GB1572394A

GB1572394A - Converter arrangements

Info

Publication number: GB1572394A
Application number: GB1602779A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1977-01-13
Filing date: 1977-01-13
Publication date: 1980-07-30

Description

(54) IMPROVEMENTS IN OR RELATING TO CONVERTER ARRANGEMENTS (71) We, THE PLESSEY COMPANY LIMITED a British Company of Vicarage Lane, Ilford, 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 converter arrangements and more specifically to DC/DC converter arrangements. It is especially concerned with DC/DC converter arrangements for use in battery powered ignition/discharge systems e.g. gas ignition circuits, cigarette lighters, photographic flash equipment etc.

In conventional gas ignition/discharge systems, a capacitor is normally charged to a higher potential than the supply voltage by means of a DC/DC converter. The capacitor is then rapidly discharged in order to produce the desired gas ignition/discharge. In some systems the voltage on the capacitor may be further boosted by discharging the capacitor into the primary of a step-up transformer, the secondary voltage of which is used to produce the desired gas ignition/discharge.

Normally, speed of operation of the system is an important consideration and it is therefore beneficial to provide a so-called speed optimised DC/DC converter. Conventional designs of converters suffer from two main disadvantages, these being the power loss in the converter and inefficient power transfer.

Normally power is lost in the converter due to losses in the chopping elements during transition and transformer losses arising from core saturation. Considering power transfer, maximum power is obtained from a battery when it is presented with a load that is equivalent to its own internal resistance.

However as the battery becomes exhausted, its internal resistance increases thus demanding an appropriate change of load if maximum power transfer is to be achieved.

Conventional designs fall into one of three categories, these being (a) where optimum performance is obtained when the battery is new; (b) where optimum performance is obtained when the battery is exhausted; and (c) where optimum performance is obtained when the battery is partially exhausted. In the case of category (a), the DC/DC converter charges the storage capacitor rapidly initially but there is a drastic deterioration in performance as the battery becomes exhausted In the case of category (b), a consistent but mediocre performance is obtained, whereas category (c) provides a compromise solution.

The present invention is concerned with a DC/DC converter where optimum performance throughout the useful life of the battery is achieved and in which certain of the power losses obtained in conventional systems are avoided.

According to the present invention there is provided a converter circuit arranged to operate from a battery the converter comprising a transformer having primary and secondary circuits; a chopper transistor connected in series with the primary circuit; means associated with the primary circuit for switching off the transistor when current in the primary circuit reaches a predetermined value and voltage sensing means for sensing the battery voltage and for inhibiting turning on of the chopper transistor during periods when the battery voltage falls below a predetermined level.

In carrying out the invention bistable drive circuit means may be provided for controlling the operation of the chopper transistor.

Conveniently the bistable drive circuit means is set to one state for controlling the operation of the chopper transistor by means of current sensing means associated with the primary circuit of the transformer means and conveniently connected in parallel with the chopper transistor, and in which the bistable drive circuit means is set to a second state for controlling the operation of the chopper transistor by the combined effect of current sensing means associated with the secondary circuit of the transformer means and said voltage sensing means for sensing battery voltage.

In one arrangement input capacitor means may be provided connected in parallel with battery supply terminals to which a battery may be connected for energising said converter and output capacitor means may also be provided connected in the secondary circuit of the transformer means, and which is caused to be charged by the successive operation of the chopper transistor in which case diode means may be provided connected in series with the output capacitor means, the current sensing means being provided connected in parallel with the diode means.

In one especially envisaged arrangement further transformer means will be provided having primary and secondary windings with spark electrodes connected to said secondary winding, further voltage sensing means being provided for sensing the voltage of the output capacitor means and for causing it to be discharged through said primary winding when the voltage of said output capacitor means exceeds a predetermined value.

In such an arrangement the further voltage sensing means may conveniently comprise a zener diode chain, and the further voltage sensing means may comprise semiconductor switch means for discharging the output capacitor means through the primary winding of the further transformer means, the semiconductor means comprising a pair of transistors connected in Darlington configuration or a thyristor.

In addition bistable means may be provided operable when the voltage of the output capacitor means exceeds the predetermined value for disabling the converter so as to obtain single-shot operation, in which case means may be provided operable on the further voltage sensing means and in conjunction with the bistable means for simulating low battery voltage when the voltage of the output capacitor means exceeds the predetermined value, thereby to disable the converter.

In one arrangement means may also be provided for preventing operation of the bistable means to enable multi-shot operation to be achieved, in which case means may be provided for selecting single shot or multi shot operation.

Advantageously the DC/DC converter according to the present invention will be constructed in integrated circuit form.

An exemplary embodiment of the invention will now be described reference being made to the accompanying drawings in which: Figure 1 is a block schematic diagram of a DC/DC converter according to the present invention; Figure 2, (a), (b) and (c) depict waveforms existing at various points in the converter points in the converter of Figure 1; Figure 3 is a circuit diagram of a gas ignition circuit arrangement incorporating the DC/DC converter of Figure 1; and Figure 4 is a circuit diagram of a modified form of the gas ignition circuit arrangement of Figure 3.

Turning initially to the DC/DC converter shown in block schematic form in Figure 1 of the drawings, this consists of a transformer 1 having oppositely wound primary and secondary windings 2 and 3 respectively, the primary winding 2 having a chopper transistor Q1 connected in series with it, the primary winding 2 and chopper transistor Q1 being connected via a switch S1 to a battery B consisting of one or more cells and having an internal resistance RB. A smoothing capacitor C1 is provided connected across the battery B when the switch S1 is closed. The secondary winding 3 of the transformer 1 is connected via a series diode D1 to a storage capacitor C2 from which is derived the H.T. (high tension) output 4 from the converter.

Control of the switching of the chopper transistor Q1 is effected by a bistable drive circuit 5, one input to which is derived from a primary current sensing circuit 6 connected across the chopper transistor Q1 and another input to which is derived via an 'AND' gate 7 from a secondary current sensing circuit 8 connected in parallel with the diode D1, and a supply voltage sensing circuit 9 which monitors the supply voltage applied to the primary winding 2 of the transformer 1.

Operation of the converter is as follows:- Assume that the switch S1 is closed and that transistor Q1 is in the 'ON' state. The current in the primary winding 2 of the transformer 1 will increase with time until it reaches the threshold level of the primary current sensing circuit 6. (This circuit is arranged to operate before core saturation is reached). At this point an output is afforded to the bistable drive circuits to turn the transistor Q1 'OFF'.

The reverse EMF thus induced in the secondary winding 3 of the transformer 1 forward biases the diode D1 and the energy stored in the magnetic field (1/2 LI2) is transferred into the storage capacitor C2. Transition losses are minimised by inhibiting transistor Q1 from turning 'ON' again until energy transfer has been completed. This is achieved by means of the secondary current sensing circuit 8 which detects diode D1 coming out of conduction and affords an output which is applied via 'AND' gate 7 to the bistable drive circuit 5 to cause the transistor Q1 to be turned 'ON'. The interconnection of the primary current sensing circuit 6 and the secondary current sensing circuit 8 with the transformer 1 is therefore in the form of a relaxation oscillator which avoids power losses obtained in conventional converters.

To obtain maximum power transfer from the battery B, the supply current drawn should be half the short circuit current of the battery. To compensate for the changing battery resistance it is therefore necessary to regulate the supply current such that the supply voltage is maintained at approximately half EMF. It is possible to obtain batteries that have a predictable and fairly constant EMF throughout their useful life and this permits the use of a simple supply voltage sensing circuit 9 to be used, the output of which is applied via the 'AND' gate 7 to the bistable drive circuit 5 and is used to increase the 'OFF' time of the transistor Q1 if the supply voltage falls below a threshold level, thus effectively reducing the mean supply current to maintain maximum power supply current to maintain maximum power In Figure 2 of the drawings there are shown typical waveforms existing at various parts of the converter of Figure 1. In Figure 2(a) there is depicted the primary current Ip which it will be seen increases with time until a threshold is reached at which time the transistor Q1 is turned 'OFF'. At this time the secondary current Is as depicted in Figure 2(b) increases sharply and then decreases until a further (zero) threshold is reached. If at this time the supply voltage Vs as depicted in Figure Z(c) is below yet a further threshold, then the transistor Q1 is maintained in its 'OFF' state until the supply voltage reaches the threshold at which time the transistor Q1 is again turned 'ON'.

Turning now to the gas ignition circuit arrangement shown in Figure 3 of the drawings, this is based on the DC/DC converter of Figure 1, and has been sub-divided by the horizontal dashed line 6, the part of the arrangement above the line 6 corresponding generally to the DC/DC converter of Figure In the arrangement of Figure 1, the transformer 1 is shown which has a transistor T14 which constitutes the chopper transistor Q1 of Figure 1, connected in series with it across the battery supply afforded by the battery B.

The battery B has connected in series with it the switch S1 and in parallel with it via the switch S1 the smoothing capacitor C1.

For the sake of convenience a description of the arrangements of Figure 3 will be given by describing a typical operating sequence.

When the switch S1 is 'closed', the battery B is connected across the capacitor C1 which charges up in a linear fashion over a short period of time. The majority of the circuit remains in an 'OFF' state until the threshold of the supply voltage sensing circuit formed by transistors Ti, T2, T3, T4 and Ts is reached.

During this initial period, the state of the bistable circuit formed by transistors Tri 7 and Tia is set by the delay in the turn 'ON' of the transistor Tia. Hence transistor T17 starts to conduct before transistor Tls and the bistable latches with transistor T17 in the 'ON' stage.

The supply voltage sensing circuit formed by transistors T1 to T5 takes the form of a high gain amplifier from the base of the transistor T1 to the output from the collector of transistor T5 the output being regulated by the action of transistor T4 to a voltage determined by resistors Rs and Rio. The threshold of the supply voltage sensing circuit is determined by the Vbe of transistor T1 and the potential divider formed by resistors R1 and R2. When the threshold is reached, transistor Ts is turned 'ON' and its collector goes positive.

This positive potential is applied to the base electrode of the chopper transistor T14 via resistor R11 which allows sufficient current to flow to cause transistor T14 to turn 'ON' and saturate. (Since it has an inductive load, the initial collector current will be zero). The primary current sensing circuit formed by transistors T12 and T13 connected in the form of a differential amplifier compares the voltage across the chopper transistor T14 which is applied to the base electrode of transistor T13 with a threshold which is determined by the Vbe of transistor T11 and the potential divider formed by resistors R17 and R1s, the junction of which is connected to the base electrode of transistor T12. When the voltage across the chopper transistor T14 is below the threshold, transistor T13 will be in the 'ON' state and transistor T12 will be in the 'OFF' state. The collector electrode of transistor T12 is connected to the base electrode of transistor Ta which is thus turned 'OFF' which in turn causes transistor Ts which is connected to it to turn 'ON'. Transistor Ts saturates and turns 'ON' multi-collector transistor Tlo which acts as an unbalanced current mirror providing the bulk of the base current required to keep chopper transistor T14 in its saturated state during its 'ON' period. Hence a positive feedback loop is provided which holds transistor T14 hard 'ON'.

During this period, charge is drawn from the smoothing capacitor C1 and the supply voltage consequently falls. It is very important that the supply voltage sensing circuit does not interrupt the 'ON' period of the chopper transistor T14 and it is therefore arranged that the supply voltage sensing circuit is maintained in its 'ON' state by feedback from transistor Tio, which is in its 'ON' state, via resistor R3 to transistor T which is thus held 'ON'.

When the voltage across the chopper transistor T14 exceeds the threshold applied to transistor Tor2, transistor T13 will be turned 'OFF' and transistor T12 will be turned 'ON'.

This causes the transistor T6 to be turned 'ON' and transistor Ts 'OFF'. The turning 'OFF' of the transistor Ts turns 'OFF' transistor Tio so that the base current supply to the chopping transistor Tr4 is extinguished.

Note, some current is still afforded by resistor R11, but this is insufficient to hold transistor Tr4 in a saturated state. As the current in the primary winding of the transformer TF1 starts to collapse then due to the reverse EMF induced in the secondary winding current is forced through the baseemitter junction of multi-collector transistor T7 into storage capacitor C2. Thus transistor T7 is turned 'ON' and one of its two collector electrodes reinforces the turn 'ON' of transistor T6 and the other causes transistor Ts to be turned 'ON' which in turn ensures that chopPer transistor T14 is turned hard 'OFF'.

When secondary current flow ceases, current flow through transistor T7 ceases, so that it turns 'OFF', which in turn causes transistor Ts to be turned 'OFF'. Thus current is again applied to chopper transistor Tr4 via resistor Rrr for the cycle to be repeated.

However, during the 'ON' period of the chopper transistor Tr4 charge was drawn from the smoothing capacitor C1 so causing the supply voltage to fall, time has to be given to allow the smoothing capacitor C1 to be recharged by the battery B. During the 'OFF' period of the transistor T14, transistor Tio is 'OFF' and no current is supplied to transistor T1 via resistor R3. This enables the supply voltage sensing circuit consisting of transistors T1 to Ts to once again become effective. If then, during the 'ON' period of the chopper transistor Tr4 the supply voltage fell below the supply threshold, then immediately the chopper transistor T14 is turned 'OFF', the transistor Tt will be turned 'OFF' and in turn the transistor Ts will be turned 'OFF'. The base current supply to chopper transistor Tr4 via resistor Rrr is removed so that chopper transistor Tr4 cannot be turned 'ON'. Thls condition is maintained until the supply voltage exceed the supply threshold at which time transistors T1 and Ts are both turned 'ON' which allows chopper transistor T14 to be turned 'ON' and the cycle repeated.

