Driver with isolation and surge signal protection

Drivers for driving light circuits with light emitting diodes include an isolation circuit with a primary part and a secondary part and a guiding circuit for guiding common mode surge signals to a reference potential. The isolation circuit may be configured to guide a first part of the common mode surge signal away from the light circuit, and the guiding circuit may be configured to guide a second part of the common mode surge signal away from the light circuit, the second part being smaller than the first part. The guiding circuit may include one or more capacitors for connecting one or more terminals of the secondary part to the reference potential, where the value(s) of the one or more capacitors is/are larger than values of a parasitic capacitance of the isolation circuit.

FIELD OF THE INVENTION

The invention relates to a driver for driving a light circuit comprising at least one light emitting diode. The invention further relates to a device.

Examples of such a device are lamps and parts thereof.

BACKGROUND OF THE INVENTION

JP 05 161258 A discloses a surge voltage absorbing circuit located between a source circuit and a rectifier circuit.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved driver. It is a further object of the invention to provide an improved device.

According to a first aspect, a driver is provided for driving a light circuit comprising at least one light emitting diode, the driver comprisingan isolation circuit comprising a primary part for receiving a primary signal from a source circuit and a secondary part for supplying a secondary signal to the light circuit, anda guiding circuit for guiding a common mode surge signal to a reference potential.

By having added an isolation circuit, a primary part for receiving a primary signal from a source circuit and a secondary part for supplying a secondary signal to the light circuit have been created that provide isolation, such as for example galvanic isolation. By having added a guiding circuit for guiding a common mode surge signal such as a common mode surge voltage or a common mode surge current to a reference potential, a common mode surge signal is guided to the reference potential. This way, the light circuit connected to the secondary part is protected against common mode surge signals. Such a combination of a driver and a light circuit can be used well in outdoor applications. These are great advantages.

A light circuit comprises one or more light emitting diodes of whatever kind and in whatever combination.

An embodiment of the driver is defined by the isolation circuit being designed to guide a first part of the common mode surge signal away from the light circuit and the guiding circuit being designed to guide a second part of the common mode surge signal away from the light circuit, the second part being smaller than the first part. One way to realize this is by coupling the guiding circuit to the secondary part of the isolation circuit. This way, for the common mode surge signal, a serial connection has been created comprising the isolation circuit and the guiding circuit.

An embodiment of the driver is defined by the guiding circuit comprisinga capacitor for connecting a first terminal of the secondary part to the reference potential. This way a common mode surge signal is guided to the reference potential at the secondary part. This capacitor forms, together with a parasitic capacitance of the light circuit, a parallel connection.

An embodiment of the driver is defined by a value of the capacitor being larger than a value of a parasitic capacitance of the isolation circuit. This way, a larger part of the common mode surge signal is guided away from the light circuit by the isolation circuit and a smaller part of the common mode surge signal is guided away from the light circuit by the capacitor. A larger capacitance value results in a smaller impedance value. Preferably, the value of the capacitor will be at least ten times larger than the value of the parasitic capacitance of the isolation circuit. Owing to the fact that, in view of the common mode surge signal, the capacitor and the parasitic capacitance of the light circuit form a parallel connection, and owing to the fact that this parallel connection and the parasitic capacitance of the isolation circuit form a serial connection, for the defined values most stress resulting from the common mode surge signal is guided away from the light circuit.

An embodiment of the driver is defined by the guiding circuit comprisinga further capacitor for connecting a second terminal of the secondary part to the reference potential.

An embodiment of the driver is defined by a value of the further capacitor being larger than a value of a parasitic capacitance of the isolation circuit.

An embodiment of the driver is defined by the driver comprisinga capacitor for connecting a first terminal of the primary part to the reference potential. This way a common mode surge signal is guided to the reference potential also at the primary part.

An embodiment of the driver is defined by the driver not comprising any capacitor that interconnects the primary part and the secondary part when having a value equal to or larger than a value of a capacitor of the guiding circuit and when having a value equal to or larger than a value of a parasitic capacitance of the isolation circuit. In a prior art situation, to guide a common mode surge signal to a reference potential, the primary and secondary parts were interconnected via a first prior art capacitor and one of both terminals of the secondary part was connected to the reference potential via a second prior art capacitor. According to this prior art situation, in a standby state a relatively large glow current may flow from the primary part to the secondary part via the first prior art capacitor. Such a relatively large glow current is not desired. According to an improved situation, this relatively large glow current is avoided, by having avoided the first prior art capacitor. Further according to this prior art situation, a relatively large voltage-to-ground-value may become present at the light circuit. Such a relatively large voltage-to-ground-value is not desired. According to an improved situation, this relatively large voltage-to-ground-value is avoided, by having avoided the first prior art capacitor.

