JP5024345B2 - Overcurrent control device - Google Patents

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 larger 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, since 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, when current control is performed by directly detecting the current flowing through the switching transistor, the detected current changes when the input voltage fluctuates, so that stable current control cannot be performed.
In this case, current detection means must be provided in series with the output load, but instantaneous peak current greater than the average current can be allowed and allowed to flow, and switching can be performed even when current flows continuously higher than the average current. It is necessary to perform current control so that a current exceeding a specified current does not flow through the 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, refer to Patent Document 1.)
In the switching power supply circuit, it is described that the voltage detection means detects the amount of current by the voltage effect due to the resistance component of the choke coil and performs overcurrent protection. (Patent Document 2)
JP 2001-119933 A JP-A-6-269159

However, in the conventional configuration, not only two current control circuits are required for the vicinity of the average current and the vicinity of the instantaneous current, but if the difference between the average current and the instantaneous current is large as described above, Since a rectifier capacitor having a large capacity is required, a situation similar to a load short-circuit occurs when the power supply is started, and the switching element may be destroyed. Furthermore, in order to solve this problem, the third current control circuit is required.

In the conventional example, an error amplifier is used in the current control circuit. In this case, if the response speed is not increased, the switching element is destroyed when the load is short-circuited. Therefore, it is necessary to select an error amplifier that can operate at high speed. It had the problem of becoming very expensive.

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 above-described problem, an overcurrent control device according to the present invention is for rectifying a transformer to which a rectangular wave is applied by switching a first transistor and a secondary output of the transformer into a DC voltage. A rectifier diode, a smoothing capacitor, and an overcurrent detection means are provided between the transformer and the smoothing capacitor on the secondary side of the transformer to detect the load current and make the DC load voltage constant. In an overcurrent control device for a switching power supply device that changes a switching frequency of one transistor, a feedback circuit that changes a switching frequency of the transistor connected to a base electrode of the first transistor and detection by the overcurrent detection means The result is fed back to the feedback circuit to control the first transistor. In a steady state, comprising a second transistor connected to the circuit, a resistor and a capacitor connected in series between the emitter electrode and the collector electrode of the second transistor, and a steady load current flows, From the time of overcurrent detection detected by the overcurrent detection means, a transition is made to a current control state in which overcurrent is suppressed by increasing the switching frequency, and the return time from the current control state to the steady state is set. The period of the switching frequency of the first transistor in the steady state is not less than one period, and the recovery time is determined by the time constant of the resistor and the capacitor connected in series .

Furthermore, the overcurrent control device of the present invention connects the potential displacement from the current detection means to the emitter electrode of a third transistor having a temperature-compensated emitter potential via a resistor. A collector that is pulled up to a reference positive potential through a resistor is connected to a base electrode of the second transistor, and a collector electrode of the second transistor is connected to a ground potential, and an emitter potential of the second transistor Control the first transistor based on changes
It is characterized by doing .

According to the overcurrent protection device of the present invention, it is possible to provide a switching power source 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.

FIG. 2 is a circuit configuration diagram of an overcurrent control device in an embodiment of the present invention The figure for demonstrating the operation waveform of the load current in the Example of this invention, a smoothing capacitor voltage, and a power supply current The figure for demonstrating the outline of the electric current operation | movement in the Example of this invention. Diagram for explaining the detected resistance current waveform in a flyback converter Schematic circuit diagram of current resonant converter The figure for demonstrating the operation waveform at the time of the overcurrent detection (in the circuit of FIG. 5) when there is no time constant circuit R300, C300 The figure for demonstrating the operation waveform at the time of overcurrent detection (in the circuit of FIG. 5) when the time constant circuits R300 and C300 are inserted.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
Example 1
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 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 switching transformer T1 through the current detection resistor R1 and at the same time to the ground potential of the load.

The position of the current detection element R1 is arranged between the secondary winding L2 of the switching transformer T1 and the smoothing capacitor C1, but between the anode electrode of the diode D1 and the secondary winding L2, the cathode electrode of the diode D1 and the capacitor The same applies to the arrangement between C1.

FIG. 1 shows an example of 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. 5, the current detection element R is interposed between the secondary winding L2 and the capacitor C1.
1 is placed.

The operation waveforms at this time will be described with reference to FIGS. In FIG. 3, the load current is IL,
The description will be made assuming that the average current of the load is Iav, the voltage across the electrolytic capacitor C2 is Vc, the current flowing through the detection resistor R1 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, FIG.
In the secondary side, the supply of power on the secondary side is dominated by the current flow loop of a.

Next, when the instantaneous peak current flows, the current is controlled to suppress the overcurrent, the current from the switching transformer T1 is limited, and the load current becomes larger than the supply current from the power source, so that the rectifying capacitor C1 is charged. The generated electric charge is discharged to supplement the load current. 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 as follows: smoothing capacitor voltage is Vc, load current is IL, application time is t, and smoothing capacitor capacity is C.
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 supply can handle an instantaneous current without supplying power far from the average current.
As described above, the switching power supply does not follow the instantaneous load current, but flows a current obtained by integrating the change in the instantaneous load current. As a result, the power supply can be designed based on the average current value, and a small power supply can be designed ignoring the instantaneous peak current.

The terminal on the transformer side of the detection resistor R1 is connected to the resistor R100 and connected to the NPN transistor Q10.
Connected to zero emitter and capacitor C100. The other end of C100 is at ground potential. The base of the transistor Q100 is connected to the emitter of the PNP transistor Q101 and the resistor R101, and the other end of R101 is connected to the reference voltage B100 having a positive potential. Transistor Q101
Is connected to the negative reference voltage B101, and the base is connected to the reference voltage B101 as a resistor R.
The voltage is divided between the ground potential at 102 and R103, and a capacitor C101 for voltage stabilization is connected between the ground potential.

The collector of the transistor Q100 has a first pull-up resistor R10 connected to the reference voltage B100.
4 and connected to the feedback circuit U1 via the transistor Q300.

The emitter of the transistor Q101 exhibits a constant voltage characteristic that becomes a reference potential that is higher than the base potential obtained by dividing the reference voltage B101 by R102 and R103 by the emitter-base voltage of the transistor Q101. On the other hand, the base of the transistor Q100 has the same potential, and the emitter has a potential lower than the emitter-base potential.

Therefore, when the load current IL flows and the current flows through the detection resistor R1, the potential on the transformer side of the resistor R1 drops according to the load current IL as compared with the ground potential. This detection resistor R1
Since there is switching noise of the rectifier diode D1 and the like and high-frequency switching noise from the load, the resistor R100 and the capacitor C100 constitute a low-pass filter and is connected to the emitter of the transistor Q100.

When the current flowing through the load L is detected by the detection resistor R1, the load current IL exceeds a predetermined threshold value Ith, and the potential dropped by the detection resistor R1 becomes equal to or higher than the base potential of the transistor Q100,
The collector current of transistor Q100 begins to flow. And the first pull-up resistor R10
4 so that the transistor Q100 operates as a constant current circuit, the collector potential drops, the transistor Q300 begins to conduct, and the feedback circuit U1 operates to suppress output power and operates as a current control circuit. To do. 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, the voltage change due to the temperature of the emitter-base voltage of the transistor is canceled due to the connection of the PNP transistor and the NPN transistor, and has a very stable characteristic with respect to the temperature change.

A resistor R300 and a capacitor C are provided between the collector and emitter of the transistor Q300.
When 300 is connected in series and the load current IL exceeds the threshold value Ith, the transistor Q
300 is turned on, acts on the feedback circuit U1 with the time constant of R300 and C300, and raises the switching frequency, thereby suppressing the power supplied from the switching transistor Q1. When the current Ir1 flowing through the detection resistor R1 is equal to or less than Ith, even if the transistor Q300 is turned off,
Until the capacitor C300 is charged with the time constants of the resistor R300 and the capacitors C300 and R301, the influence of the current control action continues on the feedback circuit U1, so that the current control state is gradually restored, and the control circuit The operation can be stabilized.

In the so-called flyback converter shown in FIG. 1, the triangular wave current waveform shown in FIG. 4 flows through the current detection resistor R1. In other words, the detected current continues to increase with time during the switching period. 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 FIG. 5 in which C3 is inserted, the current Ir1 shown in FIG. 6 flows to the current detection resistor R1 without R300 and C300.

As described above, in the case of current control for the purpose of protecting the switching element, the response of the current control needs to be faster than the switching cycle. Therefore, when the current Ir1 exceeding the threshold value Ith shown by the broken line in FIG. The emitter voltage Ve of 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. In this figure, current control works when it falls below Ves. 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. 7 as in FIG. In FIG. 7, when the overcurrent is detected, the switching frequency is increased and the current control mode for suppressing the overcurrent is entered, but the current control mode is gradually shifted to the constant voltage control mode which is a steady state. in this way,
Since the current control operation is continued and the frequency modulation operation is gently performed after the operation, 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,
When an overcurrent is detected, the switching frequency once rises, but after that, it has a predetermined time constant, and the transition from the current control state to the steady-state constant voltage operation is made gradual, and the steady-state switching is gradually performed. Since the frequency returns, the switching frequency modulation operation becomes stable.

According to the overcurrent protection device of the present invention, the current control device according to the present invention damages the switching element with a single overcurrent protection device even for a load in which an instantaneous current that is many times the average current flows. Therefore, it is useful as an output current control circuit for a switching power supply in which the instantaneous current of the load current is larger than the average current.

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 D1, D2 Secondary side rectifier diode C1 Secondary side smoothing capacitor R1 Current detection resistor C3 Resonance capacitor R100 Low-pass filter resistor C100 Low-pass filter capacitor Q100 NPN transistor Q101 PNP transistor R101 Pull-up resistor B100 Positive reference voltage B101 Negative reference voltage R102, R103 Negative reference voltage dividing resistor C101 Voltage stabilization capacitor R104 First pull-up resistor Q300 PNP transistor R300 Time constant resistor for response of current control operation R301 Resistance to feedback circuit C300 Current control operation Current flowing through the voltage across Ir1 detecting resistor R1 of constant capacitor time for response IL the load current Ith threshold current Iav load average current Vc electrolytic capacitor C2 to start the current control operation

Claims (2)

A transformer to which a rectangular wave is applied by switching the first transistor;
A rectifier diode and a smoothing capacitor for rectifying the secondary output of the transformer into a DC voltage;
Overcurrent detection means is provided between the transformer and the smoothing capacitor on the secondary side of the transformer,
In the overcurrent control device of the switching power supply device that detects the load current and changes the switching frequency of the first transistor to make the DC load voltage constant.
The switching frequency of the transistor connected to the base electrode of the first transistor
A feedback circuit that changes the number,
By feeding back the result detected by the overcurrent detection means to the feedback circuit,
A second transistor connected to the feedback circuit to control the first transistor;
A resistor and a capacitor connected in series between the emitter electrode and the collector electrode of the second transistor;
With
In a steady state where a steady load current is flowing, from the overcurrent detection time detected by the overcurrent detection means, a transition is made to a current control state in which the overcurrent is suppressed by increasing the switching frequency,
The return time from the current control state to the steady state is not less than one cycle period of the switching frequency of the first transistor in the steady state,
The recovery time is determined by the time constant of the resistor and capacitor connected in series.
,
An overcurrent control device. The potential displacement from the current detection means has a temperature compensated emitter potential through a resistor.
Connected to the emitter electrode of the third transistor
Connecting a collector to be pulled up to a reference positive potential of the third transistor via a resistor to a base electrode of the second transistor;
The collector electrode of the second transistor is connected to ground potential;
Control the first transistor based on the emitter potential change of the second transistor
To do,
The overcurrent control device according to claim 1.

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