1 and D2 is taken to a further switch S2b to a terminal 4, an electrolytic capacitor of large value being connected across the output of the bridge rectifier circuit.With switches S2a and S2b closed terminal 3 provides a positive d.c. voltage of 300 volts with respect to terminal 4. The arrangement shown is intended for use in a television receiver where the switches S2a and S2b are co-operating electronic switches which are opened when the receiver is set to the stand-by condition to remove this high d.c. voltage from the majority of the receiver's circuitry which is energised from terminals 3 and 4. If the supply from terminals 3 and 4 is applied to a switched mode power supply then the function of switches S2a and S2b could be provided by the switching device in such a supply which would be turned off by lack of drive pulses in the stand-by condition.

In parallel with the a.c. input to the bridge rectifier circuit are provided two series-connected diodes D5 and D6 poled as shown which together with diodes D1 and D2 form a second bridge rectifier circuit. The junction of diodes D5 and D6 produces a train of positive going unidirectional half sine wave shaped pulses of 340 volts peak by rectification of the mains supply, the waveform of the voltage appearing on the line A leading from this junction being as shown in Fig. 2a. The unidirectional pulses are applied to a series voltage regulator circuit comprising a resistor R1, an npn transistor T1 and an electrolytic capacitor C2 connected in the manner shown, the positive d.c. output from the regulator circuit being derived from a terminal 5 with respect to a terminal 6 which is connected to the junction of diodes D1 and D2.A potential divider comprising a resistor R2, a diode D8 and a voltage reference diode D7 is energised from the unidirectional pulses, bias for the base of transistor T1 being derived from the junction of diodes D7 and D8 which sets the output voltage present at terminal 5. In the absence of the remaining circuitry shown in Fig. 1 transistor T1 would conduct substantially all the time and if the output voltage at terminal 5 was say of the order of 1 6 volts then even with a small current drain from the voltage regulator circuit the power dissipated by current limiter resistor R1 and transistor T1 would be excessive. In such an arrangement diode D8 would not be required.

In order to overcome the above excessive dissipation in the series regulator circuit the conduction of transistor T1 is controlled by a control circuit comprising the remaining components shown in Fig. 1 such that transistor T1 is non-conducting for a substantial period around the peak of each unidirectional pulse present on line A. These pulses are applied to a potential divider comprising resistors R3 and R4, the series arrangement of a voltage reference diode D9 and the base-emitter junction of a second npn transistor T2 being connected in parallel with resistor R4 whilst a resistor R5 is conne.::ted in parallel with the base emitter junction of transistor T2 to prevent the base of this transistor being isolated when diode D9 is non-conducting.The voltage present at the junction 3 between resistors R3 and R4 is shown in Fig. 2b from which it will be seen that the voltage at this point during each unidirectional pulse rises until it reaches the reference voltage of 75 volts of the diode D9 (in practice, the reference voltage of diode D9 plus the small voltage across the base emitter junction of transistor T2), is then held constant at the reference voltage and then falls to its zero level. When diode D9 conducts the voltage on the base of transistor T2 causes this previously non-conducting transistor to conduct. The collector of transistor T2 is connected to the junction C between resistor R2 and diode D8 such that the emitter-collector path of transistor T2 is in parallel with diodes D7 and D8.When transistor T2 conducts the potential at junction C is taken substantially to the emitter potential of transistor T2 to reduce the voltage at the base of transistor T1 and stop that transistor conducting. At other times junction C is substantially at the reference voltage of diode D7. The voltage at junction C is shown in Fig. 2c, whilst the voltage on the lead D connected to the collector of transistor T1 is shown in Fig. 2d. E represents the current flow in the collector circuit of transistor T1, this current being shown in Fig. 2e and from this figure it will be seen that with transistor T1 non-conducting for a substantial period around the peak of each unidirectional pulse the current through that transistor is zero for a corresponding period.

Current only flows through transistor T1 for relatively short periods adjacent the beginning and end of each unidirectional pulse which current has a peak value which is greater than that which would flow through transistor T1 if that transistor were conducting for the whole of each pulse.

Figs. 2f and 2g respectively show the voltage and current of Figs. 2d and 2e when transistor T1 conducts on an enlarged scale, the current reducing to zero during those periods when the unidirectional pulses are at or below the d.c. but voltage whilst the voltage falls to substantially the output voltage.

Although the peak current through transistor T1 is relatively high for short periods during each unidirectional pulse the overall power consumption in the regulator circuit is substantially reduced compared with the situation where this transistor would be conducting during the whole of each unidirectional pulse. Under certain approximations it can be shown that the power consumption is proportional to 1 - cos 8 0 where 8 is the phase angle over which transistor T1 conducts and which is determined by the reference voltage of diode D9 and the ratio between the resistors R3 and R4.

