Abstract:
A voltage regulator of a vehicle AC generator includes a switching element connected between a battery and a field coil, a control circuit for controlling the switching element according to battery voltage, a power circuit for supplying the control circuit with a constant voltage and a reverse-current blocking diode having an anode connected through an outside power line to the battery and a cathode connected to the power circuit. Even if a large negative surge voltage is generated in a power line connected to a battery, and applied to the power circuit, the output voltage, i.e. V DD , does not widely fluctuate, so that devices included in the voltage regulator, such as a comparator or an oscillator, can operate properly.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority from Japanese Patent Application 2000-238627, filed Aug. 7, 2000, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage regulator of a vehicle AC generator and, particularly. a voltage regulator having a semiconductor switch element for controlling field current to be supplied to a field coil of a vehicle AC generator. 
     2. Description of the Related Art 
     A recent voltage regulator of a vehicle AC generator is provided with various electronic circuits, such as a comparator, an operational amplifier, an oscillating circuit and/or the like, disposed in an IC chip. A constant voltage power circuit is also provided in order to supply constant voltage power to such circuits. 
     On the other hand, a number of electro-magnetic actuators have been mounted in a vehicle. If two or more electro-magnetic actuators are turned off concurrently, a large negative surge voltage, such as shown in FIG. 7, is generated in a power line connected to a battery, and the negative surge voltage may be applied to a high-side terminal of the constant voltage circuit. In such a case, the output voltage, i.e. V DD , widely fluctuates, resulting in that the comparator and the oscillator can not operate properly. 
     SUMMARY OF THE INVENTION 
     Therefore, a main object of the invention is to provide an improved voltage regulator that is free from the above stated problems. 
     According to a main feature of the invention a voltage regulator of a vehicle AC generator includes a switching element connected between a battery and a field coil, a control circuit for controlling the switching element according to terminal voltage of the battery, a power circuit for providing a constant voltage from power supplied thereto and supplying the control circuit with the constant voltage and a reverse-current blocking diode having an anode connected through an outside power line to the battery and a cathode connected to the power circuit. Even if a large negative surge voltage is generated in a power line connected to a battery and applied to the power circuit, the output voltage, i.e. V DD , does not widely fluctuate, so that devices included in the voltage regulator, such as a comparator or an oscillator, can operate properly. 
     Preferably, a portion of the control circuit, the power circuit and the reverse-current blocking diode are integrated into an IC chip and separated by insulation layers. However, the reverse-current blocking diode may be formed at a portion separate from the IC chip. The IC chip and the portion at which the reverse-current blocking diode is formed may be disposed in a hybrid IC unit. 
     It is also preferable that the reverse-current blocking diode is fixed to a first conductive support plate and the power circuit is fixed to a second conductive support plate, and that 
     the first and second conductive support plates are thermally and electrically insulated from each other. 
     Further, each of the first and second support plates may be comprised of one of leads of a lead frame, the reverse-current blocking diode may have a cathode electrode connected to the power circuit by a bonding wire, and the reverse-current blocking diode, the power circuit and the bonding wires are molded together with resin. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a circuit diagram of a voltage regulator of a vehicle AC generator according to a preferred embodiment of the invention; 
     FIG. 2 is a schematic cross-sectional view of a control circuit shown in FIG. 1; 
     FIG. 3 is a circuit diagram of a variation of a power circuit shown in FIG. 1; 
     FIG. 4 is a plan view of a variation of the control circuit shown in FIG. 1; 
     FIG. 5 is a schematic cross-sectional side view cut along line A—A in FIG. 4; 
     FIG. 6 is an enlarged schematic cross-sectional view of a reverse-current blocking diode; and 
     FIG. 7 is a wave form of a negative surge voltage. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A voltage regulator of a vehicle AC generator according to a preferred embodiment of the invention is described with reference to FIGS. 1 and 2. 
     In FIG. 1, an AC generator includes a stator having a stator coil  1  and a rotor having a field coil  2 . When the rotor rotates, AC power is generated in the stator coil  1 . The AC power is rectified by a rectifier circuit  3  so that DC power is supplied to a battery  4  and an electric load  6 . Field current to be supplied to the field coil  2  is controlled by a control circuit  8 . 
     The control circuit  8  has a switching element  11  for controlling the field current supplied to the field coil  2 . The switching element  11  is a source-follower type N-channel MOSFET that has a base and a gate. The switching element  11  controls the field current according to the voltage level applied to the base thereof Reference numeral  12  is a flywheel diode connected in parallel with the field coil  2 . The gate of the switching element  11  is charged by a charge pump circuit  14  until the voltage level of the gate becomes as high as a prescribed level and is discharged by a gate-discharging transistor  17 . 
     Terminal voltage of the battery  4  is applied to the control circuit  8  via a battery connection terminal S and is divided by resistors  20  and  21 . The divided voltage is applied to a low-side terminal of a comparator  19 . A capacitor  22  is connected to the low-side terminal of the comparator  19  to bypass ripple components of the battery voltage. 
     The divided voltage is compared by the comparator  19  with a reference voltage Vr. If the battery voltage is lower than a prescribed voltage level, the comparator  19  provides an AND circuit  16  and an inverter  18  with a Hi-level signal. Consequently, the inverter  18  turns off the gate-discharging transistor  17  to stop discharging the electric charge from the gate of the switching element  11 . At the same time, the AND circuit  16  applies an output signal of an oscillation circuit  15  to drive the charge-pump circuit  14 , which charges the gate of the switching element  11  to boost the voltage of the gate to a prescribed voltage level, thereby, to turn on the switching element  11 . As a result, the field current is increased, and the output power of the AC generator is increased. 
