Abstract:
A drive circuit of first and second switches includes a first series circuit having a capacitor and a primary winding of a transformer and connected to both ends of a pulse signal generator, a first secondary winding of the transformer to apply a voltage to a control terminal of the first semiconductor switch based on the pulse signal, the first secondary winding being wound in a direction opposite to the primary winding, a second secondary winding of the transformer to apply a voltage to a control terminal of the second semiconductor switch based on the pulse signal, the second secondary winding being wound in the same direction to the primary winding, and a third semiconductor switch that turns on when the pulse signal is stopped, to shorten an ON period of the first semiconductor switch.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a drive circuit for alternately turning on first and second semiconductor switching elements that are connected in series. 
     2. Description of the Related Art 
       FIG. 1  illustrates an example of a drive circuit for driving semiconductor switching elements according to a related art. In  FIG. 1 , a first switching element Q 1  of an n-type MOSFET for high side and a second switching element Q 2  of an n-type MOSFET for low side are connected in series. Both ends of this series circuit are connected to a DC power source VDC. A connection point of the first and second switching elements Q 1  and Q 2  is connected to a load  1 . 
     The first and second switching elements Q 1  and Q 2  are controlled according to a control signal, i.e., a pulse signal generated by a pulse signal generator  2 . Both ends of the pulse signal generator  2  are connected to a series circuit including a capacitor C and a primary winding P of a transformer T 1 . The transformer T 1  has the primary winding P, a first secondary winding S 1 , and a second secondary winding S 2 . The first secondary winding S 1  and primary winding P are oppositely wound and the second secondary winding S 2  and primary winding P are wound in the same direction. 
     The first secondary winding S 1  of the transformer T 1  is connected to a first driver  3   c . Based on a voltage provided by the first secondary winding S 1 , the first driver  3   c  applies a first drive signal to a gate of the first switching element Q 1 . The second secondary winding S 2  of the transformer T 1  is connected to a second driver  3   d . Based on a voltage provided by the second secondary winding S 2 , the second driver  3   d  applies a second drive signal to a gate of the second switching element Q 2 . 
     Operation of the drive circuit illustrated in  FIG. 1  will be explained with reference to a waveform diagram of  FIG. 2 . 
     At time t 0 , the pulse signal generator  2  generates a pulse signal PL, which is transferred through the capacitor C and the primary winding P of the transformer T 1  to the first and second secondary windings S 1  and S 2 . The first and second secondary windings S 1  and S 2  are oppositely wound, and therefore, pulse signals that are inverted from each other are sent to the first and second drivers  3   c  and  3   d . The first driver  3   c  applies the first drive signal Hg to the gate of the first switching element Q 1  and the second driver  3   d  applies the second drive signal Lg to the gate of the second switching element Q 2 . The waveform diagram of  FIG. 2  is based on that the low-side second secondary winding S 2  is wound in the same direction as the primary winding P. 
     When the pulse signal PL is low, the first drive signal Hg is high to turn on the first switching element Q 1  to supply power of the DC power source VDC to the load  1 . When the pulse signal PL is high, the second drive signal Lg is high to turn on the second switching element Q 2  to discharge energy of the load  1 . 
     At time t 10 , the pulse signal generator  2  stops generation of the pulse signal PL, to stop the drive circuit. At this time, the second secondary winding S 2  wound in the same direction as the primary winding P provides a negative output voltage to turn off the second switching element Q 2 . 
     On the other hand, the first secondary winding S 1  that is oppositely wound to the primary winding P provides a positive output voltage. When this voltage exceeds a threshold voltage Vth of the gate of the first switching element Q 1 , the first switching element Q 1  turns on for a period tON. Thereafter, an excitation inductance L (not illustrated) of the transformer T 1  and the capacitor C cause an LC resonance. Due to the consumption of resonant energy, a voltage Vc across the capacitor C gradually decreases. 
     There is another related art disclosed in Japanese Unexamined Patent Application Publication No. 2002-320376 (Patent Document 1). This related art is a method of driving a power switch element. According to the related art, a switching regulator drives through a drive transformer the power switch element with a pulse signal. When stopping the pulse signal, the related art gradually decreases the duty, i.e., the pulse width of the pulse signal, or the voltage of the pulse signal, thereby protecting the power switch element from being damaged by a voltage free oscillation caused by an inductance of the drive transformer and input and output capacitors. 
     SUMMARY OF THE INVENTION 
     During the resonant period in which the LC resonance occurs in the related art of  FIGS. 1 and 2 , the first secondary winding S 1  provides a positive voltage to extend the ON period of the first switching element Q 1  and cause an overcurrent. 
     To solve this problem, the present invention provides a drive circuit for driving semiconductor switching elements according to a pulse signal, capable of preventing an ON period of the switching elements from elongating when the pulse signal is stopped. 
