Electrical DC-DC power converter with magnetically coupled switch control circuit

[0001] This application is a national stage application under 35 U.S.C. §371 of International Application No. PCT/IB2007/052997 filed on Jul. 30, 2007, and published in the English language on Feb. 7, 2008, as International Publication No. WO/2008/015626, which claims priority to European Application No. 06118451.1, filed on Aug. 4, 2006, incorporated herein by reference.

[0002] The present invention relates to an electric DC-DC power converter, for preconditioning a fully rectified sinusoidal power signal wherein voltage peaks of the rectified signal may be higher than the desired output voltage.

[0003] Electric DC-DC power converters are known in the art, and may be used to power a lamp. These converters comprise an input section and an output section, each section comprising at least a switch, for example embodied by a FET or any other suitable power switch. The converter is operated by controlling the switches, which may be performed by digital control circuitry. A difficulty in design and realization of these converters is, that at least one of the switches, usually a switch in the input section, is connected to a high voltage terminal. A control signal for such a switch, which is referred to as a floating switch in the art, should then also have at least a high voltage component, which is difficult to realize by digital control circuitry. It is known in the art to use a level shift IC for operating electronics at relatively high voltages, or to use large transformers. A disadvantage of the use of a level shift IC is that the voltage range for which a level shift IC can be applied is limited, which prohibits use of these components in power converters operating at a few hundred Volts. Large transformers have the disadvantage of being heavy and expensive.

[0004] It is an object of the present invention to provide an electric DC-DC power converter comprising a control circuit for a floating switch, overcoming at least one of the above-mentioned disadvantages.

[0005] The invention provides a power converter according to claim 1. The power converter comprises: an input section with a controllable first switch and a diode, an output section comprising a controllable second switch and a second diode, and a transformer with a primary winding and a secondary winding, the secondary winding being at least part of a control circuit for controlling the first switch. The primary winding of the transformer is configured to conduct a current that comprises an essentially constant component that is related to the mean power that is transferred from the input to the output, and an essentially alternating component that is related to switching of the second switch. As a result of the alternating voltage component across the primary winding, a synchronously alternating voltage is induced in the secondary winding, which can be used to control the first switch essentially synchronously with the second switch. Because only very little power is required to control the first switch, the transformer does not need to be dimensioned large or heavy.

[0006] In an embodiment of a power converter according to the present invention, the secondary winding of the transformer is directly connected to a control terminal of the first switch, resulting in a topology with very few components.

[0007] In another embodiment of a power converter according to the present invention, the secondary winding of the transformer is coupled to a control terminal of the first switch by a capacitor, with the advantage that operating of the first switch is enabled, even when a voltage at the input section is very low, for example about zero.

[0008] In yet another embodiment of a power converter according to the present invention, the secondary winding of the transformer is coupled to a control terminal of the first switch by a series connection of a capacitor and a resistance, with the advantage that no disturbing high frequency components in the voltage across the secondary winding are input to the control terminal of the first switch.

[0009] In a further embodiment of a power converter according to the present invention, a zener diode is coupled to a control terminal of the first switch, protecting at least the control terminal to damage due to an overvoltage.

[0010] In yet a further embodiment of a power converter according to the present invention, a shunt resistance is present across the first switch, facilitating easy start up of the power converter.

[0011] In another further embodiment of a power converter according to the present invention, a signal processing circuitry is coupled to a control terminal of the first switch, which provides an optimal switching of the first switch.

[0012] In still a further embodiment of a power converter according to the present invention, a control circuit is present for generating a control signal for the second switch.

[0013] In again another embodiment of a power converter according to the present invention, a square wave signal is used as a control signal for the second switch, offering the advantage of very precise switching of the first switch.

[0017] In the drawings, like reference numbers refer to like components. FIG. 1 shows a schematic diagram of a simplified power converter 100 according to the present invention. The converter 100 comprises a first input terminal 10 and a second input terminal 20, for receiving an input power, which is in general a DC voltage, and in particular a rectified alternating voltage. Between the first input terminal 10 and the second input terminal 20 a series connection of a controllable first switch 30 and a first diode 40 is connected, the first switch 30 and first diode 40 having a common node 42. The first switch 30 has a gate (terminal) 31, and channel terminals 32 and 33. Between a first output terminal 50 and second output terminal 60 a series connection of a controllable second switch 70 and a second diode 80 are connected, the controllable second switch 70 and second diode 80 having a common node 81. A gate (terminal) 71 of the second switch 70 is connected to a control signal generator 74. Between node 42 and node 81 a primary winding 91 of transformer 90 is provided. The secondary winding 92 of transformer 90 is connected between node 42 and gate 31.

