JPH10136644A - Switching power circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power circuit which can surely operate as a protective function from short circuit under load by a control circuit. SOLUTION: A choke coil L1 is connected between the junction between a smoothing capacitor C2 and a diode D1 and an output terminal 2, and a capacitor C3 is connected between the output terminal 2 and a ground. A diode D2 is connected in parallel with the choke coil L1, which forms a filter circuit together with this capacitor C3. Moreover, the detected voltage for constant voltage control to be fed back to a control circuit 3 and protection from short circuiting is made as the voltage between the terminals of the smoothing capacitor C2. By making such a constitution, a current flows to the choke coil L1 on the condition that the load is normal, and the ripple reduction for the output voltage is made. When the load becomes in short circuit condition, a current flows to the diode D2, and the voltage between the terminals of the smoothing capacitor C2 is lowered, so that operating the function of sure protection from short circuit becomes possible.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit that can reliably protect a power supply circuit from short-circuits.

[0002]

2. Description of the Related Art In a switching power supply circuit for controlling the amount of energy passing through an on / off operation of a switching element, a voltage value at a predetermined point in the circuit is detected, and a signal corresponding to the voltage value is fed back to a control circuit. To perform constant voltage control of the output voltage. Here, various protection systems are often configured inside the control circuit in addition to the control system that controls the operation of the switching elements. As this protection system, for example, a short-circuit protection circuit typified by a timer latch system is provided which prevents a power supply circuit from being damaged by an excessive output current flowing when output terminals are short-circuited. FIG. 2 shows an example of a conventional switching power supply circuit incorporating a control circuit having such a short-circuit protection function. The switching power supply circuit shown in FIG. 2 has the following configuration.

In FIG. 2, reference numerals 1 and 2 indicate an input terminal and an output terminal on the high potential side, respectively, and the input terminal and the output terminal on the low potential side are not shown because they are connected to the ground. A switching transistor Q1 and a primary winding N1 of a transformer T are connected in series between the input terminal 1 and the ground, and a capacitor C is connected between the input terminal 1 and the ground.
1 is connected. One end of the secondary winding N2 of the transformer T is connected to the anode of the diode D1 as a rectifying element, and the other end is connected to ground. A smoothing capacitor C2 is connected between the cathode of the diode D1 and the ground, a choke coil L1 is connected between the cathode of the diode D1 and the output terminal 2, and a capacitor C3 is connected between the output terminal 2 and the ground. It is connected. This switching transistor Q1, transformer T, diode D1
The smoothing capacitor C2 constitutes a basic switching type power supply circuit, and the choke coil L1 and the capacitor C3 form a filter circuit for reducing the ripple of the output voltage.

A series circuit of resistors R1 and R2 is connected in parallel between the terminals of the smoothing capacitor C2, and a connection point between the resistors R1 and R2 is connected to the inverting input terminal (-) of the error amplifier ER. The reference voltage source REF is connected to the positive input terminal (+) of the error amplifier ER, and the output terminal of the error amplifier ER is connected to the inverting input terminal (−) of the comparator COMP. The positive input terminal (+) of the comparator COMP is an oscillation circuit OS
C, and the output terminal of the comparator COMP is a buffer B
It is connected to the base of the switching transistor Q1 via UF. Error amplifier ER and buffer BU
A short-circuit protection circuit SCP is provided between F. This error amplifier ER, reference voltage source REF, comparator COMP,
Oscillation circuit OSC, buffer BUF, and short-circuit protection circuit SC
The control circuit 3 is constituted by P.

The basic operation of the control circuit 3 in the power supply circuit shown in FIG. 2 is as follows. First, in the error amplifier ER, the reference voltage input to the positive input terminal (+) and the smoothing capacitor C2 input to the inverting input terminal (-)
Are detected, and a voltage level signal VL corresponding to the difference between the two voltages is generated. This voltage level signal VL is input to the comparator COMP, where it is compared with the triangular wave voltage from the oscillation circuit OSC. Then, a pulse signal PO having a pulse width corresponding to the voltage level signal VL is generated at the output terminal of the comparator COMP.
The signal is input to the base of the switching transistor Q1 via the UF. The switching transistor Q1 performs on / off operation in response to the pulse signal PO, and the switching transistor Q1
A voltage is induced in the next winding N2 so that a predetermined output voltage is obtained.

Here, when the external load is short-circuited and the voltage across the terminals of the smoothing capacitor C2 is greatly reduced, as a part of a constant voltage control operation for keeping the output voltage constant, the error amplifier ER detects the detection voltage and the reference voltage. The voltage level signal VL is increased according to the difference. Then the short circuit protection circuit SC
P transmits a latch signal for shutting off the output to the buffer BUF when the voltage level signal VL becomes larger than a predetermined value. When the buffer BUF receives the latch signal, it blocks the passage of the pulse signal PO input from the comparator COMP to the base of the switching transistor Q1, and stops the operation of the switching transistor Q1. As a result, energy is not transmitted from the primary side to the secondary side of the transformer T, and an excessive current is prevented from flowing due to a load short circuit.

[0007]

In recent years, in order to meet demands for downsizing and cost reduction of circuits, small-sized coil components are often used for the choke coil L1 in FIG. A coil component having a small shape is generally manufactured by winding a fine wire, and therefore has a large internal resistance. Here, FIG.
When the power supply circuit shown in (1) is in the operating state, if the load is normal, the current passing through the choke coil L1 is small, and the voltage drop between terminals due to the product of the internal resistance of the choke coil L1 and the current is small. However, when the load is short-circuited, a large current flows through the choke coil L1, and a large voltage drop occurs between the terminals of the choke coil L1 due to the large current and a large internal resistance. Then choke coil L
1 has a low potential on the output terminal 2 side,
The potential on the side of the first smoothing capacitor C2 will be kept high.

In the power supply circuit shown in FIG. 2, the detection point of the voltage for performing the short-circuit protection while controlling the output voltage to be constant is between the terminals of the smoothing capacitor C2. However, as can be seen from the above description, the voltage between the terminals of the smoothing capacitor C2 is not exactly the same as the output voltage due to the choke coil L1. In particular, when the load is short-circuited when the current is large, the difference becomes large, so that the voltage between the terminals of the smoothing capacitor C2 may not decrease to a voltage at which the short-circuit protection circuit SCP can detect that the load is short-circuited. There is. If the operation of the power supply circuit is continued in a state where the load is short-circuited, the load on each component of the circuit increases, and there is a risk that any of the components may be damaged.

In order to avoid such a situation, a voltage detection point for performing constant voltage control and short-circuit protection is determined by connecting a capacitor C which constitutes a filter circuit and is closest to the output terminal 2.
It is also conceivable that the distance is between the terminals No. 3 and the terminal No. 3. However, if an attempt is made to detect the voltage between the terminals of the capacitor C3, the inductance component of the choke coil L1 causes a phase rotation in a feedback loop for constant voltage control formed in the circuit of FIG. The output voltage may fluctuate unstablely without voltage control. Accordingly, the present invention provides a switching power supply circuit that can reliably operate a short-circuit protection function added to a control circuit in a switching power supply circuit provided with a filter circuit on the output side of the circuit in order to reduce output voltage ripple. The purpose is to gain.

[0010]

According to the present invention, a switching element connected in series to an inductance component is turned on and off, and a voltage appearing on a predetermined winding of the inductance component is rectified by a rectifying element and a smoothing capacitor. In a switching power supply circuit that obtains a desired output voltage by smoothing, a switching element is turned on or off according to a voltage between terminals of a smoothing capacitor.
A control circuit that controls the OFF operation and detects a short-circuit state from the voltage across the terminals of the smoothing capacitor to activate the short-circuit protection function, a filter circuit having a choke coil connected to the output side of the smoothing capacitor, and a filter circuit. A diode is connected in parallel with the choke coil.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A rectifying element and a smoothing capacitor are connected to a predetermined winding of an inductance component such as a transformer, and a filter circuit having a choke coil and a capacitor is connected to the output side of the smoothing capacitor. Then, a diode having a forward direction from the smoothing capacitance to the output terminal side of the power supply circuit is connected in parallel to the choke coil of the filter circuit. Means for detecting a voltage between terminals of the smoothing capacitor is provided, and a signal corresponding to the voltage between terminals of the smoothing capacitor is input to an input terminal of an error amplifier which is a component of a control circuit having a short-circuit protection function.

[0012]

FIG. 1 shows an embodiment of a switching power supply circuit according to the present invention in which a short-circuit protection function added to a control circuit is reliably operated when a load is short-circuited.
The circuit shown in FIG. 1 is characterized in that a diode D2 having a forward direction from the smoothing capacitor C2 to the output terminal 2 is connected in parallel to the choke coil L1. The circuit of FIG. 2 described in the section of the prior art is the same. In the circuit of FIG. 1 having such a configuration, the following operation is performed by the action of the diode D2.

First, when the load condition is normal, the voltage generated in the secondary winding N2 of the transformer T is rectified and smoothed by the diode D1 and the smoothing capacitor C2, and the ripple is further reduced by the choke coil L1 and the capacitor C3. The output is reduced and supplied to an external load from the output terminal 2.
Here, if the load is normal, the current supplied from the output terminal 2 to the outside is small, and therefore, the voltage drop between the terminals of the choke coil L1 represented by the product of the current and the internal resistance is small. Further, the voltage between the terminals of the smoothing capacitor C2 includes some ripples. However, the magnitude of the ripple is about several tens mV, which is much smaller than the forward voltage drop of the diode element. Therefore, the current flows through the choke coil L1 due to the forward drop voltage of the diode D2, and almost no current flows through the diode D2, and does not adversely affect the operation of the filter circuit including the choke coil L1 and the capacitor C3. That is, when the load is normal, the operations of the circuits of FIGS. 1 and 2 are substantially the same.

Next, when the load is short-circuited, the voltage appearing at the output terminal 2 decreases, and the current supplied from the output terminal 2 to the outside increases. Then, the voltage drop appearing between the terminals of the choke coil L1 increases, and the diode D2 conducts. Due to the conduction of the diode D2, the output terminal 2
And the voltage between the terminals of the smoothing capacitor C2 have substantially the same value, and actually differ by the forward voltage drop of the diode D2. As a result, the voltage across the terminals of the smoothing capacitor C2 decreases without being affected by the internal resistance of the choke coil L1 when a load short circuit occurs. When the voltage between the terminals of the smoothing capacitor C2 decreases, the short-circuit protection circuit SCP provided inside the control circuit 3 operates, and the on / off operation of the switching transistor Q1 stops as a result of the short-circuit protection operation.

In the circuit shown in FIG. 1, the diode D2 connected in parallel to the choke coil L1 is a diode element.
Although shown as one, a plurality of diode elements may be connected and used in order to obtain a voltage difference (forward drop voltage) required for current blocking during normal operation. Further, instead of a normal diode element, a constant voltage diode element may be connected in parallel to the choke coil L1. Further, although the power supply circuit shown in FIG. 1 is a switching power supply circuit using a transformer, it goes without saying that the present invention can be applied to a switching power supply circuit using no transformer, for example, a chopper type.

[0016]

As described above, according to the present invention, a filter circuit having a choke coil is further provided on the output side of a smoothing capacitor, and a constant voltage control of output voltage and short-circuiting are performed by detecting a voltage between terminals of the smoothing capacitor. In a switching power supply circuit for protection, a diode element is connected in parallel to a choke coil of a filter circuit. According to this configuration, normal operation can be performed without impairing the function of the filter circuit when the load is normal, and when the load is short-circuited, the terminal voltage of the smoothing capacitor is substantially equal to the output voltage. Value, and the short-circuit protection function of the control circuit can be activated. Therefore, according to the present invention,
When the load is short-circuited, a highly reliable switching power supply circuit in which the short-circuit protection function operates reliably can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an embodiment of a switching power supply circuit according to the present invention.

FIG. 2 is a circuit diagram of a conventional switching power supply circuit.

[Explanation of symbols]

 Reference Signs List 1 input terminal 2 output terminal 3 control circuit REF reference voltage source OSC oscillation circuit ER error amplifier COMP comparator BUF buffer SCP short-circuit protection circuit T transformer Q1 switching transistor C2 smoothing capacitor (smoothing capacitance) D1 diode L1 as rectifying element Filter circuit Choke coil to configure C3 Capacitor to configure filter circuit D2 Diode element

Claims (1)

[Claims] 1. A switching element connected in series to an inductance component is turned on and off, a voltage appearing in a predetermined winding of the inductance component is rectified and smoothed by a rectifier and a smoothing capacitor, and a desired output voltage is obtained. In the switching power supply circuit, the on / off operation of the switching element is controlled in accordance with the voltage between the terminals of the smoothing capacitor, and at the same time, the short-circuit state is detected from the voltage between the terminals of the smoothing capacitor to activate the short-circuit protection function. A switching power supply circuit, comprising: a control circuit; a filter circuit having a choke coil connected to the output side of the smoothing capacitor; and a diode element connected in parallel to the choke coil of the filter circuit.