     If the battery voltage is higher than the prescribed voltage level, the charge-pump circuit  14  stops charging the gate of the switching element  11 . At the same time, the inverter  18  turns on the gate-discharging transistor  17  to turn off the switching element  11 . As a result the field current is reduced and the output power of the AC generator is reduced. Thus, the battery voltage is controlled at a level decided by the reference voltage and the voltage divided by the resistors  20  and  21 . 
     Operation of the voltage regulator is described below. 
     Even if a key switch  5  is turned on while the engine stops the AC voltage is not generated as the AC generator. Therefore, a generation detecting circuit  31 , which detects a phase voltage of a phase-coil, turns on a switching element  30  to drive a warning lamp  40 . If the engine starts and the AC generator starts generation, the generation detecting circuit  31  detects the phase voltage and turns off the switching element  30 , thereby turning off the warning lamp  40 . The warning lamp  40  can be driven by another signal applied to one of other devices. 
     A power circuit  10  is a series circuit of a constant voltage diode  101  and a current limiting element or resistor  102 . The power circuit  10  is energized by the battery  4  via the diode  9  and the key switch  5  to supply various portions of the control circuit  8  with the constant voltage V DD . 
     If the electric load  6  is an inductive load, the electric load  6  generates a negative surge voltage which temporarily lowers the potential of the IG terminal. The reverse-current blocking diode  9  prevents the negative surge voltage from being applied to the power circuit  10 . 
     The reverse-current blocking diode  9  is formed at an electrical isolation type integrated circuit that is comprised of other circuits including the current-limiting resistor  102 . The reverse-current blocking diode  9  and other circuits are separated by insulation layers. Reference numeral  200  indicates a low-density P-type base plate, reference numerals  201 ,  202  indicate oxidized layers forming a plurality of insulated N-type island regions on the base plate  200 . High-density N-type layers are respectively formed at the bottom of the island regions. Low-density N-type epitaxial embedded layers are also formed on the high-density N-type layers. P-type anode regions  205 , P-type resistor regions  206 , high-density P-type contact region  207  and  208 , and high-density N-type contact regions  209  are also formed one after another, as shown in FIG.  2 . The reverse current-blocking diode  9  is formed at the right island region, and the current-limiting resistor  102  is formed at the left island region in FIG.  2 . Thus, the reverse-current blocking diode  9  and other circuits are integrated into one chip, so that the voltage regulator can be made compact. 
     A variation of the power circuit  10  is described with reference to FIG. 3, in which reference numeral  105  represents a parallel capacitor, reference numeral  103  represents an emmitter-follower transistor and reference numeral  104  is a collector resistor. The parallel capacitor  105  controls fluctuation of the potential of an internal power line  300 . 
     A variation of the control circuit  8  is described with reference to FIGS. 4-6. 
     The reverse-current blocking diode  9  is fixed to a lead  510  of a lead frame. As shown in FIG.  6 . the reverse-current blocking diode  9  is formed on a P-type base plate  91  to provide a PN junction by N-type diffusion. 
     Reference numeral  50  is a heat sink, reference numeral  52  indicates a bonding wire, numeral  53  indicates a mold member, numeral  54  is a conductive member, numeral  105  is a chip capacitor, and numerals  510 - 517  respectively indicate leads. Reference numeral  80  is an IC chip on which the control circuit is formed, and numeral  10   a  is a portion on which the power circuit  10  is formed. The IC chip  80  is fixed to the heat sink. The reverse-current blocking diode  9  is comprised of a P-type base plate  91 , an N-type region  92  formed on the base plate  91  and a metal electrode  93  fixed to the N-type region  92 . The chip of the reverse-current blocking diode  9  is fixed to the lead  510 . 
     The metal electrode  93  is made of the same material as the bonding wires  52  to prevent the metal electrode  93  from chemically combining with the bonding wires  52 . It is also preferable to use the same material for members connecting the IC chip  80  with the heat sink as the material for the conductive member  54 . The chip capacitor used for the parallel capacitor  105  is disposed between a lead  516  and a lead  517 . The member for connecting the capacitor  105  is made of the same material as the connecting member for the reverse-current blocking diode  9 . However, it is not necessary to concurrently fix the capacitor  105  when the reverse-current blocking diode  9  is fixed. This arrangement prevents deterioration due to metal junction. If the voltage regulator is abnormally heated by the AC generator by accident, all the connecting members may be melted and disconnect the elements of the control circuit  8 . Therefore, current is not supplied to the power circuit of the regulator, so that a highly safe voltage regulator can be provided. 
     All the above members are molded with the thermally non-conductive mold member  53 . Therefore, a compact voltage regulator can be provided, and the reverse-current blocking diode  9  is thermally isolated from other elements of the control circuit that includes the switching element  11 . The switching element  11  can be formed at a chip separated from the chip of the control circuit  8 . Even if the switching element  11  is heated when passing the field current, the reverse-current blocking diode  9  is not heated, so that leak current of the reverse-current blocking diode  9  can be limited at a low level. 
     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.