JPH0376111B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a switching power supply device that protects against problems such as heat generation, smoke, ignition, and component failure due to abnormal current that occurs when output terminals are short-circuited.

Structure of the conventional example and its problems FIGS. 1 and 2 show the conventional example. The fundamental difference between FIG. 1 and FIG. 2 lies in the method of supplying operating power (hereinafter referred to as B voltage) to the control circuit of the switching power supply. The one in Figure 1 is the main output transformer 8
The voltage obtained by rectifying the voltage appearing in the winding is set as the B voltage of the control circuit. The one in Figure 2 is
A small capacity auxiliary power supply 19 dedicated to the B voltage is separately provided.

In recent years, there has been a strong demand for smaller size and lower cost switching power supplies, and the B voltage supply system shown in FIG. 1, which satisfies these demands, has come into widespread use.

Hereinafter, a conventional switching power supply device will be explained with reference to FIGS. 1 and 2.

FIG. 1 shows an overcurrent protection circuit diagram of a conventional switching power supply device. In the figure, 1 is
AC power supply, 2 is a bridge rectifier diode, 3 is a smoothing capacitor, and 1, 2, and 3 create a DC voltage. 4 is a resistance for starting operation, 5 is a rectifier circuit that creates the B voltage of the primary side control circuit, 6 is the primary side control circuit, which includes an oscillation circuit, a voltage detection circuit, a pulse width modulation circuit,
It consists of a pulse width output circuit, a drive circuit, a pulse width limiting circuit, etc. The a terminal of the primary side control circuit shows the B voltage, the b terminal shows the primary side reference potential, the c terminal shows the pulse width limit adjustment terminal, the d terminal shows the voltage detection terminal, and the e terminal shows the drive output terminal.

7 is a switching output transistor (switching output circuit), 8 is a main switching output transformer (switching transformer), 10 is a rectifier and smoothing circuit for the regulated output voltage V ₀ , 11 is a load for the output voltage, and 12 is a sub output A rectifying and smoothing circuit for the voltage _V1 , 13 a load for the output voltage, 14 a voltage adjustment circuit, 15 a photocoupler, and the output voltage _V0 is stabilized by 6, 14 and 15. 16 is an overcurrent detection circuit, 20 and 21 are overcurrent detection resistors, 1
7 is a photocoupler, 18 is an output voltage cutoff circuit (switching circuit), and resistors 30 and 31 of the output cutoff circuit.
The pulse width is limited by the partial pressure ratio of .

Next, the operation of the overcurrent protection circuit will be explained. The overcurrent activates the amplifier A ₂ or the amplifier A ₃ , and current flows through the photodiode of the photocoupler 17 .
Therefore, the phototransistor of the photocoupler 17 becomes conductive, and the thyristor 33 becomes conductive, so that the transistor 32 becomes conductive.
conducts and the c terminal of the control circuit becomes high level.
When the c terminal becomes high level, the pulse width becomes zero and the output circuit is cut off. An operational problem with this overcurrent protection circuit occurs when the AC power is turned on while the load 11 is short-circuited. Since the output voltage V ₀ is used as the B voltage of the amplifiers A ₂ and A ₃ of the overcurrent detection circuit 16, if the load 11 is short-circuited, the value of the voltage V ₀ becomes 0V and the amplifier A ₂ ,
_A3 doesn't work. Therefore, there was a problem that the overcurrent protection circuit stopped working.

In order to solve this problem, it is sufficient to add an auxiliary power supply 19 dedicated to the B voltage as shown in FIG. With load 11 short-circuited by the auxiliary power supply
Even if the AC power is turned on, the B voltage is supplied, so
Protection circuit works normally. However, as mentioned at the beginning, in order to meet the demands for miniaturization and low cost, it is necessary to solve the above problems using the circuit system shown in FIG.

Purpose of the invention The present invention solves the above-mentioned conventional problems.
Overcurrent protection operation of the main output circuit in the circuit system shown in Figure 1, that is, the multi-output switching power supply system that does not use the auxiliary power supply unit and uses the rectified voltage of the secondary output circuit as the rectified voltage of the protection circuit. It is inexpensive and has an overcurrent protection circuit that can operate reliably even when the AC power is turned on with the load of the main output circuit short-circuited. Small and
The purpose is to provide a highly safe, multi-output power supply device.

Structure of the Invention The switching power supply device according to the present invention connects a current detection resistor and the cathode terminal of a photodiode in the photocoupler input section to one terminal of the secondary main output winding of a switching transformer, and performs protection switching to the anode terminal of the photodiode. Connect the current detection resistor and the output terminal of the overcurrent comparison amplifier of the sub-output circuit consisting of the plurality of secondary side sub-output windings and the rectifying and smoothing circuit of the above-mentioned switching transformer, and connect the above-mentioned current detection resistor and protection switching resistor. It is equipped with a protection circuit characterized in that it is connected to a reference potential, and with this configuration, overcurrent protection operation of the main output circuit and overcurrent protection operation of a plurality of sub output circuits can be performed, and the main output circuit When the load is short-circuited
This ensures reliable overcurrent protection even when AC power is turned on.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 shows an overcurrent protection circuit for a switching power supply device according to an embodiment of the present invention.
Components having the same structure as in FIG. 1 are given the same numbers. 16a indicates an overcurrent detection circuit for sub-output voltage _V1 .

The cathode of the photodiode of the photocoupler 17 is connected to the connection point between the detection resistor 20 and the stabilized output winding of the main transformer 8 . On the other hand, the anode of the photodiode is connected to the protection switching resistor 22, and the other end of the protection switching resistor 22 is connected to the reference potential on the secondary side (the resistor 22 and the photodiode form a series connection circuit). With this circuit configuration, the output terminal of the amplifier A ₃ (overcurrent comparison amplifier) of the sub-output overcurrent detection circuit can be connected to the anode side of the photodiode.

The operation of the switching power supply device configured as described above will be explained below.

A load current I ₀ flows through the overcurrent detection resistor 20, and a voltage drop E ₀ occurs in the direction shown in the figure. The current _ID flowing through the photodiode of the photocoupler 17 due to this voltage is expressed by the following equation, where _VF is the forward voltage of the photodiode.

E ₀ = I ₀ × R ₂₀ (1) I _D = (E ₀ −V _F )/R ₂₂ (2) However, R ₂₀ : Resistance value of overcurrent detection resistor 20 R ₂₂ : Resistance of protection switching resistor 22 Value From equations (1) and (2), if the photodiode current of the photocoupler that operates the output cutoff circuit 18 is I _F ,
The protection circuit will operate when the load current I ₀ reaches the value of the following formula.

I ₀ = (I _F · R ₂₀ + V _F ) / R ₂₀ (3) From equation (3), the load current I ₀ at which the protection circuit operates is determined by the photodiode V _F , I _F and the resistors 20 and 22. It can be seen that this is not related to the value of the output voltage V ₀ . Therefore, with the load 11 short-circuited, AC
The overcurrent protection circuit operates even when the power is turned on.

Next, an explanation will be given of the overcurrent protection operation when only the sub output circuit becomes overloaded. During normal operation, the main output voltage is divided and a positive voltage is applied to the negative input terminal of overcurrent comparator amplifier A3 (hereinafter referred to as comparison amplifier A3), and the potential (reference potential) of the positive input terminal of comparator A3 is By setting the voltage higher than that, the output voltage of the comparator A3 becomes a low level, and the photodiode of the photocoupler is cut off, so that the overcurrent protection operation of the sub-output circuit is not performed.

When an overcurrent flows through the sub-output circuit, the voltage drop across the overcurrent detection resistor 21 increases, and the potential at the negative input terminal of comparator A3 decreases, becoming slightly lower than the potential at the positive input terminal of comparator A3. When it goes down, comparator A
The output voltage of No. 3 becomes high level.

The high level output voltage of the comparator A3 is applied to a parallel circuit of the photodiode of the photocoupler and the protection switching resistor 22. Since the protection switching resistor can be set sufficiently larger than the impedance of the photodiode, sufficient current flows through the photodiode due to the high-level output voltage of comparator A3, and the photocoupler transistor is turned on, thereby preventing overload in the sub-output circuit. The current protection circuit will operate. In this way, the function of the protection switching resistor 22 is to determine the value of the output current (I 0 ) at which the overcurrent protection circuit operates, as shown in equation (3), in the main output circuit protection operation, and to determine the value of the output current (I ₀ ) at which the overcurrent protection circuit operates, and to In the protective operation, no current flows through the resistor itself, but instead shunts the current through the photodiode. In this way, protection switching resistor 2
2 switches the overcurrent protection operation between the main output circuit and the sub output circuit.

