JP2878029B2 - Switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse width control type switching power supply, and more particularly to load protection of a pulse width control type switching power supply when an automatic recovery type overcurrent protection circuit operates.

[0002]

2. Description of the Related Art An example of a block diagram of a conventional pulse width control type switching power supply device is shown in FIG. 4 and a signal waveform diagram of a main part of the conventional example shown in FIG. 4 is shown in FIG. 4, reference numeral 1 denotes an input voltage source, reference numeral 2 denotes a transformer, and reference numeral 3.
Is a switching transistor, 4 is a rectifier diode, 5 is a smoothing capacitor, 6 is an output voltage terminal (+), 7 is an output voltage terminal (-), 8 is a load of a switching power supply, 9 is a photocoupler. , 17 are control resistors for controlling the current flowing through the photocoupler 9, 10 is an AND circuit, 11 is for pulse width control (P
WM) Comparison circuit, reference numeral 12 is a fundamental wave oscillation circuit, reference numeral 13
Denotes an overcurrent protection circuit, reference numeral 15 denotes a circuit operation switch, and reference numeral 16 denotes a power supply of the control circuit.

The fundamental wave oscillating circuit 12 includes first and second comparators 21 and 22, a first reference power supply 23 connected to the inverting input of the first comparator 21, and a second comparator 22. , A second reference power supply 24 and a first comparator 2
1 is a set signal and the output signal of the second comparator 22 is a reset signal.
, A frequency setting capacitor 29, and a first and a first for supplying a charging current and a discharging current of the frequency setting capacitor 29, respectively.
It has second constant current sources 26 and 27 and a first switch circuit 28 that is closed when the non-inverted output of the first flip-flop (RS type) 25 is at a high level and is open when it is at a low level. doing. The overcurrent protection circuit 13 is
A third comparator 31 that inputs a fundamental wave signal generated by the fundamental wave oscillation circuit 12 to an inverting input terminal and inputs a third reference power supply 32 to a non-inverting input terminal;
A current detection resistor 36 for converting the source current of
A fourth comparator 34 for inputting a detection voltage generated in the current detecting resistor 36 to a non-inverting input and inputting a fourth reference power supply 35 to an inverting input; an output signal of the fourth comparator 34 as a set signal; And a second flip-flop 33 that uses the output signal of the third comparator 31 as a reset signal.

[0004] The operation of the conventional switching power supply device shown in FIG. 4 will be described. When the power supply voltage is applied to each block by closing the circuit operation switch 15,
The photocoupler 9 detects an output voltage output between the output voltage terminals (+) 6 and (−) 7 and outputs a voltage proportional to the output voltage to an inverting input of the pulse width control (PWM) comparison circuit 11. On the other hand, the fundamental wave signal of the fundamental wave oscillation circuit 12 is inputted to the non-inverting input of the pulse width control comparison circuit 11.

Here, the operation of the fundamental wave oscillation circuit 12 is as follows. Both the non-inverting input of the first comparator 21 and the inverting input of the second comparator 22 are connected to the frequency setting capacitor 2.
9 and the reference voltage V _H of the first reference power supply 23 connected to the inverting input of the first comparator 21.
2 is set to a higher potential than the reference voltage _{VL of} the second reference power supply 24 connected to the non-inverting input. Now, the output voltage of the non-inverted output of the first flip-flop 25 is low,
When the switch circuit 28 is in an open circuit state, the second constant current source 27 is in an open circuit state.
Constant current I ₁ of 6 flows as charging current to the frequency setting capacitor 29, the potential of the capacitor 29 rises, and reaches up to the reference voltage V _H of the first reference power supply 23 output voltage of the first comparator 21 Is at a high level, a set signal is input to the first flip-flop 25, and the non-inverted output of the flip-flop 25 is at a high level,
Is in a closed circuit state. When the constant current I _{2 of} the second constant current source 27 is set and I ₂ is set so that I ₂ > I ₁ , the constant current I ₂ of the _second constant current source and the constant current I of the first constant current source are set. _Since the difference current I ₂ −I _{1 of 1} is discharged from the frequency setting capacitor 29 as a discharge current, the potential of the capacitor 29 continues to decrease and the second reference power supply 24
Reference voltage V _L until it reaches the output voltage of the second comparator 22 is reset signal to the first flip-flop 25 becomes high level input, a non-inverting output of the flip-flop 25 is at a low level, First switch circuit 2
8 is in an open circuit state and the above operation is repeated. Therefore,
The fundamental wave signal of the fundamental wave oscillating circuit 12 has amplitude V _H −V _L ,
A triangular fundamental signal having a charge time constant (V _H -V _L ) · C _T / I ₁ and a discharge time constant (V _H -V _L ) · C _T / I ₂ is obtained. Where _CT is the frequency setting capacitor 2
9 is the capacitance value. Next, the fundamental wave signal of the fundamental wave oscillating circuit 12 is compared with the output voltage of the photocoupler 9 by the pulse width control comparing circuit 11. If the voltage of the fundamental wave signal is smaller than the output voltage of the photocoupler 9, the low level On the other hand, if the voltage of the fundamental signal is higher than the output voltage of the photocoupler 9, a high-level signal is applied to one input of the AND circuit 10.

The operation of the overcurrent protection circuit 13 is as follows. The source current of the switching transistor 3 is detected by the current detecting resistor 36, and the detected voltage is input to the non-inverting input of the fourth comparator 34. A fourth reference power supply 35 set to operate within a range where the switching transistor 3 is not destroyed is input to the inverting input of the fourth comparator 34. If the detected voltage is lower than the reference voltage of the fourth reference power supply 35, that is, in a normal operation state, the output voltage of the fourth comparator 34 becomes low level, so that the second flip-flop (RS type) 33 Is also at a low level, the inverted output of the flip-flop 33 is at a high level, and a high-level signal is input to the other input of the AND circuit 10. On the other hand, when the source current of the switching transistor 3 increases and the detection voltage of the current detection resistor 36 becomes higher than the reference voltage of the fourth reference power supply 35, the output voltage of the fourth comparator 34 becomes high level, Since the set signal is input to the second flip-flop 33, the inverted output of the flip-flop 33 is at the low level, and a low-level signal is input to the other input of the AND circuit 10. The output voltage becomes low level, and the switching transistor 3 is cut off. Here, the reset signal of the second flip-flop 33 is obtained as follows. Fundamental wave oscillation circuit 1
The second fundamental wave signal is input to the inverting input of the third comparator 31, and the third reference power supply 32 is connected to the non-inverting input. The reference voltage V _L ′ of the third reference power supply 32 is
If the potential is set slightly higher than the reference voltage V _{L of}
Since the output voltage of the third comparator 31 becomes a high level near the peak value on the lower side of the fundamental wave signal of the fundamental wave oscillation circuit 12, a reset signal is input to the second flip-flop 33. That is, the reset signal of the second flip-flop 33 is always generated once for each period of the fundamental wave signal of the fundamental wave oscillation circuit 12.

Therefore, as described above, in the normal operation state, the output voltage of the photocoupler 9 and the fundamental wave oscillation circuit 1
The pulse width control comparison circuit 11 compares the fundamental signal with the output signal of the photocoupler 9 and the other input of the AND circuit 10 is always at a high level. And
If the voltage of the fundamental wave signal is smaller than the output voltage of the photocoupler 9, the switching transistor 3 is cut off, and the conduction time of the switching transistor 3 is set in accordance with the output voltage between the output voltage terminals (+) 6 and (-) 7. By controlling, a stable output voltage is generated between the output voltage terminals (+) 6 and (-) 7.

In an abnormal operation state in which the source current of the switching transistor 3 becomes excessive, as described above,
Since the inverted output of the second flip-flop 33 goes low and is input to the other input of the AND circuit 10,
The switching transistor 3 is shut off to protect this transistor from destruction. As described above, the reset signal is input to the reset terminal of the second flip-flop 33 every cycle, so that the overcurrent protection circuit 13 operates every cycle of the fundamental wave signal of the fundamental wave oscillation circuit 12. A reset signal is applied, and the overcurrent protection is released in each cycle.

