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 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 with one or more outputs whereby the number of discrete components forming the light source according to the present invention is limited to an absolute minimum.

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.

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 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The long lifetime of LED light may make the idea of a lamp outdated.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 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 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 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. 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 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 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.

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.

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.

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 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 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 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

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 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 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

- 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

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 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.

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 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 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 comprise a plurality 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 LEDs operatively connected to each output terminal in an antiparallel configuration. Connection of LEDs in an antiparallel configuration to one or more outputs of a piezoelectric transformer is advantageous in that the component count is reduced to a minimum.

Alternative configurations of LED arrangements may also be applicable. For example a plurality of LEDs forming a chain of series connected LEDs may be advantageous when such chains of series connected LEDs is/are operatively connected to a number of output terminals. Moreover, a plurality of chains of LEDs may be connected in parallel. Such parallel connections of chains of LEDs may be operatively connected to a number of output terminals. The chains of series connected LEDs may contain an equal number of LEDs or they may contain an unequal number of LEDs. The number of LEDs in each chain can vary from 1 to more than 100 depending on the preferred driving voltage.

The one or more semiconductor-based light emitting devices may comprise LEDs emitting light at different wavelengths. Thus, combinations of for example red, 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 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 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.

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 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.

The piezoelectric transformer is characterized by having a frequency dependent phase shift between an input and an output signal and/or between one or more input or output signals from the piezoelectric transformer. Such phase lag between transformer input signals (transformer drive signals) and transformer output signals is of particular relevance in connection with frequency tracking of the piezoelectric transformer.

In a second aspect the present invention relates to a method for driving a piezoelectric transformer of a light source according to the first aspect of the present invention, wherein an excitation frequency of the piezoelectric transformer is adjusted as to achieve a desired phase-shift between an input and an output signal and/or between one or more input or output signals from the piezoelectric transformer.

By making the excitation frequency of the piezoelectric transformer dependent on the phase- shift, the sensitivity in soft switching ability and gain to process variations, variations in temperature, variations in load etc. is reduced compared to a fixed frequency oscillator. It is advantageous to apply frequency tracking because the gain, output power and soft switching ability of a piezoelectric transformer based power converter is very sensitive to any variation in the excitation frequency. Moreover, tracking of the resonance frequency of a piezoelectric transformer eliminates the influence from process variations, variations in temperature etc. Especially for a magnetic-less power stage the sensitivity to any variation is large, given the narrow frequency band at which zero voltage switching can be achieved.

It is a characteristic feature of piezoelectric transformers that any input or output state, whether it being current or voltage, contains information about the characteristic resonance frequencies of the transformer. One of the signals containing information about the phase- shift may be derived from a network of one or more inductors, transformers, capacitors, nonlinear devices operated in a semi linear region or resistors added in series or parallel to any input or output of a piezoelectric transformer. More details regarding this will be given in one of the following paragraphs.

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,

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,

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 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,

Fig. 12 illustrates the input waveform to a piezoelectric transformer with a half-bridge 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 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 load,

Fig. 17 shows a transfer function of a piezoelectric transformer,

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, 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,

Fig. 23 illustrates a current source charge pump power factor correction circuit,

Fig. 24 shows a half-bridge power stage driving a zero voltage switching optimized piezoelectric transformer, and

Fig. 25 shows a measured phase lag between input voltage and output voltage of a piezoelectric transformer. 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, 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 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, operatively connected to electrodes of a piezoelectric transformer are supplied 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 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.

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 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 R _L = 10Ω to R _L = ∞, where R _L denotes a resistive load. The current varies 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 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 sources.

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 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 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 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.

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 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 compact design can be reached. The shape of the piezoelectric transformer is by 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 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 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 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 extends to a region below the piezoelectric material 7 as depicted 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 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 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 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 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 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 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.

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 via appropriate wiring. Moreover, the PCB is mechanically connected to the 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, 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 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 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 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.

