Abstract:
In a voltage booster circuit, a switching transistor turns on and off a flow of an electric current in a booster coil thereby to boost a voltage supplied form a d.c. power source. A back-up circuit is connected to the booster coil to store the boosted voltage as an output voltage. A current detection circuit detects an electric current flowing in the switching transistor and turns off the switching transistor, when the detected current reaches a predetermined level. An off-period control circuit variably controls an off-period of the switching transistor after the switching transistor is turned off. The off-period control circuit varies the off-period in accordance with a voltage difference between the output voltage and an upper limit threshold voltage. The off-period of the switching transistor is increased as the output voltage increases toward the upper limit threshold voltage

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based on and incorporates herein by reference Japanese Patent Application No. 2004-35519 filed on Feb. 12, 2004.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a voltage booster circuit and a voltage boosting method, which boosts a d.c. power source voltage and has a back-up function, and a passenger protection system including the voltage booster circuit and mounted on a vehicle.  
       BACKGROUND OF THE INVENTION  
       [0003]     In recent vehicles a passenger protection system such as an airbag or a seat belt pretensioner is used. In case of the airbag system, an inflator is activated to inflate an airbag when an acceleration sensor detects a vehicle collision. Since the voltage of a storage battery of the vehicle is not sufficient to activate the inflator by its associated circuits and devices, a voltage booster circuit is used to supply a higher voltage to the circuits and devices. The voltage booster circuit is typically a DC-DC converter, which boosts a battery voltage, and has a back-up circuit to supply sufficient electric power to activate the inflator even when the electric connection with the battery is disconnected due to a collision.  
         [0004]     JP 2001-178118A discloses one exemplary airbag system. In this system, a DC-DC converter circuit has a switching transistor, a booster coil and the like. The switching transistor is turned on and off at a fixed frequency, specifically with a fixed on-period and a fixed off-period, to boost a battery voltage and store the boosted voltage in a capacitor of the back-up circuit. This DC-DC converter circuit, however, needs a long time to charge the capacitor. Further, the fixed frequency to turn on and off the switching transistor is set to about several tens of kHz to several hundreds of kHz to ensure switching operation of the switching transistor even under a low battery voltage condition. This switching operation of the transistor at the high frequency tends to generate noises.  
       SUMMARY OF THE INVENTION  
       [0005]     It is therefore an object of the present invention to provide a voltage booster circuit and a passenger protection system using the voltage booster circuit, which quickly charges a back-up circuit and generates less noises when a switching transistor for a voltage boosting operation turns on and off.  
         [0006]     It is another object of the present invention to provide a voltage boosting method.  
         [0007]     According to the present invention, a switching transistor turns on and off a flow of an electric current in a booster coil thereby to boost a voltage supplied form a d.c. power source. A back-up circuit is connected to the booster coil to store the boosted voltage as an output voltage. A current detection circuit detects an electric current flowing in the switching transistor and turns off the switching transistor, when the detected current reaches a predetermined level. An off-period control circuit variably controls an off-period of the switching transistor after the switching transistor is turned off. The off-period control circuit varies the off-period in accordance with a voltage difference between the output voltage and an upper limit threshold voltage. The off-period of the switching transistor is increased as the output voltage increases toward the upper limit threshold voltage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:  
         [0009]      FIG. 1  is a block diagram of a passenger protection system according to an embodiment of the present invention;  
         [0010]      FIG. 2  is a detailed circuit diagram of a DC-DC converter circuit according to the embodiment;  
         [0011]      FIG. 3  is a signal diagram of an operation of the DC-DC converter circuit shown in  FIG. 2 ; and  
         [0012]      FIG. 4  is a signal diagram of a current flowing in a switching transistor in the DC-DC converter circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     Referring first to  FIG. 1 , a passenger protection system is constructed as an airbag system and comprises a d.c. storage battery  10 , an ignition switch  12 , a DC-DC converter circuit  20 , a back-up circuit  60 , a collision determination unit  65  and a passenger protection unit  70 .  
         [0014]     The DC-DC converter circuit  20  is connected to the battery through a diode  14  to boost a voltage of the battery  10 . The back-up circuit  60  includes a back-up capacitor  61 , and connected to both the battery  10  through a diode  13  and the output terminal of the DC-DC converter circuit  20 . The back-up circuit  60  supplies an electric power to the collision determination unit  65  and the passenger protection unit  70  in place of the battery  10 , when the battery  10  is disconnected from the above circuits and units  20 ,  60 ,  65  and  70 . The diodes  13  and  14  are for restricting electric currents from flowing in reverse.  
