JP2002252971A - Switching power unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a synchronous rectifying switching power unit, which can perform a proper switching operation even if an output current drops to below a critical point. SOLUTION: The switching power unit comprises first and second transistors 11, 12 connected in series between one terminal and the other terminal of a DC power source 10, a diode 13 connected in parallel to the second transistor 12, an inductor 16 connected between the node of the first and second transistors 11, 12 and one terminal of a load 15, an output capacitor 17 connected in parallel to the load 15, a control circuit 18 generating a control signal C for controlling on/off of the first and second transistors 11, 12, and a means for turning the transistor 12 off, irrespective of the control signal C, in response to the situation where the output current flowing in the load 15 is below a prescribed value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply, and more particularly, to a synchronous rectification type switching power supply capable of performing an appropriate switching operation even when an output current Io falls below a critical point. About.

[0002]

2. Description of the Related Art A step-down converter is one type of switching power supply for converting an input voltage to a predetermined output voltage. A step-down converter is a switching power supply for generating an output voltage lower than an input voltage, and generally uses a diode as a rectifying element.

FIG. 3 is a circuit diagram of a conventional switching power supply device using a rectifier diode.

As shown in FIG. 3, a conventional switching power supply using a rectifier diode has a transistor 2 and a rectifier diode 3 connected in series between both ends of a DC power supply 1, and the transistor 2 is , Based on the control signal C generated by the control circuit 4. The pulse width of the control signal C is determined based on the output voltage Vo, and is controlled so that the output voltage Vo is constant.

[0005] Here, the inductor current IL flowing through the inductor 5 varies depending on the size of the connected load 6, and the state is such that the inductor current IL always flows and the inductor current IL intermittently flows. Divided. A state in which the inductor current IL is constantly flowing is called a “continuous state”, and this state is established when the connected load 6 is heavy (when the output current Io is large). On the other hand, a state in which the inductor current IL is intermittently flowing is called a “discontinuous state”, and this state is established when the connected load 6 is light (the output current Io is small). The boundary between a continuous state and a discontinuous state is called a “critical state”.

FIGS. 4A to 4C show the inductor current I in a continuous state, a critical state, and a discontinuous state, respectively.
FIG. 9 is a waveform chart showing a waveform of L.

[0007] As shown in FIGS. 4A to 4C, the inductor current IL has a waveform that rises while the transistor 2 is on and falls while the transistor 2 is off. As shown in FIG. 4A, when the connected load 6 is heavy and the output current Io is large, the lower end of the inductor current IL always keeps 0 A or more, and the waveform in which the DC is superimposed Has become.
Such a state is a continuous state.

However, when the connected load 6 is lighter than this, the output current Io is smaller than this, and as shown in FIG. 4B, the lower end of the inductor current IL is 0 A. Such a state is a critical state. The value of the output current Io when the lower end of the inductor current IL is 0 A is generally called a “critical point”.

When the connected load 6 becomes lighter, the output current Io becomes smaller than the critical point, and the inductor current IL flows intermittently as shown in FIG. Such a state is a discontinuous state.

Incidentally, the step-down converter using the rectifier diode 3 as shown in FIG. 3 has a disadvantage that the loss generated in the rectifier diode 3 is relatively large. Rectifier diode 3
May be replaced with a transistor.

FIG. 5 is a circuit diagram of a conventional synchronous rectification type switching power supply using a transistor in place of the rectifier diode 3.

As shown in FIG. 5, the conventional synchronous rectification type switching power supply includes a first transistor 8 and a second transistor 9 connected in series between both ends of a DC power supply 1. The first transistor 8 and the second transistor 9 are controlled by a control signal C generated by the control circuit 4.
Are driven in opposite phases to each other. That is, during the period when the control signal C is at the high level, the first transistor 8 is turned on, the second transistor 9 is turned off,
During a period when the control signal C is at a low level, the first transistor 8 is turned off and the second transistor 9 is turned on.

According to the synchronous rectification type switching power supply having the above configuration, the loss can be reduced as compared with the switching power supply using diode rectification by reducing the ON resistance of the second transistor 9.

[0014]

As described above, if a synchronous rectification type switching power supply is constructed using the second transistor 9 instead of the rectifier diode 3, the loss can be reduced, but both transistors are used. In the state where the output current Io is equal to or lower than the critical point, the current flows through the inductor 5 in the opposite direction. In this case, since the reverse current is supplied from the output-side capacitor 7, the output voltage Vo decreases. Therefore, in this state, the control circuit 4 adjusts the pulse width of the control signal C so as to increase the output voltage Vo.