The DC/DC converter thus far described operates so that successive cycles charge up the storage capacitor C2, and when sufficient charge is stored (the stored energy being related to the voltage across capacitor C2 by the expression E = 1/2 CV2) the voltage across the capacitor causes the initial breakdown of the zener diode chain ZD, to ZD, and the Vbe of transistor Tri9. The initial collector current through transistor T1s passes through resistors R3r, R32 and R34 to turn 'ON' transistor T23. The combination of the transistors Tie and T20 form a thyristor which latches 'ON', driving the power Darlington pair formed by transistors T21 and T22 into heavy conduction. These discharge the capacitor C2 through the primary winding 10 of a step-up transformer 11 causing a spark to be generated across spark electrodes SP connected to the secondary winding 12 of the transformer 11.

When the transistors Tie and T20 are conducting, current is also supplied to transistors T16 and Tie via resistors R25 and R29 respectively, which are thus turned 'ON'. This causes the bistable circuit formed by transistors T17 and Tie to change state and causes the transistor T16 to be held 'ON' continuously, which causes the supply voltage sensing circuit to be turned 'OFF' by shunting the resistor R1 in the base circuit of transistor Ti.

The circuit is reset when switch S1 is released and then may be operated again to produce a further spark. The operation thus obtained is single-shot operation, in that one spark will be generated each time the switch S1 is operated.

In some applications however, it may be desirable to provide for continuous operation i.e. sparks are produced successively each time the switch S1 is operated. This may be achieved in the arrangement shown in Figure 3 by closing a second switch S2 which simulates transistor T17 being turned on permanently. Transistor T16 is now only switched when transistors Tie and T20 are conducting. When transistors Tie and T20 have stopped conducting, transistor T16 is turned 'OFF' allowing the supply voltage sensing circuit to operate normally.

In one particular application it is envisaged that the switches S1 and S2 be mechanically coupled to provide an arrangement in which, say, single spark-off and multi-spark operation is selected. This may conveniently be achieved using a single 3-position switch.

It is envisaged that the gas ignition arrangement shown in Figure 1 of the drawings be constructed where possible using integrated circuit techniques in which case it is envisaged that all the circuitry to the left of the vertical dashed line be in integrated form leaving the transformers, capacitors and switches to be connected in discrete form.

It is also envisaged that instead of including the power Darlington transistors T21 and T22 in the integrated circuit that an external discrete thyristor may be used in which case the circuit arrangement may be changed to that shown in Figure 4 of the accompanying drawings.

Various other modifications to the circuit arrangement described may be made without departing from the scope of the invention.

For example:- 1. The method shown for sensing the current in the primary circuit is by means of monitoring the collector emitter saturation voltage of the chopper transistor Q1. Alternative methods are: (a) Monitoring the base emitter voltage of transistor Q1.

(b) Sensing the saturation of the transformer TF1 by the collapse of the voltage across the primary winding.

(c) By monitoring the voltage across a resistor placed in series with the chopper transistor.

2. In the practical circuit the secondary current of transformer TF1 is rectified by the base emitter junction of a P.N.P. transistor.

The current is then detected by using the fact that the collector will conduct when a current is forward biasing the base emitter junction.

Alternative methods are: (a) Using an N.P.N. transistor in a similar way to that described above.

(b) Detecting that diode D1 is conducting by monitoring the forward bias voltage across it.

(c) Detecting that diode D1 is non-con ducking by the presence of a reverse voltage across it.

(d) By the insertion of a second device (diode, transistor or resistor) placed in series with the main rectifying element D1, and used in one of the aforementioned ways to detect the secondary current.

3. Capacitor Ci may be placed before or after the switch S1.

4. The supply sensing circuit uses a Vbe of a transistor as a reference level which is scaled up by the use of a potential divider.

Alternative methods are: (a) Using one or more diodes (in conduction) to produce a reference level.

(b) The use of a zener diode to produce a reference level.

5. Instead of a high gain amplifier being used a Schmitt trigger circuit may be incorporate.

These converter arrangements also form the subject of our co-pending UK Patent application 1472/76 (Serial No. 1572393).

WHAT WE CLAIM IS: 1. A converter circuit arranged to operate from a battery the converter comprising a transformer having primary and secondary circuits; a chopper transistor connected in series with the primary circuit; means associated with the primary circuit for switching off the transistor when current in the primary circuit reaches a predetermined value and voltage sensing means for sensing the battery voltage and for inhibiting turning on of the chopper transistor during periods when the battery voltage falls below a predetermined level.

