AU2007229437A1

AU2007229437A1 - Short Circuit Protection

Info

Publication number: AU2007229437A1
Application number: AU2007229437A
Authority: AU
Inventors: Koichi Hayashi
Original assignee: HPM Industries Pty Ltd
Current assignee: HPM Industries Pty Ltd
Priority date: 2006-10-25
Filing date: 2007-10-23
Publication date: 2008-05-15

Description

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AUSTRALIA

Patents Act 1990o COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: SHORT CIRCUIT PROTECTION This invention is described in the following statement:- Short Circuit Protection Field of the Invention This invention relates to short-circuit protection circuits for a dimmer output, and more particularly, short-circuit protection circuits which use field effect transistors as current sensing elements.

Background of the Invention Dimmers are commonly used to control power levels delivered to the load, for example, a light source. Circuits of this type may include a protection module to shield the load from circuit faults such as a short circuit or inrush currents.

In some existing protection modules, a current sensing resistor is in series with a regulating transistor. As the current increases, the voltage drop across this resistor causes the operation of the regulating transistor to limit the conduction of a pass transistor in series with the load. There are also more sophisticated designs, for example, designs where a silicon controlled rectifier is also used in the module. The main feature in many of these existing designs is that the current sensing resistor is used to adjust the point at which the protection module activates, or in other words, the maximum allowable current.

The current sensing resistors and accompanying capacitive components generate heat and occupy space in the circuit. Furthermore, resistors with different resistance ratings need to be chosen for circuits meant for different loads. Depending on their resistance ratings, resistors can be expensive.

Designs that simplify or maximize the efficiency of the circuit can achieve financial gains by reducing the production cost of the circuit.

While the following teachings of the present improvement are applied to a dimmer controlled lighting appliance, it will be understood that the same improvement may be applied to other electronic circuits.

Objects and Summary of the Invention Accordingly, it is an object of the present invention to simplify a shortcircuit protection circuit.

It is another object of the present invention to make the short-circuit protection circuit adaptable to loads with various power-rating requirements.

It is another object of the present invention to reduce the cost of manufacturing a short-circuit protection circuit.

Accordingly, there is provided a short-circuit protection circuit comprising a first current sensing element for connection with a positive polarity of an external load, and a second current sensing element for connection with a negative polarity of the external load. A protection module is in connection with the first and second current sensing elements, the protection module being adapted to break a conduction path between the external load and the first or second current sensing element when a shortcircuit occurs. The protection module further comprises a programmable unijunction transistor (PUT).

There is further provided a short-circuit protection circuit comprising a first transistor connected to an external load, a second transistor connected to the external load, the first and second transistors being current sensing elements for the load. The first and second transistors are connected directly to ground. The first and second transistors supply a signal to a protection module. A dimmer circuit that can control a maximum amount of current allowed in the circuit.

Brief Description of the Drawing Figures In order that the invention be better understood, reference is now made to the following drawing figures in which: Figure 1 is a circuit diagram showing a prior art circuit; Figure 2 is a circuit diagram showing an embodiment of the invention.

Best Mode and Other Embodiments of the Invention Figure 1, taken from the prior art, shows a circuit diagram 10 for a load 11 powered by an AC source 12, and controlled by a dimmer circuit 15. The circuit shown has short-circuit protection elements comprising two modules 13, 14, one for each polarity. For instance, for the current polarity defined by diode Dii, the module 13 comprises a first resistor R61, a second resistor R71, a first capacitor C41, and a silicon-controlled rectifier (SCR) Q31 which acts as a switch for the protection module.

In designs of the type shown in figure 1, the current sensing element R61 needs to be chosen specifically for different loads 11. The power-rating of the overall circuit accounts for the heat generated by the modules 13, 14. In this design, in addition to the switching SCRs, another transistor such as a bipolar junction transistor Q51 is needed as a switch controllable by the dimmer circuit.

Figure 2 is a diagram for a circuit that includes a present embodiment of the proposed protection circuit. The circuit shown in figure 2 has a load 27 powered by an AC supply Vi. Two conduction paths, one for each current polarity, can be traced by following the two diodes Di, D2, and two preferably metal-oxide-semiconductor type field effect transistors (FET) Q2 and Q3. In this example, the FET's Q2 and Q3 are respectively depletion and enhancement type devices. As will be explained, it is the FET Q2 and Q3, which unlike resistors may be controlled to operate at various levels, which are used as current sensing elements.

In the embodiment shown in figure 2, Q2 and Q3 are p-type transistors and both of the FET source electrodes 28, 29 connect to ground 26. The dimmer circuit 25 drives the FET gate electrodes 21, 32. In other embodiments, n-type transistors may be used, provided that appropriate circuit and component modifications are made.

Consider the current leaving V1 and travelling towards the load 27. This current is the same current travelling across Q3. A portion of this current enters and powers the dimmer circuit 25 through a resistor R1, while the rest flows through another resistor R2. The dimmer circuit 25 drives the gate electrode 32 of Q3 with a voltage magnitude larger than Q3's threshold voltage, causing the current to flow from the source 29 to the drain 30 of Q3, and then back towards the load 27 to complete the conduction loop.