Connection of LEDs in for example an antiparallel configuration to one or more outputs of a piezoelectric transformer is advantageous in that the component count is reduced to a minimum. LEDs however, being expensive as they are, do however not exhibits the highest possible emission efficacy when supplied with a pulsed current and as such the same light output can be achieved using less LEDs when driven with a direct current. For that reason one could be motivated to place a DC rectifier and a supply decoupling circuit in-between each output of a piezoelectric transformer and a matching network comprising one of more LEDs in a parallel, series or matrix configuration.

The most commonly known DC rectifier configurations are the 2 diode half-wave and the 4 diode full-wave rectifier configuration followed by one or more DC smoothing capacitors - cf. Fig. 5. The exact rectifier diode configuration and the possible inclusion of one ore more inductors to future smoothen the current may be implemented in various ways. However, the rectifier configuration should respect the charge-second balance of each output of a piezoelectric transformer. Thus, the average current of each transformer output should be zero, which excludes the use of certain rectifier configurations such as the 2 diode full-wave rectifier commonly used with magnetic transformers comprising a split output.

The typical forward voltage of a LED is in the range from 2.5 - 4.5 V and the forward voltage of a rectifier diode is in the range of 0.35 - 1.7 V depending on the type and its voltage ranging. Given that the voltage drop across a rectifier circuit is comparable to the voltage drop of one or more LEDs, the DC supply helps to improve the efficacy of the LEDs, but the overall light output emitted may not be improved due to the energy lost in the added rectifier circuit. The imminent solution to this is to replace one or more rectifier diodes with a transistor controlled in a manner as to reduce the forward voltage drop compared to the passive diode configuration, hence active rectification.

A transistor controlled as an active diode, can be considered as a one quadrant device in terms of its location in an I-V chart, which resembles the performance of an ideal diode. Depending on the complexity of the control circuit, strict timing requirements or for other apparent reasons, operation in only one quadrant might not always be feasible or preferred. Concerning the power loss in the active rectifier, operation in more than one quadrant can still be very efficient, as long as each transistor is controlled in an ON/OFF fashion, more specifically exclusively in a zero-current or zero-voltage state. Consequently the power loss is reduced to whatever is dictated by parasitics is present in the circuit, which makes active rectification an efficient way of supplying a direct current to one or more LEDs which are characterized by a low forward voltage drop. It should be noted that the internal rectance of a piezoelectric transformer output is transformed up or down by a rectifier circuit depending on the configuration, but the current limiting properties hereof etc. are still present.

One example of active rectification for a resonant converter circuit is "Design and control of an LCL-filter- based three-phase active rectifier" IEEE transactions on industry applications [0093-9994] Liserre yr:2005 vol:41 iss: 5 pg. : 1281.

In its simplest form, an output electrode of a piezoelectric transformer can be 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 charge balance applies to each output electrode of a 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, 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 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.

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 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.

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, f _r , of the three piezoelectric transformers are 120 kHz, 123 kHz and 319 kHz.

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 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 C _d i of a given electrode:

#L = ^

C& _* :

The transformer "Noliac-2005-09-05-A" with f _r =319kHz from Fig. 11 has a peak 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 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 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 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 V _cc together 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 the parameters for the specific transformer, the supply voltage V _cc to the excitation stage (Xl 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-Voltaαe-Switchinq

Fig. 12 illustrates the input waveform to a piezoelectric transformer with a half-bridge 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 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.

PMOSFEI \i - 2Q/ - /Q ₁ V .' _c ^• 2 _c

In hard-switching mode the power loss in the switches is dominated by switching 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 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 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 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 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:

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.

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 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 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%.

