JPH09271167A - Synchronous rectifier circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce rectification loss in a rectifier part, and realize the zero voltage switching of the main switch, in a self-excited ringing choke converter. SOLUTION: In a self-excited ringing choke converter, the secondary winding N2 of a main transformer T1 is connected with a filter capacitor C1 and a load R0 , via a synchronous rectifier Q2 constituted of an MOSFET. The synchronous rectifier Q2 is so connect that it is driven by the generated voltage of a fitting winding NG formed on the secondary side of the main transformer T1 . Zero voltage switching at the time of turn-on of the main switch Q1 is attained, on the basis is the inverse direction current of the main switch Q1 which is generated by inverses excitation of the main transformer T1 from changed voltage of the filter capacitor C1 in the delay time of driving at the time of OFF of the synchronous rectifier Q2 caused by the driving winding NG.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited ringing choke converter, and more particularly to a synchronous rectification device which uses a synchronous rectification element in a rectification section and controls the rectification element by the output of a drive winding on the secondary side. It is about circuits.

A self-exciting ringing choke converter controls a main switch by an output of a base winding provided in a main transformer without externally controlling switching, thereby performing voltage conversion operation while self-oscillating. It is something to do.

In such a self-excited ringing choke converter, in order to improve efficiency, it is required to reduce rectification loss and realize zero voltage switching when the main switch is turned on.

[0004]

2. Description of the Related Art FIG. 5 shows a configuration of a conventional circuit, in which T ₁ is a main transformer, N ₁ is a primary winding, N ₂ is a secondary winding, and N _B is It is a base winding. Q ₁ is a main switch composed of a MOSFET, D ₁ is a rectifying element composed of a Schottky barrier diode, C ₁ is a smoothing capacitor, R ₀ is a load, and E _in is an input power supply. SO
The SC is a self-excited oscillating unit, and has a function necessary for the self-excited oscillating operation in the self-excited ringing choke converter.

[0005] During the on main switch Q _1, the input power supply E _in to the primary winding N ₁ of the main transformer T ₁ is the primary current I _Q1 flows, energy is stored in the main transformer T _1. When the main switch Q ₁ is off, the voltage V _T1 ² generated in the secondary winding N ₂ by the energy stored in the main transformer T ₁ is rectified by the rectifying element D ₁ and smoothed by the smoothing capacitor C ₁ to load the load. A DC voltage V ₀ is generated at R ₀ .

At this time, the self-excited oscillating section SOSC applies a bias to the main switch Q ₁ and at the same time, the base winding N _B
By controlling the turn-off of the main switch Q ₁ on the basis of the generated voltage, the self-oscillation of the ringing choke converter is performed, and the self-excited oscillation is activated when the power is turned on.

FIG. 6 is a timing chart showing the operation of the conventional circuit.
Chart, V_Q1Is the main switch Q₁ Voltage of I_Q1
Is the main switch Q₁ Current, V_T1 ^Two Is the secondary winding N_Two No electricity
Pressure, I _RIs a rectifying element D₁ Is the current. In the figure, G
Indicates the ground potential.

In FIG. 6, the voltage V _Q1 of the main switch Q ₁ is
, Corresponding to the main switch to Q ₁ off, the sum of the power supply voltage E _in the reverse voltage of the primary winding N _1, when the on main switch Q _1, the zero voltage. Current I _Q1 of the main switch Q ₁ is, flows corresponding to the ON of the main switch Q _1, whereby the energy is stored in the main transformer T _1, during turn-not the voltage of the main switch Q ₁ is turned to zero Therefore, a large surge current is generated by short-circuiting the charging voltage of the output capacitance of the FET corresponding to this voltage.

On the other hand, the main transformer T₁ On the secondary side of the
Itchi Q₁ When the main is turned on, the main transformer T ₁ Energy stored in
Main switch Q by Rugie₁ When off, rectifying element D
₁ Through the current I_RFlows, the secondary winding N_Two Voltage V_T1 ^Two 
Is generated.

[0010]

In the conventional ringing choke converter, as the rectifying element on the secondary side,
The rectification loss is reduced by using a Schottky barrier diode with a small forward voltage drop.

However, even in the case of the Schottky barrier diode, there is a limit in reducing the forward voltage drop. The Schottky barrier diode performs a rectifying action by the junction between a semiconductor and a metal, and it is possible to reduce the forward voltage drop to some extent depending on the type of metal, but this increases the leakage current in the reverse direction, for example. Therefore, the total loss increases. Therefore, the forward voltage drop of the rectifier cannot be reduced below a certain level,
There is a limit to the improvement of the conversion efficiency of the converter due to this.

Further, in the conventional ringing choke converter, when the main switch is turned on, a large current flows due to a short circuit of the charging voltage of the output capacitance of the FET before the main switch voltage V _Q1 becomes 0, and the current, Turn-on loss occurs due to voltage multiplication.

