JPH09205768A - Synchronous rectifier circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably operate a converter by conducting a MOSFET for rectification only when the voltage at the control terminal of a control circuit installed to the gate terminal of the MOSFET is higher than the voltage at the output terminal. SOLUTION: An output voltage stabilizing control circuit 6 constantly maintains the output voltage Vo across terminals 11 and 12. When an electric current continuously flows to a choke coil 9, the waveform of the voltage Vs across the terminals 14 and 17 of the secondary winding of a transformer 5 becomes rectangular. The transformer 5 generates a voltage Vs (Vs=Ns/Np×Vi, where Ns/Np and Vi respectively represent the winding ratio and primary-side input voltage of the transformer 5) from the secondary side. A forward voltage is applied to the parasitic diode 15 of a MOSFET 7 and the potentials at the drain and source becomes equal to each other. When the current flowing to the choke coil 9 becomes zero, the conduction of a commutation-side diode 8 is completed and the bias voltage Vd between the MOSFET control terminal 14 and the terminal 12 rises, resulting in the turning off of the MOSFET 7. Therefore, the occurrence of an excessively large current and an excessively high voltage can be prevented.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a rectifying MOSFET.
The present invention relates to improvement of a synchronous rectification converter, and particularly to maintaining normal operation of the converter.

[0002]

2. Description of the Related Art In a voltage conversion circuit such as a DC / DC converter as shown in FIG. 4, a main switch 4 is an output voltage stabilization control circuit whose pulse width is modulated so that the voltage of an output terminal 11 is kept constant. Driven by the signal from 6.
A rectangular wave voltage is applied between the primary windings of the transformer 5 in synchronization with the operation of the main switch 4, and a rectangular wave voltage is also generated on the secondary winding side by the action of the transformer. This is MO
A smoothing circuit including a rectifying element including the SFET 7 and the diode 8, a choke coil 9, and a capacitor 10 rectifies and smoothes the voltage to obtain an averaged voltage. MOSFET7
And the diode 8 respectively conduct when the main switch 4 is in the on state and in the off state.

[0003]

The rectifying MOSFET 7 used as the rectifying element is controlled by the transformer 5 as follows.
Is performed in synchronization with the voltage appearing between the secondary windings of the. When the voltage of the output terminal 11 rises due to the fluctuation of the DC power supply 1 or the fluctuation of the load connected to the output terminal 11, and the signal from the stabilization control circuit 6 that detects this causes the main switch 4 to stop, the choke coil 9 Current gradually decreases, and when the current becomes close to zero, the conduction of the commutation diode 8 also ends. Then, by the voltage of the output terminal 11 maintained by the charges accumulated in the output capacitor 10, the rectifying MO
A bias voltage that turns on SFET7
A reverse current is generated between the sources and flows from the output terminal 11 to the secondary side of the transformer 5. This reverse current abnormally excites the transformer 5 and the choke coil 9 and finally reaches a saturation state, and the MOSFET 7 and the main switch 4
Excessive current flows to. Further, when the transformer is reset, an excessive voltage may be applied to the main switch and the rectifying MOSFET due to the energy of the transformer that is abnormally excited.

To solve this problem, the rectifying MOSF is used.
ET is prevented from being turned on unnecessarily, and M is applied to fluctuations in the input voltage and fluctuations in the load connected to the output terminal.
Provide a converter that operates stably without imposing a burden on the OSFET 7 and the main switch 4.

[0005]

In order to achieve the above-mentioned object, an embodiment of the present invention has a rectifying M as shown in FIG.
The control circuit 13 is provided at the gate terminal of the OSFET 7, and this end point is used as the control terminal 14 of the new rectifying MOSFET 7. The control circuit 13 becomes conductive on condition that the voltage of the control terminal 14 is higher than the voltage of the output terminal 11,
The rectifying MOSFET is controlled in synchronization with the conductive state. This allows rectifying MOSFE
T7 is turned on only when the current of the choke coil 9 increases toward the output terminal 11, and the reverse current described above does not occur.

The control circuit 13 attached in FIG. 1 raises the voltage of the control terminal 14 at which the rectifying MOSFET 7 is turned on to eliminate the disturbance of the synchronous operation.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 shows an example of a circuit in which the present invention is implemented in a one-stone type forward converter, in which a Zener diode 18 is used as a control circuit. Since the Zener diode 18 has a property of conducting at a certain voltage or more, it has an effect of raising the voltage of the control terminal 14 at which the rectifying MOSFET 7 is turned on.

The output terminal voltage Vo (voltage between terminals 11 and 12) is kept constant by the output voltage stabilization control circuit 6. When the current of the choke coil 9 continuously flows, the voltage Vs (14, 1,
The voltage between the 7 terminals is a rectangular wave, and the winding ratio of the transformer 5 is set to the primary side: the secondary side = Np: Ns, the primary side input voltage (2,
When the voltage between the three terminals) is Vi, a voltage of Vs = Ns / Np × Vi is generated. Since a forward voltage is applied to the parasitic diode 15 of the rectifying MOSFET 7 to make it conductive, the potential between the drain and source of the rectifying MOSFET 7 becomes substantially equal, and therefore the bias voltage Vd (14, 12) of the rectifying MOSFET control terminal 14 is made. The voltage between terminals is V
equal to s. In the forward converter, Vd is higher than the output voltage Vo when the main switch 4 is on, and Vd> Vo, which is the condition for turning on the rectifying MOSFET 7 after the zener diode 18 is attached, satisfies the conventional zener diode. MO not installed and rectifying MO
The operation of SFET 7 is equal.

On the other hand, when the current of the choke coil 9 becomes zero, the conduction of the commutation side diode 8 also ends, and the rectifying MO
Bias voltage Vd (14, 12) of the SFET control terminal 14
The voltage between terminals) rises. However, the condition that the rectifying MOSFET 7 is turned on by the action of the Zener diode 18 is Vd> Vo, and the rectifying MOSFET 7 is not turned on in the state where a reverse current can flow in the choke coil 9.

The resistor 16 surely turns off the rectifying MOSFET 7 against a minute leakage current when a voltage equal to or lower than the Zener voltage is applied to the Zener diode 18, and at the time of light load, the transformer 5 and the main switch MOSFE.
Rectification MOSFET due to an oscillating voltage waveform generated on the secondary side of the transformer 5 due to resonance caused by the output capacitance of T4.
For the case where 7 is turned on, rectifying MOSFE
There is an effect that the charge accumulated in the parasitic capacitance between the gate and the source of T is discharged to quickly turn it off.

FIG. 3 shows an example of a circuit in which the present invention is implemented in a one-stone type forward converter, in which an n-channel MOSFET 19 is used as a control circuit. MOSF
The ET 19 is controlled by a voltage applied to the gate terminal of the MOSFET 19 that divides the bias voltage Vd (voltage between terminals 14 and 12) of the rectifying MOSFET control terminal 14 by the resistors 20 and 21. MOSFET when Vd> Vo
If the values of the resistors 20 and 21 are determined so that 19 is turned on, the operation content of the rectifying MOSFET is the same as that when the Zener diode is used as the control circuit.

[0012]

According to the present invention, the reverse current of the choke coil is prevented so that the transformer and the choke coil are not abnormally excited. As a result, an excessive current in the main switch and the rectifying MOSFET is prevented, and the converter can operate stably without applying an excessive voltage to the main switch and the rectifying MOSFET even when the transformer is reset.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of a synchronous rectification converter of the present invention.

FIG. 2 is a semiconductor circuit diagram of an embodiment of the synchronous rectification converter of the present invention.

FIG. 3 is a semiconductor circuit diagram of an embodiment of a synchronous rectification converter of the present invention.

FIG. 4 is a semiconductor circuit diagram of an example of a conventional synchronous rectification converter.

[Explanation of symbols]

 1 ... DC power supply, 2 ... DC power supply input terminal, 3 ... Input side ground terminal, 4 ... Main switch, 5 ... Transformer, 6 ... Output voltage stabilization control circuit, 7 ... Synchronous rectification n-type MOSFET, 8 ... Diode, 9 ... Choke coil, 10 ... Capacitor, 11 ... Output terminal, 12 ... Output side ground terminal, 13 ... Rectification MOSFET control circuit, 14 ... Rectification MOSFET control terminal, 15 ... MOSFET parasitic diode.

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor Shinichi Kuwahara 180 Totsuka-cho, Totsuka-ku, Yokohama-shi, Kanagawa Hitachi Communication Systems Co., Ltd.

Claims (3)

[Claims] 1. A rectifying MOSFE utilizing the voltage appearing on the secondary side of the transformer when a DC voltage is converted into a rectangular wave pulse by a switching element and applied to the primary side of the transformer.
In a converter circuit for controlling T, a rectifying MOS is used as a control circuit set so that the rectifying MOSFET is turned on when the secondary voltage is higher than the output voltage.
A synchronous rectification circuit characterized by being connected to a gate terminal of an FET. 2. The synchronous rectification circuit according to claim 1, wherein a Zener diode is used as a control circuit for the rectification MOSFET. 3. The synchronous rectification circuit according to claim 1, wherein an n-channel MOSFET is used as a control circuit for the rectification MOSFET.