JP2004274865A - Overcurrent protection circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To operate an overcurrent protection circuit, even if an overcurrent is generated at start, and to accurately detect the overcurrent generated in normal operation. <P>SOLUTION: A main current element 11a where a main current flows to a power MOS transistor 11, and a detection element 11b where a detection current Ik geared to the main current element 11a flows are made, and a first detection circuit 16 which detects an overcurrent, based on the current flowing into the detection element 11b, and a second detection circuit 18 which detects the overcurrent, based on the current flowing to the main current element 11a are provided, and a gate control circuit 13 is made not to operate, when the first detection circuit 16 and the second detection circuit 18 have both detected an overcurrent. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an overcurrent protection circuit for protecting an overcurrent from flowing through a power transistor and destroying the power transistor.
[0002]
[Prior art]
A switching power supply circuit is often used because a required DC voltage can be easily obtained from a commercial power supply as a power supply for a television receiver or audio equipment. When an overcurrent flows through a power transistor used in such a switching power supply circuit or the like, the power transistor is destroyed. Therefore, it is necessary to detect and protect the overcurrent from flowing.
[0003]
FIG. 3 is a circuit diagram of a conventional overcurrent protection circuit. A pulse signal is applied to the gate electrode of the power MOS transistor 1 via the gate control circuit 2. The collector and source of the protective transistor 3 are connected between the gate control circuit 2 and the ground.
[0004]
The gate electrode of the protection transistor 3 is connected to the drain electrode of the power transistor 1. The load resistor 4 is connected to the drain electrode of the power transistor 1.
[0005]
When the current flows between the drain and the source of the power transistor 1, the voltage of the drain electrode increases. Therefore, the voltage V _{DS at} the drain electrode terminal of the power transistor 1 is V _DS = I _DS × R
Here, _IDS is the drain-source current R of the power transistor 1 and the resistance value of the load resistance.
[0006]
If the load resistor R and 1 [Omega, the drain-source current I _DS of the power transistor 3 is 1A, the terminal voltage V _DS of the drain electrode terminal of the power transistor 1 becomes 1V. Drain-source current _I when the _DS flows a 1A or more current, the terminal voltage _{V DS} of the drain electrode terminal is equal to or higher than 1V. Set the terminal voltage V _DS is the overcurrent becomes more 1V, turns off the protective transistor 3, in order to not operate the gate control circuit 2, the power transistor 1 is protected against destruction is inoperative.
[0007]
By the way, as shown in FIG. 4, since the resistance between the drain and source electrodes of the power transistor 1 has an infinite resistance before the power is turned on, the drain electrode terminal of the power transistor 1 becomes 1 V or more. This means that the protection transistor 3 is turned on and the gate control circuit 2 is turned off because of the same state as when the overcurrent has flown, so that the protection transistor 3 cannot be started.
[0008]
Therefore, for example, the capacitor 5 is connected during the period from T0 to T1 when the power is turned on as shown in FIG. 4 so that the protection transistor 3 is not turned on during that period. However, when the load is short-circuited or the like, an overcurrent may flow through the power transistor 1 during the above-described power-on period from T0 to T1. As described above, since the protection transistor 3 is forcibly turned off during the period from the activation states T0 to T1, the protection operation against overcurrent does not work and the power transistor 1 is destroyed.
[0009]
FIG. 5 removes such a drawback. The power MOS transistor 7 has a main current element 7a and a detection element 7b on the same semiconductor chip. The main current element 7a and the detection element 7b are divided and connected in parallel so that the unit cell becomes W: 1 (for example, 100: 1).
[0010]
As shown in the equivalent circuit of FIG. 6, the main current element 7a and the detection element 7b have their gate electrodes connected, and a pulse-like gate signal is applied. The drain electrode is also connected, and a current flows to the load 8. The detection resistor 9 is connected to the source electrode of the detection element 7b, but the source electrode of the main current element 7a is grounded as it is so that there is no loss. An overcurrent detection transistor 10 is connected to a connection point between the source electrode of the detection element 7b and the detection resistor 9.
