Abstract:
A power control circuit includes a switching regulator for converting voltage of external electric power into a medium voltage, a series regulator for converting the medium voltage to a prescribed voltage of output power to be supplied and a circuit for operating either the switching regulator or the series regulator under a protective condition if overvoltage or overcurrent is detected. The switching regulator includes a first transistor, a smoothing circuit and a first control circuit for controlling switching operation of the first transistor. The series regulator includes a second transistor and a second control circuit for controlling conductivity of the second transistor to regulate voltage of the output power to be constant, an overvoltage detecting circuit for detecting an overvoltage of the medium voltage, and an overcurrent detecting circuit for detecting an overcurrent supplied to the series regulator.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is based on and claims priority from Japanese Patent Application 2002-307431, filed Oct. 22, 2002, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a power control circuit of an on-vehicle electronic controller and, particularly, a power control circuit composed of a switching regulator and a series regulator. 
   2. Description of the Related Art 
   A power control circuit that is constituted of a switching regulator or a series regulator has been known, and disclosed in patents, such as JP-A-9-37545. In such a power control circuit, voltage of an external power source is reduced to supply stable electric power at a desired voltage. 
   The switching regulator includes a switching transistor which is connected in series with a power source, and switches the switching transistor on and off to supply necessary power to a smoothing circuit. This switching regulator can operate at low and favorable power consumption but at low and unfavorable control accuracy. 
   On the other hand, the series regulator includes a power transistor which is connected in series with a power source to control the conductivity of the power transistor so that the transistor output voltage to be supplied to a smoothing circuit can be regulated. This series regulator can operate at high and favorable accuracy but at high and unfavorable power consumption. 
   JP-A-6-335238 discloses a composite control circuit in which the switching regulator and the series regulator are connected in series. The switching regulator reduces voltage of an external power source to a medium voltage that is slightly higher than a desired voltage at low power consumption, and the series regulator reduces the medium voltage to a desired voltage at high accuracy. 
   However, if an excessive amount of current or overcurrent flows in the series regulator by accident, the switching regulator of the above composite control circuit may not stably operate without a protection circuit, which is expensive and bulky. 
   SUMMARY OF THE INVENTION 
   A main object of the invention is to provide an improved power control circuit which can solve the above-described problem. 
   Another object of the invention is to provide an improved power control circuit composed of a switching regulator and a series regulator which is resistive to overcurrent, overvoltage and also overheating. 
   According to a main feature of the invention, a power control circuit includes a switching regulator for converting voltage of external electric power into a medium voltage, a series regulator for converting the medium voltage to a prescribed voltage of output power to be supplied, and protecting means for operating one of the switching regulator and the series regulator under a protective condition if one of the overvoltage and overcurrent is detected. The switching regulator includes a first transistor, a smoothing circuit connected to the first transistor and a first control circuit for controlling switching operation of the first transistor. The series regulator includes a second transistor and a second control circuit for controlling conductivity of the second transistor to regulate voltage of the output power to be constant, an overvoltage detecting circuit for detecting an overvoltage of the medium voltage, and an overcurrent detecting circuit for detecting an overcurrent supplied to the series regulator. Because the overvoltage detecting circuit detects the medium voltage, surge pulses coming from outside are absorbed by the smoothing circuit of the switching regulator and may not badly affect the protecting means or operation of the switching regulator or the series regulator. 
   In the above power control circuit, the protecting means turns off the first transistor if the overvoltage detecting circuit detects voltage higher than a prescribed threshold level. Preferably, the protecting means turns off either the second transistor if the overvoltage detecting circuit detects voltage higher than a prescribed threshold level, or the first transistor if the overcurrent detecting circuit detects current larger than a prescribed threshold level. The protecting means may turn off the second transistor if the overcurrent detecting circuit detects current larger than a prescribed threshold level. 
   The power control circuit may further include a feedback circuit connected between the overvoltage detecting circuit and the first control circuit, in which the overvoltage detecting circuit includes a voltage dividing circuit for dividing an output voltage of the switching regulator, and connected to the feedback circuit. 
   The power control circuit may further comprises an overheat detecting circuit for detecting overheating of the first transistor, in which the protecting means operates either the switching regulator, or the series regulator under a protective condition if the overheat detecting circuit detects overheating of the first transistor. The protecting means may operates the first control circuit to turn off the first transistor if the overheat detecting circuit detects the temperature of the first transistor to be higher than a threshold temperature level. The protecting means may operate the second control circuit to turn off the second transistor if the overheat detecting circuit detects the temperature of the first transistor to be higher than a threshold temperature level. The overheat detecting circuit may include a current source and a diode which is disposed near the first transistor and connected to the current source, in which the overheating of the first transistor is detected if the forward voltage of the diode becomes higher than a threshold voltage level. 

   
     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 power control circuit according to a preferred embodiment of the invention; 
       FIG. 2  is a block diagram of an on-vehicle electronic control apparatus; 
       FIGS. 3A and 3B  are time charts showing current and voltage waveforms of the power control circuit when abnormal current flows in the power control circuit; and 
       FIGS. 4A ,  4 B,  4 C,  4 D,  4 E and  4 F are voltage and current waveforms at various portions of the power control circuit in protecting operation when abnormal voltage is applied and half-short-circuit current flows in the power control circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An electronic control apparatus that includes a power control circuit according to a preferred embodiment, which is shown in  FIG. 1 , will be described with reference to  FIG. 2 . 
   As shown in  FIG. 2 , the electronic control apparatus  50  is constituted of the power control circuit  1 , an input circuit  51 , a drive circuit unit  53  and a microcomputer  55 . The input circuit  51  is connected to various sensors and includes a filter and/or an A/D converter to take in various signals from the sensors. The drive circuit unit  53  includes a plurality of drive circuits Dr 1 –Drn which are respectively connected to outside electronic devices LD 1 –LDn. The microcomputer  55  is connected between the input circuit  51  and the drive circuit unit  53  to control the drive circuits Dr 1 –Drn according to the signals from the sensors. The power control circuit  1  is powered by an outside power source G to supply the input circuit  51 , the drive circuit unit  53  and the microcomputer  55  with electric power of prescribed voltage. 
   The voltage (e.g. 12 V) of the electric power supplied from the outside power source G is much higher than the output voltage of the power control circuit  1  so that stable electric power of a constant voltage (e.g. 5 V) can be supplied to the above-described circuits  51 ,  53 ,  55 . 
   As shown in  FIG. 1 , the power control circuit  1  includes a lowpass filter  2 , a MOSFET  4 , a smoothing circuit  5 , a bipolar transistor  7 , an overheat detecting circuit  8 , a current detecting circuit  12 , an overvoltage detecting circuit  13 , an SW-reg control circuit  14  and an SE-reg control circuit  15 . The FET  4 , the overheat detecting circuit  8 , the voltage dividing circuit  11 , the overvoltage detecting circuit  13 , the SW-reg control circuit  14 , the SE-reg control circuit  15  are integrally formed on a regulator IC  10 . 
   The lowpass filter  2  is constituted of a coil L 1  and a capacitor C 1  and removes high frequency noises flowing from the outside power source. The smoothing circuit  5  includes a diode D, a coil L 2 , a capacitor C 2  and smoothes the output voltage of the FET  4 . The overheat detecting circuit  8  includes a constant current circuit and a temperature sensing diode which is disposed near the FET  4 . The overheat detecting circuit  8  detects the forward voltage of the temperature sensing diode to thereby detect overheating of the FET  4 . The current detecting circuit  12  includes a current detecting resistor  6  and an operational amplifier and detects the voltage across the current detecting resistor  6  and current  11  flowing through the same. The overvoltage detecting circuit  13  includes a voltage dividing circuit  11  and a comparator which compares the voltage divided by the voltage dividing circuit  11  with a reference voltage Vref to detect an overvoltage. The voltage dividing circuit  11  divides the output voltage V 3  of the smoothing circuit  5 . The SW-reg control circuit  14  controls the duty ratio of the gate voltage of the FET  4  so that the voltage divided by the voltage dividing circuit  11  can be kept constant. The SW-reg control circuit  14  also controls the gate voltage of the FET  4  to turn off when the overheat detecting circuit  8  detects overheating of the FET  4 . The SW-reg control circuit  14  forms a switching regulator together with the FET  4  and the smoothing circuit  5 . The SE-reg control circuit  15  controls the base current of the transistor  7  so that the output voltage of the transistor  7  can be kept constant. The SE-reg control circuit  15  also controls the base current of the transistor  7  to control the output current when the current detecting circuit  12  detects overcurrent and to turn off the transistor  7  when the overvoltage detecting circuit  13  detects overvoltage. The SE-ref control circuit forms a series regulator together with the transistor  7 . 