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 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-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.

where n is the conversion ratio of the piezoelectric transformer (see Fig. 10), C _d i and C _d2 is the equivalent input and equivalent output capacitance respectively and η is the efficiency of the transformer. In practice C _d i in the equation should be replaced by the parallel capacitance of C _d i 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 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 "C _d2 " of the piezoelectric transformer is at least 13% and more reasonable 35% larger than the equivalent input capacitance "C _d i" times conversion ratio "n" square. This corresponds to a ZVS factor of V _P = 100% and V _P = 120%, respectively. By taking the effective parasitic output capacitance of the power stage into account, 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 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 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 k ₃₃ , the ZVS 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 V _P = 100% and V _P = 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 IWk _3x approach 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 k ₃₁ . 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 secondary electrode volume. This is just the opposite case than for a transformer operated in its thickness mode. A disc and a square type transformer both operate most efficiently by transferring the energy at a resonance frequency that utilizes the k ₃₁ coefficient, given a material with isotropic properties.

In general any type of piezoelectric transformer can be adjusted for unconditionally zero- voltage-switching ability if a relation between the mechanical layout and the equivalent parameters n, C _d i and C _d2 according to the notation in Fig. 10. can be found and wherein C _d2 is at least 13% and more reasonable 35% larger than C _d i times conversion ratio n square.

For maximum efficiency is a requirement that the piezoelectric transformer is 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.

The present invention aims 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 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 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.

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 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 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 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.

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 -2OdB. 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 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.

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 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 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 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. In one embodiment, the present invention involves exciting the piezoelectric transformer with a substantially constant 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 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 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 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 V _cc . This supply voltage can either be derived from a DC source or 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 voltage source charge pump power factor correction circuit and Fig. 23 illustrates a current source charge pump power factor correction circuit, The capacitor C _PF c denoted in both figures is selected in relation to C _d i in order to achieve a power factor approaching unity.

The high frequency harmonics generated by a rectification stage is removed by 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. The light intensity of the light source described in the present invention has so far been assumed to be constant, in that the current supplied to each LED has been limited to a certain preset value by the inherent reactance of a piezoelectric transformer output. A common property of an incandescent light bulb is that it is easily dimmable by chopping up or in other ways by alternating the 50-60Hz input mains voltage, etc. by using a triac dimmer. A piezoelectric transformer based LED luminary can also be made compatible with these types of dimming. This can be achieved if the VCC supply decoupling capacitor shown in Fig. 20 and 21, placed after the input mains rectifier, is made significantly small. If the LEDs are connected directly to one or more outputs of a piezoelectric transformer, flickering might occur. If a DC rectifier followed by a smoothing circuit is placed in-between a network of one or more LEDs and an output of a piezoelectric transformer, flickering can be avoided despite the use of dimming.

More advanced types of dimming or methods for achieving a precise fixed light output can be employed in an open-loop fashion or by closed loop control based on either a feedback signal from the output side or measurement on the primary side. The power delivered to the LEDs can be adjusted by changing the excitation frequency or by burst mode (ON/OFF) modulation. The inherent current limiting capability of the piezoelectric transformer is still exploited. I.e. a feedback used to control the output power can etc. be derived from only one transformer output and the power delivered at any other output will be adjusted proportionally. With open-loop dimming using frequency or burst mode modulation, the current is also limited without the need for a discrete current limiting associated with each group of LEDs. As such the piezoelectric transformer is short circuit proof and also inherently sets an upper current limit for each output. Feedback control or open-loop control of the light output can therefore be implemented in a simple way.

As already mentioned the piezoelectric transformer is characterized by having a frequency dependent phase shift between an input and an output signal and/or between one or more input or output signals from the piezoelectric transformer. Such phase lag between transformer input signals (transformer drive signals) and/or transformer output signals is of particular relevance in connection with frequency tracking of the resonance frequency of the piezoelectric transformer. Applying frequency tracking is advantageously because the gain, output power and soft switching ability of a piezoelectric transformer based power converter is very sensitive to any variation in the excitation frequency. Moreover, tracking of the resonance frequency of a piezoelectric transformer eliminates the influence from process variations, variations in temperature etc. Especially for a magnetic-less power stage the sensitivity to any variation is large, given the narrow frequency band at which zero voltage switching can be achieved. Thus, in an embodiment of the present invention frequency tracking by varying the excitation frequency is applied. As stated above it is a characteristic feature of piezoelectric transformers that any input or output state, whether it being current or voltage, contains information about the characteristic resonance frequencies of the transformer.

