WO2009004529A2

WO2009004529A2 - Synchronous self-driven power converter

Info

Publication number: WO2009004529A2
Application number: PCT/IB2008/052526
Authority: WO
Inventors: Georg Sauerlaender; Jeroen Snelten
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2007-06-29
Filing date: 2008-06-25
Publication date: 2009-01-08
Also published as: TW200919921A; WO2009004529A3

Abstract

The present invention relates to a synchronous self-driven power converter for converting a dc input voltage (Vin) to a dc output voltage (Vo) and/or a dc output current (io) for supplying a load (LO), comprising: a chopper unit (S) for chopping said dc input voltage (Vin) into an ac intermediate voltage (VD), a rectifier unit for rectifying said ac intermediate voltage and for outputting said rectified output voltage to said load, said rectifier unit comprising a bipolar transistor (T; T4) as the rectifying element, whose base is coupled to a control current supply terminal for providing a control current to the base of said bipolar transistor for converting said bipolar transistor into a conductive state, when said chopper unit is in its OFF state.

Description

Synchronous self-driven power converter

FIELD OF THE INVENTION

The present invention relates to a synchronous self-driven power converter for converting a dc input voltage to a dc output voltage or a dc output current for supplying a load.

The present invention relates further to a driver for providing a dc driving voltage to a load, in particular to an LED unit, a backlighting unit, an LCD unit or a rear combination lamp unit, and to a method of operating a synchronous self-driven power converter for converting a dc input voltage to a dc output voltage or a dc output current for supplying a load.

BACKGROUND OF THE INVENTION

The present invention generally relates to very simple and cost-effective drivers, e.g. for (O)LED modules. The cost price of electronic drivers for (O)LEDs is the key factor in many applications (LCD Backlighting, Automotive, General Illumination, etc). A typical low cost driver topology is the so-called self-oscillating buck converter (SOPS) as, for instance, described in M. Ossmann, "Simple cheap converters for the classroom", EPE'2001, 9^th European Conference on Power Electronics and Applications, Graz, Austria, 27-28

August 2001. One reason why it is seldom used despite its low cost is its limited efficiency of 70%-75%.

Synchronous Rectification recently has become a very popular way of increasing the efficiency of many converter topologies with low output voltages. Here, a MOSFET switch is used that replaces the rectifier diode(s) in the output stage, reducing the voltage drop across the rectifier (and therefore, the losses) from typically 70OmV-IOOOmV to 10OmV or less.

MOSFETs, however, are difficult to be applied in low output voltage SOPS, among others because of their high gate voltage requirements of 5V- 10V. Another problem can emerge from the MOSFET-intrinsic body diode. WO 01/60167 A2 discloses a flashlight which includes a switched mode converter operating to convert the output of an energy store to a fixed voltage for supply to a bulb, such that the bulb has constant brightness throughout the useful life of the energy store. Generally, the switching transistors are MOSFETs. However, bipolar transistors may also be used with, if necessary, an external anti-parallel diode. The synchronous switches are driven by a controller, adding quite some complexity and cost to the circuitry.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a synchronous self-driven power converter for converting a dc input voltage to a dc output voltage or a dc output current for supplying a load, and a corresponding method of operating a synchronous self- driven power converter.

In a first aspect of the present invention a synchronous self-driven power converter is presented comprising: a chopper unit for chopping said dc input voltage into an ac intermediate voltage, a rectifier unit for rectifying said ac intermediate voltage and for outputting said rectified output voltage to said load, said rectifier unit comprising a bipolar transistor as the rectifying element, whose base is coupled to a control current supply terminal for providing a control current to the base of said bipolar transistor for converting said bipolar transistor to a conductive state, when said chopper unit is in its OFF state. In a further aspect of the present invention, a corresponding method of operating a synchronous self-driven power converter is presented comprising the steps of: - chopping said dc input voltage into an ac intermediate voltage rectifying said ac intermediate voltage by a rectifier unit, said rectifier unit comprising a bipolar transistor, outputting said rectified output voltage to said load, switching the coupling of said output voltage to said rectifier unit on and off, and providing a control current to the base of said bipolar transistor for converting said bipolar transistor to a conductive state, when said chopper unit is in its OFF state. In a still further aspect of the present invention, a driver is presented for providing a dc driving voltage and/or a dc driving current to a load, in particular to an LED unit, a backlighting unit, an LCD unit or a rear combination lamp unit, said driver comprising a synchronous self-driven power converter as claimed in claim 1 for converting a dc input voltage to said dc driving voltage and/or said dc driving current.