The fact that the driver does not comprise any capacitor that connects the primary part and the secondary part to each other does of course not exclude a presence of a parasitic capacitance as usually present between the parts.

An embodiment of the driver is defined by the isolation circuit comprising an isolation transformer, the primary part comprising a primary winding of the isolation transformer, and the secondary part comprising a secondary winding of the isolation transformer. Isolation transformers provide galvanic isolation.

An embodiment of the driver is defined by the driver further comprisinga rectifier circuit for rectifying the primary signal with inputs to be coupled to outputs of the source circuit and with outputs coupled to inputs of the primary part.

The inputs of the primary part may be identical to the terminals of the primary part or not. Outputs of the secondary part may be identical to the terminals of the secondary part or not.

An embodiment of the driver is defined by the driver further comprisinga metal housing to be connected to the reference potential, the isolation circuit being fully surrounded by the metal housing. Such a metal housing is also known as a Faraday cage or a Faraday shield.

An embodiment of the driver is defined by the reference potential being protective earth.

According to a second aspect, a device is provided comprising the driver and further comprising the light circuit connected to the secondary part.

An embodiment of the device is defined by a value of a capacitor of the guiding circuit being larger than a value of a parasitic capacitance of the light circuit. Again, owing to the fact that, in view of the common mode surge signal, the capacitor and the parasitic capacitance of the light circuit form a parallel connection, and owing to the fact that this parallel connection and the parasitic capacitance of the isolation circuit form a serial connection, for the defined values most stress resulting from the common mode surge signal is guided away from the light circuit.

An insight is that a common mode surge signal is to be guided away from a light circuit. A basic idea is that a guiding circuit is to be used for guiding a common mode surge signal to the reference potential.

A problem to provide an improved driver has been solved. A further advantage is that a relatively large glow current to the light circuit is avoided and that a relatively large voltage-to-ground-value at the light circuit is avoided.

DETAILED DESCRIPTION OF EMBODIMENTS

In theFIG. 1, an embodiment of a prior art driver is shown. The prior art driver for driving a light circuit2with at least one light emitting diode comprises an isolation circuit, here in the form of an isolation transformer11,12that provides galvanic isolation, but other kinds of isolation circuits are not to be excluded. The isolation circuit comprises a primary part here in the form of a primary winding11for receiving a primary signal from a source circuit4(seeFIG. 3) and a secondary part here in the form of a secondary winding12for supplying a secondary signal to the light circuit2. The prior art driver further comprises a first prior art capacitor51that connects the primary and secondary parts with each other and a second prior art capacitor52that connects a secondary side of the first prior art capacitor51to protective earth. Further, a rectifier circuit31is present for rectifying the primary signal. Inputs of the rectifier circuit31are to be coupled to outputs of the source circuit and outputs of the rectifier circuit31are coupled to inputs of the primary part.

In theFIG. 2, a behavior of the prior art driver is shown. A common mode surge signal3such as a common mode surge voltage or a common mode surge current experiences a serial connection of a first parallel connection and a second parallel connection. The first parallel connection comprises the first prior art capacitor51and a parasitic capacitance41of the isolation transformer11,12. The second parallel connection comprises the second prior art capacitor52and a parasitic capacitance42of the light circuit2. Owing to the fact that the first and second prior art capacitors51,52will have values that do not differ too much from each other, and owing to the fact that the parasitic capacitances41,42will have values smaller than the values of the first and second prior art capacitors51,52, the impedances of the parasitic capacitances41,42will have values larger than the values of the impedances of the first and second prior art capacitors51,52, and the common mode surge signal3will put relatively much stress on the light circuit2, as indicated by the relatively large exclamation mark.

According to this prior art situation, in a standby state a relatively large glow current may flow from the primary part to the secondary part via the first prior art capacitor51. Such a relatively large glow current is not desired. Via the first prior art capacitor51, a relatively large voltage-to-ground-value may become present at the light circuit2. Such a relatively large voltage-to-ground-value is not desired too.

InFIG. 3, an embodiment of an improved driver1is shown. The improved driver1differs from the prior art driver shown in theFIG. 1in that the improved driver1comprises a guiding circuit for guiding a common mode surge signal3to a reference potential. Preferably, the isolation transformer11,12is designed to guide a first part S1of the common mode surge signal3away from the light circuit2and the guiding circuit is designed to guide a second part S2of the common mode surge signal3away from the light circuit2, the second part being smaller than the first part.