[0018] In operation, an input voltage V_in is present across input terminals 10 and 20, and an output voltage V_out is present across the output terminals 50 and 60. The output voltage is kept essentially constant at a desired level, for example by a capacitor or a capacitive load (not shown) connected between the output terminals 50, 60. The control signal generator 74 provides a switching signal to the gate 71 of the second switch 70, which causes a current to flow through the primary winding 91. At an instance when the first switch 30 and the second switch 70 are switched conductive, the voltage V_in is present across primary winding 91 and a current flows from the first input terminal 10 through the first switch 30 via the primary winding 91 through the second switch 70 to the second output terminal 60. When the second switch 70 is subsequently switched non-conducting, the second diode 60 will commutate and the current will flow from the primary winding 91 to the first output terminal 50. The voltage over the primary winding 91 will then decrease from V_in to V_in−V_out. Herein, it is assumed that V_in−V_out is always smaller than V_in, so that repeatedly switching switch 70 on and off leads to a square-wave voltage across the primary winding 91, and because of the magnetic coupling between the primary and secondary transformer windings 91, 92 also to a square-wave voltage V_s across the secondary winding 92, wherein V_s equals the voltage across the primary winding 91 multiplied by the turns ratio between the primary winding 91 and the secondary winding 92. The voltage across the primary winding 91 is a square wave of which the average value is essentially kept zero, by operating switch 70 in order to keep the current through the primary winding at an essentially constant value. A DC component is missing in V_s, since transformer 90 can not transform a DC component of a voltage. When the voltage across the primary winding 91 decreases due to the switching of the second switch 70, the voltage V_s induced in the secondary winding 92 also decreases from a high value to a low value. The turns ratio of the transformer 90 is selected such that a low value of V_s is low enough to switch off the first switch 30, and the value of V_s is high enough to switch on the first switch 30.

[0019] FIG. 2 shows a diagram of a power converter 200 according to an embodiment of the present invention. The power converter 200 comprises the power converter 100 as illustrated in FIG. 1, supplemented with additional electrical components. In FIG. 2 a first parasitic capacitance 41 and a second parasitic capacitance 75 are shown. It should be noted that these parasitic capacitances may be present as well when realizing a practical embodiment of the power converter 100 as illustrated in FIG. 1. The second parasitic capacitance 75 causes the commutation of the second diode 80 to take place smoothly, while the first parasitic capacitance 41 causes the commutation of the first diode 40 to take place smoothly.

[0020] An optional zener diode 94 may be applied to prevent damages to the first switch 30 due to overvoltages, since it limits the voltage between the gate terminal 31 and the source terminal 33 of the first switch 30 to a safe value. Overvoltages may in particular occur when input voltage V_in reaches low values, e.g. about 0 Volt. An optional DC blocking capacitor 95 is present to filter any DC component that arises when the input voltage V_in drops and becomes smaller than the output voltage V_out, having the advantage that no disturbing DC components in the voltage V_s across the secondary winding 92 are input to the gate 31 of the first switch 30. The DC blocking capacitor 95 also has the advantage that the voltage between the gate 31 and the channel terminal 33 of the first switch 30 becomes negative and the first switch 30 becomes non-conducting under this condition. A further advantage of the DC blocking capacitor is that it may increase the gate-source voltage of the first switch 20 at a zero-crossing of the input voltage V_in, thus enabling switching of the first switch 30 at a zero-crossing of the input voltage V_in.

[0021] A resistance 93 may be added to form a filter in conjunction with DC blocking capacitor 95 for suppressing undesired harmonic content of a square wave voltage V_s, with the advantage that no disturbing high frequency components in the voltage across the secondary winding 92 are input to the gate 31 of the first switch 30. Furthermore, a signal processing circuit 96 is shown, which is configured to process voltage V_s induced in the secondary winding 92 before it is applied to gate 31 of the first switch 30. The signal processing circuit 96 may for example be configured to delay the square wave voltage at the gate terminal 31 of the first switch 30, with respect to the switching signal at the gate terminal 71 of the second switch 70, provided by the control signal generator 74. In general signal processing circuit 96 may be configured to provide an optimum switching of the first switch 30. The control signal generator 74 may be configured to keep the output voltage V_out at a desired value, for example by applying pulse width modulation. A load capacitor 85 may be applied for keeping V_out at the desired level. When V_out is at a relatively low value, a current flowing through the second diode 80 when switch 70 is not conducting will charge load capacitor 85 until it has reached a desired value, which value can be enforced by e.g. a duty cycle of the second switch 70. When V_out is relatively high, no current will be able to flow through the second diode 80, and the load capacitor 85 will be discharged by a load current.

[0022] The first switch 30 is furthermore shunted by a shunt resistance 34, which is applied for starting up the power converter 200, i.e. facilitating easy start up of the power converter. When the shunt resistance 34 is omitted, no current path is formed when switch 70 is operated, and additional circuitry is necessary for switching the first switch 30 conductive.

[0023] In a practical embodiment the input voltage V_in may be between 347 and 480 Volts RMS, the output voltage V_out may be 480 Volts. Then the first switch 30 may be a 900 Volt MOSFET. The first diode 40 should be a high voltage diode, but it may also be embodied by a series connection of a plurality of diodes. The second switch 70 and the second diode 80 may be high-voltage components, for example 600 Volt components. An output power of 310 Watt may be achieved, which makes the power converter applicable for powering e.g. a lamp, or even up to 3 lamps. The second switch 70 is operated at a frequency of about 50 kHz to about 250 kHz. The second switch 70 may be a separate component, or it may be integrated in a control IC, which may further comprise the functionality of the control signal generator. In general, besides converter shown in the above examples, the invention may be applied in a variety of power converters, such as e.g. a DC-DC converter for additional power supply applications.

[0024] As required, detailed embodiments of the present invention are disclosed herein, and it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0025] Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. The terms “a” or “an”, as used herein, are defined as one or more than one. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily by means of wires.