Although a two-output switching power supply has been described in Fig. 3, in a multi-output switching power supply with three or more outputs, the overcurrent detection circuit output terminals of multiple sub-output circuits are connected to the comparator A3 shown in Fig. 3. By connecting it in parallel to the output terminal, that is, to the anode terminal of the photodiode of the photocoupler, an overcurrent protection circuit for a multi-output switching power supply can be easily constructed. Further, by connecting the output terminal of the overvoltage detection circuit to the anode terminal of the photodiode, an overvoltage protection circuit can be easily constructed.

Comparing this embodiment with the conventional example described above, the structure of the overcurrent protection circuit of the secondary side main output circuit is different.

In this embodiment, the current detection resistor 2 is
The voltage generated at zero causes a current to flow through the photodiode in the photocoupler, whereas in the conventional example, the output voltage of an overcurrent comparison amplifier causes current to flow through the photodiode in the photocoupler. A stable B power supply for the amplifier circuit is required. The B power supply in the conventional example in Figure 1 is supplied from the main output voltage, but when the AC power is turned on with the load in short state, the output voltage is 0V, so the B voltage of the overcurrent protection circuit also becomes 0V. ,
Overcurrent protection circuit does not work. In order to solve this problem, the conventional example shown in Figure 2 supplies the B voltage of the overcurrent protection circuit using a small-capacity auxiliary power supply, and the switching power supply is operated while the load is being shunted. Even if the overcurrent protection circuit is turned on, the overcurrent protection circuit operates without any problem because the stable B voltage is supplied by the auxiliary power supply device.

However, an auxiliary power supply device is required, and the number of parts increases, which is disadvantageous in reducing the size and cost of the switching power supply device. According to this embodiment,
Even when the AC power is turned on with the load 11 in short state, the voltage generated in the overcurrent detection resistor 20 allows current to flow through the photodiode in the photocoupler without going through the overcurrent comparison and amplification circuit, resulting in stable overcurrent. A current protection circuit can be realized.

Therefore, the same performance as the circuit configuration using a separate DC power supply shown in FIG. 2 can be achieved, contributing to miniaturization and cost reduction of the switching power supply.

Effects of the Invention As described above, according to the present invention, there is no problem in a multi-output switching power supply device equipped with a control circuit in which the power source obtained by rectifying the output pulse voltage of the main switching output transformer is used as the B power source. It is now possible to configure an overcurrent protection circuit. Therefore, the switching power supply device having the above circuit configuration can be easily used, and excellent effects can be obtained in terms of size reduction and cost reduction.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of a conventional switching power supply, Fig. 2 is a circuit diagram of a conventional switching power supply with auxiliary power supply, and Fig. 3 is a circuit diagram of a switching power supply according to an embodiment of the present invention. be. 1... AC power supply, 2... Diode, 6... Primary side control circuit, 7... Switching output transistor, 8... Main switching output transformer,
10, 12... Rectifier smoothing circuit, 11, 13... Load, 14... Voltage adjustment circuit, 16... Overcurrent detection circuit, 17... Photocoupler, 18... Output voltage cutoff circuit.

Claims (1)

[Claims] 1. A switching output circuit is connected to one terminal of the primary winding of a switching transformer, and a DC voltage obtained by rectifying and smoothing an AC power source is connected to the other terminal of the primary winding. A control circuit for controlling the above-mentioned switching output circuit, and a switching circuit for stopping the operation of this control circuit by conduction of the phototransistor in the output section of the photocoupler, A rectifier and smoothing circuit is connected to one terminal, a load is connected between the output terminal of the rectifier and smoothing circuit and a reference potential, and a current detection resistor and the input section of the photocoupler are connected to the other terminal of the secondary side main output winding. The cathode terminal of the photodiode is connected to the anode terminal of the photodiode, and the protection switching resistor and the plurality of two of the switching transformers are connected to the anode terminal of the photodiode.
The output terminal of the overcurrent comparison amplifier of the sub-output circuit consisting of the next-side sub-output winding and the rectifying and smoothing circuit was connected, and one terminal of the above-mentioned current detection resistor and protection switching resistor was connected to the reference potential. A switching power supply device characterized by: