JP3396605B2 - Limiting circuit of synchronous rectifier circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronous rectifier circuit for a switching power supply, and more particularly to a synchronous rectifier circuit using a field effect transistor (MOSFET) for a rectifying element and a commutating element. The limiting circuit. 2. Description of the Related Art In a rectifying operation of a synchronous rectifier circuit, when a power supply is started / stopped or a change occurs in input / output, a rectifying element and a commutating element are separated by a signal delay of a commutating element control section. At the same time, conduction (hereinafter, referred to as short circuit) may occur. During this short-circuit period, an extremely large current flows, causing a large damage to the element. In order to shorten such a short-circuit period, a limiting circuit that limits the pulse width of a driving pulse that drives a commutation element of a synchronous rectifier circuit is used. FIG. 7 shows a configuration of a conventional limiting circuit of a synchronous rectifier circuit. In the figure, reference numeral 50 denotes a synchronous rectification control circuit that supplies a drive pulse to a commutation element 70 that constitutes a synchronous rectification circuit. The synchronous rectification control circuit 50 includes a control circuit 52 that generates a drive pulse, and a control circuit 52. And a drive circuit 54 for amplifying the generated drive pulse to a predetermined level. Reference numeral 60 denotes a limiting circuit.
Is a commutation element 70 (in this example, an N-channel MOSFET
It is. A) a detection circuit 62 for detecting whether the rectification operation of the synchronous rectifier circuit has entered a short-circuit period based on the drain-source voltage VDS, and a synchronous rectification control circuit 50. The commutation element 7 connected between the commutation element 70 and the commutation
And a switch circuit 64 for limiting the pulse width of the drive pulse output from the drive circuit 54 of the synchronous rectification control circuit 50 when 0 is short-circuited. In this case, in the conventional limiting circuit, the function of the switch circuit for limiting the pulse width of the drive pulse output from the synchronous rectification control circuit has been realized by using the threshold voltage VTH unique to the switching element used in the switch circuit. . [0005] However, in the above-described conventional synchronous rectification circuit limiting circuit, the function of the switch circuit for simply limiting the pulse width of the drive pulse output from the synchronous rectification control circuit is added to the switch circuit. Since this is realized by using the threshold voltage VTH specific to the switching element used, there are various problems depending on the switching element used in the switch circuit. That is,
When a bipolar transistor (BJT) is used as a switching element constituting a switch circuit, an N-channel MO as a commutation element 70 in the conventional example shown in FIG.
While the SFET is off, the drain-source voltage VDS of the MOSFET is high. Detection circuit 62
Is supplied between the base and the emitter of a bipolar transistor (BJT) as a switching element constituting a switch circuit, so that during the period when the MOSFET 70 is in a non-conductive state, the voltage VDS is used as a switching element. There is a problem that a base current always flows through the bipolar transistor (BJT) and a large power loss occurs. When a field-effect transistor (FET) is used as a switching element constituting a switch circuit, a threshold voltage VTH for turning on the field-effect transistor is as high as about 2 to 4 V.
The MOSFET as the commutation element 70 is short-circuited,
When the voltage VDS between the drain and the source of the commutation element 70 becomes equal to or lower than the threshold voltage VTH, the field effect transistor as a switching element constituting the switch circuit becomes non-conductive, and the commutation element 70 may be made non-conductive. This makes it impossible to perform the function as a limiting circuit. The present invention has been made in view of such circumstances, and provides a synchronous rectification circuit limiting circuit that reduces power loss and shortens a short circuit period in a commutation element of the synchronous rectification circuit. With the goal. [0010] The present invention provides a field effect transistor as a rectifying element and a commutating element. In a synchronous rectification circuit limiting circuit for limiting a pulse width of a drive pulse supplied from the synchronous rectification control circuit to the commutation element of the synchronous rectification circuit using the rectifier circuit, a drain-source voltage of a field effect transistor as the commutation element A voltage detecting means for detecting the voltage detection means, and a low operating voltage switching means for turning off the field effect transistor when the rectifying operation of the synchronous rectifier circuit enters a short-circuit period based on the detection result of the voltage detecting means. When the rectification operation of the synchronous rectification circuit enters a short-circuit period, the synchronous rectification control circuit performs the switching. And a capacitor for limiting the output current flowing through the stage, the voltage detecting means and connected to said field effect transistor between the switching means becomes nonconductive, said when the short period has ended state detection means and the switching means And a circuit disconnecting means for disconnecting the circuit. In the limiting circuit of the synchronous rectifier circuit having the above configuration,
When the voltage between the drain and the source of the field effect transistor as the commutation element is detected by the voltage detecting means, the switching means having a low operating voltage performs the rectifying operation of the synchronous rectifier circuit based on the detection result of the voltage detecting means. When a short circuit period is entered, the field effect transistor is turned off. On the other hand, the capacitor limits the output current flowing from the synchronous rectification control circuit to the switching means when the rectification operation of the synchronous rectification circuit enters a short circuit period,
Circuit disconnecting means connected between the voltage detecting means and the switching means disconnects the voltage detecting means and the switching means when the field effect transistor becomes non-conductive and the short circuit period ends. According to the present invention , even when the pulse width of the driving pulse supplied from the synchronous rectification control circuit to the commutation element constituting the synchronous rectification circuit becomes extremely long due to the delay time of the circuit element or the like, the switching operation is performed. By using, for example, a bipolar transistor having a low threshold voltage VTH as a switching means having a low operating voltage necessary to perform the operation, it is possible to enhance the ability to detect a short-circuit state of a commutation element constituting a synchronous rectification circuit. Therefore, the short circuit period of the commutation element can be suppressed within a short time. Further, a circuit disconnecting means for disconnecting the connection between the voltage detecting means and the switching means after the short-circuit period ends, and the synchronous rectifying control circuit switches the switching means when the rectifying operation of the synchronous rectifying circuit enters the short-circuit period. The power loss can be greatly reduced by adding a capacitor that limits the output current flowing through the power supply. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration of a limiting circuit of the synchronous rectifier circuit according to the first embodiment of the present invention. In FIG. 1, a synchronous rectification control circuit 10 has a function of supplying a drive pulse to a commutation element 30 constituting the synchronous rectification circuit, a control circuit 12 for generating a drive pulse, and a drive pulse output from the control circuit 12. And a drive circuit 14 for amplifying the power. The limiting circuit 20 for limiting the pulse width of the driving pulse output from the synchronous rectification control circuit 10 includes a field effect transistor MOSFET 1 as a commutation element 30.
The detection circuit 22 detects that the rectification operation of the synchronous rectifier circuit has entered the short-circuit period in which the rectifying element and the commutating element constituting the synchronous rectifier circuit are simultaneously in a conductive state by detecting the drain-source voltage VDS of the synchronous rectifier circuit. (Corresponding to the state detecting means or the voltage detecting means of the present invention), and a switching operation for turning off the commutation element 30 when the rectification operation of the synchronous rectification circuit enters a short-circuit period. A switch circuit 24 having a low operating voltage (corresponding to the switching means of the present invention), a detection circuit 22 and a switch circuit 2
4 and a disconnection circuit 26 for disconnecting the connection between the detection circuit 22 and the switch circuit 24 when the short circuit period ends (corresponding to a circuit disconnecting means of the present invention).
And In the above configuration, when a drive pulse is generated by the control circuit 12, the drive circuit 14 amplifies the power of the drive pulse, and the commutation element constituting the synchronous rectification circuit via the switch circuit 24 of the limiting circuit 20. The field effect transistor MOSFET1 as 30 is driven. The detection circuit 22 includes a field effect transistor MO
The switch circuit 24 detects the drain-source voltage VDS of the SFET 1 and immediately sets the gate voltage of the field-effect transistor MOSFET 1 to a low potential immediately after the short-circuiting period, thereby turning off the field-effect transistor MOSFET 1. When the MOSFET 1 is turned off and the short circuit period ends, the disconnection circuit 26
The function of the switch circuit 24 is stopped by disconnecting the connection between the switch circuit 24 and the switch circuit 24. Next, a specific configuration of the limiting circuit of the synchronous rectifier circuit according to the first embodiment of the present invention shown in FIG. 1 is shown in FIG.
Shown in In the figure, an output terminal of the synchronous rectification control circuit 10 is a field effect transistor MOSF as a commutation element 30.
It is connected to the gate (G) of ET1. The source (S) of the field effect transistor MOSFET1 is grounded. One end of the resistor R1 forming the detection circuit 22 is connected to the drain (D) of the field effect transistor MOSFET1, and the other end of the resistor R1 is connected to the drain of the field effect transistor MOSFET2 forming the separation circuit 26. The gate of the field effect transistor MOSFET2 is connected to the gate of the field effect transistor MOSFET1. Field-effect transistor MOSFET
The source 2 is connected to the base (B) of the bipolar transistor BJT1 forming the switch circuit 24 and one end of the resistor R2. The other end of the resistor R2 is grounded. The collector (C) of the bipolar transistor BJT1 is connected to the gate of the field effect transistor MOSFET2,
The emitter (E) is grounded. Next, the operation of the limiting circuit of the synchronous rectifier circuit shown in FIG. 2 will be described with reference to the operation waveforms of each part shown in FIG. When the gate-source voltage VGS of the field effect transistor MOSFET 1 as the commutation element 30 becomes high potential, the field effect transistor MOSFET 1 becomes conductive (FIGS. 3A and 3B) and the drain current ID.
Flows from the source to the drain of the field-effect transistor MOSFET 1 (FIG. 3C), so that the drain-source voltage VDS of the field-effect transistor MOSFET 1 becomes slightly negative potential (FIG. 3B). At this time, since the base-emitter voltage VBE of the bipolar transistor BJT1 is about 0.7 V, the field-effect transistor MO
The gate-source voltage VGS of the SFET 2 is almost the same as that of the field effect transistor MOSFET 1 (FIG. 3).
(D)), the field effect transistor MOSFET2 is in a conductive state (FIG. 3 (e)). However, since the drain-source voltage VDS of the field effect transistor MOSFET1 is a negative potential (FIG. 3B), no base current flows through the bipolar transistor BJTl (FIG. 3H), and the bipolar transistor BJT1 is non-conductive. The conductive state is maintained (FIG. 3 (g)). At this time, when the rectifying operation of the synchronous rectifier circuit enters the short-circuit period (T1) at time t1, the drain current ID of the field effect transistor MOSFET1 flows from the drain to the source (FIG. 3 (c)), and the field effect transistor M
The drain-source voltage VDS of the OSFET 1 is about 1-2.
V (FIG. 3B). Field-effect transistor MOS
Since the FET 2 is in a conductive state (FIG. 3E), the resistance R
1. A base current flows through the field effect transistor MOSFET2 to the bipolar transistor BJTl (FIG. 3).
(H)), the bipolar transistor BJT1 becomes conductive, that is, the collector-emitter voltage VCE becomes almost zero (FIG. 3 (g)). As a result, the gate-source voltage VGS of the field effect transistor MOSFET 1 is fixed at a low potential (FIG. 3A), and the field effect transistor MOSFET 1 is turned off (FIG. 3B). The field-effect transistor MOSFET2 also becomes non-conductive with a slight delay due to the delay time (FIG. 3).
(D), (e)), the base current supplied to the bipolar transistor BJT1 disappears (FIG. 3 (h)), and the bipolar transistor BJTl also becomes non-conductive. In FIG. 3, a period T2 is a period in which the output of the synchronous rectification control circuit 1 is originally at a high potential, and
Is a shortened period of the short-circuit period in which a short-circuit state occurs. According to the limiting circuit of the synchronous rectifier circuit according to the first embodiment of the present invention, for example, the switching means having a low operating voltage necessary for performing the switching operation includes:
By using a bipolar transistor having a low threshold voltage VTH, the ability to detect the short-circuit state of a commutation element constituting a synchronous rectification circuit can be increased, and therefore, the short-circuit period of the commutation element can be extremely reduced. In addition, since the circuit disconnecting means for disconnecting the connection between the voltage detecting means and the switching means after the short-circuit period is provided, the power loss can be greatly reduced. Next, FIG. 4 shows a configuration of a limiting circuit of a synchronous rectifier circuit according to a second embodiment of the present invention. The limiting circuit of the synchronous rectifier circuit according to the present embodiment is different from the limiting circuit of the synchronous rectifier circuit according to the first embodiment in configuration in that the synchronous rectifier circuit performs synchronous rectification when the rectifying operation enters a short circuit period. A current limiting circuit 40 (corresponding to a current limiting means of the present invention) for limiting the output current flowing from the control circuit 10 to the switch circuit 24 'is connected to the drive circuit 14 and the switch circuit 24' constituting the synchronous rectification control circuit 10. It is a point added between
Other configurations are basically the same. In the above configuration, when the drive pulse is generated by the control circuit 12, the drive circuit 14 amplifies the power of the drive pulse, and the current is limited by the current limit circuit 40 of the limit circuit 20 and the synchronous rectifier circuit via the switch circuit 24 '. Field effect transistor MOSFET1 as a commutation element 30 constituting
Drive. The detection circuit 22 is a field effect transistor MO
The switch circuit 24 'detects the drain-source voltage VDS of the SFET1, and the switch circuit 24' lowers the gate voltage of the field effect transistor MOSFET1 immediately after the short-circuit period,
The field effect transistor MOSFET1 is turned off. At this time, the output current of the synchronous rectification control hand warmer 10 is limited by the current limiting circuit 40. After this, the disconnecting circuit 26
Stops the function of the switch circuit 24 'by disconnecting the connection between the detection circuit 22 and the switch circuit 24' when the field effect transistor MOSFET1 becomes non-conductive and the short circuit period ends. Next, a specific configuration of the limiting circuit of the synchronous rectifier circuit according to the second embodiment of the present invention shown in FIG.
Shown in In the figure, the output terminal of the synchronous rectification control circuit 10 is connected via a capacitor C1 constituting a current limiting circuit 40 to a field effect transistor MOSFE as a commutation element 30.
It is connected to the gate (G) of T1. The source (S) of the field effect transistor MOSFET1 is grounded. One end of the resistor R1 forming the detection circuit 22 is connected to the drain (D) of the field effect transistor MOSFET1, and the other end of the resistor R1 is connected to the drain of the field effect transistor MOSFET2 forming the separation circuit 26. The gate of the field effect transistor MOSFET2 is connected to the output terminal of the synchronous rectification control circuit 10. The source of the field effect transistor MOSFET2 is a bipolar transistor B constituting the switch circuit 24 '.
It is connected to the base (B) of JTl and one end of the resistor R2. The other end of the resistor R2 is grounded. The collector (C) of the bipolar transistor BJT1 is connected to the gate of the field effect transistor MOSFET2, and the emitter (E) is grounded. Further, the cathode of the clamping diode D1 forming the switch circuit 24 'is connected to the collector (C) of the bipolar transistor BJT1,
The anode is grounded. Next, the operation of the limiting circuit of the synchronous rectifier circuit shown in FIG. 5 will be described with reference to the operation waveforms of the respective parts shown in FIG. When the gate-source voltage VGS of the field effect transistor MOSFET 1 as the commutation element 30 becomes high potential, the field effect transistor MOSFET 1 becomes conductive (FIGS. 6A and 6B), and the drain current ID
Flows from the source to the drain of the field-effect transistor MOSFET 1 (FIG. 6C), so that the drain-source voltage VDS of the field-effect transistor MOSFET 1 becomes slightly negative potential (FIG. 6B). At this time, since the base-emitter voltage VBE of the bipolar transistor BJT1 is about 0.7 V, the field-effect transistor MO
The gate-source voltage VGS of the SFET 2 is almost the same as that of the field effect transistor MOSFET 1 (FIG. 6).
(D)), the field effect transistor MOSFET2 is in a conductive state (FIG. 6 (e)). However, since the drain-source voltage VDS of the field effect transistor MOSFET1 is a negative potential (FIG. 6B), no base current flows through the bipolar transistor BJTl (FIG. 6H), and the bipolar transistor BJT1 is non-conductive. The conductive state is maintained (FIG. 6 (g)). At this time, when the rectification operation of the synchronous rectifier circuit enters the short-circuit period (T1) at time t1, the drain current ID of the field effect transistor MOSFET1 flows from the drain to the source (FIG. 6 (c)), and the field effect transistor M
The drain-source voltage VDS of the OSFET 1 is about 1-2.
V (FIG. 6B). Field-effect transistor MOS
Since the FET 2 is in a conductive state (FIG. 6E), the resistance R
1. A base current flows through the field effect transistor MOSFET2 to the bipolar transistor BJTl (FIG. 6).
(H)), the bipolar transistor BJTl becomes conductive, that is, the collector-emitter voltage VCE becomes almost zero (FIG. 6 (g)). As a result, the gate-source voltage VGS of the field-effect transistor MOSFET 1 is fixed at a low potential (FIG. 6A), and the field-effect transistor MOSFET 1 is turned off (FIG. 6B). At the same time, the current output from the synchronous rectification control circuit 10 charges the capacitor C1 through the capacitor C1 and the bipolar transistor BJT1, but when the charging of the capacitor C1 is completed, the capacitor C1 is output from the synchronous rectification control circuit 10.
No current flows through the bipolar transistor BJT1 through the transistor (FIG. 6 (i)). The field effect transistor MOSFET2 becomes non-conductive when the time T2 elapses from the time when the field effect transistor MOSFET1 becomes non-conductive (FIGS. 6 (d) and 6 (e)). The supplied base current disappears (see FIG. 6).
(H)), the bipolar transistor BJTl is also turned off. In FIG. 6, a period T2 is a period in which the output of the synchronous rectification control circuit 10 is originally at a high potential, and
0 is a shortened period of the short-circuit period in which the short circuit occurs. According to the synchronous rectification circuit limiting circuit according to the second embodiment of the present invention, the pulse width of the drive pulse supplied from the synchronous rectification control circuit to the commutation element constituting the synchronous rectification circuit is a circuit element or the like. Even when the switching time becomes extremely long due to the delay time of the synchronous rectifier circuit, for example, a bipolar transistor having a low threshold voltage VTH is used as a switching means having a low operating voltage necessary for performing a switching operation. The ability to detect the short-circuit state of the commutation element can be enhanced, and therefore the short-circuit period of the commutation element can be suppressed within a short time. In the first and second embodiments of the present invention, a bipolar transistor BJT forming a switch circuit is used.
1 is directly connected to the gate of the MOSFET 1 as a commutation element, but is not limited to this, and the bipolar transistor BJT1 forming the switch circuit is connected to the M
A switch or the like added separately without being directly connected to the gate of the OSFET 1 may be configured as a control signal generating element for turning off the MOSFET 1. Further, even if the detection circuit 22 is constituted by using a comparator or the like, it is possible to operate similarly to the first and second embodiments. According to the present invention , the pulse width of the drive pulse supplied from the synchronous rectification control circuit to the commutation element forming the synchronous rectification circuit is determined by the delay time of the circuit element and the like. Even in the case where the commutation element is extremely long, the commutation element constituting the synchronous rectification circuit can be short-circuited by using a bipolar transistor having a low threshold voltage VTH as a switching means having a low operating voltage necessary for performing a switching operation. The state detection ability can be enhanced, and therefore, the short-circuit period of the commutation element can be suppressed within a short time. Further, a circuit disconnecting means for disconnecting the connection between the voltage detecting means and the switching means after the short-circuit period is completed, and an output flowing from the synchronous rectification control circuit to the switching means when the commutation element enters the short-circuit period. By adding a capacitor for limiting the current, power loss can be significantly reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a configuration of a limiting circuit of a synchronous rectifier circuit according to a first embodiment of the present invention. FIG. 2 is a circuit diagram showing a specific configuration of a limiting circuit of the synchronous rectifier circuit according to the first embodiment of the present invention. FIG. 3 is a waveform chart showing an operation state of a limiting circuit of the synchronous rectifier circuit shown in FIG. FIG. 4 is a block diagram showing a configuration of a limiting circuit of a synchronous rectifier circuit according to a second embodiment of the present invention. FIG. 5 is a circuit diagram showing a specific configuration of a limiting circuit of a synchronous rectifier circuit according to a second embodiment of the present invention. FIG. 6 is a waveform chart showing an operation state of the limiting circuit of the synchronous rectifier circuit shown in FIG. FIG. 7 is a block diagram showing a configuration of a conventional limiting circuit of a synchronous rectifier circuit. [Description of Signs] 10 Synchronous rectification control circuit 12 Control circuit 14 Drive circuit 20 Limiting circuit 22 Detection circuit 24 Switch circuit 24 'Switch circuit 26 Isolation circuit 30 Commutation element 40 Current limiting circuit 50 Synchronous rectification control circuit 60 Limiting circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/00-3/44 H02M ⁷ /00-7/40 H03K 17/00-17/70 H01L 27 / 06-27/08 G05F 1/00-3/30 H02P 6/00-6/02

Claims (1)

(57) [Claims] 1. A field effect transistor as a rectifying element and a commutating element.
Synchronous with the commutation element of a synchronous rectifier circuit using a transistor
Limit pulse width of drive pulse supplied from rectification control circuit
In the limiting circuit of the synchronous rectifier circuit Drain of a field effect transistor as the commutation element
A voltage detecting means for detecting a source-to-source voltage; The synchronous rectification circuit based on the detection result of the voltage detection means.
When the commutation of the circuit enters the short circuit period, the
A low operating voltage switch that turns off the transistor
Means for When the rectification operation of the synchronous rectification circuit enters a short circuit period
Flow from the synchronous rectification control circuit to the switching means
Limit output currentCapacitorWhen, Connected between the voltage detecting means and the switching means;
The field effect transistor becomes non-conductive, and the short
At the end of the connection period, the state detection means and the switch
Circuit disconnecting means for disconnecting the connection between the A limiting circuit for a synchronous rectifier circuit, comprising: