JPS6356167A - Overcurrent protecting circuit for switching regulator - Google Patents

Abstract

PURPOSE:To improve reliability, by a method wherein a pulse width is controlled so that the absolute value of a difference between a first overcurrent protecting voltage, proportional to a load output current, and a second overcurrent protect ing voltage, proportional to a load output voltage, is kept at a given value. CONSTITUTION:A transformer 2 is equipped with a primary coil 3 and a secon dary coil 5 while an auxiliary coil 24 is provided in the side of the primary coil. The primary coil 15a of a current transformer 15 and a switching element or a transistor 4 are connected to the primary coil 3 in series. Rectifying and smoothing circuit 8-11 are connected to the secondary coil 5 and a load 12 is provided with a given output voltage Vo from an output terminal. A first overcurrent protecting voltage Va, proportional to a load current 10, is obtained from the secondary coil 15b of the current transformer 15 and a second overcurrent protecting voltage Vb, proportional to the output voltage Vo for the load 12, is obtained from the auxiliary coil 24. A voltage difference between both of the voltages Va, Vb is compared with a reference voltage Vr1 in a comparator 20 and is outputted to a pulse width control circuit 34 to realize an overcurrent protecting characteristic or the characteristic of 7-shape.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an overcurrent protection circuit for a switching regulator that suppresses excessive current flowing in an overload state to prevent damage to the circuit.

(Prior Art) As a conventional switching regulator known as an ON-ON type, for example, the one shown in FIG. 8 is known.

In FIG. 8, 1 is an input voltage source using a rectifier circuit etc. that converts AClooV into a predetermined DC voltage. The voltage is applied to the primary winding 3, and a switching transistor 4 is connected in series to the primary winding 3.

The switching of the transistor 4 is controlled by a pulse width control circuit 34 operating based on a triangular wave valve 1 voltage regulator 36 so as to maintain the load output voltage O at a constant voltage.

On the other hand, the voltage induced in the secondary winding 5 of the transformer 2 is rectified by diodes 8 and 9, and further smoothed by a choke coil 10 and a capacitor 11 to supply a constant output voltage VO to a load 12.

The secondary winding voltage vin, secondary current 1irl, and discharge current of the smoothing circuit of the 0N-ON type switching regulator transformer 2 are as shown in the signal waveform diagram of FIG. 9, and the primary side switching Element (transistor 4)
Assuming that the on time within the period T determined by the on and off of is TOn and the off time is 1-Off, the output voltage Vo becomes a constant voltage given by Vo = VinxTon/T.

On the other hand, in order to control the pulse width of the transistor 4 as a switching element, the output voltage VO is input to an amplifier 13 set to a reference voltage Vr2, and the amplifier 13 is connected to the reference voltage Vr2.
Output the comparison output with r2 to the pulse width control circuit 34,
The transistor on time Ton within the period T is controlled so that the output voltage (1)0 becomes a constant voltage determined by the reference voltage r2.

Furthermore, as an overcurrent protection circuit, a current transformer 15 is connected in series to the primary winding 3, and the detected voltage (secondary voltage) of the current transformer 15 is connected to a resistor 16. A circuit comprising a diode 17 and a capacitor 18 converts the output current (2) into an overcurrent detection voltage Va proportional to 0, which is input to a comparator 20 having a reference voltage Vr1 set.

The operation of such an overcurrent protection circuit is such that when the output current IO exceeds the specified current determined by the reference voltage Vr1, the amplifier 1
3 changes to H level, and the pulse width control circuit 34 narrows the pulse width of the output pulse to the specified pulse width, and limits the output current to the specified current to prevent damage to the circuit due to overcurrent. become.

(Problems to be Solved by the Invention) However, in the overcurrent protection circuit in such a conventional 0N-ON type switching regulator,
Since the current transformer 15 extracts only the overcurrent detection voltage ya proportional to the output current 1o and performs the overcurrent protection operation by current limiting, even if the overcurrent protection operation is performed and the output voltage drops to approximately zero volts. A constant current, which corresponds to the overcurrent limit value, flows through the load, and it was not possible to obtain the so-called F7 characteristic in which the output current also decreases as the output voltage decreases.

Of course, in order to perform an overcurrent protection operation with a unique characteristic between the output voltage and output current, in addition to detecting an overcurrent protection voltage proportional to the output current IO, an overcurrent protection voltage proportional to the output voltage VO must be detected. It is sufficient to detect the current protection voltage, but it is not possible to detect the output voltage on the secondary side, which is isolated by a transformer, and connect it directly to the pulse width control circuit on the primary side, so an isolator such as a photocoupler is required. As a result, the circuit configuration becomes complicated and the cost becomes relatively high.

(Means for Solving the Problems) The present invention has been made in view of such conventional problems, and in addition to the overcurrent protection voltage proportional to the output current of the current transformer, special By detecting the overcurrent protection voltage proportional to the output voltage without using any circuit means and without damaging the insulation between the primary and secondary sides, the overcurrent protection characteristics that are the 7 characteristics of F can be easily obtained. The purpose of this invention is to provide an overcurrent protection circuit for a switching regulator.

In order to achieve this object, the present invention provides a first overcurrent protection voltage ya proportional to the load output current ■0 from the induced voltage of the current transformer connected in series with the primary winding of the transformer.
An auxiliary winding is provided on the primary side of the transformer, and a second overcurrent protection voltage vb proportional to the load output voltage Vo is detected from the induced voltage of the auxiliary winding. The pulse width of the switching element is controlled so that the absolute voltage difference fat Va - Vb I with the overcurrent protection voltage becomes a constant value.

(Function) According to the configuration of the present invention, the first overcurrent protection voltage Va is proportional to the load output current IO from the current transformer, and the second overcurrent protection voltage Va is proportional to the Q load output voltage Vo from the primary side auxiliary winding. The overcurrent protection voltage can be easily detected, and the absolute value of the first and second voltage difference I Va −Vb I
By controlling the pulse width to maintain a constant value, when the load output voltage Vo decreases due to overcurrent, the output current ■0 also decreases in response to the decrease in the load output voltage ■0. The following overcurrent protection operation can be realized.

Also, a second overcurrent protection voltage v proportional to the load output voltage 0
Since b is detected by providing a primary side auxiliary winding in the transformer, the second overcurrent protection voltage b, which is proportional to the load output voltage Vo, is insulated by the transformer itself, and the second overcurrent protection voltage b is proportional to the load output voltage Vo. Connecting the current protection voltage directly to the pulse width control circuit on the primary side does not damage the insulation between the primary and secondary sides, and there is no need to use special circuit elements such as photocouplers, which increases reliability. It is expensive and can also be made inexpensive.

(Example) FIG. 1 is a circuit diagram showing one embodiment of the present invention.

First, the configuration will be described. Reference numeral 1 denotes an input voltage source, and for example, a rectifier circuit or the like that converts commercial AC 100V into a specified DC voltage is used. 2 is a transformer that insulates the primary side and the secondary side, and has a primary winding 3 and a secondary winding 5, and further includes an auxiliary winding 24 on the primary side in the present invention. A primary winding 15a of a current transformer 15 is connected in series to the primary winding 3 of the transformer 2, and a transistor 4 as a switching element is further connected in series.

A rectifier circuit consisting of diodes 8 and 9 is connected to the secondary winding 5 of the transformer 2, and the rectified voltage by the diodes 8 and 9 is applied to a smoothing circuit consisting of a choke coil 10 and a capacitor 11, and is applied to a load 12 from an output terminal. On the other hand, a constant output voltage VO is supplied.

A diode 1 is connected to the output of a secondary winding 15b of a current transformer 15 in which the primary winding 15a is connected in series with the primary winding 2 of the transformer and a transistor 4 as a switching element.
7 and a resistor 16 are connected, the voltage induced in the secondary winding 15b of the current transformer 15 is rectified by a diode 17, and a first overcurrent protection voltage proportional to the output current IO to the load 12 is applied across the resistor 16. Va is generated.

That is, the current transformer 15, the diode 17, and the resistor 16 constitute a first voltage detection circuit that detects a first overcurrent protection voltage Va that is proportional to the load output voltage (2o).

On the other hand, the voltage induced in the auxiliary winding 24 provided on the primary side of the transformer is rectified by a diode 25, and the rectified DC voltage by the diode 25 is divided by resistors 26 and 27, and then passed through a resistor 28 to a capacitor. 30 is charged, and the detected voltage vb charged in the capacitor 30 becomes a second overcurrent protection voltage proportional to the output voltage Φ to the load 12.

Therefore, the auxiliary winding 24 of the transformer, the diode 25, the voltage dividing resistors 26, 27, the resistor 28 and the capacitor 30 create a second overcurrent protection voltage vb proportional to the load output voltage VO.
A second voltage detection circuit is constructed to detect the voltage.

A first overcurrent protection voltage Va proportional to the load output current Io generated across the resistor 16 and a second overcurrent protection voltage V proportional to the load output voltage VO charged in the capacitor 30.
b is input to the differential amplifier 32, and the differential amplifier 32 detects the voltage difference between the two (Va-Vb). The output of the differential amplifier 32 is input to the comparator 20 which has set the reference voltage Vrl, and the comparator 20 controls the H level output by pulse width when the output voltage (Va-Vb) from the differential amplifier 32 exceeds the reference voltage ■r1. The pulse width control circuit 34 receives the H level output from the comparator 20 and changes the comparison reference level for the triangular wave oscillation signal from the triangular wave oscillator 36 to narrow the pulse width of the pulse output to the transistor 4. Operate. By controlling the pulse width of the output pulse by the pulse width control circuit 34 based on the 1-level output of the comparator 20, the output (Va-Vb) of the differential amplifier 32 is kept at a constant value determined by the reference voltage Vr1 of the comparator 20. Pulse width control to maintain the value will be performed when overcurrent is detected.

Note that the pulse width control circuit 34 is supplied with the output of the amplifier 13 to which the output voltage VO is input;
compares the output voltage VO with the set reference voltage Vr2, and based on the comparison output of the amplifier 13, the pulse width control circuit 34 performs pulse width control to maintain the constant voltage determined by the output voltage O@reference voltage Vr2. Become. Also, amplifier 1
Since the output No. 3 is an output from the secondary side, it is isolated from the primary side within the pulse width control circuit 34.

Next, the operation of the embodiment shown in FIG. 1 will be explained.

First, during constant voltage operation when the output current IO to the load 12 is within the specified current, the first overcurrent protection voltage ■a proportional to the output current IO obtained from the differential amplifier 32
Since the difference (Va - Vb) between the output voltage and the second overcurrent protection voltage Vb proportional to the output voltage O is less than the reference voltage Vr1 set in the comparator 20, the overcurrent is detected as the H level output of the comparator 20. No detection signal is obtained, and the pulse width control circuit 34 controls the pulse width of the transistor 4 so that the output voltage VO to the amplifier 13 becomes a constant voltage determined by the reference voltage Vr2.

Next, if the output current IO flows excessively due to a short circuit or the like on the load 12 side, the output voltage Vo will drop as the output current IO increases. An increase in the output current in such an overcurrent state is detected by the current transformer 15, and the first overcurrent detection voltage ya proportional to the output current IO occurring across the resistor 16 increases, and at the same time the output voltage V
As O decreases, the induced voltage in the auxiliary winding 24 on the primary side of the transformer also decreases, and the output voltage V charging the capacitor 30 decreases.
The second overcurrent holding voltage vb, which is proportional to O, decreases. As a result, the voltage difference (Va-Vb) caused by the differential amplifier 32 increases and exceeds the reference voltage Vr1 of the comparator 20,
Comparator 20 produces an H level output as an overcurrent detection output. The pulse width control circuit 34 that receives the H level output from the comparator 20 operates to narrow the pulse width of the pulse output to the transistor 4, that is, the on time, and the output current IO is suppressed. In this way, the pulse width control that narrows the on time of the transistor 4 narrows the pulse width (on time) so that the output (Va-Vb) of the differential amplifier 32 maintains a constant value determined by the reference voltage Vrl of the comparator 20. Therefore, the output voltage VO follows the decrease in the output current ■0, that is, the first overcurrent detection voltage Va.
, that is, pulse width control is performed to lower the second overcurrent detection voltage vb, and as a result, the overcurrent protection operation according to the so-called 7 characteristics in which both the output voltage VO and the load current IO shown in FIG. 2 decrease. will be carried out.

FIG. 3 is a circuit block diagram showing another embodiment of the present invention, and this embodiment removes the diode 17 provided on the secondary side of the current transformer 15 in the embodiment of FIG. The charging resistor 28 for the capacitor 30 on the line 24 side is either made with a small resistance value or is removed by shorting both ends as shown by the dotted line in the figure, and the other circuit configuration is the same as the embodiment shown in FIG. It will be the same.

The embodiment shown in FIG. 3 is characterized in that the differential voltage (Va-Vb) taken out by the differential amplifier 32 is not affected even if the input voltage from the pano voltage source 1 fluctuates.

That is, since there is no rectifying diode on the secondary side of the current transformer 15, the first overcurrent detection voltage a, which is proportional to the output current io generated across the resistor 16, changes proportionally to the voltage fluctuation of the input voltage [1]. wake up

That is, when the diode 17 is provided on the secondary side of the current transformer 15 as in the embodiment shown in FIG. 1, as shown in FIG.
The pulse width of the transistor 4 is controlled to narrow n. On the other hand, when the input voltage is low, the pulse width of the transistor 4 is controlled to widen the on time Ton. Even if control is performed, the voltage Va generated across the resistor 16 by rectification by the diode 17 remains a constant voltage proportional to the output current IO.

On the other hand, if the diode 17 provided on the secondary side of the current transformer 15 is removed as shown in the embodiment shown in FIG. 3, the signal waveform shown in FIG. Current transformer - causes changes in primary current Ic and secondary voltage Va.

That is, the primary current IC of the current transformer 15 is the same as in FIG. 4, but the secondary voltage Va has no direct current component due to the removal of the diode, so the on time Ton and off time within the on/off period T are different. When the AC signal waveform for O■ is generated so that the area of time T off is equal, and pulse width control is performed to narrow the on time TOn as the input voltage increases, a secondary When the voltage ■a increases and conversely the input voltage decreases and pulse width control is performed to widen the on-time Ton, the secondary voltage Va begins to decrease as shown on the right side of FIG.
As a result, a first overcurrent detection voltage a that is proportional to fluctuations in the input voltage source is obtained.

On the other hand, by setting the resistor 28 provided on the auxiliary winding 24 side to a small value or short-circuiting it as shown by the dotted line, the charging time constant for the capacitor 30 is lowered, so that the output voltage VO generated at the capacitor 30 is reduced. The proportional second overcurrent detection voltage vb detects changes that follow voltage fluctuations of the input voltage source 1. As a result, the input voltages Va and vb to the differential amplifier 32 are both the voltage y of the input voltage source 1.
The voltage difference (Va-Vb) output from the differential amplifier 32 is mutually corrected and is not affected by the voltage fluctuation of the input voltage source 1.

FIG. 6 is a circuit diagram showing another embodiment of the present invention, in which the output current IO is different from the embodiment of FIG.
a power transformer 15 that generates a voltage Va proportional to
The negative side of the circuit consisting of the diode 7 and the resistor 8 is separated from the negative side of the input voltage source 1, and the positive side is connected to the positive side of a capacitor 13 that generates a voltage proportional to the output voltage vO, thereby forming a secondary circuit of the current transformer 15. The negative side of is input to the differential amplifier 32 to take out the voltage difference (VF6-(Va-Vb)) with the reference voltage (auxiliary voltage source) VF6 created based on the input voltage source 1. It is characterized by By removing the reference voltage 3 of the differential amplifier 32 and directly connecting it to the negative side of the input voltage source 1, this differential voltage is connected to the negative side of the secondary circuit of the current transformer 15 and the input voltage source 1 from the differential amplifier 32. It is also possible to directly generate a voltage difference (Va-Vb) with respect to the negative side of the voltage and apply it to the comparator 20.

FIG. 7 is a circuit diagram showing another embodiment of the present invention, and this embodiment adds a circuit that is not affected by voltage fluctuations of the input voltage source 1 shown in FIG. 3 to the embodiment of FIG. 6. In this combination, the diode 17 provided on the secondary side of the current transformer 15 in the embodiment shown in FIG. The resistor 28 is removed by shorting it.

(Effects of the Invention) As explained above, according to the present invention, when a short-circuit accident occurs on the load side, the overcurrent protection characteristic becomes the so-called 7 characteristics of "f" in which the output current also decreases as the output voltage decreases. can be obtained.

In addition, in detecting the output voltage to obtain the overcurrent protection characteristic (characteristic 7), since the detection voltage proportional to the output voltage is obtained from the primary side auxiliary winding of the transformer, the output current from the current transformer is The proportional detection voltage and the detection voltage for overcurrent protection operation can all be detected on the primary side, and the detection voltage of the output current and output voltage can be detected on the primary side without damaging the insulation between the primary and secondary sides by the transformer. The overcurrent protection characteristic of F7 based on the above is realized, and since there is no particular need for isolation means for separating the primary side and the secondary side, the circuit configuration can also be simplified.

Furthermore, as an effect unique to this embodiment, by removing the rectifying diode provided in the secondary side detection circuit of the current transformer and reducing the time constant of the capacitor that charges the rectified voltage of the auxiliary winding, the input voltage can be reduced. Accepts source voltage fluctuations
It is possible to obtain a differential voltage for no overcurrent protection operation.

In addition, as an effect specific to the embodiment, the detection voltage and output are proportional to the output current based on the fixed potential of the negative side of the input voltage source or the positive side of the reference voltage as an auxiliary voltage source created based on the input voltage source. Since the overcurrent detection voltage given by the difference voltage with the detection voltage proportional to the voltage can be obtained,
Connection with a pulse width control circuit can be easily made.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing one embodiment of the present invention, Fig. 2 is a graph showing overcurrent protection characteristics obtained in the embodiment of Fig. 1, and Fig. 3 is another embodiment of the present invention. Circuit diagram showing 4th
The figure is a signal waveform diagram showing the relationship between the current transformer secondary current Tc and the secondary rectified voltage Vc, which are controlled by pulse width control with respect to voltage fluctuations of the input voltage source in the embodiment of FIG.
Figure 5 shows the pulse width control according to fluctuations in the input voltage source when the diode on the secondary side of the current transformer is removed in the embodiment shown in Figure 3.
Signal waveform diagram showing the relationship between C and secondary rectified voltage VC, No. 6
, 7 is a circuit diagram showing another embodiment of the present invention, FIG. 8 is a circuit diagram showing a conventional example, and FIG. 9 is a circuit diagram showing the secondary winding voltage Vi due to pulse width control in the conventional example. It is a signal waveform diagram showing line current Iin and smoothing circuit current Id. 1: Input voltage source 2: Transformer 3 Secondary winding 4: Transistor 5: Secondary winding 8.9.17, 25: Diode 10: Choke coil 11. 30: Capacitor 13: Amplifier 15: Current transformer 16. 26, 27, 28: Resistor 20: Comparator 32: Differential amplifier 34: Pulse width control circuit 36: Triangular wave oscillator

Claims (1)

[Scope of claims] In a switching regulator that controls the pulse width of a primary winding insulated by a transformer using a switching element so as to maintain a load output voltage on the secondary side at a constant voltage, a first voltage detection circuit that detects a first overcurrent protection voltage proportional to the load output current based on a detected voltage of a connected current transformer; a second voltage detection circuit that detects a second overcurrent protection voltage proportional to the load output voltage based on the load output voltage; and a switching element configured to maintain a voltage difference between the first and second overcurrent protection voltages at a constant value. An overcurrent protection circuit for a switching regulator, comprising an overcurrent control circuit that controls the pulse width of the switching regulator. (2) The first voltage detection circuit includes a circuit that rectifies the detected voltage of the current transformer with a diode and converts it into a DC voltage, and the second voltage detection circuit includes a circuit that rectifies the detected voltage of the current transformer and converts it into a DC voltage, and the second voltage detection circuit includes a circuit that converts the detected voltage of the current transformer into a DC voltage. 2. The overcurrent protection circuit for a switching regulator according to claim 1, further comprising a circuit that charges a capacitor after rectifying the voltage and outputs the capacitor charging voltage. (3) The first voltage detection circuit includes a circuit that outputs the detected voltage of the current transformer as it is, and the second voltage detection circuit rectifies the induced voltage of the primary side auxiliary winding and then converts it to a predetermined divided voltage. Convert it to voltage and charge the capacitor,
The overcurrent protection circuit for a switching regulator according to claim 1, further comprising a circuit that outputs the capacitor charging voltage.