Abstract:
A drive circuit that supplies electric power to an electric load from a DC electric source includes a pair of series-connected first and second MOSFETS of the same conduction type, a pair of clamp circuits respectively connected between the drains and gates of the first and second MOSFETS, a series circuit of a first resistor and a switch, a first test terminal; a second test terminal connected to a joint of the first and second MOSFETS, a third test terminal for operating the switch, a fourth test terminal connected a joint of the first resistor and the switch; and a second resistor connected between the gate of the second MOSFET and the first test terminal. The switch and the first resistor are connected between the gate and the source of the first MOSFET to close when the drive circuit is normally operated and to open when it is given a high voltage test.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is based on and claims priority from Japanese Patent Application 2004-89197, filed Mar. 25, 2004, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a drive circuit that is connected between an electric power source and an electric load and a method of applying a high voltage test on the drive circuit.  
         [0004]     2. Description of the Related Art  
         [0005]     JP-A-2001-160748 or U.S. Pat. No. 6,392,463 B1, which is a counterpart of the former, discloses a drive circuit that includes series connected MOSFET type transistors (hereinafter referred to as MOSFET or MOSFETS) for high-side or low-side driving. The drive circuit includes a clamp circuit that protects the MOSFETS when a high voltage is applied to the drive circuit. Such a clamp circuit is a series circuit of a backflow preventing diode and a zener diode that is connected between the drain and the gate of an N-channel MOSFET for low-side driving.  
         [0006]     In order to provide a drive circuit with a fail safe function, the same MOSFETS may be connected in series for high-side driving or low-side driving. In this case, the drive circuit can properly drive an electric load connected to the drive circuit even if one of the transistors short-circuits. However, if the clamp circuit that is disclosed in the above publication is merely connected to this drive circuit, both the MOSFETS may simultaneously turn on when a high voltage is applied to the drain terminal of the MOSFETS even if the gate signal is not applied to the gate terminal. In other words, it is impossible to keep the MOSFETS turning off when a high voltage is applied to the drain terminal.  
       SUMMARY OF THE INVENTION  
       [0007]     In view of the above described problem, an object of the invention is to provide a drive circuit that will not simultaneously turn on when high voltage is applied thereto.  
         [0008]     According to a feature of the invention, a drive circuit for supplying electric power to an electric load from a DC electric source includes a pair of series-connected first and second MOSFETS of the same conduction type, a pair of clamp circuits respectively connected between the drains and gates of the first and second MOSFETS, a series circuit of a first resistor and a switch connected between the gate and the source of the first MOSFET, a first test terminal, a second test terminal connected to a joint of the first and second MOSFETS, a third test terminal for operating the switch, a fourth test terminal connected to a joint of the first resistor and the switch and a second resistor connected between the gate of the second MOSFET and the first test terminal.  
         [0009]     When the drive circuit is normally operated, the switch is closed so that the first and the second MOSFETS will not simultaneously turn on until the gate signal becomes twice as high as a voltage level for the clamp circuit to start clamping.  
         [0010]     In the above drive circuit the first and second MOSFETS may be N-channel or P-channel MOSFETS. The switch may be a semiconductor having a control terminal connected to the third test terminal and a resistor connected between the control terminal and the source of the first MOSFET.  
         [0011]     If the first and second MOSFETS are N-channel MOSFETS, the switch may be a PNP transistor having a base terminal connected to the third test terminal and a resistor connected between the base terminal and the source of the first MOSFET. On the other hand, the switch may be an NPN transistor having a base terminal connected to the third test terminal and a resistor connected between the base terminal and the source of the first MOSFET, if the first and second MOSFETS are P-channel MOSFETS. When the drive circuit is operated, no signal is necessary to apply to the third test terminal.  
         [0012]     Another object of the invention is to provide an improved method of applying test voltage.  
         [0013]     According to another feature of the invention, a method of applying a high voltage test on the drive circuit that has N-channel MOSFETS includes a step of opening the switch, a step of grounding the drain of the first MOSFET, the second test terminal and the source of the second MOSFET, and a step of applying test voltage to the first and fourth test terminals.  
         [0014]     According to another feature of the invention, a method of applying a high voltage test on the drive circuit that has P-channel MOSFETS includes a step of opening the switch, a step of grounding the first and fourth test terminals and a step of applying test voltage to the drain of the first MOSFET, the second test terminal and the source of the second MOSFET.  
         [0015]     With one of the above methods, it is easy to apply test voltage between the gates and the sources of the pair of MOSFETS and between the gates and the drains thereof at the same time. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings:  
         [0017]      FIG. 1  is a circuit diagram illustrating a drive circuit according to the first embodiment of the invention;  
         [0018]      FIG. 2  is a circuit diagram illustrating a drive circuit according to the second embodiment of the invention;  
         [0019]      FIG. 3  is a circuit diagram illustrating a drive circuit according to the third embodiment of the invention;  
         [0020]      FIG. 4  is a circuit diagram illustrating a drive circuit according to the fourth embodiment of the invention;  
         [0021]      FIG. 5  is a circuit diagram illustrating a drive circuit according to the fifth embodiment of the invention;  
         [0022]      FIG. 6  is a circuit diagram illustrating a drive circuit according to the sixth embodiment of the invention;  
         [0023]      FIG. 7  is a circuit diagram illustrating a drive circuit according to the seventh embodiment of the invention;  
         [0024]      FIG. 8  is a circuit diagram illustrating a drive circuit according to the eighth embodiment of the invention;  
         [0025]      FIG. 9  is a circuit diagram illustrating a drive circuit according to the ninth embodiment of the invention; and  
         [0026]      FIG. 10  is a circuit diagram illustrating a drive circuit according to the tenth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     A drive circuit  21  according to the first embodiment of the invention will be described with reference to  FIG. 1 .  
         [0028]     The drive circuit  21  includes a pair of first N-channel MOSFETS  3  and a second N-channel MOSFETS  4  that are connected in series to each other and disposed between an electric load  2  and a ground GND, drivers  7 ,  8 , a pair of first and second clamp circuits  9 ,  10 , a first resistor  11 , a second resistor  12 , a switch  22 , four test terminals M 1 , M 2 , M 3 , M 4 , etc. The MOSFETS  3 ,  4  respectively have gates, drains and sources. The gates are respectively connected to the drivers  7 ,  8  so that the same logical gate signal is applied to turn on or off current supplied to the electric load  2  from a battery  1 . The drain of the first MOSFET  3  is connected to a low-side terminal of the electric load  2 , and the source of the second MOSFET  4  is connected to the ground. The other terminal (high-side terminal) of the electric load  1  is connected to the positive side (high-side) terminal of the battery  1 .  
         [0029]     The first clamp circuit  9  is connected between the drain and gate of the first MOSFETS  3 , and the second clamp circuit  10  is connected between the drain and gate of the second MOSFETS  4 . The first resistor  11  is connected between the gate of the first MOSFET  3  and the switch  22 . The resistor  12  is connected between the gate of the second MOSFETS  4  and the test terminal M 1 . The test terminal M 2  is connected to a joint of the first and second MOSFETS  3 ,  4 , and the test terminal M 3  is a terminal to operate the switch  22 . The test terminal M 4  is connected to a joint of the first resistor  11  and the switch  22  and to the test terminal M 2  via the switch  22 .  
         [0030]     In the case that the electric load  2  is a DC motor, the switch  22  is closed. In operation, a high level gate signal to render the gates of the first MOSFET  3  and the second MOSFET  4  to be higher in potential than the source is applied to the gates so that the first MOSFET  3  and the second MOSFET  4  can turn on.  
         [0031]     If a load dump surge voltage is generated at the positive terminal of the battery  1  and applied to the drain of the first MOSFET  3  due to loose connection or incomplete connection of the battery  1 , the MOSFETS  3 ,  4  do not turn on because the clamp circuits  9 ,  10  are connected in series via the resistor  11  and the switch  22  between the drain of the first MOSFET  3  and the gate of the second MOSFET  4 . That is, the second MOSFET  4  does not turn on because the gate signal thereof is much lower than a voltage level that is twice as high as the voltage level to start clamping of the clamp circuits  9 ,  10 .  
         [0032]     In the case of a screening or burning test, the switch  22  is opened. Then, a high voltage is applied to the test terminals M 1  and M 4  while the drain of the first MOSFET  3 , the test terminal M 2  and the source of the second MOSFET  4  are grounded. Therefore, it is possible to apply a high voltage to the oxidized film of the gates of the MOSFETS  3 ,  4 .  
         [0033]     Thus, the MOSFETS  3 ,  4  do not simultaneously turn on even when a high voltage is applied to MOSFETS  3 ,  4 .  
         [0034]     A drive circuit  24  according to the second embodiment of the invention will be described with reference to  FIG. 2 . Incidentally, the same reference numeral indicates the same or substantially the same part, portion or component as that described in the description of the precedent embodiment of the invention.  
         [0035]     The drive circuit  24  has a switch circuit that includes a PNP transistor  23  and a resistor  60  instead of the switch  22 . The test terminal M 3  is connected to the base of the transistor  23 , and the resistor  60  is connected between the base and the collector of the transistor  23 .  
         [0036]     The transistor  23  is turned off by applying a high level signal on the test terminal M 3  when a high voltage test is made, and turned on when the drive circuit  24  is operated without a signal on the test terminal M 3  to drive the electric load  2 .  
         [0037]     A drive circuit  27  according to the third embodiment of the invention will be described with reference to  FIG. 3 .  
         [0038]     The drive circuit  27  has the same structure as the second embodiment except for zener diodes  25 ,  26 , which are respectively connected in parallel with the resistors  11 ,  12 . The zener diodes  25 ,  26  protect the MOSFETS  3 ,  4  from excessively high voltage.  
         [0039]     A drive circuit  29  according to the fourth embodiment of the invention will be described with reference to  FIG. 4 .  
         [0040]     The drive circuit  27  has a P-channel MOSFET  28  instead of the transistor  23 . The operation is substantially the same as the third embodiment.  
         [0041]     A drive circuit  34  according to the fifth embodiment of the invention will be described with reference to  FIG. 5 .  
         [0042]     The drive circuit  27  has the same structure as the second embodiment except for the clamp circuits  9 ,  10 . The first clamp circuit  9  is composed of a backflow preventing diode  30  connected to the drain of the first MOSFET  3  in the direction to block current flowing from the gate to the drain thereof and a zener diode  31  connected in the other direction. The second clamp circuits  10  is composed of a backflow preventing diode  32  connected to the drain of the second MOSFET  4  in the direction to block current flowing from the gate to the drain thereof and a zener diode  33  connected in the other direction.  
         [0043]     A drive circuit  44  according to the sixth embodiment of the invention will be described with reference to  FIG. 6 .  
         [0044]     The drive circuit  44  includes a pair of series-connected P-channel MOSFETS  35 ,  36  between the battery  1  and the electric load  2 , the other side terminal of which is connected to the ground, drivers  42 ,  43 , a pair of clamp circuits  38 ,  41 , resistors  37 ,  39 , a switch  40 , test terminals M 1 , M 2 , M 3 , M 4 , etc. The gates of the MOSFETS  35 ,  36  are respectively connected to the drivers  42 ,  43  so that the same logical gate signal is applied to turn on or off current supplied to the electric load  2  from the battery  1 . The source of the MOSFET  35  is connected to the positive terminal of the battery, and the drain of the MOSFET  36  is connected to the high-side terminal of the electric load  2 . The other terminal (low-side terminal) of the electric load  1  is connected to the ground.  
