Patent Publication Number: US-9419608-B2

Title: Active diode driver

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/KR2014/001965, filed Mar. 10, 2014 and published as WO2014/178529 A1 on Nov. 6, 2014, which claims the benefit of Korean Patent Application No. 10-2013-0048528, filed on Apr. 30, 2013, the entire disclosures of which are incorporated herein by reference for all purposes. 
     TECHNICAL FIELD 
     The present invention relates to an active rectifier using an active diode, and more particularly to, technology for operating a switch of an active rectifier. 
     BACKGROUND ART 
       FIG. 1  is a diagram illustrating a circuit of an active rectifier using a conventional active diode. 
     An active rectifier  10  substitutes the conventional bridge diode circuit by using the metal-oxide-semiconductor field-effect transistor (MOSFET), and can reduce a conduction loss by using a MOSFET, thereby being integrated by using IC technology. The active rectifier  10  composed of an active diode  20  is composed of M1, which is a switch of the MOSFET, and due to the characteristics of the MOSFET, a parasitic diode D 1  is located between drain and source as illustrated in  FIG. 1 . A comparator  21  turns on M1 if VKA, which is the voltage between cathode (K) and anode (A) (VKA), is lower than 0 V. Since the comparator  21  cannot provide, for itself, the sufficient current to operate the M1 at a high speed, the comparator  21  operates the M1 by using a gate driver  22  that has the sufficient capability to provide a current. 
       FIG. 2  is a waveform diagram illustrating an operation when the active diode in  FIG. 1  operates. 
     If a VKA becomes lower than 0, an output of a comparator  21  becomes high, and M1 is turned-on through a gate driver  22 . Here, due to the delay time td1+td2 of both the comparator  21  and the gate driver  22 , after the VKA becomes lower than zero and td1+td2 passes, the M1 is turned-on. Thus, since a current flows through D 1  during the period of td1+td2, the efficiency of the active diode  20  is slightly reduced. Also, electromagnetic interference (EMI) may occur, which is caused by the sudden change of the VKA as the M1 is turned-on. 
       FIG. 3  is a diagram illustrating an active diode performing a turn-on delay compensation using offset;  FIG. 4  is a diagram illustrating an operation waveform in the case the delay compensation using offset is performed; and  FIG. 5  is a diagram illustrating a phenomenon that a hard turn-on of M1 is performed when a VKA is greater than 0 and lower than Voff. 
       FIG. 3  is a case of a comparator  21  being operated by applying an offset. Because the output of the comparator  21  becomes high due to Voff before VKA becomes zero, the point in time when M1 is turned-on is brought to an earlier time after td1+td2 as illustrated in  FIG. 4 , so as to acquire more ideal characteristics of a diode. However, as in  FIG. 5 , even when the VKA is greater than 0 and lower than Voff and such a condition is maintained, there may be the phenomenon that the M1 is turned-on so as to reversely turn on an active diode  20 . Here, a MOSFET is fully turned-on, thus Rdson being very low, which is the resistance between drain and source, so that a lot of currents may flow. Such a phenomenon may cause a loss that is not made by a normal operation of a diode but by an abnormal operation thereof. 
     TECHNICAL PROBLEM 
     The purpose of the present invention is to provide a technical solution to effectively operate a switch used in an active rectifier. 
     TECHNICAL SOLUTION 