However, as in the case where the load 6 is a battery and charges it, or in the case where a plurality of synchronous rectification type switching power supplies are provided in parallel to the same load 6, a voltage is applied to the output side. When the source is present, the output current Io falls below the critical point and the inductor 5
In some cases, the output voltage Vo does not decrease even if a current flows in the reverse direction. In this case, the control signal C
May not be properly controlled.

FIG. 6 is a waveform diagram showing a waveform of the inductor current IL when the pulse width control of the control signal C is not properly performed.

As shown in FIG. 6, when the output voltage Vo does not decrease despite the reverse current flowing through the inductor 5, the control circuit 4 can set the control signal C to an appropriate pulse width. However, as a result, the reverse current gradually increases, and in some cases, the switching power supply device itself may be destroyed.

Therefore, an object of the present invention is to provide an output current I
An object of the present invention is to provide a synchronous rectification type switching power supply device capable of performing an appropriate switching operation even when o becomes lower than a critical point.

[0019]

SUMMARY OF THE INVENTION The object of the present invention is as follows.
First and second transistors connected in series between one terminal and the other terminal of the DC power supply, a diode connected in parallel to the second transistor, and a node of the first and second transistors An inductor connected between the load and one terminal of a load, an output capacitor connected in parallel with the load, and a control signal for controlling on / off of the first and second transistors based on an output voltage. A non-insulated switching power supply device comprising: a control circuit that generates the second transistor in response to an output current flowing through the load being equal to or less than a predetermined value, regardless of the control signal. This is achieved by a switching power supply device further comprising means for turning off.

In a preferred embodiment of the present invention, the predetermined value is a critical point.

In a further preferred aspect of the present invention, the means is responsive to an output current flowing through the load exceeding the predetermined value, based on the control signal, to determine whether the first transistor and the first transistor are connected to each other. The second transistors are driven in opposite phases to each other.

In a further preferred aspect of the present invention, the means includes: a current detection means for generating a detection voltage corresponding to the output current; a reference voltage generation means for generating a reference voltage; Comparing means for comparing the voltage with the voltage and generating a stop signal based on the voltage, and interrupting means for interrupting the control signal supplied to the second transistor in response to the stop signal.

In a further preferred aspect of the present invention, the diode is constituted by a parasitic diode of the second transistor.

In a further preferred aspect of the present invention, a voltage source is connected in parallel with the load.

In a further preferred aspect of the present invention, the other terminal of the DC power supply and the other terminal of the load are directly connected.

The object of the present invention is also to provide a first and a second transistor connected in series between both ends of a DC power supply;
A non-insulated switching power supply device including at least an inductor connected in series to a load and an output capacitor connected in parallel to the load, and performs synchronous rectification when an output current exceeds a critical point. If the output current is below the critical point, diode rectification is performed.

According to the present invention, since the reverse current flowing through the inductor is prevented, the switching power supply device is not destroyed by the excessive reverse current, and
In particular, it is possible to provide a switching power supply device suitable for charging a battery or driving the same load in parallel.

[0028]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
A preferred embodiment of the present invention will be described in detail.

FIG. 1 is a circuit diagram showing a switching power supply according to a preferred embodiment of the present invention.

As shown in FIG. 1, the switching power supply according to the present embodiment includes a first transistor 11 and a second transistor 12 connected in series between both ends of a DC power supply 10, and a second transistor. 12, a capacitor 14 connected in parallel with the DC power supply 10, and an inductor connected between nodes of the first transistor 11 and the second transistor 12 and one end of the load 15. 16, an output capacitor 17 connected in parallel to the load 15, a control circuit 18 for monitoring the output voltage Vo and adjusting the pulse width of the control signal C so that the output voltage Vo becomes a constant value, and a first transistor 11. A current detection circuit 19 for detecting the value of the output current Io based on the flowing current; and a stop for generating a stop signal S based on the detection result of the current detection circuit 19. A signal generation circuit 20, a logical sum circuit 21 to which the control signal C and the stop signal S are input, a buffer 22 that receives the control signal C and drives the first transistor 11, and receives an output from the logical sum circuit 21 And an inverter 23 for driving the second transistor 12.

The switching power supply according to the present embodiment comprises a negative terminal of a DC power supply 10 on the input side and a load 1 on the output side.
5 is directly connected to the negative terminal without being insulated by a transformer, and constitutes a so-called non-insulated switching power supply device.

