JPH04366772A - Overcurrent detection circuit of dc power supply - Google Patents

Abstract

PURPOSE:To enable detection of high sensitivity through simple circuit constitution and to set a detecting resistance at low resistance values so as to reduce current losses by connecting between the pair of emitters of a transistor the detecting resistance for detecting output direct current. CONSTITUTION:An input DC voltage Vin supplied between a positive input terminal 1 and a negative input terminal 2 is turned on and off by a switching transistor Q3 and is smoothed by a diode D1 and a chock coil CH and output as DC output voltages Vout of different voltage values between a positive output terminal 3 and a negative output terminal 4. In a current area wherein direct output current I0 is normal, output voltage Vs generated at the output resistance R2 of a detection circuit is set in such a way as not exceeding the threshold voltage of a control circuit 5. When the current I0 exceeds a specified value the voltage Vs of the resistance R2 exceeds the threshold voltage of the circuit 5 but even when the variation of the voltage drop of a detecting resistance R3 is several mV the voltage Vs is greatly changed. As a result, the circuit 5 recognizes overcurrent and performs control in such a way as to shorten the ON period of the transistor Q3, thereby protecting the circuit elements.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent detection circuit for a DC power source such as a DC-DC converter or an AC-DC converter.

0002

[Prior Art] Conventionally, DC-DC converters and AC-D
Overcurrent detection in a DC power supply such as a C converter is shown in Figure 4.
This was done using the circuit shown in FIG.

The circuit of FIG. 4 uses an operational amplifier OP to reduce the voltage drop that occurs across a detection resistor R3 inserted in series with the DC current supply line (detection resistor R3 and output DC current I0).
It detects the product of

The circuit of FIG. 5 uses a transistor Q10 to connect a detection resistor R3 inserted in series to a DC current supply line.
This is to detect the voltage drop (product of detection resistor R3 and output DC current I0) occurring across both ends.

The circuit shown in FIG. 6 uses a current transformer CT, the primary side of the current transformer CT is inserted into a DC current supply line, and a transistor Q11 is provided on the secondary side to detect the output DC current I0. This is what I did.

[0006]

[Problems to be Solved by the Invention] By the way, in the circuit using the operational amplifier OP shown in FIG. is required and the cost is high.

Furthermore, in a circuit that directly detects the voltage drop that occurs across the detection resistor R3 in the transistor Q10 shown in FIG. The sensitivity is several hundred mV, which is poor sensitivity. This means that
The resistance value of the detection resistor R3 is required to be large, resulting in large power loss.

Furthermore, since the circuit using the current transformer CT shown in FIG. 6 has a structure in which wires are wound around the core, it is difficult to miniaturize the circuit and the cost is high. Another disadvantage is that direct current cannot be detected.

As described above, with the circuits shown in FIGS. 4 to 6, it is not possible to realize a small, highly sensitive, and low-cost overcurrent detection circuit.

On the other hand, FIG. 7 shows a current mirror circuit in which the bases and emitters of a pair of transistors Q1 and Q2 with the same characteristics are connected in common, and each transistor Q1 is connected from a current source 7 on the collector side of one transistor Q1. , Q2 have the property that if equal base currents are supplied to the bases of transistors Q1 and Q2, equal collector currents will flow through each of transistors Q1 and Q2. The present inventor discovered that in this current mirror circuit, when a resistor is inserted between the emitters of a pair of transistors, and a voltage drop is applied by passing a current through the resistor, the equilibrium state of the current mirror circuit is disrupted and the voltage drops to the collector side. It was found that large voltage fluctuations were caused.

SUMMARY OF THE INVENTION In view of the above points, it is an object of the present invention to provide a small, low-loss, and low-cost overcurrent detection circuit for a DC power supply using the above-described current mirror circuit.

0012

[Means for Solving the Problems] In order to achieve the above object, the overcurrent detection circuit for a DC power supply of the present invention includes a current mirror circuit in which the bases or gates of the pair of transistors are connected in common. It has a configuration in which a detection resistor is connected between the emitter or the source, and the detection resistor is inserted in series with the DC current supply line.

[0013]

[Operation] The operating principle of the overcurrent detection circuit for a DC power supply according to the present invention will be explained with reference to FIG. In the current mirror circuit of Fig. 3, transistors Q1 and Q2 on the same chip with uniform characteristics are used.
A detection resistor R3 is connected between the emitters of the detection resistor R3.
are inserted in series into the DC current supply line through which the DC output current I0 flows. One end of the bias resistor R1 is connected to the collector of the transistor Q1, and one end of the output resistor R2 is connected to the collector of the transistor Q2.
A DC supply voltage Vcc is applied between the other end of the transistor Q1 and the emitter of the transistor Q1.

In the circuit of FIG. 3, each transistor Q1
, Q2 base current Ib1, Ib2, collector current Ic
1, Ic2, the output DC current I0, the voltage drop ΔV of the detection resistor R3, the output voltage Vs across the output resistor R2, etc., are shown in the following equations (1) to (10).

