FIELD OF THE INVENTION

The invention relates to the field of electric power conversion, and more specifically to AC-DC conversion of electric power, such as from an AC mains power supply to a DC load, e.g. an LED (Light Emitting Diode) lighting load.

BACKGROUND OF THE INVENTION

Solid state lighting (SSL) is of interest for residential, automotive and professional applications, such as public lighting, traffic systems, horticulture, stadium lighting, stage lighting, etc. Since solid state lamps, such as LEDs, generally cannot be supplied from a battery or from an AC mains voltage source directly, electronic drivers are needed. For efficiency reasons, electronic drivers are usually operated in a switched mode in high power applications. The electronic drivers convert the available DC or AC voltage into a DC current for the solid state lamps. Moreover, these electronic drivers have to control and stabilize the current in the solid state lamps, which are typically connected in series to form a string of lamps.

It can be expected that solid state lighting in particular becomes important for high power lighting systems, which are applied to illuminate buildings, sport stadiums and streets, or are used within green houses. In these applications the power exceeds 1 kW and the energy has to be taken from a single-phase or three-phase mains voltage source. US Patent No. 4,298,869 proposes to supply LED strings directly from single- phase or three-phase AC mains voltage. In a LED string, a series connection of a plurality of LEDs and one or more resistors is made to adapt the load to the AC voltage and to limit the current. By anti-parallel connection of LED strings, a current of positive polarity flows via a first LED string, and a current of negative polarity flows via another LED string connected anti-parallel to the first LED string.

Although this approach is simple and cheap, it shows some important disadvantages.

Any current flow through a LED string requires a certain value of the mains voltage determined by a threshold voltage of the series connected LEDs. This leads to pulsating currents and causes low frequency harmonics in the mains circuit. Moreover, all fluctuations of the mains voltage immediately influence the LED current and consequently also the intensity of the light produced by the LEDs. There are also high losses caused by the series resistors, which reduces the overall efficiency of the lighting application. If the average LED current has to be stabilized or controlled between a nominal value and zero, an electronic driver is required, which has to be realized by a power electronics circuit based on transistor switches and inductive and capacitive components for energy storage. This is already subject in many publications of the power semiconductor industry. In case of high power (e.g. 1 kW and more), the LED driver circuit not only has to stabilize the current in the LED load but also has to avoid mains interactions due to low frequency harmonics. This is typically solved by a two stage driver topology.

In such a topology, the first stage is a preconditioner or PFC rectifier which draws a sinusoidal current from the mains. The second power stage is a DC to DC converter which stabilizes the current in a LED string. In an example topology, the second power stage may be based on a simple buck converter.

A large storage capacitor is needed in the DC link as the LED load has to be fed continuously by DC power while the power flow from the AC line is pulsating with the double mains frequency. The difference is repetitively fed into or drawn from the storage (buffer) capacitor.

Since the large capacitances needed is such applications can only be realized by electrolytic capacitors being sensitive to high temperatures, the life time of the driver circuit is limited especially at high ambient temperatures.

For high power applications it is also important that the load is equally distributed to all phases of the three-phase mains supply. Applying e.g. many balanced single phase LED load driver circuits would mean that in case of a failure of one unit, the three- phase mains supply is asymmetrically loaded.

SUMMARY OF THE INVENTION It would be desirable to provide a three-phase AC/DC driver circuit that does not contain an electrolytic capacitor in the power circuit. It would also be desirable to provide a three-phase AC/DC driver circuit that has a balanced load of the three-phase power supply. It would also be desirable to provide a three-phase AC/DC driver circuit that does not substantially produce harmonics in the three-phase power supply. To better address one or more of these concerns, in a first aspect of the invention a driver circuit is provided, comprising: a first, a second and a third input terminal for receiving three-phase AC input power from a three-phase power supply; two output terminals for supplying DC output power to a load; a three-phase bridge type switching circuit connecting the first, second and third input terminals through respective first, second and third unidirectionally current conducting switching elements to a positive one of the output terminals, further connecting the first, second and third input terminals through respective fourth, fifth and sixth unidirectionally current conducting switching elements to a negative one of the output terminals, the first, second and third switching elements being arranged to conduct current to the positive output terminal, and the fourth, fifth and sixth switching elements being arranged to conduct current from the negative output terminal; a diode having a cathode connected to the positive output terminal, and having an anode connected to the negative output terminal; and a control circuit for controlling the switching of the switching elements. In an embodiment of the driver circuit, in operation, the control circuit controls the switching of the switching elements by sequentially: switching the first switching element on, while alternately switching the fifth and the sixth switching elements on; switching the sixth switching element on, while alternately switching the first and the second switching elements on; switching the second switching element on, while alternately switching the fourth and the sixth switching elements on; switching the fourth switching element on, while alternately switching the second and the third switching elements on; switching the third switching element on, while alternately switching the fourth and the fifth switching elements on; and switching the fifth switching element on, while alternately switching the first and the third switching elements on. In a lighting application, the driver circuit supplies a LED string.

