DK176870B1

DK176870B1 - A light source comprising piezoelectric transformer

Info

Publication number: DK176870B1
Application number: DKPA200900248A
Authority: DK
Inventors: Kasper Sinding Meyer; Miroslav Havranek
Original assignee: Noliac As
Priority date: 2009-02-24
Filing date: 2009-02-24
Publication date: 2010-02-08
Also published as: WO2010097407A1

Description

DK 176870 B1

LIGHT SOURCE APPLYING A PIEZOELECTRIC TRANSFORMER FIELD OF THE INVENTION

The present invention relates to a light source involving a piezoelectric transformer and a number of semiconductor-based light emitting devices, such 5 as for example light emitting diodes (LEDs) or semiconductors that exhibit electroluminescence. The light source of the present invention utilises the inherent current limiting properties of piezoelectric transformers whereby the number of discrete components forming the light source according to the present invention is limited to an absolute minimum.

10 BACKGROUND OF THE INVENTION

Lightning-class LEDs can deliver the brightness, efficiency, lifetime, colour temperature and white-point stability required for general illumination. LED-based luminaries reduce the total-cost-of-ownership though maintenance avoidance due to a lifetime exceeding 50k-hours and reduce energy costs.

15 The most common installed luminary type is recessed downlights based on incandescent, halogen or fluorescent lamps. Most of these fixtures are being used for directional light applications, but are based on lamps that emit in all directions. Downlights using non-reflector lamps typically have an efficiency of around 50%. Thus, half of the light produced by the lamp is wasted inside the 20 fixture. In contrast, lighting-class LEDs offers efficient, directional light with a lifetime exceeding 50k-hours. Additionally, the efficiency of the LED itself exceeds the efficiency of any incandescent, halogen or fluorescent luminary.

The long lifetime of LED light may make the idea of a lamp outdated. Lighting-class LEDs do no fail catastrophically like traditional light bulbs. Instead, they 25 can provide at least 50k-hours of useful lifetime before they gracefully degrade below 70% of their initial light output. A life-time of 50k-hour is equal to 5.7 years if the LED is left on continuously. If the light source is switched off DK 176870 B1 2 regularly, the lifetime of the LED can well exceed three decades as shown in Fig.

1.

The performance of commercial white-light LEDs is improving rapidly. Novel new device architectures with improved photon-extraction efficiencies are being 5 developed, which in turn increases the brightness and output power. The technology now passes the requirements for widespread deployment in solid-state lighting.

Thus, numeral advantages speak for applying LEDs in a wide range of applications.

10 However, in order to apply LEDs widely a number of technical challenges have to be addressed and solved.

It is well-known that LEDs are semiconductor components having operating voltages between 1.5 V and 4 V. LEDs exhibit such tolerances that they are not allowed to be connected in direct parallel configuration. The operating voltage 15 and operating current of LEDs are strongly temperature dependent.

If one wants to apply LEDs the following issues need to be addressed: The driving voltage from the mains (typically around 110 V AC or 240 V AC) is many times higher than the typical operating voltage of LEDs. Thus, some kind of transformation circuit must be used in order to reduce the voltage level of the 20 mains. An assembly involving a number of LEDs coupled in series will increase the total power and the operating voltage level of the assembly. Thus, the series connection of LEDS is, in most cases, not sufficient due to the required quantity of LEDs. Therefore, some kind of transformer for reducing the voltage level of the mains is still necessary.

25 For temperature reasons it is not possible to let electrical power flow freely through LEDs. To comply with this an active current regulating circuit is typically used.

DK 176870 B1 3

Moreover, LEDs cannot be directly connected in parallel. A balancing resistor which serves as an additional impedance is needed. Such balancing resistors adds to the circuit complexity and they reduce the overall electrical efficiency due to heat dissipation in such balancing resistors.

5 It is another characterising feature of LEDs that they have a light-emitting area which is small in comparison with fluorescent lamps. This implies that waste heat is concentrated to small area. To ensure that LEDs do not overheat appropriate cooling of the LEDs is required.

White light can be made by combining three light colours - typically a red, a 10 green and a blue light source. LEDs with these colours have different electrical parameters, such as for example different operating voltages. To comply with this individual driving circuits for each light source are required. This adds to the circuit complexity of the driving electronics.

The combination of the above-mentioned technical challenges relating to the use 15 of LEDs in commercially available light source suitable for mass production has been a technical barrier over the recent years.

It may be seen as an object of embodiments of the present invention to provide a light source applying semiconductor-based light emitting devices.

It may be seen as a further object of embodiments of the present invention to 20 reduce the component count of the provided light source to a minimum.

It may be seen as an even further object of embodiments of the present invention to provide a solution to the above-mention heating problem of for example LEDs, when such LEDs are incorporated into a compact light source.

DESCRIPTION OF THE INVENTION

25 It has been found by the present inventors that piezoelectric transformers are ideal for driving semiconductor-based light sources, such as for example LEDs or DK 176870 B1 4 other semiconductors that exhibit electroluminescence. The reason for this being that piezoelectric transformers provide inherent current limiters that can be tailored to match the specifications of LEDs connected directly to, and thereby driven by, a piezoelectric transformer. The piezoelectric transformer may be 5 designed to an arbitrary number of output terminals to which the LEDs may be connected directly. Each output terminal may be designed to specifically match the electrical properties of the LED or the LEDs that are to be connected to a given output terminal.

So, in a first aspect the present invention relates to a light source comprising 10 - a piezoelectric transformer comprising an input terminal and at least one output terminal, and - one or more semiconductor-based devices that exhibit electroluminescence being connected to the at least one output terminal of the piezoelectric transformer 15 wherein the piezoelectric transformer is configured in such a way that an inherent electrical property associated with the at least one output terminal sets a predetermined upper limit to an output current available from said at least one output terminal.

Preferably, the piezoelectric transformer is driven near its resonance frequency 20 and it serves both as a transformer for generating voltages levels usable for the one or more semiconductor-based devices that exhibit electroluminescence, and as driving electronics for the one or more semiconductor-based devices that exhibit electroluminescence connected to an output terminal or to output terminals of the piezoelectric transformer.

25 Thus, the piezoelectric transformer may have a single input terminal for receiving a drive signal from an associated power stage. Alternatively, the piezoelectric transformer may comprise a plurality of input terminals for DK 176870 B1 5 receiving drive signals from a single power stage or from a plurality of power stages.

Similarly, the piezoelectric transformer may comprise a single or a plurality of output terminals. Each output terminal may be connected to one or more 5 semiconductor-based light emitting devices being, for each output terminal, connected in series or in parallel as explained in further details below.

The inherent electrical property associated with the at least one output terminal may involve an internal reactance of the at least one output terminal.

As previously mentioned the piezoelectric transformer may comprises a plurality 10 of the output terminals, and wherein an internal reactance is associated with each output terminal.

The one or more semiconductor-based light emitting devices may comprise at least one LED operatively connected to each output terminal. Alternatively, the one or more semiconductor-based light emitting devices may comprise a pair of 15 LEDs operatively connected to each output terminal in an antiparallel configuration. Alternative configurations of LED arrangements may also be applicable.

The one or more semiconductor-based light emitting devices may comprise LEDs emitting light at different wavelengths. Thus, combinations of for example red, 20 green and blue LEDs form a white-light source. Alternative, the one or more semiconductor-based light emitting devices may comprise LEDs emitting light at essentially the same wavelength. The chosen wavelength may be in principle be any wavelength available.

The one or more semiconductor-based light emitting devices may 25 advantageously be arranged on a surface portion of the piezoelectric transformer so as to provide proper cooling of the one or more semiconductor-based light emitting devices. In particular, the one or more semiconductor-based light emitting devices may be arranged on one or more electrodes of the piezoelectric DK 176870 B1 6 transformer. The one or more electrodes will function as heat sinks for guiding heat away from the one or more semiconductor-based light emitting devices.

Besides the one or more electrodes additional heat sinks may be provided if so required.

5 In one embodiment of the present invention a pair of antiparallel coupled LEDs is operatively connected to each secondary electrode and to a common electrode.

Each pair of antiparallel coupled LEDs may be arranged on a surface portion of the piezoelectric transformer so as to provide efficient cooling.

A power stage operatively connected to the input terminal of the piezoelectric 10 transformer may be provided as well. Moreover, the piezoelectric transformer may comprise an additional output terminal in the form of a feedback electrode in order to provide a feedback signal to a power stage controller adapted to control the power stage.

Preferably, the piezoelectric transformer is optimized for zero voltage switching.

15 BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained in further details with reference to the accompanying drawings, wherein

Fig. 1 shows life-times of LEDs,

Fig. 2 shows an equivalent electrical circuit of a piezoelectric transformer, 20 Fig. 3 shows a characteristic gain of piezoelectric transformer versus frequency and load resistance,

Fig. 4 shows the open-loop efficiency and gain in relation to frequency for a piezoelectric transformer terminated with a matched load,

Fig. 5 shows a complete light source including a power stage, DK 176870 B1 7

Fig. 6 shows a complete light source including a power stage and one LED,

Fig. 7 shows a complete light source including a power stage and two antiparallel LEDs,

Fig. 8 shows a complete light source including a power stage and two legs of 5 series coupled LEDs,

Fig. 9 shows a complete light source including a power stage and one leg of series coupled LEDs,

Fig. 10 shows an equivalent resonant circuit of a piezoelectric transformer,

Fig. 11 shows efficiency curve for three piezoelectric transformers, 10 Fig. 12 illustrates the input waveform to a piezoelectric transformer with a halfbridge excitation,

