GB1590966A

GB1590966A - Power supply arrangement

Info

Publication number: GB1590966A
Application number: GB51361/77A
Authority: GB
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1977-12-09
Filing date: 1977-12-09
Publication date: 1981-06-10
Also published as: JPS5489416A; DE2852942C3; DE2852942A1; IT7830663A0; AU523772B2; FR2411519A1; ES475797A1; DE2852942B2; FR2411519B1; JPS58161471A; AU4223278A; JPS5928106B2; IT1192292B

Description

(54) POWER SUPPLY ARRANGEMENT (71) We, PHILIPS ELECTRONIC AND ASSOCIATED INDUSTRIES LI MITED, of Abacus House, 33 Gutter Lane, London, EC2V 8AH, a British Company, 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: The present invention relates to a circuit arrangement for producing a sawtooth current having a forward period and a flyback period through a supply transformer, and is a modification of the circuit arrangement described and claimed in our Patent Specification 1,458,984.

Our above mentioned Patent Specification relates to a circuit arrangement which is in effect the combination of a switched mode power supply circuit and a line output circuit using a common switching means for both circuit functions.

The present invention provides a circuit arrangement for producing a sawtooth current having a forward period and a flyback period through a supply transformer, the primary winding of said transformer forming part of a resonant circuit which also comprises a forward capacitor and a flyback capacitor, switching means for applying in operation the voltage present across the forward capacitor to the primary winding at the frequency of said sawtooth current during its forward period, which switching means comprises a first diode and a controlled switch connected in parallel with the said diode via a second diode, first and second terminals for connection to a direct voltage source, and a coil one end of whose winding is d.c. connected to the first of said terminals whilst the other end of said winding is d.c. connected through at least part of said primary winding and said controlled switch to said second terminal, said coil winding additionally being connected to the resonant circuit via a third diode, said transformer having a secondary winding connected to rectifying means, the arrangement being such that in operation the sawtooth current flows through the first diode during one part of the forward period and through the second diode and the controlled switch during the other part of the forward period, current which is supplied to the controlled switch from the direct voltage source flowing through the coil to cause energy to be stored therein whilst current flowing through the coil and the third diode during the cut-off period of the controlled switch restores energy loss in the resonant circuit, a d.c. supply being derived from said rectifying means.

With the present invention it has been realised that the circuit arrangement of our above mentioned Patent Specification may be modified such that it operates as a power supply and not as a line output circuit. The invention therefore also provides a modification of the circuit arrangement as claimed in Claims 1 of patent specification serial number 1,458,984, in which the line deflection coil is replaced by the primary winding of a supply transformer, the transformer having a secondary winding connected to rectifying means from which, in operation, a d.c. supply is derived. By including a transformer in the present circuit arrangement it is possible to provide d.c. isolation between a d.c. source energising the circuit arrangement and circuits energised by the circuit arrangement.

The above and other features of the invention will now be described, by way of example with reference to the accompanying drawing in which: Figure 1 is a diagram of a circuit arrangement according to the invention, and Figure 2 shows waveforms for explaining the operation of the circuit arrangement.

The switched-mode power supply circuit arrangement of Figure 1 shows a switching means in the form of a transistor Tr of the npn type whose collector is connected to one end of the primary winding L of a transformer T. The other end of winding L is connected to one end of a choke L' whose other end is connected to the positive terminal of a direct voltage source Vg, whilst the emitter of transistor Tr is connected to the negative terminal of source VB. Source Vg is, for example, a rectifier circuit which rectifies the alternating voltage of the electrical mains.

The anode of a diode D2 is connected to the negative terminal whilst the cathode of a diode D3 is connected to the positive terminal. The cathode of diode D2 is connected to the anode of diode D3 and to the anode of a diode D1 whose cathode is connected to the collector of transistor Tr. The junction point A between the three diodes is connected through a forward capacitor C1 to the junction between choke L' and primary winding L, whilst a flyback capacitor C2 is arranged in parallel with the winding L.

A driver stage Dr for supplying switching pulses is connected to the base of transistor Tr, the repetition frequency of which is equal to that of the line time base of a television receiver, not shown, of which the power supply of Figure 1 is a part. Line frequency signals which are generated in known way are applied to stage Dr therefor.

During a first part of the line period diode D1 conducts whilst transistor Tr is cut off. A current i flows through primary winding L and also through capacitor C1 and diode D1.