[0009]

In the conventional switching power supply of the pulse width control system, the abnormality of the source current of the switching transistor 3 is detected by the overcurrent protection circuit 13 every one cycle of the fundamental wave signal of the fundamental wave oscillation circuit 12. , The operation is stopped, and the operation is stopped. Therefore, until the source current of the switching transistor 3 reaches the normal operation state,
The operation of the overcurrent protection circuit 13 is repeatedly performed.

However, when the overcurrent protection circuit 13 operates and the switching transistor 3 is cut off, the overcurrent protection circuit 13, the AND circuit 10, and the switching transistor 3 each have a delay time. There is always a delay time between the detection of the excessive current and the turning off of the transistor 3. If the period of the fundamental wave signal of the fundamental wave oscillation circuit 12 is sufficiently long in comparison with the delay time, the energy stored in the secondary side of the transformer 2 is completely discharged, so that the output terminal (+) When the output voltage between 6 and (-) 7 decreases, the load current of the load 8 does not increase. That is,
The relationship between the output voltage between the output voltage terminals (+) 6 and (−) 7 and the output current of the load 8 has a drooping characteristic or a square-shaped characteristic. On the other hand, if the period of the fundamental wave signal of the fundamental wave oscillation circuit 12 is short and the above-mentioned delay time is short enough to be not ignored, the energy stored in the secondary side of the transformer 2 is completely discharged. Therefore, even if the output voltage between the output terminals (+) 6 and (-) 7 decreases, the load current of the load 8 increases due to the stored energy. That is, the relationship between the output voltage between the output terminals (+) 6 and (-) 7 and the output current of the load 8 becomes L-shaped, and the load 8 may be destroyed by the influence of the excessive current.

In particular, in recent years, in order to reduce the size and increase the efficiency of the switching power supply device, the period of the fundamental wave signal tends to become shorter and shorter. There is a problem that the load of the power supply device is destroyed. Further, since the switching transistor 3 is in a state where an excessive current continues to flow during the delay time, there is a problem that the thermal design of the switching transistor 3 and the safety operation area have no margin.

Therefore, a first technical problem of the present invention is to prevent a load of a switching power supply from being destroyed by an excessive current in an abnormal operation state in which a source current of a switching transistor increases and an overcurrent protection circuit operates. A switching power supply device is provided.

A second technical problem of the present invention is that even if the above-mentioned delay time exists, a sufficient margin for the thermal design and the safe operation area of the switching transistor can be secured, so that the degree of freedom in design can be increased. It is an object of the present invention to provide a switching power supply device that can perform the switching.

[0014]

A switching power supply device according to the present invention includes an automatic recovery type overcurrent protection circuit that operates with a detection signal that detects an overcurrent and stops operation with a fundamental wave signal of a fundamental wave oscillation circuit. A switching power supply device of a pulse width control type having a time constant conversion switch circuit for changing a time constant of charging and discharging of a fundamental wave signal of the fundamental wave oscillation circuit when the overcurrent protection circuit detects an overcurrent. It is characterized by.

[0015]

In the present invention, when the overcurrent protection circuit detects the overcurrent state of the switching transistor, the time constant conversion switch circuit changes the time constant of the discharge of the fundamental wave signal of the fundamental wave oscillation circuit.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of a switching power supply according to an embodiment of the present invention, and FIGS. 2 and 3 are operation waveform diagrams of the switching power supply shown in FIG. In FIGS. 1 to 3, the same reference numerals are given to the parts common to the conventional example of FIG.

In FIG. 1, reference numeral 1 denotes an input voltage source, reference numeral 2 denotes a transformer, reference numeral 3 denotes a switching transistor, reference numeral 4 denotes a rectifier diode, reference numeral 5 denotes a smoothing capacitor, reference numeral 6 denotes an output voltage terminal (+), and reference numeral 7 denotes Output voltage terminal (-), reference numeral 8 denotes a load of the switching power supply, reference numeral 9
Is a photocoupler, 17 is a control resistor for controlling the current flowing through the coupler, 10 is an AND circuit, 11 is a pulse width control (PWM) comparison circuit, 12 is a fundamental wave oscillation circuit, 13 is an overcurrent. A protection circuit, reference numeral 14 indicates a switch circuit for time constant conversion, reference numeral 15 indicates a circuit operation switch, and reference numeral 16 indicates a power supply of the control circuit.

The fundamental wave oscillating circuit 12 includes first and second comparators 21 and 22, a first reference power supply 23 connected to an inverting input of the first comparator 21, and a second comparator 22. A second reference power supply 24 connected to the non-inverting input of the first comparator 21 and a first flip-flop 25 using the output signal of the first comparator 21 as a set signal and the output signal of the second comparator 22 as a reset signal
A first and a second constant current sources 26 and 27 for supplying a charging current and a discharging current to the first and second frequency setting capacitors 29 and 30 and the frequency setting capacitor, respectively; Has a first switch circuit 28 which is closed when the non-inverted output is at a high level and is open when it is at a low level.

The overcurrent protection circuit 13 is a fundamental wave oscillation circuit 12
A third comparator 31 for inputting a fundamental wave signal generated at the inverting input to a non-inverting input and a third reference power supply 32 for a non-inverting input;
A current detecting resistor 36 for converting a source current of the switching transistor 3 into a voltage, and a fourth comparator 34 for inputting a detection voltage generated at the resistor 36 to a non-inverting input and inputting a fourth reference power supply 35 to an inverting input. And a second flip-flop 33 that uses the output signal of the fourth comparator as a set signal and the output signal of the third comparator as a reset signal. Up to this point, the configuration is the same as that of the conventional example of FIG.

The switching power supply circuit according to the embodiment of the present invention differs from the conventional example in that the switching power supply circuit has a time constant conversion switch circuit 14 and a second frequency setting capacitor 30. The time constant conversion switch circuit 14 operates so that the non-inverted output of the second flip-flop of the overcurrent protection circuit 13 is in a closed circuit state when it is at a high level and an open circuit state when it is at a low level.

In the normal operation state, as described in the conventional example of FIG. 4, the inverted output of the second flip-flop 33 of the overcurrent protection circuit 13 is at the high level and the non-inverted output is at the low level. The constant conversion switch circuit 14 is an open circuit. Accordingly, since the second frequency setting capacitor 30 of the fundamental wave oscillation circuit 12 is in an open circuit state, FIG.
Of As described in the conventional example, the fundamental wave signal of the fundamental wave generating circuit 12, the amplitude V _H -V _L, the charging time constant (V _H -
_{V L) C T1 / I 1} , the discharge time constant _{(V H -V L) C T1} /
(I ₂ −I ₁ ). Here, V _H is the first reference power supply 2
3, the reference voltage V _L is the reference voltage of the second reference power supply 24, V _H > V _L , C _T1 is the capacitance value of the first frequency setting capacitor 29, and I ₁ and I ₂ are the first and _second , respectively. The constant current value of the second constant current source is I ₂ > I ₁ . That is, in the normal operation state, the fundamental wave signal of the fundamental wave oscillation circuit 12 does not depend on the second frequency setting capacitor 30 but depends only on the first frequency setting capacitor 29.

When the source current of the switching transistor 3 increases, the detection voltage of the current detecting resistor 36 of the overcurrent protection circuit 13 becomes higher than the reference voltage of the fourth reference power supply 35, and an excessive current flows, resulting in an abnormal operation state. , The output voltage of the fourth comparator 34 becomes high level, and the set input of the second flip-flop 33 becomes high level. Therefore, the inverted output of the flip-flop 33 is at a low level, the non-inverted output is at a high level, and one input of the AND circuit 10 is at a low level, so that the switching transistor 3 is cut off to prevent the source current from becoming excessive. . In this state, since the non-inverted output of the second flip-flop 33 of the overcurrent protection circuit 13 is at a high level,
The time constant conversion switch circuit 14 is a closed circuit, and the period of the fundamental wave signal of the basic oscillation circuit 12 depends on the sum of the first and second frequency setting capacitors 29, 30. From the point in time when the source current of the transistor 3 is detected to be excessive, the period of the fundamental wave signal of the fundamental wave oscillation circuit 12 becomes longer than in the normal state as shown in FIGS. That is, the time constant at the time of discharge is (V _H −
V _L ) (C _T1 + C _T2 ) / (I ₂ −I ₁ ) and the time constant at the time of charging change to (V _H −V _L ) (C _T1 + C _T2 ) / I _1, and FIG. FIG. 3 shows a case where the setting capacitor has abnormal operation during discharging, and FIG. 3 shows a case where abnormal operation has occurred during charging. Thus, by providing the time constant conversion switch circuit 14 and the second frequency setting capacitor 30 and arbitrarily setting the value of the capacitor 30, the source current of the switching transistor 3 exceeds the specified value. The period of the fundamental wave signal of the fundamental wave oscillation circuit 12 in the abnormal state can be set to an arbitrary value.

[0023]

As described above, according to the present invention, the fundamental wave oscillation circuit is provided in an abnormal operation state in which the source current of the switching transistor is increased due to the provision of the time constant conversion switch circuit and the overcurrent protection circuit operates. The time constant of charge and discharge of the fundamental wave signal can be set to be significantly longer than that in a normal state, so that an overcurrent protection circuit, an AND circuit,
The energy stored in the secondary side of the transformer can be completely discharged for a certain delay time in the switching transistor, so when the output voltage between the output voltage terminals (+) and (-) drops, the load The current does not increase,
The relationship between the load current and the output voltage has a drooping characteristic or a fold-back characteristic, and has an effect that the load of the switching power supply device is not destroyed by an excessive current.

Further, according to the present invention, in the abnormal operation state even if the delay time is present, the period of the fundamental wave signal of the fundamental wave oscillation circuit becomes sufficiently long, and the duty ratio of the delay time to the period of this signal becomes sufficient. Since the switching transistor can be set small, there is an effect that the degree of freedom in design is increased because a sufficient margin is provided for the thermal design and the safe operation area of the switching transistor.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing an example of a signal waveform in each section of the embodiment shown in FIG.

FIG. 3 is a diagram illustrating another example of a signal waveform in each unit of the embodiment illustrated in FIG. 1;

FIG. 4 is a block diagram of a switching power supply circuit according to a conventional example.

5 is a signal waveform diagram in each part of the switching power supply circuit of FIG.

[Explanation of symbols]

 Reference Signs List 1 input voltage source 2 transformer 3 switching transistor 4 rectifier diode 5 smoothing capacitor 6 output voltage terminal (+) 7 output voltage terminal (-) 8 load of switching power supply 9 photocoupler 10 AND circuit 11 pulse width control comparison circuit 12 fundamental wave Oscillation circuit 13 Overcurrent protection circuit 14 Time constant conversion switch circuit 15 Circuit operation switch 16 Power supply of control circuit 17 Control resistor 21 First comparator 22 Second comparator 23 First reference power supply 24 Second reference Power supply 25 First flip-flop 26 First constant current source 27 Second constant current source 28 First switch circuit 29 First frequency setting capacitor 30 First frequency setting capacitor 31 Third comparator 32 Third reference power supply 33 Second flip-flop 34 Fourth comparator 35 Fourth reference power supply

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H02M 3/00-3/44

Claims (1)

(57) [Claims] 1. A pulse width control type switching power supply having an automatic recovery type overcurrent protection circuit which operates with an overcurrent detection signal and stops operation with a fundamental wave signal of a fundamental wave oscillation circuit. A switching power supply device comprising: a time constant conversion switch circuit that changes a time constant of charging and discharging of the fundamental signal based on an overcurrent detection signal of a protection circuit.