Fig. 13 displays the efficiency of a power stage operated in hard-switching mode,

Fig. 14 displays the voltage at the output note of the power switches in Fig. 5 to 15 9,

Fig. 15 displays the zero voltage switching ability for two different piezoelectric transformers,

Fig. 16 displays the efficiency of a power stage operated in zero-voltage-switching with a piezoelectric transformer connected to a matched electrical 20 load,

Fig. 17 shows a transfer function of a piezoelectric transformer, DK 176870 B1 8

Fig. 18 illustrates the required variation in frequency in relation to the electrical load for a piezoelectric power converter employing frequency modulation as a means of providing a constant output voltage, 5 Fig. 19 shows burst mode modulation,

Fig. 20 shows full bride rectified,

Fig. 21 shows a half bridge rectifier

Fig. 22 illustrates a voltage source charge pump power factor correction circuit, and 10 Fig. 23 illustrates a current source charge pump power factor correction circuit.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of examples in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, 15 the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention utilizes that piezoelectric transformers have inherent current limiters. Thus, electrical currents provided from piezoelectric 20 transformers are naturally limited due to the reactance of the transformer. The inherent current limiting properties of piezoelectric transformers may advantageously be utilized by various light sources, such as for example LEDs or other semiconductor-based devices that exhibit electroluminescence.

Diodes, such as LEDs or other light generating semiconductor-based devices, 25 operatively connected to electrodes of a piezoelectric transformer are supplied DK 176870 B1 9 from vibrating energy of the piezoelectric material under each electrode section.

The amount of available vibrating energy is limited.

Piezoelectric transformers are electrical energy converters based on acoustic coupling, analogous to magnetic transformers based on coupling through a 5 mutual inductance. The physical construction is a sandwich construction of one or more piezoelectric actuators and piezoelectric sensors. Transfer of energy can occur at any arbitrary frequency. However, energy transfer is most effectively achieved at or in the close proximity of a mechanical resonance frequency of the device.

10 Piezoelectric transformers come in a number of different shapes and sizes, each designed for a specific type of application in mind. They differ in the mechanical resonance mode being utilized, efficiency, power density and target voltage rating. Common for all piezoelectric transformers is that they have at least one input electrode and one output electrode that serves as a path for energy 15 transfer.

The electrical gain from an input electrode to an output electrode is a function of the excitation frequency and the electrical load. For a transformer of the type "Noliac 2005-09-05-A" the gain varies with a factor of 84 over a range from RL = 10Ω to Rl = oo, where RL denotes a resistive load. The current varies 20 accordingly. Each output of a piezoelectric transformer can be characterized as having a high output impedance. In combination with semiconductor components that exhibit electroluminescence and other nonlinear devices that are inherently current driven contrary to voltage driven, the strong load dependent nature of piezoelectric transformers can be advantageous for 25 supplying energy to these types of loads.

The limiting impedance of a piezoelectric transformer is not real in that it is in fact a reactance. This means that currents through diodes, such as LEDs, are limited without losses. This is fundamentally different from prior art systems where resistors are applied to order to limit currents through various light 30 sources.

DK 176870 B1 10

Piezoelectric transformers can be made with multiple output electrodes.

Individual diodes, such as LEDs, can be driven independently of each other in that each output can be tailored to match a given light source connected to that particular output. Thus, the current limiting properties of each output can 5 matched to the individual light sources. The current limiting properties of each output can be controlled by dimensioning the size of the individual electrodes in accordance with given requirements. Thus, one output electrode can be matched to drive a red LED, another output electrode can be matched to drive a green LED, whereas a third electrode can be matched to drive a blue LED. In this way 10 a white-light source is provided by driving the red, the green and the blue LEDs with different operating currents. Alternatively, a matrix of similar LEDs can be driven individually and thereby avoid the sensitivity to variations in forward voltages due to product variations or differences in die temperatures.

Each output electrode of a piezoelectric transformer taps into the same source of 15 mechanical vibration energy and are therefore coupled to some extend. Because the correlation factor between each electrode is limited, the current limiting nature of piezoelectric transformers apply to each electrode. If a load, such as LEDs or laser diodes, in a given electrode section fails this particular section stops lighting. However, other electrode sections are basically not affected.

20 Moreover, the relatively high output impedances of piezoelectric transformers make them short circuit proof. Thus, if for example one LED fails and leaves an open circuit end or a short-circuited end the remaining outputs of the piezoelectric transformer will still be functioning.

The unipoled type piezoelectric transformer shown in Fig. 2 is an example of a 25 piezoelectric transformer that can easily be configured with multiple outputs.

Referring to Fig. 2 diodes, such as LEDs, can be mounted directly onto the electrodes of the piezoelectric transformer, e.g. of an unipoled type. This implies the following advantages: 1) the piezoelectric transformer serves as a heat sink, and 2) termination wires for each output is essentially avoided whereby a more 30 compact design can be reached. The shape of the piezoelectric transformer is by DK 176870 B1 11 no means limited to the shape depicted in Fig. 2. Thus, the piezoelectric transformer can shaped as a disc, a ring, a tube, a hemisphere, a rectangle etc..

