JP2000350440A - Stepdown switching power source circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce loss in a step-down switching power source circuit which uses an N-type MOSFET as a switching element and has an overcurrent protecting circuit using a voltage drop due to on-resistance generated in the N-type MOSFET. SOLUTION: A switching element is constituted of an N-type MOSFET 11. A bootstrap circuit 15 generates a voltage which is to be applied to the gate of the N-type MOSFET. A control circuit 12 is provided with an overcurrent protecting circuit which detects a voltage drop due to an on- resistance of the MOSFET 11. When the detected voltage exceeds a first specified voltage, the protecting circuit gradually limits the on time of the MOSFET 11. When the detected voltage reaches a second voltage higher than the first voltage, the protective circuit makes the on time zero or sufficiently short. As a result, the loss of a stepdown switching power source circuit is reduced, and cost reduction and overheat protection of the switching element can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a step-down switching power supply circuit, and more particularly to a step-down switching power supply circuit having an overcurrent protection circuit utilizing a voltage drop caused by an on-resistance generated in a switching element.

[0002]

2. Description of the Related Art FIG. 5 shows a step-down switching power supply circuit that performs overcurrent protection, particularly output short-circuit protection, by utilizing a voltage drop caused by an on-resistance generated in a switching element. Here, the ON resistance means that when the switching element is in an ON state,
That is, it indicates the internal resistance during conduction. In addition,
The cause of the voltage drop when the switching element conducts is not necessarily due to the ON resistance alone, but may be due to the inductance component of the connection between the inside and outside of the switching element. Make it represented by resistance. The basic concept of the step-down switching power supply circuit shown in FIG.
No. 1734.

In FIG. 5, a step-down switching power supply circuit 1 includes a DC power supply 2 and a P-type MO as a switching element.
It comprises an SFET 3, a control circuit 4 including an overcurrent protection circuit, a diode 5, an inductor 6, a capacitor 7, and output terminals 8 and 9. Here, the positive electrode of the DC power supply 2 is connected to the drain of the P-type MOSFET 3 and the P-type MOS
The source of the FET 3 is connected to the output terminal 8 via the inductor 6. The negative electrode of the DC power supply 2 is connected to the output terminal 9. The control circuit 4 is connected to the drain and source of the P-type MOSFET 3 and has a P-type MOSFET
It is also connected to the gate, which is the control terminal of T3, and is also connected to the negative electrode of DC power supply 2. The diode 5 has a cathode connected to the source of the P-type MOSFET 3 and an anode connected to the negative electrode of the DC power supply 2. Further, the capacitor 7 is connected between the output terminals 8 and 9. It should be noted that the symbol of the P-type MOSFET 3 also indicates the equivalently built-in diode at the same time.

In the step-down switching power supply circuit 1 configured as described above, the P-type MOSFET 3 is connected to the control circuit 4
As a result, a voltage lower than the voltage output from the DC power supply 2 is output between the output terminals 8 and 9. At this time, the control circuit 4 controls the P-type MOSFET 3
Of the P-type MOSFET 3 when the P-type MOSFET 3 is on, the potential of the source and the drain of the P-type MOSFET 3 is detected.
A voltage drop due to the on-resistance of the OSFET 3 can be detected. The control circuit 4 monitors the voltage drop due to the on-resistance, and when the output current increases and the voltage drop becomes a certain value or more, the P-type MOSF
The output current can be limited by shortening the ON time of ET3, and overcurrent protection and output short-circuit protection can be performed.

As described above, by detecting the voltage drop due to the ON resistance of the switching element and performing the overcurrent protection and the output short-circuit protection, a current detection resistor inserted in series with the output to detect the output current is not required. Thus, the loss of the step-down switching power supply circuit can be reduced.

[0006]

However, the P-type M
OSFETs are more expensive than N-type MOSFETs, and have a relatively high on-resistance, so that there is a problem that the switching element has a relatively large loss when it is on. On the other hand, when an N-type MOSFET is used, it is necessary to make the gate potential higher than the source potential, so that there is a problem that it is difficult to use the step-down switching power supply circuit. Japanese Patent Application Laid-Open No. 6-311734 discloses that an N-type MOSFET can be used as a switching element, but does not disclose any specific configuration.

