Abstract:
An integrated synchronous rectifier package comprising a controlled switching device having two main current carrying terminals and a control terminal, a control circuit for sensing the direction of current through the controlled switching device main current carrying terminals and for generating a control signal provided to the control terminal to turn on the controlled switching device when current flows in a first direction through the controlled switching device and to turn off the controlled switching device when current flows in a second opposite direction through the controlled switching device, the control circuit and controlled switching device being contained within a single package having no more than four external electrical connections.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit and priority of U.S. Provisional Application Ser. No. 60/517,333 entitled “INTEGRATED SMART RECTIFIER” filed Nov. 4, 2003, the entire disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to electrical rectification circuits, and in particular, to synchronous rectifiers which are controlled rectification devices. More particularly, the present invention relates to an integrated synchronous rectifier which can be substituted for a non-synchronous rectifier in a rectification circuit with minimal circuit charge. 
   Output rectification in off-line isolated power supplies is often implemented by use of Schottky or PIN diodes. The typical disadvantage is that every diode has a nominal threshold voltage and therefore high conduction losses. Synchronous rectification has been introduced for flyback converters and in other applications where a MOSFET or other controlled switch is driven by an appropriate drive circuit to emulate the operation of a diode. The disadvantage of the MOSFET synchronous rectifier is the added complexity in the design of the rectification stage, which in the case of normal diode rectification, is very simple, consisting of only a diode. In particular, to substitute a synchronous rectifier for a diode requires the addition of the drive circuitry for the synchronous rectification device, leading to increased complexity and a larger number of circuit components. 
   Applicant is aware of the following prior art: 
   U.S. Pat. No. 6,353,544 uses a current transformer to sense current, making it impracticable for an integrated solution. U.S. Pat. No. 6,442,048 is another prior art example, which cannot be used to provide a one package integrated solution with a minimal external terminal count. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a synchronous rectifier which can be simply substituted for the normal non-synchronous rectification diode. Diodes have two terminals, a cathode and anode. It is an object of the invention to provide a synchronous rectifier which can be substituted for the two terminal diode device and which has a minimum number of terminals, the minimum being an anode, cathode and a power connection. In the case where the ground connection is common with the anode or cathode, the device according to the invention can have a minimum of three terminals, anode, cathode and power supply terminal. 
   It is accordingly an object of the present invention to integrate control circuitry and an active switch in a single package with a minimal number of pins in order to obtain a so-called “smart” rectifier. 
   The objects of the invention are achieved by an integrated synchronous rectifier package comprising: a controlled switching device having two main current carrying terminals and a control terminal, a control circuit for sensing the direction of current through the controlled switching device main current carrying terminals and for generating a control signal provided to the control terminal to turn on the controlled switching device when current flows in a first direction through the controlled switching device and to turn off the controlled switching device when current flows in a second opposite direction through the controlled switching device, the control circuit and controlled switching device being contained within a single package having no more than four external electrical connections. 
   The objects of the invention are also achieved by a synchronous rectifier package comprising a controlled switching device having two main current carrying terminals and a control terminal, a control circuit for sensing the direction of current through the controlled switching device main current carrying terminals and for generating a control signal provided to the control terminal to turn on the controlled switching device approximately when current begins to flow in a first direction through the controlled switching device and to turn off the controlled switching device approximately when current begins to flow in a second opposite direction through the controlled switching device, and the control circuit and controlled switching device being contained within a single package having no more than four external electrical connections. 
   The objects of the invention are also achieved by an integrated circuit package comprising, a substrate, a MOSFET and its intrinsic body diode (or a separate parallel doiode) disposed on the substrate and having source and drain terminals connected to external lead terminals, a control integrated circuit disposed on the MOSFET for controlling the MOSFET and having power connections coupled to the external lead terminals and a control terminal of the MOSFET for controlling the conduction state of the MOSFET, and the MOSFET and the control integrated circuit being housed in a common package and the external lead terminals bringing the source and drain terminals and the power connections of the integrated circuit externally of the package. 
   The present invention is not only limited to the case where two discrete dice are co-packaged. A monolithic solution is also possible where the controller and the power MOS are both on the same substrate. The tradeoffs of the ‘hybrid’ vs. ‘monolithic’ solutions need to be compared, but they both represent an optimal embodiments of the invention. 
   Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWING(S) 
     The invention will now be described in greater detail in the following detailed description with reference to the drawings in which: 
       FIG. 1  shows a typical prior art diode rectification circuit in a flyback converter circuit; 
       FIG. 2  shows a typical prior art flyback converter circuit employing synchronous rectification; 
       FIG. 3  shows a circuit according to the invention for implementing an integrated synchronous rectifier in a flyback converter circuit; 
       FIG. 3A  shows the block diagram of the control circuitry for the active rectifier switch of  FIG. 3 ; 
       FIG. 3B  shows the transfer function of the circuit of  FIG. 3A ; 
       FIG. 4  shows the circuit of  FIG. 3  incorporating the integrated smart rectifier into the circuit as a single circuit component; 
       FIG. 5  is a mechanical drawing of the smart rectifier of  FIG. 4 ; 
       FIG. 6  shows details of another embodiment of the smart rectifier of  FIG. 4 ; 
       FIG. 7  is a cross-sectional view of the structure of the smart rectifier; and 
       FIGS. 8 and 9  show operation of the invention in two modes of operation. 
   

   Other objects, features and advantages of the present invention will be apparent from the detailed description which follows. 
   DETAILED DESCRIPTION OF THE INVENTION 
   With reference now to the drawings,  FIG. 1  shows a typical prior art flyback converter circuit with a diode rectification stage in the secondary of the flyback transformer. The circuit includes an FET  10  driven by a pulse width modulation stage  15 . The FET  10  switches current through a primary P of a flyback transformer T. The secondary s output is rectified by a diode D and the output voltage is provided to an output storage capacitor C and to the load. The primary of the transformer P is coupled to a voltage source B+. A circuit comprising a diode D 1  capacitor C 1  and resistor R 1  is provided to dampen oscillations. 
     FIG. 2  shows the circuit of  FIG. 1  employing a synchronous rectification stage comprising an FET  30  including an active rectifier switch controlled by a synchronous rectifier (SR) drive circuit  40 . 
   Typical drawbacks of the circuit of  FIG. 1  are that the diode D has a voltage drop on the order of 0.7 volts and therefore high conduction losses. The drawback of the circuit of  FIG. 2  is that it is much more complex than the simple circuit of  FIG. 1  requiring SR drive circuitry  40  to drive the synchronous rectifier, thus requiring many more components and connections. 
   According to the invention, it is desired to provide a smart rectifier which comprises a synchronous rectifier but with a minimum number of pins and thus, without additional complexity, can be substituted for the diode rectifier of  FIG. 1 . It is anticipated that the minimum number of pins required would be three, that is, the anode and cathode connections normally required by the diode and in addition, a power supply pin to provide power. It is also within the scope of the invention to provide a four terminal device also including a ground connection where it is necessary to isolate the ground connection from the cathode or anode diode connections. 
   In order to emulate the diode rectification function, it is necessary to determine the direction of flow of the current through the rectifier. When the sign of the current is reversed, the control should turn off the active switch promptly. In order to perform this function, the control needs to sense the current in the active switch. For this purpose, the on-resistance of the active switch can be used to generate a voltage proportional to the switch current. 
     FIG. 3  shows an implementation of a synchronous rectifier package  50  having three connections, drain, source and power VCC. In the circuit of  FIG. 3 , the integrated synchronous rectifier is designated as  50 . It includes a control circuit  60  and an active switch  70  comprising, for example, a MOSFET. The control circuit  60  requires a common or ground connector which is connected to the source of the switch  70  and a power supply pin VCC. The other pin is the drain of the active switch  70 . In the circuit of  FIG. 3 , the drain functions as the cathode and the source as the anode of the simple diode rectifier. The device  50  only requires three external electrical connections as the ground connection is common with the source. It accordingly can be easily substituted for a two terminal diode. Only one additional connection, power VCC, is necessary, and this is easily accommodated as the power source VCC is present in the circuit anyway, as shown. 
     FIG. 3A  shows details of the control circuit  60 . The drain of the active switch  70  is coupled to the input VD. The source is connected to the input VS. The drain-source voltage is monitored by a Schmidt trigger circuit  90 . When the drain-source voltage is negative, corresponding to current flowing in the direction I of  FIG. 3 , the body diode of switch  70  will start conducting the current I developing a voltage drop across the terminals D and S. When V TH2  is selected to be smaller in absolute value than the conduction threshold of the body diode V F , as soon as the body diode starts conducting, the trigger circuit  90  turns on, providing a high level to the AND gate  100 . At the same time, the one shot  120 , which is negative edge triggered, remains in a state whereby its Q not output is high, thereby enabling AND gate  100  and providing a supply voltage Vgate to turn on the MOSFET  70 . This corresponds to conduction of the switch  70  during the positive AC half cycle. 
   During the negative AC half cycle, current will attempt to flow in the direction opposite the current I shown in  FIG. 3 . However, the circuit  60  of  FIG. 3A  will turn off the switch  70  just before the current begins to flow in the opposite direction during the negative half cycle. Once the voltage VDS reaches V TH1  near the beginning of the negative half cycle, the trigger circuit  90  will go low, providing a low input to the AND gate  100  and turning off the gate drive Vgate. At the same time, the one shot  120  will generate a negative going pulse to the gate  100 . This negative pulse will blank the Schmidt trigger circuit  90  for a programmed amount of time T BLANK . The reason for blanking the comparator  90  is to avoid further false triggering. In fact, as soon as the gate is turned off, since V TH1  is negative, there will still be some amount of current flowing through the body diode. 
   That current, once the MOSFET  70  is off, would build a voltage in excess of V TH2  and therefore try to turn the gate of  70  back on, as can be seen in  FIG. 8 , possibly causing high frequency resonance. 
   The one shot monostable  120  is in place to avoid this condition. 
   The selection of the pulse duration T BLANK  is important. On one hand it must be selected long enough to avoid spurious triggering, on the other, if too long, it will limit the maximum operating switching frequency for this circuit. 
   Therefore a third threshold is used. V TH3  is positive and large enough to determine when the positive half cycle is completed and the negative cycle is started. Once V TH3  is exceeded the one shot is immediately reset, without waiting for the pulse T BLANK  to end. This action will restore the circuit in its initial condition and prepare it for the next active cycle. 
   In an exemplary embodiment, V TH1  is −20MV, V TH2  is −200 mv and V TH3  is 5V. 
     FIG. 3B  shows the switching function of the circuit of  FIG. 3A . For voltages VDS in excess of V TH1 , Vgate is off, showing that the active switch  70  is turned off when current would attempt to flow in the direction opposite the current I shown in  FIG. 3 . When VDS is less than V TH2 , then current is flowing in direction I and the active switch  70  is turned on as shown by the voltage Vgate in  FIG. 3B . 
   The thresholds V TH1  and V TH2  need to be chosen so that the voltage drop across the active switch falls between these two thresholds in normal operation, and so that V TH2  is lower (in absolute value) than the body diode conduction threshold voltage. 
   This is generally not a problem since the body diode forward voltage drop is generally one order of magnitude larger than the ‘on’ voltage across the active switch. 
     FIG. 4  shows the circuit of  FIG. 3  wherein it is indicated by a single component  50  including three connections, cathode, anode and a power supply connection VCC. The anode also functions as the ground connection for the control circuit which is integrated in the package  50 . The control circuit and the active switch are integrated into a single power package with only three pins. 
     FIG. 5  shows a mechanical drawing of the smart rectifier  50 . The smart rectifier includes the switch  70  with its gate contact as shown. The control IC  60  is packaged on top of the switch  70 . The control IC and switch are wire bonded to the three terminals A/S, K/D and VCC. 
     FIG. 6  shows another implementation of the smart diode  50  comprising the switch  70  and the control IC  60 . 
     FIG. 7  shows the copacking of the control IC  60  and the switch  70 . The control IC  60  is isolated from the switch  70  by a kapton film or adhesive tape. The switch  70  is attached to a lead frame  80  via a soldered die attach  85 . 
   The advantages of the invention are that it allows a quick replacement of diode rectifiers with actively controlled rectifiers, preferably based on MOSFET devices, which in turn dramatically reduces conduction and leakage losses. The resulting smart rectifier will typically have only one additional pin VCC compared to a single diode, resulting in minimal system complexity increase. Smart rectifiers according to the invention can also be placed in parallel since for all purposes they act as diodes and can be used for output synchronous rectification. They can also be used for OR-ing functions. 
     FIGS. 8 and 9  show operation of the circuit in two modes of operation.  FIG. 8  shows DCM (discontinuous conduction mode) and  FIG. 9  shows CCM (continuous current mode). In these figures, the following definition apply:
         V TH1 : Turn-off threshold—when current decays below a certain level the switch can be turned off, allowing conduction in the body diode.   V TH2 : Turn-on threshold—as soon as V DS  become negative, the body diode will start conducting. These typically have voltage drops much higher than the MOSFET on-state.   V TH3 : Reset threshold—when the cycle is completed and the V DS  voltage will be positive and start to increase again, the internal one shot will be reset. This way the system is ready for next cycle.       
   Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore, the present invention should be limited not by the specific disclosure herein, but only by the appended claims.