[0011]
Each time a pulsed gate signal is applied to the gate electrode, the main current element 7a and the detection element 7b of the power MOS transistor 7 are turned on, and supply the load current Id to the load 8 via the drain / source of the main current element 7a. Then, a detection current Ik proportional to the load current Id also flows through the drain / source of the detection element 7b of the power MOS transistor 7.
[0012]
When an excessive load current Id flows between the drain and the source of the main current element 7a, the detection current Ik also increases, the detection potential of the detection resistor 9 connected to the detection element 7b also increases, and when the overcurrent set value is exceeded, the detection current Ik increases. The current detection transistor 10 is turned on. As a result, the gate control circuit 2 is deactivated, so that the power MOS transistor 7 is also deactivated and is protected from overcurrent.
[0013]
Since the detection potential of the detection resistor 9 connected to the detection element 7b does not exceed the overcurrent set value during normal startup, the overcurrent detection transistor 10 is not turned on. When an overcurrent flows at the time of startup due to a short circuit of the load or the like, the detection potential of the detection resistor 9 exceeds the overcurrent set value, the overcurrent detection transistor 10 is turned on, and the gate control circuit 2 is deactivated. You. When the gate control circuit 2 is deactivated, the power transistor 7 is also deactivated and is protected from overcurrent.
[0014]
However, since a voltage drop (detection voltage VK) occurs between the source terminal S1 of the main current element 7a and the detection terminal S2 of the detection element 7b, the voltage V _DS1 applied between the drain and source of the main current element 7a. And the voltage V _DS2 applied between the drain and the source of the detection element 7b differs by the detection voltage VK. That is, V _DS 2 = V _DS 1−VK
It becomes. Therefore, the current actually flowing through the detection element 7b does not depend on the division ratio (100: 1) of the unit cell of the main current element 7a and the detection element 7b, and the overcurrent flowing through the power transistor 7 is not accurately detected. Become.
[0015]
[Patent Document 1]
JP 2001-2111059 A
[Problems to be solved by the invention]
As shown in FIG. 3, a collector / source of a protection transistor 3 is connected between the gate control circuit 2 connected to the gate electrode of the power MOS transistor 1 and the ground. Then, the drain voltage of the power MOS transistor 1 is applied to the base electrode of the protection transistor 3. When an overcurrent occurs due to a load short circuit or the like, and the drain voltage exceeds a set value, the protection transistor 3 is turned on, the gate control circuit 2 is deactivated, and the power MOS transistor 1 is turned off to protect from overcurrent. I have.
[0017]
As described above, in the overcurrent protection circuit that detects the drain electrode of the power MOS transistor 1, the overcurrent protection is not activated in order to prevent a malfunction at the time of startup. Therefore, when an overcurrent flows during startup, the overcurrent protection circuit does not operate and the power MOS transistor is destroyed.
[0018]
Further, as shown in FIG. 5, a power MOS transistor 7 is provided with a main current element 7a and a detection element 7b on the same semiconductor chip, and the main current element 7a and the detection element 7b are divided so as to be 100: 1 and connected in parallel. I do. When the detection voltage generated at the detection resistor 9 connected to the source electrode of the detection element 7b exceeds a set value, the overcurrent detection transistor 10 is turned on to protect the overcurrent. A potential difference occurs between the terminals between the current element 7a and the detection element 7b, and the detection terminal of the detection element cannot accurately detect overcurrent.
[0019]
[Means for Solving the Problems]
The present invention enables the overcurrent protection circuit to be operated even when an overcurrent occurs at the time of startup, and enables the overcurrent to be accurately detected.
A gate-controlled power transistor that has a main current element through which a main current flows and a detection element through which a detection current corresponding to the main current flows, and supplies a load current to a load; and an overcurrent based on the current flowing through the detection element. A first detection circuit for detecting, a second detection circuit for detecting an overcurrent based on a current flowing through the main current element, and both the first detection circuit and the second detection circuit have detected an overcurrent. Sometimes, an overcurrent protection circuit including a gate control circuit for disabling a power transistor is provided.
[0020]
Further, the present invention has a main current element through which a main current flows, a detection element through which a detection current according to the main current flows, and a gate control type power transistor that supplies a current flowing through the main current element to a load; A first detection circuit for comparing a detection voltage of the connected detection resistor with a reference voltage, a second detection circuit for comparing a voltage to a load connected to the main current element with a reference voltage, A protection signal generating circuit for generating an overcurrent protection signal when the output signal from the first detection circuit and the output signal from the second detection circuit and the gate signal applied to the gate electrode of the power transistor are applied together. Provided is an overcurrent protection circuit for inactivating a gate control circuit by an overcurrent protection signal of a signal generation circuit, thereby protecting the power transistor from overcurrent that inactivates the power transistor.
[0021]
Further, the first and second detection circuits are operational amplifiers, and provide an overcurrent protection circuit that determines the overcurrent detection value by selecting the magnitude of a reference voltage applied to one input terminal.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described in detail with reference to FIGS.
[0023]
In FIG. 1, a power MOS transistor 11 is provided with a main current element 11a through which a main current flows in the same semiconductor chip and a detection element 11b through which a detection current according to the main current flows. The main current element 11a and the detection element 11b are divided and connected in parallel so that the unit cell becomes W: 1 (for example, 100: 1).
[0024]
FIG. 2 shows an equivalent circuit of the power MOS transistor 11. The main current element 11a and the detection element 11b have drain electrodes connected thereto and are both connected to the load 12. Similarly, the gate electrodes of the main current element 11a and the detection element 11b are also connected, and both are connected to the gate control circuit 13. A pulse-like gate signal G is generated from the gate control circuit 13 and applied to the gate electrodes of the main current element 11a and the detection element 11b to turn on / off the power MOS transistor 11 so that a predetermined load voltage can be obtained. I have.
[0025]
The detection resistor 14 is connected to the source electrode of the detection element 11b of the power MOS transistor 11, but the source electrode of the main current element 11a is grounded as it is to prevent loss.
[0026]
The + terminal of the first detection circuit 16 composed of the first operational amplifier is connected to the detection terminal S2 of the detection resistor 14, and the-terminal is connected to the reference voltage source 17. When the detection voltage Vk detected at the detection terminal S2 applied to the + terminal is higher than the reference voltage ref1 of the reference voltage source 17 applied to the-terminal, an output signal is generated from the first detection circuit 16.
[0027]
The + terminal of the second detection circuit 18 composed of a second operational amplifier is connected to the drain electrode terminal D of the main current element 11a of the power MOS transistor 11, and the-terminal is connected to the reference voltage source 19. Similarly to the first detection circuit 16, when the drain voltage Vd of the power MOS transistor 11 applied to the + terminal is higher than the reference voltage ref2 of the reference voltage source 19 applied to the-terminal, an output signal is generated from the second detection circuit 18. I do.
[0028]
Therefore, the reference voltage ref1 of the first reference voltage source 17 and the reference voltage ref2 of the second reference voltage source 19 are selected to have such a magnitude that an overcurrent can be detected.
[0029]
The output signals of the first detection circuit 16 and the second detection circuit 18 and the gate signal applied to the gate control circuit 13 are applied to the input terminal of the protection signal generation circuit 20, and these signals applied to the input terminal are all high. When it reaches the level, an overcurrent protection signal is generated.
[0030]
The overcurrent protection circuit of the present invention has the above-described configuration. When a power switch (not shown) is turned on, VDD is applied, and the power MOS transistor 11 is turned on. Further, a pulse-like gate signal is generated from the gate control circuit 13 and applied to the main current element 11a of the power MOS transistor 11 and the gate electrode of the detection element 11b. The main current element 11a and the detection element 11b of the power MOS transistor 11 are turned on each time a gate signal is applied to the gate electrode, and supply a load voltage of a necessary magnitude to the load 12 by selecting a duty of the gate signal.
[0031]
As shown in FIG. 2, when the main current element 11a and the detection element 11b have a unit ratio of, for example, 100: 1, the resistance between the drain and source of the main current element 11a is 1Ω, and the drain current of the detection element 11b is -The resistance between the sources is 100Ω.
Therefore, the ratio of the load current Id flowing between the drain and source of the main current element 11a to the detection current flowing between the drain and source of the detection element 11b is 100: 1.
[0032]
In the activated state, the drain voltage Vd of the drain electrode terminal D is high, and the drain voltage Vd applied to the + input terminal of the second detection circuit 18 becomes higher than the reference voltage ref2 of the reference voltage source 19, so that an output signal is generated and protection is performed. An output signal is applied to the signal generation circuit 20.
[0033]
However, during normal startup, the detection current Id flowing between the drain and source of the detection element 11b of the power MOS transistor 11 is small, and the detection voltage Vk detected at the detection terminal S2 of the detection resistor 14 is small. Since the detection voltage Vk applied to the + input terminal of the first detection circuit 16 generated at the detection terminal S2 by the detection current flowing through the detection element 11b at the time of startup is made smaller than the reference voltage ref1 of the reference voltage source 17, the first No output signal is generated from the detection circuit 16.
[0034]
Therefore, since the overcurrent protection signal is not generated from the protection signal generation circuit 20, the gate control circuit 13 continues the normal operation and applies the gate signal to the power MOS transistor 11. The power MOS transistor 11 increases the load voltage on every time a gate signal is applied.
[0035]
If the load 12 is short-circuited at the time of starting, an overcurrent Id flows through the main current element 11a of the power MOS transistor 11. Then, the detection current Ik proportional to the unit ratio also flows through the detection element 11b. In order to activate the overcurrent protection when the overload current Id flows 1 A or more, the detection current Ik is 0.01 A or more. Therefore, if the resistance value of the detection resistor 14 is 1Ω, the detection voltage Vk becomes It becomes 0.01V or more.
[0036]
Therefore, if the reference voltage of the reference voltage source 17 is set to 0.01 V or less, an output signal is generated from the first detection circuit 16 when the detection current becomes 0.01 A or more. As described above, since the output signal is also generated from the second detection circuit 18 at the time of activation, the protection control signal is output to the protection signal generation circuit 20 when the pulse signal applied to the gate control circuit 13 is applied. Therefore, the gate control circuit 13 is deactivated. Therefore, no gate signal is generated from the gate control circuit 13 and the power MOS transistor 11 is turned off, so that the power MOS transistor 11 is protected from overcurrent.
[0037]
In a normal operation state, the power MOS transistor 11 repeats the on / off operation by the gate signal from the gate control circuit 13 to supply a required load voltage to the load 12.
[0038]
In a normal operation state, the drain voltage Vd of the power MOS transistor 11 is low and smaller than the reference voltage ref2 of the reference voltage source 19 connected to the second detection circuit 18, so that an output signal is generated from the second detection circuit 18. do not do.
[0039]
On the other hand, the detection current Ik also flows between the drain and source of the detection element 11b of the power MOS transistor 11 in proportion to the main current Id. When the detection current Ik flows, a detection voltage Vk is generated at the detection terminal S2. The detection voltage Vk detected at the detection terminal S2 is applied to the + input terminal of the first detection circuit 16.
[0040]
Since the detection voltage Vk is smaller than the reference voltage ref1 of the reference voltage source 17 in the normal state, no output signal is generated from the first detection circuit 16. However, in the overcurrent recognition state, which is the upper limit of the normal state, the output signal is generated from the first detection circuit 16 because it is set to be higher than the reference voltage ref1 of the reference voltage source 17.
[0041]
In this overcurrent recognition state, no output signal is generated from the second detection circuit 18 yet, so no overcurrent protection signal is generated from the protection signal generation circuit 20 and no overcurrent protection operation is performed.
[0042]
When an overcurrent (for example, 1 A or more) flows through the power MOS transistor 11 for some reason, the drain voltage Vd (1 V or more) of the power MOS transistor 11 increases, and the reference voltage source 19 connected to the second detection circuit 18 , The second detection circuit 18 generates an output signal.
[0043]
Further, in this overcurrent state, as described above, the detection voltage Vk generated at the detection terminal S2 is set to be higher than the reference voltage ref1 of the reference voltage source 17, so that an output signal is also generated from the first detection circuit 16. ing.
[0044]
Therefore, when an output signal is generated from the second detection circuit 18, the output signals from the first detection circuit 16 and the second detection circuit 18 applied to the input terminal of the protection signal generation circuit 20 are added. An overcurrent protection signal is generated when a pulse signal is applied to 20.
[0045]
When the overcurrent protection signal from the protection signal generation circuit 20 is applied to the gate control circuit 13, the gate control circuit 13 is deactivated. Therefore, no gate signal is generated from the gate control circuit 13 and the power MOS transistor 11 is turned off, so that the power MOS transistor 11 is protected from overcurrent.
[0046]
【The invention's effect】
An overcurrent protection circuit according to the present invention forms a main current element through which a main current flows through a power MOS transistor and a detection element through which a detection current according to the main current flows, and detects an overcurrent based on a current flowing through the detection element. And a second detection circuit for detecting an overcurrent based on the current flowing in the main current element, and the first detection circuit and the second detection circuit both detect the overcurrent as the gate control circuit. Sometimes not work.
[0047]
Therefore, in the case where the overcurrent is detected by the drain voltage of the power MOS transistor as in the related art, the overcurrent detection function has to be disabled since the malfunction occurs at the time of startup. Even if an overcurrent occurs sometimes, the overcurrent can be detected and the power MOS transistor can be prevented from being destroyed.
[0048]
Also, when an overcurrent flows through the power MOS transistor during normal operation, the overcurrent is detected by the drain voltage of the power MOS transistor and an overcurrent protection operation is started. It is possible to eliminate the delay of the overcurrent start operation when the overcurrent is detected by the flowing detection current.
[Brief description of the drawings]
FIG. 1 is a block diagram of an overcurrent protection circuit according to the present invention.
FIG. 2 is an equivalent circuit diagram of a power MOS transistor used in the overcurrent protection circuit of the present invention.
FIG. 3 is a block diagram of a conventional overcurrent protection circuit.
FIG. 4 is a waveform diagram illustrating an operation of a conventional overcurrent protection circuit.
FIG. 5 is a block diagram of a conventional overcurrent protection circuit.
FIG. 6 is an equivalent circuit diagram of a power MOS transistor used in a conventional overcurrent protection circuit.
[Explanation of symbols]
Reference Signs List 11 power MOS transistor 11a main current element 11b detection element 13 gate control circuit 14 detection resistor 16 first detection circuit 18 second detection circuit 20 protection signal generation circuit

Claims (3)

A gate-controlled power transistor that has a main current element through which a main current flows and a detection element through which a detection current according to the main current flows, and supplies a load current to a load;
A first detection circuit that detects an overcurrent based on a current flowing in proportion to a main current in the detection element,
A second detection circuit that detects an overcurrent based on a current flowing through the main current element;
An overcurrent protection circuit, comprising: a gate control circuit for disabling a power transistor when both the first detection circuit and the second detection circuit detect an overcurrent. A gate-controlled power transistor having a main current element through which a main current flows and a detection element through which a detection current corresponding to the main current flows, and supplying a load current to a load;
A first detection circuit that compares a detection voltage generated at a detection resistor connected to the detection element with a reference voltage;
A second detection circuit that compares a voltage to a load connected to the main current element with a reference voltage;
A protection signal generating circuit for generating an overcurrent protection signal when an output signal from the first detection circuit and the second detection circuit and a gate electrode signal applied to the gate electrode of the power transistor are both applied. ,
An overcurrent protection circuit characterized in that a gate control circuit is deactivated by an overcurrent protection signal generated by the protection signal generation circuit, thereby deactivating a power transistor to protect against overcurrent. 3. The overcurrent protection according to claim 2, wherein the first and second detection circuits are operational amplifiers, and determine a detection value of the overcurrent by selecting a magnitude of a reference voltage applied to one input terminal. circuit.

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