Abstract:
An electronic circuit and method for sampling negative coil current in a power converter includes a zero crossing detector and a sample-and-hold circuit. A switch determines whether a charging or discharging current is flowing through a coil.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims priority to U.S. Provisional Application, 61/826,398 filed on May 22, 2013 by the same inventors as the present application, which is incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to power supplies, and, more specifically, to peak-current controlled switching power converter and method which allows for controlling output current of the power converter by monitoring peak current in a power switch. Furthermore, the present invention relates to zero-voltage switching power converters. And even more specifically, the present invention relates to improving current control accuracy in peak-current control of zero-voltage switching power converter. 
         [0003]    Zero-voltage switching is a method of eliminating switching power losses in a switching power converter by turning a power switch on or off at zero volts across it. 
         [0004]    Peak -current control, a scheme in which the output of a switch-mode power supply (SMPS) is controlled by choice of the peak current in a switching transistor, finds wide applications due to its ease of implementation, fast transient response and inherent stability. One simple example of peak-current control can be applied to a zero-voltage switching converter of a buck type operating near boundary-conduction mode. The term Boundary Conduction Model (BCM) is typically referred to a mode of operation of a switching power converter, were charging cycle of an inductive element begins immediately upon discharging it to zero current. 
         [0005]    In the BCM buck converter, peak current in the switching transistor is representative of approximately double of its output current. However, in a zero-voltage switching buck converter, a negative current swing develops in the inductor due to resonant switching transitions and rectifier diode reverse recovery effects. In the presence of this negative current, controlling peak current produces an error with respect to average output current. This error affects accuracy of the current control loop and diminishes benefits of the peak-current control method. 
         [0006]    Therefore, it would be desirable to provide a system and method that overcomes the above problems. 
         [0007]    In  FIG. 1 , a prior art LED driver  100  of a BCM buck type powering, a plurality of LEDs  200  is illustrated. The driver  100  includes an input voltage source  101 , a control switch  102 , a rectifier diode  104 , an output filter inductor  103 , and an output filter capacitor  120 . The driver also includes a control circuit. consisting of a current sense resistor  105 , a comparator  106  with a reference voltage REF, a zero-current detector circuit  107 , and a pulse width modulation PWM) flip-flop  108 . In operation, the switch  102  is activated when a zero current condition is detected in the inductor  101  The switch  102  is switched off when the current sense signal at the resistor  105  meets the reference voltage REF. 
         [0008]    In  FIG. 2 , a waveform  201  of current in the inductor  103  of the prior art LED driver is illustrated. An average current value of the waveform  201  equals the DC current in the plurality of LEDs  200 . The approximate average of the waveform  201  equals half of the voltage at REF divided by the resistance of  105 . An error results from the negative swing of the waveform  201 . 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    In the various embodiments of the present disclosure, a boundary condition mode power converter is provided having a zero crossing detector having an input node and an output node, a sample and hold circuit coupled to the output node of the zero crossing detector, a switch coupled to the input node of the zero crossing detector, a coil coupled to the switch and to the input node of the zero crossing detector, and a current sense element coupled to the switch and to the sample and hold circuit. 
         [0010]    In another embodiment, a buck converter has a zero crossing detector having an input node and an output node, a sample and hold circuit coupled to the output node of the zero crossing detector, a switch coupled to the input node of the zero crossing detector, a coil coupled to the switch and to the input node of the zero crossing detector, a current sense element coupled to the switch and to the sample and hold circuit, and a load coupled to the zero crossing detector and to the coil. 
         [0011]    A method for sensing current in a zero-voltage switching power converter, comprises providing an input voltage at a input node of a zero crossing detector, providing an output voltage of the zero crossing detector wherein the output voltage rises contemporaneously with the input voltage, reaching substantially zero, providing the output voltage to a sample-and-hold circuit, and providing a current sense voltage to the sample-and-hold circuit. 
         [0012]    This brief summary describes the general nature of the disclosure so that it may be readily understood. A more complete understanding of the disclosure, including its many advantages, can be obtained by reference to the following brief description of the drawings, the drawings themselves, the detailed description of the various embodiments and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a prior art circuit diagram; 
           [0014]      FIG. 2  is a current time form related to the prior art circuit of  FIG. 1 ; 
           [0015]      FIG. 3  is a circuit diagram according to one embodiment of the present disclosure; 
           [0016]      FIG. 4  is a circuit diagram of a zero crossing detector (ZCD) according to one embodiment of the present disclosure; 
           [0017]      FIG. 5  is a current wave form according to one embodiment of the present disclosure; and 
           [0018]      FIG. 6  is an alternative circuit diagram according to an embodiment of he present disclosure. 
       
    
    
       [0019]    The following detailed description describes the present embodiments with reference to the drawings. In the drawings, reference numerals label elements of the present embodiments, wherein like numerals indicate like elements. These reference numerals are reproduced below in connection with the discussion of the corresponding figures. 
       DETAILED DESCRIPTION 
       [0020]    Referring to  FIG. 3 , power converter topology  300  in accordance with an embodiment of the present disclosure is shown operating near BCM with zero-voltage switching. The power converter  300  comprises a switch  302 , a coil  303  (which may be an inductor), a diode  304 , a current sensing resistor  305 , a zero crossing detector (ZCD)  307 , a sample-and-hold circuit  309  and a diode  311 . 
         [0021]    The switch  302  may be a transistor, such as a MOSFET. As illustrated in  FIG. 3 , the switch  302  has drain (D), gate (G) and source (S) terminals. When the gate is biased, the switch  302  is closed and conducting current. When the gate is unbiased, the switch  302  is open and in a non-conducting mode. 
         [0022]    The diode  311  may represent an intrinsic body diode of the switch  302 , i.e. an anti-parallel diode. As illustrated in  FIG. 3 , the diode  311  has an anode which may be connected to the source terminal of the switch  302  and a cathode, which may be connected to the drain terminal of switch  302 . 
         [0023]    The sample-and-hold circuit  309  is illustrated to sample negative current sense voltage at the resistor  305  when a zero-voltage condition is detected across the switch  302  by the ZCD circuit  307 . The sample-and-hold circuit  309  outputs sampled negative current sense voltage V Sneg . A cathode of the diode  304  may be connected to voltage V 1 , i.e. the input voltage for a load in the case of a buck converter topology. An anode of the diode  304  may be connected to the drain terminal D. An input node (labeled “IN”) of the ZCD circuit  307  may be connected to the drain D. An output node (labeled “OUT”) of the ZCD  307  may be connected to the sample-and-hold circuit  309 . The sample-and-hold circuit  309  may also be connected to the switch  302  and to the current sensing resistor  305 . 
         [0024]    One terminal of the coil  303  may be connected to voltage V 2 , i.e. an output voltage of the load in the case of a buck converter topology. The other terminal of the coil  303  may be connected to the drain terminal D. The input node of the ZCD circuit  307  may be connected to the drain terminal D. The coil may charge when the switch  302  is conductive and discharge when the switch  302  is non-conductive. 
         [0025]      FIG. 4  illustrates one embodiment of the ZCD circuit  307  according to the present disclosure. The ZCD circuit has input node IN and output node OUT. Input node IN may be connected to differentiator capacitor  601 . An input voltage to the ZCD  307  may represent a voltage at a drain terminal D of the transistor  302 . Resistor  602  may be added to limit the current through capacitor  601 . A pull-up element or resistor  603  may be connected to V BIAS . Diodes  604  and  605 , i.e. diode clamp, may be included to limit voltage at the output node OUT between the approximately potential of V BIAS  and the approximately ground potential. 
         [0026]      FIG. 5  illustrates operation of the power converter  300  combined with the ZCD circuit  307 . Waveform  402  represents current sense voltage at the resistor  305 . Waveform  403  represents voltage at a drain terminal D of the switch  302 . Time moment  401  designates when voltage at the switch  302  drops to zero and the diode  311  becomes forward-biased. While the switch  302  or the diode  311  are conductive, the current sense voltage at the resistor  305  reflects the current in the coil  303 . Current through the coil  303  may reverse direction as a function of reverse recovery of the diode  304 , as well as parasitic capacitance present at the drain terminal D. This parasitic capacitance may be contributed by output capacitance of the switch  3   05 , junction capacitance of the diode  304 , inter-winding capacitance of the coil  303 , and stray capacitance of wiring connecting these elements. 
         [0027]    The diode  3   1   1  may become forward-biased as a result of the current in the coil  303  reversing its direction. As the diode  311  becomes forward-biased, complete current of the coil  303  becomes available for measuring at the sense resistor  3   05 . A waveform  404  represents voltage at the output node OUT of the ZCD circuit  307 . Time moment  401  is detected as a rising edge of the voltage  404 , generated by the pull-up resistor  603  once current in the differentiator capacitor  601  drops below the pull-up current of the resistor  603 . This moment may occurs following the diode  3   11  conduction. The sample-and-hold circuit  309  samples the corresponding negative voltage drop across the sense resistor  305  at the time moment  401 . That is, when the MOSFET  302  body diode conducts, negative current developed in the coil  303  appears at the current sense resistor  305 . At this moment, the corresponding negative current sense voltage V Sneg  may be sampled at the resistor  305 . 
         [0028]    Referring to  FIG. 6 , a buck converter  600  representing one embodiment of the power converter  300  is illustrated. In addition to the elements of the power converter  300  described above in  FIG. 3 , the buck converter  600  further comprises input voltage source  101 , the plurality of LEDs  200  which may be connected to diode  304  and to inductor  303 . An output filter capacitor  320  may also be included. In the buck converter  600 , average current of the coil  303  is substantially equal to the current of the plurality of LEDs  200 . Therefore, the corresponding negative current sense voltage V Sneg  can be used for the purpose of accurate control over the current in the plurality of LEDs  200 . 
         [0029]    Although the present disclosure has been described with reference to specific embodiments, these embodiments are illustrative only and not limiting. Many other applications and embodiments of the present disclosure will be apparent in light of this disclosure and the following claims. References throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Therefore, it is emphasized and should be appreciated that two or more references to “an embodiment” or “one embodiment” or “an alternative embodiment” in various portions of this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics being referred to may be combined as suitable in one or more embodiments of the disclosure, as will be recognized by those of ordinary skill in the art.