These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a circuit diagram of an embodiment of a power supply circuit of the present invention, supplying power to a LED string. Figure 2 shows graphs of normalized phase to neutral voltages of a three- phase mains supply.

Figure 3 (a) illustrates a time control sequence of a switching element Tl of the power supply circuit of Figure 1. Figure 3(b) illustrates a time control sequence of a switching element T5 of the power supply circuit of Figure 1.

Figure 3(c) illustrates a time control sequence of a switching element T6 of the power supply circuit of Figure 1.

Figure 3(d) illustrates a voltage Ud generated by switching the switching elements indicated in Figures 3(a), 3(b), and 3(c).

Figure 3(e) illustrates a current flowing in the switching element Tl indicated in Figure 3 (a).

Figure 3(f) illustrates a current flowing in the switching element T5 indicated in Figure 3(b). Figure 3(g) illustrates a current flowing in the switching element T6 indicated in Figure 3(c).

Figure 3(h) illustrates a current flowing in a circuit element Do of the power supply circuit of Figure 1.

Figure 4(a) illustrates a string of LEDs being powered by a voltage Uo and a current Io .

Figure 4(b) illustrates an equivalent circuit diagram of the string of LEDs shown in Figure 4(a).

Figure 4(c) illustrates an electrical characteristic of the string of LEDs shown in Figure 4(a). Figure 5 illustrates a block diagram of control circuit components of a power supply circuit of the present invention.

Figure 6 shows a voltage Ud and current Il as a function of time, as well as a phase to neutral voltage UlO of the three-phase power supply.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 depicts a three-phase mains supplied driver circuit. A three-phase mains power supply is represented by three voltage sources 10 having a common node 0 and terminals r, s, and t supplying three 120° phase shifted alternating voltages each having a substantial sinusoidal shape. The mains power supply is connected to a driver circuit at its terminals r, s, and t. The driver circuit drives a load 20, which in the embodiment shown is a LED string comprising eight series-connected LEDs. Any other number of LEDs, or parallel circuits of LED strings may be driven by the drive circuit. Instead of a LED string, any other load requiring or accepting a DC voltage may be driven. Consequently, where this description refers to a LED string, it may be substituted by various other kinds of loads.

The driver circuit comprises filter circuitry comprising inductors Lf, capacitors Cf, and an inductor Ld all having relatively small inductance and capacitance, respectively. The inductors Lf and the capacitors Cf are used to low pass filter all high-frequency voltage and current harmonics. The inductor Ld is used to smooth a load current Io.

In the driver circuit, filtered three-phase voltage and current is supplied at nodes 1, 2, and 3 to a three-phase bridge-type converter comprising three legs in which switching elements embodied as IGBT (Insulated-Gate Bipolar Transistor) transistors Tl, T2, T3, T4, T5 and T6, and a freewheeling diode Do are contained. Each transistor T1-T6 is connected in series to a respective diode D1-D6. Where reference is made to the diode Do, it may be substituted by any other kind of unidirectionally conducting element, like a (controlled) transistor.

Node 1 is connected to a collector of transistor Tl and an emitter of transistor T4. Node 2 is connected to a collector of transistor T2 and an emitter of transistor T5. Node 3 is connected to a collector of transistor T3 and an emitter of transistor T6. Emitters of transistors T1-T3 are connected to anodes of diodes D1-D3, respectively. Collectors of transistors T4-T6 are connected to cathodes of diodes D4-D6, respectively. Cathodes of diodes D1-D3 are connected to a node A, and anodes of diodes D4-D6 are connected to a node B. An anode of diode Do is connected to node B, and a cathode of diode Do is connected to node A. Through the nodes A and B, a series connection of filter inductor Ld and LED string 20 is fed.

It is to be noted that the order of the transistor Tx and the corresponding diode Dx (x = 1, 2, 3, 4, 5, 6) in the series connection thereof may be reversed. It is further noted that other types of transistors, like MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) or bipolar transistors may be used, or components having the rectifying function of the diode integrated in the switching element, such as in GTO (Gate Turn-Off) thyristors, may be used.

The switching elements may be assisted in switching by providing appropriate snubber circuits (not shown). The operation of the driver circuit is as follows.

The driver circuit may be seen as a buck converter in which repetitively two terminals of the three-phase mains supply are switched (connected) to the series connection of the inductor Ld and the load 20. To set an on- state of the driver circuit, one of the transistors T1-T3 and one of the transistors T4-T6 is turned on, i.e. connects one of the nodes 1-3 to the node A and connects another one of the nodes 1-3 to the node B. There are six possible states to form an on-state, where a voltage Ud across the diode Do may be formed by U12, U13, U23, U21, U31 or U32, where Uxy indicates a phase to phase voltage between terminals x and y (x, y = 1, 2, 3). For a continuous power flow all possible phase to phase voltages of the three-phase mains supply are successively connected to the load 20 through the terminals A and B. These respective phase to phase voltages should have a positive value (i.e. Ud(t) > 0) to effect a load current Io to flow. In this case, no current flows through diode Do (i.e. Id = 0).

To set an off-state, at least all transistors T1-T3 or all transistors T4-T6 should be turned off, in which case a load current Io flows through the diode DO (i.e. Id = Io).

The switched voltage Ud can further be pulse-width modulated by pulse-width modulating the phase-to-phase voltages U12, U13, U23, U21, U31 and U32, and by a freewheeling state in which Ud = 0.

Taking into account an object to generate a substantially constant output voltage Uo for each switching period in which a pair of transistors is active, an on-off ratio of the transistors T1-T6 has to be adapted continuously to the prevailing phase to phase voltage. The following switching sequence is chosen.

Within one period of the mains voltage T = 1/fM (e.g. fM = 50 or 60 Hz), each one of the transistors T1-T6 is continuously in an on-state during a time toN for l/6 ^th of the period: toN = T/6 = 1/(6. fu) or for a phase angle of 60°. In an embodiment, those transistors are turned on for that phase range of 60° which lead to a maximum available voltage at the freewheeling diode Do. Such a performance can e.g. be achieved by synchronizing the 60° on- state of each transistor to a phase voltage, such as a phase to neutral (0) voltage of the three-phase mains supply. Within a 60° on- state of one of the transistors T1-T3 or one of the transistors

T4-T6, the pair of opposite transistors of the legs of the driver circuit connected to the other phases are turned on successively at a high switching frequency fs = 1/τ » fM. This switching frequency may be kept constant. In an embodiment, this switching frequency is above the audible range (i.e. > 20 kHz, up to 100 kHz or 1 MHz). In another embodiment, this switching frequency is at least 400 Hz. When a substantially constant load current Io is desired, the on-time of the pair of switching transistors is pulse-width modulated such that an average value of the voltage Ud(t) remains constant for all switching periods within the 60° on- state, and thus in fact for the whole mains period. An appropriate modulation scheme and the HF filter components Lf, Cf lead to substantially sinusoidal currents in the mains supply 10.

Figure 2 shows graphs of the normalized three phase-to -neutral voltages UlO, U20 and U30 as a function of a phase angle α. For purposes of illustration, U10(α) = UM'Cos(a), U20(α) = UM-cos(α-120°), and U30(α) = UM-cos(α-240°), where U _M represents a peak value of the phase to neutral mains voltage.

With such a three-phase mains supply, the switching sequence may be selected as shown in Table I below.

Table 1

For a more detailed understanding of the operation of the driver circuit of Figure 1, as an example the phase range of -30° < α < 30° will be considered. In the other phase ranges, the operation of the driver circuit is essentially similar for the indicated transistors.

In the phase range of -30° < α < 30°, the transistor Tl is in an on-state, and the transistors T5 and T6 are switched at a high switching frequency. Transistors T2, T3, and T4 are deactivated. For the phase range under consideration, Figures 3 (a) - 3(h) illustrate transistor control signals, diode Do voltage and load current.

As can be seen in Figure 3(a), transistor Tl is continuously in the on-state in the phase range of -30° < α < 30°. Figure 3(b) shows that transistor T5 is in an on- state for a time period t2, while Figure 3(c) illustrates transistor T6 being in an on-state for a subsequent time period t3. In a time period τ, as illustrated in Figure 3(a), 3(b) and 3(c), both transistors T5 and T6 are in an off-state for a time period (τ - 12 - 13) = to.

As can be seen in Figure 3(d), if either transistor T5 or transistor T6 is in the on- state, the corresponding phase-to-phase voltage is connected to the diode Do: Ud = UlO - U20 or Ud = U10-U30, respectively. If both transistors T5 and T6 are in an off-state, Ud = 0. Figures 3(e), 3(f), 3(g) and 3(h) illustrate the (constant) load current Io flowing through transistor Tl during a time tl (tl = t2 + t3), through transistor T5 during a time t2, through transistor T6 during a time t3, and through diode Do during a time to, respectively. The load current Io flows via transistor Tl and via either transistor T5 or transistor T6.

