JP4290662B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply device having an overload protection function that detects an overload when an abnormality occurs in a secondary side output and reduces output power.

  In recent years, a switching power supply in which a switching element and its control circuit are formed on the same semiconductor chip has been required to have a technique for giving the control circuit more functions and reducing the number of parts as a power supply as much as possible.

  One of such technologies is overload protection (see, for example, Patent Document 1). Overload protection is a function that suppresses output voltage and output current in the event of an abnormality such as a short on the secondary side. Especially in the method called the U-shaped protection, both the output voltage and output current are reduced as much as possible. Reduce the output power when the side is abnormal.

  FIG. 4 is a circuit example of a conventional switching power supply semiconductor device 30 having an overload protection function and a switching power supply using the same.

  This conventional semiconductor device for switching power supply is composed of a switching element 1 such as a power MOSFET and a circuit for performing switching control of the switching element 1, and includes a high voltage terminal (DRAIN terminal) 61 of the switching element 1, a GND terminal. 62, a VCC terminal 63 that receives power supply from the auxiliary winding 40D of the transformer 40, a VDD terminal 64 that serves as a power supply line of the internal circuit, an FB terminal 65 that receives a feedback signal of the secondary output, and a CL terminal that performs overload detection 66 terminals are provided.

  In the switching power supply semiconductor device 30, the drain current detection circuit 6 converts the drain current flowing through the switching element 1 into a voltage signal and outputs the voltage signal to the drain current detection comparator 8.

  The oscillation circuit 9 determines the turn-on timing of the switching element 1 and outputs a signal of 100 kHz during normal operation.

  Reference numeral 3 denotes a starting current source connected to the DRAIN terminal 61. Reference numeral 2 denotes a regulator, which supplies current from the startup current source 3 to the VCC terminal 63 and the VDD terminal 64 during startup. When the VCC voltage reaches the VCC startup voltage, the VCC terminal 63 supplies power to the VDD terminal 64. Switch.

  Reference numeral 7 denotes a start / stop circuit, which stops the oscillation of the switching element 1 when the VDD voltage is equal to or lower than the VDD start voltage.

  Reference numeral 10 denotes a first RS flip-flop, to which the output of the oscillation circuit 9 and the drain current detection comparator 8 is connected. The oscillation circuit 9 determines the turn-on of the switching element 1 and the drain current detection comparator 8 determines the turn-off.

  The feedback signal control circuit 11 is connected to the FB terminal 65, converts a current signal flowing into the FB terminal 65 into a voltage signal, and is connected to the inverting input terminal of the drain current detection comparator 8.

  That is, current mode control is performed in which the drain current flowing through the switching element 1 is controlled according to the current flowing through the FB terminal 65.

  Further, the clamp circuit 12 is connected to the inverting input terminal of the drain current detection comparator 8, and the clamp voltage variable circuit 13 connected to the CL terminal 66 is connected to the clamp circuit 12. When the current flowing into the CL terminal 66 decreases, the clamp voltage variable circuit 13 converts this into a voltage signal, and the clamp circuit 12 controls the maximum drain current ILIMIT in proportion to the CL terminal current. Further, the clamp voltage variable circuit 13 outputs an overload detection signal CL_low to the oscillation circuit 9 when the current ICL of the CL terminal 66 becomes a certain level or lower, and the oscillation frequency is reduced from 100 kHz to 12 kHz.

  Reference numeral 40 denotes a power conversion transformer. The primary winding 40A is connected to the DRAIN terminal 61, and the auxiliary winding 40D is connected to the VCC terminal 63 via the rectifier diode 31 and the smoothing capacitor 32. The secondary winding 40 </ b> B is connected to the secondary rectifier diode 50 and the secondary smoothing capacitor 51.

  58 is a secondary side constant voltage control circuit using the secondary side IC for constant voltage control, and operates so as to keep the secondary side output at a constant voltage. 57 is a load. When the load 57 is a light load and the secondary side output voltage exceeds the rated voltage, the secondary side constant voltage control circuit 58 causes a current to flow to the photocoupler 35 that is a control signal transmission circuit, and the FB to which the photocoupler 35 is connected. A current flows through the terminal 65. 52 is a constant voltage diode, 53 is a capacitor, and 54, 55 and 56 are resistors.

  Reference numeral 33 denotes a capacitor for stabilizing the VDD voltage.

  An overload detection resistor 34 is connected to the VCC terminal 63 and the CL terminal 66.

  The operation of the switching power supply unit configured as described above will be described below.

  First, when AC power is supplied to the input terminal Vin of the switching power supply, the DC power is rectified and smoothed by the rectifier circuit 70 and the input smoothing capacitor 71 and supplied to the primary winding 40A of the power conversion transformer 40.

  Further, current is supplied from the DRAIN terminal 61 to the VCC terminal 63 via the power conversion transformer 40, and the smoothing capacitor 32 connected to the VCC terminal 63 is charged. When the VCC voltage rises to some extent, current is also supplied from the VCC terminal 63 to the VDD terminal 64, and the stabilization capacitor 33 connected to the VDD terminal 64 is charged. When the VDD voltage reaches the starting voltage VUV, the starting / stopping circuit 7 operates to start oscillation of the switching element 1 via the NAND circuit 5 and the inverter 4, and at the same time, supply of current from the drain to VCC is stopped.

  When the oscillation is started, the secondary output of the transformer 40 rises, and the auxiliary winding voltage also rises. Therefore, the current is supplied to the VCC terminal 63 from the auxiliary winding 40D.

  A signal from the secondary constant voltage control circuit 58 is input to the FB terminal 65 via the photocoupler 35. When the secondary output voltage exceeds the rating, the feedback current IFB flows to the FB terminal 65. The clamp circuit 12 determines the overcurrent protection level ILIMIT according to IFB, and controls the drain current of the switching element 1.

  Next, when an overload occurs, the secondary output voltage decreases, and the VCC voltage connected to the auxiliary winding 40D also decreases in proportion thereto. When the VCC voltage decreases, the current flowing into the CL terminal 66 via the overload detection resistor 34 decreases. The clamp voltage variable circuit 13 detects this, and outputs an ILIMIT drop signal to the clamp circuit 12 and a frequency drop signal CL_low to the oscillation circuit 9.

As a result, the output power is rapidly reduced.
JP 2003-333843 A

  When the load suddenly changes from light load to overload, first, the FB current increases in order to keep the secondary output voltage at the rated voltage, and the drain current flows up to the maximum current specified for the product, causing full oscillation. However, if the secondary output continues to decrease, the VCC voltage also gradually decreases and shifts to the overload mode.

  However, when the transformer leakage inductance is large, the VCC voltage does not decrease immediately even if the secondary output voltage decreases, so the overload mode is not entered. During that time, the FB current continues to flow, and the switching element 1 Had the problem of oscillating at the maximum current.

  In view of the above, an object of the present invention is to provide a switching power supply device having an overload protection function that operates reliably even when a load suddenly changes.

The switching power supply device of the present invention includes a transformer having a primary winding, a secondary winding, and an auxiliary winding,
Switching means for switching a DC voltage input to the primary winding and outputting a pulse voltage to the secondary winding and the auxiliary winding;
An output circuit that outputs a DC voltage obtained by rectifying and smoothing the pulse voltage of the secondary winding to the load;
A feedback circuit that outputs a feedback signal that feeds back the output voltage of the output circuit;
A control circuit for controlling the switching operation of the switching means based on the feedback signal;
A rectifying circuit for rectifying and smoothing the current pulse of the auxiliary winding;
When the auxiliary winding voltage output from the rectifier circuit decreases to the overload detection level in proportion to the output voltage, the control circuit is controlled according to the decrease in the auxiliary winding voltage, and the power supplied to the secondary winding is reduced. With overload protection means to reduce,
Overload protection means, upon overload, have a overload discharge circuit for forcibly lowering the auxiliary winding voltage,
The overload discharge circuit is a secondary side start detection circuit that detects when the output circuit reaches the rated voltage and starts, an overload detection circuit that detects that the load of the output circuit is overloaded, and is supplemented by discharge And a discharge means for reducing the winding voltage .

