Second Embodiment

With reference to FIG. 4, there will be described the structure of a rectifier circuit in a second embodiment of the present invention, and a method for controlling the rectifier circuit. The present embodiment is an embodiment of a more specific circuit structure in the first embodiment.

As illustrated in FIG. 4, the rectifier circuit in the present embodiment includes rectifying MOSFETs QR1 and QR2, a control circuit 1 connected to a drain terminal and a source terminal of the rectifying MOSFET QR2 and to a gate terminal and a source terminal of the rectifying MOSFET QR1, and a control circuit 2 connected to a drain terminal and a source terminal of the rectifying MOSFET QR1 and to a gate terminal and a source terminal of the rectifying MOSFET QR2.

The control circuit 1 includes a depression type N-channel MOSFET QS1 having a drain terminal connected to the drain terminal of the rectifying MOSFET QR2 and a source terminal connected to the gate terminal of the rectifying MOSFET QR1, a voltage-dividing resistance R1 connected between the gate terminal and the source terminal of the N-channel MOSFET QS1, and a voltage-dividing resistance R2 connected between the source terminal of the rectifying MOSFET QR1 and the gate terminal of the N-channel MOSFET QS1.

Further, the control circuit 2 includes a depression type N-channel MOSFET QS2 having a drain terminal connected to the drain terminal of the rectifying MOSFET QR1 and a source terminal connected to the gate terminal of the rectifying MOSFET QR2, a voltage-dividing resistance R3 connected between the source terminal and the gate terminal of the N-channel MOSFET QS2, and a voltage-dividing resistance R4 connected between the gate terminal of the N-channel MOSFET QS2 and the source terminal of the rectifying MOSFET QR2.

The rectifying MOSFETs QR1 and QR2 are N-channel MOSFETs having respective sources connected to a DC low-voltage terminal.

The control circuit 1 and the control circuit 2 respectively include a MOSFET QS1 for controlling an output voltage based on the drain-source voltage Vds2 of the rectifying MOSFET QR2, which is the input voltage to the control circuit 1, and a depression type N-channel MOSFET QS1, QS2 as a switch for controlling an output voltage based on the drain-source voltage Vds1 of the rectifying MOSFET QR1, which is the input voltage to the control circuit 2.

A method for determining the threshold voltages Vref1 and Vref2 will be described. As an example, a method for determining the threshold voltage Vref1 will be described. Incidentally, it is assumed that the gate threshold voltage of the MOSFET QS1 is Vth11, the on-resistance value of the MOSFET QS1 is rqs1, and the resistance values of the resistance R1 and the resistance R2 are r1 and r2, respectively. When the input voltage to the control circuit 1, namely the drain-source voltage Vds2 of the rectifying MOSFET QR2 is 0, the gate-source voltage of the MOSFET QS1 is 0, and the MOSFET QS1 is on since the MOSFET QS1 is a depression type N-channel MOSFET. Thereafter, when Vds2 is increased, the current flowing from the drain of the MOSFET QS1 to the source thereof is increased. At this time, a voltage of Vds2×(r1+r2)/(r1+r2+rqs1) is outputted as the output voltage from the control circuit 1, as the gate-source voltage of the MOSFET QR1. Therefore, by selecting the MOSFET QS1 and the resistances R1 and R2 such that the values of r1 and r1 are sufficiently larger than rqs1, it is possible to approximate the drain-source voltage Vds2 of the rectifying MOSFET QR2, which is the input voltage to the control circuit 1, and the gate-source voltage of the rectifying MOSFET QR1, which is the output voltage from the control circuit 1, to the same value.

When the drain-source voltage Vds2 of the rectifying MOSFET QR2 is increased, the current flowing from the drain of the MOSFET QS1 to the source thereof is increased, but the voltage drop caused across the resistance R1 is increased, which decreases the gate-source voltage of the MOSFET QS1 (since it is a negative voltage, the absolute value thereof increases by an amount corresponding to the voltage drop caused across the resistance R1), thereby increasing the on-resistance of QS1. As a result, when the drain-source voltage Vds2 of the MOSFET QS2 is equal to or larger than a certain voltage, the current flowing from the drain of the MOSFET QS1 to the source thereof has a certain constant value.

At this time, the voltage drop caused across the resistance R1 and the voltage drop caused across the resistance R2 are constant and, therefore, the gate-source voltage Vgs1 of the rectifying MOSFET QR1 is the sum of the voltage drop across the resistance R1 and the voltage drop across the resistance R2, which is constant. The constant voltage at this time is the threshold voltage Vref1 of the control circuit 1.

The threshold voltage Vref1 can be approximated to Vth11×(r1+r2)/r1. In order to obtain a desired threshold voltage Vref1, the gate threshold voltage Vth11 of the MOSFET QS1 and the resistance values r1 and r2 of the resistances R1 and R2 may be selected. The threshold voltage Vref2 can be similarly determined.

Third Embodiment

With reference to FIGS. 5 and 6, there will be described the structure of a rectifier circuit in a third embodiment of the present invention, and a method for controlling the rectifier circuit. As illustrated in FIG. 5, the rectifier circuit in the present embodiment includes enhancement type P-channel MOSFETs as rectifying MOSFETs QR1 and QR2, and depression type P-channel MOSFETs as MOSFETs QS1 and QS2.

In the rectifier circuits in the first embodiment (FIG. 1) and the second embodiment (FIG. 4), the rectifying MOSFETs QR1 and QR2 are constituted by enhancement type N-channel MOSFETs, and QR1 and QR2 are provided with a common source terminal. Therefore, the rectifier circuits in FIGS. 1 and 4 cannot be applied as two rectifying MOSFETs HQ in the high-side, which are not provided with a common source terminal, in a rectification bridge constituted by four enhancement type N-channel MOSFETs as illustrated FIG. 6.

For coping therewith, in the rectifier circuit in the present embodiment illustrated in FIG. 5, enhancement type P-channel MOSFETs are used as QR1 and QR2, which enables providing QR1 and QR2 with a common source-side terminal even when they are applied as the two rectifying MOSFETS HQ in the high-side in FIG. 6.

The rectifying MOSFETs QR1 and QR2 are constituted by P-channel MOSFETs having respective sources connected to a DC high-voltage terminal. Further, the MOSFETs QS