If the capacitance of capacitor C1 is sufficiently large, the voltage V0 thereacross may be assumed to be substantially constant during the whole line period, so that the variation of current i, which variation is represented in Figure 2a, can be assumed to be substantially linear, during the part (to2) of the forward period (to-t3) when diode D1 conducts. At the same time the energy stored in choke L' by a current i' flowing therethrough during the foregoing period causes this current i' to be maintained.

Current i' flows also through capacitor C, and diode D3 with a substantially linear variation as represented in Figure 2b. Because both diodes D1 and D3 are conducting the voltage at the collector of transistor Tr is substantially equal to VB with respect to the voltage at the negative terminal of the source, which is the reference voltage of the circuit arrangement, and so is the voltage at junction point A (Figure 2c).

At an instant t1 transistor Tr is rendered conducting by means of a switching pulse applied to its base and originating from driver stage Dr. Current i continues to flow in the same manner it did before instant t1 but current i' continues to flow in the same manner it did before instant t1 but current i' flows now through capacitor C1, diode D1, transistor Tr and source Vn. Both currents flow thus through diode D1. Before instant t1, current I' decreases, whereas it increases after instant t1 since it originates from source Vg and thus draws energy therefrom.

The voltage at the collector of transistor Tr and hence that at point A becomes substantially zero at instant t1, so that the voltage at the junction between choke L' and winding L, which was equal to Vg + V0 before instant t1, becomes equal to Vo after this instant. The voltage across choke L' was equal to -Vo when considered in Figure 1 from the right side to the left side of the choke and assumes now the value VB-Vo, which determines the slope of the positive going ramp of Figure 2b.

From the foregoing it appears that energy is delivered to the circuit arrangement by means of current i' after instant t1 which energy compensates for the losses and for loads to which power is being supplied. A satisfactory operation will be obtained if current i' does not become zero during the period, i.e. for a given choice for the inductance of choke L'. Because the inductance of winding L is much smaller than that of choke L' the current i becomes zero. This occurs at an instant t2 after instant t1, after which instant current i reverses its direction and consequently flows through diode D1 in the direction opposite to that of current i', the voltages across choke L' and winding L remaining the same as before instant t2, i.e.

Vg - VO and VO, respectively.

A short time after instant t2 and during the forward period, at an instant at which both currents flowing through diode D1 have the same absolute value, diode D1 ceases to conduct. Current i' flows now through winding L and transistor Tr whilst current i flows through transistor Tr, diode D2 and capacitor C1. This situation and also that of the voltages considered above remains unchanged as long as transistor Tr conducts.

At a given instant a negative going voltage edge is applied to the base of transistor Tr which cuts off at an instant t3 which is the end of the forward period and the start of the flyback period. Both currents considered in Figure 2 continue to flow in the same direction as before but fall in value.

Current i' flows again through capacitor C1 and diode D3 so that the voltage at junction point A returns to substantially the value VB whilst the voltage at the junction between choke L' and primary winding L assumes the value VB + VO. The voltage across choke L' is again equal to -VO and current i' decreases in a substantially linear way.

Current i flows after instant t3 through capacitor . This capacitor is chosen to have a capacitance such that it forms with winding L and other inductances in parallel therewith a tuned circuit whose tuning frequency is higher than the line freqency.

A substantially sinusoidal oscillation is thus produced at the collector of transistor Tr and reaches a maximum value whereafter it decreases. Current i decreases after instant t3 in a substantially sinusoidal manner, becomes zero at an instant t4 during the flyback period whereafter it reverses its direction.

Since the capacitance of capacitor C1 is much higher than than of capacitor C2, capacitor C2 may be considered as being in parallel with diode D1, which shows that the voltage across this capacitor is substantially zero before instant t3. This voltage becomes zero again at an instant t5 at the end of the flyback period after instant t4, which causes diode D1 to conduct again. The oscillation ceases, the voltage across capacitor C1 being applied to winding L so that the current i therethrough flows in a substantially linear way. Instant t5 is thus the commencement of a new period.