The present invention will in the following be explained with reference to the use of LEDs. However, the present invention is by no means limited to the 5 combination of piezoelectric transformers and LEDs - other types of semiconductor-based devices that exhibit electroluminescence can be equally applicable.

LEDs may advantageously be configured in an antiparallel configuration and mounted directly to an output of the piezoelectric transformer 1. This is depicted 10 in Fig. 2 for an unipoled piezoelectric transformer with multiple outputs. It should be noted that the piezoelectric transformer of the present invention is by no means limited and bound to the layout and the design of the unipoled piezoelectric transformer shown in Fig. 2.

In the upper drawing of Fig. 2 the antiparallel coupled LEDs 2 are operatively 15 connected to a common electrode 3 and secondary electrodes 4 - the latter surrounding a primary electrode 5. A feed-back electrode 6 is provided for generating a suitable feed-back signal to an associated power stage. In the lower drawing of Fig. 2 the piezoelectric material 7 is depicted. The common electrode 3 also extend to a region below the piezoelectric material 7 as depicted 20 in Fig. 2.

As depicted in Fig. 2 the anodes "A" and a cathodes "K" of the LEDs are oppositely connected to the electrodes of the piezoelectric transformer.

Optionally, a single LED can be replaced by a number of LEDs coupled in series or replaced by a single LED coupled in series other types of light sources. The 25 series connection of light sources provides an increased impedance towards the piezoelectric transformer.

LEDs are inefficient when driven above a certain current level. In an antiparallel configuration the peak current is limited by the threshold at which the LEDs DK 176870 B1 12 become inefficient - hence the average current to each LED is lower than this threshold. LEDs driven with efficiency in mind will have the highest flux output pr. substrate area using a DC current contrary to a pulsed current. Hence there is a trade-off between using a power stage configuration using a rectifier and a 5 smoothing capacitor as in Fig. 5 compared to the simple direct connection of one or more LEDs to an electrode output on a PT as in Fig. 2.

According to the present invention no balancing is needed for currents. Similarly, no assorting of LEDs for uniform performance is required. In the antiparallel configuration of two LEDs the electrical current that flows through a LED in one 10 direction has exactly the same medium value as the current that flows through the other LED in the opposite direction.

Referring now to Fig. 3 a ring-shaped piezoelectric transformer is depicted. As depicted in Fig. 3 pairs of LEDs 8 are connected to each of the secondary electrodes 8. Each pair of LEDs has one LED being electrically and mechanically 15 connected to a secondary electrode 9, and another LED being electrically and mechanically connected to a common electrode 10. The LEDs are connected to the respective electrodes by soldering or gluing. Appropriate wires 11 are provided for completing the wiring of the LEDs. The piezoelectric material 12 support the common electrode 10 and the secondary electrodes 9. Reference 20 numeral 13 is associated with the input terminal of the piezoelectric transformer, said input terminal being connected to an associated power stage (not shown).

Again, it should be noted that the piezoelectric transformer of the present invention is by no means limited and bound to the layout and the design of the piezoelectric transformer shown in Fig. 3.

25 Referring now to Fig. 4 an alternative configuration of a light source is depicted.

As depicted in Fig. 4 a sandwich construction comprising a metal core printed circuit board (PCB), a piezoelectric transformer and a heat sink is provided. The light emitting devices, here a plurality of LEDs, are arranged on the PCB. The PCB is electrically connected to the electrodes of the piezoelectric transformer 30 via appropriate wiring. Moreover, the PCB is mechanically connected to the DK 176870 B1 13 piezoelectric transformer - the latter also being mechanically connected to a separate heat sink so that heat generated by the LEDs can be led away from the PCB.

Alternatively, the LEDs can be mounted directly on a PCB having a substrate, 5 such as an aluminum substrate attached thereto. A piezoelectric transformer driving the LEDs is also mounted on the PCB at its nodal points. This allows that the piezoelectric transformer can vibrate freely.

Fig. 5 shows an example of a complete piezoelectric transformer based resonant converter light source power stage comprising a half-bridge MOSFET excitation 10 stage, a piezoelectric transformer (represented by its equivalent circuit), a full bridge rectifier, a decoupling capacitor and a load consisting of a series connection of three LEDs. The piezoelectric transformer will function as a pulsed AC source towards the rectifier and the output capacitor serves as a temporary energy reserve that smoothes out the current to the LEDs. The high output 15 impedance of the transformer will ensure that the current supplied to the LEDs is limited and largely insensitive to their forward voltage.

