Patent Publication Number: US-9848467-B2

Title: Driver for driving a load

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/EP2015/060884, filed on May 18, 2015, which claims the benefit of European Patent Application No. 14187630.0, filed on Oct. 3, 2014, and Chinese Patent Application No. PCT/CN2014/078982, filed on May 30, 2014. These applications are hereby incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates in general to the field of lighting, particularly LED lighting. The present invention relates more particularly to a driver for an LED lamp, although the driver can also be used for other types of load. 
     BACKGROUND OF THE INVENTION 
     LED lighting technology is developing rapidly. Especially, LEDs become available at decreasing prices. For use in LED lighting appliances, there is a general desire to provide low-cost LED drivers. Reducing the costs can for instance be done by reducing the number of components, and single-stage driver architectures are preferred. On the other hand, with increasing LED power, the drivers must meet more stringent requirements relating to distortion of the line current. Although low line current distortion is feasible with single stage architectures, there often is a trade-off between load regulation and line regulation, line-current-distortion and output ripple (flicker) and the corresponding buffer size and cost. 
     A well-known single-stage driver topology is the BiFRED topology (Boost Integrated Flyback Rectifier/Energy storage DC/DC converter). 
       FIG. 1A  is a block diagram schematically showing a BiFRED converter  1  powered from mains  2  for driving an LED load L. Reference numeral  3  indicates a rectifier, reference numeral  4  indicates an EMI filter. The actual converter comprises a series arrangement of a first diode D 1 , a first inductor L 1 , a storage capacitor C 1  and a second inductor L 2  connected between first and second input terminals  5  and  6 . The input terminals  5  and  6  are connected to the output of the filter  4 . 
     It is noted that the order of first diode D 1  and first inductor L 1  may be different. It is further noted that the order of storage capacitor C 1  and second inductor L 2  may be different. It is further noted that the direction of the first diode D 1  determines the direction of current flow, and hence determines the mutual polarity of the input terminals. For sake of convenience, first input terminal  5  will be termed “high” input terminal while second input terminal  6  will be termed “low” input terminal. 
     Reference numeral A indicates a node between first inductor L 1  and the series arrangement of storage capacitor C 1  and second inductor L 2 . A controllable switch S 1  is connected between the node A and the low input terminal  6 . 
     The converter  1  further comprises, connected in parallel to the second inductor L 2 , a series arrangement of a second diode D 2  and a parallel arrangement of an output capacitor C 2  and the LED load L. Reference numerals  9   a  and  9   b  indicate output terminals for connecting the load. It is noted that the converter can also be used for other types of load. 
     Reference numeral  8  indicates a control device for the switch S 1 . The control device controls the switch S 1  to be either conductive (first state) or non-conductive (second state), and alternates between these two states at a certain repetition frequency. 
     The basic operation is as follows. During the first state, the switch is conductive and the first inductor L 1  is charged from rectified mains via the switch S 1 . The energy in the first inductor L 1  is magnetic energy which is proportional to the inductor current. The inductor current is increasing. 
     During the second stage, the switch is un-conductive, the inductor current continues to flow, discharging the first inductor L 1  and charging the storage capacitor C 1 . The current in the first inductor L 1  decreases, while the voltage over the storage capacitor C 1  increases. The charging current from L 1  to C 1  also flows partly through the second inductor L 2  and partly via the second diode D 2  to power the LED and to charge the output capacitor C 2 . 
     During the first stage, the storage capacitor C 1  also discharges over the second inductor L 2 , via the switch S 1 . During the second stage, the energy stored in the second inductor L 2  will be used to charge output capacitor C 2  and to power the LED. 
       FIG. 1B  is a schematic diagram showing an alternative embodiment of the converter, indicated by reference numeral  11 . In this alternative embodiment, the second inductor L 2  is the primary winding of a transformer T 1  which has a secondary winding L 3  connected to the second diode D 2 . An advantage of using such transformer is that the primary and secundary windings may be mutually isolated such as to provide an insulation between input and output, and the respective numbers of turns may have a ratio higher than 1 such as to provide in a voltage increase at the output, but otherwise the operation is the same as described above. 
     For a correct operation of the converter, the timing of the switching moments from first state to second state and from second state to first state is important. The control device may operate at an arbitrary high frequency, but in view of the fact that the charging current is derived from rectified mains, the current in the load may have a frequency component (ripple) equal to twice the mains frequency. Typically, the mains frequency is for instance 50 Hz (Europe) or 60 Hz (USA), and consequently the LED light output may have a ripple frequency of 100 or 120 Hz. This is observable, and therefore it is desirable that the magnitude of the ripple current is as low as possible. 
     Further, the power drawn from the mains must be proportional to the power consumed by the load L, and this is achieved by adapting the duty cycle of the switching control, wherein an increase in the relative duration of the first state corresponds to an increase in power. 
     A typical approach in Prior Art Single-Stage PFC LED Drivers is to place the buffering, or 100 Hz/120 Hz flicker filtering, at the output of the DC/DC converter because placing significant buffering at the input of the converter would depreciate the power-factor and increase the line current distortion. The output buffer typically consists of a large output capacitor C 2  which forms a time constant with the dynamic resistance of the LEDs. To improve LED efficacy, LED manufacturers have consistently reduced the dynamic resistance of LEDs over the last decade, which has caused output buffer size and cost to increase significantly. 
     SUMMARY OF THE INVENTION 
     The present invention aims to provide a new design of the switch control device  8  that can be built with a low number of relatively simple components and therefore has low costs, while at the same time adequately and reliably providing the functions of output current ripple reduction, output current regulation, line current regulation, and line current shaping to reduce line current distortion, and also providing a high power factor. 
     In one aspect, the present invention provides a driver for driving a load, the driver having BiFRED topology and comprising: 
     a first input terminal and a second input terminal; 
     a pair of output terminals for connecting the load;
         a BiFRED converter coupled to the input terminals, comprising
           a first inductor, coupled to a first input terminal;   a first capacitor in series connection with the first inductor;   a controllable switch between the interconnection of the first inductor and the first capacitor and the second input terminal;   a second inductor coupled between the first capacitor and the second input terminal;   a control device for controlling the controllable switch;
 