In the present embodiment, a protection circuit is further added to the components mentioned in the two preceding paragraphs. The protection circuit here comprises a programmable unijunction transistor Qi (PUT) whose gate electrode 24 is connected to the FET gate electrodes. The PUT (Q1) is thus also driven by a gate pulse of the dimmer circuit A capacitor Ci is connected to the PUT anode electrode 22. Thus C1 builds up an electric field at the anode electrode 22, using the charges flowing through R2. The PUT anode 22 is also connected to the exit port of both diodes 33. The voltage at the PUT anode 22 is hence proportional to the voltage across the FET Q3. The PUT cathode electrode 23 outputs a current into the dimmer circuit. The value of C1 can be adjusted so that it acts as a filter to stop response to very fast transient voltages which can cause unnecessary activation of the protection circuit The following paragraphs will discuss the operations involving Q3, but it should be understood from the construction of the circuit that the same principles apply to the operations involving Q2, and will not be repeated. It is understood that components with appropriate electrical specifications are selected so that the relationship described in the following is sustained.

When the FET is operating in the triode region the resistance from Source to Drain is essentially constant over the normal current operating range but at higher currents, the FET operates in saturation zone, and the resistance from Source to Drain increases with increasing current. This means the rate is rise of voltage, Source to Drain, will be higher than the rate of rise of current. This rising voltage during transient current spikes is used to act as a current sensing element. The dimmer circuit 25 controls the voltage at the FET gate 32, and consequently the power delivered to the load 27.

When a short circuit occurs or when there is too much inrush current, there is a sudden increase in the current across the FET drain and source electrodes 30, 29. There is hence an increase in the voltage across the FET Q3 and consequently the voltage at the PUT anode 22. When the voltage magnitude at the PUT anode 22 becomes sufficiently, for example o.6 or 0.7 volt higher than the voltage magnitude at the PUT gate 24, the PUT switches on (or "fires") and the FET is forced off. The conduction path for the load 27 is thus broken.

When the aforementioned increase disappears or dissipates to a level such that the PUT anode voltage is again lower than the PUT gate voltage, the PUT switches off and allows the FET to restart and resume normal operation.

In the design thus disclosed, because the FET gate voltage and the PUT gate voltage are controlled by the dimmer circuit, it is possible to control the "triggering point" of the PUT by adjusting the gate pulse voltage driving the FET from the dimmer circuit. In other words, by controlling the dimmer circuit, it is possible to adjust the maximum amount of current that is allowed before the protection circuit switches on and interrupts conduction, without replacing any circuit protection components. The protection circuit is hence more versatile than the prior art.

In a preferred embodiment, the PUT cathode 23 is electrically connected to the dimmer circuit via the diode D3, and the dimmer circuit receives a current input from the PUT cathode 23 whenever a non-trivial current spike occurs. "Non-trivial" here means that the current spike is large enough to cause the FET to be turned off. It is further preferred that a microprocessor chip 34 is embedded in the dimmer circuit such that the dimmer circuit 25 turns off entirely after it senses for example five consecutive nontrivial current spikes.

Owing to fast response times of the PUT, circuit shut downs and restarts can be done in only a short time with a minimal amount of interruption.

Note that in the embodiment shown in figure 2, not only is the circuit simplified and the number of transistors reduced compared to the prior art, the two FET transistors double as current sensing elements. By reducing the number of components the manufacturing cost of the circuit is reduced. By reducing the number of resistors, related heat-generation is also reduced, and the overall circuit can handle a higher load power-rating. By using transistors rather than resistors as current load (current sensing) elements, a savings in the silicon real-estate silicon area) is further realized. This savings in area can be used to implement circuits serving different functions and thus increase the commercial attractiveness of the unit.

While the present invention has been disclosed with reference to particular details of construction, these should be understood as having been provided by way of example and not as limitations to the scope or spirit of the invention.

Claims

2. The circuit of claim 1, wherein: a PUT anode voltage is controlled by a voltage across the first or second current sensing element.
3. The circuit of either one of claims 1 or 2, wherein, the first and second current sensing elements are transistors.
4. The circuit of claim 3, wherein, the first current sensing element is a depletion type field effect transistor and the second current sensing element is an enhancement type field effect transistor. The circuit of either one of claims 3 or 4, wherein, the first and second current sensing elements operate in saturation.
6. The circuit of any one of claims 3 to 5, wherein, a gate of the PUT is in connection with a gate of the first current sensing element and with a gate of the second current sensing element.
7. The circuit of claim 6, wherein, the gates of the first and second current sensing elements receive an adjustable drive signal, wherein a change in a level of the drive signal causes a change in a maximum amount of current allowed in the circuit.
8. The circuit of claim 7, wherein, the drive signal is adjustable via controlling a dimmer circuit.
9. The circuit of any one of claims 1 to 7, further comprising, a dimmer circuit for controlling the external load. 1o. The circuit of claim 9, wherein, the dimmer circuit receives a PUT cathode signal when a conduction path through the first or second current sensing element is broken.
11. The circuit of either one of claims 9 or lo, further comprising, a microprocessor chip embedded in the dimmer circuit, the chip causing the first or second current sensing element to stop conducting electricity when a series of current spikes occurs.
12. A short-circuit protection circuit, comprising: 0 a first transistor connected to an external load; o a firstecond transistor connected to than external load; 0 a second transistor connected to the external load; O the first and second transistors being current sensing elements for the load; the first and second transistors being connected directly to ground; the first and second transistors supplying a signal to a protection module; and a dimmer circuit that can control a maximum amount of current allowed in C' the circuit. cil 0 13. The circuit of claim 12, wherein, the protection module comprises a unijunction transistor.
14. The circuit of claim 13, wherein, the unijunction transistor is a programmable unijunction transistor. The circuit of claim 14, wherein, the dimmer circuit supplies an adjustable drive signal to a gate of the unijunction transistor.
16. The circuit of claim 15, wherein, the drive signal is sent to a gate of the first transistor and a gate of the second transistor. 16. The circuit of any one of claims 12 to 15, wherein, the first and second transistors are field effect transistors.