If the rectifier in Fig. 5 is replaced by LEDs and the output capacitor and the output load is replaced by a short circuit, the power stage can be simplified while still functioning as a light source. In this case the spatial light output would 20 become pulsed, but since the human eyes is insensitivity to variations above about 100Hz the operation frequency of the piezoelectric transformer will typically be about three decades higher. Hence there is no need for a smoothing capacitor nor a rectifier stage placed before the LEDs.

In its simplest form, an output electrode of a piezoelectric transformer can be 25 terminated with two diodes in an antiparallel configuration, comprising at least one LED as shown in Fig. 6. However, using two LEDs as shown in Fig. 7 is more efficient. Alternatively each antiparallel diode can be replaced by a series connection of diodes as shown in Fig. 8. If the number of diodes is uneven as in Fig. 9, the current supplied to each diode will still be equal because the capacitor DK 176870 B1 14 charge balance applies to each output electrode ofa piezoelectric transformer, i.e. the average output current is zero.

Series connection of LEDs provides an increased impedance towards an electrode output of a PT. For a lower impedance towards an electrode output, 5 LEDs can be connected in parallel. Preferably these LEDs are from the same production batch and are thermally coupled to insure an even current distribution.

In a resonant converter the piezoelectric transformer is operated at and around its primary resonance mode. Within that limited frequency band the piezoelectric 10 transformer can be represented electrically in the form of the equivalent resonant circuit in Fig. 10 and the equivalent parameters can be obtained from measurements on the physical device using the partial differential equation method, the finite element modeling method or 1 dimensional transmission line equivalent models etc.

15 If this fitting process is done correct, the equivalent circuit in Fig. 10 will be a valid representation of the piezoelectric transformer and the properties derived from this circuit will be consistence with the properties of the real device in the proximity of the resonance frequency.

The open-loop efficiency of a piezoelectric transformer is a function of the 20 excitation frequency, the mechanical damping, the dielectric loss and the electrical load. The dielectric loss and the mechanical damping are considered device specific parameters and only the excitation frequency together with the electrical load can be altered for a given transformer, among these the load has the largest impact.

25 The open-loop efficiency curve for a piezoelectric transformer with a variable load and a constant excitation frequency is very dependent on the absolute value of the electrical load. This is illustrated for three different piezoelectric transformers in Fig. 11. The resonance frequencies, fr, of the three piezoelectric transformers are 120 kHz, 123 kHz and 319 kHz.

DK 176870 B1 15

The plot of the efficiency for each transformer in Fig. 11 is characterized by a parabolic curve with one distinct maximum point given a logarithmic scale on the x-axis. Only if the load is equal to the maximum point or in the proximity of the maximum point, the transformer can be operated efficiently. This is 5 characterized as a matched load and is specified as a resistive load with a value equal to the absolute value of the impedance of the output capacitance Cdi of a given electrode: I*L = p— <-d 2^'

The transformer "Noliac-2005-09-05-A" with fr =319kHz from Fig. 11 has a peak 10 efficiency of 98% at an electric load of 13Ω. Compared to this, the loss will be increased by 50% given a mismatched load at 6Ω or 40Ω that corresponds to an efficiency of 97%.

The impedance seen from an output electrode of a piezoelectric transformer towards a configuration of LEDs as in Fig. 5 to 9 is not a fixed value but a 15 function of a number of parameters that can be controlled. This leaves a number of options to choose from. A matched load can be emulated towards the piezoelectric transformer as to maximize the efficiency of the transformer.

Alternatively a non-matched load can be emulated towards the piezoelectric transformer as to compromise on efficiency in favor for improved zero voltage 20 switching abilities.

The emulated impedance seen from an output electrode of a piezoelectric transformer towards a configuration of LEDs as in Fig. 5 to 9 can be explained in the following way: The diode configuration at the output effectively clamps the peak output voltage of the transformer to the forward voltage of the diode 25 configuration. The voltage waveform at the output of the transformer is therefore to a large extent independent on the operating parameters of the circuit. On the contrary the output current of the piezoelectric transformer is a strong function of the excitation voltage amplitude derived from VCc together DK 176870 B1 16 with the excitation frequency. Because there is only a small correlation between the output waveform and the output current of the transformer, the impedance towards the transformer can be adjusted in a controlled manner.

The exact output impedance seen by the transformer is a complex function of 5 the parameters for the specific transformer, the supply voltage VCc to the excitation stage (XI and X2 in Fig 5 to 9), the voltage at witch the output of the transformer is clamped at, together with a sensitivity to the excitation frequency.

Zero-Voltage-Switching 10 Fig. 12 illustrates the input waveform to a piezoelectric transformer with a halfbridge excitation as depicted in Fig. 5 to 9 is operated in hard-switching mode.

The charging and discharging of the input capacitance of the transformer through the switches, induce a current in the switches concurrent with a voltage drop across them, which gives rise to joule heating. This power loss can be 15 quantified as a function of the supply voltage, the switching frequency and the input capacitance of the piezoelectric transformer as stated below: The influence from the output capacitance of the switches can typically be neglected.

Pmospev* = 2Q/ = /QiV?c

In hard-switching mode the power loss in the switches is dominated by switching 20 losses and can be considered almost constant. The power delivered to the piezoelectric transformer is dependent on small variations in the excitation frequency and the electrical load.

Fig. 13 displays the efficiency of a power stage operated in hard-switching mode connected to a piezoelectric transformer (Noliac 2005-09-05-A in this case) with 25 a matched electrical load. The efficiency peaks at the physical resonance frequency, where the most power is transferred to the load, but the numerical value is still a low 32% because of the high hard-switching losses. In the close DK 176870 B1 17 proximity of the resonance frequency, the power stage efficiency drops below 10%.

Any power stage topology, where one or more of the nodes of the switch or the switching elements, is connected directly to a piezoelectric transformer, should 5 never be operated in hard switching mode, unless the efficiency is not a concern. Adding one or more series or parallel inductors to a power stage can resolve the problem with hard-switching losses and is commonly used in the state of the art. This approach can however have its side effects. Adding inductors to a power stage can introduce new problems such as increased 10 conduction losses in the power switches and introduce new sources of power loss due to the increased number of passive components.

A power stage as eg. the half-bridge type depicted in Fig. 5 to 9 can also be operated in zero-voltage-switching mode without the aid of inductors, but this can only be achieved under very special operating conditions - namely: 15 It requires the utilization of the reactive mechanical energy, oscillating back and forth in a piezoelectric transformer excited by a power stage, as a means to charge and discharge the dielectric input capacitance of the device. This is only possible if the load is unmatched i.e. less damping or if the efficiency if the transformer is sacrificed for a design with more reactive energy.

20 Fig. 14 displays the voltage at the output note of the power switches in Fig. 5 to 9, when operated with a specific dead-time period, a matched electric load and a specific frequency of zero-voltage-switching mode.

Fig. 15 displays the zero voltage switching ability for two different piezoelectric transformers in relation to a relative frequency axis. If zero voltage switching 25 can be achieved (Vp > 100%) the ability will be limited to a small frequency band.

Fig. 16 displays the efficiency of a power stage operated in zero-voltage-switching with a piezoelectric transformer connected to a matched electrical DK 176870 B1 18 load. The efficiency peaks above the physical resonance frequency in accordance with Fig. 15, where to phase-shift between current and voltage is optimal for zero-voltage-switching. Compared to the hard switched case in Fig. 13 the peak half-bridge excitation stage efficiency has been increased from 32% to 99%.

5 Almost every piezoelectric transformer can achieve inductor-less zero-voltage-switching although it might require a termination with an unmatched load which provides less damping but also compromises on efficiency in order to achieve a state of soft-switching. For high efficiency the transformer should be terminated with a matched load, because this is the point at which the maximum amount of 10 energy is extracted from the piezoelectric transformer. This does however also mean that a matched load is the condition that enforces the largest possible amount damping to the transformer, in which a matched load becomes a worst-case scenario in terms of zero-voltage-switching ability. If the ZVS (zero-voltage-switching) factor VP in the equation below is above 100%, zero-voltage-15 switching can be achieved even with a matched load and as such the equation provides a measure for unconditionally zero-voltage-switching ability with respect to the electrical load seen by the output of the piezoelectric transformer.

T/ 1 cdl 32λ/6 VP = -r—-Γη n2 Cdl 9π2 where n is the conversion ratio of the piezoelectric transformer (see Fig. 10) , Cdi 20 and Cd2 is the equivalent input and equivalent output capacitance respectively and η is the efficiency of the transformer. In practice Cdi in the equation should be replaced by the parallel capacitance of Cdi and the effective parasitic output capacitance of the controllable switches in the power stage.

Assuming an efficiency approaching 100% and a parasitic capacitance of the 25 power stage approaching zero, any piezoelectric transformer can be adapted for unconditionally zero-voltage-switching with respect to any load impedance, given that the equivalent output capacitance "Cd2" of the piezoelectric DK 176870 B1 19 transformer is at least 13% and more reasonable 35% larger than the equivalent input capacitance "Cdi" times conversion ratio "n" square. This corresponds to a ZVS factor of VP = 100% and VP = 120%, respectively. By taking the effective parasitic output capacitance of the power stage into account, 5 the ZVS factor will be decreased and that is why a reference ZVS factor 120% is more reasonable for a balanced design.

A common property for different types of piezoelectric transformers is that the mechanical dimensions cannot be optimized for both high efficiency and a high ZVS factor. A design optimized solely for efficiency will typically have a ZVS 10 factor between 10-45% using a matched load (which is a worst-case scenario in terms of damping) and it requires a power stage with one or more series or parallel inductors in order to avoid hard switching. By increasing the ZVS factor in such a design to 120% would typically increase the loss in the transformer by 50%, but when the efficiency of the power stage and the optional series or 15 parallel inductors is taken into account, the efficiency of the complete converter will be many times greater using a transformer optimized for unconditionally zero-voltage-switching.