Some examples are given below:

1. The derivative of the input voltage to the transformer summed with an ON-state current through the switching devices provides an estimate of the equivalent electrical LCR branch resonance current.

2. The output voltage of the transformer provides information about he LCR branch resonance current, but lags behind the former.

3. If a network of one or more inductors, transformers, capacitors, non-linear devices operated in a semi linear region, or resistors are added in series or parallel to any input or output terminal of a piezoelectric transformer they will contain information about the characteristic resonance frequencies of the transformer characterized by their transfer- functions.

4. Inductive or capacitive devices will also increase the order of the system, i.e. add one or more resonance frequencies to the system which again can be measured at any input or output. For example by adding a series inductor to the input of a piezoelectric transformer the mechanical resonance frequencies of the transformer can be located by measuring the voltage across the inductor or the current through it. The inductor will also add a pair of complex conjugate poles to the system having a resonance frequency that should be carefully placed as not to interfere with the mechanical resonance frequency of interest of the transformer. The added resonance frequency will also be present when measuring any input or output of the piezoelectric transformer.

Referring now to Fig. 24 and 25 - assuming that a piezoelectric transformer is operated at a frequency at which soft switching ability is maximized, and assuming that the load is matched, the following provides an estimated reference between some of the states:

- The equivalent electrical LCR branch resonance current lags about 55 degrees behind the input voltage.

- The output voltage lags about 100 degrees behind the input voltage. If the excitation frequency is raised above this point, the phase lag for the different states becomes larger. If the excitation frequency is lowered below this point, the phase lag for the different states becomes smaller.

Thus, any input or output signal (voltage or current) contains information about all states (resonance frequencies) of a piezoelectric transformer. It has been stated that there can be a frequency dependent phase shift from one input or output to another input or output, e.g. from the input voltage to the output voltage. Similarly, the information from one or more input or outputs can be combined with a network of components to gain information about phase shift and amplitude.

The phase shift is the most valuable information about what frequency the system is excited in relation to the location of the resonance frequency of interest. The most well know example of a controller that exploit this information to track the resonance frequency is a Phase Locked Loop (PLL). The concurrent phase shift between two input/outputs is compared with a reference and the trajectory of adjustment is based hereupon.

The phase shift between two signals is found by multiplying them either by using a continuous multiplier, or by using a more brute force action applying appropriate logic gates. The outcome is a signal representing the sum and the difference frequencies of the two original signals. The signal component with a frequency corresponding to the sum of the original two input frequencies is removed in a low-pass filter. This leaves a signal representing the difference in frequency between the two. If two signals have the same frequency but a only difference in phase, the low-pass filtered version of the two signals multiplied, is a DC value proportional to the phase-shift. Compated to a reference for the wanted phase shift the information can be used to control an oscillator.

A PLL frequency tracker is not the only type of control system that can be employed to make a piezoelectric transformer based power converter insensitive to process variations, temperature etc., but common for all types of systems is that the phase shift is used to control the excitation frequency.

Fig. 24 shows a half-bridge power stage driving a zero voltage switching optimized piezoelectric transformer terminated with a resistive load. The excitation frequency is chosen as to optimize the soft switching ability.

Fig. 25 shows a measured phase lag between input voltage and output voltage of a piezoelectric transformer. The output is terminated with a mated load and the excitation frequency is chosen so as to optimize the zero voltage switching ability. In this case this corresponds to a phase lag of -100° between the input signal (upper signal) and the output signal (lower signal).