1. Field of the Invention
The present invention relates to a synchronous rectifier circuit for use with a switching power supply, such as a serial resonance converter, or the like.
2. Description of the Related Art
A synchronous rectifier circuit is known which uses a synchronous rectification element, such as an MOSFET, or the like, for synchronously rectifying a secondary side wiring current of a switching power supply device, such as a serial resonance converter, or the like (for example, refer to Patent Document 1). In FIG. 7, as an example of switching power supply device using such a synchronous rectifier circuit, a serial resonance converter is shown.
The serial resonance converter shown in FIG. 7 includes a transformer T1; switch elements Q1 and Q2; a resonance capacitor Cr; a resonance reactor Lr; synchronous rectification elements QSR1 and QSR2; synchronous rectification control circuits IC1 and IC2; and an output capacitor Co.
To the primary side circuit of the transformer T1, the switch elements Q1 and Q2 are connected, and by alternately turning-on the switch elements Q1 and Q2, a voltage of a square wave is applied to a serial resonance circuit composed of the resonance capacitor Cr, the resonance reactor Lr, and the excitation inductance Np of the transformer T1.
The secondary side rectification circuit is a synchronous rectifier circuit which is composed of the synchronous rectification elements QSR1 and QSR2 and the synchronous rectification control circuits IC1 and IC2 . The synchronous rectification elements QSR1 and QSR2 are insulated-gate field-effect transistors (FET), being connected across the secondary wiring Ns1, Ns2 and the negative polarity side terminal (GND) of the output capacitor Co of the transformer T1, respectively. The synchronous rectification elements QSR1 and QSR22 may be of another type of semiconductor switch, such as a bipolar transistor, an IGBT, or the like. Further, reference symbols Da1 and Da2 denote diodes which are connected in parallel with the synchronous rectification elements QSR1 are QSR2 respectively, being parasitic diodes in the synchronous rectification elements QSR1 and QSR2 composed of an FET, respectively. These diodes Da1 and Da2 may be individual diodes which are configured separately from the synchronous rectification elements QSR1 and QSR2.
The synchronous rectification control circuit IC1 has a differential voltage detection function, and as shown in FIG. 8, detects the current iSR flowing through the synchronous rectification element QSR1 as the drain-source voltage (VD-VS) of the synchronous rectification element QSR1 for performing gate control. In other words, the synchronous rectification control circuit IC1 monitors the saturation voltage VRds—on which is generated when a current iSR flows through the on-resistance Rds—on upon the synchronous rectification element QSR1 being on. And with the saturation voltage VRds—on being compared with the turn-on threshold voltage VTH2 and the turn-off threshold voltage VTH1, the current iSR flowing through the synchronous rectification element QSR1 is detected, and on the basis thereof, a gate signal (VGATE) is outputted. The synchronous rectification control circuit IC2 has the same configuration as that of the synchronous rectification control circuit IC1.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-292571
However, in the case where a synchronous rectification element, the on-resistance of which, in recent years, has been reduced, is used as QSR1, QSR2 for making synchronous rectification, since a resistance component, such as the on-resistance Rds—on, is small, a voltage drop due to the inductance component Llead of a bonding wire, a lead, or the like, can influence the synchronous rectification. FIG. 9(A) shows an equivalent circuit when the synchronous rectification element QSR1, QSR2 is on.
The inductance component Llead changes the impedance Zds—on of the synchronous rectification element QSR1, QSR2 into an impedance of advanced phase as shown in FIG. 9(B), which is expressed by the following expression.
                                          Z            ds_on                    =                                    R              ds_on                        +                          j              ⁢                                                          ⁢              ω              ⁢                                                          ⁢              Llead                                      ⁢                                  ⁢                  θ          =                      arctan            ⁡                          (                                                ω                  ⁢                                                                          ⁢                  Llead                                                  R                  ds_on                                            )                                                          [                  Math          ⁢                                          ⁢          1                ]            
FIG. 10 indicates respective voltage drops of VRds—on and VZds—on generated when the current iSR is caused to flow through the on-resistance Rds—on and the impedance Zds—on which is determined from “Math 1”, respectively. From FIG. 10, it can be seen that the phase of VZds—on is advanced as compared to that of VRds—on. Therefore, if the impedance Zds—on of the synchronous rectification element QSR1 is used for gate control of the synchronous rectification control circuit IC1, as shown in FIG. 11, the advanced phase of the saturation voltage VZds—on causes the VRds—on to reach the turn-off threshold voltage VTH1 far before the current iSR becoming zero, resulting in the gate signal (VGATE) being turned off.
Therefore, a sufficient gate width cannot be obtained, and the period tVF during which the current flows through the diode D1 for the synchronous rectification element QSR1 is increased, thereby it is difficult to enhance the converter efficiency. Reference symbol Vf denotes a forward voltage across the diode D1.
From now on, it can be expected that the on-resistance of the synchronous rectification element QSR1, QSR2 will be further reduced, or by parallel connection of these synchronous rectification elements QSR1 and QSR2 , the saturation voltage VRds—on will be increasingly decreased. For example, with the FETs in recent years, the on-resistance Rds—on is as low as several milliohms, and the inductance component Llead is as small as several nanohenries, the saturation voltage VZds—on being also as low as several millivolts to dozen millivolts or so, thereby they have presented a problem of being embedded in the noise, or the like, resulting in easily malfunctioning to be unserviceable. In addition, there has been a problem that, if the reduction in on-resistance causes the inductance component Llead to be further actualized to provide a more advanced phase impedance, as shown in FIG. 11, it will become impossible to obtain a sufficient gate width, and expect the advantage of the synchronous rectification.
The present invention has been made in view of the above problems of the prior art, and it is an object of the present invention to solve such problems by providing a synchronous rectifier circuit which, even if a synchronous rectification element having a low on-resistance is used, a synchronous rectifying operation can be performed, being not influenced by the inductance component.