FIELD OF THE INVENTION

This invention relates to electrical power supply apparatus. BACKGROUND TO THE INVENTION

It is known to provide a power supply which operates as a forward converter, in which a transformer has its primary winding connected in series with a capacitor to define a resonant circuit. Two transistor switches are connected so as to switch current from a source in a bidirectional manner to the resonant circuit. The current oscillates in the resonant circuit so as to change its direction each half cycle, the transistors being switched on for respective half cycles of the oscillation. The flux produced by the primary winding induces current in the secondary winding of the transformer, which is rectified to provide a d.c. output supply.

SUMMARY OF THE INVENTION

In accordance with the present invention a power supply apparatus is provided wherein an oscillatory current is established in a parallel L.C. resonant circuit.

According to the invention there is provided an electrical power supply apparatus comprising: input means to receive an input supply source; a parallel L.C. resonant circuit to receive current from the input means; switching means operative cyclically to maintain anoscillatory current flow in the resonant circuit; output means for providing an output from the resonant circuit; a feed path for feeding current from the input means to the resonant circuit; current limiting means for limiting current flowing in the feed path , and energy extraction means providing a current extraction path additional to the feed path to extract energy from the current limiting means.

The invention has the advantage that a substantially unidirectional current from a source can be cyclically switched to the resonant circuit by switching means to maintain a bidirectional oscillatory current flow in the resonant circuit. Only one switching means needs to be provided. Energy storing means may be provided to store energy flowing into

the resonant circuit from the source when the switching means is conductive, with the effect of limiting the current flowing into the resonant circuit. The extraction means may extract energy from the storing

5 means as a current before the next turn on period of the switching means commences. The apparatus is thereby protected from overcurrent at switch on by the current limiting means, and is protected from over voltage at switch off, by virtue of the provision of the

10 energy extraction means. The current from the extraction path may be returned to the apparatus to enhance the efficiency thereof.

The current limiting and energy extraction means may comprise a flyback transformer having its primary

- _j c connected in series with the resonant circuit. The secondary of the flyback transformer provides the current extraction path and may deliver current to the output of the apparatus or back to the resonant circuit, or back to the source. 0 The resonant circuit may include the primary of a transformer device which is arranged to induce by forward conversion, current into a secondary thereof, the secondary delivering current to a reservoir capacitor. 5 In accordance with a second aspect of the invention, the capacitor in the parallel LC resonant circuit is clamped during forward conversion to a voltage which is a function of the voltage of the

reservoir capacitor.

The clamped voltage may be equal to the turns ratio of the transformer times the reservoir capacitor voltage or may equal the reservoir capacitor voltage. Also, the transformer device may induce current by flyback conversion into a secondary which is then delivered to a reservoir capacitor thereby reducing the energy stored in the resonant circuit. The flyback transformer may also deliver current to the reservoir capacitor.

From a third aspect, the invention provides an electrical power supply apparatus comprising input means to receive an input supply source; a parallel L.C. resonant circuit; switching means operative cyclically to maintain an oscillatory current flow in the resonant circuit; output means for providing an output from the resonant circuit; and a unidirectional feed path arranged to feed current in only one direction from the input means in such a manner as to cause a bidirectional current flow in the resonant circuit. BRIEF DESCRIPTION OF THE DRAWINGS.

In order that the invention may be more fully understood various embodiments thereof will now be described by way of example with reference to the accompanying drawings wherein:

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Figure 1 is a schematic circuit diagram of an embodiment of power supply apparatus in accordance with the invention:

Figure 2 illustrates various waveforms produced in operation of the apparatus of Figure 1; wherein Fig 2a shows the collector current of transistor

TRl,

Fig.2B shows the voltage of the primary LI of transformer Tr,, Fig.2C shows the primary voltage of flyback

Transformer T _™ ,

Fig.2D shows the collector voltage of transistor

TRl, and

Fig.2Ξ shows the current across the resonant primary LI;

Figure 3 illustrates in generalised block diagrammatic form a power supply apparatus in accordance with the invention;

Figures 4 to 14 are schematic circuit diagrams of alternative forms of power supply apparatus in accordance with the invention; and

Figure 15 is a detailed circuit diagram of a power supply apparatus in accordance with the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figure 1, the apparatus according to the invention comprises a transformer T_ having a primary winding Ll connected in parallel with a capacitor Cl to define a resonant circuit. The resonant circuit is connected in series with the primary winding of a flyback transformer „ which is connected in series with a bipolar switching trans¬ istor TRl. The secondary L2 of transformer T_ is connected through diode Dl to reservoir capacitor C2 connected in parallel with an output load shown schematically as a resistor Rl. The secondary of the flyback transformer „ is connected through a diode pump D2 to the reservoir capacitor C2. The apparatus according to the invention as shown in Figure 1 thus includes a parallel LC resonant circuit fed with a unidirectional current from a source V. under the control of switching transistor TRl. The unidirectional current fed from the source V. produces a bidirectional resonant current in the resonant circuit and thus a bidirectional magnetising current flows in the primary of trans¬ former T_. Non-magnetising (forward conversion) current flows through the primary during the period which the voltage across the resonant circuit is clamped by its secondary and Dl to the reflected voltage on C2. This induces a unidirectional current

in the secondary which is rectified by Dl and charges up the reservoir capacitor C2 by forward conversion.

The transistor TRl is switched cyclically at or approximately at the resonant frequency of the resonant circuit Cl, Ll so as to maintain the bidirectional oscillatory current therein. When the transistor TRl switches off a flyback voltage is induced in the sec¬ ondary of the flyback transformer T _p , this flyback voltage producing a current which feeds through diode D2 to charge capacitor C2.

The flyback transformer T _p has the function of limiting the current flowing through the primary cir¬ cuit when the transistor is on by storing magnetic energy using its primary self-inductance, and trans- ferring the^ energy it has stored to the output capacitor when TRl switches off, so as to extract energy from the primary circuit. The provision of the flyback transformer results in two advantages. Firstly, it acts as a non-dissipative primary current limiter. Secondly, the flyback transformer has the advantage of dumping energy stored therein during the on period of TRl, into the capacitor C2, after switch off there¬ by improving the efficiency of the converter.

The flyback transformer can be replaced by other non-dissipative means for limiting current flow in the primary circuit (by energy storage) and for extracting the energy before TRl next switches on, as will be explained in more detail hereinafter.

Reference will now be made to Figure 2 which shows waveforms occurring in the circuit of

Figure 1 during operation thereof. When the transistor TRl is turned on at time t _Λ o, current starts flowing in the primary of flyback transformer Tp and builds up in the curved manner shown in Figure 2A. The voltage across the primary LI of transformer T _R is shown in Figure 2B. This voltage initially falls and then at time t-,, becomes clamped to the reflected level of C2. i.e. N _α it. VO

Where _R is the turns ratio of LI to L2 and V is the output voltage across C2.

At this point, the transformer T _R acts by forward conversion (with a reservoir output), and passes current to the output load Rl whilst transistor TRl remains conducting.. During this period, the current which is flowing through the primary of the transformer Tp stops charging up the capacitor Cl of the resonant circuit because it is now charged to the reflected level of C2, to which it is clamped by the secondary of T _R and Dl, and instead flows through the primary LI of transformer T _R thereby inducing the secondary L2 to deliver current to the output load Rl. When this occurs, the primary of the flyback transformer Tp develops a constant voltage:

^V i - ^V o This can be expressed as :

-9-

where iis defined as V QNπ _D

V.

I

At time . the transistor TR1 switches off so that the primary current (Figure 2k ) falls to zero and the flyback transformer primary voltage falls to -V ₀ Np (where Np is the turns ratio of T„ ) due to the secondary of T„ clamping it to the output reservoir capacitor C2. Current stops flowing through both the primary of the transformer T _R and T _p so the transformer T _R stops forward converting and an approximately sinusoidal magnetising current (E) waveform is established in the primary L1, due to the fact that it is part of the parallel resonant circuit LI , CT.

At switch off of the transistor TR1, a flyback voltage is induced in the secondary of transformer Tp which causes a current to flow through diode D2 to the capacitor C2. When the flyback transformer runs out of energy at time t,, the output current through diode D2 reaches zero and the primary of the flyback transformer Tp then returns to zero voltage. Consequently, the collector of transistor TR1 experiences a drop in voltage of N _p V due to the primary of the flyback transformer Tp being in series with the switch TR1.

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The converter then remains in a quiescent state until the next time that transistor TRl is switched on.

It will be seen that X is a design choice selected by the turns ratio N _D of transformer T _D for

V. ^κ a given value of ? . In a flyback or foward converter A ha °s to be unity since ^v -i= = flr. o However, in the apparatus according to the invention, due to the flexibility, introduced by the provision of the inductance of the primary of the flyback transformer Tp, it is possible to choose the output voltage reflected onto the primary of the transformer T„ to be different from the input voltage i.e. λ ^ i Figure 2 shows the waveforms for A = 1- 2, 1.0 and 0.8. A determines the ratio of power output from T _R and Tp. A is defined so that when the output from Tr, increases, increases.

Referring now to Figure 3 _> the apparatus according to the invention is shown in block diagrammatic form. The parallel LC resonant circuit is shown schematically by block 1. The current limiting, energy storing and extraction means (e.g.the flyback transformer Tp) is shown by block 2 and has a separate current extraction path 3 to extract stored energy.

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A switching means (e.g. the transistor switch TRl) is shown by block 4. Block 1 is connected to an output stage 5. Block 2 can also be connected to an output stage. It will be seen that blocks 1, 2 and 4 can be series connected in any order so as to achieve the present invention.

Various modifications of the arrangement shown in Figure 1 will now be described.

Referring to Figure 4 this shows an arrangement similar to that of Figure 1 but in which the capacitor Cl is connected in parallel with the secondary winding L2 of the transformer T _R . Alternatively, the capacitor Cl could be replaced by two capacitors one in parallel with the primary of T _R and the other in parallel with the secondary.

In the embodiment of Figure 1, the current limiting, storing and extraction means comprises a flyback transformer Tp having its primary connected in series with the primary of the transformer T _R and the secondary of the transformer Tp being connected to the output stage of the converter. However, other means can be utilised. For example, as shown in Figure 5 the transformer Tp its secondary connected through a diode D3 to the primary side of the converter so as to return the stored energy to the source, thereby limiting the voltage

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developed across the transistor TRl when it switches off. Alternatively, the arrangement of Figure 6 may be adopted- in which the transformer Tp is a forward converter instead of a flyback. Tp has two secondary windings, one being connected through diode D3 to the primary side of the circuit so as. to return magnetising energy to the source, and the other which comprises a forward conversion secondary, being connected in parallel with a free¬ wheeling diode D5, and in series with a current limiting inductance L3 connected to the capacitor C2 through diode D4. Alternatively, the forward conversion output can be fed back to the input i.e. the primary side of the circuit.

Another embodiment is shown in Figure 7 which includes an extra secondary on transformer T _R ,comprising a flyback winding L4 connected in series to a diode pump D6 which feeds current to the reservoir capacitor C2. The effect of this additional secondary is to extract energy from the resonant circuit after transistor TRl has switched off and when Cl is charged with opposite polarity, thereby limiting the collector voltage of the transistor after switch off, and flyback converting energy from resonant circuit to the output. This embodiment can be so arranged that substantially equal amounts of energy are transferred from the resonant circuit by flyback and forward conversion.

Another form of the circuit of Figure 7 is shown in Figure 8. In this arrangement, the forward conversion and the flyback conversion are achieved by means of a single secondary for the transformer T_, in combination with a bridge rectifier BRl. In use, the secondary L2 and the bridge rectifier BRl cooperate to charge Cl by forward conversion and then when TRl switches off an opposite polarity flyback current is induced in L2 which is passed by the bridge rectifier to the capacitor C2.

Another embodiment of the invention is shown in Figure 9 wherein the forward converter output stage has a freewheeling output including inductor L5 and diode D7, to feed the reservoir capacitor C2. In Figure 10, a non-isolated a.c. output form of converter is shown. In this embodiment, the inductor coil LI is not isolated from the output stage by the transformer structure T _R . Instead, the parallel resonant circuit comprising capacitor Cl and inductor LI is directly connected to the output. Stored energy is extracted from T _p by means of its secondary, which is arranged to return the extracted energy to the input side of the circuit when the transformer TRl switches off. In Figure 11 a d.c. non-isolated form of the converter is shown. The resonant circuit Cl, Ll is connected directly to the reservoir capacitor C2

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through a rectifying diode Dl. The flyback trans¬ former „ has its secondary connected through diode D2 to charge the capacitor C2 in response to switch- off of transistor TRl. In Figure 12 a multi-tap output form of the converter of Figure 1 is shown. The secondaries of T _R and _p are provided with tapping points so that separate output stages can be provided each similar to that shown in Figure 1 across respective resistors R. and Ri • It will be appreciated that negative out¬ put voltages could be produced by appropriate modific¬ ation of the circuitry. Further, any given output need only receive current from one of T _R and _p .

Figure 13 shows a circuit similar to that of Figure 5, but in which two switching elements and two flyback transformers are provided in parallel. Thus two bipolar switching transistors TRlA, TRlB are provided, which are cyclically switched by a common base drive. Each transistor TRlA, TRlB is connected in series with the resonant circuit Cl, Ll and the primary of a respective flyback transformer T_A, T _p B, the secondaries of which feed the extracted flyback current through diodes D3A, D3B to the input to the resonant circuit Cl, Ll. This circuit provides a low cost way of increasing the power handling capability of the circuit since it enables a relat¬ ively high cost high power transistor to be replaced

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by two transistors which together can handle the same power but which in combination cost less than the equivalent single high power transistor.

