JP2008167572A - Current detecting circuit and power supply unit using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein, since the detection current of an excessive current protective circuit is a constant value, irrespective of the ambient temperature, when the ambient temperature rises, the detection current exceeds the limit of a delating curve, having a possibility to cause problems, such as heating or firing. <P>SOLUTION: A voltage signal, wherein voltage drops as the ambient temperature rises, is applied between one end of a current-detecting resistor and one input terminal of a comparator, for allowing a voltage-restricting element to restrict the voltage signal from becoming more than the constant value, and a constant voltage is applied between the other end of the resistor and the other input terminal of the comparator. Furthermore, the current detection circuit is used for the overcurrent detection circuit of the power supply unit. Since when the ambient temperature rises, the detection current drops, the detection current is prevented from exceeding the limit of the delating curve to eliminate problems, such as heating or firing. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a current detection circuit suitable for use in a power supply apparatus in which allowable power varies depending on mounting conditions and ambient temperature, and a power supply apparatus using the current detection circuit.

  FIG. 4 shows the configuration of a DC power supply device incorporating an overcurrent protection circuit. In FIG. 4, the voltage control circuit 10 converts the input alternating current into direct current, reconverts the direct current into alternating current with a predetermined frequency, and applies it to the primary side of the transformer 11. The alternating current generated on the secondary side of the transformer 11 is rectified by the diode 12, smoothed by the capacitor 13, and output to the load 14. The voltage control circuit 10 controls the alternating current applied to the transformer 11 so that the output voltage Vout is constant. The voltage control circuit 10 and the transformer 11 constitute a voltage control unit.

  The current detection circuit 20 includes a comparator 21, a reference voltage source 22 connected to the inverting input terminal of the comparator 21, and a resistor 23 inserted in a feedback path for current flowing through the load 14. A voltage drop proportional to the current flowing through the load 14 is generated at both ends of the resistor 23. This voltage drop is input to the non-inverting input terminal of the comparator 21. The output of the comparator 21 is input to the voltage control circuit 10.

  In such a configuration, when the output current of the power supply device is small and the voltage drop across the resistor 23 is smaller than the voltage across the reference voltage source 22, the output of the comparator 21 is at a low level. The voltage control circuit 10 performs a normal operation and maintains the output voltage Vout at a constant value.

  When the output current increases and the voltage drop across the resistor 23 becomes larger than the voltage across the reference voltage source 22, the output of the comparator 21 changes to a high level, and the voltage control circuit 10 decreases the output voltage Vout. For this reason, the output current decreases. In this way, the output current is controlled to be equal to or less than a predetermined value so that no overcurrent is output, thereby preventing accidents such as burning of the power supply device.

  In such a power supply device, the maximum current that can be output is limited by the heat generated by the device itself. That is, the maximum output current varies depending on the mounting position of the apparatus, the presence or absence of a cover that is a part of the housing, or the ambient temperature. This maximum output current is specified by the derating curve.

  FIG. 5 shows an example of a derating curve. The horizontal axis in FIG. 5 is the ambient temperature, and the vertical axis is the ratio of the maximum output current to the rated output current (maximum current that can be output at room temperature). 30 to 33 are derating curves, 30 is a case where the power supply is mounted vertically and no cover is attached, 31 is a case where the power supply is attached horizontally and no cover is attached, 32 is a cover where the power supply is attached vertically , 33 is a derating curve when the power supply is mounted horizontally and a cover is attached.

  In curve 33, when the ambient temperature reaches 40 ° C., the maximum output current starts to decrease. Similarly, the maximum output current begins to decrease at an ambient temperature of 45 ° C. for curve 32, an ambient temperature of 50 ° C. for curve 31, and an ambient temperature of 55 ° C. for curve 30. The reason why the derating curves differ depending on the attachment method and the presence or absence of the cover is that the heat dissipation characteristics differ depending on these.

JP-A-6-38518

  However, such a power supply device has the following problems. Since the rated output current is set with a margin, the detection current of the normal current detection circuit 20 is set to 110 to 130% of the rated output current. That is, the detection current of the current detection circuit 20 is a fixed value, and does not change depending on the device mounting method and the ambient temperature.