The amplitude of the oscillating voltage produced during the flyback period between instants t3 and tS across winding L is much higher than voltage Vg depending on the ratio of the period between these instants to the whole line period, whereas the voltage across winding L during the rest of the line period being equal to VO, does not substantially vary. The amplitude of the flyback oscillation can be kept below the rating of the transistor by a proper choice of the tuning frequency. Oscillations with substantially the same shape over the line period are produced on secondary windings of transformer T. Figure 1 shows two secondary windings L1 and L2 connected to rectifiers D4 and Dg, respectively, by means of which these oscillations are rectified. The d.c. voltage obtained by means of rectifier D5 is smoothed by a capacitor C3 and can serve for the d.c. power supply for parts of the television receiver such as a line deflection circuit whilst the high d.c. voltage obtained by means of rectifier D4 is applied in known way to the final electrode of a picture display tube, not shown, which also forms part of the receiver. The winding sense of the various secondary windings and the conductivity directions of the relevant rectifiers can be chosen in known way so that a rectifier conducts either during a part of the time interval between instants t3 and t5 or during a part of the rest of the line period.

One of the d.c. voltages produced at the secondary side side of transformer T, for example the voltage across capacitor C3 is fed back to driver stage Dr in which a comparison circuit and a pulse duration modulator ensure in known way that the conductivity time t1 to t3 of transistor Tr is varied in such a manner that the voltage across C3 and consequently the other d.c.

voltages remain constant. D.c. isolation may be obtained between the primary and secondary windings of transformer T such that the secondary windings may have one of their ends d.c. connected to the chassis of the television receiver whereas the primary winding is d.c. connected to the mains. In such a case, the feedback path referred to above to the driver circuit for transistor Tr must include one or more transformers.

A further secondary winding L3 of transformer T may be provided for driving the line deflection circuit output transistor Tr' of the television receiver. A condition therefor is that transistor Tr' is driven into conduction during at least the second half of the line scan period, whereas, as known, it may also be driven into conduction during a part or the whole of the first half without the deflection circuit being adversely affected thereby although it must be cut off during the line flyback period, this period depending on the flyback tuning of the deflection circuit. Transistor Tr' is in Figure 1 of the npn type, its base lead includes in known manner a coil L4. The winding sense of winding L3 is chosen such that the flyback oscillation contained in the drive signal has a negative polarity. Due to the action of coil L4 the switching off of transistor Tr' is delayed for a given time after instant t3 and so is its switching on after instant t5. The latter must not occur before the flyback period has been completed.

It is clear that a positive polarity for the flyback oscillation could be used to drive transistor Tr'. However this measure could have the drawback that the top of the oscillation would be clipped by the base current of transistor Tr', so that the negative polarity is to be preferred.

From Figure 2a it appears that the instant t2 at which current i passes through the value zero does not lie, in general, in the middle of the linear part of the curve represented in this figure. This means that current i has a d.c. component. From Figure 1 it will be clear that this d.c. component is nothing other than that of current i' and therefore depends on the losses of the circuit arrangement and on the energy to be delivered to the various secondary circuits. Its value can therefore be considerable, which may be a drawback since it may cause a magnetic saturation of transformer T though such a saturation may be reduced by using for transformer T a core with an air gap. The d.c. component of current i may, however, be reduced by connecting the right hand end of choke L' to a tap on winding L, the connection of capacitors C1 and C2 remote from diode D1 remaining connected to the upper end of winding L.

There will be no d.c. component through winding L at all if the right hand end of choke L' is directly connected to the collector of transistor Tr. In such a case, however, the voltage across choke L' is higher than in the case of Figure 1 so that the inductance thereof has to be higher. It will be observed that the conduit of the embodiment obtained with this direct connection is the same as that represented in Figure 5 of our Patent Specification No. 1,458,984 with the only significant difference that primary winding L in the present Figure 1 replaces the line deflection coil Ly of the prior specification in a line deflection arrangement. As known, only a small d.c. current may flow for centering purposes through a deflection coil, so that the present Figure 1 or as modified with a tap as mentioned above would not be suitable for a line deflection arrangement. It will also be appreciated that the present Figure 1 is a modification of Figure 5 of our prior specification with the re-arrangement of the resonant circuit. Figures 3,4 and 6 of the prior specification may also be modified such that they can operate as power supplies rather than deflection circuits.