In the circuit of Fig. 1 the diode D8 is used to protect transistor T1 when reverse-biased. This diode could be replaced by a resistor but in any case would not be required in the conventional regulator circuit.

The time at which transistor T1 is cut-off and then on is determined by the reference voltage of diode D9. If this reference voltage were reduced the peak currents through the regulator transistor T1 would increase and thus this voltage is chosen as a compromise to keep the peak currents within a reasonable limit.

With the arrangement of Fig. 1 the resistor R2 supplying bias current to transistor T1 consumes (comparatively) appreciable power. The base of this transistor must draw adequate current when transistor T1 is conducting during which periods the voltage of the unidirectional pulses is relatively low. (Transistor T1 for most of the time when conducting must be kept above the knee of its collector voltage/current characteristic to ensure stablisation.) Fig. 3 shows a modification of part of Fig. 1 where like references between these Figures indicate like components. The current through resistor R2 is reduced by including a current amplifier between the junction of diodes D7 and D8 and the base of transistor T1.The amplifier comprises a third npn transistor T3 whose base is connected to the above junction, whilst its emitter is connected to the base of transistor T2, the collector of transistor T3 being connected through a resistor R6 to line A. The current amplifier circuit of transistor T3 operates in conventional manner and will not further be described.

The current consumption through resistor R2 may alternatively be reduced by the modification of the circuit of part of Fig. 1 as shown in Fig. 4 where again like references between these figures indicate like components. Here a series switch for the base current of transistor T1 comprises a further npn transistor T4 which has its emitter connected to the base of transistor T1. The collector of transistor T4 is connected to the junction of a series circuit comprising a resistor R8 and an electrolytic capacitor C3 connected across the lines conveying the unidirectional pulses, resistor R8 determining the mean current for the base of transistor T1 whilst capacitor C3 provides the peaks for that current.The collector of transistor T2 is connected through a resistor R7 to the base of transistor T4 to control the switching of this transistor and hence the switching of transistor T1. When transistor T2 is non-conducting transistor T4 conducts and the voltage across capacitor C3 is held at the reference voltage of diode D7 so causing transistor T1 to conduct. When transistor T2 conducts transistor T4, and hence transistor T1, is held non-conducting as the base of transistor T4 is reduced to substantially the voltage on the emitter of transistor T2. The modified circuit of Fig. 4 does not use the diode D8 as in Figs. 1 and 3 but its function is provided by resistor R7.

Fig. 5 shows a modification of the circuit of Fig. 3 where again like reference numerals indicate like components between these two figures. In Fig. 5 it will be seen that diode D7 is formed by the series connection of two voltage reference diodes D7' and D7" each having a reference voltage of half that of D7 in Fig. 3 in order to provide improved temperature stability.

Also it will be seen that resistor R4 is now formed by two serially connected resistors R4' and R4" with resistor R4" of substantially half the value of resistor R4'. In parallel with resistor R4" is provided the series connection of a diode D10 poled in the manner shown and an electrolytic capacitor C4, a resistor R9 shunting the diode D10. The arrangement of these components is to reduce, at switch-on, the time taken for the voltage at terminal 5 to rise to its stablised level. At switch-on capacitor C4 is effectively a short circuit and hence diode D10 conducts to provide an effective short circuit across resistor R4".The voltage at point B is therefore lower than in the case where resistors R4' and R4" jointly form the bottom resistor of the potential divider and thus it takes longer for the voltage at point B to reach the reference voltage of the diode D9 and hence before transistor T2 conducts. Transistor T1 is therefore held conducting for a longer period during the initial unidirectional pulses and in fact nearer to the peak of these pulses to increase the charge to capacitor C2 and hence more quickly build up the voltage across that capacitor. Over the first few cycles from the a.c. supply capacitor C4 is charged until the voltage across it is such as to prevent diode D10 from conducting at which time the voltage at point B is once again determined by the resistor R4' and R4" in combination with resistor R3.Resistor R9 provides a discharge path for capacitor C4 when the circuit is switched off.