         [0015]     The collision determination unit  65  includes a constant voltage power circuit  66 , an acceleration sensor  67  and a collision determination circuit  68 . The acceleration sensor  68  is held operative with the constant voltage from the power circuit  66  and detects an acceleration (deceleration) applied to a vehicle. The determination circuit  67  determines a collision of the vehicle in response to the detected acceleration.  
         [0016]     The passenger protection unit  70  includes a safety sensor  71 , an igniter drive circuit  72 , a squib  73  and an airbag (not shown). The igniter drive circuit  72  turns on to activate the squib  73  to inflate the airbag in response to an output signal from the collision determination circuit  68  indicative of a vehicle collision.  
         [0017]     The DC-DC converter circuit  20  includes, as shown in  FIG. 2 , an input smoothing capacitor  21 , a reference voltage circuit  22 , a voltage booster coil  26 , a diode  27 , an output smoothing capacitor  28 , a current detection circuit  30 , a off-timer variable control circuit  35 , a drive circuit  50 , a voltage booster switching transistor  55  and the like.  
         [0018]     The drive circuit  50  is a NOR gate for controlling the switching transistor  55 . It receives the output signals of the current detection circuit  30  and the off-timer variable control circuit  35 . The NOR gate produces a low level signal (“ 0 ”) and a high level signal (“ 1 ”), when at least either input is at the high level and when both inputs are at the low level, respectively. The switching transistor  55  turns on and off in response to the high level signal and the low level signal applied from the drive circuit  50 , respectively.  
         [0019]     The reference voltage circuit  22  includes a resistor  23  and a Zener diode  24 , and provides a voltage with the output voltage Vout of the DC-DC converter circuit  20 . The coil  26  stores therein electric energy supplied from the battery  10 . The transistor  55  is turned on and off by the driver circuit  50  to charge the capacitor  61  in the back-up circuit  60  with the energy stored in the coil  26  through the diode  27 .  
         [0020]     The current detection circuit  30  is for determining the time point to turn off the switching transistor  55 . It includes a shunt resistor  31 , a comparator  32  and the like. The comparator  32  receives a first reference voltage and a detection voltage from the shunt resistor  31 . The shunt resistor  31  is connected in series with the switching transistor  55 , and hence the detection voltage corresponds to a current Itr flowing in the switching transistor  55 . The comparator  32  compares those two input voltages and produces a low level signal (“ 0 ”) each time the current Itr reaches a predetermined threshold current level Ith. This low level signal is inverted and then applied to the drive circuit  50  thereby to turn off the switching transistor  55 .  
         [0021]     The off-timer variable control circuit  35  is for controlling the off-period of the switching transistor  55 , that is, the time point to turn on the switching transistor  55 , variably with a voltage difference between a second reference voltage and the output voltage Vout of the DC-DC converter circuit  20  produced at the output terminal  29 . The control circuit  35  includes an operational amplifier  37 , a pair of current mirror circuits  38 ,  39 , a transistor  40 , a capacitor  41 , a comparator  43 , a NOR gate  45  and the like.  
         [0022]     The operational amplifier  37  amplifies the voltage difference between the two input voltages of the control circuit  35 . The current mirror circuits  38  and  39  convert a voltage indicative of the input voltage difference to a current, which charges the capacitor  41 . As the input voltage difference increases, the charging current for the capacitor  41  also increases. Therefore, the voltage across the capacitor  40  applied to the comparator  43  increases with an increase in the input voltage difference of the amplifier  37 .  
         [0023]     The comparator  43  compares the output voltage of the capacitor  41  and a third reference voltage. The comparator  43  produces a low level signal (“ 0 ”). This low level signal is inverted and applied to the NOR gate  45 . The NOR gate  45  responsively produces a low level signal. Since the NOR gate  45  is connected to the drive circuit  50 , the drive circuit  50  produces a high level signal (“ 1 ”). With this high level signal, the switching transistor  55  turns on. At the same time, that is, at the start of the on-period of the switching transistor  55 , the transistor  40  turns on and discharges the capacitor  41 . Thus, the off-period of the switching transistor  55  is controlled to a shorter period as the input voltage difference of the control circuit  35  increases.  
         [0024]     In operation, as the output voltage Vout of the DC-DC converter circuit  20  is much lower than a limit threshold voltage Vth as shown with A in  FIG. 3 , the switching transistor  55  repeats turning on and off in response to the signal from the off-timer variable control circuit  35 . The timer (transistor  40  and capacitor  41 ) in the variable control circuit  35  starts to measure the off-period of the switching transistor  55 , when the switching transistor  55  turns off. The transistor  55  remains turned off until the measured period reaches a period t 2 . After the period t 2 , the switching transistor  55  turns on again.  
