JP3259128B2 - Synchronous rectification circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronous rectifier circuit that performs rectification by turning on and off in synchronization with an induced voltage of a secondary winding of a transformer. The synchronous rectifier circuit replaces the rectifier-side diode and the flywheel-side diode with a field-effect transistor (MOS FET) having a small on-resistance, and turns on one of them in accordance with the polarity of the induced voltage of the secondary winding of the transformer. The other is turned off. Therefore, power loss can be reduced as compared with the case where a diode is used. Improvements in the efficiency and characteristics of such a synchronous rectifier circuit have been demanded.

[0002]

2. Description of the Related Art FIG . 2 is an explanatory view of a conventional example, in which a DC power supply E and a switching transistor Q1 are connected to a primary winding N1 of a transformer T1, and the switching transistor Q1 is turned on and off by a pulse width control circuit PWM. Under control, the current flowing through the primary winding N1 is turned on and off. A rectifier-side synchronous rectifier transistor Q2 and a flywheel-side synchronous rectifier transistor Q3 are connected to the secondary winding N2 of the transformer T1.
And a smoothing circuit composed of a choke coil L and a capacitor C. S is a source, D is a drain, G is a gate, and d1 to d3 are parasitic diodes formed between the n-layer and the p-layer in the transistor substrate.

The pulse width control circuit PWM compares the rectified and smoothed DC voltage with a set reference voltage, and applies a drive signal having a pulse width corresponding to the error voltage to the gate of the switching transistor Q1, thereby to form a primary voltage of the transformer T1. The current flowing through the winding N1 is controlled. Due to the induced voltage of the secondary winding N2 generated when the switching transistor Q1 is on, the gate G of the rectification-side synchronous rectification transistor Q2 has a positive polarity, the source S has a negative polarity, and the rectification-side synchronous rectification transistor Q2 is on. . Also, the gate G of the flywheel transistor Q3 has a negative polarity and the source S has a positive polarity,
The flywheel-side synchronous rectification transistor Q3 is turned off. Therefore, the current due to the induced voltage of the secondary winding N2 charges the capacitor C via the rectification-side synchronous rectification transistor Q2 and the choke coil L, and flows from the output terminal to a load (not shown).

Next, when the switching transistor Q1 is turned off, resonance phenomena due to the exciting inductance of the primary winding of the transformer T1, the winding capacitance, the parasitic capacitance of the switching transistor Q1, the parasitic capacitance of the rectification-side synchronous rectification transistor Q2, and the like. Is generated, the transformer T1 is self-reset, and a voltage of the opposite polarity is generated in the secondary winding N2. The rectification-side synchronous rectification transistor Q2 is turned off, the flywheel-side synchronous rectification transistor Q3 is turned on by the induced voltage of the secondary winding N2, and current flows through the choke coil L.

Therefore, the rectification side synchronous rectification transistor Q2
Rectified current flows when is turned on, current flows due to stored energy in the choke coil L when the flywheel-side synchronous rectifying transistor Q3 is turned on, and current flows with a small on-resistance, thereby reducing power loss. Can be.

The synchronous rectifying transistor Q on the flywheel side
3 has the same polarity as that of the flywheel-side diode, so that the flywheel-side synchronous rectifier transistor Q3 has the transformer T
1 due to the induced voltage of the secondary winding N2, if the switching transistor Q1 is not kept on during the off period,
The commutation current of the choke coil L flows through the parasitic diode d3. Since the forward voltage of the parasitic diode d3 is equal to or higher than that of a normal pn junction diode, for example, about 0.9 V, the power loss increases.

[0007] The parasitic diode d3 of the flywheel-side synchronous rectification transistor Q3 corresponds to a pn junction diode, the switching transistor Q1 is turned on, and the rectification-side synchronous rectification transistor Q2 is turned on by the induced voltage of the secondary winding N2 of the transformer T1. When turned on, the parasitic diode d3 of the flywheel-side synchronous rectifier transistor Q3
The recovery current flows through the secondary winding N2
Is short-circuited, and the current flowing through the switching transistor Q1 increases.

Therefore, a configuration shown in FIG. 3 has been proposed.
You. In the figure, T1 is a transformer, N1 is a primary winding,
N2 is a secondary winding, Q1 is a switching transistor, Q
2 is a rectification side synchronous rectification transistor, and Q3 is a flywheel.
L side synchronous rectification transistor, L is a choke coil, C is
Capacitor, PWM is pulse width control circuit, E is DC power
Source, R1 is a resistor, d1 to d3 are parasitic diodes, SD
1 and SD2 are Schottky diodes. This show
The diode SD1 and the diode SD2 are respectively parasitic diodes.
Set the same polarity on the rectifying side so that
Rectifier transistor Q2 and synchronous rectifier on flywheel side
It is connected in parallel with the transistor Q3.

A DC power supply E is connected to the primary winding N1 of the transformer T1.
And the switching transistor Q1, the pulse width
The control circuit PWM controls the switching transistor Q1.
ON / OFF control and switching transistor Q1
Adjust the induced voltage of the secondary winding N2 of the transformer T1 when turned on.
Between the gate G and the source S of the flow-side synchronous rectifier transistor Q2
To the synchronous rectifying transistor on the rectifying side.
Q2 is turned on, and the induced voltage of the secondary winding N2 is choked.
Smoothing by smoothing circuit consisting of coil L and capacitor C
And apply this smoothed DC voltage to a load (not shown).
Supply.

When the switching transistor Q1 is turned off,
Then, as described above, the excitation inductance of the transformer T1
Resonance due to the stray capacitance of the coil and winding and the parasitic capacitance of each part
State self-reset voltage is generated in the secondary winding N2,
The self-reset voltage is
The voltage applied between the gate G and the source S of the star Q3 is
The rywheel-side synchronous rectifier transistor Q3 is turned on.
You.

As a result, the accumulated energy of the choke coil L is reduced.
Flywheel with low on-resistance due to commutation current due to energy
Since the current flows through the side synchronous rectification transistor Q3,
Loss can be reduced. Also fly for some reason
Fast turn-off of the wheel-side synchronous rectifier transistor Q3
In the event that the power goes down, via the Schottky diode ZD2
Since the commutation current flows, the current flows through the parasitic diode d3.
Power loss compared to when
You.

In this case, the rectifying side synchronous rectifying transistor Q
The value of the resistor R1 connected to the gate G of the
Of the self-reset voltage induced in the secondary winding N2 of the
The waveform can be formed into a desired shape. Thereby,
Synchronous with the turning off of the switching transistor Q1
During the period in which the first rectifying transistor Q2 is turned off,
Continue turning on the side synchronous rectifier transistor Q3
be able to.

FIG . 4 shows the relationship between the voltage and current in the configuration of FIG.
FIG. 8 is a measurement waveform diagram showing the switching transistor Q1.
Drain current I _D and the drain-source waveform of the voltage V _DS
Is connected to the gate G of the rectification side synchronous rectification transistor Q2.
The results obtained by changing the value of the resistance R1 are shown in FIGS.
(E). V _E indicates the voltage of the DC power supply E. Again
The rain-source voltage V _DS is the secondary winding of the transformer T1.
This is equivalent to the self-reset voltage induced in N2.

FIG . 4A shows the case where R1 = 0, that is,
In the case of the configuration shown in FIG. 2, the input voltage is 50.02.
V, the input current is 1.2525 A, and the output voltage is 5.005.
V, the output current is 10.004 A, and the efficiency is 79.9.
2%. The drain current _ID is the switching current .
When the transistor Q1 turns on, the flow
The in-source voltage V _DS becomes zero.

The switching transistor Q1 is
The drain-source voltage V _DS immediately before turning on is DC
It is reduced to a voltage V _E of the power supply E. That is, self reset
The voltage turns on the switching transistor Q1.
It does not continue until before. Because of that, the switch
Flywheel before the switching transistor Q1 turns on.
The side synchronous rectifier transistor Q3 is turned off,
The current flows through the parasitic diode d3 or as shown in FIG.
In this case, the current flows through the Schottky diode ZD2.
You. Therefore, the flywheel-side synchronous rectifier transistor Q
Power loss increases compared to the case where 3 continues the ON state
I do.

(B) shows a case where R1 = 3.18Ω.
Show. In this case, the input voltage is 50.04V and the input current is
1.2210A, output voltage is 5.005V, output current is
At 10.003 A, the efficiency was 81.94%. Immediately
That is, the efficiency is improved as compared with the case where R1 = 0. or
The drain-source voltage V _DS is equal to the switching transistor.
The voltage V of the DC power supply E until just before the star Q1 turns on.
_The state higher than _E continues and the self-reset voltage continues
You can see that it has occurred. Thereby switching
Flywheel until just before transistor Q1 turns on
The synchronous rectifier transistor Q3 to the ON state
Can be.

(C) shows a case where R1 = 3.6Ω.
Show. In this case, the input voltage is 50.03V and the input current is
1.1911A, output voltage is 5.005V, output current is
At 10.003 A, the efficiency was 84.01%. or
As in the case of (b), the switching transistor Q1
Self-reset voltage continues until just before turn-on
As a result, the flywheel-side synchronous rectifier transistor Q
3 keeps the ON state and reduces the commutation current
Flow through the wheel-side synchronous rectifier transistor Q3
Can be.

(D) shows a case where R1 = 8.4Ω.
In this case, the input voltage is 50.06 V and the input current is
1.1786A, output voltage is 5.005V, output current is
At 10.004 A, the efficiency was 84.86%. this
Even if the self-reset voltage is the switching transistor
It occurs continuously until just before Q1 turns on, and fly
Wheel-side synchronous rectifier transistor Q3 keeps on
can do.

(E) is the case where R1 = 20.9Ω.
Where the input voltage is 50.07 V and the input current is 1.18.
32A, output voltage is 5.005V, output current is 10.0
At 04A, the efficiency was 84.51%. Also in this case,
The self-reset voltage is set by the switching transistor Q1.
It occurs continuously just before turning on the flywheel.
To keep the ON state of the side synchronous rectification transistor Q3
However, in contrast to (d), switching
Drain saw just before transistor Q1 turns on
Since the inter-voltage V _DS increases, the transistor Q1 turns.
Efficiency decreases because on-loss increases.

The gate of the synchronous rectifying transistor Q2 on the rectifying side
By connecting the resistor R1 to the
Immediately before the switching transistor Q1 is turned on.
To the flywheel-side synchronous rectifier
Transistor Q3 and switching transistor Q1 is off.
During the period of the
Commutation current via synchronous rectifier transistor Q3 on wheel side
Current can flow, the parasitic diode d3 or
A commutation current flows through the Schottky diode ZD2
Power loss can be reduced as compared with the case.

The efficiency is affected by the value of the resistor R1.
And connect Schottky diodes SD1 and SD2.
, Through the parasitic diodes d2 and d3
Even if a state where current flows occurs, the forward
Flows to the side of the Schottky diode SD1, SD2
Will be.

FIG . 5 is an explanatory diagram of a configuration provided with an auxiliary transformer.
And the same reference numerals as those in FIG. 3 indicate the same parts.
Transformer, N11 is a primary winding, N12 is a secondary winding
You. The primary winding N11 of the auxiliary transformer T2 is connected to a transformer.
T1 is connected in parallel with the primary winding N1, and the auxiliary transformer T
2 to the flywheel side synchronous rectifier transformer N12.
It is connected to the gate and source of the transistor Q3.

A pulse width control circuit PWM (not shown )
The switching transistor Q1 is turned off by these drive signals.
And the induced voltage of the secondary winding N2 of the transformer T1
Therefore, the rectification side synchronous rectification transistor Q2 is turned on,
A smoothing circuit composed of a choke coil L and a capacitor C
A DC voltage is output via the power supply. At that time, the auxiliary transformer T
2, the induced voltage of the secondary winding N12 is the same as that on the flywheel side.
Polarity to maintain the initial rectifier transistor Q3 in the off state.
You.

Also, a switch is controlled by a pulse width control circuit PWM.
When the switching transistor Q1 is turned off, the
The polarity of the induced voltage of the secondary winding N2 is inverted, and the
Current transistor Q2 is turned off, and the auxiliary transformer T2
Of the flywheel side due to the induced voltage of the secondary winding N12.
The first rectifying transistor Q3 is turned on. By doing so,
The commutation current due to the stored energy of the inductor L is low
Resistor R3 on the flywheel side
Flow through Secondary winding of auxiliary transformer T2 in this case
The induced voltage of N12 is the switching voltage of switching transistor Q1.
Because it will continue until just before turn-on,
Rectifier transistor Q3 is rectifier side synchronous rectifier transistor
During the off-period of Q2, a commutation current flows by continuing the on-state
be able to.

[0025]

The structure of the conventional example shown in FIG .
As described above, the synchronization adjustment on the flywheel side
Loss due to the parasitic diode d3 of the current transistor Q3
is there. It is In the configuration shown in Matazu 3, Schottky die
The rectifier side synchronous rectification transistor Q
2 and the flywheel-side synchronous rectifier transistor Q3.
By connecting each in parallel, the parasitic diode
And the resistor R1 is connected to the rectifier side synchronous rectifier transistor.
Connected to the gate of the transistor Q2
During the off-period of Q1, the flywheel type synchronous rectifier transformer
The commutator current is caused to flow by continuously turning on the transistor Q3.
Can be set, but it is not easy to set the resistance value of the resistor R1.
There's a problem.

Further, in the configuration shown in FIG .
Primary winding of the auxiliary transformer T2 in parallel with one primary winding N1
N11, and the induced voltage of the secondary winding N12 is
Marked on the gate of the synchronous rectifier transistor Q3 on the rywheel side
The rectifying-side synchronous rectifying transistor Q2 is turned off.
During the period, the flywheel-side synchronous rectifier transistor Q3
And the commutation current can be caused to flow while the ON state is maintained. I
However, the primary winding N of the transformer T1 and the auxiliary transformer T2
1, N11 has different number of turns and impedance
To the primary windings N1 and N11 connected in parallel, respectively.
There is a problem that a current flows according to the difference between the induced voltages. Departure
Ming aims to solve the aforementioned problems and improve efficiency.
Target.

[0027]

Means for Solving the Problems] synchronous rectifier circuit of the present invention, with reference to FIG. 1, the primary winding of the transformer T1
Switching transistor Q1 connected in series with line N1
And connected in series to the secondary winding N2 of the transformer T1.
The rectification side synchronous rectification transistor Q2 and the secondary winding N2
In parallel with the rectification side synchronous rectification transistor Q2
Connected flywheel side synchronous rectifier transistor Q3
A synchronous rectifier circuit of the forward converter having an auxiliary transformer for applying the induced voltage of the gate in the secondary winding N12 of the flywheel side synchronous rectification transistor Q3
ON at the same time as T2 and switching transistor Q1,
The auxiliary switching transistor Q4 that is turned off
The primary winding N11 of the auxiliary transformer T2 and the auxiliary switch.
A series circuit with the switching transistor Q4 and a transformer T1.
Between the primary winding N1 and the switching transistor Q1.
And a column circuit connected in parallel to a DC power supply .

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG . 1 illustrates an embodiment of the present invention.
In the figure, T1 is a transformer, N1 is a primary winding, N2 is a secondary winding.
Next winding, Q1 is switching transistor, Q2 is rectification
Side synchronous rectifier transistor, Q3 is flywheel side synchronous
Rectifier transistor, Q4 is auxiliary switching transistor
, T2 is an auxiliary transformer, N11 is a primary winding, N12 is
Secondary winding, L is choke coil, C is capacitor, E is
Indicates a DC power supply.

The auxiliary transformer T2 is compared with the transformer T1.
It can be made small and small
The auxiliary switch is connected in series with the primary winding N11 of the auxiliary transformer T2.
Switching transistor Q4 and the secondary winding N12.
The induced voltage is changed to the flywheel side synchronous rectifier transistor Q3.
Of the switching transformer.
Along with the transistor Q1, the auxiliary switching transistor Q4
From the pulse width control circuit PWM (not shown)
On / off control is performed.

Therefore, the switching transistor Q1 is
When ON, the auxiliary switching transistor Q4 is also ON.
The primary winding N1 of the transformer T1 and the auxiliary transformer T2
Current flows from the DC power source E to the primary winding N11 of the
Rectification-side synchronization by the induced voltage of the secondary winding N2 of the
Current transistor Q2 is on and the secondary winding of auxiliary transformer T2
Synchronous rectification on flywheel side by induced voltage on line N12
The transistor Q3 is turned off.

When the switching transistor Q1 is turned off,
Then, the auxiliary switching transistor Q4 is also turned off.
Rectified by the induced voltage of the secondary winding N2 of the transformer T1.
Side synchronous rectifier transistor Q2 is off, auxiliary transformer T2
The flywheel side by the induced voltage of the secondary winding N12
The synchronous rectification transistor Q3 is turned on. At this time,
The primary winding N11 of the transformer T2 is
It is in a state of being separated from the winding N1.

Therefore, the primary winding N1 of the transformer T1,
The number of turns of the auxiliary transformer T2 with the primary winding N11 and the impedance
The primary windings N1 and N11
Make the current flowing between them zero to prevent an increase in loss
Can be. Also, flywheel-side synchronous rectifier transistor Q
3 immediately before the switching transistor Q1 is turned on.
Up to low on-resistance flywheel-side synchronous rectifier
The ON state of the star Q3 can be continued.

[0033]

As described above, the present invention provides
Auxiliary transformer in synchronous rectifier circuit of
The auxiliary switching transistor is connected to the primary winding N11 of the switch T2.
Connected to the auxiliary switching transistor Q4.
4 and a switch connected to the primary winding N1 of the transformer T1
ON / OFF control of the transistor Q1 at the same time
The induced voltage of the secondary winding N12 of the transformer T2 is
Applied to the gate of the gate-side synchronous rectifier transistor Q3.
The synchronous rectifier transformer on the flywheel side
The transistor Q3 is turned on by the switching transistor Q1.
It is possible to continue the on state until just before the on,
Therefore, low on-resistance flywheel side synchronous rectifier transformer
A commutation current can flow through the transistor Q3. It
Can reduce power loss and improve efficiency
it can.

The primary winding N of the auxiliary transformer T2 having a small capacity
11 has the auxiliary switching transistor Q4 connected.
As a result, the primary winding N1 of the transformer T1 and the auxiliary transformer
The number of turns of the primary winding N11 with the primary winding N11 and the impedance are
Even if they differ, the current between the primary windings N1 and N11
No reflux, thus preventing an increase in losses
Can be

[Brief description of the drawings]

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

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

FIG. 3 is an explanatory diagram of a configuration proposed earlier.

FIG. 4 shows the voltage and voltage depending on the value of a resistor R1 connected to a gate .
It is a flow measurement waveform diagram.

FIG. 5 is an explanatory diagram of a configuration provided with an auxiliary transformer.

[Explanation of symbols]

Q1 Switching transistor Q2 Rectification side synchronous rectification transistor Q3 Flywheel side synchronous rectification transistor Q4 Auxiliary switching transistor T1 Transformer N1 Primary winding N2 Secondary winding T2 Auxiliary transformer N11 Primary winding N12 Secondary winding L Choke coil C Capacitor

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

Claims (1)

(57) [Claims] (1)A switch connected in series with the primary winding of the transformer
To the switching transistor and the secondary winding of the transformer
A rectification-side synchronous rectification transistor connected in series;
Parallel to the winding and the rectifying side synchronous rectifying transistor
And the flywheel-side synchronous rectifier transistor connected to
HaveIn the synchronous rectifier circuit of the forward converter
To the gate of the flywheel-side synchronous rectification transistor
An auxiliary transformer for applying an induced voltage of the secondary winding, ON / OFF control simultaneously with the switching transistor
An auxiliary switching transistor, The primary winding of the auxiliary transformer and the auxiliary switching
A series circuit with a transistor, a primary winding of the transformer,
A series circuit with the switching transistor;
Has a configuration connected in parallel to the power supply Characterized in that
Period rectifier circuit.