wherein said control device comprises:
   
           a first sensing element for sensing the current through said switch and providing a first output signal proportional to said current, said current being the sum of the charging current of the first inductor and discharging current of the first capacitor;   a reference device for providing a reference signal;   comparing element for comparing said first output signal with said reference signal, and for switching said switch to a non-conductive state in each transition of the controllable switch&#39;s oscillation when said first output signal is equal to or higher than said reference signal.       

     An advantage of this aspect is the output current ripple is greatly reduced. 
     In a preferred embodiment, the BiFRED converter comprises: 
     a first series arrangement of a first diode and the first inductor, the first series arrangement having one end connected to the first input terminal and having an opposite second end connected to a first node; 
     a second series arrangement of the first capacitor and the second inductor, the second series arrangement having one end connected to the second input terminal and having an opposite second end connected to the first node; 
     the controllable switch connected between the first node and the second input terminal, wherein when the switch is conductive the first inductor is charged by the input terminals and the first capacitor discharges to charge the second inductor, and when the switch is non-conductive the first inductor discharges to charge the first capacitor, and said switch is adapted to oscillate for converting power; 
     an output capacitor connected between said output terminals; 
     a second diode connected in series with said output capacitor; wherein the series arrangement of second diode and output capacitor is connected in parallel with said second inductor, or in parallel with a third inductor inductively coupled to said second inductor; 
     and said control device further comprises:
         a second sensing element for sensing the current through said second diode, and for providing a second output signal for switching said switch to a conductive state when said current through said second diode reaches zero.       

     A driver of this design has the advantages of including a relatively simple and low-cost control circuit that provides high performance without needing an additional isolated feedback loop, and that guarantees that the output portion of this circuit is always working in boundary conduction mode. 
     In a preferred embodiment, said first inductor has an inductivity selected such that, in the non-conductive state of said switch, the current in said first inductor reaches zero before the second sensing element switches said switch to the conductive state. This guarantees that the input portion of this circuit is always working in discontinuous mode. 
     In a particular embodiment, said second sensing element comprises a sensing inductor inductively coupled to said second inductor. This provides a simple and low-cost manner of implementing the second sensing element. 
     In a particular embodiment, said first sensing element comprises a sensing resistor connected between the switch and the second input terminal. This provides a simple and low-cost manner of implementing the first sensing element. 
     In a preferred embodiment, the switch comprises a transistor or a FET, having a first current path terminal coupled to said node, having a second current path terminal coupled to the second input terminal via a sensing resistor, and having a control terminal. This provides a simple and low-cost manner of implementing the switch. 
     Further advantageous elaborations are mentioned in the dependent claims. 
     It is noted that document US2002/0154521 discloses an insulated BiFRED converter comprising a measuring resistor RS in series with the controllable switch S 1 , such that the charging current of the first inductor L 1  and the discharging current of the storage capacitor C 1  pass through this measuring resistor RS and the sum current develops a measuring voltage over the measuring resistor RS, which measuring voltage is used as a control input signal for the control device ST. However, the document is silent on how to process this control input signal for providing the actual control output signal for the controllable switch S 1 . And US 20050168199A1 discloses a cuk type converter with a sensing resistor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: 
         FIG. 1A  is a block diagram schematically showing a non-insulated BiFRED converter according to prior art; 
         FIG. 1B  is a block diagram schematically showing an insulated BiFRED converter according to prior art; 
         FIG. 2  schematically shows a circuit diagram of an exemplary switch control device according to the present invention; 
         FIG. 3  schematically shows a circuit diagram of another exemplary switch control device according to the present invention; 
         FIG. 4A  is a graph illustrating current waveforms in the converter; 
         FIG. 4B  is a graph comparable to  FIG. 4A  but on a larger time scale; 
         FIG. 4C  is a graph illustrating current waveform envelope in the converter; 
         FIG. 4D  is a graph illustrating input voltage and current waveform for the converter; 
         FIG. 5  is a graph illustrating measured output current as a function of the supply voltage for an experimental specimen of the converter according to  FIG. 2 ; 
         FIG. 6A  is a graph illustrating measured output current as a function of time for an experimental specimen of the converter according to  FIG. 2 ; 
         FIG. 6B  is a graph illustrating measured frequency content for this experimental specimen of the converter according to  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  schematically shows a circuit diagram of a switch control device  20  according to the present invention. This inventive switch control device can be used in any of the converters of  FIGS. 1A and 1B . In the exemplary embodiment shown, the switch S 1  is implemented as a bipolar transistor, but alternative implementations are also possible, for instance a MOSFET. A sensing resistor  21  is connected in the switched current path between the emitter terminal of switch S 1  and the low input terminal  6 . The collector terminal of switch S 1  is connected to node A via a second resistor  22 , but this is not essential and this resistor may also be omitted. A bias resistor  23  connects the base terminal of switch S 1  to the high input terminal  5 . A voltage limiter  24 , here embodied as a zener diode, is connected between the base terminal of switch S 1  and the low input terminal  6 . 
     Further, between the base terminal of switch S 1  and the low input terminal  6 , a series arrangement is connected of a fourth resistor  28 , an auxiliary capacitor  26  and a feedback inductor  25 . A third diode  27  is connected in parallel to the auxiliary capacitor  26 , having its cathode directed towards the base terminal of switch S 1 . The feedback inductor  25  is magnetically coupled to the second inductor L 2 , having the same direction as the second inductor L 2 , so an increasing current in second inductor L 2  will cause an increasing voltage induced over feedback inductor  25 . 
     The operation is as follows. 
     Initially, on power up, the auxiliary capacitor  26  is empty, so the voltage at the base terminal of switch S 1  is zero and the switch S 1  is non-conductive. The auxiliary capacitor  26  will receive a small charging current via the bias resistor  23 , causing the voltage at the base terminal of switch S 1  to rise. When this voltage reaches the base-emitter threshold voltage, the switch S 1  will start to become conductive. As described above, the storage capacitor C 1  will discharge over the second inductor L 2 , which causes a positive voltage to be induced over the feedback inductor  25 . This positive voltage is fed to the base terminal of switch S 1  to accelerate the transition to its conductive state. The third diode  27  parallel to the auxiliary capacitor  26  allows for extra base current to be provided, via a current path that bypasses the impedance of the auxiliary capacitor  26 . 
