AU2008204723A1

AU2008204723A1 - A DC/DC converter

Info

Publication number: AU2008204723A1
Application number: AU2008204723A
Authority: AU
Inventors: Paul Thompson
Original assignee: GDI LLC
Current assignee: PAKTON DEVELOPMENTS Pty Ltd
Priority date: 2007-01-10
Filing date: 2008-01-10
Publication date: 2008-07-17
Also published as: WO2008083434A1

Description

WO 2008/083434 PCT/AU2008/000011 1 A DC/DC CONVERTER FIELD OF THE INVENTION The present invention relates to direct current (DC) circuits, and DC/DC converters therefore, and in particular to DC/DC converters to power the capacitive discharge circuits in electric fence energisers. Although the invention will be described with particular reference to energisers, it will be appreciated that the invention may be employed with DC/DC converters that are used in other applications. The term converter covers both step-up converters, which take input current at a certain voltage and increase the voltage of the output current, and step-down converters, which take input current at a certain voltage and decrease the voltage of the output current. The term inverter is sometimes used to describe a DC/DC converter which is a step-up converter, however it is also used to describe electrical and electronic circuits that convert DC current to Alternating Current (AC). Because of this ambiguity of terminology this specification uses the term converter to mean both step-up and step-down DC/DC converters. Step-up converters can sometimes be referred to as boost converters or boost circuits. The term set-up converter is preferred in this specification. Step-down converters can sometimes be referred to as buck converters or buck circuits. The term set-down converter is preferred in this specification. BACKGROUND TO THE INVENTION Electric fences are widely used on farms to restrict the movement of both farm and feral animals. Such fences normally include a number of uninsulated wire conductors strung on supporting posts from which they are insulated. The conductors are coupled to an energiser that periodically outputs a high voltage pulse to energise the conductors so that an animal will receive a small electric shock if it contacts the energised conductors. Electric WO 2008/083434 PCT/AU2008/000011 2 fences are also used for perimeter security in domestic and industrial premises and detention centres. Energisers that are used to energise electric fences may be mains or battery powered. Most modern energisers include a discharge capacitor, a capacitor charging circuit for charging the capacitor to a high potential (e.g. several hundred volts), and a capacitor discharging circuit for discharging the capacitor through a step-up output transformer to produce a very high potential output pulse (e.g. several thousand volts) that is used to energise the fence conductors. The capacitor charging circuit is typically a voltage converter circuit that converts the relatively low supply voltage powering the energiser to the high voltage required to charge the capacitor. The capacitor discharging circuit typically includes a semiconductor switch and a step-up output transformer that are both coupled to the capacitor such that the capacitor is able to be discharged through the transformer's primary winding by closing the switch to thereby produce a high voltage pulse across the transformer's secondary winding that can be used to energise the fence conductors. The semiconductor switch is usually a Silicon Controlled Rectifier (SCR). These devices are favoured because of their ability to withstand the high voltage while switched off and conduct the high peak currents when switched on and while discharging the capacitor. The overall control of the energisers charge and discharge cycle can be performed by analogue oscillators, digital logic and timers or by a micro-controller. The most popular energisers are small battery powered units used for applications including strip grazing and temporary fencing. Strip grazing or 'ration' grazing, is a grazing management technique involving allowing grazing animals only partial access to a pasture. This reduces the grazing pressure on the pasture. Battery powered electric fence energisers typically use a DC/DC converter to build the high voltage for the discharge capacitor. The DC/DC converter is commonly the flyback topology, using a ferrite transformer. Strobe lights and camera flashes also use DC/DC converters and capacitor discharge circuits.

WO 2008/083434 PCT/AU2008/000011 3 PRIOR ART An example of a prior art capacitor discharge circuit is shown in Figure 1. A disadvantage of using the flyback converter topology is that the cost and complexity is higher than that for a non-isolated step-up converter circuit. The Flyback converter is a DC/DC converter with a galvanic isolation between the input and the output. It has the same structure as the step-up converter, with a transformer instead of an inductor. The DC/DC transformer is up to ten times more expensive than the simple inductor used in a step-up converter circuit. Since the selection of off-the-shelf transformers is much narrower than that of equivalent inductors, transformers generally cost more than inductors of equivalent energy and energy density. The customer base for transformers is smaller, yet the set of possible transformer configurations is much larger than that of corresponding inductor configurations. As a result, designs based on custom transformers are often necessary. Standard non-isolated step-up converters are limited in that they can produce output voltages equal to or above the input. The step-down converter can produce output voltages equal to or below the input. Non-isolated step-up converters are not usually used in capacitor discharge circuits because a DC current path exists between the input supply and the discharge device. Since the discharge device is normally an SCR, which requires the current to fall to a low level before it switches off, the SCR would latch up and not switch off again. In a previous invention by the inventor, (Patent 2003100627), the DC current path was removed or restricted by means of a switch or simple RC circuit to limit the current flow. Both of these methods are inferior in cost or efficiency to the present invention.

WO 2008/083434 PCT/AU2008/000011 4 SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a DC/DC converter suitable for providing either step-up or step-down conversion of input voltage. According to a second aspect of the present invention there is provided a DC/DC converter suitable for providing either step-up or step-down conversion of input voltage comprising an inductor, a capacitor, a switch and an oscillator. According to a third aspect of the present invention there is provided an apparatus for building the high voltage for a capacitor discharge circuit by using a modified step-up converter circuit. Preferably the apparatus uses a capacitor to couple the standard step up converter inductor to the rectifier. Preferably the capacitor blocks DC current from flowing directly from the input to the output. Preferably an extra diode is added to provide a discharge circuit to keep the blocking capacitor from fully charging. In one embodiment the apparatus is arranged to charge a capacitor in the capacitor discharge circuit of a battery powered electric fence energiser. In another aspect of the invention there is provided an apparatus for converting DC power from a power source to a load such that the converter produces an output voltage that may be continuously varied from below the input voltage level to above the input voltage level. BRIEF DESCRIPTION OF FIGURES In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate typical preferred embodiments of the invention and wherein: Figure 1 is a schematic diagram of a prior art arrangement for a capacitor discharge circuit. Figures 2 and 3 are schematic diagrams of apparatus according to the present invention. Figure 3 - a step-up and step-down non-isolated DC/DC WO 2008/083434 PCT/AU2008/000011 5 converter circuit for producing an output voltage from an input voltage that may be continuously varied from below the input voltage level to above the input voltage level DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference to Figure 2 there is depicted a block diagram of an electric fence energiser using a capacitor discharge circuit according to the present invention. The apparatus of Figure 2 includes an oscillator 1 to operate a switch 2 (which may be a BJT transistor or a MOSFET, but is not limited to being so) to turn the switch on, which builds a current in the inductor 3. When the switch switches off the inductive flyback causes current to flow through capacitor 4, diode 5 and charges the main capacitor 6 of the capacitor discharge circuit. Another diode 7 ensures that any charge built on capacitor 4 is removed when the transistor turns on again. Capacitor 4 is large enough so that it is essentially a short circuit at the high frequency of the flyback pulse and therefore presents minimal voltage drop or losses. When the voltage on capacitor 6 is high enough the timing circuit 8 fires the main switch 9 which discharges the main capacitor 6 through the output transformer 10 producing the electric fence output pulse. The capacitor 4 stops any DC current from flowing directly from the DC supply through to the SCR, thus allowing it to turn off when capacitor 4 is fully discharged. The SCR turns off and the cycle repeats. For higher power levels the peak current in the switch immediately after it turns on, due to the discharging of capacitor 4, may exceed the limits of the switch 2 and or diode 7. The addition of another small inductor Figure 3, 8 anywhere in the current path from ground through diode 7, capacitor 4 and switch 2 back to ground, may be used to control the rate of rise of and therefore the peak of this current spike.

Claims

1. A step-up and step-down non-isolated DC/DC converter circuit for producing an output voltage from an input voltage that may be continuously varied from below the input voltage level to above the input voltage level comprising an input voltage connection, a first inductor, a blocking capacitor, a switch, an oscillator and an output voltage connection in which the oscillator operates the switch to switch on so as to build a current in the first inductor and when the switch switches off an inductive flyback causes current to flow through the blocking capacitor.

2. A step-up and step-down non-isolated DC/DC converter circuit as claimed in claim 1 further comprising a second inductor, the second inductor operating to limit current spikes in the switch from the blocking capacitor.

3. A step-up and step-down non-isolated DC/DC converter circuit as claimed in claim 2 wherein the second inductor is magnetically coupled to the first inductor.

4. An electric fence comprising conductors, an energiser for energising the conductors, the energiser further comprising a discharge capacitor, a discharge capacitor charging circuit for charging the discharge capacitor, a discharge capacitor discharging circuit for discharging the discharge capacitor, and a transformer with a primary and secondary winding, in which the discharge capacitor is charged by the discharge capacitor charging circuit, and the discharge capacitor is discharged by the discharge capacitor discharging circuit through the primary winding of a step-up transformer to energise the fence conductors.

5. An electric fence as claimed in claim 4 in which the discharge capacitor charging circuit comprises a step-up and step-down non isolated DC/DC converter circuit for producing an output voltage from an input voltage that may be continuously varied from below the input voltage level to above the input voltage level comprising an input voltage connection, a first inductor, a blocking capacitor, a switch, an WO 2008/083434 PCT/AU2008/000011 7 oscillator and an output voltage connection in which the oscillator operates the switch to switch on so as to build a current in the first inductor and when the switch switches off an inductive flyback causes current to flow through the blocking capacitor.

6. An electric fence as claimed in claim 5 in which the step-up and step-down non-isolated DC/DC converter circuit operates only to step up the input voltage to produce an output voltage to a level above the input voltage.

7. An electric fence as claimed in claim 6 in which the step-up and step-down non-isolated DC/DC converter circuit further comprises a second inductor, the second inductor operating to limit current spikes in the switch from the blocking capacitor.

8. An electric fence as claimed in claim 7 wherein the second inductor is magnetically coupled to the first inductor.