JP3292697B2 - Overcurrent detection 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 protection circuit for preventing an overcurrent from flowing to a load, and more particularly to a power MOSFET.
And the like.

[0002]

2. Description of the Related Art FIG. 4 is a circuit diagram of a conventional overcurrent detection circuit having a power MOSFET. The overcurrent detection circuit of FIG.
Power MOSFET1, NPN transistor Q1, and resistor R1
And a constant current source 2.

The power MOSFET 1 has a drain terminal connected to the output terminal OUT, and a gate terminal connected to the collector terminal of the transistor Q 1 and the constant current source 2. Power MOSFET 1
Is divided into two parts, a main source part and a sub-source part, the main source part is grounded, and the sub-source part is connected to the base terminal of the transistor Q1 and one end of the resistor R1. The other end of the resistor R1 and the emitter terminal of the transistor Q1 are grounded. The dimension ratio between the main source portion and the sub-source portion of the power MOSFET 1 is set to N: 1.

Next, the operation of the overcurrent detection circuit shown in FIG. 4 will be described. A load 10 to be subjected to overcurrent detection is connected to the drain terminal of the power MOSFET 1 and a pulse-like bias voltage is supplied to the gate terminal. Since the dimension ratio between the main source portion and the sub-source portion of the power MOSFET 1 is set to N: 1, a current N times larger than that of the sub-source portion flows through the main source portion.

In the circuit of FIG. 4, when the current flowing through the load 10 increases, the current flowing through the main source and the sub-source also increases, and the voltage across the resistor R1 increases. When the voltage across the resistor R1 exceeds a predetermined voltage (approximately 0.6 V), the transistor Q1 turns on and the gate current of the power MOSFET 1 is drawn to the collector of the transistor Q1. With such negative feedback control, the gate voltage of the power MOSFET 1 is controlled to be substantially constant, and as a result, the drain current is also controlled to be substantially constant, and the current flowing to the load 10 is limited.

The control current Is flowing through the main source of the power MOSFET 1 is expressed by the following equation (1). Is = N · Isense = N · Vbe / R1 (1) where Isense is the current flowing through the sub-source portion, and Vbe is the base-emitter voltage when the transistor Q1 is on.

However, in the case of the circuit of FIG. 4, the overcurrent cannot be detected unless the voltage across the resistor R1 exceeds the voltage Vbe (= about 0.6 V). On the other hand, power MOSFET1
Normally operates at a gate voltage of about 2V. Thus, the operating voltage of the transistor Q1 and the power MOSFET 1
4 has a problem that the control current flowing through the main source of the power MOSFET 1 cannot be adjusted with high accuracy because of the large voltage difference from the operating voltage.

FIG. 5 is a circuit diagram of a conventional overcurrent detection circuit in which the disadvantage of the circuit of FIG. 4 is improved. The circuit of FIG. 5 is obtained by adding a transistor Q2 and a constant current source 3 to the configuration of FIG.

The base terminals of transistors Q1 and Q2 are connected to each other, the emitter terminal of transistor Q2 is connected to the sub-source portion, and the emitter terminal of transistor Q1 is grounded. The constant current source 3 is connected to the collector terminal and the base terminal of the transistor Q2, and the constant current source 2 is connected to the collector terminal of the transistor Q1 and the gate terminal of the power MOSFET 1. The emitter area ratio of transistors Q1 and Q2 is set to 1: K.

Next, the operation of the circuit of FIG. 5 will be described. When the current flowing through the load 10 increases, the current flowing through the main source portion and the sub-source portion of the power MOSFET 1 also increases, and the voltage across the resistor R1 increases. Thereby, the transistors Q1, Q
2, the control current is controlled such that the collector current of the transistor Q1 matches the output current of the constant current source 2, and the control current Is is controlled to be constant as in FIG.

The control current Is flowing through the main source of the power MOSFET 1 is expressed by equation (2). Is = N · VT · ln (K × Idrive / Io) / R1 (2) As shown in the equation (2), the output current Idr of the constant current sources 2 and 3
ive, Io and the emitter area ratio K of the transistors Q1 and Q2.
Is given, the control current Is has a constant value determined by the value of the resistor R1.

In the circuit of FIG. 5, transistors Q1, Q2
Performs negative feedback control with the difference voltage between the base-emitter voltage Vbe of the power MOSFET 1, the current flowing through the load 10 can be detected with a slight voltage applied across the resistor R1, and the main source and sub-source of the power MOSFET 1 can be detected. The ratio of the flowing current substantially matches the dimension ratio of the main source section and the sub source section.