  Assume that a power supply is used at 40% of the rated output current. Referring to the derating curve in FIG. 5, if the ambient temperature is 40 ° C. or less, there is a sufficient margin regardless of the mounting method, and no problem occurs.

  However, when the ambient temperature exceeds 55 ° C., it crosses the derating curve 33 when the cover is installed horizontally. Therefore, an abnormal temperature rise occurs, and in the worst case, there is a possibility that an accident such as destruction of the power supply device, heat generation due to the abnormal temperature rise, smoke generation, or fire may occur. As described above, since the detection current of the current detection circuit 20 is fixed at 110 to 130% of the rated output current, there is a problem that the current detection circuit 20 cannot prevent these accidents.

  Accordingly, an object of the present invention is to provide a current detection circuit in which a detection current changes depending on an ambient temperature, and a power supply device using the current detection circuit.

In order to solve such a problem, the invention according to claim 1 of the present invention,
A comparator having two input terminals, the output of which is determined by the magnitude of the voltage applied to these input terminals;
A resistor arranged in a path through which a current to be detected flows;
A variable voltage source for applying an output voltage having a negative temperature coefficient between one end of the resistor and one input terminal of the comparator;
A voltage limiting element for limiting the one input terminal voltage of the comparator to a certain value or less;
A reference voltage source for applying a constant voltage between the other end of the resistor and the other input terminal of the comparator;
Is provided. A characteristic that the detected current decreases as the temperature rises can be realized.

The invention according to claim 2 is the invention according to claim 1,
The variable voltage source includes a temperature sensor that outputs a voltage having a negative temperature coefficient and a reference voltage source that outputs a constant voltage. The degree of freedom in selecting the reference voltage source can be increased.

The invention described in claim 3
A voltage controller that outputs DC power of a constant voltage and that prevents an overcurrent from being output by lowering the output voltage when an overcurrent detection signal is input;
The current detection circuit according to claim 1 or 2, wherein an output current of the voltage control unit is detected and the overcurrent detection signal is output.
Is provided. No accidents such as overheating and ignition occur even when the ambient temperature rises.

The invention according to claim 4
A voltage controller that outputs DC power of a constant voltage and that prevents an overcurrent from being output by lowering the output voltage when an overcurrent detection signal is input;
A resistor disposed in a path through which an output current of the voltage control unit flows;
A variable voltage source for applying an output voltage having a negative temperature coefficient between one end of the resistor and one input terminal of the comparator;
A voltage limiting element for limiting the one input terminal voltage of the comparator to a certain value or less;
A voltage divider that divides the output voltage of the voltage controller and outputs the divided voltage to the other input terminal of the comparator;
Is provided. No accidents such as overheating and ignition occur even when the ambient temperature rises.

The invention according to claim 5 is the invention according to claim 4,
The variable voltage source includes a temperature sensor that outputs a voltage having a negative temperature coefficient and a reference voltage source that outputs a constant voltage. The degree of freedom in selecting the temperature sensor can be increased.

As is apparent from the above description, the present invention has the following effects.
According to the first, second, third, fourth and fifth aspects of the present invention, a voltage signal whose voltage decreases as the ambient temperature increases is applied between one end of the resistor for detecting the current and one input terminal of the comparator. The voltage signal is limited by a voltage limiting element so that the voltage signal does not exceed a certain value, and a constant voltage is applied between the other end of the resistor and the other input terminal of the comparator. Further, this current detection circuit was used as an overcurrent detection circuit of the power supply device.

  Conventionally, the detection current of the overcurrent was constant regardless of the ambient temperature, so if the ambient temperature rises, the range restricted by the derating curve will be exceeded, causing damage to the power supply, heat generation due to abnormal temperature rise, ignition, etc. There was a possibility of an accident. According to the present invention, the value of the detected current can be reduced as the ambient temperature increases. Therefore, even when used in a state where the ambient temperature is high, the derating curve is not exceeded, accidents such as heat generation and ignition can be reliably prevented, and the power supply device can be protected.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a configuration diagram of a power supply device using an embodiment of a current detection circuit according to the present invention. The same elements as those in FIG. 4 are denoted by the same reference numerals, and description thereof is omitted. The configuration and operation of the power supply device are the same as those in FIG. In FIG. 1, reference numeral 40 denotes a current detection circuit, which includes a comparator 21, reference voltage sources 42 and 44, a temperature sensor IC 41, a Zener diode 43, and a resistor 23.

  The temperature sensor IC 41 has three terminals: a power supply terminal S, a ground terminal G, and an output terminal O. The temperature sensor IC 41 incorporates a temperature sensing element, and when a predetermined voltage is applied between the power supply terminal S and the ground terminal G, the voltage between the output terminal O and the ground terminal G changes depending on the internal temperature of the power supply device. This voltage changes linearly with respect to the internal temperature, and has a negative temperature coefficient that decreases as the internal temperature increases. The temperature coefficient of the both-ends voltage of the reference voltage sources 42 and 44 and the Zener voltage of the Zener diode 43 is sufficiently smaller than the temperature coefficient of the temperature sensor IC 41. The temperature sensor IC 41 and the reference voltage source 42 constitute a variable voltage source. Further, the Zener diode 43 has a function of a voltage limiting element.

  The connection point between the cathode of the diode 12 and one end of the capacitor 13 is IS, and the connection point between the other end of the capacitor 13 and the secondary side of the transformer 11 is IG. IS is a positive output of the power supply device, and IG is a common potential point of the circuit.

  The power supply terminal S of the temperature sensor IC 41 is connected to IS, and the output terminal O is connected to the inverting input terminal of the comparator 21. The ground terminal G of the temperature sensor IC 41 is connected to one end of the reference voltage source 42, and the other end of the reference voltage source 42 is connected to IG. That is, the temperature sensor IC 41 and the reference voltage source 42 are connected in series, and both ends of this series circuit are connected between IS and IG. A voltage obtained by adding the voltage across the reference voltage source 42 and the output voltage of the temperature sensor IC 41 is applied to the inverting input terminal of the comparator 21. A Zener diode 43 is connected between the output terminal O of the temperature sensor IC 41 and IG.

  One end of the resistor 23 is connected to the IG, and the other end is connected to the output terminal GND of the power supply device. Therefore, the output current I of the power supply device flows in the resistor 23 in the direction of the arrow. The reference voltage source 44 is connected to the non-inverting input terminal of the comparator 21 and the other end (non-IG side) of the resistor 23. Therefore, a voltage obtained by adding the voltage drop across the resistor 23 and the voltage across the reference voltage source 44 is applied to the non-inverting input terminal of the comparator 21. The output of the comparator 21 is input to the voltage control circuit 10.

Next, the operation of this embodiment will be described. The voltage across the reference voltage sources 42 and 44 is V _R42 and V _R44 , the Zener voltage of the Zener diode 43 is V _Z , the temperature detected by the temperature sensor IC 41 is θ (° C.), and the output voltage of the temperature sensor IC 41 is V (Θ), the resistance value of the resistor 23 is R.

When the voltages of the non-inverting input terminal and the inverting input terminal of the comparator 21 with respect to IG are V ₊ and V ₋ , respectively,
V ₊ = R · I + V _R44
V ₋ = V (θ) + V _R42
become. However, since the Zener diode 43, V _- is limited to ≦ _{V Z.}

V ₋ > V ₊ , that is, I <(V (θ) + V _R42 −V _R44 ) / R
In this case, the output of the comparator 21 is at a low level. The output voltage of the power supply device is controlled to a constant value by the voltage control circuit 10.

V ₋ <V ₊ , that is, I> (V (θ) + V _R42 −V _R44 ) / R
Then, the output of the comparator 21 changes to a high level. The voltage control circuit 10 reduces the output voltage of the power supply device so that no overcurrent is output.

As a result, the maximum output current I _MAX is expressed by the following equation (1).
I _MAX = (V (θ) + V _R42 −V _R44 ) / R (1)
As described with reference to FIG. 5, since the derating curve is a downward-sloping curve, the maximum output current decreases as the ambient temperature increases. In this embodiment, since a sensor having a negative temperature coefficient is used as the temperature sensor IC 41, V (θ) decreases as the temperature θ increases, and the maximum output current I _MAX decreases from the equation (1). Therefore, the maximum output current I _MAX can be adjusted to the derating curve by adjusting the temperature coefficient of V (θ).

In the equation (1), V (θ) increases as the temperature θ decreases, and I _MAX eventually becomes larger than the rated output current of the power supply device. Therefore, a Zener diode 43 is inserted between the output terminal O and IG to regulate V (θ) + _VR42 to a certain voltage or less. If the rated output current is I _S and the Zener voltage of the Zener diode 43 is V _Z , the Zener voltage V _Z may be selected from the following equation (2).
V _Z = I _S · R + V _R44 ··· (2)

  Next, the operation of this embodiment will be described with reference to FIG. FIG. 2 is a graph showing a derating curve and an overcurrent value detected by the current detection circuit 40. The horizontal axis represents the internal temperature of the power supply device, and the vertical axis represents the ratio to the rated output current. The internal temperature of the power supply device is the same as the temperature θ detected by the temperature sensor IC41.

  In FIG. 2, 50 is a derating curve. Although the derating curves 30 to 33 in FIG. 5 are divided into a plurality of curves depending on the mounting state of the power supply device or the like, if the temperature inside the power supply device is taken as a variable, it can be collected into a single curve 50. The derating curve 50 has a characteristic that the maximum output current decreases from an internal temperature of 70 ° C.

  A curve 51 represents a change in overcurrent detected by the current detection circuit 40. This curve 51 is set to a value of 120% of the derating curve 50. That is, the detection current at normal temperature is set to 120% of the rated output current, and the characteristic is set such that the internal temperature decreases from 70 ° C. with the same gradient of the derating curve 50. This gradient, starting temperature, and the like can be achieved by adjusting the temperature coefficient of the temperature sensor IC 41, the voltage across the reference voltage sources 42 and 44, and the resistance value of the resistor 23.

  Next, a specific example for realizing the curve 51 will be described. The detection current of the current detection circuit 40 is 12A at room temperature, and the characteristic decreases with a gradient from 70 ° C. to 0.4 A / ° C. This characteristic corresponds to a power supply device with a rated output current of 10A.

As the temperature sensor IC 41, an element having an output voltage of 1.713 V at 0 ° C. and a temperature coefficient of −8.2 mV / ° C. is used. An example of such a temperature sensor IC is S-8120C manufactured by Seiko Instruments Inc. The relationship between the temperature θ (° C.) and the output voltage V (θ) is expressed by the following equation (3).
V (θ) = 1.713−0.0082 · θ (3)

The resistance value of the resistor 23 can be determined from the temperature coefficient of the temperature sensor IC 41 and the slope of the curve 51. When the resistance value of the resistor 23 is R, the resistance value R must satisfy the following formula (4).
−0.0082 (V / ° C.) / R (Ω) = − 0.4 (A / ° C.) (4)
From the above equation (4), R = 0.0205Ω.

From the equation (3), V (θ) at 70 ° C. is
V (θ) = 1.713−0.0082 · 70 = 1.139V
become. Since the maximum output current I _{MAX at} this time is 12 A, from the above equation (1),
12 = (1.139 + V _R42 −V _R44 ) /0.0205
become. Therefore, V _R42 −V _R44 = −0.893V. Accordingly, the reference voltage source 42 with a voltage at both ends of 2.500 V and the reference voltage source 44 with a voltage at both ends of 3.393 V are used.

Since the detected current at room temperature is 12 A, the Zener voltage V _Z = 3.639 V can be obtained by substituting I _S = 12, V _R44 = 3.393, and R = 0.00205 in the equation (2).

  If the reference voltage source 44 has a voltage at both ends of 0.893 V, the reference voltage source 42 can be eliminated. However, when the reference voltage source 42 is used, the degree of freedom in determining the reference voltage source can be increased. Therefore, a commercially available standard reference voltage source can be used. In FIG. 1, the temperature sensor IC 41 and the reference voltage source 42 are connected in series and these voltages are added. However, they can be added using an adder.

  FIG. 3 shows another embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted. In FIG. 3, reference numeral 60 denotes a current detection circuit, which includes a comparator 21, a reference voltage source 42, a temperature sensor IC 41, a Zener diode 43, and resistors 23, 61 and 62. Resistors 61 and 62 are connected in series, and both ends of the series circuit are connected to IS and GND, respectively. The voltage at the connection point between the resistors 61 and 62 is input to the non-inverting input terminal of the comparator 21. The resistors 61 and 62 constitute a voltage divider.

  In this embodiment, instead of the reference voltage source 44, the output voltage of the power supply device is divided by resistors 61 and 62, and this divided voltage is input to the non-inverting input terminal of the comparator 21. In this embodiment, when the voltage control circuit 10 fails and an excessive voltage is generated, the output of the comparator 21 becomes a high level, so that it can also be used as an excessive voltage detection circuit. However, when an excessive current flows and the output voltage decreases, the output of the comparator 21 shifts to a low level. Therefore, when used as an overcurrent protection circuit, a mechanism is required to latch the output when the output of the comparator becomes high.

  In these embodiments, the power supply device is used as a part of the overcurrent protection circuit, but the present invention is not limited to this. It can also be used as a current detection circuit for other devices. Further, the voltage across the temperature sensor IC 41 is input to the inverting input terminal of the comparator 21 and the voltage across the reference voltage source 44 is input to the non-inverting input terminal. In this case, the output level of the comparator 21 is reversed.

  Moreover, although the AC input power supply apparatus has been described in these embodiments, it may be a DC input power supply such as a DC-DC converter, and is applicable not only to an insulation type but also to a non-insulation type power supply apparatus. You can also.

It is a block diagram which shows one Example of this invention. It is a characteristic view showing a temperature characteristic of a detection current. It is a block diagram which shows the other Example of this invention. It is a block diagram of the conventional power supply device with an overcurrent protection circuit. It is a characteristic view which shows an example of a derating curve.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Voltage control circuit 11 Transformer 12 Diode 13 Capacitor 14 Load 21 Comparator 23, 61, 62 Resistance 40, 60 Current detection circuit 41 Temperature sensor IC
42, 44 Reference voltage source 43 Zener diode

Claims (5)

A comparator having two input terminals, the output of which is determined by the magnitude of the voltage applied to these input terminals;
A resistor arranged in a path through which a current to be detected flows;
A variable voltage source for applying an output voltage having a negative temperature coefficient between one end of the resistor and one input terminal of the comparator;
A voltage limiting element for limiting the one input terminal voltage of the comparator to a certain value or less;
A reference voltage source for applying a constant voltage between the other end of the resistor and the other input terminal of the comparator;
A current detection circuit comprising:   2. The current detection circuit according to claim 1, wherein the variable voltage source includes a temperature sensor that outputs a voltage having a negative temperature coefficient and a reference voltage source that outputs a constant voltage. A voltage controller that outputs DC power of a constant voltage and that prevents an overcurrent from being output by lowering the output voltage when an overcurrent detection signal is input;
The current detection circuit according to claim 1 or 2, wherein an output current of the voltage control unit is detected and the overcurrent detection signal is output.
A power supply device comprising: A voltage controller that outputs DC power of a constant voltage and that prevents an overcurrent from being output by lowering the output voltage when an overcurrent detection signal is input;
A resistor disposed in a path through which an output current of the voltage control unit flows;
A variable voltage source for applying an output voltage having a negative temperature coefficient between one end of the resistor and one input terminal of the comparator;
A voltage limiting element for limiting the one input terminal voltage of the comparator to a certain value or less;
A voltage divider that divides the output voltage of the voltage controller and outputs the divided voltage to the other input terminal of the comparator;
A power supply device comprising:   5. The power supply apparatus according to claim 4, wherein the variable voltage source includes a temperature sensor that outputs a voltage having a negative temperature coefficient and a reference voltage source that outputs a constant voltage.

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