JP2001086742A - Switching power circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To very simply protect the overcurrent of a low-voltage side output system at low costs, in a switching power circuit that has a plurality of DC output systems for rectifying voltage being outputted to the secondary side of a converter transfermer, an output current limiting means for actuating, when a converter transformer primary side current reaches a setting value for protecting a primary side overcurrent, and a feedback means for low-voltage side output system voltage stabilization. SOLUTION: This switching power circuit has a clamp menas ZD1 for clamping the output voltage of a high-voltage side output system to a prescirbed voltage value, and a circuit RS3 formed by an element with a resistance component connected to a front stage side of a feedback means. When a low-voltage side output current reaches a prescribed limit value, the clamp menas ZD1 is actuated by a circuit RS3 for actuating an output current limitating means, and the overcurrent of a lower-voltage side output system can be protected by mainly adding the clamp menas ZD1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching device suitable for a power supply circuit of an electrophotographic printer or the like having a plurality of DC output systems obtained by rectifying and smoothing a plurality of voltages output at mutually different values. It relates to a power supply circuit.

[0002]

2. Description of the Related Art Conventionally, as a power supply circuit of an electrophotographic printer or the like, a plurality of converters which output different values to the secondary side of the converter transformer when the primary switching means on the primary side of the converter transformer is turned off are turned off. There is a flyback type switching power supply circuit having a plurality of DC output systems obtained by rectifying and smoothing the respective voltages. Such a switching power supply circuit usually has a feedback function added to the output system on the low voltage side in order to stabilize the output voltage. For protection against overcurrent, the primary side of the converter transformer is provided with an output current limiting function by detecting the current on the primary side.

[0003]

However, the above conventional circuit has the following problems. As shown in FIG.
For overcurrent protection, 1
A switching power supply circuit having an output current limiting function based on detection of a secondary current and having two DC output systems that output voltages having mutually different values will be described as an example. In such a conventional circuit, overcurrent protection on the primary side is performed by monitoring the total current value to both output terminals OUT1 and OUT2.

Here, the current Ip flowing through the primary winding N1 of the converter transformer T is a secondary high potential (output terminal O).
If the current flowing through the UT2) side winding N4 is I2, the current flowing through the low potential (output terminal OUT1) side winding N2 is I1, and the number of turns of the windings N1, N2, N4 is n1, n2, n4, Equation (1) is obtained. Ip = I2 · (n4 / n1) + (I1 + I2) · (n2 / n1) (1)

The current Ip is switched by a switching transistor Q1 which is a main component of a main switching means on the primary side of the converter transformer T.
This passes through a primary-side overcurrent detection resistor RS1 connected to the emitter of the switching transistor Q1.

For this reason, when Ip · RS1 rises to a voltage (for example, about 0.6 V) at which the primary side overcurrent cutoff transistor Q3 can be turned on, the transistor Q3 is turned on and the base current of the switching transistor Q1 is reduced. The current Ip flowing through the switching transistor Q1 is reduced. Therefore, even when the output current becomes excessive, the switching power supply circuit suppresses the output current and protects itself.

In the case of a single output (one output terminal OUT), a sufficient effect can be obtained only by the above-mentioned primary-side overcurrent protection, but when a plurality of outputs are provided, an overcurrent detection means is required on the secondary side. It becomes. That is, a single output, for example, output terminal O
In the case of only UT1, the above equation (1) is as follows: Ip = I1 · (n2 / n1) (2), which is proportional to the current I1. For this reason, the accuracy of the overcurrent detection of the winding N2 (output terminal OUT1) is sufficient with the primary-side overcurrent detection means (the resistor RS1), and the secondary-side overcurrent detection means is not required.

However, in the case of plural outputs, for example, the output terminal O
When the output voltage of the UT1 is 5V and the output voltage of the output terminal OUT2 is 24V, n4: n2 = (24-5): 5 =
Since the ratio is 19: 5, it is apparent from the above equation (1) that the factor that changes the current Ip is dominated by the current I2 rather than the current I1.

