DEVICE FOR INCREASING A DIRECT-CURRENT VOLTAGE FROM A FIRST VALUE TO A SECOND VALUE

Abstract: A device is used for doubling a direct-current voltage, for example doubling +12 volt to 24 volt. The device comprises a transformer and change-over devices in the form of a pair of transistors. A direct-current voltage with a first value is connected to a central tap on the winding. The ends of the winding are connected to change-over devices which alternately bring about changes in potential of the said ends and thereby create direct-current voltage signals with the higher value by means of the transformer's counter electromotive force.

Description: Device for increasing a direct-current voltage from a first value to a second value. The present invention relates to a device for increasing a direct-current voltage from a first value, for example 12 volts, to a second value, 24 volts. The device comprises a transformer or coil provided with an iron core and change-over devices. The device can be used as voltage doubler, for example for vehicles, weapons, etc. The use of transformer arrangements is already known in connection with voltage conversion for increasing a direct-current voltage by means of counter electromotive forces that arise in the winding(s) during changes in current direction. The known equipment can comprise chokes and relatively large capacitors. In addition, the known equipment often comprises pure voltage regulators where there is a need for some form of filter arrangement for the outgoing voltage, which together comprises relatively large equipment with a large external volume. There is a need for a simply constructed DC/DC transducer which, in spite of its simple construction, has a high level of efficiency and high duty cycle, and can also be constructed with relatively small external dimensions. In addition, there is a requirement for the transducer to be able to operate with high output powers if so required. The object of the present invention is to solve all or parts of this problem. The principal characteristic of a device or transducer according to the invention is that the direct-current voltage with the first value is connected or is able to be connected to a central tap on the winding and that the ends of the winding are connected to change-over devices as mentioned in the introduction, which changeover devices are arranged to alternately bring about changes in potential of the said ends and thereby to create direct-current voltage pulses with the second value by means of the transformer's or the coil's counter electromotive force. ln an embodiment of the conception of the invention, the respective change-over devices comprise a pair of transistors. The respective change-over devices or pairs of transistors can thereby operate with a change-over ratio which is as close to 50% as possible, that is the distance between the outgoing direct current pulses is as short as possible without the danger arising that both change-over devices or pairs of transistors conduct at the same time with a resultant risk of damage. The said embodiments can also comprise the ends of the windings being connected to an output for the direct-current voltage pulses via diodes, which preferably consist of Schottky diodes or corresponding diodes. In a preferred embodiment, the output voltages on the cathode sides of the diodes are twice the direct-current voltage of the lower value minus the voltage drop across the respective diode. A smoothing choke can additionally be arranged before the winding's central tap in order to smooth the direct-current voltage of the first value. The device can additionally comprise a monitoring unit that ensures that the respective change-over devices or pairs of transistors conduct for such a long time during the generation of the direct- current voltage of the second value that it compensates for effects of any differences in resistance and/or inductance in the respective halves of the windings on the transformer and/or the resistances in the change-over devices or the pairs of transistors. The monitoring unit can also ensure that the change-over devices or the pairs of transistors work well when starting up. In a preferred embodiment, there is a regulator which is arranged to effect an optimal duty cycle for the direct-current voltage of the second value. A sawtooth voltage is used in the device, which sawtooth voltage controls the switching-on and -off times of the change-over devices or the pairs of transistors. The regulator or monitoring unit can thereby affect the gradient of the sawtooth voltage in such a way that the change-over time for the respective change-over device or pair of transistors assumes a value that allows the changes in the magnet flows to be equally large for the respective change-overs. The regulator can also detect current in the winding halves of the transformer or coil, for example by means of two measuring resistances or corresponding devices. The voltage or voltages which thereby arise across the resistances or corresponding devices affect the switching-on and -off times of the change-over devices or the pairs of transistors and in this way prevent the maximal desired current value from being exceeded. A fault indication voltage generated by the regulator can thereby control the switch-off level in the respective change-over device or pair of transistors. Additional embodiments of the invention are apparent from the following subsidiary claims. By means of the invention, a transducer is realized which in total gives small losses, as certain losses arise only in utilized diodes and change-over devices. The reduction of external dimensions is possible by the outgoing voltage not needing to be filtered to any high degree and by chokes and large capacitors being able to be eliminated. In a preferred embodiment, the transducer operates as a pure voltage doubler and therefore does not need to be provided with any complicated regulating function. In addition, technically well-tested components can be used. The transformer is constructed as a part of a circuit board, which guarantees a small external volume in total. DESCRIPTION OF FIGURES A currently proposed embodiment of a device that has the significant characteristics of the invention is described below with reference to the attached figures in which: Figure 1 shows in detailed schematic form a constructive embodiment of the invention, Figure 2 shows in diagrammatic form the embodiment of a sawtooth voltage that is generated by a regulator incorporated in Figure 1 , wherein a first version of the sawtooth voltage is indicated by an unbroken line and a second version of the sawtooth voltage is indicated by a dotted line, Figure 3 shows in diagrammatic form the switching-off point that is determined by the sawtooth voltage and a switching-off level determined by a fault indication voltage, and Figure 4 shows in diagrammatic form the period times for a change-over device or a pair of transistors incorporated in Figure 1. In Figure 1 a transformer is indicated by L1 and two Schottky diodes by D1 and D2. In addition, there is a regulator REG of type UC 2825 A which can be obtained on the general market. The device also comprises a switch output stage SSL, which in turn comprises two pairs of transistors Q1 , Q2 and Q3, Q4 arranged in switch connections. In addition, there is a monitoring unit OVE which can be considered to be incorporated in or to interact with the said regulator REG. The said monitoring unit comprises among other things a unit U1 and a transistor Q5. In addition, there is a smoothing choke L2 which is connected to the midpoint of the transformer L1. The transformer is a so-called autotransformer. The midpoint is indicated by M. The respective ends N, N' are pulsed by the pairs of transistors Q1 , Q2 and Q3, Q4 respectively. The duty cycle for the switching per transistor pair is to be as close to 50% as possible. An incoming voltage of +12 volts, that is the positive pole of a voltage source SK, is connected to the said midpoint M via the said smoothing choke L2. When activated, the said pairs of transistors Q1, Q2 and Q3, Q4 cause the winding ends N and N' respectively to be connected to zero potential NO and NO' respectively. The transformer is thereby connected in such a way that 50% duty cycle per pair of transistors can be obtained. The output voltage on the cathode side of D1 and D2 is thereby 2 times the input voltage minus the voltage drop across the diode D1 and D2 respectively. In the ideal case, no or little filtration of the output voltage is thereby required, as it is then in principle a pure DC voltage. The said outgoing voltage Eut is essentially equal to + 24 volts. The said regulator REG is set in such a way that the maximal duty cycle is attained without the pairs of transistors Q1 , Q2 and Q3, Q4 respectively through-conducting with damage as a result. In accordance with the invention concept, special measures have been taken at the transformer connection L1 so that this is not DC-magnetised, which would result in damage to the switch output stage SSL. Such DC-magnetising could occur due to the magnet flow changes in the transformer not being equal when the respective pairs of transistors switch. Causes of this include differences in the resistance/inductance in the respective winding halves of the transformer. The difference in "ON" resistance for the respective pairs of transistors can also have an effect. The said special measures therefore consist of protecting the switch output stage SSL and in the preferred embodiment the invention therefore operates with a monitoring function that is implemented by the monitoring unit OVE. This unit comprises according to the above, the transistor Q5 and the unit U1 which guarantee that the respective pairs of transistors each conduct for a sufficiently long time. The transistor Q5 also operates when starting up the device in such a way that the output stage has a soft start. In accordance with what follows, the regulator REG brings about a sawtooth voltage and the unit U1 is arranged to affect the gradient of this sawtooth voltage in such a way that the switch times of the respective pairs of transistors are designed to permit the magnet flow changes through the iron cores of the transformer to be the same size when the output stage switches. The sawtooth voltage which is generated by the regulator together with the surrounding components shown in the figure controls in turn the on- and off-times of the switch transistors. In addition, the regulator REG operates with a current-limiting function where the current in the respective winding halves of the transformer L1 is detected by two measurement resistances. The on- and off-times of the switch output stage are affected by the voltage across these resistances in such a way that the maximal desired current value is never exceeded. As the circuit according to Figure 1 and also the regulator REG are constructed in a known way, all the components will not be described here in detail, but only their connection and interaction essential for the object of the invention. The regulator REG comprises a comparator function (PWM comparator) to which connection is made via the inputs 1 and 7. The comparator function controls the abovementioned switch device's connections and disconnections via the output 14 of the regulator. In the on position, the electromotive voltage Ein is connected to the inductance L and an outgoing voltage value (Eout) of 2 times Ein will thereby arise. By means of known summation and feedback functions an Itot is hereby attained. The resistance measurement function (including R1 and R2) can thereby be implemented with the latter current and after measurement a high-pass function is arranged, and also an integration function, in which the capacitor C6 is involved. The integration function is obtained at the regulator's output 6 and forms the average value of the flow change during the respective half period. The said output 6 is connected to the input 7 and the connection point is also connected to the comparator function via the input 1 between the resistances R6 and R5, across which the said Eout is connected. If the flow increases linearly during a half period, it is sufficient if the average value of the flow < Λ the saturation value. The soft start function effected by the unit OVE is effected by means of the transistor Q5, the control input of which is connected across the resistance R7 and the capacitor C3. The resistances R13 and R14 are connected to the anode of the transistor, and one side of the capacitor C3 is connected to the connection point of the resistances R13 and R14. The cathode of the transistor is connected to the connection point of the resistance R4 and the capacitor C17, the other side of which in turn is connected to the capacitor C4 and also to the series-connected resistances R11 and R12. When Ein is switched on (is connected) in accordance with the above, there is a positive pulse on the base or the control input of the transistor Q5, which thereby starts to conduct. The capacitor C17 is hereby charged up during the start-up procedure to approximately the correct voltage. During the continued normal operation the transistor Q5 is blocked. Figure 2 shows two different sawtooth voltages initiated by the regulator REG in Figure 1 , RT (indicated by an unbroken line) and RT (indicated by a dotted line). The choice of, that is the gradient of, the sawtooth voltage is selected by means of the unit U1 in the monitoring unit shown in Figure 1. In Figure 2 the sawtooth voltage is related to a constant voltage level KN. In figure 3, a fault indication voltage emanating from the above irregularities in the transformer's resistance or inductance has caused a voltage level NKN which is changed in relation to the voltage level in Figure 2. This change or fault indication voltage controls the switch-off level in the pairs of transistors Q1 , Q2 and Q3, Q4 respectively. The switch-off points are indicated in Figure 3 by FP and FP' respectively, which are related to the sawtooth voltages RT and RT' respectively. With reference to Figure 3 above, the fault indication voltage on input 1 controls the switch-off level (0-50%). Normally, the voltage is just below 50%. The fault indication voltage tries to keep the output voltage in question constant. The pulse length (see Figure 4) depends partly on the gradient of the capacitor voltage VC6, and partly on the fault indication voltage (INV). The invention is not restricted to the embodiments described above for the purpose of exemplification, but can be modified within the scope of the following patent claims.

