JP2005312203A - Overcurrent controller - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an overcurrent controller which never does damage on a switching element, with one current protector even to load where a momentary current over an average current flows. <P>SOLUTION: An overcurrent controller for a switching power unit, which changes its switching frequency, is equipped with a diode which rectifies the current detected in a current transformer CT1. A control transistor Q100 is equipped with a time constant circuit which is bias-controlled by the potential geared to its detection current. When a current transformer CT1 detects an overcurrent over a load current in stationary state, this controller puts it in time-current control states that are determined by the time constant circuit of the control transistor Q100. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to an overcurrent control device, and in particular, can be suitably used for an output current control circuit of a switching power supply in which an instantaneous current of a load current is larger than an average current.

In an overcurrent control device for a conventional switching power supply device, a current control circuit prevents the output current from flowing beyond a certain value so that the switching power supply will not be destroyed even if there is an abnormality in the output load and a short circuit occurs. Is generally used.
The output load current varies depending on the load form, and some of them output an instantaneous peak current larger than the average current. In general, for an element that switches at a period of several tens of microseconds to several hundreds of microseconds, this peak current may range from several milliseconds to several tens of milliseconds, that is, several tens to several thousand pulses.
A switching power supply in which such a load current flows requires a large-scale power supply that can supply the peak current as a steady continuous load when an instantaneous peak current flows through a switching element such as a switching transistor, transformer, or rectifier diode. Therefore, an instantaneous peak current greater than the average current often supplies a load current with the charge accumulated in the rectifying capacitor and does not flow to the switching element.
In this case, the larger the difference between the average current and the instantaneous peak current, the larger the capacity of the rectifier capacitor described above is required. As similar conditions continue, current control circuits become increasingly important.
Therefore, a method is adopted in which the current flowing through the switching transistor is directly detected by a current transformer and current control is performed. However, an instantaneous peak current greater than the average current can be allowed to flow, and more current flows continuously than the average current. Even in this case, it is necessary to control the current so that a current exceeding a specified current does not flow through the switching element.
For this reason, the current control circuit that operates when the current flows further than the instantaneous peak current and the current control circuit that operates at a current equal to or higher than the average current below the instantaneous current are combined, and a timer is provided for the latter current control circuit. Therefore, it can cope with an overcurrent in the vicinity of a continuous average current. (For example, Patent Document 1)
In addition, a description is disclosed in which a power-up circuit is disabled, and a load current is temporarily increased to be equal to or higher than a rated output current to supply power to a load having a large current specification. (Patent Document 2)
JP 2001-119933 A Japanese Patent Laid-Open No. 5-95673

However, in the conventional configuration, it is necessary to increase the detection current so that the current control circuit does not work even when the difference between the instantaneous current and the average current is very large. In this case, when an instantaneous current flows, the switching element may be destroyed. Therefore, there is a problem that a very expensive switching element must be used even in a device having a small average current. In addition, even when an expensive switching element is used, if the average current is very small, damage other than the switching element may occur when the average current abnormally increases below the pulse current. In order to solve this problem, the second current control circuit is required.
The present invention solves the above-described conventional problems, and provides an overcurrent control device having no stress on a switching element in a switching power supply having a load current in which an instantaneous current larger than an average current flows with a simple configuration. Objective.

In order to solve the conventional problems, an overcurrent control device according to the present invention is a switching transformer to which a rectangular wave is applied by switching a transistor, and for rectifying a secondary output of the switching transformer into a DC voltage. In an overcurrent control device of a switching power supply device that changes a switching frequency according to the detected current value, the detection element that detects the current is a current transformer. The diode that rectifies the current detected by the current transformer and the control transistor that is bias-controlled by a potential corresponding to the detected current include a time constant circuit, and a steady-state load current is generated by the current transformer. When overcurrent is detected, the time constant circuit of the control transistor Ri determined time, in which is characterized in that the current control state.
The present invention also provides a switching transformer to which a rectangular wave is applied by switching a transistor, a full-wave rectification rectifier circuit for rectifying the secondary output of the switching transformer to a DC voltage, and a current detection And an overcurrent control device of a switching power supply device that changes a switching frequency according to the detected current value, wherein the detection element that detects the current is a current transformer, and the secondary side of the switching transformer A diode arranged between the middle point of the winding and the ground point, which rectifies the current detected by the current transformer, and a control transistor bias-controlled by a potential according to the detected current includes a time constant circuit, If an overcurrent exceeding the steady state load current is detected by the current transformer, the control transistor Time determined by the time constant circuit of the motor, in which is characterized in that the current control state.

According to the overcurrent protection device of the present invention, it is possible to provide a switching power supply that does not damage a switching element with a single current protection device even for a load in which an instantaneous current that is many times the average current flows. it can.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows an overcurrent control apparatus according to a first embodiment of the present invention.

  In FIG. 1, the primary side includes a primary winding L1 of a switching transformer T1 connected to a DC power source B1 and a collector of a switching transistor Q1 connected thereto. The emitter of the switching transistor Q1 is connected to L100 of the current detection element current transformer CT1, and the opposite end is connected to the negative electrode of the DC power supply B1. A rectangular wave whose frequency or pulse width is controlled by the feedback circuit U1 is input to the base of the switching transistor Q1.

  Electric power is transmitted to the secondary side via the switching transformer T1, and a rectangular wave corresponding to the winding ratio is generated in the secondary winding L2. One end of the secondary winding L2 is connected to the anode of the diode D1, and the cathode is connected to the positive electrode of the smoothing capacitor C1 and the load. The negative electrode of the capacitor C1 is connected to the other end of the secondary winding L2 of the switching transformer T1, and at the same time is connected to the ground potential of the load.

  The position of the current detection element CT1 is disposed between the emitter of the switching transistor Q1 and the negative electrode of the DC power supply B1, but between the anode electrode of the diode D1 and the secondary winding L2 of the switching transformer T1, and the cathode electrode of the diode D1. The same current detection operation can be performed even if it is arranged between the capacitor C1 and the same current detection operation is performed even if it is arranged between the secondary winding L2 of the switching transformer T1 and the smoothing capacitor C1. be able to.

  FIG. 1 shows an example in which the voltage detected by the current detection element CT1 is rectified by a half-wave rectification method. However, current detection with higher accuracy can be performed by bridge rectification as shown in FIG. it can.

  FIG. 1 shows an example in which the switching transistor is a one-stone converter, but the same applies to a two-stone converter. For example, in the case of FIG. 6, the same highly accurate current detection operation can be performed even in the configuration in which the current detection element CT1 is arranged between the emitter of the switching transistor Q2 and the primary side of the switching transformer T1.

  In the case of FIG. 6, the same highly accurate current detection operation can be performed in the configuration in which the current detection element CT1 is arranged between the emitter of the switching transistor Q2 and the negative electrode of the DC power supply B1.

  FIG. 1 shows an example of the secondary wave rectification that is the output of the switching transformer T1 in the half wave rectification circuit method, but the same applies to the case of the full wave rectification method. For example, in the case of FIG. 7, the same detection current detection operation can be performed even in the configuration in which the current detection element CT1 is arranged between the intermediate point position of the secondary winding L2 and the ground side of the capacitor C1.

  The operation waveforms at this time will be described with reference to FIGS. In FIG. 3, the load current is IL, the average load current is Iav, the voltage across the electrolytic capacitor C2 is Vc, the current flowing through the detection element CT1 is Ir1, and the current control threshold current is Ith.

  When a steady load current is flowing, the period of FIG. In this case, the power supplied to the switching power supply is the power supplied with the current equal to the load current, that is, the power supply on the secondary side in FIG. It becomes operation.

Next, when the instantaneous peak current flows, the current is controlled to suppress the overcurrent, and the current supplied via the switching transformer T1 is limited, and the load current becomes larger than the supply current from the power source. An attempt is made to compensate the load current by discharging the charge charged in the capacitor C1. In FIG. 3, the current flow loop of b is dominant. Therefore, the voltage Vc of the smoothing capacitor C1 starts to drop. The drop voltage at this time is Vc as the smoothing capacitor voltage, IL as the load current, t as the application time, and C as the smoothing capacitor capacity.
Vc = IL × t / C
Therefore, in order to maintain the smoothing capacitor voltage Vc within a predetermined voltage range, the capacity of the smoothing capacitor derived from the above equation is required.

  Thereafter, when the instantaneous peak current stops flowing, the power source continues to charge the smoothing capacitor in a constant current state. In FIG. 3, the current flow loop of c is dominant. In this way, the power source does not supply a current greatly deviating from the average current from only the switching transformer T1, that is, by supplying the current through the switching transformer T1 and the discharge current from the smoothing capacitor C1, thereby instantaneously. Can deal with typical currents.

  In this way, the switching power supply does not follow the instantaneous load current, but supplies a current obtained by integrating the instantaneous load current change to the load. As a result, power supply design based on the average current value becomes possible, current design in the vicinity of the average current value much smaller than the instantaneous peak current becomes possible, and the power supply device can be miniaturized. For example, when the peak current value is 10 A, it is possible to design a power supply apparatus corresponding to an average current value of about 2 A that is much smaller than that.

  When the switching transistor Q1 is turned on, a voltage induced by the voltage across L100 of the current transformer CT1 serving as a detection element is output to L101. One end of L101 is connected to the cathode of the diode D100, and the voltage output to L101 is rectified to a negative potential. One end of L101 not connected to D100 is connected to the negative potential of the reference voltage B100, and is connected to the anode of D100 via a resistor R101 serving as a first dummy load. The anode of D100 is connected to the anode of diode D101 via R100, which is a second dummy resistor, and the cathode is connected to the base of PNP transistor Q100, and is higher than the emitter-base voltage of Q100 via pull-up resistor R102. It is connected to the positive potential of the reference voltage B100 as a potential. The emitter of the transistor Q100 is connected to the feedback circuit U1 through the resistor R301 and is connected to one end of C300 through the R300. The other end of C300 is connected to the negative potential of B100.

  The diode D101 uses a Zener diode, and the base potential of the transistor Q101 is temperature compensated. Further, Q100 may be configured to have a stable temperature characteristic by complementary connection of an NPN transistor and a PNP transistor.

When load current IL flows, current Ir1 flows between the emitter and collector of switching transistor Q1 to supply power to the secondary side, and current Ir1 also flows through L100 of current transformer CT1 between the emitter and the negative potential of the DC power supply. Flows. At this time, a voltage having the same waveform as the current waveform shown in FIG. 4 is generated at both ends of L100 by the resistance component between L100. As for the generated voltage, a voltage having the same waveform, which is induced by the current transformer CT1 in a ratio of the number of turns of L100 and L101, is output to L101. The voltage output to L101 is rectified to a negative potential by D100. The voltage after rectification can be set to a desired negative potential value by the turn ratio of the current transformer CT1 and R101 which is a dummy resistor.
During normal operation, the base of the transistor Q100 is pulled up by the reference voltages B100 and R102, and is off.

  When the voltage rectified to a negative potential becomes equal to or higher than the VF voltage of D101, current flows from the cathode of D101 connected to the base of transistor Q100 to the anode side, and transistor Q100 begins to conduct, so that output power is suppressed in feedback circuit U1. Acting as a current control circuit. That is, the feedback circuit U1 limits the output current of the switching transistor Q1 and performs a current control circuit operation.

  At this time, by setting D100 to bridge rectification as shown in FIG. 5, the negative potential on the cathode side of D101 can be stably output with high accuracy, so that highly accurate current detection can be provided.

  Further, a resistor R300 and a capacitor C300 are connected in series between the collector and the emitter of the transistor Q100. When the load current IL exceeds the threshold value Ith, the transistor Q300 is turned on and the time constant of R300 and C300 is applied to the feedback circuit U1. By acting and increasing the switching frequency, the power supplied from the switching transistor Q1 is suppressed. When the current Ir1 flowing through the detection element CT1 becomes equal to or less than Ith, the current control action is applied to the feedback circuit U1 until the capacitor C300 is charged with the time constants of the resistor R300, the capacitors C300, and R301 even when the transistor Q300 is turned off. By continuing the influence, the return from the current control state can be performed slowly, and the operation of the control circuit can be stabilized.

  In the so-called flyback converter shown in FIG. 1, the triangular wave current waveform shown in FIG. 4 flows to the current detection element CT1. That is, the detected current continues to increase over time during the switching period of the detected current. For this reason, once the detected current exceeds the threshold value Ith, there is no problem even if the current control operation is performed at that time, but the resonant capacitor between the primary winding L1 of the transformer T1 and the switching transistor Q1 in the circuit diagram of FIG. In the case of the current resonance type converter as shown in FIGS. 6 and 7 in which C3 is inserted, the current Ir1 shown in FIG. 8 flows to L100 of the current detection element CT1 without R300 and C300 in FIG.

  As described above, in the case of current control for the purpose of protecting the switching element, the current control response needs to be faster than the switching cycle. Therefore, when a current exceeding the threshold value Ith indicated by the broken line in FIG. The emitter voltage Ve of the transistor Q300 immediately decreases as shown in the lower diagram of FIG. This acts on the feedback circuit, increases the switching frequency, and suppresses the output current. As a result, the current becomes equal to or higher than Ith in one pulse cycle period and then becomes lower than Ith again. Therefore, the current control is performed after the current control is performed within the period of one pulse cycle, and the current control is not performed.

  That is, since the current control state instantaneously shifts to the constant voltage operation in the steady state and the current control state is instantaneous, the switching frequency is also unstable.

  This operation is extremely unstable because the switching frequency is not stable in a circuit that controls the output power by modulating the switching frequency.

  Therefore, by providing a time constant with the resistor R300 and the capacitor C300, the transition to the current control operation is performed quickly with a short time constant, and the current control operation returns with a time constant of one pulse period or more. . That is, for the return, the time constant of the resistor R300 and the capacitor C300 is given and the transition from the current control operation to the constant voltage operation that is steady state control is performed gradually.

  The operation at this time is shown in FIG. 9 as in FIG. In FIG. 9, when an overcurrent is detected, the switching frequency is increased and the current control mode in which the overcurrent is suppressed is entered, but the current control mode is gradually shifted to a constant voltage control mode that is a steady state. As described above, the current control operation is continued, and the frequency modulation operation is gently performed after the operation, so that the operation stability can be brought about.

  That is, when an overcurrent is detected, an unstable operation of the switching frequency such that the switching frequency increases, then decreases, and increases can be eliminated. In the present invention, once an overcurrent is detected, the switching frequency once increases. After that, however, it has a predetermined time constant, and the transition from the current control state to the constant voltage operation in the steady state is made gradual. Therefore, the switching frequency modulation operation becomes stable.

The overcurrent control device according to the present invention can control a load in which an instantaneous current that is many times the average current flows with one overcurrent protection device without damaging the switching element. This is useful as an output current control circuit for a switching power supply in which the instantaneous current is larger than the average current.

1 is a circuit configuration diagram of an overcurrent control device according to Embodiment 1 of the present invention. The figure for demonstrating the operation waveform of the load current, the smoothing capacitor voltage, and the power supply current in Example 1 of this invention The figure for demonstrating the outline of the electric current operation | movement in Example 1 of this invention. Diagram for explaining the current waveform of the sensing element in a flyback converter Schematic circuit diagram when current detection rectification of the sensing element is realized by the bridge method Schematic circuit diagram of a two-stone current resonant converter Schematic circuit diagram of full-wave rectification method for two-stone type current resonant converter The figure for demonstrating the operation waveform at the time of overcurrent detection in case there is no time constant circuit R300 and C300. The figure for demonstrating the operation waveform at the time of overcurrent detection at the time of inserting the time constant circuits R300 and C300 of the overcurrent control apparatus in Example 1 of this invention.

Explanation of symbols

B1 Input DC power supply T1 Switching transformer L1 Primary winding of switching transformer L2 Secondary winding of switching transformer Q1 Switching transistor U1 Feedback circuit CT1 Current transformer L100 Primary winding of current transformer L101 Secondary winding of current transformer D1, D2 Two Secondary side rectifier diode C1 Secondary side smoothing capacitor C3 Resonant capacitor R100 Low-pass filter resistor C100 Low-pass filter capacitor Q100 PNP transistor R102 Pull-up resistor B100 Positive potential reference voltage R300 Time constant resistor for current control response R301 Resistance to feedback circuit C300 Time constant capacitor for response of current control operation IL Load current Ith Threshold current for starting current control operation Iav Average current Vc of load Current flowing through the voltage across Ir1 detecting element CT1 of capacitor C1

Claims (8)

A switching transformer to which a rectangular wave is applied by switching a transistor, a rectifier diode and a smoothing capacitor for rectifying the secondary output of the switching transformer into a DC voltage, a detection element for detecting a current, and the detection In the overcurrent control device of the switching power supply device that changes the switching frequency according to the current value
The detection element for detecting the current is a current transformer, a diode for rectifying the current detected by the current transformer, and a control transistor bias-controlled by a potential corresponding to the detection current includes a time constant circuit, When an overcurrent exceeding a load current in a steady state is detected by the current transformer, the overcurrent control device is placed in a current control state for a time determined by a time constant circuit of the control transistor.   The overcurrent control device according to claim 1, wherein the time constant circuit includes a series connection element of a resistor and a capacitor.   The current transformer has a primary side connected in series with the emitter electrode of the switching transistor, a diode for rectifying the current flowing to the secondary side of the current transformer, and a resistor for obtaining a potential corresponding to the rectified current The overcurrent control device according to claim 1, wherein the control transistor is bias-controlled according to a potential obtained by the resistor.   2. The overcurrent control device according to claim 1, wherein a rectification method of a diode that rectifies a current flowing on a secondary side of the current transformer is either a half-wave rectification method or a full-wave rectification method.   The current transformer is characterized in that a return time from a detected overcurrent to a steady-state constant voltage operation when an overcurrent is detected is at least one period of the switching frequency. The overcurrent control device according to 1.   The control transistor has a base potential that is temperature-compensated by a PNP transistor, and a potential displacement from the current detection means is connected to a base electrode of the control transistor via a resistor and connected to an emitter electrode of the PNP transistor. The overcurrent control device according to claim 1, wherein the switching transistor of the switching power supply device is controlled through a time constant circuit. A switching transformer to which a rectangular wave is applied by switching the transistor, a full-wave rectification rectifier circuit for rectifying the secondary output of the switching transformer into a DC voltage, a detection element for detecting current, In the overcurrent control device of the switching power supply device that changes the switching frequency according to the detected current value,
The detection element for detecting the current is a current transformer, and is arranged between the middle point of the secondary winding of the switching transformer and the ground point,
A diode that rectifies the current detected by the current transformer and a control transistor that is bias-controlled by a potential corresponding to the detected current include a time constant circuit, and an overcurrent that exceeds a steady-state load current by the current transformer And detecting a current, the current control state is maintained for a time determined by a time constant circuit of the control transistor.   The detection element, wherein an overcurrent is detected when the detection element is detected, a return time from a detected overcurrent to a constant voltage operation in a steady state is set to at least one period of the switching frequency. 8. The overcurrent control device according to 7.

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