For this reason, the overcurrent detection value of the output terminal OUT1 greatly changes depending on the output current value of the output terminal OUT2. As described above, when a plurality of outputs are provided, the overcurrent detection and cutoff means is also provided on the secondary side. Is required. However, this type of conventional overcurrent detection and cutoff means includes a resistor R1
2. An overcurrent detection resistor RS2, the transistor Q4, an overcurrent cutoff photocoupler (PD2, PSCR1) driven by turning on the transistor Q4, and the like using a comparator (not shown), etc. As described above, there is a problem that the circuit configuration is complicated and the cost is high.

The present invention has been made to solve the above-mentioned problems of the prior art.

[0011]

The present invention employs the following structure to solve the above-mentioned problems. <Configuration 1> A plurality of DC output systems are provided, each of which rectifies and smoothes a plurality of voltages output at mutually different values to the secondary side of the converter transformer by the operation of the switching means on the primary side of the converter transformer. An output current limiting means which operates when a current on the primary side of the converter transformer reaches a set value to perform overcurrent protection on the primary side; and an output system on a low voltage side among the plurality of DC output systems. Feedback means for stabilizing the output voltage of the plurality of DC output systems, clamping means for clamping the output voltage of the output system on the high voltage side of the plurality of DC output systems to a predetermined voltage value, and a feedback circuit connected to the preceding stage of the feedback means. A circuit formed of an element having a resistance component, wherein when the output current of the output system on the low voltage side reaches a predetermined limit value, the circuit activates the clamping means, and Switching power supply circuit, characterized in that actuating the output current limiting means by the operation means.

<Structure 2> A plurality of DC outputs obtained by rectifying and smoothing a plurality of voltages output at mutually different values to the secondary side of the converter transformer by the operation of the switching means on the primary side of the converter transformer. A feedback means for stabilizing the output voltage of the low-voltage output system of the plurality of DC output systems, and detecting the output voltage of the high-voltage output system of the plurality of DC output systems, A shutoff means for stopping the operation of the primary side switching means on the basis of the detection; and a circuit formed of an element having a resistance component connected in front of the feedback means, wherein the low voltage side A switching power supply circuit, wherein the shut-off means is operated by the circuit when the output current of the output system reaches a predetermined limit value.

<Structure 3> The circuit according to claim 1 or 2, wherein the circuit formed by an element having a resistance component connected upstream of the feedback means uses a voltage / current characteristic of a rectifier diode. Switching power supply circuit.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. << Example 1 >><Structure of Example 1> FIG. 1 is a diagram showing Example 1 of a switching power supply circuit according to the present invention. In the figure, the rectifier circuit 21 is composed of, for example, a diode bridge,
C) Rectify the input. Smoothing capacitor C1
Is for smoothing the output of the rectifier circuit 21 and applying it to the switching regulator 22. The switching regulator 22 includes a converter transformer T having a primary winding N1 and an auxiliary winding N3 on the primary side and secondary windings N2 and N4 on the secondary side, and a primary winding of the converter transformer T. N
1, a switching transistor Q having a collector and an emitter connected in series and constituting a main configuration of main switching means.
1 is provided.

Here, the converter transformer T is composed of 1
The secondary winding N1 and the auxiliary winding N3 have the same polarity, and
The secondary winding N1, the auxiliary winding N3 and the secondary windings N2, N4 are wound with opposite polarities. The switching transistor Q1 has a base connected to the high-potential-side input line HL via a starting resistor R1, and one end of an auxiliary winding N3 of the converter transformer T via a diode D1 having the illustrated polarity and a base resistor R2 in this order. Connected to.

The diode D1 is provided to prevent a backflow of the starting current from the starting resistor R1, and is connected in parallel with the diode D1 to a resistor R3 for improving the re-ignition (ON) characteristic.
And a series circuit of a capacitor C2.

One end of a secondary winding N2 of the converter transformer T is connected to the anode of a rectifying diode D2, and the cathode of the rectifying diode D2 is connected to a smoothing capacitor C2.
3 is connected to the secondary 0V side output terminal (common output terminal) OUT0 of the converter transformer T.

On the secondary side of the converter transformer T, there is provided a circuit for detecting an error between the output voltage and the set voltage. That is, the resistor R4, the light-emitting side photodiode PD1 and the shunt regulator SR constituting the photocoupler are provided between the secondary 0V side output terminal (common output terminal) OUT0 and the low potential side output terminal OUT1 of the converter transformer T. In addition to being connected in series, a series circuit of voltage dividing resistors R5 and R6 is connected. The voltage dividing points (connection points) of the voltage dividing resistors R5 and R6 are connected to the reference of the shunt regulator SR. Also,
A capacitor C4 is connected between the reference and the cathode of the shunt regulator SR.

The collector of the switching transistor Q2 is connected to the base of the switching transistor Q1 in order to change the base current corresponding to the output voltage. The base of the switching transistor Q2 has, via a base resistor R7, a diode D3 having an anode connected to one end of an auxiliary winding N3 of the converter transformer T and a light receiving side phototransistor PTr paired with the photodiode PD1. Are connected in series,
Only when the switching transistor Q1 is conducting, the switching transistor Q2 via the base resistor R7.
The base current is made to flow.

In order to increase the responsiveness of the phototransistor PTr during conduction, this phototransistor PTr
A resistor R8 and a capacitor C5 in parallel with each other, a resistor R9 between the collector of the phototransistor PTr and the low-potential-side input line LL, and a resistor between the base of the switching transistor Q2 and the low-potential-side input line LL. The capacitor C6 is connected.

The auxiliary winding N3 is connected in parallel with a resistor R10 for suppressing the effect of leakage magnetic flux. A snubber circuit composed of a diode D4, a capacitor C7 and a resistor R11 is connected in parallel to the primary winding N1.

Here, in a power supply circuit of an electrophotographic printer or the like, for example, 5 V or 3.
3V power supply and 2 for actuator (motor, etc.)
A power supply such as 4V is required. The secondary winding N2 of the converter transformer T is used for a logic circuit, and is configured to output a voltage V1 of 5 V to an output terminal OUT1.

The secondary winding N4 additionally connected to the secondary winding N2 is used for an actuator, and is configured to output a voltage V2 of 24 V to an output terminal OUT2. Therefore, the secondary winding N4 is wound in the same polarity as the secondary winding N2, one end thereof is connected to the anode of the rectifying diode D5, and the cathode of the rectifying diode D5 is connected to the output terminal OUT2. In addition, 0 on the secondary side of the converter transformer T is connected via the smoothing capacitor C8.
It is connected to the V-side output terminal (common output terminal) OUT0.

Here, the winding voltage of the secondary winding N2 is VN2
Assuming that the voltage drop in the forward direction of the rectifying diode D2 is VF1, VN2 = V1 + VF1. Therefore, the output voltage V2 is such that the forward voltage drop of the rectifier diode D5 is VF2, the number of turns of the secondary winding N2 is n2, and the number of turns of the secondary winding N4 is n4.
Then, V2 = (1 + n4 / n2) · VN2-VF2 = (1 + n
4 / n2) · (V1 + VF1) −VF2. By setting the number of turns n4 of the secondary winding N4 to an appropriate value, a desired output voltage V2, in this case, 2
4 V can be output from the output terminal OUT2.

The resistor RS1 is provided for detecting the primary side overcurrent, and the transistor Q3 is provided for cutting off the overcurrent when the primary side overcurrent is detected by the resistor RS1 (the resistor RS1 and the transistor Q3 are provided respectively). The main configuration of the output current limiting means). The resistor RS3 is provided between the cathode of the rectifying diode D2 and the output terminal OUT1 for detecting a secondary side overcurrent. The Zener diode ZD1 is provided between the output terminals OUT2 and OUT0 for secondary-side voltage clamping.

<Operation of Specific Example 1> Next, the operation of the specific example 1 will be described. When the AC input voltage is applied to the rectifier circuit 21, a base current flows through the switching transistor Q1 via the starting resistor R1, and the switching transistor Q1 is made conductive (turned on), so that the collector is connected to the primary winding N1 of the converter transformer T. Electric current flows. Since the primary winding N1 and the secondary winding N2 are wound with opposite polarities,
A voltage of opposite polarity is applied to the rectifier diodes D2 and D5 on the secondary side, and no current flows.

When the switching transistor Q1 is turned on and an input voltage is applied to the primary winding N1, the auxiliary winding N3 has a winding ratio of the auxiliary winding N3 to the primary winding N1. A determined induced voltage is generated in a direction for conducting the diode D1. This causes the diode D1 to conduct,
The base current due to the induced voltage flows through the switching transistor Q1 via the base resistor R2, and increases the collector current of the switching transistor Q1 at a constant slope determined by the inductance of the converter transformer T. When the collector current reaches a certain value given by the product of the base current and the current amplification factor of the switching transistor Q1, the operation region of the switching transistor Q1 shifts to the active region as the base current becomes insufficient. Then, the voltage between the collector and the emitter is increased.

As described above, when the input voltage applied to the primary winding N1 decreases, the voltage generated in the auxiliary winding N3 decreases, and the base current of the switching transistor Q1 decreases. Due to this positive feedback, the switching transistor Q1 is instantaneously turned off (off).

At the moment when the switching transistor Q1 is turned off, the primary winding N1, the secondary windings N2, N
4 and the auxiliary winding N3, the switching transistor Q
A voltage having a polarity opposite to that until 1 is turned off is generated, and the energy stored during the period when the switching transistor Q1 is on is discharged from the primary winding N1. At this time,
Rectifier diode D connected to secondary windings N2 and N4
2, The current flowing through D5 increases instantaneously and then gradually decreases. Output voltages from the rectifying diodes D2 and D5 are smoothed by smoothing capacitors C3 and C8 and applied to a load (not shown) connected to the output terminals OUT1 and OUT2.

In this way, the stored energy is 2
When the current is released from the secondary windings N2 and N4, the emission ends, and the current values of the rectifying diodes D2 and D5 become 0, and further undershoot occurs and drops below 0 instantaneously, the primary winding N1 Voltages of opposite polarities are generated in the secondary windings N2 and N4 and the auxiliary winding N3. Therefore, the auxiliary winding N
3, a base current flows to the switching transistor Q1, the switching transistor Q1 is turned on again, and energy is stored again. Hereinafter, the same operation is repeated.

Here, the voltage V1 of the output terminal OUT1 is
Is fed back to the primary side by the photodiode PD1, and is controlled so that the output voltage V1 becomes the set voltage. Hereinafter, such stabilization of the output voltage V1 will be described.

Now, when the output voltage V1 becomes higher than the set voltage, here 5V, the shunt regulator SR is turned on and a current flows through the photodiode PD1. As a result, the phototransistor PTr is turned on, and while the base current is supplied from the auxiliary winding N3 to the switching transistor Q1, a base current is supplied to the switching transistor Q2 via the diode D3 to turn on the switching transistor Q2. And the base current of the switching transistor Q1 is reduced.

When the base current decreases, the upper limit of the collector current of the switching transistor Q1 decreases,
The switching transistor Q1 is shifted to the active region in a shorter time than when the switching transistor Q2 is not turned on, and the energy transmitted to the secondary winding N2 is reduced. As a result, the output voltage V1 decreases and is stabilized. In the switching regulator 22 having the above-described configuration, the value of each element is set so as to change only in the direction in which the output voltage increases, and a stabilizing (voltage increasing) operation for a decrease in the output voltage V1 is unnecessary.

The snubber circuit constituted by the diode D4, the capacitor C7 and the resistor R11 absorbs a spike voltage generated by the leakage inductance of the primary winding N1 in the converter transformer T when the switching transistor Q1 is turned off. This protects the switching transistor Q1.

That is, the switching transistor Q1
The spike voltage generated when the switch is turned off reaches several hundred volts, and if there is no snubber circuit, the switching transistor Q1 is destroyed. Here, since the spike voltage is rectified by the diode D4 of the snubber circuit, smoothed by the capacitor C7, and generated by the resistor R11 to dissipate energy, the switching transistor Q1 is not destroyed.

Next, the overcurrent detection and cutoff (overcurrent protection) operation on the secondary side of the converter transformer T will be described with reference to FIG. FIG. 2 shows the output current I of the output terminal OUT1.
It shows the change of the voltage VN2 of the secondary winding N2, the voltage V2 of the output terminal OUT2, and the primary winding current Ip with respect to the change of o1, the output current Io1 on the horizontal axis, and the secondary winding voltage VN2 on the vertical axis. The output voltage V2 and the primary winding current Ip are shown.

Now, when the voltage V1 of the output terminal OUT1 is 5V
It is assumed that the stabilization control is constant. In the circuit of the present invention shown in FIG. 1, if the forward voltage drop of the diodes D2 and D5 is neglected, the secondary winding voltage VN2 becomes the output terminal O
This is a value obtained by adding the voltage drop of the resistor RS3 to the voltage V1 of the UT1.

The number of turns of the secondary windings N2, N4 is n2, n4
Then, VN4 = (n4 / n2) · VN2, and the voltage V2 of the output terminal OUT2 is V2 = VN2 + VN4 = {1+ (n4 / n2)} · VN
It becomes 2.

As described above, the secondary winding voltage VN2 is a value obtained by adding the voltage drop of the resistor RS3 to the output voltage V1 (V1
+ Io1 · RS3), which increases with an increase in the output current Io1, and the output voltage V2 also increases as Io1 · RS3 · ｛1+
(N4 / n2)}.

The output voltage V2, that is, VN2 + VN4
Is the Zener voltage VZD1 of the Zener diode ZD1.
, A current flows through the Zener diode ZD1,
Apparently, the load current flows to the output terminal OUT2, and the primary winding current Ip sharply increases.

Since the overcurrent detection value of the primary winding current Ip is determined by the value of the resistor RS1, the overcurrent detection value is set to an appropriate value capable of detecting the overcurrent, and the Zener is set within an allowable range of the output voltage V2. Zener voltage VZD1 of diode ZD1
Is selected, the overcurrent detection at the desired current value on the secondary side of the converter transformer T, specifically, the overcurrent detection of the output terminal OUT1 can be performed by the value (set value) of the resistor RS3 from n4: n2. .

This overcurrent detection is finally performed by the resistor RS1 through which the primary winding current Ip flows, and its value (overcurrent detection value), in other words, the converter transformer T
Is applied to the primary side overcurrent cutoff transistor Q3 when the secondary side overcurrent occurs. As a result, the transistor Q3 is turned on, the base current of the switching transistor Q1 is shunted, and the switching transistor Q1 is turned off to turn off the primary winding current I
p, and thus the output current Io1 of the output terminal OUT1 is cut off to protect the circuit.

As described above, when the output voltage V2 reaches the Zener voltage VZD1 of the Zener diode ZD1, a current flows through the Zener diode ZD1. That is, the Zener diode ZD1 applies a predetermined voltage (Zener voltage V) between the output terminals OUT2 and OUT0.
The output voltage V2 is clamped by ZD1) or more. If the output voltage V2 abnormally rises for some reason, the load connected to the output terminal OUT2 is protected.

<Effects of Embodiment 1> According to Embodiment 1, the overcurrent detection and interruption (overcurrent protection) on the secondary side of the converter transformer T is extremely simple and low-cost in that only the addition of the zener diode ZD1 is required. Can be realized. In addition, the overcurrent detection and cutoff on the secondary side of the converter transformer T is performed by the resistor RS1 and the transistor for detecting and cutting off the overcurrent on the primary side in addition to the Zener diode ZD1 and the resistor RS3 on the secondary side. Q3 is also used. That is, the resistor RS1 and the transistor Q3 are also used for detecting and blocking overcurrent on the secondary side.
There is also an advantage that the primary side overcurrent detection and cutoff means of the converter transformer T is effectively used for the secondary side overcurrent detection and cutoff means.

A Zener diode ZD1 is connected between the output terminals OUT2 and OUT0 (output terminal OU).
A voltage clamping means is provided at T2), and the output voltage V2 which is higher than a predetermined voltage is clamped. Therefore, even if the output voltage V2 rises abnormally, the load connected to the output terminal OUT2 is protected and breakage is avoided. There is also an effect.

<< Embodiment 2 >><Structure of Embodiment 2> FIG. 3 is a diagram showing Embodiment 2 of the switching power supply circuit according to the present invention. In FIG.
The same or corresponding parts as those in FIG. 1 are denoted by the same reference numerals. The basic configuration of the specific example 2 is the same as the specific example 1 (FIG. 1) described above, but differs from the specific example 1 in the following points.

First, a series circuit of a Zener diode ZD2, a resistor R13, and a light emitting photodiode PD3 constituting a photocoupler is provided between the output terminals OUT2 and OUT0, instead of the Zener diode ZD1 in FIG. . On the primary side of the converter transformer T, a light-receiving-side photothyristor PSCR2 that forms a photocoupler with the photodiode PD3 is connected between the base and the emitter of the switching transistor Q1. The Zener diode ZD2, the resistor R13, and the photocoupler (PD3, PSCR2) constitute blocking means.

<Operation of Specific Example 2> Next, the operation of the specific example 2 will be described. The operation of the second embodiment other than the above configuration is the same as that of the first embodiment, and the description thereof is omitted here. As described above, the output terminal OU
If the output current Io1 of T1 increases, the output terminal OUT2
V2 is Io1 · RS3 · {1+ (n4 / n2)}
Just rise. When the output voltage V2 exceeds the Zener voltage VZD2 of the Zener diode ZD2, the output terminal O
From UT2, Zener diode ZD2 and resistor R13
The current flows through the photodiode PD3 through the photodiode PD3, and the photodiode PD3 emits light.
Turn on 2.

Once turned on, the photothyristor PSCR2 continues to be supplied with current through the resistor R1 to maintain the on state, short-circuits the base and emitter of the transistor Q1 and keeps the transistor Q1 off, and operates the circuit. (The output current Io1 of the output terminal OUT1 is cut off).

A low potential (output terminal OUT1) can be obtained by selecting a Zener diode ZD2 whose Zener voltage is close to the maximum value of the output allowable range of the output terminal OUT2 and selecting the value of the resistor RS3 from the value of n2: n4. A desired overcurrent detection value of the current I1 flowing through the side winding N2 can be obtained.

For example, when the output of the output terminal OUT1 is 5
V, the current I2 flowing through the high-potential (output terminal OUT2) side winding N4 such that the output of the output terminal OUT2 is 24V,
If the current Ip obtained by the above equation (1) becomes dominant, the detection value of the primary side overcurrent detection means (resistor RS1) is adjusted to this value, so that the overcurrent detection of the current I2 can be substituted. It is possible to do.

<Effects of Embodiment 2> According to Embodiment 2 described above, the cutoff means operates not only as overcurrent detection and cutoff (overcurrent protection) means, but also in part (the series circuit). Also performs secondary side overvoltage detection. That is, the output voltage V2
Reaches the Zener voltage VZD2 of the Zener diode ZD2 (for convenience, the voltage drop due to the resistor R13 is ignored), a current flows through the Zener diode ZD2. In addition, the Zener diode ZD2 clamps the output voltage V2 that is equal to or higher than the predetermined voltage (Zener voltage VZD2) between the output terminals OUT2 and OUT0. The load connected to the output terminal OUT2 can be sufficiently protected.

Further, unlike the specific example 1 in which the Zener diode ZD1 tends to increase in size during the overcurrent detection operation and the Zener diode ZD1 tends to increase in size, the specific example 2 reduces the loss of the Zener diode ZD2. Less, the zener diode ZD2 can be small. Moreover, in the specific example 2, a desired overcurrent detection value can be obtained relatively freely as described above, and the output terminal OUT
2, the overcurrent detection value of the output terminal OUT2 when the overcurrent detection is also performed can be set relatively freely.

In the specific example 1 described above, the Zener diode ZD1 is used alone as the voltage clamping means. However, as shown in FIG. 4, a thyristor SCR1, a transistor Q41, or the like is connected to the Zener diode ZD41 or ZD42 as a booster. A capacity that can withstand the loss during current protection may be obtained.

In the second embodiment, the overvoltage detecting means is provided on the secondary side of the converter transformer T. However, a voltage substantially proportional to the output voltage V2 of the output terminal OUT2 appears from the auxiliary winding N3 as a negative voltage. It is also possible to provide the overvoltage detecting means on the primary side of the converter transformer T by paying attention to. For example, as shown in FIG. 5, a negative voltage from the auxiliary winding N3 is rectified by a diode D51 and a capacitor C51.
An overvoltage detecting means is formed on the primary side so as to smoothen and turn on the triac TR51 through the Zener diode ZD51 and the resistor R51. According to this, it is possible to detect overvoltage on the secondary side on the primary side without using a photocoupler using the expensive photothyristor PSCR2 as the light receiving side element.