     With the switch S 1  in its conductive state, the charging current of the first inductor L 1  and the discharging current of the storage capacitor C 1  together flow through the sensing resistor  21 , causing the voltage drop over the sensing resistor  21  and hence the voltage at the emitter terminal of switch S 1  to rise. Consequently, the voltage level at the base terminal of switch S 1  rises (being the emitter voltage plus the forward voltage Vbe between base and emitter). On the other hand, the voltage level at the base terminal of switch S 1  is limited by the Zener diode  24 . When the voltage level at the base terminal of switch S 1  reaches the breakdown voltage of the Zener diode, the base voltage of switch S 1  can rise no further, and the rising emitter voltage will cause the switch S 1  to make a transition to its non-conductive state. 
     In this non-conductive state of switch S 1 , the first inductor L 1  is discharged to charge the storage capacitor C 1 , and the second inductor L 2  is discharged to charge the output capacitor C 2  and power the load L, as described above. The voltage over the feedback inductor  25  is now negative, keeping the switch S 1  in its non-conductive state. With the discharging of the second inductor L 2 , the magnitude of the current in the second diode D 2  reduces. When this magnitude becomes zero, the voltage across the feedback inductor  25  will become positive, causing a positive voltage at the base of switch S 1  and hence turning the switch S 1  to its conductive state. The above switching cycle repeats itself. 
     It will thus be seen that the converter is self-oscillating. The minimal current in the feedback inductor  25  is zero Amp, hence the converter operates in the Critical Discontinuous Mode. 
     Thus the timing of the switching cycle is based on two mechanisms. A first mechanism controls when the switch S 1  is made conductive: this is the Critical Discontinuous Mode. A second mechanism controls when the switch S 1  is made non-coductive: this mechanism is based on maximizing the summation of the charging current of the first inductor L 1  and the discharging current of the storage capacitor C 1 , i.e. the peak-value of this summation is always constant. These two control mechanims in combination ensure a constant output current irrespective of input voltage and output voltage without any additional control-loop for output-current control being needed. 
       FIG. 3  is a diagram comparable to  FIG. 2 , showing a switch control device  30  that is a further elaboration of the switch control device  20  of  FIG. 2 . Instead of the Zener diode  24  being connected directly to the base terminal of switch S 1 , a switching accelerator circuit  35  is connected between the Zener diode  24  and the base terminal of switch S 1 , which switching accelerator circuit  35  comprises a diode  34  having its cathode connected to the cathode of the Zener diode  24  and having its anode connected to the base terminal of a first transistor  31 . The first transistor  31  has its emitter terminal connected to the base terminal of switch S 1 , and has its collector terminal connected to the low input terminal  6  via a fifth resistor  32 . A second transistor  33  has its base terminal connected to the collector terminal of the first transistor  31 , has its collector terminal connected to the base terminal of the first transistor  31 , and has its emitter terminal connected to the low input terminal  6 . The circuit  35  is to detect the breakdown current in the zener  24  as an indicator that the switch current is going to be zero, and operates to make the switch current zero as quickly as possible. 
     In the conductive state of the switch S 1 , with the increasing current through sensing resistor  21 , the emitter voltage of switch S 1  rises, hence the base voltage of switch S 1  rises, as mentioned above. Initially, the first transistor  31  is non-conductive. The base voltage of the first transistor  31  follows the emitter voltage of the first transistor  31 , which is equal to the base voltage of switch S 1 . When the base voltage of switch S 1  reaches the breakdown voltage of the Zener diode  24  plus the forward voltage of the diode  34 , the Zener diode  24  will breakdown and draw a current in the first transistor  31  so that this first transistor  31  makes a transition to its conductive state. As a consequence, the base voltage of the switch S 1  is pulled down and the switch S 1  is turned off. Further, the voltage drop over the fifth resistor  32  rises and the second transistor  33  becomes conductive, shorting the Zener diode and accelerating the switching off of the switch S 1  and the discharging of the auxiliary capacitor  26 . 
     An advantage of the control device  30  is that it achieves a faster switch off of the switch S 1 . Consequently, the delay between the moment when the voltage across sensing resistor  21  reaches the switch-off value as determined by the Zener diode  24  on the one hand, and the moment when the switch S 1  actually becomes non-conductive, is reduced, so the switching timing is more accurately related to the current detection and the regulation is better. 
     As already mentioned earlier, during the second stage, the current in the first inductor L 1  decreases and also the current in the second inductor L 2  reduces. At a certain moment in time, these currents become zero, but this timing depends on the component values. In a preferred embodiment, the inductance value of the first inductor L 1  and the inductance value of the second inductor L 2  are chosen such that the reducing current in the first inductor L 1  always reaches zero before the reducing current in the second inductor L 2  reaches zero. C 1  value is big enough to ensure it&#39;s only the function of transfer energy from L 1  to LED Load, so the target is to design L 1  and L 2  value ensure L 1  always works on discontinuous mode. As for L 1  and L 2  relationship, it depends on input voltage(terminal  5  and  6 ) and output voltage(terminal  9   a  and  9   b ). so it isn&#39;t simple to say L 1 &lt;L 2 . 