   In the above-described power control circuit  1 , high frequency noises of input power are removed by the lowpass filter  2 , and medium voltage V 3 , which is lower than the input voltage, is provided by the switching regulator that is constituted of the FET  4 , the smoothing circuit  5  and the SW-reg control circuit  14 . The medium voltage V 3  is further reduced by the series regulator that is constituted of the transistor  7  and the SE-reg control circuit  15  to provide supply voltage V 4 . 
   If overcurrent or a big surge pulse flows in the power control circuit  1  due to a short-circuit or some other accident is detected by the current detecting circuit  12  at time t 1 , the SE-reg control circuit  15  reduces the base current of the transistor  7 , as shown in  FIGS. 3A and 3B . As a result, the supply voltage V 4  of the transistor  7  under control is lowered to control the output current  11 . 
   After the overcurrent is removed at time t 2 , the SE-reg control circuit  15  returns to its normal control operation. 
   If an overvoltage is applied to the power control circuit  1  through a power line at time t 11  and the output voltage V 3  of the switching regulator circuit is boosted up as shown in  FIGS. 4A ,  4 B and  4 C, the overvoltage detecting circuit  13  sends a signal to the SE-reg control circuit  15 , which cuts off the base current of the transistor  7 . 
   Thereafter, the SW-reg control circuit  14  controls the gate voltage of the FET  4 . If the output voltage V 3  of the switching regulator circuit lowers and the output voltage of the voltage dividing circuit  11  becomes lower than the reference voltage Vref of the overvoltage detecting circuit  13 , the SE-reg control circuit  15  returns to its normal driving operation to keep the supply voltage V 4  constant, as shown in  FIG. 4D . Incidentally, the reference voltage Vref can be changed by providing the comparator of the overvoltage detecting circuit  13  with a hysteresis to detect whether the output voltage V 3  returns to normal or not. It is also possible to detect the recovery by measuring a recovering time which is set beforehand. The surge pulses except very big surge pulses are usually removed when it passes through the filter  2 , the FET  4  and the smoothing circuit  5 . 
   If a half shortcircuit accident takes place in the power line and an abnormal amount of current that is not so large as the amount to be detected by the current detecting circuit  12  flows for a certain period of time as shown in  FIG. 4E , the FET  4  is heated and the temperature of the FET  4  rises to a highest allowable level (overheat level). The SW-reg circuit  14  turns off the FET  4  at time t 12  if the temperature detecting circuit  8  detects such a temperature rise of the FET  4 , as shown in  FIG. 4F . 
   Thereafter, the normal on-off control operation of the SW-reg control circuit  14  resumes when the temperature of the FET  4  lowers to an allowable level or after a time period for the FET  4  to cool down passes, at time t 13 . 
   The FET  4  and the transistor  7  can be respectively controlled by the SW-reg circuit  14  and the SE-reg circuit  15  according to various signals from portions of the power control circuit  1 , as shown in dotted lines in  FIG. 1 . 
   The overheat detecting circuit  8  may include an overheat determining circuit for directly controlling the FET  4 . The FET  4  and the overheat detecting circuit  8  can be separated from the regulator IC  10  and integrated together into a chip  20 , which is encircled by a dotted line in  FIG. 1 . The regulator IC  10  may also include the bipolar transistor  7 . The regulator IC  10  may include a plurality of transistors  7  and a corresponding number of SE-reg circuits  15  that is jointly controlled by the current detecting circuit  12  and the overvoltage detecting circuit  13 . In this case, each composite circuit including the transistor  7  and the overheating circuit  15  is connected to the output side of the current detecting resistor  6 . 
   The regulator IC  10  may also include a plurality of the current detecting circuits  12  and the corresponding transistors  7  connected thereto, and a corresponding number of SW-reg circuits  14  that is jointly controlled by the overvoltage detecting circuit  13 . In this case, each the current detecting resistor  6  is series-connected to the output side of the smoothing circuit  5 . 
   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.