JPH0611187B2 - Switching power supply - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply, in which a duty ratio set value of a maximum duty ratio setting circuit provided in a control circuit for giving pulse width control to a switching element is set to an overcurrent. By controlling by the output of the protection circuit, the responsiveness and the detection accuracy are excellent, and the circuit can be surely protected from damage due to load short circuit or sudden load change.

2. Description of the Related Art FIG. 6 is a diagram showing a general configuration of a conventional switching power supply of this type. A main switching element composed of a transistor or a field effect transistor, 2 is a transformer for power conversion, 3 is a DC power source obtained by rectifying and smoothing a commercial AC power source, 4 is an output circuit, 5 is a load The DC power supplied from the DC power supply 3 through the winding 21 of the transformer 2 is switched by the switching element 1, and the switching output is taken out from the winding 21 side of the transformer 2 to the winding 22 side and wound. The output circuit 4 connected to the line 22 rectifies and smoothes it to convert it into DC power, and supplies DC power to the load 5.

6 is an output voltage error detection circuit, 7 is an overcurrent protection circuit, 8 is a pulse width modulation circuit, 9 is a maximum duty ratio setting circuit, 1
Reference numeral 0 is a drive circuit, which constitutes a control circuit for controlling the switching operation of the switching element 1.

The output voltage error detection circuit 6 divides the output voltage V ₀ supplied to the load 5 by the resistors 61 and 62 to divide the output voltage V ₀ into the error amplifier 6
3 is input to one input end of the error amplifier 63, and the error component with the reference voltage source 64 input to the other input end of the error amplifier 63 is amplified,
Output through the diode 65.

The overcurrent protection circuit 7 includes a current detection transformer 71 that detects the pulse current Ic flowing through the switching element 1, and the pulse current Ic flowing through the primary winding 71a thereof is connected between the secondary winding 71b and its terminal. The voltage is converted into a voltage signal by the resistor 72 and detected, and the peak value of the detected voltage signal is peak-held by the diode 73 and the capacitor 74. The peak-held value is input to the amplifier 75 and supplied from the reference voltage source 76. The difference from the reference voltage is amplified and output through the diode 77.

The output terminal of the output voltage error detection circuit 6 and the output terminal of the overcurrent protection circuit 7 are wired OR connection, and the wired OR output is input to the pulse width modulation circuit 8.

The pulse width modulation circuit 8 includes an oscillator 81 that generates a triangular wave having a constant period, comparators 82 and 83, a NOR circuit 84, and the like.

The maximum duty ratio setting circuit 9 divides the DC operating power supply Vcc by the resistors 91 and 92, and supplies the divided voltage to the comparator 82 as the maximum duty setting voltage E ₉ . The comparator 82 compares the maximum duty setting voltage E ₉ given from the maximum duty ratio setting circuit ₉ and the triangular wave E ₈₁ given from the oscillator 81, as shown in FIG. As shown in, the maximum duty setting voltage E ₉ produces a comparative output E ₈₂ which becomes high level in the range exceeding the triangular wave E ₈₁ . This comparison output E
₈₂ is inverted by the NOR circuit 84 as shown in FIG. 7 (c) and is given to the control input terminal of the switching element 1 through the drive circuit 10 to set the maximum duty ratio (τ / T) of the switching element 1. It

In the comparator 83, the logical sum input Eor of the signals given from the output voltage error detection circuit 6 and the overcurrent protection circuit 7
And the triangular wave E ₈₁ given by the oscillator 81,
A comparison output E ₈₃ that is high in the range where the OR input Eor exceeds the triangular wave E ₈₁ is generated. The comparison output E ₈₃ is inverted by the NOR circuit 84 and input to the switching element 1 through the drive circuit 10, and the duty ratio of the switching element 1 is pulse width controlled to a value according to the output current and the output voltage.

Therefore, the NOR circuit 84 uses the pulse width modulated by the maximum duty ratio setting circuit 9 and the comparator 82 and the pulse width modulated by the output voltage error detection circuit 6, the overcurrent protection circuit 7 and the comparator 83 as input conditions, and negates it. Performs a logical sum operation. The switching element 1 is
The NOR output of the NOR circuit 84 is the narrower pulse width of the pulse widths of the overcurrent protection circuit 7 or the output voltage error detection circuit 6 and the maximum duty ratio setting circuit 9 which gives the maximum duty. As described above, the pulse width is controlled.

Problems to be Solved by the Invention As described above, conventionally, the pulse width by the overcurrent protection circuit 7 or the output voltage error detection circuit 6 and the pulse by the maximum duty ratio setting circuit 8 provided independently of these Since the circuit configuration is such that the pulse width of the switching element 1 is controlled so that the narrower pulse width of the width is obtained, the following problems occur.

(A) When configuring the overcurrent protection circuit 7, the diode 73, the capacitor 74, and the amplifier 75 that configure the peak hold circuit are indispensable, and the charging time of the peak hold capacitor 74, the slew rate characteristic of the amplifier 75, etc. Affected, poor responsiveness, and unable to follow abrupt load changes. Therefore, an overcurrent that exceeds the safe operation area of the switching element 1 may temporarily flow, and the switching element 1 may be damaged or destroyed.

(B) The amplifier 7 has the overcurrent drooping characteristic of the output current and the output voltage.
It depends on the gain of 5, and its improvement is limited. For example, as shown in FIG. 8, it has an overcurrent drooping characteristic with a tail.

Means for Solving the Problems In order to solve the above-mentioned conventional problems, the present invention provides a transformer for power conversion, a switching element for switching a DC input provided through an input winding of the transformer, In a switching power supply comprising a circuit for converting a switching output extracted to an output winding of the transformer into a direct current, and a control circuit for applying pulse width modulation control to the switching element, the control circuit includes the switching element. A maximum duty ratio setting circuit for setting a maximum duty ratio of the above, and an overcurrent protection circuit for detecting a load current or a current proportional thereto and controlling a duty ratio setting value of the maximum duty ratio setting circuit when an overcurrent is detected. It is characterized by

The pulse width control by the output voltage error detection circuit is performed separately from the pulse width control by the overcurrent protection circuit and the maximum duty ratio setting circuit.

Action Since the switching power supply according to the present invention is configured as described above, the maximum duty ratio of the switching element is set by the maximum duty ratio setting circuit to prevent saturation of the power conversion transformer, and at the same time when overcurrent is detected. The duty ratio setting value of the maximum duty ratio setting circuit can be controlled by the overcurrent detection signal from the overcurrent protection circuit in the direction in which the pulse width becomes narrower to perform the overcurrent protection.

The control of the duty setting value by the overcurrent protection circuit may be performed by controlling the maximum duty setting voltage of the maximum duty ratio setting circuit, and the conventional peak hold circuit and amplification circuit are unnecessary. Therefore, there is no response delay due to the charging time of the peak hold capacitor and the slew rate characteristic of the amplifier, and the response is fast,
It becomes possible to sufficiently follow a sudden change in load.

Further, the overcurrent drooping characteristic is not influenced by the gain of the amplifier, and a sharp overcurrent drooping characteristic which does not have a tail is obtained.

First Embodiment FIG. 1 is a block diagram of a switching power supply according to the present invention, and the same reference numerals as those in FIG. 6 denote the same components. Reference numeral 11 is a maximum duty ratio setting circuit, and 12 is an overcurrent protection circuit.

The maximum duty ratio setting circuit 11 divides the DC power supply Vcc by a resistance voltage dividing circuit, inputs the maximum duty setting voltage E ₁₁ to the comparator 82 of the pulse width modulation circuit 8, and inputs the triangular wave E ₈₁ from the oscillator _81. Compare with.
In this case, the maximum duty ratio setting and the pulse width modulation resulting therefrom are the same as in the conventional case.

The overcurrent protection circuit 12 includes a current detection circuit 121 that detects a pulse current Ic flowing in the switching element 1 or a current proportional thereto, and an overcurrent detection circuit 122 that detects an overcurrent from a current detection signal of the current detection circuit 121. It is composed of

When the overcurrent detection circuit 122 detects an overcurrent, the detection signal is given to the maximum duty ratio setting circuit 11.
As a result, the voltage division ratio of the maximum duty ratio setting circuit 11,
That is, the duty ratio setting voltage E ₁₁ is controlled so that the pulse width becomes narrower, and an overcurrent protection action is obtained.

FIG. 2 is a diagram for explaining the maximum duty setting by the maximum duty ratio setting circuit 11 and the duty ratio setting value control operation by the overcurrent protection circuit 12. Overcurrent protection circuit 1
When there is no detection signal from 2, the triangular wave E of the oscillator 81
₈₁ and the set voltage E _{11 of the} maximum duty ratio setting circuit 11 are compared by the comparator 82, and become high level in the range where the set voltage ₁₁ is higher than the triangular wave E ₈₁ (FIG. 2B).
The comparator output E ₈₂ as shown in (1) is obtained, and the maximum duty ratio is set by the circuit operation similar to the conventional one.