     An active diode driver to operate a switch of an active rectifier using an active diode according to an exemplary embodiment so as to solve the above-mentioned technical problems first controls a soft turn-on of the switch and secondly controls a hard turn-on of the switch. 
     The active diode driver first controls a soft turn-on of the switch when the voltage between the opposite ends of a parasitic diode of the switch is close to zero; and the active diode driver secondly controls a hard-turn on of the switch when the voltage between the opposite ends of the parasitic diode reaches zero. 
     The active diode driver includes: a near zero-cross detector (NZCD) that generates an output signal when the voltage between the opposite ends of the parasitic diode of the switch is close to zero; a zero-cross detector (ZCD) that detects a point in time when the voltage between the opposite ends of the parasitic diode is zero, and generates an output signal; and a gate driver that receives the output signal from the NZCD so as to control a soft turn-on of the switch, and then additionally receives the output signal from the zero-cross detector so as to control a hard turn-on of the switch. 
     The NZCD generates an output signal when the voltage between the opposite ends of the parasitic diode is lower than the trigger voltage of the NZCD. 
     The NZCD includes a second switch that, if a threshold voltage is the same as the trigger voltage and a gate voltage is lower than the threshold voltage, is turned off so as to increase a drain voltage and generate an output signal. 
     ADVANTAGEOUS EFFECTS 
     The active driver according to the present invention controls a gate operation voltage in dual steps by using a NZCD and an existing ZCD, thereby naturally turning on the switch of the active rectifier to reduce the conduction loss, enhance electromagnetic interference (EMI), and acquire efficiency not to flow an overcurrent even when a voltage of the active diode is maintained near zero. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute 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  is a diagram illustrating a circuit of an active rectifier using a conventional active diode. 
         FIG. 2  is a waveform diagram illustrating an operation when the active diode in  FIG. 1  operates. 
         FIG. 3  is a diagram illustrating an active diode performing a turn-on delay compensation using offset. 
         FIG. 4  is a diagram illustrating an operation waveform in the case the delay compensation using offset is performed. 
         FIG. 5  is a diagram illustrating a phenomenon that a hard turn-on of M1 is performed when VKA is greater than 0 and lower than Voff. 
         FIG. 6  is a diagram illustrating an active diode driver circuit for the control of MOSFET turn-on according to an exemplary embodiment of the present invention. 
         FIG. 7  is a diagram illustrating MOSFET soft turn-on performed by the circuit in  FIG. 6 . 
         FIG. 8  is a diagram illustrating an operation for restraining the generation of an excessive reverse-current in the circuits in  FIG. 6 . 
         FIG. 9  is a diagram illustrating the detailed example of  FIG. 6 . 
         FIG. 10  is a diagram illustrating a gate driver according to an exemplary embodiment. 
         FIG. 11  is a diagram illustrating an operation waveform of the gate driver illustrated in  FIG. 10 . 
         FIG. 12  is a diagram illustrating a simulation result of a circuit illustrated in  FIG. 9 . 
         FIG. 13  is an enlarged diagram illustrating VKA and Vg waveforms at a point in time when an M1 is turned-on in the simulation waveform in  FIG. 12 . 
     
