JP3066720B2 - Synchronous rectification circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a synchronous rectifier circuit using a field effect transistor (MOS FET) as a rectifier. The forward voltage of the diode is about 0.5 V, whereas the on-resistance of the field effect transistor is about 10 mΩ or less. For example, when a current of about 20 A flows, the diode has However, the power loss in the field effect transistor is about 4 W, and the power loss can be reduced to about 40% in this case by using the field effect transistor. Therefore, there is known a synchronous rectifier circuit in which a field effect transistor is connected instead of the rectifying diode connected to the secondary winding of the main transformer, and on / off control is performed in accordance with the timing of the induced voltage of the secondary winding. Have been.

[0002]

2. Description of the Related Art FIG. 2 is an explanatory view of a conventional example, in which a DC power source E is connected to the middle point of the primary winding of a main transformer T11.
The switching transistor Q1 is connected to both ends of the primary winding.
1 and Q11 ', and the switching transistors Q11 and Q11' are alternately turned on and off by a pulse width control circuit PWM, so that the voltage induced in the secondary winding of the main transformer T11 is full-wave rectified. First, the sources S of the second synchronous rectification transistors Q12 and Q13 are connected, the induction voltage of the auxiliary winding of the main transformer T11 is connected to the gates G of the synchronous rectification transistors Q12 and Q13, and the drain D is connected. The flywheel diode D11 is connected in common, and the choke coil L
11 and a capacitor C11 are connected.

The output DC voltage from the smoothing circuit is detected by a pulse width control circuit PWM and compared with a set reference voltage.
A drive signal having a pulse width corresponding to the error voltage is formed and applied to the gates of the switching transistors Q11 and Q11 ', and the ON periods of the switching transistors Q11 and Q11' are controlled so as to stabilize the output DC voltage.

[0004] Switching transistors Q11, Q1
A voltage is induced in the secondary winding and the auxiliary winding of the main transformer T11 in response to turning on and off of 1 '. For example, when the source S side of the first synchronous rectification transistor Q12 has a positive polarity, the gate G has a positive polarity with respect to the source S,
The first synchronous rectification transistor Q12 is turned on. At that time, the gate G of the second synchronous rectification transistor Q13
Becomes negative with respect to the source S, and the second synchronous rectification transistor Q13 is turned off. When the polarity of the induced voltage of the secondary winding is reversed, the first synchronous rectification transistor Q12 is turned off and the second synchronous rectification transistor Q13 is turned on, so that the induced voltage of the secondary winding is full-wave rectified. Can be. Then, the choke coil L11 and the capacitor C
11 and supplies a stabilized DC voltage to a load (not shown).

The first and second synchronous rectification transistors Q
During a period in which both 12 and Q13 are off, a current continuously flows through the choke coil L11 via the flywheel diode (commutation diode) D11. Also in this case, since a current flows through the diode D11 and power is consumed, a Schottky diode having a low forward voltage may be used.

[0006]

In a bridge type or push-pull type inverter, a flywheel diode (commutation diode) D11 is replaced with a field effect transistor to perform a synchronous operation, thereby reducing power loss. It is conceivable to reduce it. However, during the period in which the first and second synchronous rectification transistors Q12 and Q13 are off, the induced voltage on the secondary side of the main transformer T11 becomes zero, so that the drive voltage of the flywheel transistor cannot be obtained. It is. Therefore, as shown in FIG. 2, the flywheel diode (commutation diode) D11 is connected to perform commutation of the choke coil L11.
An object of the present invention is to secure an operation power supply with a simple configuration, provide a flywheel transistor, and perform synchronous rectification even during a flywheel period to reduce rectification power loss.

[0007]

A synchronous rectifier circuit according to the present invention is connected to secondary windings N2a and N2b of a main transformer T1 of a bridge-type or push-pull type inverter and alternately turned on and off to form a secondary rectifier circuit. First and second synchronous rectification transistors Q2 and Q3 for rectifying and outputting the induced voltage of the winding, a choke coil L1 and a capacitor C1 for applying the output voltages of the first and second synchronous rectification transistors Q2 and Q3. , A flywheel transistor Q4 that is turned off during the on-period of the first and second synchronous rectification transistors Q2 and Q3 and is turned on during the off-period, and a drive circuit that drives the flywheel transistor Q4 When,
And a capacitor C2 that rectifies and charges the induced voltage of the secondary winding of the main transformer T1 and serves as an operating power supply for the drive circuit.

