JP2017099083A - Overcurrent protection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive current protection circuit of extremely small power loss which prevents breakdown of a switching element by detecting and interrupting an excessive current generated in the switching element at high speed.SOLUTION: When placing a current detection inductor 101 at a certain position for the electrode of a switching element Q5, an induction electromotive force of the current detection inductor 101 is generated by mutual induction when an overcurrent occurs, and a voltage is induced in the R12. When the base-emitter potential of a switching element Q1 exceeds the diffusion potential, a base current begins to flow, and a collector current of hfe times thereof begins to flow. When the voltage drop in the R5 exceeds -0.6 V, base current of a switching element Q3 begins to flow. When the switching element Q3 is turned ON, the base current also flows to a switching element Q2 which is thereby turned ON, and short-circuiting the gate-source of the switching element Q5 thus turning the switching element Q5 OFF.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an overcurrent protection circuit that detects an overcurrent at a high speed when an inverter circuit or the like used in solar power generation or a storage battery system malfunctions, and prevents breakage of a switching element.

  For example, in the case of a single-phase two-wire system, a grid-connected inverter in a photovoltaic power generation or stationary storage battery system is configured by an H-bridge circuit as shown in FIG. 8, and an AC-DC by appropriately switching the switching elements Q1 to Q4. Conversion and regenerative-powering operations are possible. In such a configuration, when the switching elements Q1 and Q2 or the switching elements Q3 and Q4 on the same arm are simultaneously turned on, the charge stored in the smoothing capacitor C1 is short-circuited. Therefore, the control circuits 801-1 and 801-2 are controlled so that the switching elements on the same arm are not simultaneously turned on even for a moment.

  However, when a long electric wire such as the commercial power system 804 is connected to the output or when a motor load or the like is connected, a surge voltage such as lightning or noise is likely to enter, and the control circuits 801-1, 801-2, The gate drive circuits 802-1 to 802-4 may malfunction. No matter how the control side logic signal is designed to be turned on simultaneously, if the arm short circuit is caused by such a malfunction caused by such disturbance, the switching element is damaged.

  Therefore, in such an inverter circuit, it is general to provide a protection circuit that detects an excessive current at the time of a short circuit by some means and forcibly switches the gate of the switching element to OFF.

G. Castino, A. Dubashi, S. Clemente, B. Pelly, “IGBT Short-Circuit Protection”, International Rectifier, Application Notes, AN-984J

As a means for detecting the current, a method of inserting a shunt resistor R in the current path is well known. However, in order to read a voltage drop when the current I flows, an I ² R joule loss occurs, resulting in a circuit power loss. Has the disadvantage of increasing.

  As another method, there is an overcurrent detection method that utilizes the fact that the saturation voltage between the positive electrode and the negative electrode of the switching element rises abnormally when an overcurrent flows. However, in this method, the detection function must be turned ON / OFF in conjunction with the ON / OFF of the switching element. For this reason, the circuit becomes complicated, it is easy to malfunction, and it is difficult to increase the reaction speed. In addition, since the saturation voltage between the positive electrode and the negative electrode of the switching element is easily affected by temperature and varies widely from switching element to switching element, it is difficult to set an appropriate protection current. For this reason, there is a case where adjustment means must be taken, and there is a drawback that the cost is increased and the space is likely to increase.

  Further, for example, in an IGBT (Insulated Gate Bipolar Transistor) or the like, there is a case where a short circuit allowable time is specified. However, when a protection circuit is assembled, it is necessary to design in consideration of the short circuit allowable time. However, this short circuit allowable time is greatly affected by the voltage between the positive electrode and the negative electrode and the gate drive voltage, and therefore there is a problem that the circuit is inevitably complicated in order to properly protect in consideration of the influence.

  Therefore, a configuration using a MOSFET (metal-oxide-semiconductor field-effect transistor) that has been considered to have a longer short-circuit withstand time than the IGBT under the same conditions is also conceivable. However, in recent years, the characteristics of high speed, high withstand voltage, and low loss, and the trench junction super junction MOSFETs and SiC MOSFETs that are frequently used for switching elements, have a shorter short-circuit withstand time than the same rated IGBT, Often not stipulated. Therefore, even if a MOSFET is used, it is not easy to simplify the circuit.

  In this way, the circuit must be complicated and the circuit size is increased, which makes it difficult to design heat dissipation in the power circuit, and is a major obstacle to miniaturization, high-speed switching, and low cost. There is.

  Therefore, there has been a demand for a high-speed and simple protection circuit that does not easily affect the structure, heat dissipation, and high-speed design, and does not easily malfunction due to overcurrent.

  The present invention has been made in view of such problems, and the object of the present invention is to prevent damage to the switching element by detecting and shutting off an excessive current generated in the switching element at a high speed. An object of the present invention is to provide an inexpensive overcurrent protection circuit with extremely low power loss.

  In order to solve the above problems, the present invention provides an overcurrent protection circuit, wherein a magnetic path disposed in the vicinity of a negative electrode or a positive electrode of a switching element, or in a vicinity of a wiring connected to the negative electrode or the positive electrode of the switching element. An inductor that is not closed, the inductor being arranged such that a magnetic path releasing direction of the inductor is a direction of a magnetic flux generated by a current flowing through the switching element, and an induced electromotive force generated in the inductor And an off circuit that turns off the switching element when a predetermined value is exceeded.

  According to a second aspect of the present invention, in the overcurrent protection circuit according to the first aspect, the off-circuit includes a full-wave rectifier circuit that rectifies the induced current generated in the inductor.

  According to a third aspect of the present invention, in the overcurrent protection circuit according to the first or second aspect, the off circuit has a state in which the switching element is turned off using a circuit in which an NPN transistor and a PNP transistor are thyristor-connected. It is characterized by maintaining.

  According to a fourth aspect of the present invention, in the overcurrent protection circuit according to any one of the first to third aspects, the off circuit turns off the switching element by short-circuiting the gate electrode and the source electrode of the switching element. It is characterized by doing.

  According to a fifth aspect of the present invention, in the overcurrent protection circuit according to any of the first to fourth aspects, the inductor is mounted on the same printed wiring board as the switching element.

  The present invention has the following effects.

  1. According to the present invention, since a current detection element such as a shunt resistor or a current transformer is not used, it is possible to detect a current without affecting the wiring of the switching element. For this reason, the adverse effect of the heat dissipation structure optimization of the switching element and the wiring inductance can be minimized.

  2. According to the present invention, since the current detection sensitivity depends on the positional relationship between the switching element and the current detection inductor, the variation in the current detection sensitivity can be minimized by fixing the mechanical positional relationship.

  3. According to the present invention, detection can be performed without considering the winding direction of the current detection inductor, an inexpensive general-purpose inductor can be used as the current detection inductor, and an overcurrent detection circuit can be realized inexpensively and simply. .

  4). According to the present invention, a steep current change such as an overcurrent can be detected at a very high speed, and the detection sensitivity has a positive temperature coefficient, so that the maximum rating of the switching element decreases with increasing temperature. A suitable overcurrent detection circuit can be configured.

It is a figure which shows the 1st mounting example of the current detection inductor with which the overcurrent protection circuit of this invention is provided. It is a figure which shows the 2nd mounting example of the current detection inductor with which the overcurrent protection circuit of this invention is provided. It is a figure which shows the 3rd mounting example of the current detection inductor with which the overcurrent protection circuit of this invention is provided. It is a figure which shows the example of the general purpose inductor used as a current detection inductor in the overcurrent protection circuit of this invention. It is a figure which shows the example of the general purpose inductor used as a current detection inductor in the overcurrent protection circuit of this invention. It is a figure explaining arrangement | positioning of the current detection inductor in case the positive electrode and negative electrode of a switching element are taken out from the same direction. It is a figure which shows the structural example of the overcurrent protection circuit which concerns on one Embodiment of this invention. It is a figure which shows the structural example of the grid connection inverter comprised by the typical H bridge circuit.

  Hereinafter, embodiments of the present invention will be described in detail.

  1 to 3 show mounting examples of a current detection inductor provided in the overcurrent protection circuit of the present invention. As the current detection inductor 101, a structure in which the magnetic path is not closed as shown in FIGS. FIG. 7 shows a configuration example of an overcurrent protection circuit according to an embodiment of the present invention.

<Current detection>
When the current i flows through the positive electrode or the negative electrode of the switching element 202, a magnetic flux φ is generated around the electrode. Assuming that the inductance component of the electrode is L, from Maxwell's law, φ = L · i
Thus, a magnetic flux φ proportional to the current is generated. Therefore, as shown in FIG. 6, if the current detection inductor 101 is arranged at a fixed position with respect to the electrode of the switching element 202, for example, about 10 mm from the electrode, a magnetic coupling circuit is formed. A relationship is established.
v = (L2−M) · di ₂ / dt + M · d (i ₁ + i ₂ ) / dt (1)

Here, v is the induced electromotive force of the current detection inductor 101, L2 is the inductance of the current detection inductor 101, i ₁ is the electrode current of the switching element, i ₂ is the current detection inductor current, and M is the electrode inductance L1 of the switching element 202. And the mutual inductance of the inductance L2 of the current detection inductor 101. That is, it can be regarded as equivalent to inserting a transformer having a coupling coefficient k = M / (L 1 · L 2) ^0.5 between the electrode of the switching element 202 and the current detection inductor 101.

  In the present invention, a current detection inductor 101 having a structure in which the magnetic path is not closed as shown in FIGS. Furthermore, the magnetic path opening direction is arranged so as to be the direction of the magnetic flux generated by the current flowing through the electrode of the switching element 202. Thereby, the coupling coefficient k can be increased, and sufficient sensitivity can be obtained for detecting a large transient current such as an overcurrent of the switching circuit. 4 includes a mold resin 111, an electrode 112, a ferrite core 113, and a magnet wire 114. The inductor 101 illustrated in FIG. 5 includes a ferrite core 115, a ceramic 116, a magnet wire 117, and an electrode 118.

  The detection sensitivity largely depends on the coupling coefficient k. However, by mounting the switching element 202 and the current detection inductor 101 on the same printed wiring board 201, the coupling coefficient k can be easily maintained at a constant value, and variation among individuals is suppressed. Can do. Further, since the surface mount component automatic mounter can mount the component with high positional accuracy, for example, when the switching element 202 and the current detection inductor 101 are surface mount components as shown in FIGS. 2 and 3, the printed wiring just below the switching element 202 is used. Stable and sufficient coupling can be obtained by disposing it near the back surface of the plate 201 or in the vicinity of the switching element 202.

  In the case of a package in which the positive electrode and the negative electrode of the switching element 202 are taken out from the same direction as shown in FIG. 6, the positive electrode current and the negative electrode current are reversed, so that the magnetic flux is at the same distance from the positive electrode and the negative electrode. Will cancel each other. For such a package, the sensitivity can be increased by arranging the current detection inductor 101 at a position where the distance ratio from the positive electrode to the negative electrode is as large as possible.

<Use of general-purpose inductor>
One of the problems in using a general-purpose inductor as a current detection element is management of the coil winding direction. Originally, general-purpose inductors are not manufactured for the purpose of capturing magnetic flux from the outside, and therefore the winding direction of the coil is not generally displayed. For this reason, it cannot generally be specified which direction of the current detected by the general-purpose inductor is.

  Therefore, in one embodiment of the present invention, as shown in FIG. 7, the output of the current detection inductor 101 is full-wave rectified by a full-wave rectifier circuit constituted by diode bridges CR1 to CR4, and the winding of the current detection inductor 101 is performed. The current output can be obtained in the same direction regardless of the direction.

<Overcurrent protection circuit>
Induced electromotive force v of the current detection inductor 101 generated by mutual induction, the mutual inductance M, the inductance L2 of the current detection inductor 101, since the coupling coefficient k is constant, the current value i ₁ flowing according to equation (1), i Only ₂ are variables.

  Therefore, the voltage V1 across the load resistor R12 of the current detection inductor 101 is induced without being substantially affected by the forward voltage Vf of the diode or its nonlinear effect even if the diodes CR1 to CR4 are inserted into the output of the current detection inductor 101. V1≈v.

  Next, the problem is whether the voltage V1 proportional to the detected overcurrent can be simply connected to protection of the switching element without causing malfunction at high speed.

