JPH0866021A - Switching power supply - Google Patents

Abstract

PURPOSE: To obtain an inexpensive switching power supply in which power loss is reduced while suppressing fluctuation in the drooping point of overcurrent protective characteristics for the input voltage. CONSTITUTION: A series circuit of the primary winding 24a of a transformer 24, a switching element 25, and a current detection resistor 37 is connected in series between lines L1 and L2 to form a primary circuit. The switching element 25 is driven while pulsating by means of a control circuit 26. The transformer 24 is additionally provided with an auxiliary winding 24b having same polarity as the primary winding 24a of the transformer 24 and connected through a rectification diode 29 with the power supply terminal Vc of the control circuit 26. The joint between the current detection resistor 37 and the switching element 25 is connected through a resistor 38 with the current detection terminal CLM of the control circuit 26. A correction resistor 39 is connected with the DC output section of the auxiliary winding 24b, i.e., between the output of the diode 29 and the current detection terminal CLM.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device having an overcurrent protection function for a primary circuit.

[0002]

2. Description of the Related Art FIG. 5 is a circuit diagram showing an example of a conventional switching power supply device. The AC power supplied from the power supply 1 is full-wave rectified by the diode bridge 2 and further smoothed by the capacitor 3, and the line L1,
The DC voltage V1 is supplied to L2. On the other hand, between the line L1 and the line L2 (ground side), the primary winding 4a of the transformer 4, the switching element 5, and the resistor 1 for current detection.
7 are connected in series to form a primary circuit.

The switching element 5 is driven in pulses by the control circuit 6, and its conduction period and non-conduction period are controlled. A resistor 7 is connected between the power supply terminal Vc of the control circuit 6 and the line L1 to supply the power supply voltage of the control circuit 6. A smoothing capacitor 8 is connected between the power supply terminal Vc and the line L2. further,
An auxiliary winding 4b having the same polarity as that of the primary winding 4a of the transformer 4 and outputting a voltage V2 lower than the supply voltage V1 to the primary winding 4a is separately provided in the transformer 4, and the auxiliary winding 4b is provided.
One end of b is connected to the line L2, and the other end of the auxiliary winding 4b is connected to the power supply terminal Vc of the control circuit 6 via the rectifying diode 9. The ground terminal G of the control circuit 6 is connected to the line L2.

The connection point between the current detecting resistor 17 and the switching element 5 is connected to the current detecting terminal CLM of the control circuit 6 via the resistor 18, and the current I1 of the primary circuit is converted into a voltage signal to control the circuit. It has been entered in 6.

On the other hand, looking at the secondary circuit of the transformer 4,
One end of the secondary winding 4c is connected to the rectifying diode 10 and the other end of the secondary winding 4c is connected to the line L3. A flywheel diode 11 is connected between the diode 10 and the line L3. Further, a smoothing choke coil 12 is provided between the output of the diode 10 and the line L3.
And the capacitor 13 are connected, and the output voltage Vo from the output terminals 14 and 15 connected to both ends of the capacitor 13,
The output current Io is output and supplied to the load (not shown). The output voltage Vo is input to an isolator 16 such as a photocoupler that insulates the primary circuit from the secondary circuit, and the isolator 16 is connected to the feedback terminal FB of the control circuit 6.

Next, the operation will be described. When energization starts from the power source 1, the DC voltage on the line L1 rises, and the resistance 7
The control circuit 6 is energized to start the control operation. The control circuit 6 outputs a rectangular wave having a predetermined duty ratio (duty ratio) from the output terminal OUT to drive the switching element 5. When the switching element 5 is repeatedly turned on and off, 1
Since the current of the secondary circuit is switched intermittently, an AC voltage is induced in the auxiliary winding 4b and the secondary winding 4c magnetically coupled to the primary winding 4a.

The AC voltage V2 induced in the auxiliary winding 4b is rectified by the diode 9, smoothed by the capacitor 8 and supplied as the power source of the control circuit 6. After that, the control circuit 6 continues to operate with the electric power supplied from the auxiliary winding 4b. The resistor 7 is for starting the control circuit 6.

On the other hand, the AC voltage induced in the secondary winding 4c is rectified by the diode 10, and the choke coil 12
And is smoothed by the capacitor 13 and converted into a DC voltage. The flywheel diode 11 supplies a current to the choke coil 12 during the off period of the secondary winding 4c. The output voltage Vo is input to the control circuit 6 via the isolator 16, and the control circuit 6 outputs the output voltage V
When o becomes high, the duty ratio of the switching element 5 is reduced to reduce the power supplied to the secondary winding 4c. Conversely, when the output voltage Vo becomes low, the duty ratio of the switching element 5 is increased to the secondary winding 4c. To stabilize the output voltage Vo.

The current I1 flowing in the primary circuit is the resistance 1
When the current I1 is higher than a predetermined value and an overcurrent state is detected, the switching element 5 is made non-conductive for a certain period by detecting Overcurrent protection of the next circuit is performed.

[0010]

In the conventional switching power supply device shown in FIG. 5, even if the load on the secondary side is constant, the input voltage V1 supplied between the lines L1 and L2 on the primary side is high. When it fluctuates in the direction, the control circuit 6 operates by the feedback signal from the secondary circuit and the primary side current I1
Becomes smaller, and as shown in FIG. 6A, the drooping start point F in the overcurrent protection characteristic of the output current Io fluctuates in the direction of becoming higher. Conversely, when the input voltage V1 changes in the lowering direction, the drooping start point F changes in the lowering direction.

In order to solve such a problem, a signal correction resistor 1 is provided between the line L1 and the current detection terminal CLM.
It is conceivable to connect 9. According to this method, since the voltage proportional to the input voltage V1 of the line L1 is input to the current detection terminal CLM of the control circuit 6, for example, when the input voltage V1 becomes high, the current I1 of the primary circuit becomes apparently large. As a result, overcurrent protection is activated faster, and Fig. 6
As shown in (b), it is understood that the fluctuation of the drooping start point F with respect to the input voltage V1 is suppressed and the overcurrent protection characteristic is improved.

However, the power source 1 is 100 V or 2
In the case of a commercial power source of 00V, the voltage V1 of the line L1 reaches 100V to 185V or a double voltage thereof, and the resistance 1
A fairly high voltage will be applied across 9.
Therefore, if the resistance value of the resistor 19 is reduced and the correction amount corresponding to the input voltage V1 is increased in order to reduce the variation amount of the drooping start point F, the power loss of the resistor 19 and A large amount of heat is generated, which causes an increase in the power rating of the resistor 19 and a decrease in power supply efficiency, resulting in an increase in the cost of the power supply device.

Further, when the short circuit failure of the resistor 19 occurs, a high voltage is applied to the current detection terminal CLM of the control circuit 6 and the resistor 18, which may cause a serious accident or fire. As a countermeasure, it is conceivable to configure the resistor 19 by connecting a plurality of resistors in series, but this leads to an increase in the number of parts and an increase in cost.

An object of the present invention is to provide a switching power supply device which is inexpensive and has a small power loss while suppressing the fluctuation of the drooping start point of the overcurrent protection characteristic with respect to the input voltage.

[0015]

The present invention is a transformer 1
The detection signal from the current detection circuit is provided with a primary circuit in which a secondary winding, a switching element and a current detection circuit are connected in series, and a control circuit for driving the switching element in a pulse shape and controlling the conduction period of the switching element. In a switching power supply device for inputting a current to a current detection terminal of a control circuit to protect an overcurrent of a primary circuit, the switching power supply device has the same polarity as that of a primary winding of a transformer and is lower than a voltage supplied to the primary winding. In the switching power supply device, a correction resistor for correcting a detection signal is connected between a DC output section of another winding that outputs a voltage and a current detection terminal of a control circuit.

[0016]

According to the present invention, the correction resistor for correcting the detection signal from the current detection circuit has the same polarity output as that of the primary winding of the transformer and is lower than the voltage supplied to the primary winding. By connecting to the DC output section of another winding that outputs a voltage, the voltage applied across the correction resistor is reduced even when the input voltage of the primary circuit is as high as commercial power. The power loss and heat generation of the correction resistor are reduced. Therefore, the correction resistor may be a resistor having a small rating, and it is safe and inexpensive. Further, since the voltage of the DC output section of the other winding is proportional to the input voltage of the primary circuit, it is possible to appropriately suppress the variation of the drooping start point of the overcurrent protection characteristic. Further, since the fluctuation of the drooping start point is small, it is possible to use the parts other than the correction resistor that are low in rating and inexpensive.

[0017]

1 is a circuit diagram showing the electrical construction of an embodiment of the present invention. AC power supplied from a power supply 21, such as a commercial power supply, is full-wave rectified by a diode bridge 22, smoothed by a capacitor 23, and a DC voltage V1 is supplied to the lines L1 and L2. On the other hand, between the line L1 and the line L2 (ground side), the primary winding 24a of the transformer 24, the switching element 25 such as a transistor, and the resistor 37 for current detection are connected in series to form a primary circuit. ing.

The switching element 25 is driven in a pulse shape by the control circuit 26, and its conduction period and non-conduction period are controlled. A resistor 27 is connected between the power supply terminal Vc of the control circuit 26 and the line L1 to supply the power supply voltage of the control circuit 26. In addition, the power supply terminal Vc and the line L2
A smoothing capacitor 28 is connected between and. Further, the auxiliary winding 24b having the same polarity output as that of the primary winding 24a of the transformer 24 and outputting the voltage V2 lower than the supply voltage V1 to the primary winding 24a is the transformer 24.
Is separately provided, one end of the auxiliary winding 24b is connected to the line L2, and the other end of the auxiliary winding 24b is connected to the power supply terminal Vc of the control circuit 26 via the diode 29 for rectification. The ground terminal G of the control circuit 26 is connected to the line L2.

The connection point between the current detecting resistor 37 and the switching element 25 is connected to the current detecting terminal CLM of the control circuit 26 via the resistor 38, and the current I1 of the primary circuit is converted into a voltage signal to control the control circuit. It has been input to 26. Further, in order to suppress the fluctuation of the drooping start point of the overcurrent protection characteristic with respect to the voltage fluctuation of the line L1, the DC output portion of the auxiliary winding 24b, that is, the output of the diode 29 and the control circuit 26.
The correction resistor 39 is connected between the current detection terminal CLM and the current detection terminal CLM.

On the other hand, looking at the secondary circuit of the transformer 24,
One end of the secondary winding 24c is connected to the rectifying diode 30, and the other end of the secondary winding 24c is connected to the line L3. A flywheel diode 31 is connected between the diode 30 and the line L3. Further, a smoothing choke coil 32 and a capacitor 33 are connected between the output of the diode 30 and the line L3, and the capacitor 3
The output voltage Vo and the output current Io are output from the output terminals 34 and 35 connected to both ends of the output terminal 3 and are supplied to a load (not shown). The output voltage Vo is input to an isolator 36 such as a photocoupler that insulates the primary circuit from the secondary circuit, and the isolator 36 feeds the feedback terminal FB of the control circuit 26.
It is connected to the.

FIG. 2 is a block diagram showing an example of the internal configuration of the control circuit 26. This control circuit 26 is a PWM
An integrated circuit (for example, M51978P manufactured by Mitsubishi Electric) that switches a current of a primary circuit by (pulse width modulation), and includes a triangular wave oscillator 61 and a comparator 62.
, A PWM latch circuit 63, a current limit detection circuit 64, a NAND gate 65 and the like.

The triangular wave oscillator 61 outputs a triangular wave having a constant cycle and also has a falling period (dead time) of the triangular wave.
The sync pulse corresponding to is output. Comparator 62
Outputs the comparison signal by comparing the levels of the triangular wave from the triangular wave oscillator 61 and the input signal of the feedback terminal FB. The PWM latch circuit 63 is set by the comparison signal from the comparator 62 and reset by the synchronizing pulse from the triangular wave oscillator 61. The NAND gate 65 inverts the logical product of the output signal of the PWM latch circuit 63 and the synchronization pulse from the triangular wave oscillator 61, and outputs a switching signal from the output terminal OUT. When the input signal of the feedback terminal FB rises in this way, the high level period of the comparison signal from the comparator 62 becomes longer, the high level period of the output signal of the PWM latch circuit 63 also becomes longer, and the switching signal output from the NAND gate 65 becomes The duty decreases. The input mode of the feedback terminal FB is the same even in the current mode.

On the other hand, the current limit detection circuit 64 is
When the signal from the current detection terminal CLM exceeds a certain level, an overcurrent signal is output to forcibly set the PWM latch circuit 63. Thus, every time the current I1 of the primary circuit increases and enters the overcurrent state, the switching signal output from the NAND gate 65 becomes low level, and the current I1 of the primary circuit is cut off. Even if the PWM latch circuit 63 is set by the overcurrent signal, the triangular wave oscillator 61
The overcurrent detection operation is performed every cycle because it is reset by the synchronizing pulse from the. The input mode of the current detection terminal CLM is the same even in the current mode.

Next, the operation will be described with reference to the timing chart of FIG. When energization starts from the power supply 21, the DC voltage of the line L1 rises, and the control circuit 26 is energized via the resistor 27 to start the control operation. As shown in FIG. 3A, the control circuit 26 outputs a rectangular wave having a predetermined duty ratio (duty ratio) from the output terminal OUT to drive the switching element 25. When the switching element 25 is repeatedly turned on and off, the current in the primary circuit is intermittently switched as shown in FIG.
An AC voltage is induced in the auxiliary winding 24b and the secondary winding 24c magnetically coupled to the secondary winding 24a. 3B shows a voltage waveform at the connection point between the primary winding 24a and the switching element 25.

The AC voltage V2 induced in the auxiliary winding 24b is rectified by the diode 29, smoothed by the capacitor 28, and supplied as the power source of the control circuit 26. After that, the control circuit 26 continues to operate with the power supplied from the auxiliary winding 24b. The resistor 27 is for starting the control circuit 26.

On the other hand, the AC voltage induced in the secondary winding 24c is rectified by the diode 30, and the choke coil 3
It is smoothed by 2 and the capacitor 33, and converted into a DC voltage. Further, during the off period of the secondary winding 24c,
The flywheel diode 31 supplies a current to the choke coil 32. It should be noted that FIG. 3E shows a current flowing through the choke coil 32 when a constant load is connected. The output voltage Vo is input to the control circuit 26 via the isolator 36, and the control circuit 26 reduces the duty ratio of the switching element 25 when the output voltage Vo becomes high to 2
The power supplied to the secondary winding 24c is reduced, and conversely the output voltage Vo
When becomes low, the duty ratio of the switching element 25 is increased to increase the power supplied to the secondary winding 24c, thereby stabilizing the output voltage Vo.

Further, as shown in FIG. 3 (d), the current I1 flowing in the primary circuit is detected by the resistor 37 and detected by the control circuit 26.
When the current I1 becomes higher than a predetermined value and an overcurrent state is detected, the control circuit 26 turns off the switching element 25 for a certain period to protect the primary circuit from overcurrent. ing.

Further, a correction resistor 39 is connected between the DC output portion of the auxiliary winding 24b and the current detection terminal CLM. Therefore, when the voltage V1 of the line L1 changes,
Since the voltage of the DC output portion of the auxiliary winding 24b also changes, the correction resistor 39 enables the same level of signal correction as in the conventional case. For example, when the input voltage V1 becomes high, the correction resistor 3
Due to the presence of 9, the current I1 of the primary circuit apparently becomes large, and the overcurrent protection operates quickly.
As shown in (b), the variation of the drooping start point F with respect to the input voltage V1 is suppressed.

Now, a specific example of the correction resistor 39 will be considered.
First, regarding the conventional example shown in FIG. 5, for example, in the case of a 100 V input switching power supply, the input allowable voltage is generally A
The range is set to C85 to 132V. If AC13
When 2V is input, the voltage V1 of the line L1 is 132 ×
1.41 = 186 Vmax is reached. In order to obtain the correction effect of the correction resistor 19, it is necessary to flow a current of about several mA. Therefore, assuming that a maximum current of about 3 mA will flow, the resistance value is estimated to be 186 V / 3 mA = 55.8 kΩ, the power consumption P is calculated to be about 0.62 W, and the power rating is a resistance of at least 2 W or more in consideration of a margin. A device is required. The detection level of the current detection terminal CLM is 0.2.
It is about V and can be ignored in the above calculation.

On the other hand, with respect to the embodiment of FIG.
The maximum output voltage of 4b is 25V, and the resistance value is 25V /
It is estimated to be 3 mA = 7.5 kΩ, and the power consumption P is about 0.0.
It is calculated as 8W, and the power rating is 1 / 6W even if a margin is expected.
A resistance element of a certain level will suffice. In the case of a switching power supply with 200V input, the power consumption is doubled. Furthermore, in the present embodiment, even if a short circuit failure of the resistor 39 occurs, at most 2
Only a voltage of about 5V is applied and the safety is high.

FIG. 4 is a circuit diagram showing the electrical construction of another embodiment of the present invention. This embodiment is similar to that of FIG. 1, except that the current detection circuit is composed of a current transformer.

The primary winding 24a of the transformer 24, the switching element 25 such as a transistor, and the primary winding 40a of the current transformer 40 are connected in series between the line L1 and the line L2 (ground side) to form a primary connection. It constitutes the circuit. A rectifying diode 41 is connected to one end of the secondary winding 40b of the current transformer 40, and the other end is connected to the line L2.
Connected to. Further, a smoothing capacitor 42 and a resistor 43 are connected between the output side of the diode 41 and the line L2, the current I1 of the primary circuit is converted into a DC signal, and the current of the control circuit 26 is further passed through the resistor 44. Detection terminal CL
It is input to M. Incidentally, as in FIG. 1, the auxiliary winding 24
A correction resistor 39 is connected between the DC output section of b and the current detection terminal CLM to suppress the variation of the drooping start point.

With this configuration, even if the current I1 of the primary circuit is large, it can be converted into a low voltage by the current transformer 24, so that the current detection terminal CLM is used.
Can be easily adapted to the detection level of, and the detection efficiency can be improved.

In the above description, the forward system is illustrated as the circuit type of the switching power supply, but the present invention is not limited to this system, and the auxiliary winding wound in the same polarity as the primary winding of the transformer. If the correction signal is applied to the current detection terminal CLM from the rectified and smoothed output of, the same effect can be obtained. Further, as the switching element 25, not only a transistor but also an FET, an SCR or the like can be used.

[0035]

As described above, according to the present invention, 1
Even if the input voltage of the next circuit is as high as a commercial power source, the voltage applied across the correction resistor becomes small, and therefore the power loss and heat generation amount of the correction resistor are reduced. Therefore, the correction resistor may be a resistor having a small rating, and it is safe and inexpensive. Further, since the fluctuation of the drooping start point of the overcurrent protection characteristic is small, it is possible to use the parts other than the correction resistor that have a small rating and are inexpensive.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an electrical configuration of an embodiment of the present invention.

FIG. 2 is a block diagram showing an example of an internal configuration of a control circuit 26.

FIG. 3 is a timing chart showing waveforms of main parts.

FIG. 4 is a circuit diagram showing an electrical configuration of another embodiment of the present invention.

FIG. 5 is a circuit diagram showing an example of a conventional switching power supply device.

6 is a graph showing overcurrent protection characteristics, and FIG.
6A shows the case without the correction resistor 19, and FIG. 6B shows the case with the correction resistor 19.

[Explanation of symbols]

 21 power supply 22 diode bridge 24 transformer 24a primary winding 24b auxiliary winding 24c secondary winding 25 switching element 26 control circuit 31 flywheel diode 32 choke coil 36 isolator 39 correction resistor 40 current transformer

Claims (1)

[Claims] 1. A primary circuit in which a primary winding of a transformer, a switching element and a current detection circuit are connected in series, and a control circuit for driving the switching element in a pulse shape and controlling the conduction period of the switching element. In a switching power supply device for inputting a detection signal from a current detection circuit to a current detection terminal of a control circuit to protect an overcurrent in a primary circuit, the switching power supply has the same polarity as that of a primary winding of a transformer, and A correction resistor for correcting the detection signal is connected between the DC output section of another winding that outputs a voltage lower than the voltage supplied to the winding and the current detection terminal of the control circuit. Switching power supply.