Abstract:
The present invention discloses a smart driver used in flyback converters adopting a transconductance amplifier to turn on a synchronous rectifier FET, and a comparator to quickly turn off the synchronous rectifier FET.

Description:
FIELD OF THE INVENTION 
     The present invention relates to flyback converters, and more particularly to flyback converters using synchronous rectification. 
     BACKGROUND ART 
     The majority of notebook power adapters use a flyback converter as shown in  FIG. 1 . To improve efficiency over diode rectification, most notebook power adapter manufacturers use synchronous rectification (SR). In other words, a synchronous rectifier FET is used to replace diode D on the secondary winding T 1  of the transformer T in the flyback converter shown in  FIG. 1 . A major disadvantage of SR over diode rectification is the higher cost, which is associated with the control signal that needs to be sent across the isolation barrier (the transformer). 
     Instead of sending the control signal across the isolation barrier, in certain prior art systems, the signal can also be derived from the voltage across the synchronous rectifier FET. One of the issues in doing this is that the voltage signal across the FET is very small compared to the dynamic range (millivolts versus tens of volts). To avoid false triggering, bandwidth is sacrificed. This leads to increased turn-off times, leading to efficiency losses. 
     Some prior art systems use a transconductance amplifier to slow the turn on of the synchronous rectifier FET, which solves the false triggering problem. However, slow turn-on makes it not useful in continuous conduction mode (CCM) applications. Furthermore, the transconductance amplifier brings a slow turn-off, which causes even more switching losses. 
     Therefore, there is an unmet need to provide a solution having a much faster turn-off and hence reduced switching losses. Moreover, the solution should be used in any type of flyback converter (CCM, discontinuous conductance mode (DCM), and quasi-resonant). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  illustrates a prior art flyback converter. 
         FIG. 2  illustrates a circuit  100  using a transconductance amplifier to turn on a synchronous rectifier FET and uses a comparator to turn off the synchronous rectifier FET in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
     Now referring to  FIG. 2 , a circuit  100  using a transconductance amplifier U 0  to turn on a synchronous rectifier FET M 1 , and using a comparator U 1  to turn off the synchronous rectifier FET M 1  is illustrated. As shown in  FIG. 2 , circuit  100  comprises the transconductance amplifier U 0  receiving a first DC offset V 1  at its non-inverting input terminal and receiving a “V D ” signal at its inverting input terminal, wherein the V D  signal is the drain signal of the synchronous rectifier FET M 1 . The drain signal V D  is also sent to the non-inverting input terminal of the comparator U 1 , while the comparator U 1  receives a second DC offset V 2  at its inverting input terminal. 
     The output of the transconductance amplifier U 0  is sent to the gate of M 1  directly, and the output of the comparator U 1  is sent to the gate of M 1  via an internal switch S 1 . When the output of the comparator U 1  is high, the internal switch S 1  is turned on, pulling the gate of M 1  low. When the output of the comparator U 1  is low, the internal switch S 1  is turned off, releasing the gate of M 1  to be controlled by the output of the transconductance amplifier U 0 . A diode D 1  is a parasitic diode that comes with M 1  and is used to clamp V D  to a certain negative voltage such as −0.7V during D 1 &#39;s turn-on. 
     In operation, if the flyback converter is in continuous current mode, when a main switch on the primary winding T 0  in the flyback converter is turned off, the diode D 1  is on immediately, which causes V D  to be negative, such as −0.7V. As a result, the output of the transconductance amplifier U 0 , i.e. V G  signal goes high gradually. When V G  increases to the on-threshold of M 1 , M 1  is turned on accordingly. With the turn-on of M 1 , the diode D 1  is off. 
     When the main switch on the primary winding T 0  in the flyback converter is turned on, V 0  goes high due to the induced voltage across the secondary winding T 1 , which causes the output of the transconductance amplifier U 0  to be low, i.e., V G  is low. In the meantime, V D  goes higher than the second DC offset V 2 , and the comparator U 1  outputs a high level signal, which turns on the internal switch S 1 , and pulls low V G . As a result, the synchronous rectifier FET M 1  is quickly turned off. 
     If the flyback converter is in DCM or quasi resonant mode, when the main switch on the primary winding T 0  in the flyback converter is turned off, circuit  100 ′s operation is same to that in CCM. However, when the main switch on the primary winding T 0  in the flyback converter is turned on, the voltage on V D  goes up slowly. Then the transconductance amplifier U 0  will cause V G  to go low and the comparator is not used. Such operation also turns off the synchronous rectifier FET M 1 . 
     To avoid the transconductance amplifier U 0  and the comparator U 1  “fighting” each other, a dead band is introduced, which is the voltage difference between V 1  and V 2 . When V D  drops below V 1 , the transconductance amplifier U 0  tries to keep V D  at the V 1  level by regulating V G . When V D  is moving so fast that the transconductance U 0  can&#39;t hold V D  to V 1 , V D  will go up and at a certain instant will hit V 2 . If that happens, the comparator U, turns on an internal switch S 1 , which swiftly pulls low V G . This will turn off M 1  and no current will flow anymore, preventing shoot through. 
     Many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. It should be understood, of course, the foregoing disclosure relates only to a preferred embodiment (or embodiments) of the invention and that numerous modifications may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims. Various modifications are contemplated and they obviously will be resorted to by those skilled in the art without departing from the spirit and the scope of the invention as hereinafter defined by the appended claims as only a preferred embodiment(s) thereof has been disclosed.