JP2008148404A - Switching power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply device which will not elongate a starting time at powering on, while keeping the rise in the temperature of a transformer to be within an allowance, when the output load is short-circuited. <P>SOLUTION: At steady state, a PWM control circuit U1, which feeds an input DC current to a secondary winding S1 by converting it to a pulse current by an on/off-operation, obtains its starting voltage from a capacitor C1 charged with the input DC current flowing through a first starting resistor R1, by taking a prescribed time; whereas, when the output load Z of a secondary circuit is short-circuited and overcurrent flows to the transformer T1, short-circuiting of a second starting resistor R10 which is short-circuited at the stationary time is released, and the starting voltage of the PWM control circuit U1 is obtained by the currents flowing via a composite resistor formed of the first and the second resistors, by taking a time that is longer than a prescribed time at the stationary time. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a switching power supply device having a circuit protection function in case of an abnormality such as a load short circuit.

  FIG. 4 is a circuit diagram showing an example of the switching power supply device. The switching power supply 10 drives the voltage (energy) rectified and smoothed by the diode DB and the aluminum electrolytic capacitor CB by the main switch Q1 via the transformer T1. The basic configuration of such a switching power supply device is described in, for example, Japanese Patent Application Laid-Open No. 2006-246681.

  The transformer T1 has at least a primary main winding P1, a secondary output winding S1, and a primary winding P2, and a primary PWM (Pulse Width Modulation) control circuit U1 that performs PWM control of ON / OFF of the main switch Q1. Works with. The PWM control circuit U1 is a switching element that converts an input DC current into a pulse current by ON / OFF operation and supplies the pulse current to the secondary winding S1. The PWM control circuit U1 has a configuration in which a voltage at the time of start-up is obtained by a voltage obtained by charging the input DC current rectified and smoothed by the aluminum electrolytic capacitor CB to the capacitor C1 via the start-up resistor R1. When the PWM control circuit U1 is activated, ON / OFF control of the main switch Q1 starts, a voltage is generated in the winding P2, and the PWM control circuit U1 operates using this VCC voltage as a power source.

  FIG. 5 is a waveform diagram showing a control operation when the output load Z in the switching power supply device 10 of FIG. 4 is short-circuited due to some abnormality. FIG. 5A shows the switching of the switch Q1 and the VCC voltage generated in the primary winding P2 thereby. FIG. 5B shows the switching of the switch Q1 when the current flowing through the transformer T1 is reduced by extending the stop period (switching stop) with respect to the intermittent operation period (switching operation) of the switch Q1, and at that time The VCC voltage generated in the primary winding P2 is shown.

  When the secondary output load Z of the switching power supply device 10 is short-circuited, an excessive current flows through the current detection resistor R2, and the PWM control circuit U1 detects the detection voltage (the voltage across the resistor R2). The overcurrent protection of U1 works, and the pulse width of the main switch Q1 is narrowed as shown in the intermittent operation period of FIG. When the pulse width is narrowed and the supply power is insufficient, the voltage generated in the primary winding P2 is lowered, the VCC voltage is also lowered, and the intermittent operation stop voltage (9 V in this case) of the PWM control circuit U1 is lowered. The PWM control circuit U1 stops the power supply and enters a standby mode (stop period).

  When the standby mode is entered, the current consumed by the PWM control circuit U1 decreases, and the current flowing through the starting resistor R1 begins to charge the rectifying capacitor (VCC capacitor) C1. When the VCC voltage rises to the intermittent operation start voltage (15V in this case) of the PWM control circuit U1, the PWM control circuit U1 is activated and outputs a pulse signal for driving the main switch Q1 again. Although the power supply starts to operate, since the output load Z is short-circuited, the pulse width is narrowed again in the intermittent operation period, the power supply is stopped, and the standby mode (stop period) is entered. Thus, the stop period and the intermittent operation period are repeated.

  However, as shown in FIG. 5A, when the stop period (Q1 switching stop) is equal to 100 ms for the intermittent operation period (Q1 switching operation) of 100 ms, the stop period is short and the transformer T1 The current flowing in the current increases, exceeding the temperature tolerance value in safety standards. In order to lower the temperature, it is necessary to suppress the current flowing through the transformer T1. In order to suppress the current, two measures are conceivable: (1) reducing the current during operation in the intermittent mode, and (2) extending the stop period.

  In order to take the above measure (1), specifically, the overcurrent protection input voltage correction resistor R3 may be changed from 94 kΩ to 80 kΩ, for example, to lower the overcurrent protection threshold (current set value). If the power is lowered to about 70 kΩ, the pulse is not expanded at the time of starting the power source, and there is a possibility of starting failure, so the overcurrent set value can only be lowered to about 80 kΩ.

  In order to take the above measure (2), specifically, the starting resistance R1 may be changed from 6 kΩ to 27 kΩ and the stop period may be extended. The constants of these parts are adjusted to optimum values according to the power supply specifications.

When these countermeasures (1) and (2) are taken, as shown in FIG. 5B, the stop period (Q1 switching stop) is extended with respect to the intermittent operation period (during Q1 switching operation), and the transformer T1 The flowing current is reduced.
JP 2006-246681 A

  However, by taking the above two measures, the temperature of the transformer T1 can be suppressed to within an allowable value, but the startup time when the power is turned on (the time from when the input voltage is turned on until the secondary output voltage rises) is reduced. It takes about 5 seconds (5 times the time before the measure). When such a degradation occurs, there is a risk that the user may be suspected of having a malfunction.

  In view of such a problem, the present invention provides a switching power supply apparatus that keeps the temperature rise of the transformer when the output load is short-circuited within an allowable value and sets the startup time at power-on to the same level as before the above countermeasures. For the purpose.

In order to solve the above-described problems, the present invention provides a transformer having a primary winding and a secondary winding;
In a switching power supply apparatus including a switching element that converts an input DC current into a pulse current by ON / OFF operation and supplies the pulse current to a secondary winding, the switching element is charged by charging the input DC current via a first starting resistor. A capacitor that generates a starting voltage for a predetermined time, a second starting resistor that is connected in series to the first starting resistor and is short-circuited in a steady state, and the output load of the secondary circuit is short-circuited and excessively passed to the transformer. When the current flows, the short circuit of the second starting resistor is released, the input DC current is charged to the capacitor via the combined resistance of the first and second resistors, and the time longer than the predetermined time in the steady state is taken. And a starting resistance increasing means for generating a starting voltage in the capacitor.

  The above switching element continues the intermittent operation of narrowing the pulse width of the pulse current from when the starting voltage is generated until the voltage decreases to a predetermined value, thereby suppressing the rise in temperature generated in the transformer due to overcurrent. .

  According to the present invention, the startup time (stop period) can be extended only when an abnormality such as an output load short-circuit occurs without extending the startup time in a steady state, and the switching current flowing through the transformer can be reduced. As a result, loss can be reduced and temperature rise can be suppressed.

  Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the figure, elements not directly related to the present invention are not shown. Similar elements are denoted by the same reference numerals. Further, signals and currents are expressed by the codes of the lines through which they pass.

  FIG. 1 is a circuit diagram of a switching power supply device showing an embodiment of the present invention, in which an additional portion indicated by a dotted line is added to the circuit diagram of FIG. Description of the parts common to FIG. 4 will be omitted, and additional parts indicated by dotted lines will be described below.

  The switching power supply device 100 of FIG. 1 is obtained by additionally connecting a second starting resistor R10 for short-circuiting the output load in series with the first starting resistor R1 to open / short the both ends of the starting resistor R10. Switch Q10, a switch Q31 for controlling ON / OFF of the base voltage of the switch Q10, comparators U10 and U11, and the like. As will be described later, these switches Q10, Q31 and comparators U10, U11 serve as means for increasing the starting resistance so that the starting resistance is changed from a single R1 state to a series combined resistance of R1 and R10 when the load is short-circuited. Yes.

  At the time of steady operation and startup, the switch Q10 is turned ON, the startup resistor R10 is short-circuited, and the VCC capacitor C1 is charged only by the startup resistor R1. If an overcurrent flows through the transformer T1 when the output load is short-circuited, the switch Q10 is turned OFF (the short-circuit of the starting resistor R10 is released), and the VCC capacitor C1 is charged via the series combined resistor of the starting resistor R1 and the starting resistor R10. . In this case, since the resistance value increases several times, a start time (stop period) longer than the start time required in the steady state is required, and the current flowing through the transformer T1 can be reduced.

  FIG. 2 is a waveform diagram showing an operation in the case where the switching power supply device shown in FIG. As shown in FIG. 2 (a), the VCC voltage is maintained at about 13V in a steady state. Since this value exceeds the threshold 10V of the comparator U10, the output U10-OUT of the comparator U10 in FIG. 2B is at a low level, and the output U11-OUT of the comparator U11 in FIG. 2D is at a high level. Since the switch Q31 is in the ON state and the switch Q10 in the next stage is also in the ON state, the starting resistor R10 is in a short state and can be regarded as not present.

  When the output load Z is short-circuited, the VCC voltage decreases as described with reference to FIGS. When the VCC voltage falls below the threshold (10V) of the comparator U10, the output U10-OUT of the comparator U10 becomes a high level (14V).

  When the VCC voltage further decreases to the intermittent operation stop voltage (9V), the PWM control circuit U1 enters the standby mode, starts to charge the VCC capacitor C1 again via the starting resistor R1, and rises to 10V, the output U10-OUT of the comparator U10 Decreases to a low level (0 V). This is repeated.

  The pulse voltage of the output U10-OUT of the comparator U10 is applied to the capacitor C21 via the resistor R24. The diode D20 is a backflow prevention diode, which prevents the electric charge charged in the capacitor C21 when the output U10-OUT of the comparator U10 is at a high level from being discharged when the output is at a low level. is there. The resistor R25 is a discharge resistor for charges stored in the capacitor C21. The voltage across the capacitor C21 is as shown in FIG.

  By setting the resistance R24 through which the charging current flows to about 150 kΩ and the resistance R25 through which the discharging current flows to about 470 kΩ, charging is performed more rapidly than discharging. As a result, when the voltage of the capacitor C21 rises and reaches the threshold (5V) of the comparator U11, the output U11-OUT becomes a low level, the switch Q31 is turned off, and the next-stage switch Q10 is also turned off.

  As a result, the starting resistance becomes a series resistance of (starting resistance R1 + starting resistance R10), and the restart time (stop period) is extended. Therefore, the current flowing through the transformer T1 in FIG. 1 is reduced, and the temperature rise of the transformer T1 is suppressed.

  On the other hand, when the output load short circuit is released, the VCC voltage returns to the steady state of 13 V, and the output U10-OUT of the comparator U10 falls to the low level (0 V). The electric charge charged in the capacitor C21 is discharged through the resistor R25 and returns to a steady state.

  According to the above operation of the embodiment of the present invention, in a steady state, the resistance value of the starting resistor R1 is about 6 kΩ, the power source starting time (VCC voltage: time from 0 V to 15 V) is about 1 second, and the output When the intermittent mode due to load short-circuiting etc. continues, the starting resistance is switched to about 30 kΩ (= starting resistance R1 + starting resistance R10), the intermittent mode stop period is extended to about 5 seconds, and the current flowing through the transformer T1 is reduced to reduce the Limit the rise to within the allowable value.

  FIG. 3 is a waveform diagram showing an operation at the time of startup of the switching power supply device of FIG. Before startup, all voltages are 0V. When the input voltage is applied, as shown in FIGS. 3A and 3B, the smoothing capacitor CB and the VCC2 voltage rise instantaneously.

  The VCC voltage shown in FIG. 3C rises from 0 V by charging the VCC capacitor C1 through the starting resistor. Since the VCC voltage at the rise of the VCC voltage is equal to or lower than the threshold (10V) of the comparator U10, the output U10-OUT of the comparator U10 outputs a high level (14V) and charges the capacitor C21 via the resistor R24. The voltage across the capacitor C21 is as shown in FIG.

  Before the voltage of the capacitor C21 reaches the threshold (5V) of the comparator U11, the time constants of R24 and C21 / R25 are set in advance so that the VCC voltage becomes equal to or higher than the threshold (10V) of the comparator U10. This period becomes dead time. Assuming that the starting resistance R1 = 6 kΩ and the VCC capacitor C1 = 100 μF, the charging time until the threshold (10V) of the comparator U10 is reached is about 0.4 seconds. Therefore, it is preferable to set it to twice or more 0.8 seconds in consideration of the margin.

  Thus, since the VCC voltage becomes equal to or higher than the threshold (10V) of the comparator U10 before the voltage of the capacitor C21 reaches the threshold (5V) of the comparator U11, the switch Q10 maintains the ON state and only the starting resistor R1. Thus, the VCC capacitor C1 is charged, and the startup time is not extended.

  In this embodiment, when an intermittent operation occurs due to a short circuit of the output load or the like, a drop / rise in the VCC voltage is detected, and when the fall / rise continues for a certain period, it is detected as an abnormal state, Extend outages and reduce losses. In this embodiment, the duration of the detected pulse voltage (output U10-OUT of the comparator U10) is detected using the time constants of R24 and C21 / R25. However, a method for detecting the duration of the detected pulse voltage is as follows. Other than the above method. For example, the number of pulses may be counted using a pulse counter, and when the specified number of times is exceeded, the switch Q10 may be turned off to extend the stop period.

  Further, the method for detecting the intermittent operation may not be the VCC voltage, but may directly detect the voltage Vds of the main switch Q1 or the VCC winding voltage.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to these embodiment. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

  The present invention can be applied to a switching power supply device having a circuit protection function in case of an abnormality such as a load short circuit.

It is a circuit diagram of a switching power supply device showing an embodiment of the present invention. It is a wave form diagram which shows operation | movement in case the switching power supply apparatus of FIG. It is a wave form diagram which shows the operation | movement at the time of starting of the switching power supply device of FIG. It is a circuit diagram which shows an example of a switching power supply device. FIG. 5 is a waveform diagram showing a control operation when an output load in the switching power supply device of FIG. 4 is short-circuited due to some abnormality.

Explanation of symbols

10, 100 Switching power supply device R1, R10 Starting resistance C1, C21 Capacitor Q1, Q10, Q31 Switch U1 PWM control circuit U10, U11 Comparator

Claims (3)

A transformer having a primary winding and a secondary winding;
A switching power supply device including a switching element that converts an input DC current into a pulse current by an ON / OFF operation and supplies the pulse current to the secondary winding;
A capacitor that charges the input direct current through a first starting resistor to generate a starting voltage of the switching element over a predetermined time;
A second starting resistor connected in series to the first starting resistor and short-circuited in a steady state;
When the output load of the secondary circuit is short-circuited and an overcurrent flows through the transformer, the short-circuit of the second starting resistor is released, and the input DC current is supplied to the capacitor via the combined resistance of the first and second resistors. And a starting resistance increasing means for causing the capacitor to generate the starting voltage over a time longer than the predetermined time in a steady state.   The switching element continues the intermittent operation of narrowing the pulse width of the pulse current from when the start-up voltage is generated until the voltage decreases to a predetermined value, thereby increasing the temperature generated in the transformer due to the overcurrent. The switching power supply device according to claim 1, wherein the switching power supply device is suppressed. The starting resistance increasing means includes
A first comparator that sets the output to a high level when the start-up voltage of the switching element decreases to a first threshold voltage;
A capacitor that is charged while the output of the first comparator is at a high level and is discharged while it is at a low level, and a capacitor that is charged more rapidly than discharging;
A second comparator for setting the output to a low level when the voltage of the capacitor rises to a second threshold voltage by the charging and discharging;
A first switch which is turned off when the output of the second comparator becomes low level;
It is connected to both ends of the second starting resistor, and is configured by a second switch that releases a short circuit of the second starting resistor by being turned off when the first switch is turned off. The switching power supply device according to claim 1 or 2.

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