  In the above configuration, the secondary side activation detection circuit detects the feedback signal.

  In the above configuration, the overload detection circuit detects the feedback signal.

  In the above configuration, the overload discharge circuit stops the discharge when the auxiliary winding voltage drops to the overload detection level.

  Since the overload discharge circuit that forcibly discharges the VCC voltage during overload is added, the conventional problem that the VCC voltage declines slowly during overload can be solved, and it can operate safely and reliably even during sudden load changes. The power loss of the switching power supply can be minimized.

  In addition, since it has a secondary-side startup detection circuit and an overload detection circuit, it can no longer start when the overload discharge circuit operates during startup, so it can not operate during startup but can operate only during overload. .

  Hereinafter, a switching power supply semiconductor device according to an embodiment of the present invention will be described with reference to the drawings.

  First, FIG. 1 shows a switching power supply semiconductor device 30 having an overload protection function according to an embodiment of the present invention and a power supply circuit example using the same.

  The semiconductor device for a switching power supply according to the present invention includes a switching element 1 such as a power MOSFET and a circuit for performing switching control of the switching element 1, and includes a high voltage terminal (DRAIN terminal) 61 and a GND terminal of the switching element 1. 62, a VCC terminal 63 that receives power supply from the auxiliary winding 40D of the transformer 40, a VDD terminal 64 that serves as a power supply line of the internal circuit, an FB terminal 65 that receives a feedback signal of the secondary output, and a CL terminal that performs overload detection 66 terminals are provided.

  In this switching power supply semiconductor device 30, the drain current detection circuit 6 converts the drain current flowing through the switching element 1 into a voltage signal and outputs the voltage signal to the drain current detection comparator 8.

  The oscillation circuit 9 determines the turn-on timing of the switching element 1 and outputs a signal of 100 kHz during normal operation.

  Reference numeral 3 denotes an activation current source for supplying an activation circuit current, which is connected to the DRAIN terminal 61. At startup, a startup current is supplied to the VCC terminal 63 via the switch 2A of the regulator 2. When the VCC voltage is equal to or lower than the set voltage after startup, a circuit current is supplied to the VDD terminal 64 via the switch 2B.

  Reference numeral 2 denotes a regulator. During startup, current is supplied from the startup current source 3 to the VCC terminal 63 and the VDD terminal 64. When the VCC voltage reaches the VCC startup voltage, the VDD terminal is connected from the VCC terminal 63 via the switch 2C. Switch to supply to 64.

  Reference numeral 7 denotes a start / stop circuit, which stops the oscillation of the switching element 1 when the VDD voltage is equal to or lower than the VDD start voltage.

  Reference numeral 10 denotes a first RS flip-flop, to which the output of the oscillation circuit 9 and the drain current detection comparator 8 are connected. The oscillation circuit 9 determines the turn-on of the switching element 1, and the drain current detection comparator 8 turns it off. To decide. Reference numeral 5 denotes a NAND circuit, and the outputs of the start / stop circuit 7 and the RS flip-flop 10 are input. Reference numeral 4 denotes an inverter which receives the output of the NAND circuit 5 and outputs it to the gate of the switching element 1.

  The feedback signal control circuit 11 is connected to the FB terminal 65, converts a current signal flowing into the FB terminal 65 into a voltage signal, and is connected to the inverting input terminal of the drain current detection comparator 8.

  That is, current mode control is performed in which the drain current flowing through the switching element 1 is controlled according to the current flowing through the FB terminal 65.