Therefore, in the present invention, the loss is reduced in a step-down switching power supply circuit having an overcurrent protection circuit using an N-type MOSFET as a switching element and utilizing a voltage drop due to an on-resistance generated in the N-type MOSFET. The purpose is to:

[0008]

To achieve the above object, a step-down switching power supply circuit according to the present invention comprises a DC power supply, a switching element, and a control circuit for controlling the switching element. In a step-down switching power supply circuit in which a DC input voltage is turned on and off by the switching element to obtain a stable output voltage, the switching element comprises an N-type MOSFET and generates a voltage to be applied to a gate of the N-type MOSFET. A bootstrap circuit;
A voltage drop due to the ON resistance of the ET is detected, and when the voltage drop exceeds a predetermined first voltage, the ON time of the N-type MOSFET is gradually limited, and a second voltage higher than the first voltage is reduced.
And an overcurrent protection circuit for reducing the on-time to zero or a sufficiently short time when the voltage reaches the above-mentioned voltage.

Further, in the step-down switching power supply circuit according to the present invention, a plurality of the N-type M
OSFETs are connected in parallel.

Further, the step-down switching power supply circuit of the present invention is characterized in that the step-down switching power supply circuit includes a synchronous rectifier circuit for performing rectification in synchronization with the on / off operation of the switching element.

With such a configuration, the step-down switching power supply circuit of the present invention can achieve low loss and low cost.

[0012]

FIG. 1 shows an embodiment of a step-down switching power supply circuit according to the present invention. 1, the same or equivalent parts as those in FIG. 5 are denoted by the same reference numerals, and description thereof will be omitted.

In FIG. 1, a step-down switching power supply circuit 10 includes an N-type MOSFET 3 in place of the P-type MOSFET 3 in FIG.
N-type MOSFET 11 is provided.
The drain of T11 is connected to the positive electrode of the DC power supply 2, and the source is connected to the inductor 6 and the cathode of the diode 5. A control circuit 12 including an overcurrent protection circuit is provided instead of the control circuit 4 in FIG.
In addition to being connected to the drain and source of the FET 11, it is also connected to the gate, which is the control terminal of the N-type MOSFET 11, and is also connected to the negative electrode of the DC power supply 2. Further, a bootstrap circuit 15 is provided in the step-down switching power supply circuit 10. The bootstrap circuit 15 includes a diode 13 and a capacitor 14 connected in series between the source and the drain of the N-type MOSFET 11. Here, the diode 13
Is arranged with the cathode connected to the capacitor 14, and the connection point between the diode 13 and the capacitor 14 is connected to the control circuit 12. Further, the N-type MOSFET 11
In the symbol, the built-in equivalent diode is clearly shown. The step-down switching power supply circuit 10 thus configured
, The bootstrap circuit 15 includes an N-type MOSF
While ET11 is off and diode 5 is conducting,
Capacitor 14 from DC power supply 2 through diode 13
Charge can be stored in the N-type MOS
When the FET 11 is on, the potential at the connection point between the diode 13 and the capacitor 14 can be higher than the source potential of the N-type MOSFET 11. Since the connection point between the diode 13 and the capacitor 14 is connected to the control circuit 12, the potential at the connection point between the diode 13 and the capacitor 14 is applied to the gate of the N-type MOSFET 11 via the control circuit 12. In this way, in the step-down switching power supply circuit 10, the bootstrap circuit 15 can generate a potential higher than the source potential of the N-type MOSFET 11 and make it a gate potential of the N-type MOSFET 11.

As described above, an N-type MOSFET having a lower on-resistance and a lower on-resistance than a P-type MOSFET is used as a switching element.
Since T can be used, cost and loss can be reduced as compared with the case where a P-type MOSFET is used.

