Abstract:
A converter-controller includes a power device, coupled to the primary coil of a transformer, a resonant circuit, coupled to the primary coil and the power device, a voltage regulator, coupled to the resonant circuit, and a control logic, coupled to the voltage regulator. The converter-controller does not need a snubber circuit. Further, the transformer structure of the converter-controller does not need additional auxiliary windings around the transformer core. The described converter-controller makes it possible to use low voltage integrated circuits in high voltage applications.

Description:
FIELD OF INVENTION 
   The present invention relates to control circuits of converters and more particularly to lossless resonant circuits for providing a low operating voltage for control circuits. 
   DESCRIPTION OF RELATED ART 
   A typical converter includes a control circuit with power devices, a transformer having a primary winding and a secondary winding, where the secondary winding is capable of generating an output voltage by stepping up or down the input, or primary, voltage. The rate of stepping up or down the input voltage is determined by the ratio of the number of windings of the primary and the secondary windings. The degree of voltage conversion is controlled by a control logic, or control circuit. While Flyback converters often work at several hundred volts, their control logic operates at about ten volts. Therefore, in the design of these controllers a low operating voltage has to be provided to the control logic. 
   Many existing Flyback converters provide low operating voltages for their control logic by using an additional auxiliary winding on the transformer core besides the primary and secondary windings. This auxiliary winding contains only a few turns, thus generating a low voltage for the control circuit. However, this auxiliary winding makes the transformer core more complex and also increases the price of the circuit. 
   Other Flyback converters use snubber circuits. These snubbers consist of a resistor—capacitor—diode circuit coupled between a control power device and the high voltage terminal in the primary circuit of the converter. These snubbers are very popular because of the simplicity of their design. However, the resistor dissipates a large amount of power, lowering the efficiency of the power conversion. 
   In some existing circuits, an RC bridge or a resistor ladder is used to provide an operating voltage for the control logic. In such circuitry, however, there is considerable power dissipation in the resistors, leading to losses in the operation of the circuitry. In some existing designs a combination of auxiliary windings and resistor circuits is applied. However, these circuits still exhibit considerable dissipation. 
   SUMMARY 
   Briefly and generally, embodiments of the invention include a converter-controller, which can be operated to control a converter. The converter has a transformer, which has a primary and a secondary windings. The converter-controller includes a power device, coupled to the primary coil of the transformer, a resonant circuit, coupled to the primary coil and the power device, a voltage regulator, coupled to the resonant circuit, and control logic, coupled to the voltage regulator. 
   Aspects of the invention include that it does not need a snubber circuit, and therefore can be operated without power dissipation. 
   Other aspects include that embodiments do not need additional auxiliary windings around the transformer core, thus having a simpler transformer structure and lowering the price of the circuit. 
   Embodiments of the invention make it possible to use low operating voltage integrated control circuits in a high voltage Flyback converter. Finally, embodiments have low electromagnetic interference (EMI), therefore generating a smooth resonance of voltage and current. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings. 
       FIG. 1  is a block diagram of a converter-controller, according to an embodiment of the invention. 
       FIG. 2  is an implementation of a converter-controller according to an embodiment of the invention. 
       FIG. 3  illustrates the definition of currents in the converter-controller circuit according to embodiments of the invention. 
       FIG. 4  illustrates various voltages and currents according to an embodiment of the invention. 
       FIGS. 5A–D  illustrate the current paths in various time intervals according to a flyback converter embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention and their advantages are best understood by referring to  FIGS. 1–5  of the drawings. Like numerals are used for like and corresponding parts of the various drawings. 
     FIG. 1  illustrates a converter-controller  100  according to embodiments of the invention. Converter-controller  100  can be operated to control a converter, which has a transformer T 1 . Transformer T 1  has a primary winding N 1  and a secondary winding N 2 . Converter-controller  100  includes a power device Q 1 , coupled to primary winding N 1  of transformer T 1 , and a resonant circuit  104 , coupled to primary winding N 1  and power device Q 1 . Converter-controller  100  further includes a voltage regulator  108 . Voltage regulator  108  is coupled to resonant circuit  104  and to a control logic U 1 . 
   In various embodiments power device Q 1  can be a MOS-FET, a bipolar junction transistor, or an Insulated Gate Bipolar Transistor (IGBT). 
   In some embodiments resonant circuit  104  includes a central node  112  with voltage Va, a resonant capacitor C 1 , coupled between central node  112  and power device Q 1 , a resonant diode D 2 , which has an anode and a cathode, the cathode of resonant diode D 2  coupled to central node  112 , and a resonant inductor L 1 , coupled between the anode of resonant diode D 2  and a ground. 
   Voltage regulator  108  includes a regulator diode D 3 , which has an anode and a cathode, the anode of regulator diode D 3  coupled to central node  112 , a regulator resistor R 1 , coupled to the cathode of regulator diode D 3 , a Zener diode D 4 , coupled between regulator resistor R 1  and a ground, and a regulator capacitor C 2 , coupled in parallel to Zener diode D 4 . 
   Control logic U 1  is coupled in parallel to regulator capacitor C 2 . Control logic U 1  is coupled to a gate of power device Q 1  (not shown). Control logic is operable to control the voltage generated in the secondary coil of the converter by controlling the ON and OFF times of power device Q 1  during switching cycles, as described below. 
   In various embodiments one or more of regulator diode D 3 , Zener diode D 4 , regulator capacitor C 2 , regulator resistor R 1 , parts or all of resonant circuit  104 , power device Q 1 , and control logic U 1  can be formed on an integrated circuit. For example, in the embodiment of  FIG. 2 , control logic U 1 , Zener diode D 4 , and power device Q 1  are formed on a single integrated circuit. 
   Converter-controller  100  further includes a high voltage link  116 , coupled to primary winding N 1 . In various embodiments high voltage link  116  can be powered by a DC source or a rectified AC source. For example, in  FIGS. 1 and 2 , high voltage link  116  is powered by a rectified AC source. 
   Central node  112  of resonant circuit  104  is coupled to high voltage link  116  through a connecting diode D 1  and regulator resistor R 1  is coupled to high voltage link  116  through a connecting resistor R 15 . 
     FIG. 2  illustrates an embodiment of the invention. Corresponding circuit elements are labeled the same as in  FIG. 1 . As mentioned before, in this embodiment control logic U 1 , Zener diode D 4 , and power device Q 1  are integrated into an integrated circuit  120 . Power device Q 1  is coupled between pins labeled Drain and Ground. Zener diode D 4  is coupled between pins labeled Vcc and Ground. An integrated circuit with these attributes is, for example, Fairchild switch KA5M0365. In other embodiments other combination of the above circuit elements can be integrated on an integrated circuit. 
   The secondary circuit, which contains secondary winding N 2 , has a typical architecture. In this embodiment secondary winding N 2  is coupled to control logic U 1  to provide a feedback signal. Besides some standard circuit elements, the feedback circuit contains integrated circuit U 2 . Integrated circuit U 2  provides a feedback signal without electrical coupling between the primary and the secondary circuit. This type of coupling is sometimes referred to as Galvanic isolation. This functionality can be achieved, for example, by employing a coupled photodiode-phototransistor pair. The photodiode emits a light signal in proportion to the current flowing through it and the phototransistor senses the emitted light and generates a feedback signal proportional to the sensed light. An example of an integrated circuit with a coupled photodiode-phototransistor pair is the Fairchild FOD2741 integrated circuit. Many other feedback circuit designs are well known in the art and can be employed in other embodiments. 
   Several types of converters are known in the arts. In the following two types of converters will be detailed, but the scope of the invention is not limited to these two types, but is understood to cover several alternatives as well. 
   A converter can be of the Flyback type or the Forward type, depending how the secondary coil is connected to the load circuit relative to the primary winding. In Flyback converters the input energy is stored in transformer T 1 , when power device Q 1  is turned ON. The energy is transferred to the load, or secondary, side when power device Q 1  is turned OFF. Forward converters operate the opposite way. The energy is transferred to the load side, when Q 1  is turned ON, and there is no power conversion when Q 1  is turned OFF. Since in Forward converters the energy is not stored in the transformer, the size of the transformer can be chosen to be smaller. The direction of windings is indicated by the black dot in the figures, as is customary. 
   When coupled to different types of converters, converter-controller  100  can be operated to control an output voltage of the converters. In some embodiments, converter-controller  100  periodically switches ON and OFF power device Q 1 , a process sometimes called a switching cycle. In these embodiments the output voltage of the converter is controlled by converter-controller  100  controlling the length of the switch-ON and switch-OFF intervals of the switching cycle. In embodiments of the invention, converter-controller  100  switches ON and OFF power device Q 1  by control logic U 1  switching the gate of power device Q 1 . 
     FIGS. 3–5  illustrate the operation of converter-controller  100 . 
     FIG. 3  illustrates the labeling of currents. The current flowing across primary winding N 1  is labeled ip, the current flowing through resonant capacitor C 1  is labeled ic, and the current flowing through power device Q 1  is labeled id. From Kirchhoff&#39;s laws in general id=ip+ic. The current in the secondary circuit is labeled is. 
     FIG. 4  illustrates the various current and voltage levels during switching cycles of a flyback embodiment. Such diagrams are sometimes referred to as timing diagrams, or waveforms.  FIGS. 5A–D  illustrate the corresponding current paths during the switching cycles. The current carrying circuit elements indicated by thickened lines. 
   The first graph of  FIG. 4  indicates the switching status of power device Q 1 . Power device Q 1  is switched OFF before time instance t 1 , then it is switched ON at time instance t 1  and switched OFF at time instance t 3 , the process controlled by control logic U 1 . The current flowing into power device Q 1  is zero, when power device Q 1  is switched OFF, i.e. before t 1  and after t 3 . In the t 1 ˜t 3  interval id differs from zero. 
     FIG. 5A  illustrates that in the t 1 ˜t 2  time interval both primary coil current ip and resonant circuit current ic are clockwise, and thus add up to a non-zero power device current id. In the interval t 1 ˜t 2 , ip steadily rises, whereas ic approximately follows a sinusoidal form, adding together to a rising peaked pattern, as shown. The resonant current ic first discharges resonant capacitor C 1 , then charges with opposite polarity in this t 1 ˜t 2  interval. This discharging-recharging process is illustrated in the fifth graph of  FIG. 4 , showing a resonant capacitor voltage Vc 1  starting from a finite negative value, go through zero, and reach a positive value of approximately the same magnitude. In this t 1 ˜t 2  time interval the voltage of central node  112 , Va, tracks the behavior of Vc 1  as shown in the sixth graph of  FIG. 4 . 
     FIG. 5B  illustrates the current paths in the interval t 2 ˜t 3 . The time instance t 2  is approximately the half-period of resonant circuit  104 , therefore, at t 2  resonant current ic would change sense. However, resonant diode D 2  prevents ic from turning negative. Therefore, in the interval t 2 ˜t 3  the resonant current remains essentially zero: ic=0. Therefore, in this interval id=ip, steadily rising, as shown. The slope of current depends on the amplitude of Vdc and the primary inductance of T 1 . Since ic=0, resonant capacitor C 1  is not charged, thus Vc 1  remains essentially constant, as shown in the fifth graph of  FIG. 4 . By t 2  central node voltage Va is pulled up to a finite value, as shown in the sixth graph of  FIG. 4 . 
     FIG. 5C  illustrates the current paths in the t 3 –t 4  interval. At time instance t 3 , power device Q 1  is switched off by control logic U 1 , controlling the gate of Q 1 . This sets power device current id=0. Kirchhoff&#39;s laws force the primary current ip across resonant capacitor C 1 , therefore, ic=−ip. Resonant diode D 2  still prevents current flow into the rest of resonant circuit  104 . However, a current path is possible across linking diode D 1 , as shown. In this time interval resonant capacitor C 1  is discharged and then recharged to restore its initial polarity, as shown in the Vc 1  graph of  FIG. 4 . If linking diode D 1  is conducting, the voltage level Va becomes the applied DC voltage Vdc until time instance t 4 , as shown in the  FIG. 4 . Finally, at t 4 , the central node voltage Va returns to its steady state value reflected from the secondary side to the primary side. 
     FIG. 5D  illustrates that the process of  FIG. 5C  goes on until resonant capacitor C 1  is recharged to its initial negative value. Once this is achieved, the primary current ip, which was recharging resonant capacitor C 1 , ceases. However, the stored energy of transformer T 1  is now released into the secondary circuit, as seen from the is waveform in  FIG. 4 . 
   In a general sense it can be said that in the time interval t 1 ˜t 3  energy is being built up in the primary circuit of the converter. Then, after time instance t 3  the energy is released from the primary circuit to the secondary circuit. 
   As seen in the Va waveform in  FIG. 4 , central node voltage Va is rectified by diode D 3  and regulated by zener diode D 4  so as to generate a required operating voltage of Vcc as shown in  FIG. 4 . This voltage Vcc is then used to power control logic U 1 . 
   As is clear from the Va waveform of  FIG. 4 , a key aspect of a Flyback converter is that the value of the voltage induced in secondary coil N 2  is determined by the length of the ON and OFF intervals. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims. That is, the discussion included in this application is intended to serve as a basic description. It should be understood that the specific discussion may not explicitly describe all embodiments possible; many alternatives are implicit. It also may not fully explain the generic nature of the invention and may not explicitly show how each feature or element can actually be representative of a broader function or of a great variety of alternative or equivalent elements. Again, these are implicitly included in this disclosure. Where the invention is described in device-oriented terminology, each element of the device implicitly performs a function. Neither the description nor the terminology is intended to limit the scope of the claims.