    
    
     BEST MODE 
     The above-mentioned and additional aspects of the present invention may be clearer through preferred embodiments, which are described with reference to attached figures. Hereinafter, the present invention is specifically described so as to help those skilled in the art easily understand and implement the present invention through these exemplary embodiments. 
       FIG. 6  is a diagram illustrating an active diode driver circuit for the control of MOSFET turn-on according to an exemplary embodiment of the present invention; and  FIG. 7  is a diagram illustrating MOSFET soft turn-on performed by the circuit in  FIG. 6 . 
     When controlling the turn-on of the switch, an active diode driver first controls the soft turn-on and secondly controls the hard turn-on. More specifically, the active diode driver first controls the soft turn-on of the switch when a voltage between the opposite ends of the parasitic diode of the switch (VKA) is close to zero, and secondly controls the hard turn-on of the switch when the VKA reaches zero, which will be specifically described below through exemplary embodiments thereof. 
     As illustrated in  FIG. 6 , an M1 turn-on controller, which is the active diode driver, includes a zero cross detector (ZCD)  100  that detects a point in time when a voltage between the opposite ends K and A (VKA) becomes zero, as the existing active diode circuit does; and further includes a near zero-cross detector (NZCD)  200 . The NZCD  200  generates an output signal when the VKA is close to zero. The NZCD  200  generates SDRV; and the ZCD  100  generates HDRV. Here, the SDRV is a signal for softly turning on the switch M1, and the HDRV is a signal for fully turning on the M1. 
       FIG. 7  illustrates an operation of a circuit illustrated in  FIG. 6 . If a VKA becomes lower than VT, which is a trigger voltage of a NZCD  200 , an SDRV signal is generated after the delay of the NZCD  200 . Such an SDRV signal and a gate driver  300  softly turn on an M1. When being softly turned-on, the M1 operates on the lower voltage than the voltage when the gate voltage is fully turned-on so that the M1 operates in the state in which Rdson is high. Thus, during such a period, the loss may be increased more than the one when the M1 being fully turned-on, but the loss is reduced than the one when only diode D 1  operates. Then, if the VKA becomes lower than zero, the ZCD  100  generates a HDRV signal, and then after the delay of the gate driver  300 , the M1 is turned-on. Accordingly, in the existing manner, while the diode is on, the M1 is turned-on so that the width of the VKA being changed is large; however, in the manner using  FIG. 6 , the VKA is more softly changed compared to the existing manner so as to reduce the EMI. In addition, the conduction loss is reduced compared to  FIG. 2 . 
       FIG. 8  is a diagram illustrating an operation for restraining the generation of an excessive reverse-current in the circuits in  FIG. 6 . 
       FIG. 8  illustrates an operation for retraining the generation of an excessive reverse-current by M1 only being softly turned-on when a VKA is greater than zero voltage and lower than VT. The VKA is the value between the VT and 0V, an NZCD  200  performs the soft turn-on. However, since an output of a ZCD  100  is not generated, the full turn-on as illustrated in  FIG. 5  is not performed. Since Rdson is comparatively high when M1 is softly turned-on, a reverse turn-on current lower than the one in  FIG. 5  is generated. Thus, unnecessary loss is reduced, and EMI is also reduced. 
       FIG. 9  is a diagram illustrating the detailed example of  FIG. 6 . 
     A circuit that is implemented including a NZCD  200  is as  FIG. 9 . In the case an M3 is always on and a VKA is greater than VB-VT, the maximum voltage of source of an M3 is clamped to VB-VT. If the VKA becomes smaller than VB-VT, the source voltage of the M3 becomes almost the same as the VKA. When a threshold voltage of a second switch M2 is VT, and the gate voltage of the M2 is lower than the VT, the M2 is turned off so as to increase the drain voltage of the M2 to generate an SDRV signal. The numeral reference  400  is a constitution of a switch voltage sense. 
       FIG. 10  is a diagram illustrating a gate driver according to an exemplary embodiment; and  FIG. 11  is a diagram illustrating an operation waveform of the gate driver illustrated in  FIG. 10 . 
       FIG. 10  is a diagram illustrating a gate operation circuit that is operable in dual steps, such as soft turn-on and full turn-on. When an SDRV is high and a HDRV is low, M1 is in a soft operation state, and then voltage Vg is increased with the voltage that has been charged in capacitor Cs. If the SDRV and the HDRV is both high, the M2 is turned-on and is provided with additional charges from VDD so that the voltage Vg is additionally increased. The Cg indicates Cgs+Cgd and is a*Cg. The gate voltage when M1 is softly turned-on is as the following Equation 1. 
     
       
         
           
             
               
                 
                   
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     The gate voltage when M1 being fully turned-on is as the following Equation 2. 
     
       
         
           
             
               
                 
                   
                     
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       FIG. 12  is a diagram illustrating a simulation result of a circuit illustrated in  FIG. 9 ; and  FIG. 13  is an enlarged diagram of a point in time when an M1 is turned-on in a simulation waveform in  FIG. 12 . 
       FIG. 12  illustrates each simulation waveform of VKA, Vg, SDRV, and HDRV; and  FIG. 13  is an enlarged diagram illustrating VKA and Vg waveforms at a point in time when an M1 is turned-on. As illustrated in  FIGS. 12 and 13 , the gate voltages are controlled in dual steps, and the VKA voltage is controlled to be softly changed below 0V. 
     While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The preferred embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention.