1. Field of the Invention
The present invention relates to a drive circuit and a power supply apparatus having a switch element such as MIS-FET (Metal Insulation Semiconductor—Field Effect Transistor), IGBT (Insulated Gate Bipolar Transistor) or the like, more particularly to an improved technique for switching the switch element at a high speed.
2. Description of the Related Art
Generally, in drive circuits having a voltage-driven type switch element such as a MIS-FET, an IGBT or the like, controllability of the drive circuits can be improved by driving a gate voltage at a high speed, thereby shortening a switching time of the switch element.
Further, when the drive circuits receive a negative voltage of a pulse signal, a method is performed which applies a reverse bias voltage to the switch element in order to abruptly turn off the switch element.
From the above view point, a variety of techniques have been disclosed relating to drive circuits for a switch element.
For example, Japanese Unexamined Patent Publication (Kokai) No. H08(1996)-149796 discloses a technology of drive circuits having a capacitor in series between a secondary winding of a transformer and a switch element, thereby applying a reverse bias voltage generated by the capacitor to a gate of the switch element, and limiting amplitude of the reverse bias voltage by a Zenor diode connected in parallel with the capacitor.
Japanese Unexamined Patent Publication (Kokai) No. H11(1999)-145810 discloses a technology of drive circuits connecting a capacitor and a diode in series between a secondary winding of a transformer and a voltage-driven type switch element, and connecting a base and an emitter of a discharging transistor to both terminals of the diode without a current-limiting resistor, thereby turning on the discharging transistor at a high speed when the switch element is off-state, which allows the gate capacitor to be discharged at a high speed.
According to the technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. H08(1996)-149796, a reverse bias voltage is generated for applying the gate of the switch element. However, when an input voltage is reversed and a gate capacitor of the switch element is discharged, a large current flows through the gate, the capacitor and the transformer (inductance). Therefore, it suffers from a disadvantage that a large level of a LC resonance appears.
Moreover, when an input voltage is reversed, a time constant of the discharge is long in a discharging loop formed by the gate (gate capacitor), the capacitor and the transformer (inductance). Therefore, it also suffers from a disadvantage that power dissipation (consumption) is large.
According to the technology disclosed in Japanese Unexamined Patent Publication No. H11-145810, a current-limiting resistor is not provided and impedance of a discharging loop from the gate capacitor is small when the switch element is off-state. As a result, it suffers from a disadvantage that a large level of a LC resonance may appear and power dissipation is large because of a large current that appears due to the small impedance.
Further, since a power loss of the discharging transistor becomes large under high temperature range, power dissipation may be even larger.