  Further, the clamp circuit 12 is connected to the inverting input terminal of the drain current detection comparator 8, and the clamp voltage variable circuit 13 connected to the CL terminal 66 is connected to the clamp circuit 12. When the current flowing into the CL terminal 66 decreases, the clamp voltage variable circuit 13 converts this into a voltage signal, and the clamp circuit 12 controls the maximum drain current ILIMIT in proportion to the CL terminal current. Furthermore, the clamp voltage variable circuit 13 outputs an overload detection signal CL_low to the oscillation circuit 9 when the current ICL at the CL terminal becomes a certain level or lower, and the oscillation frequency is reduced from 100 kHz to 12 kHz.

  The overload discharge circuit 20, which is a feature of the present invention, is connected to the feedback signal control circuit 11 and the clamp voltage variable circuit 13.

  The overload discharge circuit 20 includes the following circuit elements. Reference numeral 21 denotes an overload determination comparator connected to the feedback signal control circuit 11. Reference numeral 22 denotes a secondary output determination comparator.

  When the FB current IFB falls below a certain level (IFBH), the overload determination comparator 21 determines that the load is overloaded and outputs a low level signal.

  The secondary-side output determination comparator 22 outputs a low-level signal when the FB current IFB starts to flow beyond a certain level (IFBL).

  FIG. 2 shows the relationship between the FB current and the current Ids flowing through the DRAIN terminal 61 of the switching element 1. Under heavy load, the FB terminal current is small and the drain current flows to the maximum. When the load becomes light and the FB terminal current increases, the drain current also decreases according to the FB terminal current. When the FB terminal current reaches IFB1, the oscillation is stopped. IFB1 is provided with a hysteresis. After the oscillation is stopped, the output voltage decreases, and when the FB terminal current recovers to IFB1 + hysteresis, the oscillation is resumed. Therefore, intermittent oscillation occurs at light loads.

  Reference numeral 23 denotes a second RS flip-flop, which uses the output of the secondary-side output determination comparator 22 as a set input and the output of the clamp voltage variable circuit 13 as a reset input.

  An AND circuit 24 is connected to the output of the overload determination comparator 21 and the output of the second RS flip-flop 23, and drives the gate of the discharge MOSFET 26, which is a discharge means such as a transistor. The FET 26 is a high voltage MOSFET because it is directly connected to VCC, which is a high voltage input terminal.

  A VCC voltage detection comparator 25 permits discharge by the discharge MOSFET 26 up to about 5 V when the VCC voltage is lowered by the discharge MOSFET 26.

  Reference numeral 40 denotes a power conversion transformer, the primary winding 40A is connected to the drain terminal 61, and the auxiliary winding 40D is rectified and smoothed by the rectifier diode 31 and the secondary side smoothing capacitor 32, and is connected to the VCC terminal 63. The secondary winding 40 </ b> B is connected to the secondary rectifier diode 50 and the secondary smoothing capacitor 51.

  58 is a secondary side constant voltage control circuit using a secondary side IC for constant voltage control, and controls so that the secondary side output voltage becomes constant. 57 is a load. When the load 57 is a light load and the secondary side output voltage exceeds the rated voltage, the secondary side constant voltage control circuit 58 causes a current to flow to, for example, the photocoupler 35 that is a control signal transmission circuit, and the photocoupler 35 is connected. A current flows through the FB terminal.

  Reference numeral 33 denotes a capacitor for stabilizing the VDD voltage.

  An overload detection resistor 34 is connected to the VCC terminal 63 and the CL terminal 66.

  The operation of the thus configured switching power supply apparatus will be described below with reference to FIG.

  In FIG. 3, the operation of each part in one embodiment of the present invention is shown in a timing chart. In order from the top, the size of the load 57, the VCC voltage (63), the VDD voltage (64), the FB terminal current (IFB) of the FB terminal 65, the output FBL of the secondary output determination comparator 22, the clamp voltage variable circuit 13 shows an output CL_low, an output Q of the second RS flip-flop 23, an output FBH of the overload detection comparator 21, and a gate signal of the discharge MOSFET 26.

