Patent Publication Number: US-2011069420-A1

Title: Protection to a power converter by using a high-voltage start-up device in a controller chip of the power converter

Description:
FIELD OF THE INVENTION 
     The present invention is related generally to a power converter and, more particularly, to a controller chip and protection method for a power converter. 
     BACKGROUND OF THE INVENTION 
     In a conventional AC-to-DC power converter, as shown in  FIG. 1 , the AC voltage VAC supplied to the power converter is filtered and rectified into a DC voltage VDC to be applied to the primary coil Lp of a transformer T 1 , and a controller chip  10  has a driver  18  to provide a control signal Vgate to switch a power switch Q 1  to convert the voltage VDC into an output voltage Vo at the secondary side. In the controller chip  10 , a high-voltage start-up device  16  is connected between a high-voltage pin HV and a power input pin VDD to start up the control chip  10  during the power-on state of the AC voltage VAC. After the controller chip  10  is turned on, an auxiliary coil Laux will generate a current that flows through a resistor R 1  and a diode D 1  to charge a power capacitor C 1 , thereby supplying electric power to the controller chip  10 . In the AC-to-DC power converter, to avoid malfunctions of the load circuit caused by under-voltage condition of the DC voltage VDC, a brownout detector  12  is added to monitor the DC voltage VDC. The brownout detector  12  includes resistors R 2 , R 3  and R 4  and a hysteresis comparator  14 . The resistors R 2 , R 3  and R 4  establish a voltage divider to divide the DC voltage VDC to generate a voltage V 1 , and the hysteresis comparator  14  compares the voltage V 1  with a threshold Vref to assert a protection signal Sc to turn off he driver  18 . As the brownout detector  12  requires the external resistors R 2 , R 3  and R 4  and the controller chip  10  needs an additional pin EN to receive the voltage V 1 , the AC-to-DC power converter is of higher cost. The brownout detector  12  also consumes additional power since the resistors R 2 , R 3  and R 4  always have a current flowing therethrough. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a controller chip for a power converter. 
     Another object of the present invention is to provide a protection method for a power converter. 
     According to the present invention, a controller chip for a power converter includes a high-voltage pin, a power input pin and a junction field effect transistor (JFET) as a high-voltage start-up device connected between the high-voltage pin and the power input pin. The JFET has a drain connected to the high-voltage pin, and a source connected to an anode of a diode whose cathode connected to both a gate of the JFET and the power input pin. A protection circuit monitors the voltage at the source to assert a protection signal, and a detector monitors the voltage at the power input pin to switch the JFET. 
     According to the present invention, a protection method for a power converter includes charging a power capacitor through a JFET at power on, turning off the JFET when the voltage at the power capacitor increases to reach a threshold, and monitoring the voltage at a source of the JFET after the JFET is turned off, to trigger a protection signal. 
     By using the high-voltage start-up device already present in the controller chip to implement protection functions for the power converter, there is no need of additional pins or external elements of the controller chip to protect the power converter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram of a conventional AC-to-DC power converter; 
         FIG. 2  is a circuit diagram of an embodiment according to the present invention; and 
         FIG. 3  shows a characteristic curve of the threshold voltage-drain voltage of a JFET. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows an application of the present invention, in which a power converter uses a controller chip  20  that implements protection to the power converter with a different mechanism from that employed in prior arts. In the controller chip  20 , a junction field effect transistor (JFET)  24  is used as a high-voltage start-up device  22 , which has a drain connected to a high-voltage pin HV, a source connected to an anode of a diode Dh, and a gate receiving a voltage JG, a current-limiting resistor Rh is connected between a cathode of the diode Dh and the gate of the JFET  24 , the cathode of the diode Dh is also connected to a power input pin VDD, a switch ST is connected between the gate of the JFET  24  and ground, a detector  26  detects the voltage at the power input pin VDD to determine a detection signal S 1  to switch the switch ST, and a protection circuit  28  monitors the source voltage JS at the source of the JFET  24  to assert a protection signal. The protection circuit  28  includes a brownout detector  30  and an over-voltage detector  34 . The brownout detector  30  has a hysteresis comparator  32  to compare the source voltage JS with a brownout threshold Vref 1  to assert a brownout protection signal BNO to turn off the driver  18 . The over-voltage detector  34  has a hysteresis comparator  36  to compare the source voltage JS with an over-voltage threshold Vref 2  to assert an over-voltage protection signal OVP for the driver  18 . 
     Based on the characteristic of the JFET  24 , the threshold voltage Vth of the JFET  24  depends on the drain voltage VDC of the JFET  24 , as shown by the characteristic curve of  FIG. 3 . Referring to  FIGS. 2 and 3 , immediately after an AC voltage VAC is supplied to the power converter at power on, the gate voltage JG of the JFET  24  will be equal to or substantially equal to the source voltage JS of the JFET  24  and thus the JFET  24  is turned on, thereby allowing a current flowing through the JFET  24 , the diode Dh and the power input pin VDD to charge the power capacitor C 1 . The detector  26  detects the voltage VDD at the power capacitor C 1 , for example by using a comparator  38  to compare the voltage VDD with a threshold UVLO. When the voltage VDD increases to reach the threshold UVLO, the detector  26  triggers the detection signal S 1  to turn on the switch ST and thereby pull the gate voltage JG of the JFET  24  to 0V. As a result, the JFET  24  is turned off, i.e., no longer conducts a current therethrough, to save power. At this moment, the voltage VDD at the power capacitor C 1  is sufficiently high to start up the controller chip  20 , so the driver  18  is turned on to provide a control signal Vgate to switch the power switch Q 1  and thereby start up the auxiliary coil Laux to supply electric power to the controller chip  20 . Thus, the start-up process of the power converter is completed. 
     After the power converter is started up, the gate voltage JG of the JFET  24  is 0V and the voltage at the power input pin VDD is higher than the threshold UVLO, such that the source voltage JS of the JFET  24  is locked at the threshold voltage Vth of the JFET  24 , as shown in  FIG. 3 . Since the threshold voltage Vth of the JFET  24  varies with the DC voltage VDC, the brownout detector  30  can identify whether or not the DC voltage VDC is excessively low by detecting the source voltage JS of the JFET  24 . For instance, assuming that the brown-in level is 114V and the brownout level is 100V, it can be known from  FIG. 3  that the source voltage JS is approximately 1.2V when the voltage VDC is 114V and the source voltage JS is approximately 1.1V when the voltage VDC is 100V. Therefore, if the hysteresis comparator  32  in the brownout detector  30  detects that the source voltage JS is higher than 1.2V, the driver  18  will work normally; however, if the hysteresis comparator  32  detects that the source voltage JS is lower than 1.1V, the brownout protection signal BNO will be triggered to turn off the driver  18 . Similarly, the over-voltage detector  34  can identify whether or not the voltage VDC is excessively high by detecting the source voltage JS. Assuming that the over-voltage level is set at 400V, it can be known from  FIG. 3  that the source voltage JS is approximately 4.4V when the voltage VDC is 400V. Hence, if the hysteresis comparator  36  in the over-voltage detector  34  detects that the source voltage JS is higher than 4.4V, the over-voltage protection signal OVP will be triggered to turn off the driver  18 . 
     In this embodiment, brownout protection and over-voltage protection are illustrated by way of example only; it is also feasible to provide other types of protection to the power converter, such as overload protection compensation by detecting the source voltage JS of the JFET  24 . In this embodiment, the controller chip  20  provides protection by monitoring the DC voltage VDC via the high-voltage pin HV; in other embodiments, it may do it by monitoring the AC voltage VAC or the drain voltage of the power switch Q 1 . 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.