Further, in the first and second specific examples, the description has been made ignoring the forward voltage drop (resistance) of the diode D2. However, the forward voltage drop VF1 of the diode D2 is determined by the output current Io.
Paying attention to the fact that the resistance R changes due to
It is also possible to omit S3 (substitute the forward resistance of the diode D2 for the resistance RS3). That is, if the overcurrent detection value of the output terminal OUT1 is, for example, several A to several tens A
When the voltage is set to be large as described above, the output current Io1 increases and the winding voltage V of the secondary winding N2 increases.
The forward voltage drop VF1 of N2 and diode D2 also increases significantly. Therefore, such a forward voltage drop V
It becomes possible to substitute the voltage drop of the resistor RS3 (the value of the resistor RS3) by the value of the resistor F3 (the value of the resistor RS3).
Can be omitted.

[Brief description of the drawings]

FIG. 1 is a diagram showing a specific example 1 of the circuit of the present invention.

FIG. 2 is a graph for explaining the operation of the circuit of the present invention shown in FIG. 1;

FIG. 3 is a diagram showing a specific example 2 of the circuit of the present invention.

FIG. 4 is a circuit diagram showing another example of the voltage clamping means in FIG.

FIG. 5 is a circuit diagram showing another example of the overvoltage detecting means in FIG.

FIG. 6 is a diagram showing a conventional circuit.

[Explanation of symbols]

21 Rectifier circuit 22 Switching regulator D2, D5 Rectifying diode C1, C3, C8 Smoothing capacitor T Converter transformer N1 Primary winding N2, N4 Secondary winding N3 Auxiliary winding Q1, Q2 Switching transistor R1 Starting resistance PD1 Output voltage V1 stabilizing photodiode PTr Output voltage V1 stabilizing phototransistor R5, R6 Output voltage V1 stabilizing voltage dividing resistor SR Output voltage V1 stabilizing shunt regulator RS1 Primary side overcurrent detection resistor Q3 Primary side overcurrent Current cutoff transistors PSCR1, PSCR2 Primary overcurrent cutoff photothyristor ZD1 Secondary side voltage clamp Zener diode ZD2 Secondary side overvoltage detection Zener diode R13 Secondary side overvoltage detection resistor PD3 Secondary side overvoltage detection photoda Eau OUT0 0V side output terminal (common output terminal) OUT1 low potential side output terminal OUT2 high potential side output terminal RS2 1 primary overcurrent detecting resistor Q4 1 primary overcurrent detection transistor PD2 1 primary overcurrent blocking

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) H02H 3/087 H02H 3/087 F term (Reference) 2C061 BB17 HX10 2H027 EK06 ZA01 5G004 AA04 AB02 BA03 BA04 DA02 DB01 DB02 DC04 DC09 DC10 EA01 FA01 5H730 AA15 AA20 BB43 BB55 CC01 DD02 DD41 EE02 EE07 EE73 FD01 FD24 FD51 FF19 XX03 XX12 XX15 XX23 XX26 XX27 XX32 XX35 XX41 XX43 9A001 KK32

Claims (3)

[Claims] 1. A plurality of DC output systems each of which rectifies and smoothes a plurality of voltages output at mutually different values to a secondary side of the converter transformer by an operation of a switching means on a primary side of the converter transformer. Output current limiting means that operates when a current on the primary side of the converter transformer reaches a set value to perform overcurrent protection on the primary side; and an output on a low voltage side of the plurality of DC output systems. Feedback means for stabilizing the output voltage of the system; clamp means for clamping the output voltage of the high-voltage output system of the plurality of DC output systems to a predetermined voltage value; and And a circuit formed of an element having a resistance component, when the output current of the low-voltage side output system reaches a predetermined limit value, the circuit operates the clamp means, Switching power supply circuit, characterized in that operating the output current limiting means by the operation of the lamp unit. 2. A plurality of DC output systems, each of which rectifies and smoothes a plurality of voltages output at mutually different values to a secondary side of the converter transformer by an operation of a switching means on a primary side of the converter transformer. Feedback means for stabilizing the output voltage of the low-voltage output system of the plurality of DC output systems; and detecting the output voltage of the high-voltage output system of the plurality of DC output systems. And a circuit formed by an element having a resistance component connected to a stage preceding the feedback unit, wherein the switching unit stops the operation of the switching unit on the primary side based on the following. A switching power supply circuit, wherein the shut-off means is operated by the circuit when the output current of the system reaches a predetermined limit value. 3. The switching device according to claim 1, wherein a circuit formed by an element having a resistance component connected before the feedback means uses a voltage / current characteristic of a rectifier diode. Power circuit.