[0013]

As described above, FIG.
Can control the control current Is flowing through the main source portion of the power MOSFET 1 to a constant current. However, the control current Is can be controlled unless the types of the constant current sources 2 and 3 and the resistance value of the resistor R1 are changed. It cannot be changed. That is, FIG.
Cannot easily change the current value for overcurrent detection. For this reason, when the type of the load 10 is different, or when the limited current value is different depending on the operating condition even with the same load 10, it is necessary to separately provide a dedicated overcurrent detection circuit, which increases the circuit scale and the components. The cost rises. The present invention has been made in view of such a point, and an object of the present invention is to provide an overcurrent detection circuit that can easily change a current value for overcurrent detection.

[0014]

According to a first aspect of the present invention, a source region of a first transistor is divided into a main source portion and a sub-source portion. While connected to a reference voltage terminal, the sub-source unit is connected to the reference voltage terminal via a resistor,
Overcurrent detection in which the gate voltage of the first transistor is negatively feedback-controlled in accordance with the voltage across the resistor so that the current flowing to a load connected to the drain terminal of the first transistor does not exceed a limit value In the circuit, second and third transistors having gate terminals connected to each other, a switch having one end connected to the gate terminal and the drain terminal of the second transistor, and a first transistor connected to the other end of the switch. And a second current source connected to a gate terminal of the first transistor and a drain terminal of the third transistor, wherein the negative feedback control circuit comprises: Each of the source terminals of the second transistors in the control circuit is connected to the sub-source section, and the third transistor in the plurality of negative feedback control circuits is connected to the sub-source section. Both transistors each source terminal connected to said reference voltage terminal, each of the switches of said plurality of negative feedback control in the circuit can be individually switched.

The invention of claim 1 will be described with reference to FIG. 1, for example.
1, the "second and third transistors" correspond to the NPN transistors Q1 and Q2, and the "switch" corresponds to the switches SW1 and SW.
2, the "plural negative feedback control circuits"
1 and 12, the “first current source” is connected to the constant current source 3 and the “second current source”
Correspond to the constant current source 2 respectively. The invention of claim 4 will be described with reference to, for example, FIG. 2. “A plurality of current supply units” corresponds to the constant current source 3.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an overcurrent detection circuit according to the present invention will be specifically described with reference to the drawings.

(First Embodiment) FIG. 1 is a circuit diagram of an overcurrent detection circuit according to a first embodiment of the present invention. In FIG. 1, the same components as those in FIG. 5 are denoted by the same reference numerals, and the following description will focus on the differences. The circuit of FIG. 1 has a power source in which a source region is divided into a main source portion and a sub source portion.
It includes a MOSFET 1, a plurality of negative feedback control circuits 11 and 12 for performing negative feedback control on a gate voltage of the power MOSFET 1 in accordance with a current flowing to a load 10, and a constant current source 2. FIG. 1 shows an example in which two negative feedback control circuits are provided.

The connection of the power MOSFET 1 is the same as in FIGS. That is, the drain terminal of the power MOSFET 1 is connected to the load 10 via the output terminal OUT, the main source section is grounded, and the sub-source section is grounded via the resistor R1. The dimension ratio between the main source section and the sub source section is set to N: 1.

The negative feedback control circuits 11 and 12 include NPN transistors (Q1 and Q2) whose base terminals are connected to each other,
(Q1 ', Q2'), constant current source 3, switch SW1,
SW2. The base terminals and the collector terminals of the transistors Q2 and Q2 'are connected to the constant current source 3 via the switches SW1 and SW2, and the collector terminals of the transistors Q1 and Q1' and the gate terminal of the power MOSFET are connected to the constant current source 2. .

The emitter terminals of the transistors Q2 and Q2 'in the negative feedback control circuits 11 and 12 are both power MOSFETs.
1 and connected to the transistors Q1 and Q
The emitter terminals 1 'are all grounded.

The emitter area ratios of the transistors (Q1, Q2) and (Q1 ', Q2') in the negative feedback control circuits 11, 12 are different for each of the negative feedback control circuits 11, 12. In FIG. 1, the emitter area ratio of the transistors Q1 and Q2 in the negative feedback control circuit 11 is 1: K1, and the emitter area ratio of the transistors Q1 'and Q2' in the negative feedback control circuit 12 is 1: K2.

Next, the operation of the circuit of FIG. 1 will be described. A pulse-like bias voltage is applied to the gate terminal of the power MOSFET 1. When the current flowing to the load 10 connected to the drain terminal of the power MOSFET 1 increases, the power MOSFET 1
, The current flowing through the main source portion and the sub source portion increases, and the voltage across the resistor R1 increases.

Switch SW in negative feedback control circuits 11 and 12
Only one of SW1 and SW2 is set to ON. In the negative feedback control circuit in which the switch is set to OFF, no current is supplied from the constant current source 3, and thus no negative feedback control is performed.

On the other hand, in the negative feedback control circuit in which the switch is turned on, negative feedback control is performed so that the current supplied from the constant current source 2 and the collector currents of the transistors Q1 and Q1 'match. As a result, the gate voltage of the power MOSFET 1 is controlled to be constant, and as a result, the control current Is flowing to the main source section is also controlled to be constant.

The control current Is when the switch SW1 in the negative feedback control circuit 11 is turned on is expressed by equation (3), and the control current Is when the switch SW2 in the negative feedback control circuit 12 is turned on is (4) It is expressed by an equation. Is = N · VT · ln (K1 × Idrive / Io) / R1 (3) Is = N · VT · ln (K2 × Idrive / Io) / R1 (4) where VT is the threshold value of the transistor. The voltage, Idrive is the current output from the constant current source 2, and Io is the current output from the constant current source 3.

As shown in the equations (3) and (4), since the emitter area ratios of the transistors in the negative feedback control circuits 11 and 12 are different from each other, the power MOSFET 1 is switched by turning on and off the switches SW1 and SW2. Can be switched and controlled. As described above, according to the present embodiment, the current value for overcurrent detection can be easily switched as needed.

In FIG. 1, two negative feedback control circuits 11, 1
Although the example where 2 is provided has been described, three or more negative feedback control circuits may be provided. In this case, the emitter area ratio of the transistor in each negative feedback control circuit may be changed for each negative feedback control circuit. Further, at least one of the switches SW1 and SW2 in the negative feedback control circuits 11 and 12 in FIG. 1 may be turned on, and a plurality of switches may be turned on at the same time.

(Second Embodiment) In a second embodiment, a plurality of current sources are provided in a negative feedback control circuit. FIG. 2 is a circuit diagram of an overcurrent detection circuit according to a second embodiment of the present invention. The circuit in FIG. 2 includes a power MOSFET 1, a resistor R1, a constant current source 2, and a negative feedback control circuit 11 connected in the same manner as in FIG.

The negative feedback control circuit 11 includes NPN transistors Q1 and Q2 whose base terminals are connected to each other, three constant current sources 3, and switches SW1 and SW1 connected to each of the constant current sources 3.
SW2 and SW3. Each constant current source 3 is a switch
It is connected to the collector terminal of the transistor Q2 via SW1, SW2 and SW3. These switches SW1, SW2, SW3
Are individually switchable. Each of the constant current sources 3 outputs the same current Io.

Next, the operation of the circuit of FIG. 2 will be described. For example, when only the switch SW1 is turned on, the current Io is supplied to the collector terminal of the transistor Q2. The control current I flowing through the main source of the power MOSFET 1 in this case
s is represented by equation (5). Is = N · VT · ln (K × Idrive / Io) / R1 (5) On the other hand, when the switches SW1 and SW2 are turned on simultaneously,
The current 2Io is supplied to the collector terminal of the transistor Q2. In this case, the control current Is flowing through the main source of the power MOSFET 1 is expressed by equation (6). Is = N · VT · ln (K × Idrive / 2Io) / R1 (6)

On the other hand, when the switches SW1, SW2, and SW3 are turned on at the same time, the current 3Io is supplied to the collector terminal of the transistor Q2. Power MOSFET 1 in this case
The control current Is flowing through the main source section is expressed by equation (7). Is = N ・ VT ・ ln (K × Idrive / 3Io) / R1 (7) As described above, in the second embodiment, the negative feedback control circuit 11
A plurality of current sources 3 and switches SW1, SW2, and SW3 are provided therein, and the switches SW1, SW2, and SW3 can be arbitrarily switched. Therefore, the value of the control current Is flowing through the main source portion of the power MOSFET 1 is switched in a plurality of ways. be able to.

FIG. 2 illustrates an example in which three constant current sources 3 are provided in the negative feedback control circuit. However, two or four or more constant current sources 3 may be provided. Further, the current values output from the respective constant current sources 3 need not always be the same.

(Third Embodiment) In FIG. 1, the transistors (Q1, Q2), (Q
1 ', Q2'), the emitter area ratio is different for each negative feedback control circuit. However, the emitter area ratios are all the same, and the current value supplied by the constant current source 3 is changed for each negative feedback control circuit. You may. FIG. 3 shows a negative feedback control circuit 11,1.
FIG. 3 is a circuit diagram showing an example in which a current value output from a constant current source 3 in each of the negative feedback control circuits 2 is different for each negative feedback control circuit. The circuit of FIG.
The fact that the emitter area ratios of the transistors Q1 and Q2 are the same in both the negative feedback control circuits 11 and 12, and that the current values output from the constant current sources 3a and 3b in the negative feedback control circuits 11 and 12 are negative feedback control. Except for the difference between the circuits 11 and 12, the circuit is common to the circuit of FIG. In FIG. 3, the constant current source 3a
The current value output from the constant current source 3b is Io1, and the current value output from the constant current source 3b is Io2.

The switch SW1 shown in FIG.
The control current Is flowing through the main source portion of the power MOSFET 1 when the switch 2 is turned off is expressed by equation (8). Is = N · VT · ln (K1 × Idrive / Io1) / R1 (3) Is = N · VT · ln (K2 × Idrive / Io2) / R1 (4) In this way, the switches SW1 and SW2 are switched. Thus, similarly to the first and second embodiments, the power MOSFET
The control current Is flowing through one main source section can be switched.

[0035]

As described in detail above, according to the present invention, a switch is provided in the feedback control circuit, and the switch is controlled to switch the current flowing through the main source of the first transistor. Since it is made possible, the current value for overcurrent detection can be easily changed. Therefore, the use range of the overcurrent detection circuit is expanded, the circuit scale can be reduced, and the component cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment of an overcurrent detection circuit according to the present invention.

FIG. 2 is a circuit diagram of an overcurrent detection circuit according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram of an overcurrent detection circuit according to a third embodiment of the present invention.

FIG. 4 is a circuit diagram of a conventional overcurrent detection circuit having a power MOSFET.

FIG. 5 is a circuit diagram of a conventional overcurrent detection circuit in which the disadvantage of the circuit of FIG. 4 is improved.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Power MOSFET 2, 3 Constant current source 10 Load 11, 12 Negative feedback control circuit Q1, Q2 NPN transistor SW1, SW2, SW3 Switch

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G01R 19/165

Claims (5)

(57) [Claims] A source region of the first transistor is divided into a main source portion and a sub-source portion, the main source portion is connected to a reference voltage terminal, and the sub-source portion is connected to the reference voltage via a resistor. Connected to the terminal, performing negative feedback control on the gate voltage of the first transistor in accordance with the voltage across the resistor, so that the current flowing to the load connected to the drain terminal of the first transistor does not exceed the limit value. In the overcurrent detection circuit described above, the second and third transistors having gate terminals connected to each other, a switch having one end connected to the gate terminal and the drain terminal of the second transistor, and A plurality of negative feedback control circuits having a first current source connected thereto; a gate terminal of the first transistor and a drain of the third transistor; A second current source connected to the plurality of negative feedback control circuits, and a source terminal of each of the second transistors in the plurality of negative feedback control circuits is connected to the sub-source unit; The source terminal of each of the third transistors in the control circuit is connected to the reference voltage terminal, and each of the switches in the plurality of negative feedback control circuits can be individually switched. Current detection circuit. 2. The overcurrent detection according to claim 1, wherein a source area ratio of said second and third transistors in said plurality of negative feedback control circuits is different for each of said negative feedback control circuits. circuit. 3. The overcurrent detection circuit according to claim 1, wherein the first current sources in the plurality of negative feedback control circuits each output a different amount of current. 4. A source region of the first transistor is divided into a main source portion and a sub-source portion. The main source portion is connected to a reference voltage terminal, and the sub-source portion is connected to the reference voltage via a resistor. Connected to the terminal, performing negative feedback control on the gate voltage of the first transistor in accordance with the voltage across the resistor, so that the current flowing to the load connected to the drain terminal of the first transistor does not exceed the limit value. In the overcurrent detection circuit, a second transistor and a third transistor whose gate terminals are connected to each other, a plurality of switches respectively connected to a gate terminal and a drain terminal of the second transistor,
A negative feedback control circuit having a plurality of second current sources provided corresponding to each of the switches; and a second feedback control circuit connected to a gate terminal of the first transistor and a drain terminal of the third transistor. A source terminal of the second transistor is connected to the sub-source portion, a source terminal of the third transistor is connected to the reference voltage terminal, and each of the plurality of switches is individually An overcurrent detection circuit characterized in that the circuit can be switched over to (1). 5. The power supply according to claim 1, wherein the first transistor is a power MOSFET.
The overcurrent detection circuit according to any one of claims 1 to 4, wherein