WO2016046180A1

WO2016046180A1 - Electric power converter

Info

Publication number: WO2016046180A1
Application number: PCT/EP2015/071680
Authority: WO
Inventors: Reinhold Elferich; David Llewellyn JOHN
Original assignee: Philips Lighting Holding BV
Current assignee: Signify Holding BV
Priority date: 2014-09-22
Filing date: 2015-09-22
Publication date: 2016-03-31
Anticipated expiration: 2017-03-22

Abstract

An electric power converter (10), in particular a class-E amplifier is disclosed for powering a load (22). The electrical power converter comprises input terminals (14, 16) for connecting the power converter to a voltage source (12) and for receiving an input voltage (V10) and an output interface (24) for providing an output voltage (V20) to the load for powering the load. The power converter further comprises a controllable switch (30) connected to the input terminals, and a voltage buffering (32) circuit including a non-linear capacitor (34) and a bias voltage element (36) for providing a bias voltage (V16) to the non-linear capacitor. The non-linear capacitor and the bias voltage element are connected in series to each other and the voltage buffering circuit is electrically connected to the controllable switch.

Description

ELECTRIC POWER CONVERTER

FIELD OF THE INVENTION

The present invention relates to an electric power converter for powering a load. The present invention in particular relates to a class E-amplifier. The present invention further relates to a lighting apparatus comprising one or more lighting devices, in particular one or more LEDs.

BACKGROUND OF THE INVENTION

In the field of electrical power converter or driver devices for offline applications such as LED drivers, solutions are demanded to achieve high efficiency, small size and high power factors.

Class-E type amplifiers allow high switching frequencies, high efficiencies, small size and require only a single switch, which is in particular advantageous for LED driver applications. Class-E type converters comprise a resonant circuit and a single switch, wherein the power amplification is mainly determined by the switching frequency of the controllable switch. During the off-time for the switch, a high voltage stress is applied across the controllable switch, which can be larger than three times the peak input voltage so that a high voltage switch is required for this type of electrical power converter; however, these high voltage switches are expensive and increase the overall technical effort and the size of the amplifier circuits.

A class-E converter which provides a linearized output voltage is e.g. known from US 4 788 634; however, the circuit for linearization of the output voltage cannot reduce the voltage peaks dropping across the controllable switch.

Known solutions to reduce the voltage stress add either harmonic tuning networks or active or passive clamping, which increase the technical effort, the costs and the power loss and have usually an overall reduced operation range.

A power amplifier and a power transmission apparatus including a variable passive element and a comparator is known from WO 2013/021748 Al. The variable passive element is connected directly or indirectly to a first terminal of a switch element and serves to increase or reduce the resonant frequency of the amplifier. The comparator compares the voltage of interest with a reference voltage and outputs a control voltage for a variable passive element based on a difference between the voltage of interest and the reference voltage.

A DC-DC converter including a resonant switching circuit connected between a supply voltage and ground is known from EP 0 788 217 A2. The switching circuit is driven by a periodic drive signal having a predetermined drive frequency and a varactor diode circuit for controlling an impedance matching circuit such that an output voltage of the DC- DC converter is substantially constant. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved electrical power converter, in particular a class-E amplifier for powering a load with reduced technical effort, and in particular, with reduced voltage stress across the switch. Further, it is an object of the present invention to provide a corresponding light apparatus.

According to one aspect of the present invention, an electrical power converter, in particular a class-E amplifier for powering a load is provided, comprising:

input terminals for connecting the power converter to a voltage source and for receiving an input voltage,

an output interface for providing an output voltage to the load for powering the load,

a controllable switch connected to the input terminals, and

a voltage buffering circuit including a non-linear capacitor and a bias voltage element for providing a bias voltage to the non-linear capacitor, wherein the non-linear capacitor and the bias voltage element are connected in series to each other, and wherein the voltage buffering circuit is electrically connected to the controllable switch.

According to another aspect of the present invention, a lighting apparatus is provided comprising one or more lighting devices, in particular one or more LEDs and an electrical power converter according to the present invention.

Preferred embodiments of the invention are defined in the dependent claims. The present invention is based on the idea to buffer high voltages dropping across the controllable switch by means of the buffer circuit, wherein the main element of the buffer circuit is a non-linear capacitor which is biased by means of a bias voltage element. The non-linear capacitor comprises a capacitance, which is dependent on the voltage dropping across the capacitor. The non-linear capacitor is biased by the bias voltage element so that the capacitance of the non-linear capacitor is large for high voltages across the controllable switch and thus the high voltages across the controllable switch in the electrical power converter and in particular a class-E converter can be effectively reduced and buffered accordingly. Consequently, the peak voltages across the controllable switch can be reduced to less than twice of the input voltage so that less expensive controllable switches can be used and the technical effort for the power converter is reduced.

In a preferred embodiment, the voltage buffering circuit is connected in parallel to the controllable switch. This is a possibility to effectively buffer the voltage dropping across the controllable switch by means of the non-linear capacitor.

In a preferred embodiment, the bias voltage has a polarity such that a voltage across the non-linear capacitor is opposed to a voltage across the controllable switch. This is a possibility to reversely bias the non-linear capacitor, so that a high voltage across the controllable switch leads to a low voltage across the non-linear capacitor so that a high capacitance can be achieved or set for buffering high voltages.

In a preferred embodiment, the bias voltage is a constant bias voltage. This is a possibility to reduce the technical effort for providing the bias voltage element and to achieve a constant operating point of the non-linear capacitor.

In a preferred embodiment, the bias voltage element is a constant voltage source. This is a possibility to provide a constant bias voltage with high reliability in order to set a reliable operating point for the non-linear capacitor.

In a preferred embodiment, the bias voltage element comprises a DC capacitor, wherein a diode is connected to the DC capacitor. This is a possibility to reduce the technical effort of the bias voltage element, since the charged DC capacitor provides the bias voltage to set the operating point of the non-linear capacitor and the diode provides a means to charge the DC capacitor.

Preferably the diode is connected in parallel to the non-linear capacitor. This is a possibility to define the charging direction of the DC capacitor in order to provide a defined bias voltage.

In a preferred embodiment, a Zener diode is connected in parallel to the DC capacitor. This is a possibility to limit the bias voltage, wherein the Zener diode is active during transients only and does not conduct during steady state operation. Hence, the efficiency is not affected.

In a preferred embodiment, the non-linear capacitor is formed by the diode connected in series to the DC capacitor. The diode usually comprises a non-linear junction capacitance, which can be utilized as the non-linear capacitor so that the technical effort for providing the non-linear capacitor is reduced. The non-linear capacitance behavior of the diode is usually effective for high frequencies so that for high frequencies an additional nonlinear capacitor can be omitted.

In a preferred embodiment, the electrical power converter comprises an inductance forming a resonant circuit with the non-linear capacitor. This is a possibility to form a resonant converter which provides a high power factor. The inductance in a Class-E converter is usually connected in series with the input terminal.

In a preferred embodiment, the non-linear capacitor has a negative capacitance-voltage characteristic. In other words the capacitance of the non-linear capacitor is large for low voltages and the capacitance decreases with increasing voltage dropping across the non-linear capacitor. This is a possibility to reduce the technical effort, since a negative capacitance-voltage characteristic can be achieved with low technical effort.

In a preferred embodiment the voltage buffering circuit is connected in parallel to the inductance. This is a possibility to stack the bias voltage onto the input voltage.

In a preferred embodiment, the non-linear capacitor is connected electrically between a drain connector of the controllable switch and the bias voltage element. This is a possibility to set the operating voltage of the non-linear capacitor to a desired value with low technical effort and to easily buffer the voltage peaks of the controllable switch so that the voltage dropping across the controllable switch can be reduced.

Alternatively, the bias voltage element is connected electrically between the drain connector of the controllable switch and the non-linear capacitor.

In a preferred embodiment, the non-linear capacitor is formed as a ceramic capacitor. This is a possibility to further reduce the technical effort and the costs for the electrical power converter.

In a preferred embodiment, the non-linear capacitor is formed as a multilayer- ceramic capacitor. This is a possibility to provide a capacitor having a precise capacitance- voltage characteristic and a high reliability.

In a preferred embodiment, the bias voltage is at least twice the input voltage. This is a possibility to provide an entirely reversed voltage dropping across the non-linear capacitor, so that a large voltage across the controllable switch leads to a low voltage across the non-linear capacitor.

As mentioned above, the non-linear capacitor which is biased by the bias voltage element can be utilized to reduce high voltage peaks at the controllable switch, since the capacitance of the non-linear capacitor can be set to an operating point such that the capacitance of the non-linear capacitor is high for high voltages at the controllable switch. Hence, the non-linear capacitor can buffer the voltage peaks so that the maximum voltage dropping across the controllable switch is reduced. Therefore, a lower maximum voltage- handling of the controllable switch is necessary, so that the technical effort for the

controllable switch can be reduced. Since the bias voltage is preferably a reverse bias voltage, the operating voltage of the non-linear capacitor is also reversed so that a high voltage dropping across the controllable switch leads to a low voltage dropping across the non-linear capacitor. This is a possibility to utilize non-linear capacitors which have a high capacitance for low voltages so that the overall technical effort is further reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings:

Fig. 1 shows a schematic block diagram of an isolated, Class E electrical power converter for powering a load,

Fig. 2 shows a schematic block diagram of an alternative embodiment of the electrical power converter for powering the load,

Fig. 3a shows a diagram illustrating the capacitance-voltage characteristic of a typical non-linear capacitor,

Fig. 3b shows a timing diagram illustrating a voltage and a current of the controllable switch of different electrical power converters including a linear capacitor or a biased non-linear capacitor,

Fig. 4 shows a schematic block diagram of an alternative connection of the buffering circuit,

Fig. 5 shows a schematic block diagram of an embodiment of the electrical power converter shown in Fig. 1 or 2, and

Fig. 6 shows a further embodiment of the electrical power converter shown in

Fig. 1 or 2.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a schematic block diagram of an electrical power converter generally denoted by 10. The electrical power converter is in this particular case formed as a class-E amplifier. The electrical power converter 10 is connected to a voltage supply 12 which provides a supply voltage V10. The electrical power converter 10 is connected to the voltage supply 12 by means of input terminals 14, 16. The electrical power converter 10 is adapted to draw an input current 110 from the voltage supply 12. The electrical power converter 10 converts the input voltage V10 to an output voltage V20 provided at output terminals 18, 20 for powering a load 22. The load 22 is in the particular embodiment shown in Fig. 1 formed as LED unit 22. The load 22 is connected to the output terminals 18, 20 via a transformer 24 comprising a primary winding 26 and a secondary winding 28

electromagnetically coupled to each other. The transformer 24 forms in general an output interface of the electrical power converter 10 for powering the load 22.

The electrical power converter 10 comprises a controllable switch 30 and a buffering circuit 32 which are connected in parallel to each other. The buffering circuit 32 comprises a non-linear capacitor 34 and a bias voltage element 36 which are connected in series to each other. The electrical power converter 10 further comprises an inductance 38 connected in series between the input terminal 14 and the controllable switch 30 and the non- linear capacitor 34. The inductance 38 forms a resonant circuit with the non-linear capacitor 34 of the buffering circuit 32.

During operation, the controllable switch 30 is pulsed and the electrical power converted by the electrical power converter 10 can be set by controlling the pulse frequency of the controllable switch 30. The controllable switch 30 controls a current 114.

The controllable switch 30 is preferably formed as a transistor and in particular as an FET-transistor, wherein the controllable switch 30 comprises a non-linear junction capacitance 40 as schematically shown in Fig. 1. During the operation a voltage V12 drops across the controllable switch 30, wherein usually according to the electrical power converters known from the prior art peak values of the voltage V12 can be three times larger than the input voltage VI 0.

The non-linear capacitor 34 is electrically in series between a drain contact of the controllable switch 30 and the bias voltage element 36 as shown in Fig. 1. The non-linear capacitor 34 could be alternatively connected between the bias voltage element 36 and ground.

The bias voltage element 36 is preferably formed as a voltage source and provides the bias voltage V16, which is preferably twice the input voltage V10. The bias voltage element 36 is connected in series to the non-linear capacitor 34 in order to ensure that the voltage V14 dropping across the non-linear capacitor 34 is low when the voltage V12 is high and vice versa. The non-linear capacitor 34 comprises a capacitance, which is dependent on the voltage VI 4, wherein the capacitance is large for low voltages V14 and reduces with increasing voltage V14 dropping across the non-linear capacitor 34. Due to the bias voltage V16, large values of the voltage V12 lead to low values of the voltage V14 dropping across the non-linear capacitor 34 so that the capacitance of the non-linear capacitor 34 is large if the voltage V12 dropping across the controllable switch 30 is large, i.e., V14=V16-V12. Hence, large values of the voltage V12 can be buffered by the large capacitance of the nonlinear capacitor 34 so that the peak values of the voltage V 12 are reduced. Consequently, the maximum values of the voltage V12 is reduced to values twice of the input voltage V10. Therefore, the controllable switch 30 can be formed with a reduced voltage-handling capability so that the technical effort and cost of the electrical power converter 10 is in general reduced.

The power converter 10 may also be any electrical power converter having a controllable switch 30 which is pulsed for power conversion, such as a DC-DC converter e.g. a buck-converter, boost-converter, buck-boost-converter etc.

Fig. 2 shows an alternative embodiment of the electrical power converter 10 shown in Fig. 1. Identical elements are denoted by identical reference numerals, wherein here merely the differences are explained in detail.

The load 22 is in this embodiment electromagnetically coupled to the inductance 38, wherein the inductance 38 forms the primary winding 26 of the transformer 24. The transformer 24 forms an output interface for powering the load 22. Hence, the output terminals 18, 20 are formed at the inductance 38 and the output voltage V20 drops across the inductance 38, which forms the primary winding 26 of the transformer 24.

Fig. 3a shows a diagram illustrating the capacitance-voltage characteristic of the non-linear capacitor 34. The capacitance has a maximum at low voltages close to zero and decreases with increasing voltages dropping across the non-linear capacitor 34. The maximum value of the capacitance is more than three times larger than the minimum value of the capacitance. The non-linear capacitor 34 having this voltage-capacitance characteristic is preferably formed as a ceramic capacitor or a multilayer-ceramic capacitor.

Due to the bias voltage V16, the voltage V14 dropping across the non-linear capacitor 34 has low values for large values of the voltage V12 so that the peaks of the voltage V12 can be effectively reduced.

In Fig. 3b a timing diagram of the voltage V12 dropping across the

controllable switch 30 and the current 114 in the controllable switch 30 are schematically shown for two cycles. The dashed lines show the voltage and the current for electrical power converters of this kind known from the prior art and the solid lines show the voltage V12 and the current 114 of the electrical power converter 10 shown in Fig. 1 or 2. The dashed line corresponds to the use of a linear capacitor and the solid line corresponds to the use of a nonlinear capacitor according to the invention wherein the capacitance is low for low voltages and high for high voltages across the controllable switch 30, respectively.

As shown in Fig. 3b, the peak values of the voltage V12 which is usually larger than 1000 V are significantly reduced to a peak value of less than 760 V. The current shown in Fig. 3b illustrates that the peak values of the current in the controllable switch 30 are roughly the same as in the prior art cases so that the same power is converted by the electrical power converter 10.

Fig. 4 shows an alternative embodiment of the electrical power converter 10. Identical elements are denoted by the reference numerals, wherein here merely differences are described in detail.

In this embodiment, the buffering circuit 32 comprising the bias voltage element 36 and the non-linear capacitor 34 is connected in parallel to the inductance 38, wherein the bias voltage element 36 is connected to the input terminal 14. The non-linear capacitor 34 is electrically connected between the drain of the controllable switch 30 and the bias voltage element 36.

It shall be understood that the embodiment shown in Fig. 4 may also be provided with a transformer output interface at the inductance 38 as shown in Fig. 2 or without a transformer output interface having a direct connection to the load 22.

Due to the direct connection of the bias voltage element 36 and the voltage supply 12, the bias voltage V16 can be stacked onto the input voltage V10.