For a pulse width modulation to be used to control the load current Io, two parameters are needed: a modulation factor m (0 < m < 1); and a phase angle α = 2-π-f _M -t (-30° < α < 30°) For these parameters, a total on-time of transistor T5 and transistor T6 is to be set according to: tl = t2 + 13 = τ-mcos(α). This assumes a synchronization to the phase to neutral voltage U10(α) = U _M 'COS((X). The on-time of the transistor T5 should be for a time period of t2 = τ-nτcos(α-120°), while the on-time of transistor T6 should be for a time period of t3 = τ-nτcos(α+120°). During each switching period τ, the current freewheeling time period to is to = τ - tl .

If the above conditions are fulfilled, the driver circuit shows the following performance. The output or load voltage is set to Uo = Ud (average) = 1.5-ΠTU _M .

A LED load current Io is determined by the set voltage Uo, and the voltage to current characteristics of the LED load. This is illustrated in Figures 4(a), 4(b) and 4(c). Figure 4(a) represents a LED load comprising, for illustrative purposes, a string of four LEDs, although the string may contain any number of LEDs, and further parallel LED strings may be present. Figure 4(b) represents an equivalent circuit diagram representing a LED load. With the aid of the equivalent circuit diagram, the LED load can be described in general by two parameters: a voltage UT, and a differential series resistance Rd = Δu/Δi, as indicated in Figure 4(c). Accordingly, the load current Io of the driver circuit in case of a LED load can be selected by: Io = (Uo - UT)ZRd. With a relatively low value of Rd, a small change of the voltage Uo leads to a large change in the load current Io . Moreover, the parameters UT and Rd vary from physical component to component, and they are also dependent from temperature. Accordingly, a current control circuit may be used, an embodiment of which is illustrated in Figure 5. In Figure 5, when compared to Figure 1, filter components Lf and Cf, as well as transistors T1-T6, diodes D1-D6, and diode Do are deemed to be comprised in block 50, with input terminals r, s, t, and output terminals A, B. In the circuit of Figure 5, the load current Io is measured by a current sensor S 1 , which may be embodied as a shunt resistor or otherwise. In a comparator 52, the load current measuring value Iom is compared to a reference current value Iref. A difference value (Iom - Iref) is fed to a current control circuit 54 which sets the modulation factor m, and supplies it to a driver control circuit 56. The driver control circuit 56 generates switching control signals for switching the transistors Tl- T6 on in predetermined time periods, as discussed above. In addition, the three-phase mains voltages are measured to determine their instantaneous values and to synchronize the pulse width modulation of the transistors T1-T6 as explained above to the frequency and the phase of the mains voltage. A mains measurement and synchronization circuit 58 fulfilling such function, and is functionally connected to the mains voltage supply and to the driver control circuit 56. By setting a value of Iref, the power supplied to the load may be varied. In case of a LED load, setting a value of Iref enables dimming of the LED load. The current ripple in the load may be reduced by connecting a capacitor Co in parallel with the load.

If the filter embodied by the inductors Lf and the capacitors Cf, the current control circuit 54, and the mains measurement and synchronization circuit 58 are properly designed by the skilled person, the mains currents become sinusoidal and in phase with the mains voltages. Figure 6 shows characteristic voltage and current waveforms of Ud(t) and Il(t) for half a mains period, where Il(t) is the AC current in the mains voltage UlO phase. It can be seen that the driver circuit draws a sinusoidal current from the mains with a unity power factor.

In summary, in accordance with the above, a driver circuit receives a three- phase AC voltage through three input terminals. The driver circuit has a bridge type three- phase switching circuit having three upper and three lower switching elements. The upper switching elements are connected to one of two DC output terminals, and the lower switching elements are connected to the other one of the two DC output terminals. A diode is connected between the output terminals. Sequentially, one of the upper switching elements in a predetermined phase is switched on while the two lower switching elements of other phases are alternately switched on, and one of the lower switching elements in a predetermined phase is switched on while the two upper switching elements of the other phases are alternately switched on.

The driver circuit of the invention does not need a storage element in the form of an (electrolytic) capacitor. The lack of such capacitor leads to an excessive lifetime of the driver circuit which is expected to be more than 100,000 hours.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention. The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

A single processor or other unit may fulfill the functions of several items recited in the claims.