JP3144673U - Overcurrent protection for switching power supply - Google Patents

Abstract

A switching power supply with an overcurrent protection function that stops an output voltage when an overload condition continues on a DC cable or the like.
An overcurrent limiting circuit is provided in a secondary side circuit, and a circuit for detecting the output voltage of the tertiary winding by rectifying and smoothing the primary side winding voltage is provided. When the voltage of the tertiary winding decreases due to, for example, an overload, it is compared with a predetermined reference voltage. When the voltage drops below the reference voltage, the time constant circuit provided in the latch circuit of the control circuit is operated to Stop operation.
[Selection] Figure 1

Description

  The present invention relates to a switching power supply, and more particularly to a circuit with an overcurrent protection function that stops an output voltage when an overload condition continues.

  In the conventional switching power supply, when an output short circuit or an abnormality occurs in the load and an overload operation is started, the output current is suppressed and the load current value is prevented from increasing. There exists patent document 1 as this prior art.

  Also, when the overcurrent operation continues or when the load is short-circuited, in the conventional switching power supply, the power supply voltage of the control circuit falls below the operating voltage due to the decrease of the primary side winding voltage as the control circuit power supply. Therefore, the switching operation of the switching power supply is stopped, and thereafter, the battery is charged from the starting circuit and restarted. This was designed to restore the device to normal operation by removing the cause of the output short circuit or overload. However, unless the cause of the output short circuit or overload is removed, the above oscillation stop and restart are repeated. Therefore, in applications such as an AC adapter, the DC cable may be bent or sandwiched between objects to cause a short circuit or impedance short circuit. However, there is a problem that the DC cable generates heat.

Here, a method is also conceivable in which the overcurrent protection characteristic is a U-shaped drooping characteristic and the overoutput current value during drooping is made smaller than the rated output current value to suppress heat generation. However, when a device with constant current load characteristics where the input current of the load is a constant value regardless of the power supply voltage applied to the load is connected, the overcurrent drooping characteristic is limited to a range smaller than the rated output current value. If it is, the output voltage will not rise. For this reason, it is necessary to set the overcurrent drooping characteristic to be equal to or higher than the rated current, and the above-mentioned problems have not been solved.
JP 05-153774 A

The problems and necessary conditions related to the overcurrent protection of the switching power supply are that there is a danger that the DC cable of the output wiring will generate heat due to the overcurrent state continuing, but the overcurrent drooping characteristic requires the constant current drooping characteristic. ing.
Therefore, since it is difficult to improve the overcurrent drooping characteristic itself in the present invention, the overcurrent protection means when the overcurrent state at the time of output short circuit or overload continues for a certain period is reviewed, and the heat generation of the DC cable is reviewed. The purpose is to provide a safe switching power supply.

  In order to achieve the above object, the switching power supply of the present invention employs the following means. The switching power supply of the present invention has a DC power supply and a series circuit of a primary winding of the transformer and a main switch element connected in series with the DC power supply, and the voltage of the secondary winding and the secondary winding of the transformer is the first. A power conversion circuit that supplies power to the load via the rectifying / smoothing circuit, an overcurrent limiting circuit that limits an output current when the load is short-circuited or overloaded to the output of the first rectifying / smoothing circuit, and stably controlling the output voltage Therefore, in the switching power source in which the control circuit for controlling on / off of the main switch element and the second rectifying / smoothing circuit constituting the driving power source for the control circuit are connected to the tertiary winding of the transformer, The smoothing capacitor has a capacity capable of supplying a voltage that allows the control circuit to operate for a first predetermined time or more even when there is no supply from the tertiary winding, and the third rectifying and smoothing circuit connected to the tertiary winding and the third winding 3 output voltage of rectifying and smoothing circuit A third winding voltage detecting circuit is provided, and the smoothing capacity of the third rectifying and smoothing circuit is smaller than the smoothing capacity of the second rectifying and smoothing circuit. When the line voltage decreases, the tertiary winding voltage also decreases at the same rate, so that the output voltage of the third rectifying / smoothing circuit decreases, and the tertiary winding voltage detection circuit compares the predetermined voltage with a predetermined reference voltage. When the voltage of the third rectifying / smoothing circuit is lower than the reference voltage, an off signal is sent to the on / off terminal of the control circuit after a second predetermined time longer than the first predetermined time to control the switching power supply The operation of the circuit is stopped.

If the overload or output short-circuit condition continues, the second rectifying and smoothing circuit voltage becomes less than the voltage at which the control circuit operates, the control circuit operation is turned off, and the power supply voltage of the control circuit is charged by the start circuit and restarted. During a period of repeated stoppages, an off signal is sent to the on / off terminal of the control circuit via the tertiary winding voltage detection circuit and the time constant circuit, and the operation of the control circuit of the switching power supply is performed. It is characterized by being stopped.

  The overcurrent protection circuit of the switching power supply according to the present invention is limited by a predetermined output current by the overcurrent limiting circuit when the load state becomes an overcurrent state. As a result, the output voltage is lowered and coupled by the transformer. The voltage of the tertiary winding also decreases at the same rate. Here, the voltage drop of the tertiary winding voltage is monitored, and when the voltage drop continues even after a predetermined time, the switching operation of the switching power supply is stopped and the power supply to the secondary load is stopped. Therefore, it is possible to prevent burnout due to overload and heat generation failure of the load wire due to overcurrent.

  Hereinafter, the most preferred embodiment of the switching power supply of the present invention will be specifically described with reference to the drawings.

  FIG. 1 is a diagram showing a configuration of a switching power supply according to the first embodiment of the present invention. The difference between the conventional example and FIG. 3 is that when a voltage drop is determined based on the detection circuit 12 for detecting the voltage of the tertiary winding and the voltage detection signal, an off signal is sent to the control circuit after a predetermined time. In the configuration having the constant circuit 11, the same reference numerals are given to other similar parts.

First, the configuration of the switching power supply will be described with reference to FIG. 1 of the present invention.
1 includes a flyback converter, and includes a rectifier circuit 2, a smoothing capacitor C1, a transformer T1, a main switching element Q1, a control circuit 10, a starting resistor R1, a secondary side rectifying and smoothing circuit 20, and an output voltage detection circuit 22. The current limiting circuit 21, the tertiary winding rectifying and smoothing circuits D3 and C3 of the transformer T1, the tertiary winding voltage detecting circuit 12, and a stop signal sending circuit 11 having a time constant circuit are included. The stop signal transmission circuit 11 is connected to the operation stop terminal of the control circuit 10.
The voltage of the AC power source 1 is rectified and smoothed to a DC voltage by the rectifier circuit 2 and the capacitor C1, and a pair of positive and negative output terminals of the DC voltage is connected to a series circuit of the primary winding N1 of the transformer T1 and the main switching element Q1.

The DC voltage is converted into an AC voltage by turning on and off the main switching element, converted into a secondary output by the transformer T1, and converted into a DC output voltage by the rectifying and smoothing circuit 20. The output voltage is feedback-controlled by the output voltage detection circuit 22 to the control circuit 10 on the primary side via the photocoupler PC1.
On the secondary side, an overcurrent limiting circuit 21 is provided. When an overload occurs, the voltage drop of the resistor R5 becomes large, the transistor Q2 is turned on, and the output current is increased by increasing the current flowing through the photocoupler PC1. The output voltage is lowered as a result of limiting the output current. Although not shown, in the flyback converter having no overcurrent limiting circuit having the overcurrent limiting circuit 21 or the like on the secondary side, particularly when the input voltage range is wide, the overcurrent droop of the maximum input voltage with respect to the minimum input voltage The point increases to about 3 to 5 times, and the overcurrent value increases as the voltage drops. Therefore, the overcurrent value increases up to 10 to 20 times at the maximum. For this reason, the secondary overcurrent limiting circuit is indispensable as a protection function for an AC adapter or the like which is an application of the present invention. By having the overcurrent limiting circuit 21 on the secondary side, the current is limited in the event of an overload, and the output voltage decreases, and the voltage of the tertiary winding N3 coupled by the transformer T1 as the output voltage decreases is the same ratio. Decrease.

Here, the voltage of the tertiary winding N3 includes a second rectifying / smoothing circuit 13 and a third rectifying / smoothing circuit 14 which are driving voltages of the control circuit 10. The second rectifying / smoothing circuit 13 charges the input voltage with the capacitor C3 via the resistor R1, temporarily supplies the charging voltage of C3 as the control circuit power supply when the switching power supply is activated, and after the on / off switching starts, the third rectifying / smoothing circuit 13 It plays a role of supplying the control circuit power supply voltage by rectifying and smoothing from the voltage of the winding. The smoothing capacity of C3 is set to a relatively large value than the C4 capacity value of the third rectifying and smoothing circuit, and the holding time from switching of the resistance R1 to the tertiary winding voltage at the time of start-up, output overload, etc. The control circuit is provided with a capacity capable of supplying a voltage capable of operating for a first predetermined time or more even when there is no supply from the tertiary winding, because the smoothing voltage of C3 is less susceptible to fluctuations.

The C4 smoothing capacity of the third rectifying / smoothing circuit is set to a capacity value smaller than the C3 smoothing capacity value of the second rectifying / smoothing circuit so that the output voltage of the secondary winding N2 on the secondary side is passed through the transformer T1. The output voltage of the third rectifying / smoothing circuit 14 can be quickly reflected by the voltage of the tertiary winding N3. The tertiary winding voltage detection circuit has means for detecting the smoothing voltage of the third rectifying / smoothing circuit 14 with a predetermined reference voltage, that is, the base voltage of the transistor Q3. The charging of the stop signal transmission circuit 11 by R8, C5 is started by turning off, and the operation of the control circuit 10 is stopped after a delay time until the off voltage of the on / off terminal of the control circuit 10 is reached. Does not lead to a reboot. Note that the delay time (second predetermined time) due to the R8, C5 time constant of the stop signal transmission circuit 11 is longer than the time constants R1, C3 at the time of switching power supply activation. Here, when the overload or the output short-circuit state continues, the second rectifying / smoothing circuit voltage becomes less than the voltage at which the control circuit operates, the operation of the control circuit is turned off, and the power supply voltage of the control circuit is charged by the starting circuit. The control circuit repeats restarting and stopping, but an off signal is output after the second predetermined time by the tertiary winding voltage detection circuit and the stop signal sending circuit, and the control circuit can be shifted to stop.
The relationship between the time constants R1 and C3 when the switching power supply is activated and the second predetermined time is as follows.
Time constant R1, C3 <R8, C5 time constant at startup of switching power supply The second predetermined time by the time constant As described above, the output short circuit or overload state does not continue continuously, and it stops after a predetermined time, thereby causing trouble due to component heat generation Can be prevented. In order to restart the power supply, the AC power supply 1 can be turned off to discharge C3 and C5 of the stop signal transmission circuit 11, remove the fault of the load, and restart the AC power supply.

  The overcurrent protection of the switching power supply according to the present invention can be safely stopped without causing abnormal heating of the DC cable or load even if an overcurrent due to an impedance short circuit due to bending of the DC cable or an overload occurs.

1 is a circuit diagram of a switching power supply according to an embodiment of the present invention. It is an overcurrent characteristic figure of a switching power supply by one example of the present invention. It is a circuit diagram of the conventional switching power supply. It is an overcurrent characteristic figure of the conventional switching power supply.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 AC power supply 2 Rectifier circuit 3 Flyback converter 10 Control circuit 11 Stop signal transmission circuit 12 Tertiary winding voltage detection circuit 13 2nd rectification smoothing circuit 14 3rd rectification smoothing circuit 20 1st rectification smoothing circuit 21 Overcurrent Limit circuit 22 Output voltage detection circuit C1 Input smoothing capacitor Q1 Main switching element R1 Start resistance C3 Capacitor D3 Diode T1 Transformer PC1 Photocoupler

Claims (2)

  A DC power supply and a series circuit of a primary winding of the transformer and a main switch element are connected in series with the DC power supply, and the voltage of the secondary winding and the secondary winding of the transformer is passed through the first rectifying and smoothing circuit. A power conversion circuit for supplying power to the load, an overcurrent limiting circuit for limiting an output current when the load is short-circuited or overloaded to the output of the first rectifying and smoothing circuit, and a main switch element for stably controlling the output voltage In a switching power supply in which a control circuit for on / off control of the power supply circuit and a second rectifying / smoothing circuit constituting the driving power supply of the control circuit are connected to the tertiary winding of the transformer, the smoothing capacity of the second rectifying / smoothing circuit is Even when there is no supply from the tertiary winding, the control circuit has a capacity capable of supplying a voltage capable of operating for a first predetermined time or more, and a third rectifying and smoothing circuit connected to the tertiary winding and a third Detects the output voltage of the rectifying / smoothing circuit A third winding voltage detecting circuit is provided, and the smoothing capacity of the third rectifying and smoothing circuit is smaller than the smoothing capacity of the second rectifying and smoothing circuit. The output voltage of the third rectifying / smoothing circuit also decreases in conjunction with a decrease in the output voltage of the smoothing circuit, and the third winding voltage detection circuit compares the output voltage with a predetermined reference voltage. When the voltage of the rectifying / smoothing circuit 3 decreases, an off signal is sent to the on / off terminal of the control circuit after a second predetermined time longer than the first predetermined time, and the operation of the control circuit of the switching power supply The switching power supply characterized by stopping.   If an overload or output short-circuit condition continues, the second rectifying and smoothing circuit voltage becomes less than the voltage at which the control circuit operates, the control circuit operation is turned off, and the power supply voltage of the control circuit is charged by the start circuit to restart and stop During the period of repeating the above, an off signal is sent to the on / off terminal of the control circuit via the tertiary winding voltage detection circuit and the time constant circuit to stop the operation of the control circuit of the switching power supply The overcurrent protection of the switching power supply according to claim 1.

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