The guiding circuit comprises for example a capacitor22for connecting a first terminal of the secondary winding12to the reference potential, a value of the capacitor22being larger than a value of a parasitic capacitance41of the isolation transformer11,12. In that case, a larger part of the common mode surge signal3is guided away from the light circuit2by the isolation transformer11,12and a smaller part of the common mode surge signal3is guided away from the light circuit2by the capacitor22. The improved driver1may further comprise a capacitor21for connecting a first terminal of the primary winding11to the reference potential. This first terminal may be one of both terminals of the primary winding11. Clearly, the driver1does not comprise any capacitor that connects the primary winding11and the secondary winding12to each other.

In theFIG. 4, a behavior of the improved driver1is shown. A common mode surge signal3experiences a first parallel connection of the capacitor21and a serial connection. The serial connection comprises a parasitic capacitance41of the isolation transformer11,12and a second parallel connection. The second parallel connection comprises the capacitor22and a parasitic capacitance42of the light circuit2. Owing to the fact that the capacitors21,22will have values that do not differ too much from each other, and owing to the fact that the parasitic capacitances41,42will have values smaller than the values of the capacitors21,22, the impedances of the parasitic capacitances41,42will have values larger than the values of the impedances of the capacitors21,22, and the common mode surge signal3will put relatively little stress on the light circuit2, as indicated by the relatively small exclamation mark.

So, a value of the capacitor22may be larger than a value of a parasitic capacitance41of the isolation transformer11,12and than a value of a parasitic capacitance42of the light circuit2.

According to the improved situation, the relatively large glow current is avoided, by having avoided the first prior art capacitor51. And when having avoided the first prior art capacitor51, the relatively large voltage-to-ground-value at the light circuit2is avoided as well.

When comparing theFIGS. 2 and 4it becomes clear that the stress resulting from a common mode surge signal3at the primary part of the isolation transformer11,12is shifted away from (the parasitic capacitance42of) the light circuit2to (the parasitic capacitance41of) the isolation transformer11,12. Isolation transformers11,12and other kinds of isolation circuits as for example shown in theFIG. 6can usually survive much more stress resulting from common mode surge signals3than light circuits2comprising light emitting diodes.

A capacitor for interconnecting the primary winding11and the secondary winding12to each other may only be present in case it has a value smaller than a value of the capacitors22,23(preferably 20% or smaller, further preferably 10% or smaller) and in case it has a value smaller than a value of the parasitic capacitance41of the isolation circuit (preferably 50% or smaller, further preferably 25% or smaller).

In theFIG. 5, a device100is shown comprising the improved driver1and the light circuit2. Here, the improved driver1further comprises a capacitor23for connecting a second terminal of the secondary winding12to the reference potential. A value of the capacitor23may be larger than values of a parasitic capacitance41of the isolation transformer11,12and a parasitic capacitance42of the light circuit2.

Further, the improved driver1may further comprise a metal housing5to be connected to the reference potential, the isolation circuit being fully surrounded by the metal housing. Such an improved driver1is extremely suitable for outdoor applications and can survive lightning surges. The reference potential may for example be protective earth or a heat sink etc.

In theFIG. 6, another embodiment of an improved driver1is shown, that differs from the embodiment shown in theFIG. 3in that this driver1comprises an isolation circuit with a primary part13and with a secondary part14. This isolation circuit for example comprises inductively coupled inductors different from the windings of a transformer.

First and second elements may be coupled directly or may be coupled indirectly via a third element. The rectifier circuit31may be located inside or outside the driver1.

Summarizing, drivers1for driving light circuits2with light emitting diodes comprise isolation circuits11-14with primary parts11,13and secondary parts12,14and guiding circuits22,23for guiding common mode surge voltages to reference potentials. The isolation circuits11-14may be designed to guide first parts of the common mode surge signals3away from the light circuits2and the guiding circuits22,23may be designed to guide second parts of the common mode surge signals3away from the light circuits2, the second parts being smaller than the first parts. The guiding circuit22,23may comprise capacitors22for connecting first terminals of the secondary parts12,14to the reference potentials, values of the capacitors22being larger than values of parasitic capacitances41of the isolation circuits11-14. The driver1does not comprise any capacitor that interconnects the primary and secondary parts11-14.