JP2004032910A - Switching circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching circuit with a simple configuration, which is capable of eliminating a shunt resistor for detecting an output current and has a protective circuit for protecting the circuit by completely detecting an overcurrent or short-circuit condition. <P>SOLUTION: This switching circuit comprises a switching control circuit, an integrator, and the protective circuit. The switching control circuit outputs a pulse signal for determining the conduction period of a switching device corresponding to the output voltage of the switching device for switching an input voltage. The integrator integrates the pulse signal and determines a voltage signal proportional to the frequency or pulse duration of the pulse signal. The protective circuit changes the frequency or pulse duration of the pulse signal if the output voltage of the integrator is larger than a reference voltage to limit the output current of the switching circuit. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a switching circuit, and more particularly to a switching circuit having an overcurrent protection circuit that does not consume power for detecting an output overcurrent or an output short circuit.
[0002]
[Related background art]
2. Description of the Related Art Recently, a switching circuit that converts input power with low loss by driving a semiconductor element switching element and outputs the converted power has been frequently used. For example, a DC-DC converter is widely applied as a device for converting a DC voltage with high efficiency. This DC-DC converter is basically configured as a so-called DC chopper that converts a DC input voltage by switching with a switching element.
[0003]
The on or off operation of the switching element used in such a DC-DC converter is ideally performed instantaneously, but actually requires some operation time. Therefore, a loss occurs in the switching element at the time of ON or OFF. In order to reduce this loss, a resonance type converter having a switching element provided with a resonance circuit composed of an LC circuit is used.
[0004]
By the way, the output overcurrent protection of this type of switching circuit exclusively detects a voltage drop occurring in a shunt resistor inserted in series between the output of the converter and the load, and this voltage drop is a predetermined constant level voltage. When the value becomes a value, the overcurrent protection circuit stops the operation of the switching circuit by judging that an overcurrent or a short circuit has occurred.
[0005]
[Problems to be solved by the invention]
However, in such an overcurrent protection circuit of a switching circuit using a shunt resistor, a current always flows through the resistor. For this reason, in a switching circuit that outputs a large current, there is a problem that the current consumption generated in the shunt resistor increases.
[0006]
For example, in a protection circuit used for a current resonance type DC-DC converter as a switching circuit, a pulse signal output from the switching control circuit is output when the output of the converter is in an overcurrent or short-circuit state. Is limited only to the frequency at the maximum output. For this reason, when the converter outputs an overcurrent or short-circuits, the converter enters a constant power output state and the output voltage decreases, but the output current increases.
[0007]
In particular, in a DC-DC converter that outputs a large amount of power, the power consumption of the shunt resistor becomes extremely large. Therefore, it is necessary to prepare a large shunt resistor that can withstand this power consumption. Furthermore, since a large-sized shunt resistor is used, the entire DC-DC converter becomes large, and heat dissipation of the shunt resistor must be taken into consideration, resulting in a problem that the accommodation space of the DC-DC converter becomes large.
[0008]
The present invention has been made in view of such circumstances, and its purpose is to eliminate the need for a shunt resistor used to detect the output current of a switching circuit, and to reliably detect output overcurrent or output short-circuit, An object of the present invention is to provide a switching circuit having a simple configuration provided with a protection circuit for protection.
[0009]
[Means for Solving the Problems]
To achieve the above object, a switching circuit according to the present invention includes a switching element that switches an input voltage,
A switching control circuit that outputs a pulse signal that determines a conduction period of the switching element according to an output voltage of the switching element and controls driving of the switching element;
An integrator for integrating the pulse signal output from the switching control circuit to obtain a voltage signal proportional to the frequency of the pulse signal or the pulse width thereof;
When the output voltage of the integrator is higher than a predetermined reference voltage, a protection circuit for changing the frequency or pulse width of the pulse signal to limit the output current output from the switching element is provided. And
[0010]
That is, the switching pulse signal output from the switching control circuit that drives the switching element is integrated by an integrator and converted into a voltage signal. When the level of this voltage signal is larger than a predetermined reference voltage value, an overcurrent or short-circuit state occurs. And the output current is limited by changing the frequency or pulse width of the switching pulse.
[0011]
Preferably, the switching element is provided with a resonance circuit at an output stage to form a current resonance type switching circuit, and is configured to be so-called PFM-controlled by a pulse signal having a predetermined pulse width whose frequency is controlled. .
Alternatively, the switching element is configured to be so-called PWM controlled by a pulse signal whose pulse width is controlled.
[0012]
More preferably, the protection circuit includes a correction unit that corrects a current limit value according to the input voltage and / or the output voltage.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a switching circuit according to an embodiment of the present invention will be described with reference to the drawings.
This embodiment shows a DC-DC converter that outputs a DC voltage by switching a DC input voltage by a semiconductor switching element.
[0014]
This DC-DC converter basically detects an output voltage with a voltage detection circuit so as to have a predetermined output voltage, and keeps the output voltage constant by feeding back the detected output voltage to a switching control circuit. Perform feedback control.
FIG. 1 is a block diagram showing a schematic configuration of a current resonance type DC-DC converter. This converter is provided with a switching element 2 that switches and outputs a voltage input from a DC power supply 1 such as a battery. The voltage and current output from the switching element 2 are pulsating, and therefore, a smoothing circuit 3 as an output circuit for performing waveform shaping (rectification and smoothing) is provided. The smoothing circuit 3 includes, for example, an inductance, a capacitor, and a diode as described later.
[0015]
An output terminal of the smoothing circuit 3 is provided with a voltage detection circuit 4 for detecting a DC voltage output from the smoothing circuit 3. Then, based on the voltage information detected by the voltage detection circuit 4, a voltage control oscillator (VCO) 5 for adjusting the frequency of the pulse signal given to the switching element 2 to maintain a constant voltage is provided. Take a role.
[0016]
Note that a resonance circuit 6 is provided in parallel between the input and output of the switching element 2 so that a sharp change in output voltage and current does not occur when the switching element 2 is turned on or off, and the resonance frequency of the resonance circuit 6 is used. It is configured to perform soft switching (zero cross operation).
Here, the present invention is characterized in that an integrator 10 for extracting a pulse signal supplied from the voltage controlled oscillator 5 to the switching element 2 and integrating the pulse signal to convert the pulse signal into a voltage signal is provided. Is provided as an input signal of the comparator 11 and the frequency of the pulse signal output from the voltage controlled oscillator 5 is variably controlled using the output signal of the comparator 11.
[0017]
Now, in the DC-DC converter configured as described above, the pulse frequency of the pulse signal of the voltage controlled oscillator 5 is determined by the voltage detection signal of the voltage detection circuit 4 that detects the DC output voltage. That is, when the load current of the DC-DC converter increases and the DC output voltage drops below the rated voltage, the voltage control oscillator 5 increases the frequency of the pulse signal applied to the switching element 2.
[0018]
On the other hand, the voltage-controlled oscillator 5 acts to reduce the frequency of the pulse signal when the load current of the converter decreases and the DC output voltage rises above the rated voltage. That is, when the output current increases and the output voltage of the converter decreases, the frequency of the pulse signal is increased. Conversely, when the output current decreases, the output voltage of the converter increases and the frequency of the pulse signal is decreased. Therefore, the output current of the converter has a characteristic proportional to the frequency of the pulse signal output from voltage-controlled oscillator 5. Note that the pulsating current output of the switching element 2 is smoothed by the smoothing circuit 3 to output a constant voltage DC.
[0019]
Now, basically, when the output of the DC-DC converter performing the DC voltage conversion operation as described above is overcurrent or short-circuited, as described above, the voltage-controlled oscillator increases in proportion to the increase in the output current of the converter. The frequency of the pulse signal output from 5 increases. This pulse signal is integrated by an integrator 10 connected to the output of the voltage controlled oscillator 5 which is a feature of the present invention, and is converted into a voltage signal. Then, this voltage signal is compared by the comparator 11 with the reference voltage output from the reference voltage generation circuit 12 connected to the DC power supply 1.
[0020]
When the comparator 11 determines that the reference voltage output from the reference voltage generating circuit 12 is higher than the voltage output from the integrator 10, the comparator 11 lowers the control voltage applied to the voltage controlled oscillator 5, and the voltage controlled oscillator 5 It acts to lower the frequency of the output pulse signal. That is, when the converter outputs the maximum current, setting the reference voltage to be applied to the comparator 11 so as to be equal to the voltage output by the integrator 10 can protect the converter output from overcurrent or short circuit. That is, when the voltage output from the integrator 10 exceeds the reference voltage given to the comparator 11, it can be determined that the DC-DC converter is in an overcurrent or short-circuit state.
[0021]
Further, when the output of the DC-DC converter is in an overcurrent state or a short-circuit state, the voltage information of the voltage drop detected by the voltage drop detection circuit 13 is given to the comparator 11 in addition to the voltage signal output by the integrator 10, so that The output current can be rapidly reduced, and the output of the converter can be reliably protected.
That is, the voltage signal output from the integrator 10 at the time of output overcurrent or output short-circuit of the converter and the voltage drop signal of the voltage detection circuit 4 are combined, and the level of the voltage signal supplied to the comparator 11 increases. Then, the control voltage applied from the comparator 11 to the voltage controlled oscillator 5 is reduced, so that the switching frequency can be suppressed lower than when only the integrator 10 is used. Therefore, the DC-DC converter can be reliably protected from the output overcurrent or the output short-circuit of the DC-DC converter.
[0022]
The switching circuit according to the present invention will be described in more detail with reference to the circuit diagram of FIG. 2 in which the switching circuit is applied to a DC-DC converter. In this circuit, a DC power supply 1 composed of a battery or the like is connected to a drain of a switching element (MOSFET) 2 via a backflow prevention diode D. The MOSFET 2 is configured to be supplied with a pulse signal output from the voltage-controlled oscillator 5 and output a switched DC from a source. Further, a series resonance circuit 6 composed of L ₁ and C ₁ that performs current resonance operation is connected to the source of the MOSFET 2. The series resonance circuit 6 has a soft switching function of suppressing a loss at the time of switching.
[0023]
Further, a smoothing circuit 3 is interposed between the series resonance circuit 6 and the load 7 in order to shape the waveform of the DC output switched by the MOSFET 2. The smoothing circuit 3 is composed of an inductance _{L 2} and capacitor _{C 2.} The diode D _F provided in the smoothing circuit 3, a freewheeling diode that forms a path for flowing the energy stored in the inductance L ₂ when the off-switching element 2 to load 7.
[0024]
The output of the smoothing circuit 3 is connected to a load 7 and a voltage detection circuit 4 for detecting the output voltage of the converter. The voltage detection circuit 4 is configured by, for example, a circuit obtained by resistance division. The voltage detection signal detected by the voltage detection circuit 4 is connected to the error amplifier 14. This voltage detection signal is used for frequency control of a pulse signal output from the voltage controlled oscillator 5, and is controlled so that the output voltage of the converter becomes a constant rated voltage.
[0025]
And the output voltage of the converter, the voltage drop detecting circuit 13 for outputting a voltage reduction signal is provided when below the allowable voltage E ₁ a predetermined. The allowable voltage E ₁ is set as the allowable limit value of the output voltage drops when converter output over-current or short circuit.
On the other hand, the pulse signal of the voltage controlled oscillator 5 is integrated by an integrator 10 including a resistor R ₃ , a capacitor C ₃ and a buffer amplifier A ₁ . With this integrator 10, a voltage signal proportional to the frequency of the pulse signal output from the voltage controlled oscillator 5 can be obtained. This voltage signal is combined with the voltage drop signal voltage drop detection circuit 13 is compared by a comparator 11 which forms the inverting amplifier A _2. The oscillation frequency of the pulse signal of the voltage controlled oscillator 5 is determined by the output voltage of the comparator 11.
[0026]
In the DC-DC converter configured as described above, when the output current increases, the output voltage of the converter decreases. This voltage drop is detected by the voltage detection circuit 4 including the voltage dividing resistors R ₁ and R ₂ , and lowers the voltage of the negative input of the error amplifier 14. For this reason, the output voltage of the error amplifier 14 rises, and acts to increase the frequency of the pulse signal of the voltage controlled oscillator 5. Then, the on-time of the MOSFET 2 becomes longer, and the output voltage rises. Conversely, when the output voltage of the converter increases, the voltage of the negative input of the error amplifier 14 increases. For this reason, the output voltage of the error amplifier 14 decreases, and the frequency of the pulse signal of the voltage controlled oscillator 5 decreases. Thus, the output voltage of the converter can be kept constant.
[0027]
Basically, when the output current of the DC-DC converter operating as described above increases, the oscillation frequency of the voltage controlled oscillator 5 is increased to maintain the output voltage. Then, when the output current increases and enters an overcurrent state, the output voltage of the integrator 10 increases, and the output voltage of the comparator 11 decreases. Then, since the voltage of the plus side input of the error amplifier 14 decreases, the output voltage of the error amplifier 14 decreases. For this reason, the oscillation frequency of the voltage controlled oscillator 5 is reduced, the conduction period of the MOSFET 2 is shortened, and the output current of the converter is suppressed.