Preferred embodiments of the invention are defined in the dependent claims. It will be understood that the method and the driver have similar and/or identical preferred embodiments as the synchronous self-driven power converter, and as defined in the dependent claims of claim 1. The present invention is based on the idea to replace the rectifier diode of the dc-dc converter, e.g. the buck converter, by a synchronous switch. Unlike the normally utilized MOSFETs, a bipolar transistor is used in place of (or in parallel with) the standard rectifier diode, making the circuit ideally suited for self-oscillating power supplies (SOPS) that traditionally give the best price-performance ratio. The solution further provides both a cheap and more efficient approach for synchronous rectification in applications with low output voltages where using MOSFETs is more difficult due to the additional driving efforts. The chopper unit, which can be a single transistor, a transistor half bridge or a transistor full bridge, serves to convert a dc input voltage into an ac voltage. Preferably, the chopper unit acts as a kind of switching unit for switching the coupling of said input voltage to said rectifier unit on and off.

The synchronous power converter proposed by the present invention is self- driven and, contrary to the circuit shown in WO 01/60127 A2, does not need any controller. The ON and OFF state is set automatically by the operation of the circuitry. This reduces the cost and complexity of the circuitry, but achieves the same performance and function as circuitry having a dedicated controller.

Preferably, this chopper unit further serves for coupling / decoupling the output voltage with respect to said rectifier unit, i.e. the coupling of the output voltage to the rectifier unit (R) is preferably activated / deactivated by the chopper unit (directly or indirectly). Here, the term "coupling is activated" relates to a state in which the synchronous rectifier is in a conductive state (= ON) and the term "coupling is deactivated" relates to a state in which the synchronous rectifier is in a non-conductive state (= OFF). The proper switching action is achieved just by utilising the right circuit-intrinsic signals that are already there. The proposed method can significantly increase the driver efficiency of a standard self-oscillating buck converter without increasing (or even decreasing) the overall driver costs. This is especially attractive for applications requiring multiple low power (O)LED drivers, like ID or 2D segmented backlights for LCD-TVs, or in applications that are very much cost driven (e.g. automotive drivers for rear combination lights RCL).

The invention thus provides a very inexpensive and efficient way of implementing synchronous rectification (and therefore, high efficiency power conversion) in almost any type of dc-dc converter.

According to a preferred embodiment, the rectifier unit further comprises one or more reactive elements, in particular one or more capacitors, inductive elements and or transformers, coupled to said chopper unit, said bipolar transistor and said load. For instance, an inductive element, e.g. a simple inductor, is provided, which is coupled in series to said bipolar transistor and said load. Said reactive element(s) serve(s) as energy storage element, which is charged while the input voltage to said rectifier unit is switched on and which is discharged while the input voltage to said rectifier unit is switched off.

According to a further embodiment an impedance unit is provided, one terminal of which is coupled to the base of said bipolar transistor for providing said control current. This additional impedance, e.g. a simple resistor, connects the transistor base to a voltage potential such that the transistor takes over the freewheeling current of the standard rectifier diode when needed.

Preferably, another terminal of said impedance unit is coupled to said output voltage. For instance, in cases where the reactive element is an inductive element, the other terminal of said impedance unit is coupled to a terminal of said inductive element which is not coupled to said bipolar transistor. In this way, the impedance element provides the control current for converting said bipolar transistor to the conductive state, when the inductive element is discharged while the output voltage is decoupled from said rectifier unit.

The control current is advantageously derived from said dc output voltage or a dc reference voltage. Which of these dc voltages is chosen depends in the first place on whether or not the dc voltage is already available in the circuit, and in the second place, on which of these voltages causes minimum rectifier losses.

In one embodiment the rectifier unit further comprises a rectifier diode coupled between the emitter and the collector of said bipolar transistor. In this embodiment, the diode is used to guarantee voltage rectification also in cases where the output voltage has not fully built up to its regulated value yet (e.g. during circuit start-up). In still another embodiment, the reactive element is a transformer for transforming said input voltage into an intermediate voltage and said chopper unit is coupled between a primary winding of said transformer and an input voltage supply unit. This embodiment is generally used in converters, where galvanic isolation is required between the output and the input voltages, or in converters, where the transformer is used for adapting certain circuit voltages in order to minimize losses and/or cost.