If the value of the inductance L2 of the current detection inductor 101 is sufficiently large with respect to the electrode inductance of the switching element, the value of the mutual inductance M is almost negligible with respect to the inductance L2, so that it is induced in R12 from the moment the overcurrent is detected. The applied voltage E1 changes as shown by the equation (2) with the lapse of time t.
E1 = V1 ・ e ^{― (R12 ・ t / L2)} (2)

  Here, t is an elapsed time from the moment when the overcurrent is detected, and V1 is a voltage induced in R12 when t = 0. For example, when R12 = 18 [Ω] and L2 = 15 [uH], even if the voltage V1 at t = 0 is 1V, the voltage decreases to about 0.3V after 1 μs.

  For this reason, the protection circuit must be able to respond reliably to this fine pulse voltage and operate so as not to react to unnecessary noise. Therefore, it is desirable that the constant of R12 is 100Ω or less.

  In one embodiment of the present invention, in order to react to a thin pulse signal, as shown in FIG. 7, the collector of the switching element Q3, which is a PNP transistor, is used as the base of the switching element Q1, which is an NPN transistor. Positive feedback is applied by so-called thyristor connection in which the collector is connected to the base of the switching element Q3. The diffusion potential of the PN junction is about 0.6 V in the case of silicon, and has a temperature coefficient of about −2 mV / ° C.

When the base-emitter potential of the switching element Q1 exceeds about 0.6 V of the diffusion potential, the base current I _bQ1 starts to flow, and the collector current I _cQ1 that is hfe times starts to flow. When the voltage drop in R5 generated by the collector current I _cQ1 exceeds −0.6 V, the base current I _bQ3 of the switching element Q3 starts to flow, and the collector current I _cQ3 of the switching element Q3 flows through R9 → R10 → R12. Constants are set so that the voltage drop generated at R5 is equal to or greater than the diffusion potential between the base and emitter of switching element Q3, and the voltage drop generated at R10 + R12 is equal to or greater than the diffusion potential between the base and emitter of switching element Q1. Then, positive feedback is applied, and the switching elements Q1 and Q3 are operated in the saturation region and are completely turned on. In operation in the saturation region, the collector-emitter potential is almost 0 V, and each collector current continues to flow.

When the switching element Q3 is turned on, the base current _{IbQ2 begins} to flow also to the switching element Q2 which is an NPN transistor, and the switching element Q5 is forcibly turned on by short-circuiting the gate and the source of the switching element Q5 on the low side. Turn off to cut off short circuit current.

  Since this circuit example is a half-bridge arm short circuit protection circuit, the object can be achieved if the circuit is placed in either the switching element Q5 or Q4. If a protection circuit is inserted in both switching elements Q4 and Q5, it is possible to protect not only the arm short circuit but also the overcurrent of the output current regardless of the direction of the current.

Normally, in the operation in the saturation region where the bipolar transistor is completely in the ON state, it takes time to shift to the OFF state, but the transition from the OFF state to the ON state is very fast. In the overcurrent protection circuit 100 according to the embodiment shown in FIG. 7, all the operations of the switching elements Q1 to Q3 from the detection of the overcurrent until the switching element Q5 is turned off are in the direction of OFF → ON. Even an inexpensive transistor having a frequency f _T of about 100 MHz can cut off the gate of Q5 in a short time of about 200 ns.

  Further, the slow transition of the switching elements Q1 to Q3 in the OFF direction is not a demerit, and unnecessary parasitic vibration or the like hardly occurs after the ON state is once turned on, so that stable protection performance can be exhibited rather.

  Further, by inserting the photocoupler PC1 or the like into the current path of the switching elements Q1 and Q3 connected to the thyristor and the control circuit and driving circuit of the switching elements Q5 and Q6, information on the occurrence of an overcurrent event can be obtained. It can be isolated and passed to the control circuit or drive circuit. The overcurrent protection circuit 100 according to the embodiment shown in FIG. 7 is configured to cut off the drive current of the direct drive circuit 701-1 directly.

<Automatic return from protection operation>
The shut-off time and whether to automatically recover from the protection operation are determined by the resistor R4, the capacitor C1, and the collector currents I _cQ1 and I _cQ3 flowing through the switching elements Q1 and Q3.

First, when the protection circuit is in the standby monitoring state, since the collector currents I _cQ1 and I _cQ3 of the switching elements Q1 and Q3 are both zero, the voltage V _{C1 of the} capacitor _C1 is charged through the resistor R4 and gradually _approaches the power supply B1.

