JPH0337385B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to an overcurrent detection and protection device that detects a passing current and energizes a current control means, thereby obtaining a so-called “F”-shaped droop characteristic. It is.

[Prior Art] Conventionally, as an output terminal short-circuit protection method in a power supply device or the like, a method that obtains a so-called "F"-shaped droop characteristic has been known. This method is known as an effective method from the viewpoint of controlling the power consumption of the current control transistor.

However, even with the conventional method with the so-called "F"-shaped droop characteristic described above, in devices intended to output large currents, not only the power consumption of the output current detection resistor increases dramatically, but also the mounted capacitance increases. This brings about various inconveniences such as enlargement of the area, making appropriate design difficult.

FIG. 1 shows an example of a constant voltage power supply with an overcurrent detection and protection circuit according to the prior art, which is the premise of the present invention. According to the illustrated circuit, the resistance value of the overcurrent detection resistor Rs is uniquely determined by the short circuit current Iso. That is, the resistance value is determined by the magnitude of the base-emitter voltage V _BE of the transistor Q2, resulting in the following drawbacks.

The power dissipated in the resistor Rs is the output current Iout
Since it is proportional to the square of , it is necessary to use a resistor for high power as the resistor Rs as a large current is output.

In order to further reduce the magnitude of the short-circuit current Iso, the value of the resistor Rs must be increased, and as a result, the power consumption in the resistor Rs increases further when the maximum load current is output. Furthermore, when the maximum load current is output, the resistor
Since the amount of voltage drop at Rs also increases, it is necessary to further increase the input voltage Ein. Under such conditions, the power consumption of transistor Q1 also increases. Thus, in the constant voltage power supply device according to the prior art, the magnitude of the short circuit current Iso is limited to about 1/4 to 1/3 of the magnitude of the maximum output current.

Base-emitter voltage of transistor Q2
V _BE not only depends on the ambient temperature, etc., but also varies depending on the individual transistors, so when setting the overcurrent detection value and short-circuit current value, it is necessary to take these temperature factors and variations into consideration. There is.

From the viewpoint of stability coefficient, the magnitude of the current flowing through the resistors R1 and R2 connected to the base of transistor Q2 is the base current of transistor Q2.
I Requires about 10 times more than _B2 . Therefore output voltage
If Eout is high, these resistors R1, R
2, the power consumption becomes large, and it is necessary to use a resistor with a capacity that can withstand this.

[Objective] In view of the above-mentioned points, an object of the present invention is to significantly reduce power consumption in current control transistors, output current detection resistors, etc. compared to conventional systems;
It is an object of the present invention to provide an overcurrent detection and protection device configured to obtain ideal so-called "foldback" characteristics.

[Structure of the Invention] In order to achieve the above object, the present invention provides an overcurrent detection and protection device that energizes a current control means in accordance with a detected current passing through an overcurrent detection resistor. a constant voltage generating means for generating a voltage; and a voltage amplifying means having the constant voltage generating means connected to a first input terminal and one end of the overcurrent detection resistor connected to a second input terminal, the voltage amplifying means The current control means is configured to be energized using the output signal of the current control means.

[Example] Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 2 shows an embodiment of a constant voltage current device to which the present invention is applied. The difference between this diagram and Figure 1 is that an operational amplifier AMP is used instead of the transistor Q2.
a resistor RB and a constant current source CC for applying a bias voltage to the non-inverting input terminal of the operational amplifier AMP,
A capacitor C, a resistor R, and a resistor Rf are connected between the output terminal and the inverting input terminal of the operational amplifier AMP, and a resistor RP and a diode D are connected to the output terminal of the operational amplifier AMP.
Further, HL indicates a hot line, and RL indicates a return line.

Note that it is also possible to apply another voltage to point A in the figure and a negative voltage to point B.

The rest of the configuration is exactly the same as the conventional circuit shown in the first figure, so a detailed explanation will be omitted.

The operation of each part of this embodiment is as follows.

First, a constant current is supplied from a constant current source CC to generate a predetermined constant bias voltage V _B between the terminals of the resistor RB. Therefore, it is also possible to divide the generated voltage using a constant voltage source (not shown) and thereby obtain the bias voltage V _B. The magnitude of this bias voltage V _B is preferably set to several millivolts to several hundred millivolts, taking into consideration the offset voltage of the operational amplifier AMP. However, if the bias voltage V _B is set to be smaller than the above offset voltage, care must be taken because the transistor Q1 may be cut off and locked in this state when the output is short-circuited.

Diode D functions to block current flowing out from operational amplifier AMP. Therefore, if the output circuit of the operational amplifier AMP is configured as an open collector type, or if the output circuit of the operational amplifier AMP is
If transistors (not shown) are connected in series on the output side of the AMP and the output circuit is of an open collector type, the diode D is not necessary. Furthermore, the resistor RP serves to protect the circuit components when a negative voltage is used (that is, when the output voltage is reversed).

A resistor R and a capacitor C connected in series across the input and output sides of the operational amplifier AMP serve to prevent parasitic oscillation when the overcurrent detection circuit is activated and causes parasitic oscillation. Furthermore, by adding a resistor Rf, it is possible to lower the overall gain of the operational amplifier AMP, thereby making parasitic oscillation less likely to occur.

The resistance value Rs of the overcurrent detection resistor RS is given by (RS) = V _B / (Iso), where Iso is the current value when the output is shorted.

According to the above-described configuration, the following features can be obtained compared to the constant voltage power supply device shown in the first diagram.

Since the bias voltage V _B can be set to a small value, the resistance value of the overcurrent detection resistor Rs can be made small.

Even when setting the short circuit current Iso to 1/4 or less of the maximum output current, there is no need to increase the resistance value of the resistor Rs, so the power consumption of the transistor Q1 is controlled and the rise in the input voltage Ein is kept low. be able to.

Temperature-dependent changes in the overcurrent detection value and short-circuit current value can be suppressed to a small level.

Since the bias current of the operational amplifier AMP is small, the resistance values of the resistors R1 and R2 can be increased. As a result, resistors R1 and R2 may require resistors with small power capacities.

The graph shown in FIG. 3 shows the output characteristics and power consumption characteristics of this embodiment based on these characteristics. "Prior art" shown in the upper part of FIG. 3 refers to the constant voltage power supply device shown in the first diagram, and "this embodiment" refers to the constant voltage power supply device shown in the second diagram. In addition, A shown in Figure 3 is a diagram showing the relationship between output voltage and output current, B is a diagram showing the power consumption characteristics of the control transistor Q1, and C is a diagram showing the power consumption characteristics of the overcurrent detection resistor RS. FIG. 3 is a diagram showing power characteristics.

Note that the scale represents a relative value (ratio) and does not represent an absolute value.

As shown in FIG. 3A, while the short circuit current in the prior art is "0.3", the short circuit current in this embodiment is as small as "0.1".

Moreover, the broken line is the output voltage vs. output current characteristic of this embodiment superimposed on the diagram of the prior art. The broken line shows the power consumption characteristics when trying to obtain the characteristics shown by the broken line using a constant voltage current device according to the prior art. Similarly, the broken line indicates the power consumption characteristic corresponding to the broken line.

In addition, in the above-mentioned embodiment, the hot line
Although the overcurrent detection and protection circuit according to the present invention is inserted on the HL side, it is possible to do the same on the return line RL side. Therefore, it goes without saying that the present invention can be applied even when using a negative voltage.

Further, the present invention is not limited to a constant voltage power supply as in this embodiment, but can be applied to a switching power supply and an overcurrent protection device other than a power supply device.

[Effects] As explained above, the present invention can reduce the resistance value of the overcurrent detection resistor and suppress the power consumption in the control transistor, etc., thereby achieving high efficiency and reliability. An efficient overcurrent detection and protection device can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a constant voltage power supply device according to the prior art, Fig. 2 is a circuit diagram showing an embodiment of a constant voltage power supply device to which the present invention is applied, and Fig. 3 explains the effects of this embodiment. FIG. Q1, Q2, Q3...transistor, R1, R
2, RS, RB, R, Rf...Resistor, C...Capacitor, D...Diode, AMP...Operation amplifier, Ein...Input voltage, Eout...Output voltage, Iso
...Short circuit current, HL...Hot line, RL...Return line.

Claims (1)

[Claims] 1. In an overcurrent detection and protection device that energizes current control means connected upstream of the output current detection resistor in response to a detected current passing through the output current detection resistor, the overcurrent detection and protection device includes: Voltage amplification means having an input end and a non-inverting input end; Constant voltage generation for applying a bias voltage of a predetermined value between the load side terminal of the output current detection resistor and the non-inverting input end of the voltage amplification means. and voltage dividing means for dividing the voltage at the current control means side terminal of the output current detection resistor and applying it to the inverting input terminal of the voltage amplification means, using the output signal of the voltage amplification means to An overcurrent detection and protection device characterized by energizing current control means. 2. The overcurrent detection and protection device according to claim 1, wherein the voltage amplification means has an output circuit that allows only one-way output current to pass through, or an open collector type output circuit.