         [0045]     The clamp circuit  38  is connected between the gate and drain of the MOSFETS  35 , and the clamp circuit  41  is connected between the gate and the drain of the MOSFETS  36 . The resistor  37  is connected between the gate of the MOSFET  35  and the test terminal M 1 . The resistor  39  is connected between the gate of the MOSFETS  36  and the switch  40 . The switch  40  is connected between the test terminal M 2 , which is connected to a joint of the series-connected MOSFETS  35 ,  36 , and the test terminal M 4 . The test terminal M 2  is connected to a joint of the first and second MOSFETS  3 ,  4 , and the test terminal M 3  is a terminal to operate the switch  22 . The test terminal M 4  is connected to a joint of the first resistor  39  and the switch  40  and to the test terminal M 2  via the switch  40 .  
         [0046]     In operation, a low level gate signal, which is lower than the source voltage, is applied to the gates of the MOSFET  35  and the MOSFET  36  to turn them on.  
         [0047]     If a big negative surge voltage is applied to the drain of the MOSFET  36 , the MOSFETS  35 ,  36  do not turn on because the clamp circuits  38 ,  41  are connected in series via the resistor  39  and the switch  40  between the gate of the MOSFET  35  and the drain of the MOSFET  36 . That is, the MOSFET  35  does not turn on because the gate signal thereof is much higher than a voltage level that is twice as low as the voltage level to start clamping of the clamp circuits  38 ,  41 .  
         [0048]     In the case of a screening or burning test, the switch  40  is opened. Then, a high voltage is applied to the test terminals M 1  and M 4  while the drain of the MOSFET  35 , the test terminal M 2  and the source of the MOSFET  36  are grounded. Therefore, it is possible to apply a high voltage to the oxidized film of the gates of the MOSFETS  35 ,  36 .  
         [0049]     Thus, the MOSFETS  35 ,  36  do not simultaneously turn on even when an excessive voltage is applied to MOSFETS  35 ,  36 .  
         [0050]     A drive circuit  47  according to the seventh embodiment of the invention will be described with reference to  FIG. 7 .  
         [0051]     The drive circuit  24  has a switch circuit that includes an NPN transistor  45  and a resistor  46  instead of the switch  40  of the sixth embodiment. The test terminal M 3  is connected to the base of the transistor  45 , and the resistor  46  is connected between the base and the collector of the transistor  45 .  
         [0052]     The transistor  45  is turned off by applying a low level signal on the test terminal M 3  when a high voltage test is made, and turned on when the drive circuit  24  is operated without a signal on the test terminal M 3  to drive the electric load  2 .  
         [0053]     A drive circuit  50  according to the eighth embodiment of the invention will be described with reference to  FIG. 8 .  
         [0054]     The drive circuit  50  has the same structure as the seventh embodiment except for zener diodes  48 ,  49 , which are respectively connected in parallel with the resistors  37 ,  38 . The zener diodes  37 ,  38  protects the MOSFETS  3 ,  4  from excessively high voltage.  
         [0055]     A drive circuit  52  according to the ninth embodiment of the invention will be described with reference to  FIG. 9 .  
         [0056]     The drive circuit  52  has an N-channel MOSFET  28  instead of the transistor  45  of the eighth embodiment. The operation is substantially the same as the eighth embodiment.  
         [0057]     A drive circuit  57  according to the tenth embodiment of the invention will be described with reference to  FIG. 10 .  
         [0058]     The drive circuit  57  has the same structure as the eighth embodiment except for the clamp circuits  38 ,  41 . The clamp circuits  38 ,  41  is composed of a backflow preventing diode  53  connected to the gate of the MOSFET  35  in the direction to block current flowing from the drain to the gate thereof and a zener diode  54  connected in the other direction. The clamp circuits  41  is composed of a backflow preventing diode  55  connected to the gate of the MOSFET  36  in the direction to block current flowing from the drain to the gate thereof and a zener diode  56  connected in the other direction.  
         [0059]     Instead of the resistors  46 ,  60 , means for turning on or off the switch circuit may be used for the drive circuits described above.  
         [0060]     In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.