For a piezoelectric transformer where the main part of its energy in transferred in its thickness mode using the electromechanical coupling k33, the ZVS 20 condition corresponds to a volume of the secondary electrode that is at least 13% and more reasonable 35% larger than the primary electrode volume i.e. a ZVS factor of VP - 100% and VP - 120%, respectively. This is the case for a ring shaped transformer. A disc or a square shaped piezoelectric transformer can also be operated effectively in a thickness mode if the ratio k33/k3i approach 25 infinity, i.e. if the material has strong anisotropic properties.

For a transformer operated in a radial or planar mode, the main part of the energy is transferred using the electromechanical coupling factor k3i. In this case the unconditional zero-voltage-switching condition is meet if the volume of the primary electrode is at least 13% and more reasonable 35% larger than the 30 secondary electrode volume. This is just the opposite case than for a transformer operated in its thickness mode. A disc and a square type DK 176870 B1 20 transformer both operate most efficiently by transferring the energy at a resonance frequency the utilizes the k3i coefficient, given a material with isotropic properties.

In general any type of piezoelectric transformer can be adjusted for 5 unconditionally zero-voltage-switching ability if a relation between the mechanical layout and the equivalent parameters n, Cdi and Cd2 according to the notation in Fig. 10. can be found and wherein Cd2 is at least 13% and more reasonable 35% larger than Cdi times conversion ratio n square.

For maximum efficiency is a requirement that the piezoelectric transformer is 10 adapted to provide the right gain at the right excitation frequency to satisfy all the conditions to emulate a matched load towards the transformer while at the same time satisfy the conditions for zero-voltage-switching. When all conditions are met both the power stage and the piezoelectric transformer will be performing at maximum efficiency.

15 It is an embodiment of the invention to emulate a matched load towards an output of a piezoelectric transformer, achieve zero-voltage switching of the power stage utilizing reactive energy from the transformer (i.e. inductor-less), operate the power stage at a constant excitation frequency within the narrow frequency band at which zero-voltage-switching can be achieved, prescribe the 20 electrode dimensioning of the piezoelectric transformer for unconditionally zero-voltage-switching ability with respect to the load and achieve all these properties concurrently.

In favor of soft switching at the expense of efficiency, it is also an embodiment of the invention to emulate a non-matched load towards an output of a 25 piezoelectric transformer, achieve conditional zero-voltage switching of the power stage utilizing reactive energy from the transformer (i.e. inductor-less) and operate the power stage at a constant excitation frequency within the narrow frequency band at which zero-voltage-switching can be achieved.

DK 176870 B1 21

In certain applications a dimmable light output would be desired. There are over-all three fundamentally different types of modulation commonly known as frequency modulation (FM), pulse width modulation (PWM) and burst mode modulation (BMM). Other modulation techniques are usually derived from these 5 three types, e.g. a combination of FM + PWM has been reported.

The most commonly applied modulation type for driving resonant converters is frequency modulation. The principle behind frequency modulation is to operate the piezoelectric transformer off resonance and control the frequency in accordance to the load. Based on the transfer function of a piezoelectric 10 transformer as shown in Fig. 17 the gain of the transformer is dependent on frequency and the load resistance. By adjusting the frequency the gain can be controlled and thereby the current supplied to one or more light emitting semiconductors at the output. Because the zero-voltage-switching bandwidth is typically very narrow, the controllability using frequency modulation is limited if 15 high switching losses should be avoided.

In order to obtain a desired gain using a specific load resistance, there exist a maximum of two frequencies at which this gain can be obtained. One frequency lies above the damped resonance frequency (the gain maximum) and one frequency below the damped resonance frequency.

20 In the top plot in Fig. 18 the two frequency solutions is plotted as a function of the load resistance for the piezoelectric transformer 2005-09-05-A by Noliac A/S using a constant gain of -20dB. If the load varies the efficiency varies as well and only when the load is matched to the transformer the efficiency is high. This is shown in the bottom plot in Fig. 18. Frequency modulation can not be 25 efficiently implemented with an inductor-less power stage (a half-bridge) since the range on controllability is non-existing due to the very limited zero-voltage switching bandwidth (as illustrated in Fig. 15). Magnetic support in the power stage using frequency modulation is required in order to avoid excessive switching losses.