Due to these causes, the conventional self-excited ringing choke converter has a problem that its efficiency cannot be sufficiently increased. The present invention is intended to solve such a problem of the conventional art, and in a self-excited ringing choke converter,
An object of the present invention is to provide a synchronous rectifier circuit that can reduce the forward voltage drop of the rectifying element and can improve the conversion efficiency by enabling zero voltage switching.

[0014]

When a Schottky barrier diode is used as the rectifying element, it is difficult to greatly reduce the rectifying loss. On the other hand, MOSF
If the synchronous rectification method is applied using ET as the rectifying element, the forward voltage drop is determined by the product of the on-resistance of the synchronous rectifying element and the current flowing through the rectifying element. It is possible to realize a rectification method in which the directional voltage drop is small and the voltage value does not have a threshold value.

Further, by delaying the turn-on of the synchronous rectifying element, the main transformer is reversely excited from the smoothing capacitor during this period, and when the main switch is turned on,
Based on this reverse excitation, the current of the main switch is made to rise from a negative value and the voltage of the main switch is set to zero, whereby zero voltage switching at the time of turning on the main switch can be achieved.

As described above, according to the present invention, in the self-excited ringinghi choke converter, reduction of rectification loss and zero voltage switching at turn-on of the main switch can be realized, so that loss of the main switch is reduced. Then, in the self-excited ringingi choke converter,
It is possible to greatly improve the conversion efficiency.

Specific means for solving the problems of the present invention will be given below.

[0018] (1) the main primary winding N ₁ of the transformer T ₁ as well as connected to the power source E _in through the main switch Q _1, the main from the base winding N _B provided in the transformer T ₁ self Fubu Self-excited ringing in which the main switch Q ₁ is driven via the SOSC to cause self-excited oscillation, and the voltage generated in the secondary winding N ₂ of the main transformer T ₁ is rectified and smoothed to obtain a DC output. In the choke converter, the main transformer T
Synchronous rectifier Q comprising _one secondary winding N ₂ of a MOSFET
₂ is connected to the smoothing capacitor C ₁ and the load R ₀ via ₂ and is connected so as to drive the synchronous rectifier Q ₂ by the generated voltage of the drive winding N _G provided on the secondary side of the main transformer T _1. , Based on the reverse current of the main switch Q ₁ generated by the reverse excitation of the main transformer T ₁ from the charging voltage of the smoothing capacitor C ₁ during the delay time of the drive when the synchronous rectifier Q ₂ is turned off by the drive winding N _G. Achieving zero voltage switching when the main switch Q ₁ is turned on.

[0019]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which the same components as those in FIG. 5 are designated by the same symbols, and Q ₂ is provided at the secondary winding output. , MOSFET
, N _G is a drive winding provided on the secondary side of the main transformer T ₁ .

In comparison with the case of the conventional circuit shown in FIG.
In the circuit of the present invention, the main transformer T₁The secondary side of
A rectifying element D consisting of a Schottky barrier diode
₁ Instead of a synchronous rectifier Q consisting of a MOSFET_Two Use
It has been. Also, the main transformer T₁ Drive to the secondary side of
Winding N_GThe synchronous rectifier is provided according to the generated voltage.
Q _Two By configuring a synchronous rectification circuit by driving
I have. Drive winding N_GThe main switch Q₁ Is off
Synchronous rectifier Q_Two Voltage is generated in the direction to drive
Synchronous rectifier Q_Two Turn on.

As described above, in the circuit of the present invention, since the synchronous rectifier composed of the MOSFET is used as the rectifying element, the forward voltage drop has no threshold value as compared with the case of the conventional Schottky barrier diode. Also, since the on-resistance is small, the rectification loss can be reduced.

In the circuit of the present invention, zero voltage switching of the main switch can be realized.

FIG. 2 is a timing chart showing the operation of the circuit of the present invention, in which the same elements as those in FIG. 6 are indicated by the same symbols.

In FIG. 2, the voltage V _Q1 of the main switch Q ₁ is
, Corresponding to the main switch to Q ₁ off, the sum of the power supply voltage E _in the reverse voltage of the primary winding N _1, when the on main switch Q _1, the zero voltage. Current I _Q1 of the main switch Q ₁ is, flows corresponding to the ON of the main switch Q _1, whereby energy is stored in the main transformer T _1.

On the other hand, the synchronous rectifier Q_Two Is the drive winding N_Gof
Main switch Q depending on voltage₁ Turned on at the opposite timing
Or turned off. Main switch Q₁ When turned on, the main tran
S ₁ The energy stored in the main switch Q
₁ When off, synchronous rectifier Q_TwoThrough the current I_RFlows
But the main transformer T₁ The energy stored in
From a certain point, the smoothing capacitor C₁ Depending on the charging voltage of
The secondary winding N_Two And synchronous rectifier Q_Two In the opposite direction via
The current flows, the main transformer T₁ Reversely excite. In this case
Main transformer T₁ Reverse excitation period t_RIs the drive winding N_G
It depends on the voltage generated (number of turns), and the more turns, the longer
It becomes.

After that, when the synchronous rectifier Q ₂ is turned off and the main switch Q ₁ is about to be turned on, the current I of the main switch Q ₁ is generated by the energy of the reverse excitation of the main transformer T _1.
Q1 begins to flow from a negative value through the FET's built-in diode. As a result, the voltage V applied to the main switch Q _1
Q1 is primarily the drain of the main transformer T ₁ of the primary-side inductance and the main switch Q ₁ - abruptly becomes zero voltage by the resonance phenomenon based on the inter-source capacitance. This achieves zero voltage switching when the main switch Q ₁ is turned on.

In general, in a ringing choke converter, even when the energy to be stored in the primary side of the main transformer T ₁ is small at the time of a light load, the pulse of the current of the main switch Q ₁ is generated due to the operation of self-excited oscillation. The width cannot be narrowed. As a result, the state becomes overexcited, and the self-sustained pulsation becomes intermittent, resulting in intermittent operation and unstable operation.

However, in the circuit of the present invention, since there is a reverse excitation period of the main transformer T ₁ , the ON period of the main switch Q ₁ is apparently wide, and therefore intermittent operation is possible even when the pulse width becomes narrow. It does not wake up and can operate stably.

FIG. 3 shows an operation example (1) in the circuit of the present invention.
FIG. 4 shows (2), and FIG. 4 shows operation examples (3) and (4) in the circuit of the present invention. Operation example (1), (2), (3),
(4) is the number of turns of the drive winding N _{G 3} , 4, 4.5,
The case of 5 is shown. At this time, the number of turns of the primary winding N ₁ is 9, the number of turns of the secondary winding N ₂ is 6, and the number of turns of the base winding N _B is 10. The gate resistance of the synchronous rectifier Q ₂ is 0. In the figure, the time between one division is 2 μs
It is.

In the case of the operation examples (1) and (2), the driving winding N _G
Less turns of, the generation voltage is too small, there is little reverse excitation of the main transformer, thus, of the main switch Q ₁ current I
_Q1 cannot rise from a negative value. Further, the voltage V _Q1 of the main switch Q ₁ cannot fall rapidly to zero voltage. Therefore, zero voltage switching is not achieved in this state.

In the case of the operation example (3), since the number of turns of the drive winding N _G is appropriate, the reverse excitation of the main transformer has an appropriate value, so that the current I _Q1 of the main switch Q ₁ has a negative value. The voltage V _Q1 of the main switch Q ₁ sharply drops to zero voltage as it rises. Therefore, in this state, zero voltage switching is achieved.

On the other hand, in the case of the operation example (4), the drive winding N
_Since the number of turns of _G is large and the generated voltage is too large, there is too much reverse excitation of the main transformer. Therefore, the current I of the main switch Q _1
Q1 has a long negative value. In this case, the voltage V _Q1 of the main switch Q ₁ suddenly drops to zero voltage, and thus zero voltage switching is achieved, but the main transformer Q ₁ has a negative value during the period when the current I _Q1 has a negative value. Since energy is not transmitted to the secondary side of, the conversion efficiency is reduced.

[0033]

As described above, according to the present invention, in a self-excited ringing choke converter, a synchronous rectifier made of MOSFET is used as a rectifying element, and a suitable drive winding is provided on the secondary side of the main transformer. Since the drive voltage is applied to perform synchronous rectification, it is possible to reduce the rectification loss of the rectification unit and achieve zero voltage switching when the main switch is turned on.
It is possible not only to reduce the switching loss and improve the power conversion efficiency of the converter, but also to achieve stable operation at a light load.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a timing chart showing the operation of the circuit of the present invention.

FIG. 3 is a diagram showing operation examples (1) and (2) in the circuit of the present invention.

FIG. 4 is a diagram showing operation examples (3) and (4) in the circuit of the present invention.

FIG. 5 is a diagram showing a configuration of a conventional circuit.

FIG. 6 is a timing chart showing the operation of a conventional circuit.

[Explanation of symbols]

T ₁ Main transformer N ₁ Primary winding N ₂ Secondary winding N _B Base winding _NG Drive winding Q ₁ Main switch Q ₂ Synchronous rectifier SOSC Self-oscillation C ₁ Smoothing capacitor R ₀ Load

Claims (1)

[Claims] 1. A primary winding of a main transformer is connected to a power supply via a main switch, and the main winding is driven from a base winding provided on the main transformer via a self-excited oscillating section to be self-excited. In a self-exciting ringing choke converter that oscillates and rectifies and smoothes the voltage generated in the secondary winding of the main transformer to extract a DC output, in which the secondary winding of the main transformer is a synchronous MOSFET. It is connected to a smoothing capacitor and a load via a rectifier, and is connected so as to drive the synchronous rectifier by the voltage generated by a drive winding provided on the secondary side of the main transformer, and is synchronized by the drive winding. Based on the reverse current of the main switch caused by the reverse excitation of the main transformer from the charging voltage of the smoothing capacitor during the drive delay time when the rectifier is off, the main switch Synchronous rectifier circuit, characterized in that to achieve zero voltage switching at turn-on of the switch.