Figure 14 illustrates another form of the circuit in which the flyback transformer Tp is replaced by an inductor coil Lp. An energy extraction path is provided back to the input of the resonant circuit Cl, Ll through diode D3, so that when transistor TRl switches off, current freewheels through dibde D3 to extract stored energy from the inductor Lp. The diode D3 may be replaced by a transistor (not shown) the base of which receives a clock waveform to switch on when transistor TRl switches off and vice versa. The transistor has the advantage of providing extra flexibility in the control of energy extraction.

Referring now to Figure 15, a practical form of apparatus according to the invention is shown in detail. An input mains supply is rectified by a bridge rectifier BR2, smoothed by capacitors C3, C4 and fed to transformer T _R , the flyback transformer Tp and switching transistor TRl, which are connected in series generally in the manner shown' in Figure 1.

The transistor TR 1 is driven by a complimentary pair TR2, TR3 which is cyclically switched by transistor TR4 that is switched by a control circuit provided with an oscillator, the control circuit being represented by integrated

- ^* ircuit IC1. The transistors TR?, TR ^' -i receive + and - 5v rail voltages from a voltage regulator chip IC2 and zener diode ZD1. The chip IC2 is powered by a start up winding L6 inductively coupled to the flyback transformer, a self power winding L7 inductively coupled to the transformer T _R and also by a kick start diode pump arrangement comprising a capacitor C5, resistor R3 and a zener diode ZD2 fed by the mains. Upon start up of the circuit, the transistor TR1 is switched off so that no currents are induced in the windings L6, L7. However, the kick start arrangement ZD1, R3, C3 provides a voltage pulse to the voltage regulator circuit IC2 once every mains cycle. This provides enough energy to operate the main transistor switch TR1 a few times when the converter is starting up and receiv¬ ing no self power from windings L6 and L7. As soon as the switching of TR1 commences, the start up winding Lβ starts providing energy to the +5v rail A since at this time the output voltage of the con¬ verter is only a small fraction of its normal operating value and so the voltage across the resonant circuit L1, Ct is relatively low. Con- sequentally the primary of the flyback transformer p sees nearly all the rectified mains voltage produced by BR2, C3, C*4 when the transistor TRT is

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swif-.ched on. The voltage on the winding L6 is a reflection of the voltage across the primary of the flyback transformer T _p . Since the voltage across the resonant circuit C1, L1 is low, the winding L7 provides no power at this stage and this is why the start up winding L6 is required. Once the output voltage reaches a signifigant fraction of its normal operating value, the main self power take off winding L7 takes over the role of powering the converter.

The secondary windings of the transformer T _R and Tp are connected generally in the manner shown in Figure 1 to a reservoir capacitor C2. ^" Also, an output ripple L.C. filter is provided comprising inductor L8 and capacitor C6.

The secondary of the transformer T _R includes a winding L4 which damps the resonant circuit L1 , C1 when TR1 is off, and flyback converts resonant energy into the reservoir capacitor C2 (in the manner described with reference to Figure 7).

• The circuit includes a negative feedback loop for controlling the magnitude of the output. The emitter voltage V1 of transistor TR1. which is indicative of flyback primary current,is fed back as an input to the control circuit IC1 for current protection purposes as will be. described

hereinafter. Also, the output voltage V ^" developed across the reservoir capacitor C2 is fed to a voltage comparator including an optical isolator chip 0C1 connected in series with a zener diode ZD3 and a parallel connected variable resistor VR1. The optical isolator 0C1 comprises a light emitting diode LED which energises a phototransistor TR5 that develops a reference voltage V2 when the voltage exceeds a reference set by ZD3 and VR1. The voltage V2 is fed back to the control oscillator of circuit IC1 to control its duty cycle.

The transistor TR1 is protected by a parallel circuit comprising a series connected capacitor C5 and resistor R2 which limits the rate of change of voltage across the transistor at switch off. Also, the circuit IC1 is arranged to terminate the duty cycle in response to the voltage V1 rising above a particular level so as to provide dynamic protection against excess current. Preferably, the dynamic, current limit threshold is arranged to be variable so as to decrease with a larger duty cycle for the circuit IC1. The threshold may be arranged in two levels, one for a small duty cycle and the other for a large duty cycle. In particular, the current limit threshold may be set to one value for the initial part of the turn on period of TR1 and to

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a smaller value for later part, of the turn on period. Also, the circuit IC1 provides thermal overload protection by terminating the duty cycle in response to continued excess current threshold overexcursion, the circuit being arranged to include a dwell period before restarting can occur, to allow the circuit to cool.