A voltage source 24 is connected to the load 15. Such voltage sources can include batteries and other switching power supplies.

The current detecting circuit 19 includes a current transformer 25 for detecting a current flowing through the first transistor 11,
A resistor 26 and a capacitor 27 are connected in parallel to the current transformer, and a diode 28 is connected between the current transformer 25 and the resistor 26. The current transformer 25 is connected to the first transistor 11
A current proportional to the current flowing through the diode 28,
This is smoothed by the capacitor 27 and converted into a detected voltage value by the resistor 26, and is supplied to the stop signal generation circuit 20. Here, since the current flowing through the first transistor 11 has a correspondence with the output current Io, the detection voltage output from the current detection circuit 19 is
This indirectly indicates the value of the output current Io.

The stop signal generation circuit 20 includes a reference voltage source 29 for generating a reference voltage, a comparator 30 for comparing a reference voltage output from the reference voltage source 29 with a detection voltage output from the current detection circuit 19. Is provided. The comparator 30 receives the reference voltage output from the reference voltage source 29 at a non-inverting input terminal, and receives the detection voltage output from the current detecting circuit 19 at an inverting input terminal.
When the detection voltage output from the reference voltage source 29 is higher than the reference voltage output from the reference voltage source 29, the output of the stop signal S is set to low level, and the detection voltage output from the current detection circuit 19 is changed to the reference voltage source 29. If the output is lower than the reference voltage output from the controller, the output of the stop signal S is set to a high level. Here, the reference voltage is set to the value of the detection voltage to be output from the current detection circuit 19 when the output current Io reaches the critical point. As a result, when the output current Io has exceeded the critical point, the stop signal S has a low level, and when the output current Io has fallen below the critical point, the stop signal S has a high level.

Next, the operation of the switching power supply according to this embodiment will be described.

FIG. 2 is an operation waveform diagram of the switching power supply according to this embodiment.

As shown in FIG. 2, during the period when the output current Io exceeds the critical point, the stop signal generation circuit 2
0, the stop signal S is at a low level, so that the buffer 2 that drives the first transistor 11
The output waveform of the control signal C coincides with the waveform of the control signal C, and the output waveform of the inverter 23 that drives the second transistor 12 is a waveform opposite to the control signal C. Thus, the first transistor 11 and the second transistor 12 are driven in opposite phases to each other as usual. In this case, the output current I
Since o is beyond the critical point, the inductor current IL flowing through the inductor 16 is in a state where DC is superimposed, and no reverse current flows through the inductor 16.

On the other hand, during a period in which the output current Io is below the critical point, the stop signal S generated by the stop signal generation circuit 20 is at a high level, so that the buffer 22 for driving the first transistor 11 Output waveform coincides with the waveform of the control signal C, but the output of the inverter 23 that drives the second transistor 12 remains low. As a result, the first transistor 11 switches in synchronization with the control signal C as usual, while the second transistor 12 is kept off. Therefore, while the first transistor 11 is off, the inductor current IL is
3, and when the current reaches 0 A, the inductor current IL stops flowing until the first transistor 11 is turned on next time. That is, a discontinuous state occurs.

As described above, according to this embodiment, when the output current Io exceeds the critical point, synchronous rectification using the second transistor 12 is performed, and the output current Io is below the critical point. In this case, since diode rectification using the diode 13 is performed, even when the voltage source 24 is connected to the output side, there is no possibility that the switching power supply device is damaged by an excessive reverse current. . In this case, the control circuit 18 outputs the output voltage V
What is necessary is just to control the pulse width of the control signal C so that o may be constant.

The present invention is not limited to the above embodiments, and various modifications can be made within the scope of the invention described in the claims, and these are also included in the scope of the present invention. It goes without saying that it is a thing.

For example, in the above embodiment, as a method of detecting the output current Io, the first transistor 11
Although the output current Io is indirectly detected by using the current detection circuit 19 for monitoring the current flowing through the circuit, the method of detecting the output current Io is not limited to this, and the output current Io is directly or indirectly detected. Any method that detects Io may be used. For example, a current flowing between a node of the first transistor 11 and the second transistor 12 and the first transistor 11, or a current flowing between the node and the second transistor
The output current Io may be detected indirectly based on the current flowing between the transistors, or the output current Io may be directly detected by providing a current detection circuit on the load 8 side. Also, the elements for detecting these currents are not limited to the current transformer, and any current detection element may be used for current detection.

The diode 13 connected in parallel to the second transistor 12 may be constituted by a component different from the second transistor 12, or may be constituted by a parasitic diode included in the second transistor 12. It may be configured.

Further, the stop signal generation circuit 20 and the OR circuit 21 may be built in the control circuit 18.

In the above embodiment, the stop signal generation circuit 20 sets the stop signal S to the high level when the output current Io is lower than the critical point. The configuration may be such that the stop signal S is at a low level when it is lower. In this case, it is necessary to use an AND circuit instead of the OR circuit 21.

[0045]

As described above, in the present invention, synchronous rectification is performed when the output current Io exceeds the critical point, and diode rectification is performed when the output current Io is below the critical point. Therefore, even when a voltage source is connected to the output side, there is no possibility that the switching power supply device is damaged by an excessive reverse current. This provides a switching power supply particularly suitable for charging a battery and driving the same load in parallel.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a switching power supply device according to a preferred embodiment of the present invention.

FIG. 2 is an operation waveform diagram of the switching power supply device according to a preferred embodiment of the present invention.

FIG. 3 is a circuit diagram of a conventional switching power supply device using a rectifier diode.

FIGS. 4A to 4C are waveform diagrams showing waveforms of an inductor current IL in a continuous state, a critical state, and a discontinuous state, respectively.

FIG. 5 is a circuit diagram of a conventional synchronous rectification type switching power supply device.

FIG. 6 is a waveform diagram showing a waveform of an inductor current IL when pulse width control of a control signal C is not appropriately performed.

[Explanation of symbols]

 Reference Signs List 1 DC power supply 2 Transistor 3 Rectifier diode 4 Control circuit 5 Inductor 6 Load 7 Capacitor 8 First transistor 9 Second transistor 10 DC power supply 11 First transistor 12 Second transistor 13 Diode 14 Capacitor 15 Load 16 Inductor 17 Output Capacitor 18 Control circuit 19 Current detection circuit 20 Stop signal generation circuit 21 OR circuit 22 Buffer 23 Inverter 24 Voltage source 25 Current transformer 26 Resistance 27 Capacitor 28 Diode 29 Reference voltage source 30 Comparator

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Tomohiko Isagawa 1-13-1, Nihonbashi, Chuo-ku, Tokyo TDC Corporation F-term (reference) 5H006 AA05 BB08 CA02 CA07 CB07 CB09 CC02 DA04 DC02 DC05 5H730 AA14 AS01 AS05 BB13 BB57 DD04 EE13 EE59 FD01 FD41 FG05 FG22

Claims (8)

[Claims] A first transistor connected in series between one terminal and the other terminal of the DC power supply; a diode connected in parallel to the second transistor; An inductor connected between a node of the second transistor and one terminal of a load, an output capacitor connected in parallel with the load, and turning on / off the first and second transistors based on an output voltage. And a control circuit for generating a control signal for controlling the non-isolated switching power supply device, wherein in response to an output current flowing through the load being equal to or less than a predetermined value, regardless of the control signal, The switching power supply device further comprising means for turning off the second transistor. 2. The switching power supply according to claim 1, wherein the predetermined value is a critical point. 3. The method according to claim 2, wherein said means is responsive to said output current flowing through said load exceeding said predetermined value to cause said first transistor and said second transistor to be in phase opposition to each other based on said control signal. The switching power supply according to claim 1, wherein the switching power supply is driven. 4. The apparatus according to claim 1, wherein said means is configured to generate a detection voltage corresponding to said output current, to generate a reference voltage, to generate a reference voltage, and to compare said detection voltage with said reference voltage. 4. A comparison device according to claim 1, further comprising: a comparison unit configured to generate a stop signal, and a blocking unit configured to block the control signal supplied to the second transistor in response to the stop signal. Item 2. The switching power supply device according to item 1. 5. The switching power supply device according to claim 1, wherein the diode is constituted by a parasitic diode of the second transistor. 6. The switching power supply device according to claim 1, wherein a voltage source is connected in parallel with said load. 7. The switching power supply device according to claim 1, wherein the other terminal of the DC power supply is directly connected to the other terminal of the load. 8. A non-current power supply including at least first and second transistors connected in series between both ends of a DC power supply, an inductor connected in series to a load, and an output capacitor connected in parallel to the load. An insulated switching power supply device that performs synchronous rectification when the output current exceeds a critical point, and performs diode rectification when the output current is below the critical point. Switching power supply.