Abstract:
A dual-switch forward power converter, and a method of operating the same, employs a self-coupled driver to achieve among other advantages higher efficiency, lower part count and component cost. In one aspect of the present invention, a power converter comprises a transformer and two switching transistors, and said transformer has two serially-connected primary windings with the first winding connected to a first switching transistor which is biased by a pulse controller, and the second winding couples the voltage across said first winding to bias the second switching transistor. In addition, the circuit on the primary side of said transformer further comprises means of dissipating magnetization current and the circuit on the secondary side comprises a rectifier and a low-pass filter.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a switch mode power supply, and more particularly to a dual-switch forward converter employing a self-coupled driver to achieve among other advantages higher efficiency, lower part count and component cost. 
       BACKGROUND OF THE INVENTION 
       [0002]    Switch mode power supplies have been widely used in a great variety of applications and appliances which require light and compact regulated power sources of high efficiency. In addition, said power supplies have high reliability and low power loss, and they can easily be configured to step up and down supply voltages in accordance with design requirements. 
         [0003]    In a switch mode power supply, power regulation is accomplished by applying pulse-width modulation to the switching transistors, in particular, the control of the on-time to off-time ratio of said transistors which operate at frequencies up to hundreds of kilo-hertz. The output voltage depends on the duty cycle of said control pulses and the input voltage, and it is essentially load independent. Moreover, changing the switching pulse width accordingly results in constant output voltage even when the input voltage varies. 
         [0004]    A plurality of power converter topologies has been devised to address the different design issues such as power level, output voltage and input-output isolation. Flyback converters, for instance, require relatively large transformer cores and switching transistors, and they are suitable for applications which require low part count and low power levels. In the popular forward topology, energy is supplied to the output capacitor while the switching transistor is conducting, and it achieves significantly better transformer utilization than the flyback design. However, forward converters employing single switching transistor suffer from the same shortcoming as the flyback design, namely the voltage across said transistor is inherently unconstrained. This results in higher voltage rating requirement for the switching transistor, and large voltage transients which must be clamped by snubber circuit or additional reset winding. The undesirable power loss in said snubber circuit results in lower power conversion efficiency. 
         [0005]    As illustrated in  FIG. 1 , a two-switch forward converter typically employs two switching transistors  406  &amp;  407  on the primary side of transformer  102 . Magnetizing current builds up during the conducting periods. When said transistors turn off and interrupt the current path, the magnetizing inductance acts as a voltage source. Reverse voltage from this inductive source forward biases and turns on the two diodes  403  &amp;  404  to maintain current flow. In sufficient time, the magnetizing inductance is depleted by this voltage until the stored energy is returned to input source  401 . Its clamped transformer voltage operation, with a maximum duty cycle of 50%, allows easy reset of transformer core. Thus, the voltage across said switching transistors is constrained to the input voltage and this allows lower-voltage and less expensive switching transistors to be used. 
         [0006]    The conventional two-switch forward configuration has one known disadvantage, namely an isolated driver circuit  101  is required to couple pulses from controller  402  to transistor  407 . Said drive circuit is commonly implemented with additional driver transformer or active semiconductor isolation device, typically together with external components, at the expense of increased bill-of-material cost, power consumption, part count, and overall circuit board estate. 
         [0007]    Accordingly, there is an imperative need for innovative gate drive circuit designs which could meet the increasing demand for high-efficiency, low-cost and compact switch mode power supplies. The power converter of the present invention satisfies the need. Other advantages of this invention are apparent with reference to the detailed description provided herewith. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with an aspect of the present invention, a power converter comprises a transformer and two switching transistors. Said transformer has two serially-connected primary windings with the first winding connected to a first switching transistor which is biased by a pulse controller, and the second winding couples the voltage across said first winding to bias the second switching transistor. Thus, the power converter of the present invention does not require a stand-alone isolation driver typically used to couple pulses generated by said controller to one of the switching transistors. In particular, said transformer couples input energy to an output capacitor when both of said switches are conducting. In addition, the circuit on the primary side of said transformer further comprises means of dissipating magnetization current and the circuit on the secondary side comprises a rectifier and a low-pass filter. 
         [0009]    In another aspect, the present invention provides a method of operating a power converter comprising the steps of coupling an input voltage to said first primary winding and periodically biasing said switching transistors by said controller and a self-coupled driver implemented with said second primary winding of said transformer; and deriving an output from an output capacitor of said low-pass filter in said circuit on the secondary side of said transformer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Preferred embodiments according to the present invention will now be described with reference to the Figures, in which like reference numerals denote like elements. 
           [0011]      FIG. 1  illustrates the schematic diagram of a typical two-switch forward converter (prior art). 
           [0012]      FIG. 2  shows the schematic diagram of a dual-switch forward converter using an isolated transformer-based gate driver. 
           [0013]      FIG. 3  shows the typical waveforms of control and gate pulses for transformer-based gate drivers. 
           [0014]      FIG. 4  depicts the schematic diagram of the dual-switch forward converter of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The present invention may be understood more readily by reference to the following detailed description of certain embodiments of the invention. 
         [0016]    One solution to said challenge of designing cost-effective forward converters is to use transformer-based gate driver circuit  201  &amp;  202  as shown in  FIG. 2  instead of using the relatively costly semiconductor isolation driver circuit  101  in  FIG. 1 . Upper switching transistor  407  is connected between the high voltage bus of input source  401  and the primary winding of transformer  102 , whereas its gate terminal is coupled to the secondary winding of a second transformer  201  which has the first end of its primary winding coupled by capacitor  202  to the output of controller  402 . The second end of said primary winding is connected to the 0V or ground return bus of input source  401 . The voltage level (with respect to 0V) at the source of transistor  407  is not constant. When transistor  407  is conducting, said voltage level is almost equal to the input voltage  401 . On the other hand when transistor  407  is not conducting, said voltage will drop to a level that almost equals to 0V or the voltage of the ground return bus of input source  401 , and this voltage starts to rise when the energy in the core is fully dissipated. As a result, said transformer  201  serves as an inductive coupling device between controller  402  and gate-to-source input of transistor  407  which have different voltage references. 
         [0017]    In spite of its advantage, the above transformer-based approach may not be suitable for applications with tight constraints on component footprint due to the considerable size of transformer  201 . In addition, gate driver transformer  201  couples the effective load of the switching transistors and other components to the output of controller  402 , thus drawing additional current from said controller. Finally, the amplitude of the gate pulses coupled to transistor  407  is inversely proportional to the duty cycle of the control pulses generated by controller  402 . This relationship, which is illustrated in the waveform diagrams in  FIG. 3 , poses another limitation to the maximum duty cycle of the pulses which controller  402  can generate without excessively driving down the amplitude of the gate pulses delivered to transistor  407 , which fails to turn on when said pulse amplitude is lower than the cut-off threshold of said transistor. Coupling capacitor  202  must have large enough capacitance, otherwise it would round-off pulses generated by controller  402  and those coupled to the gate of transistor  407 , which further limits the duration of the turn-on time or the maximum duty cycle of control pulses. 
         [0018]    The present invention provides a dual-switch forward power converter using an efficient yet simple self-coupled driver configuration in the primary side of the converter circuitry. Said self-coupled driver does not require any semi-conductor device or stand-alone transformer as the isolation driver circuit  101 ,  201  &amp;  202  (refer to  FIGS. 1 &amp; 2 ).  FIG. 4  illustrates the schematic diagram of said dual-switch forward converter having self-coupled driver. Controller  402  generates periodic pulses alternating between a predetermined high voltage and a predetermined low voltage at a duty cycle which is predetermined or variable in accordance with the predetermined or detected magnitudes of input voltage  401  and/or the output voltage across capacitor  414 , and said alternating voltage levels are capable of turning on and off the switches used in the forward converter. Although the description of the switches used in the present invention is referenced to a type of field effect transistors, it is apparent to those skilled in the art that said switches can be implemented with a variety of devices including but not restricted to field-effect transistors (FETs), metal-Oxide semiconductor field-effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs) and bipolar junction transistors (BJTs). 
         [0019]    In  FIG. 4 , controller  402  is connected to the gate terminal of first switching transistor  406 . The source of said transistor is tied to the 0V return bus of input source  401 . Second switching transistor  407  has its drain terminal connected to the high voltage power bus of said input source  401 . Transformer  415  comprises two sets of primary winding  408  and  409  having N p1  and N p2  number of turns respectively. First set of primary winding  408  has two of its ends connected respectively to the drain of said first transistor  406  and the source of said second transistor. Second primary winding  409  has two of its ends connected respectively to the gate and source terminals of said second transistor  407 . A first magnetization current path is formed by first diode  403  with its cathode and anode connected respectively to the high power bus of said input source  401  and the drain of said first transistor  406 , and a second current path is formed by second diode  404  with its cathode and anode connected respectively to the source of said second transistor  407  and said 0V power bus of said input source  401 . A resistor  405  is connected across the drain and source terminals of said second transistor  407 . Finally, secondary winding  410  of transformer  415  having N s  number of turns is connected to a rectifying circuit comprising a third diode  411  in series with inductor  413  and capacitor  414 . Second end of said secondary winding  410  is connected to the anode of a fourth diode  412  whose cathode is tied together with the cathode of said third diode  411 . An output voltage source is derived across said capacitor  414 . 
         [0020]    Referring again to  FIG. 4 , controller  402  outputs a continuous stream of pulses of pre-determined amplitude and duty cycle to turn on and off first switching transistor  406  alternately. No isolation driver circuit is required between said controller  402  and second switching transistor  407 . When said first transistor  406  is conducting, resistor  405  provides a path for a very small amount of current to first transformer primary winding  408 . The voltage across said first primary winding (V Np1 ) almost equals to the voltage V in  of input source  401 : V Np1 ≈V in . The voltage across second primary winding  409  is proportional to that across said first primary winding  408 : V Np2 =nV Np1  with n denoting the turn ratio N p2 /N p1 . Turn ratio is chosen such that V Np2  is sufficiently large to turn on said second transistor  407  (when said first transistor  406  is conducting), which has, at this stage, a much lower drain-source resistance than resistor  405 . Thus, said second primary winding  409  draws almost all its current flow through said second switching transistor  407 . When said first switching transistor  406  is not conducting, the magnetization current of transformer  415  flows through first and second diodes  403  &amp;  404  and back to said input source  401 . At this stage, the voltage across said first primary winding  408  is very close to that of input source  401 , but with the polarity reversed: V Np1 ≈−V in . The voltage across said second primary winding  409  is therefore V Np2 =−nV Np1 . Thus, said second switching transistor  407  is reverse-biased and it stops conducting. Accordingly, both switching transistors  406  &amp;  407  in the dual-switch forward power converter of the present invention are capable of being turned on and off simultaneously. The voltage across fourth diode  412  is a pulse-width-modulated voltage which jumps between nV Np1  and a level close to 0V. The low-pass filter formed by inductor  413  and output capacitor  414  produces an average value from said voltage across said fourth diode  412 . The output voltage depends on the duty cycle of the control pulses and the input voltage, and it is essentially load independent. 
         [0021]    The choice of components affects the overall conversion efficiency of any switch mode power supplies including said dual-switch forward converter of the present invention. Switching transistors must have low resistance during conducting cycles and should meet the required bandwidth, voltage and current ratings with safety margin. Suitable transistor types include but not restricted to FET, MOSFET, IGBT and BJT. When bipolar transistors are used to replace FETs as the switches, the base, emitter and collector terminals of the bipolar transistors replace respectively the gate, source and drain terminals of the corresponding FETs. Diodes should preferably have low on-resistance, low forward voltage drop and they should meet the required forward and breakdown voltage and current requirements. Higher switching frequency allows smaller size of the inductor to be used, at the expense of higher switching losses of the switching transistors. 
         [0022]    It is evident to those skilled in the art that the dual-switch forward converter of the present invention provides many advantages over the widely used designs employing isolation driver circuit. Firstly, the additional second primary winding  409  is inexpensive and secondly it does not take up additional circuit board estate; thirdly, the power dissipated in said winding and resistor  405  is insignificant compared to the typical power consumption of a semiconductor isolator chip; and that an isolation driver circuit, whether it is active or passive, adds additional load to said pulse controller. Finally, compared to forward converters employing transformer-based drive circuits, the self-coupled driver of the present invention does not pose additional limitation on the maximum duty cycle of the control pulses. 
         [0023]    While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the invention scope is not so limited. Alternative embodiments of the present invention will become apparent to those having ordinary skill in the art to which the present invention pertains. Such alternate embodiments are considered to be encompassed within the spirit and scope of the present invention. Accordingly, the scope of the present invention is described by the appended claims and is supported by the foregoing description.