JPS5814705Y2 - switching power supply circuit - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a switching power supply circuit used in various electronic devices, and in particular detects the presence or absence of circuit components to prevent destruction of switching elements and the like.

Generally, a switching power supply circuit performs a soft start operation to prevent destruction of switching elements due to rush current at startup.

FIG. 1 shows the configuration of a conventional switching power supply circuit. For example, in FIG. Line 6a and switching transistor 3
It is grounded via the main electrode and resistor 14 and connected to the starting circuit 4, soft start circuit 5, error amplifier 6, comparator 7, driver 8, lamp signal generation circuit 9, and protection circuit such as overcurrent detection circuit 10. It gives power when starting.

The secondary winding 2b of the transformer 2 is connected to the rectifier circuit 11,
A stabilized DC output voltage is obtained on the output side of the rectifier circuit 11.

Also, a part of the DC output voltage of the rectifier circuit 11 is transferred to the detection circuit 26.
isolator 12 such as a photocoupler or transformer for insulating between the primary and secondary sides of the transformer 2.
is supplied to an error amplifier 6 through which it is compared with a reference voltage to produce an error signal.

Further, the transformer 2 has a tertiary winding 2C, the output end of which is connected to each circuit after the error amplifier 6 via a diode 13, and the DC voltage rectified by the diode 13 is applied to these circuits in a steady state. Give it as a power source.

The output side of the starting circuit 4 is connected to the input side of the soft start circuit 5, and the output side of the soft start circuit 5 is connected to the output side of the error amplifier 6 and to one input end of the comparator 7. The output side of the ramp signal generating circuit 9 is connected to the other input terminal of the ramp signal generating circuit 9.

In addition, the input side of the overcurrent detection circuit 10 is connected to the emitter resistor 14 of the switching transistor 3, and the voltage generated in this resistor is compared with a reference voltage.
The output side is connected, for example, to an electronic switch 15 provided between the comparator 7 and the driver 8, and in the event of an overcurrent, a signal of, for example, a low bell is generated on the output side to open the electronic switch. .

Next, the operation of the conventional switching power supply circuit configured as described above will be explained with reference to the signal waveforms shown in FIGS. 2 and 3.

Now, when the power is turned on, the starting circuit 4 is activated, and the potential at the connection point A is increased by the capacitor 41 toward the operation start voltage V5 as shown in FIG. 2A.

During this time, the Zener diode 42 of the starting circuit 4 turns off the transistors 43, 44, and 45, so the transistor 51 of the soft start circuit 5 turns on, and the transistor 52 turns on.
Also turns on.

Therefore, the current from the node A flows through the transistor 52, the diode 53, and the resistor 54, so that the potential at the node B also increases in substantially the same manner as that at the node A, as shown in FIG. 2B.

Thereafter, the potential of the capacitor 41 gradually decreases toward the stable operating voltage (2), but transistors 43 and 44 turn on when they reach a predetermined bias level, and 45 also turns on, so that the transistors of the soft start circuit 5 51.5
2 is turned off, and as a result, the potential at the connection point B gradually decreases toward the stable operating voltage VR by the soft start capacitor 55.

Therefore, since the voltage at the connection point B that changes in this way is supplied to one input terminal of the comparator 7, the output side of the comparator 7 is A pulse width modulated (PWM) signal as shown in FIG. 2C, whose pulse width gradually changes, is supplied to the oscillator.

It can be seen that the driver 8 drives the switching transistor 3 by this pulse width modulation signal, so that an emitter current as shown in the second diagram is obtained at the connection point, and the switching transistor 3 starts operating slowly.

Therefore, by connecting this operation, a stable DC output voltage can be obtained on the output side of the rectifier circuit 11.

Further, when an abnormal condition occurs, for example, an overcurrent, this overcurrent is detected by the overcurrent detection circuit 10, and the electronic switch 15 is opened and the switching operation is stopped.