The step-down switching power supply circuit 1
At 0, the overcurrent protection circuit can be activated by utilizing the voltage drop due to the on-resistance generated in the N-type MOSFET, which is a switching element, as in the case of using the P-type MOSFET. Here, FIG. 2 shows the relationship between the voltage drop due to the on-resistance generated in the N-type MOSFET 11 of the step-down switching power supply circuit 10 and the output voltage, and will be described with reference to FIG. That is, the overcurrent protection circuit included in the control circuit 12 performs the N-type MO control when the voltage drop due to the on-resistance generated in the N-type MOSFET 11 exceeds the first voltage V1.
When the output voltage is gradually reduced by shortening the ON time of the SFET 11, reducing the number of times of ON, or changing the switching frequency, and further reaching the second voltage V2 higher than the first voltage V1. N-type MOSFE
The output is cut off or set to a sufficiently small value by setting the ON time of T11 to zero or a sufficiently short time.

At this time, as described above, the overcurrent protection circuit included in the control circuit 12 includes the N-type MOSFET 1
No. 1 utilizes a voltage drop due to on-resistance. The on-resistance of the N-type MOSFET 11 generally has a positive temperature characteristic. Therefore, even if the output current does not necessarily change, the on-resistance increases even when the junction temperature of the N-type MOSFET 11 increases due to an increase in the internal temperature of the step-down switching power supply circuit 10 and the like. Also increase. As a result, this N-type MOSFET
The overcurrent protection circuit included in the control circuit 12 operates even when the on-resistance increases due to an increase in the junction temperature of the control circuit 11. This means overheating protection of the N-type MOSFET 11, that is, the switching element.

As described above, the step-down switching power supply circuit 10 of the present invention has an overheat protection function for the switching element in addition to the overcurrent protection function.

Note that the bootstrap circuit in the step-down switching power supply circuit of the present invention is not limited to a configuration including a capacitor and a diode connected in series, but generates a potential higher than the source potential of the N-type MOSFET 11. Any configuration may be used as long as it can be supplied to the control circuit 12.

FIG. 3 shows another embodiment of the step-down switching power supply circuit of the present invention. 3, the same or equivalent parts as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.

In FIG. 3, a step-down switching power supply circuit 20 includes three N-type MOSFETs as switching elements.
The ETs 21, 22, and 23 are configured by connecting the drain to the positive electrode of the DC power supply 2 and connecting the source to the inductor 6 and the cathode of the diode 5, respectively. The control circuit 24 including the overcurrent protection circuit includes an N-type MOSFET 21,
It is connected to the drains and sources of the N-type MOSFETs 22 and 23, is also connected to the gates that are the control terminals of the N-type MOSFETs 21, 22, and 23, and is further connected to the negative electrode of the DC power supply 2.

In the step-down switching power supply circuit 20 configured as described above, an N-type M
The three OSFETs 21, 22, and 23 are connected in parallel, and are controlled by the control circuit 24 to be turned on and off at the same time. Therefore, each of the N-type MOSFETs 21, 22, 23
Although the value of each on-resistance does not change, by connecting three in parallel, the value of the on-resistance can be reduced to about 1/3 as a whole.
Losses due to the on-resistances of 22 and 23 can be reduced. Thus, the loss of the step-down switching power supply circuit 20 itself can be reduced.

Also, three N-type MOSFETs 21, 2
2 and 23 are connected in parallel, and even if there is some variation in the on-resistance of each N-type MOSFET,
Since T generally has a positive temperature characteristic, any one of T
Current concentration does not occur in the two N-type MOSFETs,
A stable overcurrent value can be obtained.

In the above embodiment, the number of N-type MOSFETs as switching elements is three.
One N-type MOSFET may be connected in parallel. In this case, the value of the on-resistance as a whole is reduced to about 1 /
Can be 2. In addition, four or more N-type MOSFETs
T may be connected in parallel. In this case, the value of the on-resistance as a whole can be further reduced.

FIG. 4 shows still another embodiment of the step-down switching power supply circuit of the present invention. In FIG. 4, the same or equivalent parts as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.

In FIG. 4, a step-down switching power supply circuit 30 includes two N-type MOSFETs as switching elements.
ET31 and ET32 are provided with the drain connected to the positive electrode of the DC power supply 2 and the source connected to the inductor 6, respectively.
The SFETs 33 and 34 are provided in parallel with the drain connected to the inductor 6 and the source connected to the negative electrode of the DC power supply 2, respectively. Among them, two N-type MOSFETs 33,
Reference numeral 34 denotes a synchronous rectification circuit 35. The control circuit 36 including the overcurrent protection circuit includes an N-type MOSFET 3
In addition to being connected to the drains and sources of the N-type MOSFETs 1 and 32, they are also connected to the gates that are the control terminals of the N-type MOSFETs 31 and 32. It is also connected to the negative electrode of the power supply 2.

In the step-down switching power supply circuit 30 configured as above, an N-type M
Two OSFETs 31 and 32 are connected in parallel, and are controlled by a control circuit 35 to be simultaneously turned on and off.
Then, the N-type MOSFETs 33, 3
4 is synchronously controlled to be turned on when the N-type MOSFETs 31 and 32 are turned off. When the N-type MOSFETs 31 and 32 are off, a current flows from the sources to the drains of the N-type MOSFETs 33 and 34,
When the switches 1 and 32 are on, the N-type MOSFETs 33 and 34 are off, so that the N-type MOSFETs 33 and 34 have the same function as the diodes. And N-type MOSFET3
3 and 34, the on-resistance of which is smaller than the forward voltage drop of the diode.
3 and 34 are connected in parallel.
The conduction loss at the time of turning on the FETs 33 and 34 is significantly reduced as compared with the case where diodes are used. Thus, the loss of the step-down switching power supply circuit 30 itself can be reduced.

It should be noted that the number of N-type MOSFETs in the synchronous rectifier circuit is not limited to two, but one N-type MOSFET.
ET or three or more N-type MOSFs connected in parallel
It may be ET.

[0028]

According to the step-down switching power supply circuit of the present invention, the switching element comprises an N-type MOSFET, a bootstrap circuit for generating a voltage applied to the gate of the switching element, and the on-resistance of the N-type MOSFET. To detect a voltage drop due to
Overvoltage protection circuit that gradually limits the on-time of the N-type MOSFET when the voltage exceeds the second voltage, and sets the on-time to zero or a sufficiently short time when the second voltage higher than the first voltage is exceeded. Accordingly, low loss and low cost of the step-down switching power supply circuit and overheating protection of the switching element can be achieved.

Further, by connecting a plurality of N-type MOSFETs in parallel to form a switching element, it is possible to further reduce the loss.

Further, by providing a synchronous rectifier circuit for performing rectification in synchronization with the on / off operation of the switching element, it is possible to further reduce the loss.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a step-down switching power supply circuit of the present invention.

FIG. 2 is an N-type MO of the step-down switching power supply circuit of FIG. 1;
FIG. 4 is a diagram illustrating a relationship between a voltage drop due to an on-resistance generated in an SFET and an output voltage.

FIG. 3 is a circuit diagram showing another embodiment of the step-down switching power supply circuit of the present invention.

FIG. 4 is a circuit diagram showing still another embodiment of the step-down switching power supply circuit of the present invention.

FIG. 5 is a circuit diagram showing a conventional step-down switching power supply circuit.

[Explanation of symbols]

2 DC power supply 5 Diode 6 Inductor 7 Capacitor 8, 9 Output terminal 10, 20, 30 Step-down switching power supply circuit 11, 21, 22, 23, 31, 32, 33, 34 N
Type MOSFETs 12, 24, 36 Control circuit 13 Diode 14 Capacitor 15 Bootstrap circuit 35 Synchronous rectifier circuit

Claims (3)

[Claims] 1. A step-down type comprising a DC power supply, a switching element, and a control circuit for controlling the switching element, wherein a DC input voltage from the DC power supply is turned on and off by the switching element to obtain a stable output voltage. A switching power supply circuit, wherein the switching element comprises an N-type MOSFET, a bootstrap circuit for generating a voltage applied to a gate of the N-type MOSFET, and the N-type MO.
A voltage drop due to the ON resistance of the SFET is detected, and when the voltage drop exceeds a predetermined first voltage, the N-type MOSFET
An overcurrent protection circuit is provided, which gradually limits the on-time of T and sets the on-time to zero or a sufficiently short time when the second voltage higher than the first voltage is reached. Step-down switching power supply circuit. 2. The step-down switching power supply circuit according to claim 1, wherein a plurality of said N-type MOSFETs are connected in parallel as said switching elements. 3. The step-down switching power supply circuit according to claim 1, further comprising a synchronous rectifier circuit that performs rectification in synchronization with an on / off operation of the switching element.