     According to an aspect of the present invention, the drive circuit for alternately turning on first and second switching elements that are connected in series includes a first series circuit including a capacitor and a primary winding of a transformer and connected to both ends of a pulse signal generator that generates a pulse signal; a first secondary winding of the transformer, configured to generate a voltage based on the pulse signal and apply the voltage to a control terminal of the first switching element, the first secondary winding being wound in a direction opposite to a direction in which the primary winding is wound; a second secondary winding of the transformer, configured to generate a voltage based on the pulse signal and apply the voltage to a control terminal of the second switching element, the second secondary winding being wound in the same direction as the direction in which the primary winding is wound; and a third switching element configured to turn on when generation of the pulse signal is stopped and shorten an ON period of the first switching element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a drive circuit for driving switching elements according to a related art; 
         FIG. 2  is a waveform diagram illustrating operation of the drive circuit of  FIG. 1 ; 
         FIG. 3  is a schematic diagram illustrating a drive circuit for driving switching elements according to Embodiment 1 of the present invention; 
         FIG. 4  is a waveform diagram illustrating operation of the drive circuit of  FIG. 3 ; 
         FIG. 5  is a schematic diagram illustrating a drive circuit for driving switching elements according to Embodiment 2 of the present invention; 
         FIG. 6  is a waveform diagram illustrating operation of the drive circuit of  FIG. 5 ; 
         FIG. 7  is a schematic diagram illustrating a drive circuit for driving switching elements according to Embodiment 3 of the present invention; and 
         FIG. 8  is a schematic diagram illustrating a drive circuit for driving switching elements according to Embodiment 4 of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Drive circuits for driving semiconductor switching elements according to embodiments of the present invention will be explained in detail with reference to the drawings. 
     Embodiment 1 
       FIG. 3  is a schematic diagram illustrating a drive circuit for driving semiconductor switching elements according to Embodiment 1 of the present invention. The drive circuit of Embodiment 1 illustrated in  FIG. 3  is characterized by a switching element Q 3  (“third semiconductor switch” stipulated in the claims) that turns on when a pulse signal from a pulse signal generator  2  is stopped and shortens an ON period of a first switching element Q 1  (first semiconductor switch), thereby preventing the ON period of the first switching element Q 1  from elongating. 
     The switching element Q 3  is an npn-type bipolar transistor having a collector (first main terminal) connected to a first end of a capacitor C, an emitter (second main terminal) connected to a second end of the capacitor C, and a base to receive a stop signal for stopping the drive circuit. 
     The switching element Q 3  may be a MOSFET or any other semiconductor switch. In  FIG. 3 , both ends of the pulse signal generator  2  are connected to a series circuit including a primary winding P of a transformer T 1 , a resistor R (not illustrated), and the capacitor C. It is possible to connect a series circuit including the resistor R and capacitor C between the collector and emitter of the switching element Q 3 . 
     In the drive circuit, a first driver  3   a  has a resistor Dr 1  having a first end connected to a first end of a first secondary winding S 1  of the transformer T 1  and a second end connected to a gate of the first switching element Q 1 . A second driver  3   b  has a resistor Dr 2  having a first end connected to a first end of a second secondary winding S 2  of the transformer T 1  and a second end connected to a gate of a second switching element Q 2  (second semiconductor switch). 
     A time constant determined by the resistor Dr 1  and an input capacitance (gate-source capacitance) of the first switching element Q 1  provides a first dead time. A time constant determined by the resistor Dr 2  and an input capacitance (gate-source capacitance) of the second switching element provides a second dead time. The first and second dead times each are a period in which the first and second switching elements Q 1  and Q 2  are both OFF. These time constants are determined not to simultaneously turn on the first and second switching elements Q 1  and Q 2 . 
     The resistors Dr 1  and Dr 2  may be eliminated by adjusting a time constant of the circuit on the primary side of the transformer T 1 . 
     Operation of the drive circuit according to Embodiment 1 will be explained with reference to a waveform diagram of  FIG. 4 . 
     From time t 0  to t 1 , the stop signal Q 3   g  is low to keep the switching element Q 3  off. Namely, operation of Embodiment 1 in the period from t 0  to t 1  is the same as that of the related art of  FIG. 2  in the period from t 0  to t 10 , and therefore, will not be explained. 
     At time t 1 , the drive circuit is inoperative and the stop signal Q 3   g  is applied to the base of the switching element Q 3  to turn on the switching element Q 3 . Then, energy accumulated in the capacitor C is discharged through the switching element Q 3 , to reduce energy transmitted to an excitation inductance L (not illustrated) of the transformer T 1 . 
     This causes a sharp drop in energy transmitted to the first secondary winding S 1  that is oppositely wound relative to the primary winding P of the transformer T 1 . As results, an output voltage of the first secondary winding S 1  of the transformer T 1 , i.e., a first drive signal Hg sharply drops below a threshold voltage Vth of the gate of the first switching element Q 1 , thereby turning off the first switching element Q 1 . 
     At time t 2 , the voltage of the first drive signal Hg becomes nearly zero. At the same time, a voltage Vc across the capacitor C decreases. Consequently, Embodiment 1 prevents the ON period of the first switching element Q 1  from elongating when generation of the pulse signal from the pulse signal generator  2  is stopped. 