     As explained in the above, controlling the switching from conductive state to non-conductive state of the switch S 1  is based on the sum of the charge current in first inductor L 1  and discharge current of storage capacitor C 1 . Switching the switch S 1  to its conductive state is based on the output current in the second diode D 2  reaching zero. As a result, the current IL 1  in first inductor L 1  and the current IC 1  in storage capacitor C 1  have complementary wave-form envelopes.  FIG. 4A  is a graph showing these currents (vertical axis in arbitrary units) as a function of time (horizontal axis in arbitrary units) obtained in an experimental embodiment of the driver. The direction from D 1  to L 1  to C 1  to L 2  is taken as positive direction, therefore IC 1  is shown as being negative. The graph shows that, during the first state from t 1  to t 2  when the switch S 1  is conductive, the magnitude of IL 1  rises from a lowest value which may be higher than zero to a highest value, L 1  values will be designed to ensure IL 1  rises from zero to highest values, 
     whereas the magnitude (i.e. absolute value) of IC 1  rises from a lowest value which may be higher than zero to a highest value. During the second state from t 2  to t 3  when the switch S 1  is non-conductive, IL 1  and IC 1  decrease to their respective lowest values. Since at time t 2  the sum of IL 1  and IC 1  will always have the same value, the highest discharge current of storage capacitor C 1  will always decrease when the highest charge current in first inductor L 1  increases and vice versa. 
       FIG. 4B  shows the same currents at a larger time scale, covering a full period of the mains.  FIG. 4B  also shows the waveform of rectified mains voltage V 5 , in arbitrary units. The figure clearly shows the complementary wave-form envelopes of the currents. Further, the figure clearly shows that the wave-form envelope of IL 1 , i.e. the line current, follows the rectified mains voltage V 5 , and is in phase therewith, which is good for power factor correction.  FIG. 4C  shows the wave-form envelope of IL 1 , obtained by measuring IL 1  via a low-pass filter, together with the rectified mains voltage V 5 , and  FIG. 4D  shows the input voltage V 3  and input current I 3  measured at the input of the rectifier  3 , which was implemented as a 
     Graetz bridge. The total harmonic distortion was found to be around 8% and the power factor was found to be about 99%. It is noted that this near-perfect current shaping is obtained without any modulation of the output-current set-point. 
     In an experiment with variable AC supply replacing the mains, the supply voltage was varied and the output current in the load L was measured.  FIG. 5  shows the measured output current (vertical axis) as a function of the supply voltage (horizontal axis). It can be seen that the output current is substantially constant over a large range of supply voltage values. The slight dependency of the output current on the input voltage was found to be due to a non-ideal behaviour of the current detector/comparator in the experimental specimen. 
     The sum-of-currents control method increases the current through second inductor L 2 /L 3  around the zero-crossings of the mains (see  FIG. 4B ). This gives a rise in output current around the zero-crossings and introduces frequency doubling of the output ripple which is beneficial because flicker and stroboscopic effects become less apparent when the frequency increases.  FIGS. 6A and 6B  are graphs showing the output current in the time domain and frequency domain, respectively, obtained in a test circuit according to  FIG. 2 . The ratio between peak and average ripple is approximately 18%. The 100 Hz component of the ripple is approximately 10% of the DC value and the 200 Hz component is also around 10% of the DC value. 
     Summarizing, the present invention provides a driver comprising: 
     
         
         a first diode and a first inductor connected in series between a first input terminal and a first node; 
         a first capacitor and a second inductor connected in series between a second input terminal and the first node; 
         a switch connected between the first node and the second input terminal; 
         a capacitor connected between output terminals; 
         a second diode connected in series with said capacitor.
 
The series arrangement of second diode and capacitor is connected in parallel with said second inductor, or in parallel with a third inductor inductively coupled to said second inductor.
 
Said switch is controlled to a non-conductive state when the current through said switch is equal to or higher than a threshold.
 
Said switch is controlled to a conductive state when the current through said second diode reaches zero.
 
       
    
     While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. 
     For instance, instead of connecting the sensed voltage of sensing resistor  21  and the reference voltage of zener diode  24  to the emitter and base terminals, respectively, of the switch transistor S 1 , it is also possible to apply these voltage to respective input terminals of a comparator and to drive the switch transistor S 1  on the basis of an output signal from such comparator. 
     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 processor or other unit may fulfil the functions of several items recited in the claims. Even if certain features are recited in different dependent claims, the present invention also relates to an embodiment comprising these features in common. Any reference signs in the claims should not be construed as limiting the scope. 
     In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.