         [0025]     As shown with D in  FIG. 4 , the transistor current Itr flowing in the switching transistor  55  increases gradually at a predetermined time constant determined by the coil  26  and the resistor  31 , when the switching transistor  55  turns on. Thus, electric current Itr flows from the battery  10  in the coil  26 . When the current Itr detected by the shunt resistor  31  reaches the threshold current Ith set by the reference voltage circuit  22 , the current detection circuit  30  turns off the transistor  55  through the drive circuit  50 .  
         [0026]     When the switching transistor  55  turns off, the current flows from the battery  10  to the capacitor  61  in the back-up circuit  60  through the diode  27  as shown with E in  FIG. 4 . Thus the capacitor  61  is charged. When the capacitor  61  is fully charged, the current from the battery  10  is stored in the coil  26  to boost the voltage by the repetition of the turn-on and turn-off of the switching transistor  55 .  
         [0027]     The boosted voltage is supplied to the collision determination unit  65  and the passenger protection unit  70  from the booster circuit  20  or the back-up circuit  60  through the terminal  29 . Therefore, even when the battery  10  is disconnected at the time of a vehicle collision, the units  65  and  70  are held operative with the voltage from the terminal back-up circuit  60 . Thus, the DC-DC converter circuit  20  and the back-up circuit  60  forms a voltage booster circuit  15  with a back-up function.  
         [0028]     When the output voltage supplied from the DC-DC converter circuit  20  or the like becomes too high exceeding the limit threshold voltage Vth, overheat is likely to arise. Further noise is likely to be generated under the normal condition in which the output voltage is at the limit threshold voltage. For those reasons, the off period of the switching transistor  55  is varied with the difference between the reference voltage and the output voltage Vout.  
         [0029]     Specifically, the operational amplifier  37  produces a large voltage difference when the output voltage Vout is far lower than the limit threshold voltage Vth as shown with A in  FIG. 3 . The current mirror circuits  38  and  39  responsively produce a large current to charge the capacitor  41 . Thus, the time period (off-period) until the capacitor  41  is discharged by the transistor  40  becomes short. This off-period t 2  is set to as short as the on-period t 1 , for example.  
         [0030]     As the off-period is short, the output voltage Vout is still lower than the limit threshold voltage Vth even at the end of the off-period. The comparator  43  produces a signal indicative of this detection to the NOR gate  45 . The current detection circuit  30  limits the transistor current Itr to Ith. Thus, the switching transistor  55  repeats turning on and off in a short cycle period. Thus, more electric current flows from the battery  10  to the coil  26  and hence the output voltage Vout is boosted rapidly.  
         [0031]     As the output voltage approaches the limit threshold voltage Vth as shown with B in  FIG. 3 , the operational amplifier  37  produces a smaller voltage difference, so that the off-period t 3  of the switching transistor  55  becomes longer than t 2  as shown with B in  FIG. 3 . As a result, the on-off cycle period of the switching transistor  55  also becomes longer.  
         [0032]     After the output voltage Vout reaches the limit threshold voltage Vth, the comparator  37  produces only a small voltage difference, so that the off-period t 4  of the switching transistor  55  becomes longer than the period t 3  as shown with C in  FIG. 3 . Thus, the on-off cycle period of the switching transistor  55  further becomes longer.  
         [0033]     According to the above embodiment, the noise generation arising from the repeated turning on and off of the switching transistor  55  is reduced for the following reasons.  
         [0034]     First, the cycle period (t 1 +t 4 ), of on-off control of the switching transistor  55  is lengthened to be less than that of the frequency range (several tens KHz to several hundreds KHz), in which radio noise will be generated more likely.  
         [0035]     In addition, a recovery current that flows from the diode  27  to the switching transistor  55  after the switching transistor  55  turned off reduces generation of the radio noise. As understood from E in  FIG. 4 , the switching transistor  55  turns on when the forward current Idi flowing through the diode  27  is null. This is because the off-periods t 3  and t 4  are sufficiently long as shown with B and D in  FIG. 3 , that is, the on-off cycle period of the transistor  55  is sufficiently long.  
         [0036]     According to the above embodiment, the voltage booster circuit may be designed, that is, the inductance of the coil  26  and the threshold of the switching transistor  55  may be determined, assuming that the voltage of the battery  10  is not at a minimum level but at a certain level. It is possible even in this instance to boost the battery voltage in a short period by shortening the off-period of the switching transistor  55  while the output voltage Vout is far lower than the threshold voltage Vth as shown in A in  FIG. 3 .  
         [0037]     The present invention should not be limited to the above embodiment, but may be modified in many ways without departing from the spirit of the invention.