Claims: CLAIMS 1. Device for increasing a direct-current voltage from a first value, for example 12 volts, to a second value, 24 volts, and comprising a transformer (L1) or coil provided with an iron core and change-over devices (SSL), characterized in that the direct-current voltage with a first value is connected or can be connected to a central tap ( ) on the winding and that the ends of the winding (N, N') are connected to the said change-over devices arranged to alternately bring about changes in potential of the said ends and thereby by means of the transformer's or the coil's counter electromotive force to create direct-current voltage pulses with the second value. 2. Device according to Claim 1 , characterized in that the respective change-over devices comprise a pair of transistors (Q1 , Q2 and Q3, Q4 respectively) and that the respective change-over devices or pairs of transistors operate with a change-over ratio which is as close to 50% as possible, that is the distance between the outgoing direct current pulses is as short as possible without the danger arising that both change-over devices or pairs of transistors conduct at the same time, with a resultant risk of damage. 3. Device according to Claim 1 or 2, characterized in that the ends of the windings (N, N') are connected to an output for the direct-current voltage with the second value via diodes, preferably Schottky diodes (D1 , D2) or corresponding diodes. 4. Device according to Claim 1 , 2 or 3, characterized in that the output voltage on the cathode sides of the diodes (D1 , D2) is twice the direct-current voltage of the lower value minus the voltage drop across either diode. 5. Device according to any one of the preceding claims, characterized in that a smoothing choke (L2) is arranged before the winding's central tap (M) in order to smooth the direct-current voltage of the first value. 6. Device according to any one of the preceding claims, characterized in that it comprises a monitoring unit (Q5, U1) which ensures that the respective change-over devices or pairs of transistors conduct for such a long time during the generation of the direct-current voltage of the second value that it counteracts differences in resistance/inductance in the respective halves of the windings on the transformer and/or resistances in the change-over devices or pairs of transistors. 7. Device according to Claim 6, characterized in that the monitoring unit (OVE) also ensures that the change-over devices or the pairs of transistors procure a soft start up when the device is started up. 8. Device according to any one of the preceding claims, characterized in that a regulator (REG, OVE) is arranged to effect an optimal duty cycle for the direct- current voltage of the second value. 9. Device according to any one of the preceding claims, characterized in that a sawtooth voltage controls the switching-on and -off times of the change-over devices or the pairs of transistors. 10. Device according to any one of Claims 6-9, characterized in that monitoring unit (OVE) affects the gradient of the sawtooth voltage in such a way that the change-over time for the respective change-over device or transistor device assumes a value that allows the changes in the flows in the iron core to be equally large for the respective change-overs. 11. Device according to any one of Claims 8, 9 or 10, characterized in that the regulator generates the saw-tooth voltage. 12. Device according to any one of Claims 8-11 , characterized in that the regulator detects the current in the winding halves of the transformer or coil by means of two measuring resistances or corresponding devices, and that a voltage or voltages which thereby arise across the resistances or corresponding devices affect the switching-on and -off times of the change-over devices and in this way prevent the maximal desired current value from being exceeded. 13. Device according to any one of the preceding claims, characterized in that it works as a direct-current voltage doubler. 14. Device according to any one of the preceding claims, characterized in that the transformer or coil constitutes a part of a circuit board on which other components are arranged, and that the transformer/coil and the components have a volume content that is optimally minimized. 15. Device according to any one of Claims 6-14, characterized in that the monitoring unit (OVE), which also serves as a compensating unit, contributes, together with small losses in only the said diodes and change-over devices, to the device's high level of efficiency. 16. Device according to any one of Claims 6- 5, characterized in that the regulator comprises a circuit that generates a control voltage (RT) that controls the respective charging current, that is, controls the respective charging period. 17. Device according to any one of the preceding claims, characterized in that a generated fault indication voltage controls the switching-off level in the respective change-over device/pair of transistors.

Inventors: LINDBLOM BORIS, LINDSTROEM STEFAN, LINDBLOM, BORIS, LINDSTROEM, STEFAN
Applicants: LINDBLOM BORIS, LINDSTROEM STEFAN, SAAB AB, LINDBLOM, BORIS, LINDSTROEM, STEFAN
Application year: 2001
Main classification: H02M3
IPC codes: H02M3