By the way, in a conventional switching power supply circuit having such a configuration, if a circuit component such as the soft start capacitor 55 of the soft start circuit 5 is not installed due to mismounting or the like, the switching transistor 3 may be damaged due to the inrush current when the power is turned on. It will be damaged.

That is, if the soft start capacitor 55 is not attached, the potential at the connection point B will rapidly drop from the operation start voltage level as shown in FIG. 3B.

Therefore, when the potential at the connection point B suddenly decreases and its level is low, a signal of this level is transmitted to the ramp signal generation circuit 9.
Since the point where the ramp signal intersects with the ramp signal from the waveform is the low part of the waveform, that is, the point where the pulse width is large, a pulse width modulated signal with a large duty ratio is obtained on the output side of the comparator 7, as shown in FIG. 3C.

As a result, a steep inrush current as shown in the third diagram flows through the switching transistor 3, resulting in damage.

Since the inrush current at this time is very fast, it is difficult to prevent it even if the overcurrent detection circuit 10 is provided.

The present invention has been made in view of these points, and provides a switching power supply circuit that can prevent damage to switching elements and the like due to missing circuit components.

An embodiment of the present invention will be described in detail below with reference to FIGS. 4 to 6.

In FIGS. 4 to 6, parts corresponding to those in FIGS. 1 to 3 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

In the example shown in FIG. 4, a transistor 16 is provided on the output side of the starting circuit 4, the base of the transistor 16 is connected to the transistor 45, the emitter is grounded, the collector is connected to the connection point A, and the transistor 16 is connected to the transistor 45 via the inverter 17. It is connected to a pulse generating means, for example a monostable multivibrator 18, which generates a single pulse in response to a reset pulse generated via the inverter 17 at power-up.

The output side of monostable multi-by break 18 is AND circuit 1
A comparator 20 is connected to one input terminal of the AND circuit 19 and the other input terminal of the AND circuit 19, and one input terminal of the comparator 20 can detect the presence or absence of a predetermined circuit component, such as a soft start capacitor 55. The other input terminal is connected to a reference potential power source 21 that is compared with the potential of the connection point B.

Further, the output terminal of the AND circuit 19 is connected to one input terminal of a NOR circuit 22, and the other input terminal of this NOR circuit 22 is connected to the output side of a protection circuit, for example, an overcurrent detection circuit 10.
The output end is connected to the input side of the memory 23.

The output side of the memory 23 is connected to the electronic switch 14 via an inverter 24, and opens and closes this switch depending on the output.

The memory 23 is also cleared by a reset pulse supplied through the inverter 17 when the power is turned on.

Further, the power terminals of the monostable multivibrator 18 and the comparator 20 are connected to the output terminal of the diode 13 and also connected to the connection point A to receive power.

The rest of the structure is the same as in FIG. Next, the operation of the switching power supply circuit according to the present invention constructed as described above will be explained with reference to the signal waveforms shown in FIGS. 5 and 6.

Now, considering the case where the soft start capacitor 55 is installed, when the power is turned on, the potentials at connection points A and B change as shown in FIGS. 5A and B, respectively, in the case of FIGS. 2A and B. It is similar to

Next, when the starting circuit 4 starts operating, the transistor 16 turns on, thereby resetting the memory 23, and the monostable multivibrator 18 generates a single pulse as shown in FIG. Supplied to the input end.

The pulse width of this single pulse is set to be shorter than the time it takes to reach the stable operating voltage VR.

On the other hand, the potential at connection point B is, for example, the detection level voltage ■.

A capacitor 55 is connected to the output side of the converter 20 to detect the
A high level signal, as shown in FIG. 5F, is generated indicating the presence of.

Therefore, signals at the levels shown in FIG. 5G and H are obtained on the output sides of the AND circuit 19 and the NOR circuit 22, respectively.

The low level signal from the NOR circuit 22 is sent to the next stage memory 2.
3, and this memory 23, while storing the low level signal, energizes the electronic switch 15 by the inverted signal through the inverter 24 and passes the pulse width modulated signal from the comparator 7 through the driver 8 to the switching transistor. 3, thereby maintaining normal switching operation.

Next, considering the case where the software capacitor 55 is not attached, the potentials at connection points A and B change as shown in FIGS. 6A and B when the power is turned on, as in the case of FIGS. 3A and B. The same is true.

When the capacitor 55 is attached, the memo 23 is similarly reset by the start of the operation of the starting circuit 4, and a single pulse as shown in FIG. 6E is generated from the monostable multivibrator 18.

On the other hand, on the output side of the comparator 20 that detects the potential at the connection point B, for example, the detection level voltage VD, a low level as shown in FIG. A level signal is generated.

Therefore, signals at the levels shown in FIG. 5G and H are obtained on the output sides of the AND circuit 19 and the NOR circuit 22, respectively.

The high level signal from the NOR circuit 22 is sent to the next stage memory 23.
While this memory 23 stores the high level signal, the signal inverted through the inverter 24 deactivates the electronic switch 15 to cut off the pulse width modulated signal from the comparator 7 to the driver 8; The switching transistor 3 is thus protected from the danger of inrush currents.

As is clear from the above description, the switching power supply circuit according to the present invention responds to a predetermined single pulse generated after the power is turned on and a predetermined detection signal generated depending on the presence or absence of the object under test. By configuring the switching operation to be controlled, it is possible to prevent damage to the switching elements and the like due to missing circuit components.

In addition, in the above embodiment, the case where a monostable multivibrator is used as a means for generating a predetermined single pulse has been explained, but the invention is not limited to this, and any circuit that generates the above-mentioned pulse after power is turned on can be used. Anything is fine.

Furthermore, although the case of voltage has been described as a detection signal corresponding to the presence or absence of a circuit component, other detection signals such as current may also be used.

Furthermore, the signal levels of the single pulse and detection signal, and the circuits that process them, are not limited to those of the above-described embodiments, and similar effects can be obtained by using other signal levels and processing circuits. Needless to say.

Furthermore, although the electronic switch 15 is provided on the output side of the comparator 7 to control its output, it may be provided in the power supply path of the comparator 7, the driver 8, etc. so as to cut off the so-called control circuit power supplies.

Furthermore, although the circuit component described above is for detecting a soft-start capacitor, the same effect can be obtained in the case of detecting other circuit components whose absence may cause destruction of switching elements, etc. I can do it.

Furthermore, in addition to the presence or absence of circuit components, extreme differences in the constants of circuit components can also be detected in the same manner.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram showing an example of a conventional switching power supply circuit, Figures 2 and 3 are signal waveform diagrams for explaining the operation of Figure 1, and Figure 4 is an implementation of the switching power supply circuit of the present invention. A configuration diagram showing an example, and FIGS. 5 and 6 are signal waveform diagrams for explaining the operation of FIG. 4. In the figure, 3 is a switching transistor, 4 is a starting circuit, 5 is a soft start circuit, 6 is an error amplifier, 7 is a comparator, 8 is a driver, 9 is a ramp signal generation circuit, 1
0 is an overcurrent detection circuit, 15 is an electronic switch, 18 is a monostable multivibrator, 20 is a comparator, and 23 is a memory.

Claims (1)

[Scope of utility model registration request] A starting circuit that outputs a control signal at a predetermined value of an input voltage that gradually increases with a predetermined time constant when the power is turned on, and software that is controlled by the control signal of the starting circuit and includes at least a capacitor that starts charging with the input voltage. A start circuit, a comparator circuit that compares the charging voltage of the capacitor of the soft start circuit with a reference voltage, and a comparator circuit that switches the input voltage to supply current to the primary side of the transformer, and a stabilized current to the secondary side of the transformer. a switching element that obtains a DC output voltage, and a gate circuit that selectively supplies a pulse width modulation signal having a pulse width corresponding to a change in the DC output voltage to the switching element; A switching power supply circuit characterized by controlling the gate circuit described above.