The other additional components in Fig. 5 form a protection circuit to guard against transistor T2 becoming open circuit or being permanently held non-conducting and hence causing transistor T1 to permanently conduct. In such an event, in the absence of the protection circuit, the voltage at point C would be the reference voltage of diodes D7' and D7" for substantially the whole time to produce a mean voltage approaching this reference voltage. The series arrangement of a resistor R 10 and a capacitor C5 is connected in parallel with transistor T2, the junction of this series arrangement being connected through a voltage reference diode Dii to the base of an npn transistor T5 and the collector of a pnp transistor T6.The two latter transistors connected in the manner shown effectively act, in known manner, as a small signal thyristor in parallel with transistor T2. Should the voltage at point C remain at the reference voltage of diode D7 capacitor C5 will be charged through resistor R10 until the reference voltage of diode D11 is just exceeded at which time this diode conducts to render conducting the previously non-conducting transistors T5 and T6. These transistors then place an effective short circuit across transistor T2 and hence diodes D8, D7' and D7" to cut transistor T1 off.

Under normal operating conditions the mean voltage at point C is insufficient to charge capacitor C5 to the reference voltage of diode D11. The two RC circuits comprising resistors R11, R12 and capacitors C6, C7 are provided to prevent spurious trigger such as could happen in a television receiver in the event of flash-over in a cathode ray tube.

In a practical embodiment combining the circuit of Fig. 5 and the appropriate portions of Fig.

1 the following components were employed: Diodes Resistors D1 Philips BYW56 R1 240 D2 ,, ,, R2 47 KE2 D3 ,, " R3 180 KS2 D4 ,, ,, R4' 100 KQ D5 ,, " R4" 56 KE2 D6 ,, " R5 47 KE2 D7' Philips BZX79B8V2 R6 1.5 K53 D7" ,, " R9 220 KE2 D8 Philips BA317 R10 220 uke2 D9 Philips BZX79C75V Ri 1 330 KS2 D10 Philips BA318 R12 68 KE2 D11 Philips BZX79C12V Capacitor Transistors C1 470 if T1 Philips BUX85 C2 2200 F T2 Philips BC548 C4 2.2 of T3 Philips BUX87 C5 220 nF T5 Philips BC548 C6 47 nF T6 Philips BC558 C7 47 nF The voltages and currents shown in Fig. 2 apply to the circuit of Fig. 5 with the above components for an A.C. mains supply of 240 volts r.m.s. to produce a d.c. output at terminal 5 of 1 5 volts.

Claims (8)

1. An electrical power supply arrangement for providing a low voltage d.c. supply from a high voltage a.c. supply, said arrangement comprising rectifying means for producing unidirectional pulses of half sine wave shape from an applied a.c. supply of sinusoidal shape, connecting means for applying said unidirectional pulses to the input of a series voltage regulator circuit from whose output the low voltage d.c. supply is derived, characterised in that said unidirectional pulses are high voltage pulses, the arrangement additionally comprising control means for rendering said regulator circuit non-conducting for a substantial period around and including the peak of each unidirectional pulse such that said regulator circuit only conducts during corresponding periods adjacent the beginning and the end of each unidirectional pulse.

2. A power supply arrangement as claimed in Claim 1, in which said regulator circuit comprises a regulator transistor whose main current path is connected between the circuit's input and output, characterised in that said control means is connected to the base of said regulator transistor periodically to render said transistor conducting and non-conducting at twice the frequency of said a.c. supply.

3. A power supply arrangement as claimed i-n Claim 2, characterised in that said control means comprises a potential divider connected to said rectifying means, the series arrangement of a voltage reference element and the base-emitter junction of a second transistor being connected across part of said potential divider, the collector of said second transistor being connected to the base of the regulator transistor and arranged such that the regulator transistor is rendered non-conducting during those periods when the instantaneous voltage across the series arrangement exceeds the reference voltage of said voltage reference device.

4. A power supply arrangement as claimed in Claim 3, characterised in that the collector of said second transistor and the base of the regulator transistor are connected to a point or points of substantially like potential on a second potential divider which potential determines the voltage of the d.c. supply from the output of said series regulator circuit.

5. A power supply arrangement as claimed in Claim 3, characterised in that the base of the regulator transistor is connected to the junction of a third transistor and a second voltage reference element present in a second potential divider also connected to said rectifying means, the collector of said second transistor being connected to the base of the regulator transistor through the base-emitter junction of said third transistor.

6. A power supply arrangement as claimed in Claim 3, 4 or 5, characterised in that the portion of the first mentioned potential divider across which said series arrangement is connected is formed by two resistive components connected in series across one of which is connected a diode and capacitor in series, so as to delay the time at which the instantaneous voltage across said series arrangement reaches the reference voltage of said first mentioned voltage reference device for a finite period after the supply arrangement is energised.

7. A power supply arrangement as claimed in Claim 3, 4, 5 or 6, characterised in that a protection circuit is connected in parallel with the emitter-collector path of said second transistor to short circuit the path of that transistor in the event of an increase of the mean voltage at the collector or said second transistor.

8. An electrical power supply arrangement substantially as herein described with reference to the accompanying drawings.