However, when the pulse current Ic flowing through the switching element 1 becomes an overcurrent state due to a load short circuit or the like, it is detected by the current detection circuit 121 and the overcurrent detection circuit 122, and the detection signal outputs the duty setting of the maximum duty ratio setting circuit 11. The voltage E ₁₁ rises as shown by the dotted line in FIG. Therefore, the time width in which the comparator output E ₈₂ is at the high level extends from the time width τ ₁ when the maximum duty is set to the time width τ ₂ , as shown in FIG. 2 (b). Since the NOR circuit 84 is connected to the subsequent stage of the comparator 82, the NOR circuit 84 inverts the comparator output E ₈₂ and outputs the pulse width modulation output E ₈₄ as shown in FIG. 2C.
Is obtained. This pulse width modulation output E ₈₄ is the drive circuit 10
Is applied to the switching element 1 through the pulse width modulation circuit 1, so that the ON time width of the switching element 1 is narrowed to the ON time width τon of the pulse width modulation output E ₈₄ , and the overcurrent control action is obtained.

FIG. 3 is a diagram showing a more specific embodiment of the switching power supply according to the present invention. In this embodiment, the current detection circuit 121 of the overcurrent protection circuit 12 inserts and connects the primary side winding of the current detection transformer 121a in series with the switching element 1 and the terminal of the secondary side winding of this current detection transformer 121a. A resistor 121b is connected between them, and the voltage appearing across the resistor 121b is rectified by the diode 121c and input to the overcurrent detection circuit 122.

In this embodiment, the overcurrent detection circuit 122 includes the transistor 1
An emitter that supplies the voltage signal Vd ₁ of the current detection circuit 121 to the base of 22a and extracts an output signal from the terminal of the capacitor 122b connected to the emitter. Follower. It is configured as a transistor circuit. 122c is a discharge resistor. The output signal Vd ₂ taken out from the terminal of the capacitor 122b passes through the diode 122d, and the maximum duty ratio setting circuit 11
Entered in.

As in the conventional case, the maximum duty ratio setting circuit 11 divides the DC power supply Vcc by the resistors 111 and 112, and sets the divided voltage E ₁₁ as the maximum duty setting voltage to the comparator 82.
However, in the present invention, the output point of the overcurrent protection circuit 12 is connected to the connection point (a) between the resistor 111 and the resistor 112, and the setting obtained at the connection point (a) is set. The voltage E ₁₁ can be controlled by the signal Vd ₂ from the overcurrent protection circuit 12. Reference numeral 113 denotes a capacitor, which performs a soft start operation to gradually widen the pulse width when the input power of the switching power supply is turned on.

In the above circuit configuration, the voltage signal Vd ₁ proportional to the pulse current Ic flowing through the switching element 1 appears in the secondary winding of the current detection transformer 121a that constitutes the overcurrent protection circuit 12. This voltage signal Vd ₁ is applied to the emitter. Follower. It is input to the overcurrent detection circuit 122 formed of a transistor circuit, and the voltage signal Vd ₁ and the base of the transistor 122a. An output voltage Vd ₂ is obtained which is approximately equal to the difference between the emitter voltage Vbe. This output voltage Vd ₂ passes through the diode 122d and the maximum duty ratio setting circuit 11
Therefore, the set voltage E ₁₁ is set to a voltage value depending on the output voltage Vd ₂ , and the above-mentioned overcurrent protection action is performed.

In the case of this embodiment, since the pulse current Ic flowing through the switching element 1 is converted into the voltage signal Vd ₁ by the transformer 121a and detected, the loss becomes smaller as compared with the case of the resistance detection method, and at the same time, The noise margin becomes large. For example, if the maximum duty setting voltage E ₁₁ at a maximum duty setting circuit 11 and about 1.5～2V, the voltage signal Vd ₁ after rectifying the detected voltage of the current detection transformer 121a operates at about 3V (peak value), the foreign A circuit having a very large noise margin with respect to noise or the like can be configured.

Further, since the maximum duty ratio setting value can be directly controlled by the detection signal proportional to the peak value of the pulse current Ic flowing through the current detection transformer 121a, there is no time delay on the circuit and the overcurrent drooping characteristic is improved. .

Moreover, the emitter. Follower. Since the duty set value of the maximum duty ratio setting circuit 11 is controlled by the output Vd ₂ of the overcurrent detection circuit 122 formed of a transistor circuit, the response due to the slew rate characteristic or the like as in the case of using the conventional amplifier. Without causing a decrease in sex
Responsiveness becomes very fast, and sufficient overcurrent protection can be obtained even against load short-circuit or sudden load change.

In this embodiment, the output circuit 4 includes the diode 41,
42, a choke coil 43, a capacitor 44, and the like, which is a choke input system. Further, the drive circuit 10 includes a transistor 101 driven by the pulse width modulation signal E ₈₄ output from the pulse width modulation circuit 8.
And the input winding is connected to the transistor 101 and the output winding is connected to the base drive circuit of the main switching element 1. And a transformer 102. Further, the output voltage error detection circuit 6 gives its output to the comparator 83 and is independent of the pulse width control system by the overcurrent protection circuit 12 and the maximum duty ratio setting circuit 11.

FIG. 4 is an electric circuit diagram of a main part in another embodiment of the switching power supply according to the present invention. This embodiment shows an overcurrent control technique suitable for a multi-output type switching power supply. That is, the transformer 2 has a plurality of output windings 22
a and 22b are provided, and when the output circuits 4a and 4b are connected to the output windings 22a and 22b to form a multi-output switching power supply, a current detection transformer is included in the circuit loop including the output windings 22a and 22b. 121a is provided respectively, the current detection circuit 121 is configured for each output, and the output end of the current detection circuit 121 is wired-OR connected to be input to the overcurrent detection circuit 122. 5a and 5b are loads of each output.

In the case of the embodiment of FIG. 4, the fact that any one of the plurality of outputs has become an overcurrent is individually detected by the current detection circuit 121 individually provided for each output system, and Since a logical sum can be input to the common overcurrent detection circuit 122 to perform overcurrent protection, the overcurrent protection circuit for a multi-output type can be simplified.

FIG. 5 is an electric circuit diagram in still another embodiment. In this embodiment, the voltage appearing in the output winding 22 of the transformer 2 is rectified by the diode 13 and divided by the resistors 14 and 15 to be added to the current detection circuit 121. The variation of the detected current value can be compensated.

As described above, the present invention, in the pulse width control type switching power supply, the duty ratio setting value of the maximum duty ratio setting circuit provided in the control circuit for applying the pulse width control to the switching element, Since the control is performed by the output of the overcurrent protection circuit, it is possible to provide a switching power supply which is excellent in responsiveness and detection accuracy and can surely protect the circuit from damage at the time of load short circuit or sudden load change.

[Brief description of drawings]

FIG. 1 is a block diagram of a switching power supply according to the present invention,
FIG. 2 is a waveform diagram for explaining the maximum duty setting by the maximum duty ratio setting circuit of the switching power supply according to the present invention and the duty ratio set value control operation by the overcurrent protection circuit, and FIG. 3 is for the switching power supply according to the present invention. FIG. 4 is an electric circuit diagram of a more specific embodiment, FIG. 4 is an electric circuit diagram of a main part of another embodiment of the switching power supply according to the present invention, and FIG. 5 is another embodiment of the switching power supply according to the present invention. 6 is an electric circuit diagram showing a general configuration of a conventional switching power supply, FIG. 7 is a waveform diagram showing its maximum duty setting, and FIG. 8 is its output current-output. It is a voltage characteristic diagram. 1 ... Switching element 2 ... Transformer for power conversion 3 ... DC power source, 4 ... Output circuit 5 ... Load, 6 ... Output voltage error detection circuit 8 ... Pulse pulsating modulation circuit 10 ... Drive circuit 11 …… Maximum duty ratio setting circuit 12 …… Overcurrent protection circuit 121 …… Current detection circuit 122 …… Overcurrent detection circuit

Claims (7)

[Claims] 1. A transformer for power conversion, a switching element for switching a DC input given through an input winding of the transformer, and a switching output extracted to an output winding of the transformer to DC. In a switching power supply including a circuit and a control circuit that applies pulse width modulation control to the switching element, the control circuit includes a maximum duty ratio setting circuit that sets a maximum duty ratio of the switching element, and a load current or An overcurrent protection circuit that detects a proportional current and controls the duty ratio setting value of the maximum duty ratio setting circuit when an overcurrent is detected. 2. The overcurrent protection circuit includes a current detection circuit that includes a current detection transformer and converts a current signal into a voltage signal and outputs the voltage signal, and an emitter-driven base that is driven by the voltage signal output from the current detection circuit. Follower. The switching power supply according to claim 1, further comprising a transistor circuit, wherein a duty ratio setting value of the maximum duty ratio setting circuit is controlled by an output of the transistor circuit. 3. The switching power supply according to claim 2, wherein the transistor circuit has a circuit configuration in which a capacitor is connected to an emitter terminal. 4. The switching power supply according to claim 2, wherein the current detection transformer is inserted in a circuit loop including the switching element and the input winding of the transformer. . 5. The switching power supply according to claim 2, wherein the current detection transformer is inserted in a circuit loop including the output winding of the transformer. 6. The transformer includes a plurality of output windings independent of each other, the current detection transformer is provided for each of the output windings, and a logical sum of voltage signals output from the current detection circuits, The switching power supply according to claim 5, wherein the switching power supply is input to the overcurrent detection circuit. 7. The switching according to claim 2, wherein the current detection transformer has a signal proportional to the DC input voltage superimposed on one end of a winding on the voltage signal output side. Power supply.