Further, the rectifier unit preferably comprises a pair of bipolar transistors, wherein said impedance unit comprises two impedance elements, whose terminals, which are not coupled to the base of the associated bipolar transistor, are coupled to one another and to a middle terminal of a split secondary winding of said transformer. This embodiment is generally used in a resonant converter using full wave rectification.

The present invention is particularly useful for applications requiring multiple drivers (like ID or 2D backlighting for LCD-TVs) or very cost-sensitive applications like rear combination lights or RCLs for automotive applications. The invention is also applicable to PWM or AM dimming circuits and many converter topologies, provides good integration possibilities and results in a very competitive driver concept.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 shows a circuit diagram of a standard self-oscillating buck converter, Fig. 2 shows the inductor current and diode voltage in the converter shown in Fig. 1,

Fig. 3 shows the diode current and diode voltage in the converter shown in

Fig. 1, Fig. 4 shows diode losses for a LED voltage of 3 V, a LED current of 5OmA and an input voltage in the converter shown in Fig. 1, Fig. 5 shows a first embodiment of a self-oscillating buck converter according to the present invention, Fig. 6 shows the collector current of the bipolar transistor and the diode voltage V_D in the converter shown in Fig. 5, Fig. 7 shows a second embodiment of a converter according to the present invention with a bipolar transistor in a standard buck converter, Fig. 8 shows a third embodiment of a converter according to the present invention with a bipolar transistor in a standard boost converter, Fig. 9 shows a fourth embodiment of a converter according to the present invention with a bipolar transistor in a standard buck-boost converter, Fig. 10 shows a fifth embodiment of a converter according to the present invention with a bipolar transistor in a standard flyback converter, Fig. 11 shows a sixth embodiment of a converter according to the present invention with bipolar transistors in a standard resonant converter,

Fig. 12 shows a seventh implementation of a converter according to the present invention, in particular a buck converter circuit for use in automotive RCL applications,

Fig. 13 shows an eighth implementation of a converter according to the present invention, in particular an AM buck converter circuit, and

Fig. 14 shows a ninth implementation of a converter according to the present invention, in particular a buck converter for hysteretic LED control.

DETAILED DESCRIPTION OF THE INVENTION

In the following, by means of an example illustrating the implementation and performance of synchronous rectification with bipolar transistors for a self-oscillating buck converter, the present invention will be explained.

Fig. 1 shows the standard self-oscillating buck converter as, for instance, disclosed in M. Ossmann, "Simple cheap converters for the classroom", EPE'2001, 9^th

European Conference on Power Electronics and Applications, Graz, Austria, 27-28 August 2001. The converter is adapted for converting a dc input voltage V_1n provided by a voltage source VS to a dc output voltage Vo or a dc output current io for supplying a load LO, which is an LED in this example. The converter comprises two transistors Tl and T3, which - together with the resistor R2 - form a current-limited switch, wherein the main current flows through Tl . When Tl is closed (ON-state), the input voltage V_1n minus the voltage drop V_R2 across the shunt resistor R2 (V_R2 being small compared to V_1n) is applied to the cathode of diode D, reverse-biasing it such that no current flows through rectifier diode D. In steady state operation, the voltage drop across the inductor is approximately V_1n - V₀ in this situation, which causes the inductor current i_L to rise linearly. When a certain peak current is reached - defined by the quotient of the emitter-base threshold voltage of about 70OmV and resistor R2 - the transistor T3 starts to conduct, taking over the current originally flowing through the base of Tl . At this point, the current through the inductor L cannot continue to flow through Tl, but is forced to flow through the diode D instead. This causes the voltage at the cathode of D to flip from V_1n to ground, which in turn switches off the base current of transistor T2 - and thus, also the current through T3. The voltage drop across the inductor L is now -V₀, thus linearily decreasing the inductor current i^ to zero. At this point, transistor T2 is turned on again through the (startup) resistor Rl and the whole cycle starts again.

It is worth noting that the peak current set by resistor R2 is actually twice the desired average current in the output LED(s) LO. Capacitor C is optional and can be used to reduce the LED ripple current. The current in transistor T2 is limited by resistor R3. The "current limited switch" is herein also called chopper, which serves for chopping the dc input voltage V_1n into an ac intermediate voltage V_D (also called diode voltage).

The layout and function of such a buck converter are generally known to the skilled person, so that no further explanation will be given here.

The current through the inductor i^ as well as the voltage across the diode V_D and the diode current io are depicted in Fig. 2 and Fig. 3, respectively. In particular, Fig. 2 shows the inductor current i^ and diode voltage V_D and Fig. 3 shows the diode current io and diode voltage VD.

In Fig. 4, also the instantaneous diode losses are visualized for a LED voltage of 3 V, a LED current of 5OmA and an input voltage of V_1n=IOV. From this Figure it becomes clear that the losses in the diode D can make the converter efficiency decrease significantly. Fig. 5 shows an embodiment of the proposed new circuit for a self-oscillating buck converter. In addition to the circuit shown in Fig. 1, it comprises a bipolar transistor T4 for synchronous rectification, the base of which is connected to the LED string voltage (the voltage at the terminal connecting the inductor L with the load LO) via a base impedance Z, i.e. the first terminal Zl of the impedance Z is coupled to the base of the bipolar transistor T4 and the second terminal Z2 is coupled to the positive terminal of the LED string voltage Vo- When the "switch" is switched off, the control current for converting the bipolar transistor T4 to a conductive state is provided via the impedance (e.g. in this embodiment a simple resistor) to the base of the bipolar transistor T4. Such a bipolar transistor T4 has a high current gain, is cheap, enables fast switching and provides a low threshold voltage of about 10OmV (depending on the type of bipolar transistor being used), e.g. 5OmV. Further, it has a much lower bias voltage (base-emitter voltage V_BE of e.g. about 0.7V) compared to a MOSFET having a high gate voltage of 5- 10V or a standard rectifier diode (Si-diode) having a forward voltage drop of 70OmV - 100OmV. In particular, when the "switch" is switched on, the voltage at the emitter of the bipolar transistor T (which equals the ac intermediate voltage V_D) corresponds to the input voltage V_1n and the base-emitter voltage V_BE of the bipolar transistor T4 is negative. When the "switch" is switched off and the ac intermediate voltage V_D is zero (or close to zero) the voltage at the emitter of the bipolar transistor becomes zero and the base-emitter voltage V_BE becomes larger than zero so that the bipolar transistor becomes conductive. In consequence the bipolar transistor T4 takes over iL that would otherwise flow through D, thereby significantly reducing the rectifier losses. The synchronous "switches" of the circuitry according to the present invention are thus self-driven.

Typical values of the elements used in the converter embodiment shown in Fig. 5 are:

L: typically lOμH to 5OmH; Z(R): typically 100Ω to lOOkΩ; C: optional, L: typically 1OnF to lOOOμF; Rl : typically lkΩ to lOOOkΩ; R3: typically 100Ω to 500kΩ;

R2: typically lmΩ to 1000Ω

It is clear that these values are just examples which may be used in a preferred embodiment. The invention is, however, in no way limited to the use of elements using these values. Other values are of course conceivable, too.

Fig. 6 shows the collector current ic of the bipolar transistor T4 and the diode voltage V_D- It can be seen that the diode forward voltage, and therefore the rectification losses, can be reduced significantly with synchronous rectification, thereby increasing the converter efficiency. The following Figures show further embodiments of the present invention, demonstrating how synchronous rectification utilizing one or more bipolar transistors can be achieved in all major dc-dc converter topologies. The switch is indicated in these Figures by S, the bipolar transistor(s) by T or Ta, Tb, respectively. Fig. 7 shows an implementation of synchronous rectification with a bipolar transistor T in a standard buck converter. The rectifier function, which is usually executed by the expensive diode which generates much forward losses and at higher temperatures also much reverse losses, has now been replaced by the transistor T, which does not have the above mentioned disadvantages.

During the freewheel time, the cathode of the diode D has a negative voltage potential and the voltage Vo is higher in potential than the cathode of the diode D. This means that the impedance Z supplies the base current of the transistor T, and the transistor T will conduct to fulfill the rectification function. The diode D may be a very inexpensive diode because its function now will only be to start up the rectification. When the rectification is fully active, the diode D will be bypassed by the collector-emitter saturation voltage of the transistor T. Without the diode D, the circuit will also work because of the fact that the voltage Vo will always be higher in voltage potential than the emitter of the transistor T because of the polarity exchange of the voltage V_L over the impedance L during the rectify- cation time. The transistor T may be any type of transistor, like a bipolar transistor or FET. Fig. 8 shows an implementation of synchronous rectification with a bipolar transistor T in a standard boost converter. Fig. 9 shows an implementation of synchronous rectification with a bipolar transistor T in a standard buck-boost converter. Fig. 10 shows an implementation of synchronous rectification with a bipolar transistor T in a standard flyback converter (half wave rectification) using a transformer Tr having a primary winding prim and a secondary winding sec. Fig. 11 shows an implementation of synchronous rectification with bipolar transistors Ta, Tb in a standard resonant converter (full wave rectification with split secondary winding sec a, sec b of the transformer Tr; the primary side of resonant converter is not shown). Fig. 12 shows a further implementation of a transistor synchronous rectifier according to the present invention, in particular a buck converter circuit, that is in use for an automotive RCL application, with an added transistor synchronous rectifier. When the peak current value, adjustable by Rl 1, has been reached, the transistor Q16 will be switched off and the V- input of the operational amplifier Ul ID will be lower than the V+ input and therefore the output of the operational amplifier Ul ID will be switched high. The result will be conduction of the transistor Q 17. The rectification cycle part has started now. This will take place until the decreasing current through the impedance L2 is low and the start-up circuit activates the transistor Q 14 again. When the transistor Q 14 has been activated again, the V- input of the operational amplifier Ul ID is higher in voltage potential then the V+ input. This will result in stopping the conduction of the transistor Q 17.

When taking for the V-input of the operational amplifier UI lD a reference voltage already present in the circuit of Fig. 12, instead of the anode voltage of the LEDl, then the conductor R9, which is proposed to be arranged outside a possible integrated circuit, may be a fixed value. The benefit will be that a number of LEDs in series will not require adaptation of the resistor R9 again. In this way the resistor R9 may be integrated.

The diode DlO, which is a very inexpensive diode, has been added to ensure that no high negative voltage could occur at the collector of the transistor Q 16. The transistor Q16 bypasses the diode DlO during normal operation. This is possible, but not necessary because the resistor R16 output and the operational amplifier Ul ID output limit this voltage.

It is noted that the V+ input of the operational amplifier Ul ID may also be connected to a reference voltage source instead of to the anode of the LEDl.

Fig. 13 shows an embodiment of an AM (amplitude modulation) buck converter circuit according to the present invention, in particular for AM dimming and the possibility of synchronous rectifier application. The diode Dl applies the control voltage for low LED light output levels while the diode D2 is intended to have a high LED light output level.

For the high LED light output level a resistor R3 determines the current peak level. When the voltage across the resistor R3 reaches 0.7V, the base of the transistor Q3 will be connected by the collector / emitter of the transistor Q2 to a higher voltage such that the transistor Q3 stops conducting and the freewheel diode D3 starts to conduct.

For the low LED light output level, the resistors R2 and R3 are the current sensing resistors. Since the resistor R2 is higher in value than the resistor R3, the main peak current trip level is determined by the resistor R2. Thus, at a lower peak current level, the transistor Q3 will be switched off.

In Fig. 13, an operational amplifier and a resistor- transistor combination as shown in Fig. 12 may also replace the freewheel diode D3. In general, there is no limitation with respect to the way in which dimming is controlled. Fig. 14 shows an embodiment of a buck converter for hysteretic LED control according to the present invention. The operational amplifier Ul 3D determines the maximum current peak value, and the operational amplifier U14D determines the minimum peak current value. The minimum current peak value may be near to zero and will then result in a critical mode converter. The switching level of the operational amplifiers U13D and U14D is determined by reference voltages to be at respectively the - input and the + input of the operational amplifiers. The voltage difference between these reference voltages will cause the situation that the transistors Ql 8 and Q67 will never conduct at the same time. A kind of dead zone will be introduced this way. The amplified sense voltages across the resistor R23 feed the other inputs of the operational amplifiers U13D and U14D. The current sense circuit around the operational amplifier U12D is a well-known means to convert current into voltage.

The amplification A_v of the sense circuit is R23/R25. The output voltage at the tUl ID of the resistor R23 determines, together with the reference voltages, the hysteretic behavior of the converter such as current ripple amplitude and, derived therefrom, frequency. The diode D28 has the function of a freewheel diode in case of non-conduction of transistor Q76, in fact during the dead time when one coil connection is floating and a high negative voltage may occur. Since the dead time is small, the diode D28 may be an inexpensive diode. The power dissipation in this diode D28 will be negligible. Thus, Fig. 14 is an example of a simple hysteretic control circuit that relies on a different approach than the basic finding (= self biasing), but also applies a bi-polar transistor instead of the Schottky diode.

According to the present invention, a modification to the standard dc-dc converters (like the SOPS) is proposed, which enables the standard rectifier diode to be replaced by a synchronous rectifier that is based on a bipolar transistor (rather than a MOSFET). As an essential element, a bipolar transistor is provided in parallel with (or instead of) the standard rectifier diode. Compared to MOSFETs, which are difficult to be applied in low output voltage SOPS due to their high gate voltage requirements of 5V- 10V, a bipolar transistor requires only a base-emitter voltage of about 70OmV. Further, a bipolar transistor does not have a (MOSFET-intrinsic) body diode. Thus, a very inexpensive and efficient way of implementing synchronous rectification (and therefore, high efficiency power conversion) in almost any type of dc-dc converter is possible. A further advantage of the present invention is that the power converter is self-driven and does not require a dedicated controller. The present invention can be applied in combined brakes, tail and turn light for automotive applications, ID or 2D backlighting for LCD-TVs, rear combination lights or RCLs for automotive applications, OLED drivers or general illumination applications.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered as illustrative or exemplary, and not as restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. Synchronous self-driven power converter for converting a dc input voltage (V_1n) to a dc output voltage (Vo) and/or a dc output current (io) for supplying a load (LO), comprising: a chopper unit (S) for chopping said dc input voltage (V_1n) into an ac intermediate voltage (VD), - a rectifier unit for rectifying said ac intermediate voltage and for outputting said rectified output voltage to said load, said rectifier unit comprising a bipolar transistor (T; T4) as the rectifying element, whose base is coupled to a control current supply terminal for providing a control current to the base of said bipolar transistor for converting said bipolar transistor to a conductive state, when said chopper unit is in its OFF state.

2. Synchronous self-driven power converter as claimed in claim 1, further comprising one or more reactive elements, in particular one or more capacitors (C), inductive elements (L) and or transformers (Tr) coupled to said chopper unit (S), said bipolar transistor (T; T4) and said load (LO).

3. Synchronous self-driven power converter as claimed in claim 1, further comprising an impedance unit (Z), one terminal (Zl) of which is coupled to the base of said bipolar transistor (T; T4) for providing said control current.

4. Synchronous self-driven power converter as claimed in claims 2 and 3, wherein another terminal (Z2) of said impedance unit (Z) is coupled to said output voltage (Vo).

5. Synchronous self-driven power converter as claimed in claim 1, wherein said control current is derived from said output voltage (Vo) or a dc reference voltage.

6. Synchronous self-driven power converter as claimed in claim 1, wherein said rectifier unit further comprises a rectifier diode (D) coupled between the emitter and the collector of said bipolar transistor (T; T4).

7. Synchronous self-driven power converter as claimed in claim 1, wherein said reactive element is an inductor (L).

8. Synchronous self-driven power converter as claimed in claim 1, wherein said reactive element is a transformer (Tr) for transforming said input voltage to an intermediate voltage, and wherein said chopper unit (S) is coupled between a primary winding (prim) of said transformer (Tr) and an input voltage supply unit.

9. Synchronous self-driven power converter as claimed in claim 8, wherein said rectifier unit comprises a pair of bipolar transistors (Ta, Tb), wherein said impedance unit comprises two impedance elements (Za, Zb), whose terminals, which are not coupled to the base of the associated bipolar transistor (Ta, Tb), are coupled to one another and to a middle terminal of a split secondary winding (Sec a, Sec b) of said transformer (Tr).

10. Driver for providing a dc driving voltage and/or a dc driving current to a load (LO), in particular to an LED unit, a backlighting unit, an LCD unit or a rear combination lamp unit, comprising a synchronous self-driven power converter as claimed in claim 1 for converting an input voltage (V_1n) to said dc driving voltage (Vo) and/or said dc driving current (io).

11. Method of operating a synchronous self-driven power converter for converting a dc input voltage (V_1n) to a dc output voltage (Vo) and/or a dc output current (io) for supplying a load (LO), comprising the steps of: chopping said dc input voltage (V_1n) into an ac intermediate voltage (V_D), rectifying said ac intermediate voltage by a rectifier unit, said rectifier unit comprising a bipolar transistor (T; T4), - outputting said rectified output voltage to said load, switching the coupling of said output voltage to said rectifier unit on and off, and providing a control current to the base of said bipolar transistor for converting said bipolar transistor to a conductive state, when said chopper unit is in its OFF state.