[0028]
Further, a voltage drop on the converter output side which occurs when the output of the converter is overcurrent or the output is short-circuited is detected by the voltage drop detection circuit 13. The voltage drop detection circuit 13 is configured to output a voltage drop signal when the output overcurrent of the converter or the voltage drop when the output is short-circuited becomes lower than a certain level (allowable voltage E ₁ ). Then, this voltage drop signal is combined with the output of the integrator 10 described above, and given to the comparator 11. For this reason, in addition to suppressing the oscillation frequency of the voltage-controlled oscillator 5 by the integrator 10 at the time of output overcurrent or output short-circuit of the converter, the voltage drop signal detected by the voltage drop detection circuit 13 is synthesized and given to the comparator 11. Thereby, the frequency of the voltage controlled oscillator 5 can be further reduced. Therefore, the current limit of the converter can be given a drooping characteristic, so that the output current of the converter can be reliably narrowed down.
[0029]
As described above, conventionally, a shunt resistor inserted between the output of the converter and the load for detecting an overcurrent or a short circuit was required. However, according to the DC-DC converter of the present invention, the switching element 2 The frequency of the pulse signal is converted into a voltage signal by the integrator 10, and based on the voltage signal, overcurrent or short circuit of the output of the DC-DC converter is detected and protected. For this reason, a shunt resistor is not required, and unnecessary power consumption generated by the shunt resistor can be reduced.
[0030]
Furthermore, in order to more reliably protect the DC-DC converter from output overcurrent or output short-circuit, the voltage drop detection circuit 13 detects and corrects the output voltage drop that occurs at the time of output overcurrent or output short-circuit of the converter. Becomes possible.
Further, a reference voltage generation circuit 12 is provided to correct the fluctuation of the DC input voltage, and outputs a reference voltage signal inversely proportional to the input voltage. The reference voltage signal is supplied to the comparator 11, and acts to lower the threshold for lowering the frequency of the voltage controlled oscillator 5 when the input voltage is high, and to increase the threshold when the input voltage is low. Therefore, it is suitable as a switching circuit applied to, for example, a vehicle battery circuit in which the voltage fluctuation of the DC power supply is large.
[0031]
In the above-described embodiment, the DC-DC converter using the voltage-controlled oscillator 5 as the switching control circuit of the switching element 2 is used. However, the present invention can be similarly applied to the PWM DC-DC converter shown in FIG. it can.
The switching circuit of this other embodiment is configured, for example, as a DC-DC converter including a PWM (pulse width modulation) control circuit 8 that generates a pulse signal that determines a conduction period of the switching element 2.
[0032]
The output voltage of the DC-DC converter configured as described above is detected by the voltage detection circuit 4 provided on the DC output side in order to maintain the output voltage constant. From the detected voltage detection signal, the signal width of the pulse signal output from the PWM control circuit 8 to the switching element 2 is determined.
That is, if the load current of the DC-DC converter increases and the DC output voltage drops below the reference output voltage, the pulse width of the pulse signal of the voltage controlled oscillator 5 with respect to the switching element 2 is extended. On the other hand, if the load current of the converter decreases and the DC output voltage rises above the reference voltage, it acts to shorten the pulse width of the pulse signal. As described above, by controlling the time of the pulse width output from the PWM control circuit 8, the conduction period of the switching element 2 is controlled to keep the output voltage of the converter constant.
[0033]
Basically, when the output of the DC-DC converter of the PWM system that performs the DC voltage conversion operation as described above becomes overcurrent or short-circuited, as described above, the PWM control circuit 8 is increased in proportion to the increase of the output current of the converter. Increases the pulse width time of the pulse signal output by This pulse signal is integrated by an integrator 10 connected to the output of the PWM control circuit 8 and converted into a voltage signal. Then, a voltage signal proportional to the width of the ON time of the pulse signal is obtained on the output side of the integrator 10. Then, this voltage signal is compared by the comparator 11 with the reference voltage output from the reference voltage generation circuit 12 connected to the DC power supply 1.
[0034]
When the comparator 11 determines that the voltage output from the integrator 10 is higher than the reference voltage output from the reference voltage generating circuit 12, the comparator 11 reduces the control voltage applied to the PWM control circuit 8, and the PWM control circuit 8 Command to shorten the pulse width of the given pulse signal.
Therefore, also in the DC-DC converter using the PWM control circuit 8, it is possible to detect overcurrent or short-circuit of the output of the DC-DC converter as in the above-described embodiment. In this case, the reference voltage applied to the comparator 11 may be set to be equal to the voltage output from the integrator 10 when the converter outputs the maximum current.
[0035]
Of course, in the case of the output overcurrent or the output short-circuit state of the DC-DC converter as in the above-described embodiment, the integrator 10 outputs the voltage information detected by the voltage detection circuit 4 so that the output current can be narrowed more rapidly. The correction may be made in consideration of the voltage signal to be applied. As described above, also in the DC-DC converter of the PWM system, the on-time of the pulse signal given to the switching element 2 is converted into a voltage signal by the integrator 10, and based on this voltage signal, the output of the DC-DC converter becomes excessive. Current or short circuit protection eliminates the need for a shunt resistor.
[0036]
Further, the output voltage drop caused by the output overcurrent or output short-circuit of the converter is detected by the voltage drop detection circuit 13 and corrected by combining with the output signal of the integrator 10. Furthermore, it is possible to more reliably protect the DC-DC converter from output overcurrent or output short circuit.
More preferably, it is desirable to use information from a temperature sensor (not shown) installed around the switching element 2 to protect the switching element by lowering the switching frequency when the temperature rises. In this case, in addition to protection against output overcurrent and short circuit of the converter, it is possible to appropriately protect the switching circuit even during normal operation from a rise in the temperature of the switching element 2 and a rise in the ambient temperature of the installation location of the converter. It becomes possible.
[0037]
The present invention can be applied to a switching circuit that performs PWM control or PFM control in addition to the DC-DC converter described above. In this case, a shunt resistor for detecting the output current is not required, and a switching circuit having a simple configuration including a protection circuit that reliably detects and protects an overcurrent or short-circuit state can be realized.
[0038]
In addition, the present invention can be implemented with various modifications without departing from the gist thereof.
[0039]
【The invention's effect】
As described above, according to the switching circuit of the present invention, after a pulse signal given to a switching element is integrated by an integrator and converted into a voltage signal, this voltage changes to a constant voltage level indicating an overcurrent or short circuit state. When it exceeds, the output of the switching circuit is protected. For this reason, useless power due to the shunt resistor for overcurrent detection is not consumed. In addition, since there is provided a correction means for correcting the output current due to a decrease in the output voltage generated at the time of an overcurrent, a great effect in practical use can be achieved such that the output overcurrent of the switching circuit can be more reliably protected. .
[Brief description of the drawings]
FIG. 1 is a block diagram showing a schematic configuration when a switching circuit according to an embodiment of the present invention is applied to a DC-DC converter.
FIG. 2 is a circuit diagram showing a specific example of a switching circuit according to one embodiment of the present invention.
FIG. 3 is a block diagram showing a schematic configuration of a switching circuit according to another embodiment of the present invention.
[Explanation of symbols]
2 Switching element 3 Smoothing circuit 4 Voltage detection circuit 5 Voltage controlled oscillator 6 Resonant circuit 8 PWM control circuit 10 Integrator 11 Comparator 12 Reference voltage generation circuit 13 Voltage drop detection circuit

Claims (4)

A switching element for switching an input voltage;
A switching control circuit that outputs a pulse signal that determines a conduction period of the switching element according to an output voltage of the switching element and controls driving of the switching element;
An integrator that integrates the pulse signal output from the switching control circuit to obtain a voltage signal proportional to the frequency of the pulse signal or its pulse width;
When the output voltage of the integrator is higher than a predetermined reference voltage, a protection circuit is provided that limits the output current output from the switching element by changing the frequency or pulse width of the pulse signal. And a switching circuit. 2. The switching device according to claim 1, wherein the switching element includes a resonance circuit at an output stage thereof to form a current resonance switching circuit, and the switching is controlled by a pulse signal having a predetermined pulse width whose frequency is controlled. 3. circuit. The switching circuit according to claim 1, wherein the switching element is switching-controlled by a pulse signal having a pulse width controlled. 4. The switching circuit according to claim 1, wherein the protection circuit includes a correction unit configured to correct a current limit value according to the input voltage and / or the output voltage. 5.

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