DK 176870 B1 22

The properties of PWM modulation for piezoelectric transformers are somewhat different from what is known from magnetic converters, although the power stages and control principles are the same. A way to analyze a piezoelectric transformer under PWM operation is to think of it as a band-pass filter. When a 5 PWM waveform is applied at the input of a piezoelectric transformer, it will only be exited by the fundamental Fourier component of that signal. Given that the amplitude of the fundamental Fourier component is dependent on the duty-cycle of the PWM waveform, the load dependent gain of a piezoelectric transformer can be compensated. As for frequency modulation the efficiency is only high at 10 the operating point at which the load is matched. Additionally, zero voltage switching of the power stage with the aid of inductors can only be achieved at a limited range of duty-cycles, which limits the desired controllability range. PWM modulation can not be efficiently implemented with an inductor-less power stage as the Half-bridge shown in Figs. 5 to 9 since the range on controllability is close 15 non-existing if high switching losses should be avoided.

Contrary to frequency modulation and PWM modulation, burst mode modulation can control the power delivered to the output, while maintaining the desired conditions for load emulation and zero-voltage-switching. The present invention involves exciting the piezoelectric transformer with a substantially constant 20 excitation frequency and operate the power stage in a low frequency alternating ON and OFF state where the duty cycle controls the power delivered to the load.

This modulation type is named burst mode modulation and an example of the input voltage to a piezoelectric transformer is illustrated in Fig. 19. In the left plot of Fig. 19 the burst frequency is approximately 100 Hz whereas the burst 25 frequency in the right plot is approximately 300 Hz.

The main advantage of burst mode modulation is that under certain operation conditions load matching can be emulated. This results in maximum efficiency.

Because the excitation frequency is kept constant it is also possible to operate the controllable power switches in a soft-switching mode with an inductor-less 30 power stage by utilizing reactive energy stored in a special piezoelectric transformer designed for this type of operation. Alternatively a non-matched load can be emulated in which the zero-voltage-switching ability is improved at DK 176870 B1 23 the expense of efficiency in which the requirements to the piezoelectric transformer are less prominent.

In Figs. 5 to 9 the energy supplied to the power stage has been indicated by the voltage node VCc· This supply voltage can either be derived from a DC source or 5 a rectified AC source. For general solid stage lighting the input could be from an ac mains rectified by a full bride rectified as illustrated in Fig. 20 or a half bridge rectifier as illustrated in Fig. 21. In order to reduce the low frequency harmonics generated by rectification, a charge pump rectification stage can be implemented in order to achieve power factor correction. Fig. 22 illustrates a 10 voltage source charge pump power factor correction circuit and Fig. 23 illustrates a current source charge pump power factor correction circuit, The capacitor CPFc denoted in both figures is selected in relation to Cdi in order to achieve a power factor approaching unity.

The high frequency harmonics generated by a rectification stage is removed by 15 means of a EMI filter not shown in Figs. 20-23.

It is an embodiment of the invention to use a charge pump power factor correction rectifier in conjunction with a piezoelectric transformer and an electrical load consisting of one or more semiconductor devices exhibiting electroluminescence such as LEDs.

Claims

A light source comprising - a piezoelectric transformer comprising an input terminal and at least one output terminal, and 5 - one or more semiconductor-based components exhibiting electroluminescence, and wherein one or more of these semiconductor-based components are connected to the output terminal (s) of the piezoelectric transformer, characterized in that the piezoelectric transformer is configured in such a way that an inherent electrical property associated with the at least one output terminal sets a predetermined upper limit for an available output current from this at least one output terminal.

A light source according to claim 1, wherein the inherent electrical property associated with the at least one output terminal involves an internal reactance at the at least one output terminal.

A light source according to claim 2, wherein the piezoelectric transformer comprises a plurality of output terminals and wherein an internal reactance is associated with each output terminal.

A light source according to any one of claims 1-3, wherein one or more of the 20 semiconductor-based components comprises at least one LED operatively connected to each output terminal.

A light source according to any one of claims 1-3, wherein one or more of the semiconductor-based components comprises a pair of LEDs operatively connected to each output terminal in an antiparallel configuration. DK 176870 B1 25

A light source according to any one of the preceding claims, wherein one or more of the semiconductor-based components comprises LEDs emitting light of different wavelength.

A light source according to any one of claims 1-5, wherein one or more of the 5 semiconductor-based components comprises LEDs emitting light of approximately the same wavelength.

A light source according to any one of the preceding claims, wherein one or more of the semiconductor-based components is arranged on a surface portion of the piezoelectric transformer to thereby obtain a suitable cooling of one or more of the semiconductor-based components.

A light source according to claim 8, wherein one or more of the semiconductor-based components is arranged on one or more of the piezoelectric transformer electrodes.

A light source according to claim 9, wherein a pair of antiparallel coupled LEDs are operatively connected to each secondary electrode and to a common electrode.

A light source according to any one of the preceding claims, further comprising a power stage operatively connected to the input terminal of the piezoelectric transformer.

A light source according to claim 11, wherein the piezoelectric transformer comprises an additional 20 output terminal in the form of a feedback electrode, thereby supplying a feedback signal to a power stage controller adapted to control the power stage.

A light source according to any one of the preceding claims, wherein the piezoelectric transformer is optimized for zero voltage switching.