Pulse generator provided with a duty-factor limiting function

Abstract

In a pulse generator, a sawtooth-shaped wave generator circuit generates a sawtooth-shaped wave by charging and discharging a capacitor. The sawtooth-shaped wave is fed to a comparator that performs pulse-width modulation on it in accordance with the voltage it receives via a terminal and thereby produces pulses. The comparator has its output grounded through a transistor that is turned on with appropriate timing by the sawtooth-shaped wave generator circuit. Thus, the maximum duty factor of the output pulses is made equal to the duty factor of the sawtooth-shaped wave.

1. A pulse generator comprising:
 a sawtooth-shaped wave generating circuit that generates a sawtooth-shaped wave voltage;  
 a pulse generating circuit that compares the sawtooth-shaped wave voltage with a first predetermined voltage by means of a first comparator and that generates pulses with such a duty factor that duration of each pulse coincides with a period during which the sawtooth-shaped wave voltage exceeds the first predetermined voltage; and  
 a limiting circuit that sets an upper  a limit on the duty factor by forcibly keeping an output of the first comparator at a fixed voltage either during a period in which the sawtooth-shaped wave voltage is rising or during a period in which the sawtooth-shaped wave voltage is decaying.  
 
     
     
       2. A pulse generator as claimed in  claim 1 ,
 wherein the sawtooth-shaped wave generating circuit has a circuit for charging and discharging a capacitor and compares a voltage across this capacitor with a second predetermined voltage by means of a second comparator so that the sawtooth-shaped wave voltage is generated by charging the capacitor when the voltage across it is lower than the second predetermined voltage and discharging the capacitor when the voltage across it is higher than the second predetermined voltage, the discharging being continued until the voltage across the capacitor becomes equal to a third predetermined voltage which is lower than the second predetermined voltage, and  
 the limiting circuit is driven by an output of the second comparator.  
 
     
     
       3. A pulse generator comprising:
 an inductance coil that receives a direct-current voltage at one end;  
 a switching transistor that is connected between another end of the inductance coil and a fixed voltage;  
 a rectifying circuit that rectifies a high voltage that appears at the other end of the inductance coil when the switching transistor is turned from on to off;  
 an amplifier that compares an output voltage of the rectifying circuit with a reference voltage and generates a first predetermined voltage;  
 a sawtooth-shaped wave generating circuit for generating a sawtooth-shaped wave voltage;  
 a first comparator that generates duty pulses by comparing the sawtooth-shaped wave voltage with the first predetermined voltage and that supplies those duty pulses to a control electrode of the switching transistor; and  
 a limiting circuit that sets an upper  a limit on the duty factor by forcibly keeping an output of the first comparator at a fixed voltage either during a period in which the sawtooth-shaped wave voltage is rising or during a period in which the sawtooth-shaped wave voltage is decaying.  
 
     
     
       4. A pulse generator as claimed in  claim 3 ,
 wherein the sawtooth-shaped wave generating circuit has a circuit for charging and discharging a capacitor and compares a voltage across this capacitor with a second predetermined voltage by means of a second comparator so that the sawtooth-shaped wave voltage is generated by charging the capacitor when the voltage across it is lower than the second predetermined voltage and discharging the capacitor when the voltage across it is higher than the second predetermined voltage, the discharging being continued until the voltage across the capacitor becomes equal to a third predetermined voltage, which is lower than the second predetermined voltage, and  
 the limiting circuit driven by an output of the second comparator.  
 
     
     
       5. A pulse generator as claimed in  claim 4 ,
 wherein the second comparator comprises:  
 a first transistor and a second transistor that together constitute a differential pair;  
 a conductor that connects the voltage across the capacitor to a base of the first transistor; and  
 a circuit that applies the second and third predetermined voltage to a base of the second transistor.  
 
     
     
       6. A pulse generator as claimed in  claim 4 ,
 wherein a clock signal is fed to an output node of the second comparator so that the sawtooth-shaped wave generating circuit and the limiting circuit operate in synchronism with the clock signal.  
 
     
     
       7. A pulse generator as claimed in  claim 4 ,
 wherein the circuit that charges or discharges the capacitor includes a circuit for supplying a constant current as a charging/discharging current to the capacitor.  
 
     
     
       8. A pulse generator as claimed in  claim 6 , wherein a one-shot pulse made from the edge of the clock signal is added to the output node of the second comparator. 
     
     
       9. A pulse generator as claimed in  claim 2 ,
 wherein the limiting circuit includes a transistor that is controlled by the output of the second comparator to connect the output of the first comparator to the fixed voltage, the transistor having one end connected to an output terminal of the first comparator, having another end connected to a fixed voltage point and having a control electrode connected to the output of the second comparator.  
 
     
     
       10. A pulse generator as claimed in  claim 4 ,
 wherein the limiting circuit includes a transistor that is controlled by the output of the second comparator to connect the output of the first comparator to the fixed voltage, the transistor having one end connected to an output terminal of the first comparator, having another end connected to a fixed voltage point, and having a control electrode connected to the output of the second comparator.  
 
     
     
       11. A pulse generator as claimed in  claim 1 , wherein the pulse generating circuit generates pulses with a period during which the sawtooth-shaped wave voltage exceeds the first predetermined voltage. 
     
     
       12. A pulse generator as claimed in  claim 11 , wherein the limiting circuit sets an upper limit on the duty factor. 
     
     
       13. A pulse generator as claimed in  claim 1 , wherein the limiting circuit sets an upper limit on the duty factor. 
     
     
       14. A pulse generator as claimed in  claim 3 , wherein the limiting circuit sets an upper limit on the duty factor.