Next, the collector current I _cQ1 of the switching element Q1 and the collector current I _cQ3 of Q3 when the protection operation is _performed are I _cQ1 = (V _C1 −V _beQ3 −V _AKPC1 −V _ceQ1 ) / R6
I _cQ3 = (V _C1 −V _beQ1 −V _ceQ3 ) / R9
Thus, the voltage charged in the capacitor C1 is gradually discharged. Here, V _BEQ1 the base of the switching element Q1, emitter diffusion _potential, V beQ3 the base of the switching element Q3, emitter diffusion potential, V _AKPC1 the forward voltage of the diode of the photocoupler _PC1, V ceQ1 the switching element Q1 V _ceQ3 is a saturation voltage between the collector and the emitter of the switching element Q3.

When the voltage of the capacitor C1 decreases, the currents of I _cQ1 and I _cQ3 also decrease. When the voltage between the base and emitter of the switching element Q1 or Q3 cannot maintain the diffusion potential, the switching elements Q1 and Q3 shift to OFF. To do. This time constant is the time until automatic return to the monitoring state.

Here, if the value of the resistor R4 is set so that the base-emitter voltages of the switching elements Q1 and Q3 become equal to or higher than the respective diffusion potentials by I _cQ1 and I _cQ3 , the protection state is maintained without automatic recovery. You can also

  As described above, since the threshold value for shifting to the protection operation of the protection circuit according to the present invention depends on the diffusion potential between the bases and emitters of the switching elements Q1 and Q3, it has a negative temperature coefficient of about −2 mV / ° C. For this reason, the maximum rating of the switching element that decreases as the temperature rises is selected by selecting the inductance L2 and load resistance R12 of the current detection inductor to appropriate values and linking the temperature of the switching elements Q1 and Q3 with the switching elements Q4 and Q5. Accordingly, it is possible to follow the threshold value of the protection current.

DESCRIPTION OF SYMBOLS 100 Overcurrent protection circuit 101 Current detection inductor 111 Mold resin 112, 118 Electrode 113, 115 Ferrite core 114, 117 Magnet wire 116 Ceramic 201 Printed wiring board 202 Switching element

In order to solve the above problems, the present invention provides an overcurrent protection circuit, wherein a magnetic path disposed in the vicinity of a negative electrode or a positive electrode of a switching element, or in a vicinity of a wiring connected to the negative electrode or the positive electrode of the switching element. An inductor that is not closed, and is arranged such that a magnetic path releasing direction of the inductor is a direction of a magnetic flux generated by a current flowing through the switching element, the transistor, and a gate electrode and an emitter of the transistor An off circuit including a resistor connected to the electrode, and the switching element is turned off when a voltage drop in the resistor generated by an induced electromotive force generated in the inductor exceeds a diffusion potential of the transistor. And an off circuit for maintaining the off state .

The invention according to claim 3, in the overcurrent protection circuit according to claim 1 or 2, wherein the transistor comprises a NPN transistor and a PNP transistor, the off-circuit thyristor connecting the PNP transistor and the NPN transistor The above-described circuit is used to maintain the switching element in an off state.

Claims (5)

A magnetic path disposed in the vicinity of the negative electrode or positive electrode of the switching element, or in the vicinity of the wiring connected to the negative electrode or positive electrode of the switching element, wherein the magnetic path release direction of the inductor flows to the switching element. The inductor arranged to be in the direction of the magnetic flux generated by the current,
An off circuit for turning off the switching element when the induced electromotive force generated in the inductor exceeds a predetermined value;
An overcurrent protection circuit comprising:   The overcurrent protection circuit according to claim 1, wherein the off circuit includes a full-wave rectifier circuit that rectifies an induced current generated in the inductor.   3. The overcurrent protection circuit according to claim 1, wherein the off circuit maintains a state in which the switching element is turned off using a circuit in which an NPN transistor and a PNP transistor are thyristor-connected.   4. The overcurrent protection circuit according to claim 1, wherein the off circuit turns off the switching element by short-circuiting a gate electrode and a source electrode of the switching element. 5.   The overcurrent protection circuit according to claim 1, wherein the inductor is mounted on the same printed wiring board as the switching element.