A capacitor C2 for operating power supplies a voltage induced by the secondary winding N2a of the main transformer T1 to a diode D2.
3 shows a case where charging is performed by rectification, and the terminal voltage of the capacitor C2 is applied to a drive circuit including transistors Q5 to Q8. When the induced voltage of the secondary windings N2a and N2b of the main transformer T is zero and both the first and second synchronous rectification transistors Q2 and Q3 are off, the transistor Q5 constituting the drive circuit is off and the transistor Since Q6 is on, transistor Q7 is on and transistor Q8 is off, flywheel transistor Q4 is on and commutation of choke coil L1 is performed.

[0009]

FIG. 1 is an explanatory view of an embodiment of the present invention.
T1 is a main transformer, N1 is a primary winding, N2a, N2
b is a secondary winding, N3a and N3b are auxiliary windings, Q1 and Q
1 'is a switching transistor, E is a DC power supply, Q
2, Q3 are first and second transistors for synchronous rectification, Q4
Are flywheel transistors for synchronous rectification, Q5
Q8 is a transistor forming a driving circuit, D1 to D3 are diodes, ZD is a Zener diode, PWM is a pulse width control circuit, R1 to R4 are resistors, C1 and L1 are capacitors and choke coils forming a smoothing circuit, and C2 is an operation. This is a power supply capacitor.

The on / off operation of the switching transistors Q1 and Q1 'by the pulse width control circuit PWM and the operation of the first and second synchronous rectification transistors Q2 and Q3 are the same as those of the conventional example shown in FIG. Duplicate description is omitted. The flywheel transistor Q4 in this embodiment is composed of first and second synchronous rectification transistors Q4.
As in the case of Q2 and Q3, a case is shown in which an n-channel field-effect transistor having a low on-resistance is used.

The transistor Q7 of the driving circuit shows a case of a p-channel field-effect transistor, and the transistor Q8 shows a case of a complementary type of an n-channel field-effect transistor. When a positive polarity voltage is applied to the gate via the diodes D1 and D2, the transistor When Q7 is off and transistor Q8 is on, and the gate applied voltage is zero, transistor Q7 is on and transistor Q8 is off.

The flywheel transistor Q4 is turned on when the transistor Q7 is turned on and the terminal voltage of the capacitor C2 is applied to the gate. When a voltage is applied to the base via the diodes D1 and D2, the transistor Q5 turns on, and the transistor Q6 turns off. When the transistor Q5 is turned off, the terminal voltage of the capacitor C2 is applied to the base of the transistor Q6 via the Zener diode ZD and the resistor R2, and when the Zener diode ZD is turned on, the transistor Q5 is turned on.
6 turns on. That is, a case is shown in which the threshold voltage of the transistor Q6 is increased so that malfunction does not occur. When the transistor Q6 turns on, the transistor Q7 turns on, the transistor Q8 turns off, and the flywheel transistor Q4 turns on.

The induced voltage of the secondary winding N2a of the main transformer T1 is applied to the capacitor C2 via the resistor R4 and the diode D3, and the capacitor C2 is charged. The terminal voltage of the capacitor C2 is connected via the resistor R2 to the collector of the transistor Q5 and further to the Zener diode ZD.
To the base of the transistor Q6, and to the complementary transistors Q7 and Q8. That is, the capacitor C2 functions as a power supply of the drive circuit.

Therefore, as described above, the main transformer T
Due to the induced voltage of the first auxiliary winding N3a, a positive polarity voltage is applied to the gate of the first synchronous rectification transistor Q2. When a negative polarity voltage is applied, the first synchronous rectification transistor Q2 is turned on, and the induced voltage of the secondary winding N2a is applied to the smoothing circuit as a rectified output. Also, the second synchronous rectification transistor Q3
Turns off.

The capacitor C2 is charged via the resistor R4 and the diode D3. The base of the transistor Q5 and the transistor Q7,
A positive voltage is applied to the gate of Q8, transistor Q5 is on, transistor Q6 is off, and transistor Q5 is off.
7 is off, the transistor Q8 is on, and the flywheel transistor Q4 is off.

The secondary winding N2a of the main transformer T1,
When the induced voltages of N2b and the auxiliary windings N3a and N3b are inverted, the first synchronous rectification transistor Q2 is turned off and the second synchronous rectification transistor Q2 is turned off.
Is turned on, and the induced voltage of the secondary winding N2b is rectified and applied to the smoothing circuit. At that time, the transistor Q is connected via the diode D2.
A positive polarity voltage is applied to the base of transistor 5 and the gates of transistors Q7 and Q8, transistor Q5 is on, transistor Q6 is off, transistor Q7 is off, transistor Q8 is on, and flywheel transistor Q4 is off.

When the induced voltages of the auxiliary windings N3a and N3b become zero, the first and second synchronous rectification transistors Q
2 and Q3 are turned off, and the output voltages of the diodes D1 and D2 become zero, so that the transistor Q5 is turned off.
Thereby, the terminal voltage of the capacitor C2 is connected to the resistor R2 and the Zener diode ZD at the base of the transistor Q6.
When the terminal voltage of the capacitor C2 exceeds the Zener voltage of the Zener diode ZD, the transistor Q6 is turned on and the transistor Q7 is turned on, so that the terminal voltage of the capacitor C2 is passed through the transistor Q7. To the gate of the flywheel transistor Q4.
Is turned on, and commutation of the choke coil L1 is performed.
The current flowing at this time flows through the flywheel transistor Q4 having a small on-resistance, so that the power loss is small.

The present invention is not limited to the above-described embodiment, but can be variously modified. The driving circuit including the transistors Q5 to Q8 drives the flywheel transistor Q4. If,
Other configurations can be applied. In addition, the capacitor C2 as an operating power source is shown to charge the induced voltage of one secondary winding N2a via the diode D3, but the induced voltage of the other secondary winding N2b can be used.
It is also possible to adopt a configuration in which charging is performed using induced voltages of both secondary windings. Also, the present invention can be applied to a multi-stone inverter such as a push-pull or bridge inverter.

[0019]

As described above, according to the present invention, the secondary winding is connected to the secondary windings N2a and N2b of the main transformer T1 of a bridge type or push-pull type inverter, and is turned on and off alternately. The first and second synchronous rectification transistors Q2 and Q3 that rectify and output the induced voltages of the lines N2a and N2b to the smoothing circuit, the flywheel transistor Q4, and the operating power supply of the drive circuit of the flywheel transistor Q4 During the period in which the first and second synchronous rectifying transistors Q2 and Q3 are both off, the induced voltage on the secondary side of the main transformer T1 becomes zero. By applying the terminal voltage of C2 to the drive circuit, the flywheel transistor Q4 can be turned on and commutation of the choke coil L1 can be performed. Will, by using a field effect transistor for low synchronous rectification on-resistance (FET), to reduce power loss, there is an advantage that it is possible to improve the overall efficiency.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a conventional example.

[Explanation of symbols]

 Q2, Q3 First and second synchronous rectification transistors Q4 Flywheel transistor T1 Main transformer Q5 to Q8 Transistor constituting drive circuit L1 Choke coil C1 Capacitor C2 Capacitor as operating power supply

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/28 H02M 3/337 H02M 7/21

Claims (1)

(57) [Claims] 1. A first and a second switch connected to a secondary winding of a main transformer of a bridge type or push-pull type inverter and alternately turned on and off to rectify and output an induced voltage of the secondary winding. A transistor for synchronous rectification, a smoothing circuit including a choke coil and a capacitor for applying an output voltage of the first and second synchronous rectification transistors, and a smoothing circuit that is turned off during an on period of the first and second synchronous rectification transistors. A flywheel transistor to be turned on during an off period; a drive circuit for driving the flywheel transistor; and a rectifying and charging an induced voltage of a secondary winding of the main transformer, and an operating power supply for the drive circuit. And a synchronous rectifier circuit comprising: