JP2001292571A - Synchronous rectifying circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize effective control of a synchronous rectifying transistor used in a switching power supply. SOLUTION: This synchronous rectifying circuit comprises a current detecting resistor R1 for detecting rectified current, a comparator U3 for comparing a terminal voltage Vcurrent of the current detecting resistor R1 with the prescribed threshold Vth and controllers U1, U2 for generating the turn-on and turn-off timings of synchronous rectifying transistors SR1, SR2 based on the output of the comparator U3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a synchronous rectifier circuit, and more particularly, to a synchronous rectifier circuit that can be used in a switching power supply device known as a flyback converter, a half-bridge resonant converter, or the like.

[0002]

2. Description of the Related Art A switching power supply using a synchronous rectifier circuit is used as a DC power supply in various devices such as a measuring device. FIG. 8 is a circuit diagram of a half-bridge resonance converter which is one of the conventional switching power supply devices of this type. Converter output transformer T1
The main transistors Q1 and Q2 are connected to the primary circuit 11 and are alternately turned on with the same duty ratio. Rectifier circuit 12 connected to the secondary side of output transformer T1
Output diode Co through diodes D1 and D2.
Is connected.

When the main transistor Q1 is turned off and the main transistor Q2 is turned on, a square wave voltage is applied to a series resonance circuit composed of the resonance reactor Lr, the exciting inductance Np of the output transformer T2, and the resonance capacitor Cr, and the series resonance is performed. A sinusoidal current flows through the circuit. A rectified current flows through the secondary side rectifier circuit 12 via the diode D2, charging the output capacitor Co and supplying the load current to the load.

When the main transistor Q2 is turned off and the main transistor Q1 is turned on, the charge stored in the resonance capacitor Cr is discharged by the energy stored in the resonance reactor Lr and the excitation inductance Np of the transformer. As a result, a rectified current flows through the secondary side rectifier circuit 12 via the diode D1 to charge the output capacitor Co and supply the load current to the load.

[0005]

In order to rectify and smooth the secondary winding current of a switching power supply, not only the above-described half-bridge type resonant converter, a rectifier circuit using a diode has conventionally been used. Used. Here, instead of a rectifier diode, an MO having a small on-resistance is used.
It is well known that a synchronous rectifier circuit that performs synchronous rectification using an SFET can reduce rectification loss in a switching power supply device. However, in such a synchronous rectifier circuit, there are few practical control circuits for efficiently controlling the synchronous rectification, and there have been few examples in which a switching power supply having a synchronous rectifier circuit is actually commercialized.

In view of the above, the present invention provides a synchronous rectifier circuit that can efficiently control and drive synchronous rectification.
Accordingly, it is an object of the present invention to realize a switching power supply device with low rectification loss and high energy efficiency.

[0007]

In order to solve such a problem, the present invention provides a synchronous rectifier circuit for a converter operating in a discontinuous current mode, in which a rectified current is detected and converted into a voltage signal. A rectifying current detector, a comparator for comparing the voltage signal with a predetermined threshold, and a controller for generating a turn-off timing signal of the synchronous rectification transistor in response to a comparison result of the comparator. And a synchronous rectifier circuit.

Further, according to the present invention, the converter is a half-bridge resonant converter, and one synchronous rectifying transistor is turned off in response to the turn-off timing signal, and the other synchronous rectifying transistor is turned off after a lapse of a predetermined time from the generation of the turn-off timing signal. The synchronous rectifier circuit, wherein a rectifier transistor is turned on.

Further, the present invention provides the synchronous rectifier circuit, wherein the rectified current detector has a CR series circuit connected in parallel to an output capacitor.

[0010] Further, according to the present invention, the rectified current detecting section may include:
The synchronous rectifier circuit includes an energy transfer capacitor connected in series with the synchronous rectifier transistor and a CR series circuit connected in parallel with the energy transfer capacitor.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to the drawings based on embodiments of the present invention.

FIG. 1 is a circuit diagram of a half-bridge type resonant converter having a synchronous rectifier circuit according to a first embodiment of the present invention. The converter according to the present embodiment is a diode D1 of the conventional converter described with reference to FIG.
D2 is replaced by synchronous rectification transistors SR1 and SR2, respectively; a resistor R1 connected in series to an output capacitor Co for detecting the current of the secondary rectifier circuit 12 is provided; This is different from the conventional converter in that a control circuit for controlling ON / OFF of SR2 is provided.

The control circuit includes a comparator U3 for comparing the terminal voltage of the resistor R1 to the inverting terminal and comparing the voltage with a threshold (Vth) input to the non-inverting terminal.
The output of the comparator U3 and the drain voltage of the corresponding synchronous rectification transistor SR1 or SR2 are input, and based on these inputs, the corresponding synchronous rectification transistor SR
1 or U2 for controlling the gate voltage of SR2
And The comparator U3 functions as a timing generator that generates turn-on / turn-off timing.

FIG. 2 is an operation waveform diagram obtained by an oscilloscope in the converter of the above embodiment. In the figure, Vgs1 and Vgs2 denote the gate voltages of the synchronous rectification transistors Q1 and Q2, respectively, and Vcurrent.
Represents the terminal voltage of the resistor R1, Vds2 represents the source / drain voltage of the synchronous rectification transistor Q2, and Vcmp1 represents the output voltage of the comparator U3. The current Id1 (Id) flowing through the synchronous rectification transistor Q1 (Q2)
2) is a source current Ids1 (Ids2) and a body diode (parasitic diode) current Isbd1 (Is2).
bd2). Timing signal Vrt / ct1
Is a signal generated in the control unit U1.

When the synchronous rectification transistor SR1 is off, SR
2 is ON, when the rectified current of the secondary rectifier circuit 12 decreases and the voltage Vcurrent falls below Vth, the output Vcmp1 of the comparator U3 goes to a high level, and the control units U1 and U2 output the timing signal V2 respectively.
rt / ct1 and Vrt / ct2 (not shown) are set to high level. The control unit U2 outputs the timing signal Vrt
Based on the rise of / ct2, the gate voltage Vgs2 of the synchronous rectification transistor SR2 is set to low level to turn off the synchronous rectification transistor SR2. Both timing signals shift to low level after a short predetermined period has elapsed. The control unit U1 controls the synchronous rectification transistor SR based on this fall of the timing signal Vrt / ct1.
Since the gate voltage of No. 1 is at a high level, the synchronous rectification transistor SR1 is turned on.

Thereafter, when the rectified current increases and then decreases, the control units U1 and U2 operate in the opposite directions, so that the synchronous rectification transistor SR1 is turned off and SR2 is turned on. This operation is repeated, and the converter generates a predetermined DC voltage while performing synchronous rectification. Source / drain voltage V of each synchronous rectification transistor SR1 and SR2
During the low level period (blanking period), the synchronous rectification transistor is on and ds1 (not shown) and Vds2 are input to the control units U1 and U2.
It acts so as not to turn on the other synchronous rectification transistor.

In the present embodiment, the turn-on and turn-off timings are obtained by monitoring the current flowing through the synchronous rectification transistor by utilizing the fact that the current is in the discontinuous mode and the oscillation frequency fluctuates. According to the configuration of the present embodiment, a general-purpose product can be used as the MOSFET driving IC and the comparator, so that there is an advantage that it can be realized at low cost.

FIG. 3 is a circuit diagram of a converter having a synchronous rectifier circuit according to a second embodiment of the present invention, and shows an example in which the present invention is applied to a flyback converter operating in a discontinuous current mode. The converter includes a main transistor Q3 connected in a primary circuit 13 connected to a primary winding of the output transformer T2 and supplying power to the output transformer T2 by switching, and a secondary winding of the output transformer T2. Synchronous rectification transistor SR3 for performing synchronous rectification in the secondary side rectifier circuit 14 connected to
, And a control unit U4 and a comparator U5.

The rectified current of the secondary rectifier circuit 14 is a resistor R
The signal is converted into a voltage signal Vcurrent by 1 and compared with a predetermined Vth by a comparator U5 to generate a turn-off timing. That is, when the rectified current falls below a predetermined value, the output of the comparator U5 goes to a high level, and on condition that the synchronous rectification transistor SR3 is turned on.
By setting the gate voltage to the low level, the synchronous rectification transistor SR3 is turned off. The turn-on timing is made using the source / drain voltage of the synchronous rectification transistor.

In the half-bridge resonant converter, the synchronous rectifier transistors SR1 and SR2 have a gradual rise in current at the time of turn-on and a small increase in loss even if the turn-on timing is slightly delayed. Producing both timings has the advantage of simplifying the control circuit. However, in the flyback converter, since the rise of the current at the time of turn-on is steep, as described above, the operation is more stable if only the turn-off timing is generated by the rectified current. The configuration of the present embodiment is also applicable to a quasi-resonant converter or a Cuk converter that operates in a discontinuous current mode.

When the synchronous rectifier circuit of the present invention is employed, the load becomes lighter during the operation of the converter and the load of the main transistor can be reduced both in the case where the turn-on signal is detected by the comparator and in the case where the turn-on signal is detected by the turn-on voltage waveform. When the ON period becomes short, the turn-on signal is not input, so that the synchronous rectification stops, and the drive loss becomes zero. That is, the loss of the converter is almost equal to that of the diode rectifier circuit. This makes it possible to reduce the power consumption during standby, and to comply with the regulation of the power saving mode.

FIG. 4 is a circuit diagram of a converter having a synchronous rectifier circuit according to a third embodiment of the present invention.
It is a modification of the embodiment. The difference from the embodiment of FIG. 1 is that an output capacitor Co is used instead of the resistor R1 of FIG.
Current detection unit CS1 connected in parallel with
DC reproducing unit V for reproducing a DC voltage signal from the output terminal
S1.

In the synchronous rectifier circuit of the present embodiment, the capacitor C1 having a smaller capacity than the output capacitor Co in the secondary rectifier circuit 12 in view of the fact that the loss in the resistor R1 increases when the load current of the converter increases. And a current detection unit CS composed of a series circuit of an appropriate detection resistor R1
1 is connected in parallel to the output capacitor Co. Further, the terminal voltage (shunt current detection voltage) Vic of the resistor R1 is led to a DC regeneration unit VS1 in which a capacitor C2 and a rectifier diode D3 are connected in series, and is converted into a DC detection voltage signal Vicdc. Monitor.

FIG. 5 is an oscilloscope waveform observed by the converter of FIG. DC detection voltage signal Vicdc
Has the same waveform as Vcurrent shown in FIG. 2 and can be used in the same manner. The obtained drain current waveform Ids of the synchronous rectification transistor SR1 has a better sine wave shape. The waveform in FIG. 5 is 220 μF for Co, 4.7 μF for C1, and 4.0 for R3 in the circuit of FIG.
This is obtained in an example using 7Ω. According to the present embodiment, the loss for detection is about 1/400 of 1.6 mW as compared with the detection by the detection resistor of FIG. For this reason, the 1 W resistor conventionally used could be replaced with a small resistor.

In this embodiment, since the anode of the diode D2 is directly connected to GND, the lowest voltage level of the obtained waveform of the DC regeneration voltage Vicdc remains at the negative voltage. By appropriately adjusting the potential, the offset potential can be adjusted.

FIG. 6 is a circuit diagram of a converter having a synchronous rectifier circuit according to a fourth embodiment of the present invention.
Shows the operation waveform in the same manner as FIG. The present embodiment is an example in which the present invention is applied to a Cuk-type converter. In taking out a rectified current, instead of inserting the resistor R1, an energy transfer capacitor Cs2 is provided.
The rectified current is converted into a voltage signal by using the CR series circuit forming the shunt circuit and is input to the comparator U5.
Other configurations in controlling the synchronous rectification transistor SR are the same as those in the control in FIG.

In many converters in which the secondary rectifier circuit is smoothed by a capacitor, the current of the smoothing capacitor is shunted,
A rectified current can be detected by DC regeneration. However, in the Cuk type converter, an output smoothing capacitor cannot be used, and thus the energy transfer capacitor Ce2 is used as described above. The current detection itself can be performed not only by the secondary side rectifier circuit but also by the primary side circuit or the auxiliary winding circuit of the output transformer. In the detection in the present embodiment, the loss was reduced to about 1/220 compared to the detection by the resistor.

As described above, the present invention has been described based on the preferred embodiment. However, the synchronous rectifier circuit of the present invention is not limited to the configuration of the above-described embodiment, but the configuration of the above-described embodiment. Synchronous rectification circuits with various modifications and alterations from are also included in the scope of the present invention.

[0029]

【The invention's effect】 [Brief description of the drawings]

FIG. 1 is a circuit diagram of a half-bridge resonance converter having a synchronous rectification circuit according to a first embodiment of the present invention.

FIG. 2 is an operation waveform diagram in the converter of FIG.

FIG. 3 is a circuit diagram of a flyback converter having a synchronous rectifier circuit according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram of a half-bridge resonant converter having a synchronous rectifier circuit according to a third embodiment of the present invention.

5 is an operation waveform diagram of the converter of FIG.

FIG. 6 is a circuit diagram of a Cuk converter having a synchronous rectification circuit according to a fourth embodiment of the present invention.

FIG. 7 is an operation circuit diagram of the converter of FIG. 6;

FIG. 8 is a circuit diagram of a half-bridge type resonant converter having a conventional synchronous rectifier circuit.

[Explanation of symbols]

 11, 13: primary side circuit 12, 14: secondary side rectifier circuit Q1, Q2: main transistor SR1 to SR3: rectifier transistor U1, U2, U4: control unit U3, U5: comparator Co: output capacitor Ce2: energy transfer capacitor Cin: input capacitor Cr: resonance capacitor C1, C2: capacitor R1, R2, R3: resistor T1, T2: output transformer Lr: resonance capacitor

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Seiichi Noguchi 2-93-2 Nakamachi, Musashino-shi, Tokyo F-term in Yokogawa Electric Corporation (reference) 5H730 AA14 BB26 BB43 BB44 BB66 DD04 EE13 FD31

Claims (4)

[Claims] 1. A synchronous rectifier circuit for a converter operating in a discontinuous current mode, comprising: a rectified current detector for detecting a rectified current and converting the rectified current into a voltage signal; and comparing the voltage signal with a predetermined threshold. A synchronous rectifier circuit comprising: a comparator; and a control unit that generates a turn-off timing signal of the synchronous rectifier transistor in response to a comparison result of the comparator. 2. The converter is a half-bridge resonant converter, wherein one synchronous rectifying transistor is turned off in response to the turn-off timing signal, and the other synchronous rectifying transistor is turned on after a lapse of a predetermined time from the generation of the turn-off timing signal. The synchronous rectifier circuit according to claim 1, wherein: 3. The synchronous rectifier circuit according to claim 1, wherein the rectifier current detector has a CR series circuit connected in parallel to an output capacitor. 4. The rectification current detection unit includes an energy transfer capacitor connected in series to a synchronous rectification transistor, and a CR series circuit connected in parallel with the energy transfer capacitor. 2. The synchronous rectifier circuit according to 1.