Abstract:
A discharge circuit for a power supply unit includes a pulse width modulator (PWM) chip, a first and second electronic switch, and a resistor. The first electronic switch receives a power good signal from the power supply unit. When a system power terminal outputs a voltage later than a stand-by power terminal, a voltage creep outputted by the PWM chip is discharged through the resistor and the second electronic switch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Relevant subject matter is disclosed in one co-pending U.S. patent applications (Attorney Docket Nos. US47539) having the same titles, which are assigned to the same assignees as this patent application. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to a discharge circuit. 
         [0004]    2. Description of Related Art 
         [0005]    During a power-on operation of a computer, a voltage creep may be generated when a system voltage is output later than a stand-by voltage. A resistor may be used to discharge the voltage creep. However, the resistor is still working when the computer is powered on, which wastes energy. 
         [0006]    Therefore, there is room for improvement in the art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Many aspects of the present disclosure can be better understood with reference to the following drawing(s). The components in the drawing(s) are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawing(s), like reference numerals designate corresponding parts throughout the several views. 
           [0008]      FIG. 1  is a circuit diagram of a first embodiment of a discharge circuit. 
           [0009]      FIG. 2  is a circuit diagram of a second embodiment of the discharge circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean “at least one.” 
         [0011]      FIG. 1  illustrates a first embodiment of a discharge circuit. The discharge circuit comprises a power supply unit  30 , a pulse width modulator (PWM) chip U 1 , four capacitors C 1 -C 4 , seven resistors R 1 -R 7 , and four transistors Q 1 -Q 4 . 
         [0012]    A gate G of the transistor Q 3  is coupled to an enable pin En and a control pin PG of the pulse width modulator (PWM) chip U 1 , to receive a control signal from the power supply unit  30 . The gate G of the transistor Q 3  is coupled to a system power terminal Vcc through the resistor R 3 , and is connected to ground through the resistor R 4 . A source S of the transistor Q 3  is connected to ground. A drain D of the transistor Q 3  is coupled to a stand-by power terminal 5VSB through the resistor R 5 , and is also coupled to a gate G of the transistor Q 4 . A source S of the transistor Q 4  is connected to ground. A drain D of the transistor Q 4  is coupled to a first terminal of an inductance L 1  through the resistors R 6  and R 7  connected parallel. A second terminal of the inductance L 1  is coupled to a phase output pin Phase of the PWM chip U 1 . 
         [0013]    A voltage pin Vc of the PWM chip U 1  is coupled to the system power terminal Vcc. The enable pin En is used to receive an enable signal Enable. A ground pin GND of the PWM chip U 1  is connected to ground. A high gate output pin Hgate of the PWM chip U 1  is coupled to a gate G of a transistor Q 1 . A drain D of the transistor Q 1  is coupled to a power terminal VIN. The power terminal VIN is connected to ground through capacitors C 1  and C 2  connected parallel. A source S of the transistor Q 1  is coupled to the phase output pin Phase. A low gate output pin Lgate is coupled to a gate G of a transistor Q 2 . A drain D of transistor Q 2  is coupled to the phase output pin Phase. A source S of the transistor Q 2  is connected to ground. A voltage output pin FB of the PWM chip U 1  is connected to ground through a resistor R 2 , and is coupled to the first terminal of the inductance L 1 . The first terminal of the inductance L 1  is connected to ground through capacitors C 3  and C 4  connected parallel. The control pin of the PWM chip U 1  is used to receive a power good signal (PWR_GD) from the power supply unit  30 . 
         [0014]    In the embodiment, the gate G of the transistor Q 3  receives the enable signal and the power good signal. During a power-on operation, the power good signal is at low-voltage level, such as logic 0, and the enable signal is at low-voltage level, if the system power terminal VCC is not outputting a system voltage, while the stand-by power terminal 5VSB outputs a stand-by voltage, the PWM chip U 1  outputs a voltage creep through the phase output pin Phase. The gate G of the transistor Q 3  receives the low-voltage level control signal, and the transistor Q 3  is turned off, and the gate G of the transistor Q 4  is at high-voltage level. Accordingly, the transistor Q 4  is turned on, and the voltage creep is discharged through the resistors R 6 , R 7  and the transistor Q 4 . 
         [0015]    When the system power terminal Vcc outputs the system voltage, the power supply unit  20  outputs a high-voltage level power good signal, such as logic 1, and the enable signal is also at high-voltage level. Accordingly, the gate G of the transistor Q 3  is at high-voltage level, and the transistor Q 3  is turned on, and the gate G of the transistor Q 3  is at low-voltage level, such as logic 0, the transistor Q 4  is turned off Accordingly, the resistors R 6  and R 7  are no longer consuming power. 
         [0016]    In other embodiments, only one of the power good signal or the enable signal is received by the gate G of the transistor Q 3 . 
         [0017]      FIG. 2  illustrates a second embodiment of the discharge circuit. In comparison to the first embodiment, the discharge circuit further comprises a delay unit  100  between transistors Q 5  and Q 6 . The delay unit  100  comprises a capacitor C 5 , a resistor R 8 , and a diode D 1 . A drain D of the transistor Q 5  is connected to ground through the resistor R 8  and the capacitor C 5  in that order. The drain D of the transistor Q 5  is coupled to a cathode of the diode D 1 . An anode of the diode D 1  is coupled to a gate G of the transistor Q 6 , and coupled to a node between the resistor R 8  and the capacitor C 5 . 
         [0018]    During a power-on operation, the power good signal is at low-voltage level, and the enable signal is at low-voltage level, if the system power terminal VCC is not outputting a system voltage, while the stand-by power terminal 5VSB outputs a stand-by voltage, the PWM chip U 1  outputs a voltage creep through the phase output pin Phase. The gate G of the transistor Q 3  receives the low-voltage level control signal, and the transistor Q 3  is turned off, and the stand-by power terminal 5VSB charges the capacitor C 5 . After the capacitor C 5  is charged to a voltage that makes gate G of the transistor Q 4  be at high-voltage level, the transistor Q 4  is turned on, and the voltage creep is discharged through the resistors R 12 , R 13  and the transistor Q 6 . 
         [0019]    In the embodiment, the transistors Q 1 -Q 6  are n-channel metal oxide field effect transistors (NMOSFET). 
         [0020]    While the disclosure has been described by way of example and in terms of preferred embodiment, it is to be understood that the disclosure is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the range of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.