In the above equation (10), kT is about 0.023 eV at room temperature (T=300°K), so several mV can be detected as the voltage drop ΔV. As a result, the detection resistor R3 for detecting the output DC current I0 can have a low resistance, and power loss can be reduced.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of an overcurrent detection circuit for a DC power supply according to the present invention will be described with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention, which is applied to a chopper type DC-DC converter. This chopper type DC-DC converter has a switching transistor Q3 and a choke coil CH inserted in series in the positive line connecting the positive input terminal 1 and the positive output terminal 3. A capacitor C1 is connected between the side input terminals 2, and a diode D1 is connected between the negative side line (common return line) connecting the negative side input terminal 2 and the negative side output terminal 4 and the collector of the switching transistor Q3. It has a connected configuration. The control circuit 5 applies an on/off drive signal to the base of the switching transistor Q3. Further, a detection resistor R3 connected between the emitters of a pair of transistors Q4 and Q5 forming the current mirror circuit 6 is inserted in series on the output side of the choke coil CH. The current mirror circuit 6 is substantially the same as that explained in FIG. 3, except that a pair of PNP transistors Q4 and Q5 of the same chip and having the same characteristics are used instead of the NPN transistors Q1 and Q2. It's different. The collectors of the transistors Q4 and Q5 are connected to the negative line through the bias resistor R1 and the output resistor R2, respectively, and the output voltage Vs appearing across the output resistor R2 is applied to the control circuit 5. Note that the detection resistor R3 is set to a resistance value such that the voltage drop is minute (for example, several mV or less) within the range of the normal DC output current I0.

In the configuration of the first embodiment described above, the input DC voltage Vin supplied between the positive input terminal 1 and the negative input terminal 2 is turned on and off by the switching transistor Q3, and further by the diode D1. and the DC output voltage Vout of different voltage values smoothed by the choke coil CH.
It is output between the positive side output terminal 3 and the negative side output terminal 4 as . When the DC output current I0 is in a normal current range, the output voltage Vs generated at the output resistor R2 of the detection circuit is set so as not to exceed the threshold voltage of the control circuit 5.

Now, when the DC output current I0 exceeds the specified current value, the output voltage Vs of the output resistor R2 will exceed the threshold voltage of the control circuit 5, but the amount of change in the voltage drop of the detection resistor R3 is Even if the voltage is mV, the output voltage Vs changes greatly. As a result, the control circuit 5 recognizes an excessive current due to a large change in the output voltage Vs, and controls the switching transistor Q3 to shorten its on period, thereby protecting each circuit element.

The configuration of the first embodiment has the advantage that it can be made smaller and cheaper than the circuit using the operational amplifier shown in FIG. It has higher sensitivity than that of the detection resistor, and can use a detection resistor R3 having a sufficiently low resistance value (for example, several tens of mΩ), resulting in good efficiency. For example, when applied to a DC power supply with an output DC voltage of 5V, the loss is approximately 0.6V in the case of Figure 5.
(voltage drop across detection resistor R3), and the loss ratio is 0.6/
5, which is 12%, whereas in the first embodiment, the loss is about 20 mV (voltage drop across the detection resistor R3), and the loss ratio is 0.02/5, which is 0.4%. The detection resistor R3
Since it is sufficient to have a low resistance value, it is possible to use a conductor pattern formed directly on a printed circuit board.

In the first embodiment described above, the detection resistor R3 is inserted into the positive side line that serves as the supply line for the output DC current I0.
It may also be inserted into the negative side line that serves as the supply line for the output DC current I0. This case is shown in a second embodiment shown in FIG. In FIG. 2, the current mirror circuit 6A is the same as the circuit explained in FIG. 3 (same as the current mirror circuit 6 in FIG. 1 except that the transistor is replaced from PNP to NPN type),
A detection resistor R3 connected between the emitters of the transistors Q1 and Q2 is inserted into the negative line (between the anode of the diode D1 and the negative output terminal 4). In this case, since the same control circuit is used, the polarity is inverted and connected using the transistor Q6. Note that the other configurations are the same as those of the first embodiment described above.

In each of the above embodiments, the current mirror circuit is constructed using bipolar transistors, but field effect transistors (MOS) other than bipolar transistors are also used.
FET, J-FET). In the case of field effect transistors, the gates may be connected in common and a detection resistor may be connected between the sources.

[0022]

As explained above, according to the overcurrent detection circuit for a DC power supply of the present invention, a detection resistor for detecting output DC current is installed between the emitters (sources) of transistors forming a pair of current mirror circuits. By connecting, highly sensitive detection can be performed with a relatively simple circuit configuration, and loss can be reduced by setting the detection resistor to a sufficiently low resistance value. Furthermore, since no operational amplifier or auxiliary power supply is required, it is possible to downsize and configure the circuit at low cost.

[Brief explanation of the drawing]

[Fig. 1] A first overcurrent detection circuit for a DC power supply according to the present invention.
FIG. 2 is a circuit diagram showing an example.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

FIG. 3 is a circuit diagram for explaining the principle of the present invention.

FIG. 4 is a circuit diagram showing a first conventional example in which overcurrent detection is performed using an operational amplifier.

FIG. 5 is a circuit diagram showing a second conventional example in which overcurrent detection is performed using one transistor.

FIG. 6 is a circuit diagram showing a third conventional example in which overcurrent detection is performed using a current transformer.

FIG. 7 is a circuit diagram showing the basic form of a current mirror circuit.

[Explanation of symbols]

1 Positive input terminal 2 Negative input terminal 3 Positive output terminal 4 Negative output terminal 5 Control circuit 6, 6A Current mirror circuit CH Choke coil CT Current transformer OP Operational amplifier Q1 to Q6, Q10 Transistor R1 Bias resistor R2 Output Resistor R3 Detection resistor

Claims (1)

[Claims] Claim 1: A detection resistor is connected between the emitters or sources of the pair of transistors of a current mirror circuit in which the bases or gates of the pair of transistors are connected in common, and the detection resistor is inserted in series with a direct current supply line. An overcurrent detection circuit for a DC power supply.