WHAT WE CLAIM IS: 1. A circuit arrangement for producing a sawtooth current having a forward period and a fly back period through a supply transformer, the primary winding of said transformer forming part of a resonant circuit which also comprises a forward capacitor and a flyback capacitor, switching means for applying in operation the voltage present across the forward capacitor to the primary winding at the frequency of said sawtooth current during its forward period, which switching means comprises a first diode and a controlled switch connected in parallel with the said diode via a second diode, first and second terminals for connection to a direct voltage source, and a coil one end of whose winding is d.c. connected to the first of said terminals whilst the other end of said winding is d.c. connected through at least part of said primary winding and said controlled switch to said second terminal, said coil winding additionally being connected to the resonant circuit via a third diode, said transformer having a secondary winding connected to rectifying means, the arrangement being such that in operation the sawtooth current flows through the first diode during one part of the forward period and through the second diode and the controlled switch during the other part of the forward period, current which is supplied to the controlled switch from the direct voltage source flowing through the coil to cause energy to be stored therein whilst current flowing through the coil and the third diode during the cut-off period of the controlled switch restores energy loss in the resonant circuit, a d.c.

supply being derived from said rectifying means.

2. A circuit arrangement as claimed in claim 1 in which a series combination of said second and third diodes connected for conduction in the same pass direction is connected in parallel with the series combination comprising said coil, said primary winding and the collector-emitter path of a transistor forming said controlled switch, said parallel combination being connected to the terminals for said direct voltage source, said first diode being connected between the junction of said second and third diodes and the junction of the collector of said transistor and said primary winding.

3. A circuit arrangement as claimed in claim 1 or 2, in which said controlled switch is switched at television line frequency, said transformer having a further secondary winding coupled to the control electrode of a further controlled switch present in a line deflection circuit for switching said further controlled switch at line frequency.

4. A circuit arrangement as claimed in claim 3, in which said line deflection circuit is energised from said rectifying means.

5. A circuit arrangement as claimed in any of the preceding claims 1 to 4, in which said transformer provides d.c. isolation between circuit elements connected to said primary winding and circuit elements connected to said first mentioned secondary winding.

6. A modification of the circuit arrangement as claimed in Claim 1 of patent specification serial number 1,458,984, in which the line deflection coil is replaced by the primary winding of a supply transformer, said transformer having a secondary winding connected to rectifying means from which, in operation, a d.c. supply is derived.

7. A circuit arrangement substantially as herein described with reference to the accompanying drawing.

8. Television display apparatus incorporating a circuit arrangment as claimed in any of the preceding Claims 1 to 7.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. from diode D1 remaining connected to the upper end of winding L. There will be no d.c. component through winding L at all if the right hand end of choke L' is directly connected to the collector of transistor Tr. In such a case, however, the voltage across choke L' is higher than in the case of Figure 1 so that the inductance thereof has to be higher. It will be observed that the conduit of the embodiment obtained with this direct connection is the same as that represented in Figure 5 of our Patent Specification No. 1,458,984 with the only significant difference that primary winding L in the present Figure 1 replaces the line deflection coil Ly of the prior specification in a line deflection arrangement. As known, only a small d.c. current may flow for centering purposes through a deflection coil, so that the present Figure 1 or as modified with a tap as mentioned above would not be suitable for a line deflection arrangement. It will also be appreciated that the present Figure 1 is a modification of Figure 5 of our prior specification with the re-arrangement of the resonant circuit. Figures 3,4 and 6 of the prior specification may also be modified such that they can operate as power supplies rather than deflection circuits. WHAT WE CLAIM IS:

1. A circuit arrangement for producing a sawtooth current having a forward period and a fly back period through a supply transformer, the primary winding of said transformer forming part of a resonant circuit which also comprises a forward capacitor and a flyback capacitor, switching means for applying in operation the voltage present across the forward capacitor to the primary winding at the frequency of said sawtooth current during its forward period, which switching means comprises a first diode and a controlled switch connected in parallel with the said diode via a second diode, first and second terminals for connection to a direct voltage source, and a coil one end of whose winding is d.c. connected to the first of said terminals whilst the other end of said winding is d.c. connected through at least part of said primary winding and said controlled switch to said second terminal, said coil winding additionally being connected to the resonant circuit via a third diode, said transformer having a secondary winding connected to rectifying means, the arrangement being such that in operation the sawtooth current flows through the first diode during one part of the forward period and through the second diode and the controlled switch during the other part of the forward period, current which is supplied to the controlled switch from the direct voltage source flowing through the coil to cause energy to be stored therein whilst current flowing through the coil and the third diode during the cut-off period of the controlled switch restores energy loss in the resonant circuit, a d.c.

supply being derived from said rectifying means.