2. A converter as claimed in claim 1, in which bistable drive circuit means is provided for controlling the operation of the chopper transistor.

3. A converter as claimed in claim 2, in which the bistable drive circuit means is set to one state for controlling the operation of the chopper transistor by means of current sensing means associated with the primary circuit of the transformer means, and in which the bistable drive circuit means is set to a second state for controlling the operation of the chopper transistor by the combined effect of current sensing means associated with the secondary circuit of the transformer means and said voltage sensing means for sensing battery voltage.

4. A converter as claimed in claim 2 or claim 3, in which current sensing means is provided connected in parallel with the chopper transistor.

5. A converter as claimed in any preceding claim, comprising input capacitor means connected in parallel with battery supply terminals to which a battery may be connected for energising said converter.

6. A converter as claimed in any preceding claim, comprising output capacitor means connected in the secondary circuit of the transformer means, and which is caused to be charged by the successive operation of the chopper transistor.

7. A converter as claimedindaim6com- prising diode means connected in series with the output capacitor means, current sensing means being provided connected in parallel with the diode means.

8. A converter as claimed in claim 6 or claim 7, comprising further transformer means having primary and secondary windings, spark electrodes connected to said secondary winding and further voltage sensing means for sensing the voltage of the output capacitor means and for causing it to be discharged through said primary winding when the voltage of said output capacitor means exceeds a predetermined value.

9. A converter as claimed in claim 8, in which the further voltage sensing means comprises a zener diode chain.

10. A converter as claimed in claim 8 or claim 9, in which the further voltage sensing means comprises semiconductor switch means for discharging the output capacitor means through the primary winding of the further transformer means.

11. A converter as claimed in claim 10 in which the semiconductor means comprises a pair of transistors connected in Darlington configuration.

12. A converter as claimed in claim 10, in which the semiconductor switch means comprises a thyristor.

13. A converter as claimed in any of claims 8 to 12 comprising bistable means operable when the voltage of the output capacitor means exceeds the predetermined value for disabling the converter so as to obtain single-shot operation.

14. A converter as claimed in claim 13,

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. voltage of the chopper transistor Q1. Alternative methods are: (a) Monitoring the base emitter voltage of transistor Q1. (b) Sensing the saturation of the transformer TF1 by the collapse of the voltage across the primary winding. (c) By monitoring the voltage across a resistor placed in series with the chopper transistor. 2. In the practical circuit the secondary current of transformer TF1 is rectified by the base emitter junction of a P.N.P. transistor. The current is then detected by using the fact that the collector will conduct when a current is forward biasing the base emitter junction. Alternative methods are: (a) Using an N.P.N. transistor in a similar way to that described above. (b) Detecting that diode D1 is conducting by monitoring the forward bias voltage across it. (c) Detecting that diode D1 is non-con ducking by the presence of a reverse voltage across it. (d) By the insertion of a second device (diode, transistor or resistor) placed in series with the main rectifying element D1, and used in one of the aforementioned ways to detect the secondary current. 3. Capacitor Ci may be placed before or after the switch S1. 4. The supply sensing circuit uses a Vbe of a transistor as a reference level which is scaled up by the use of a potential divider. Alternative methods are: (a) Using one or more diodes (in conduction) to produce a reference level. (b) The use of a zener diode to produce a reference level. 5. Instead of a high gain amplifier being used a Schmitt trigger circuit may be incorporate. These converter arrangements also form the subject of our co-pending UK Patent application 1472/76 (Serial No. 1572393). WHAT WE CLAIM IS:

1. A converter circuit arranged to operate from a battery the converter comprising a transformer having primary and secondary circuits; a chopper transistor connected in series with the primary circuit; means associated with the primary circuit for switching off the transistor when current in the primary circuit reaches a predetermined value and voltage sensing means for sensing the battery voltage and for inhibiting turning on of the chopper transistor during periods when the battery voltage falls below a predetermined level.

14. A converter as claimed in claim 13,

comprising means operable on the further voltage sensing means and in conjunction with the bistable means for simulating low battery voltage when the voltage of the output capacitor means exceeds the predetermined value, thereby to disable the converter.

15. A converter as claimed in claim 13 or claim 14, comprising means for preventing operation of tebistable means to enable multi-shot operation to be achieved.

16. A converter as claimed in claim 15, in which means is provided for selecting singleshot or multi-shot operation.

17. A converter as claimed in any preceding claim constructed in integrated circuit form.