Fig. 5 shows a schematic block diagram of an embodiment of the electrical power converter shown in Fig. 1. Identical elements are denoted by the identical reference numerals wherein here merely the differences are explained in detail.

In this embodiment, the bias voltage element 36 is formed as a DC-capacitor connected to the non-linear capacitor 34. The buffering circuit 32 further comprises a diode 42 connected in parallel to the non-linear capacitor 34 so that the bias voltage element 36 can be charged accordingly and can provide a sufficiently high bias voltage VI 6. The diode 42 also comprises a non-linear junction capacitance 44 which is in Fig. 5 schematically shown.

In a particular embodiment, the buffering circuit 32 further comprises a Zener diode 46 which is connected in parallel to the bias voltage element 36 and reversely directed to the diode 42. The Zener diode 46 limits the bias voltage V16 and is only active during the transient and does not conduct during the steady state operation of the electrical power converter 10. Hence, the efficiency of the electrical power converter 10 is not affected.

Fig. 6 shows a further embodiment of the electrical power converter 10 for high frequencies. Identical elements are denoted by identical reference numerals, wherein here merely the differences are explained in detail.

The buffering circuit 32 shown in the block diagram of Fig. 6 merely comprises the diode 42 and the non-linear junction capacitance 44 which are connected in parallel to each other, and connected in series to the bias voltage element 36 which is formed as a capacitor corresponding to the embodiment shown in Fig. 5. For high frequencies above 100 MHz, the non-linear junction capacitance 44 increases so that the junction capacitance 44 of the diode 42 can act as the desired non-linear capacitor 34 with the respective degree of non-linearity. Hence, the technical effort of the electrical power converter 10 can be further reduced for high frequencies above 100 MHz.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. 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.

Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. Electric power converter (10), in particular class-E amplifier for powering a load (22), comprising:

input terminals (14, 16) for connecting the power converter to a voltage source (12) and for receiving an input voltage (V10),

- an output interface (24) for providing an output voltage (V20) to the load for powering the load,

a controllable switch (30) connected to the input terminals, and a voltage buffering (32) circuit including a non-linear capacitor (34) and a bias voltage element (36) for providing a bias voltage (VI 6) to the non-linear capacitor, wherein the non-linear capacitor and the bias voltage element are connected in series to each other, and wherein the voltage buffering circuit is electrically connected to the controllable switch.

2. Electrical power converter as claimed in claim 1, wherein the voltage buffering circuit is connected in parallel to the controllable switch.

3. Electrical power converter as claimed in claim 1, wherein the bias voltage has a polarity such that a voltage (V14) across the non-linear capacitor is opposed to a voltage (V12) across the controllable switch.

4. Electrical power converter as claimed in claim 1, wherein the bias voltage is a constant bias voltage.

5. Electrical power converter as claimed in claim 1, wherein the bias voltage element is a constant voltage source.

6. Electrical power converter as claimed in claim 1, wherein the bias voltage element comprises a DC-capacitor and wherein a diode (42) is connected to the DC- capacitor.

7. Electrical power converter as claimed in claim 6, wherein the diode is connected in parallel to the non-linear capacitor.

8. Electrical power converter as claimed in claim 6, wherein a Zener diode (46) is connected in parallel to the DC capacitor and connected reversely to the diode.

9. Electrical power converter as claimed in claim 6, wherein the non-linear capacitor is formed by the diode connected in series to the DC capacitor.

10. Electrical power converter as claimed in claim 1, wherein the electrical power converter comprises an inductance (38) forming a resonant circuit with the non-linear capacitor.

11. Electrical power converter as claimed in claim 10, wherein the voltage buffering circuit is connected in parallel to the inductance.

12. Electrical power converter as claimed in claim 1, wherein the non-linear capacitor has a negative capacitance-voltage characteristic.

13. Electrical power converter as claimed in claim 1, wherein the non-linear capacitor is connected electrically between a drain connector of the controllable switch and the bias voltage element.

14. Electrical power converter as claimed in claim 1, wherein the non-linear capacitor is formed as a ceramic capacitor.

15. Lighting apparatus comprising one or more lighting devices (22), in particular one or more LEDs and an electrical power converter (10) as claimed in claim 1 for powering the lighting devices.