     Embodiment 2 
       FIG. 5  is a schematic diagram illustrating a drive circuit for driving semiconductor switching elements according to Embodiment 2 of the present invention. The drive circuit of Embodiment 2 illustrated in  FIG. 5  is characterized in that a switching element Q 3  for preventing an increase in an ON period is connected in series with a resistor R 1  and the series circuit is connected to both ends of a second secondary winding S 2  of a transformer T 1 . 
     The resistor R 1  has a resistance value that is set so that a voltage generated by a first secondary winding S 1  of the transformer T 1  does not exceed a threshold voltage Vth of a first switching element Q 1 . 
     Operation of the drive circuit according to Embodiment 2 will be explained with reference to a waveform diagram of  FIG. 6 . 
     Operation of the drive circuit in a period from t 0  to t 1  is the same as that of the drive circuit of Embodiment 1 in the period from t 0  to t 1  illustrated in  FIG. 4 , and therefore, will not be explained. 
     At time t 1 , the drive circuit becomes inoperative by stopping a pulse signal generator  2  and a stop signal Q 3   g  is applied to a base of the switching element Q 3 , to turn on the switching element Q 3 . Then, energy accumulated in the second secondary winding S 2  of the transformer T 1  is discharged through the switching element Q 3  and resistor R 1  and is consumed by the resistor R 1 . 
     As results, an output voltage of the second secondary winding S 2  of the transformer T 1 , i.e., a second drive signal Lg sharply drops. This results in sharply decreasing an output voltage of the first secondary winding S 1  of the transformer T 1 , i.e., a first drive signal Hg below the threshold voltage Vth of the gate of the first switching element Q 1 , thereby turning off the first switching element Q 1 . 
     At time t 2 , the voltage of the first drive signal Hg becomes substantially zero. In this way, Embodiment 2 prevents an increase in an ON period of the first switching element Q 1  when transmission of a pulse signal from the pulse signal generator  2  is stopped. 
     Embodiment 3 
       FIG. 7  is a schematic diagram illustrating a drive circuit for semiconductor switching elements according to Embodiment 3 of the present invention. Embodiment 3 is characterized in that a switching element Q 3  for preventing an increase in an ON period is connected in series with a resistor R 1  and the series circuit is connected to both ends of a first secondary winding S 1  of a transformer T 1 . 
     The resistor R 1  has a resistance value that is set so that a voltage generated by the first secondary winding S 1  of the transformer T 1  will not exceed a threshold voltage Vth of a first switching element Q 1 . 
     When the drive circuit becomes inoperative by stopping a pulse signal generator  2 , a stop signal Q 3   g  is applied to a base of the switching element Q 3 , to turn on the switching element Q 3 . Then, energy accumulated in the first secondary winding S 1  of the transformer T 1  is discharged through the switching element Q 3  and resistor R 1  and is consumed by the resistor R 1 . 
     As results, an output voltage from the first secondary winding S 1  of the transformer T 1 , i.e., a first drive signal Hg sharply drops below the threshold voltage Vth of the gate of the first switching element Q 1 , thereby turning off the first switching element Q 1 . In this way, Embodiment 3 prevents an increase in the ON period of the first switching element Q 1  when transmission of a pulse signal from the pulse signal generator  2  is stopped. 
     Embodiment 4 
       FIG. 8  is a schematic diagram illustrating a drive circuit for semiconductor switching elements according to Embodiment 4 of the present invention. Embodiment 4 is characterized in that a switching element Q 3  for preventing an increase in an ON period is connected in series with a resistor R 1  and the series circuit is connected to both ends of a primary winding P of a transformer T 1 . 
     The resistor R 1  has a resistance value that is set so that a voltage generated by a first secondary winding S 1  of the transformer T 1  will not exceed a threshold voltage Vth of a first switching element Q 1 . 
     When the drive circuit becomes inoperative by stopping a pulse signal generator  2 , a stop signal Q 3   g  is applied to a base of the switching element Q 3 , to turn on the switching element Q 3 . Then, energy accumulated in the primary winding P of the transformer T 1  is discharged through the switching element Q 3  and resistor R 1  and is consumed by the resistor R 1 . 
     This causes a sharp drop in energy transmitted to the first secondary winding S 1  that is oppositely wound relative to the primary winding P of the transformer T 1 . As a result, an output voltage from the first secondary winding S 1  of the transformer T 1 , i.e., a first drive signal Hg sharply drops below the threshold voltage Vth of the gate of the first switching element Q 1 , thereby turning off the first switching element Q 1 . In this way, Embodiment 4 prevents the ON period of the first switching element Q 1  from elongating when transmission of a pulse signal from the pulse signal generator  2  is stopped. 
     The present invention is applicable to a lighting apparatus that has a resonant half-bridge converter to light discharge lamps. 
     As mentioned above, the present invention turns on an ON-period-elongation-preventive switching element when a pulse signal is stopped, to shorten an ON period of a first semiconductor switching element and prevent the ON period of the first semiconductor switching element from elongating. 
     This application claims benefit of priority under 35 USC §119 to Japanese Patent Application No. 2009-142229, filed on Jun. 15, 2009, the entire contents of which are incorporated by reference herein. Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the teachings. The scope of the invention is defined with reference to the following claims.