GB2154386A - Protection - Google Patents

Abstract

A protective circuit (15) is suitable for use with main apparatus (10) having a component (Q5) to be protected against excessive current flow therethrough and/or a short circuit. Current through the component (Q5) is detected by series-connected sensing means (R7) which provides an output in response to excessive current flow, to cause diverting means (Q3, Q4) connected between main current and drive terminals of the component (Q5) to become conductive so as to divert current flow from the component (Q5), so protecting the component (Q5) from excessive current flow. The diverting means (Q3, Q4) switches off after a time determined by capacitive discharge within the protective circuit (15), but switches on again if excessive current again flows. The protective circuit (15) is intended primarily for protecting switching transistor components (Q5, Q6) of a push-pull voltage converter (10), and is capable of protecting more than one such component via "mirror image" effects in a transformer (T2) associated with the components (Q5, Q6). <IMAGE>

Description

SPECIFICATION Protective circuit This invention relates to a protective circuit of the kind, hereinafter referred to as being of the kind specified, suitable for use with a main apparatus having a component to be protected against flow of excessive current therethrough and/or a short circuit.

In one application of the invention, the main apparatus is in the form of a circuit having semi-conductor switching means, and may for example be a voltage converter circuit such as those disclosed in our copending British patent application No. 8221480.

According to a first aspect of the invention, we provide a protective circuit of the kind specified comprising means for diverting current from the component to be protected in response to detection of excessive current flow through said component.

According to a second aspect of the invention we provide a protective circuit of the kind specified comprising diverting means for rendering inoperative a drive device associated with the component to be protected in response to detection of excessive current flow through said component, thereby causing cessation of said excessive current flow.

In a protective circuit in accordance with either or both of said aspects of the invention, the diverting means may comprise a diverting transistor having a main current path connected substantially in parallel with said component or said drive device, the diverting transistor being normally non-conductive but switching-on in response in excessive current flow through said component.

Said excessive current may be sensed by a sensing means in series with the component to be protected, the sensing means providing an "output" in response to said current sufficient to cause switching on of the diverting transistor when said current becomes excessive.

The protective circuit may comprise transistor latch means operated by said "output" to cause said switching on of the diverting transistor.

The sensing means may comprise a resistance through which the current flows.

Alternatively or additionally, the sensing means may comprise a current sensing transformer.

A rectifying means may be provided to cause current through the sensing means to be unidirectional.

The diverting transistor, after being switched on, may switch off after a time at least partially determined by the protective circuit, so that normal operation of the main apparatus may continue if said excessive current is no longer sensed to flow.

Said time may be determined by the time constant of a discharge circuit for a capaci tance in the protective circuit.

The invention also provides the combination of a main apparatus comprising a main electri cal circuit and a protective circuit in accor dance with the invention.

Said main electrical circuit may be a push pull voltage converter circuit.

Said push-pull voltage converter circuit may be of the kind described in our copending British patent application No. 8221480.

In any such combination, the component protected by the protective circuit may be a semi-conductor switching component.

The protective circuit may be capable of switching off more than one such component substantially simultaneously.

Said substantially simultaneous switching off may be achieved by "mirror image" ef fects in a transformer associated with said components.

Several embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings wherein: FIGURE 1a shows a first main apparatus including a first protective circuit embodying the invention; FIGURE ib shows current variation in a portion of the apparatus of Figure 1 a; FIGURES 2a, 2b and 2c show alternative details of the protective circuit shown in Fig ure 1a; FIGURE 3 shows a second main apparatus including a second protective circuit embody ing the invention; FIGURE 4 shows a third main apparatus including a third protective circuit embodying the invention; and FIGURE 5 shows a fourth main apparatus including a fourth protective circuit eimbody ing the invention.

Referring first to Figure 1 a of the drawings, the main apparatus comprises a half-wave I push-pull voltage converter circuit 10 having alternately conductive switching transistors Q5 and Q6 respectively which provide an alternat ing current through a primary winding T1/1 of an output transformer T1 in response to connection of input terminals + V,, - V, to a DC voltage source. The alternating current in the winding T1/1 is transformed by the trans former T1 and the transformed current in the secondary winding T1/2 is rectified by a bridge rectifier REC 1 to provide a DC output at output terminals + VO, - V,.

A current drive transformer T2 having a saturable core, in response to saturation of which core switching of the transistors Q5 and Q6 occurs, has a primary winding T2/1 through which said alternating current flows so as to induce positive current feed-back in the windings T2/2 or T2/3 according to whichever of the transistors Q5 and Q6 re I spectively is conductive. The alternating cur rent also flows through a current sensing resistance R7 of a bridge rectifying means REC 2 comprising diodes D3, D4, D5 and D6. The current through R7 gives rise to a unidirectional "output" voltage across R7.

The resistance R7 and the rectifying means REC 2 are part of the protective circuit 1 5.

The protective circuit 1 5 further comprises an auxiliary winding T1 /X of the output transformer Ti which provides a DC supply to two transistors Ol and Q2 of the circuit 15, this supply being obtained by the transistors via a rectifying diode D1 and a smoothing capacitor C3. When the current through R7 becomes excessive, due to excessive current flow through either OS or Q6, the voltage applied to the base of the transistor Q1 via a resistor R6 becomes sufficient to switch on Q1.The transistors Ol and Q2 are connected to form a latch circuit such that when the transistor Q1 is switched on, it is held on by a tie down connection of its base to one side of the auxiliary supply via the transistor 02.

Diverting transistors Q3 and Q4, which are normally non-conductive, are then switched on, via respective resistors R4 and R5, so as to short circuit the base and emitter terminals of the transistor 05, and also the base and emitter terminals of the transistor Q6 via a "mirror image effect" in the windings T2/2 and T2/3. Consequently, the transistors Q5 and Q6 are turned off, thereby causing cessation of said current flow, and preventing sustained, possibly damaging, flow of excessive current.

When the transistors Q5 and Q6 are switched off, no current flows through primary winding T1/1 and hence the induced voltage across T1 /X becomes zero. The capacitor C3 then discharges until the voltage across it is insufficient to sustain latching of Ol and Q2, whereupon the transistors Q1 and 02 delatch, and the transistors Q3 and Q4 switch off.

Current is then able to flow again through the transistors Q5 and 06. If excessive current is not subsequently detected via the resistor R7, the circuit 10 will then operate normally. However, if for example a short circuit existed across the output + VO, -V0 and has not been removed, the current will again become excessive, at a rate determined by a inductor L1 in the output circuit, and the protective circuit 1 5 will again operate to switch off the transistors Q5 and 06. This cycle of operations will repeat until any short circuit is removed. The output current (ordinate) is shown in Figure 1 b as a function of time (abscissa).Between times tA and tB the output current rises at a rate determined by L1 and becomes excessive at tBw at which time the protective circuit 1 5 causes the current to become zero. The time interval from t8 to a third time tA, tAt being a later time at which the circuit 1 5 permits the transistors OS and Q6 to conduct again, is determined by the time constant of the discharge circuit of the capacitor C3. Thus in the event of a sustained output short circuit, the current 10 is caused to switch on and off to sustain a safe current level, the switching rate being determined by C3 (and other components of the discharge circuit thereof) and L1.

A capacitor C4 is provided in the circuit 1 5 to smooth voltages applied to the base of the transistor 01, thereby preventing the transistor Q1 from being switched on undesirably and unnecessarily during a normal brief transient high current which may flow when the circuit 10 is switched on.

A capacitor C10 smooths the output voltage at +VO' -V0.

Although the diverting transistors Q3 and Q4 are shown in Figure 1 a to be both of the n-p-n type, they could alternatively both be of the p-n-p type, arranged as shown in Figure 2a, or could be p-n-p and n-p-n respectively, arranged as shown in Figure 2b, or could be n-p-n and p-n-p respectively, arranged as shown in Figure 2c.

Figures 3, 4 and 5 show push-pull inv,erter circuits each provided with an alternative protective circuit in accordance with the invention. Components corresponding functionally to those already described with reference to Figures la and 1 b are identically or similarly labelled, and the preceding description is to be deemed to apply.

Figure 3 shows a full-wave bridge voltage converter circuit 20 and an associated second protective circuit 25 in accordance with the invention. The right-hand half of the circuit 20 (as seen in the drawing) is substantially the same as the circuit 10 of Figure 1 a, except that corresponding components now additionally bear the designation "a" while the lefthand half of the circuit 20 is substantially a laterally inverted image of said right-hand half and corresponding components bear a designation "b" instead of "a". Components common to both halves of the circuit 20 do not have such additional designations. Resistors R7a and R7b respectively sense excessive current flow in the current path through transistors 05b and 06a or the current path through Q5a and Q6b, and the circuit 25 responds to said excessive current flow by switching off both "legs" Q5b, 06b and 05a, Q6a of the circuit 20, thereby causing cessation of the current flow. If a cause of excessive current flow is eliminated, the circuit 20 resumes normal operation, while if for example a short circuit is maintained, the output current switches on and off at a safe current level at a rate determined by an inductance L1 and time constants of the discharge circuits of capacitors C3a and C3b.

Figure 4 shows a third voltage converter circuit 30 and an associated third protective circuit 35 embodying the invention. The cir cuit 30 differs from the circuit of Figure la in that the transistors Q5 and Q6 are repositioned so that their main current paths are in circuit branches connected in parallel across the input voltage, and instead of employing a single output transformer primary winding, a split output transformer primary winding having two portions Ti/i a and T1/1 b respectively is employed, one of said portions being in series with each of said main current paths, so that alternate conduction by the transistors OS and Q6 causes an alternating output to be induced in the secondary winding T 1/2.The resistor R7 for sensing the current through the transistors OS and O6 is connected in the return current path, leading from the transistors OS and Q6 to the terminal - V1, and so carries only unidirectional current. Thus there is no need to employ a rectifying means equivalent to REC 2 of Figure 1 a. The current feed-back transformer T2 of the circuit has a primary winding comprising two portions T2/1 a and T2/lb respectively in respective main current paths.

This circuit also employs voltage feed-back via windings T1 /VF.

In response to excessive current flow through the resistor R7, a diverting transistor Q30, functionally similar to Q3 or Q4 of Figure 1a, is switched on so as to short circuit the current feed-back winding T2/3, thereby removing drive from the transistor 06, and by a "mirror image effect" via the winding T2/2 also removing drive from the transistor Q5.

Thus flow of current through OS and Q6 ceases. When the circuit 35 again permits current flow after at least partial discharge of the capacitor C3, this process will be repeated if excessive current again flows. Otherwise, the circuit 30 will function normally.

Figure 5 shows a fourth voltage converter circuit 40 associated with a fourth protective circuit 45 embodying the invention. While the circuits of Figure ia. 3 and 4 are suitable for a variety of input voltages, for example high voltages of the order of several hundreds of volts or more, the circuit of Figure 5 is primarily, although not exclusively, intended for operation with a low input voltage supply, for example a 1 2 volt DC motor vehicle battery.Excessive current flow through a sensing resistor R7 in series with the transistor Q6 causes a latch transistor Q1 (which in the circuit 45 also acts as a diverting transistor) to be latched on by the transistor 02, thereby causing shorting of the base of the transistor Q6, thereby rendering the drive winding T2/3 inoperative, so as to switch off the transistor Q6 and, by a "mirror image effect" via the transformer T2. also switching off the transistor Q5. Thus said excessive current flow ceases. As with the other circuits hereinbefore described, current will flow again when the capacitor C3 has discharged sufficiently, but the transistors Q5 and Q6 will be switched off again by the circuit 45 should the current again become excessive.Current flow through the resistor R7 is unidirectional, and so there is no need to employ a rectifying means such as REC 2 of Figure 1a.

Current flows through R7 in the circuit 45 only when current is flowing through the transistor Q6, and so if the current becomes excessive during a conduction period of Q5, the circuit 45 will not cause cessation of current flow until a similarly excessive current has started to flow through 06. In a low voltage push-pull circuit, such an arrangement is not likely to cause difficulties, since low voltage transistors are often capable of sustaining excessive currents briefly.

The circuits 10, 20, 30 and 40 are voltage converters of the kind described in our copending application No. 8221480.

However, the protective circuits hereinbefore described are also suitable for use with other kinds of main circuits wherein protection against excessive current flow or short circuit is required or desirable. Such other main circuits may be other kinds of voltage converters, or may be other kinds of other electrical circuits.

Although the protective circuits described have used a resistance R7 to sense excessive current, such excessive current may alternatively or additionally be sensed via a current sensing transformer.

In general, use of protective circuits in accordance with the invention is preferable to use of a conventional fuse. The protective circuits are capable of being faster-reacting than conventional fuses, and have the further advantage that should an excessive current flow, which would have "blown" a conventional fuse, the protective circuits in accordance with the invention are able to cause cessation of said current without destruction of any part of the circuits, so that when a cause of excessive current flow is removed or rectified, normal operation of the main circuits may resume immediately, without incurring any delay such as would be necessary for replacement of a blown conventional fuse.

Claims (15)

1. A protective circuit of the kind specified comprising means for diverting current from the component to be protected in response to detection of excessive current flow through said component.

2. A protective circuit of the kind specified comprising diverting means for rendering inoperative a drive device associated with the component to be protected in response to detection of excessive current flow through said component, thereby causing cessation of said excessive current flow.

3. A protective circuit according to claim 1 or claim 2 wherein the diverting means comprises a diverting transistor having a main current path connected substantially in parallel with said component or said drive device, the diverting transistor being normally nonconductive but switching on in response to excessive current flow through said component.

4. A protective circuit according to claim 3 wherein said excessive current is detected by a sensing means in series with the component to be protected, the sensing means providing an "output" in response to said current sufficient to cause switching on of the diverting transistor when said current becomes excessive.

5. A protective circuit according to claim 4 comprising a transistor latch means operated by said "output" to cause switching on of the diverting transistor.

6. A protective circuit according to claim 4 or claim 5 wherein the sensing means comprises a resistance through wlhich the current flows.

7. A protective circuit according to any one of claims 4 to 6 wherein the sensing means comprises a current sensing transformer.

8. A protective circuit according to any one of claims 4 to 7 wherein a rectifying means is provided to cause the current through the sensing means to be unidirectional.

9. A protective circuit according to any one of claims 3 to 8 wherein the diverting transistor switches off after a time at least partially determined by the protective circuit.

1 0. A protective circuit according to claim 9 wherein said time is determined by the time constant of a discharge circuit for a capacitance in the protective circuit.

11. The combination of a main apparatus coimprising a main electrical circuit and a protective circuit according to any one of claims 1 to 10.

1 2. The combination according to claim 11 wherein said main electrical circuit is a pushpull voltage converter circuit.

1 3. The combination according to claim 1 2 wherein said voltage converter circuit is of the kind described in our copending British patent application No. 8221480.

1 4. The combination according to any one of claims 11 to 1 3 wherein the coimponent protected by the protective circuit is a semiconductor switching component.

15. The combination according to claim 14 wherein the semi-conductor switching component is a switching transistor.