  First, when AC power is supplied to the input terminal Vin of the switching power supply, the DC power is rectified and smoothed by the rectifier circuit 70 and the input smoothing capacitor 71 and supplied to the primary winding 40A of the power conversion transformer 40.

  Further, current is supplied from the DRAIN terminal 61 to the VCC terminal 63 via the power conversion transformer 40, and the smoothing capacitor 32 connected to the VCC terminal 63 is charged. When the VCC voltage rises to some extent, current is also supplied from the VCC terminal 63 to the VDD terminal 64, and the stabilization capacitor 33 connected to the VDD terminal 64 is charged. When the VDD voltage reaches the starting voltage VUV, the starting / stopping circuit 7 operates to start oscillation of the switching element 1 via the NAND circuit 5 and the inverter 4, and at the same time, supply of current from the drain to VCC is stopped.

  When oscillation starts, the secondary output rises to the rated voltage, and a current flows through the photocoupler 35. The feedback signal control circuit 11 detects this, inputs a set signal to the second RS flip-flop 23, and the output of the second RS flip-flop 23 becomes high level.

  When the load 57 receiving the secondary output is overloaded, the FB current decreases, the overload determination comparator 21 outputs a high level signal, and the discharge MOSFET 26 is turned on by the AND circuit 24.

  When the discharge MOSFET 26 is turned on, the VCC voltage is lowered. When the VCC voltage is lowered to 5 V, the VCC voltage detection comparator 25 detects this, and the discharge MOSFET 26 is turned off. Thereafter, the overload protection function is activated by the current flowing into the CL terminal 66, as in the conventional switching power supply semiconductor device having the overload protection function.

  The present invention is characterized by distinguishing between startup and overload after startup.

  Immediately after the primary-side VCC voltage rises and oscillation starts, the secondary-side output does not rise, so no current flows through the photocoupler 35, and the output of the secondary-side output determination comparator 22 is low. Thus, the discharging MOSFET 26 is not turned on.

  The set signal is input to the second RS flip-flop 23 only when the secondary output reaches the rated voltage and the FB current becomes equal to or less than IFBL. Further, when an overload is detected by the CL current, a reset is input to the second RS flip-flop 23. With such a configuration, an overload is erroneously detected at the time of start-up, and the overload discharge circuit 20 does not operate, and the VCC voltage can be lowered only during an overload.

  According to the embodiment of the present invention, in the switching power supply that performs overload protection using the voltage drop of the auxiliary winding voltage at the time of overload of the auxiliary winding voltage that is proportional to the output voltage to the load, By forcibly reducing the auxiliary winding voltage by the discharge circuit, overload protection can be reliably performed even when the auxiliary winding voltage is difficult to decrease during a sudden load change. Furthermore, since the discharge circuit distinguishes between overloading at startup and overload after startup, it is possible to prevent problems such as failure to start due to erroneous detection of overload at startup.

  The switching power supply of the present invention has an effect of reliably detecting an abnormality in the secondary side output and minimizing the power loss of the switching power supply, and is useful for a switching power supply.

1 is a circuit diagram showing a switching power supply device using a switching power supply semiconductor device having an overload protection function according to an embodiment of the present invention. FIG. It is a figure which shows the relationship between FB terminal current and DRAIN terminal current Ids of the switching element 1 in the semiconductor device for switching power supplies which concerns on one Embodiment of this invention. It is a timing chart of each part waveform in a switching power supply concerning one embodiment of the present invention. It is a circuit diagram of the switching power supply device provided with the conventional overload protection function.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Switching element 2 Regulator 3 Start-up current source 4 Gate driver 5 AND circuit 6 Drain current detection circuit 7 Start / stop circuit 8 Drain current detection comparator 9 Oscillation circuit 10 RS flip-flop 11 Feedback signal control circuit 12 Clamp circuit 13 Clamp Voltage variable circuit 20 Overload discharge circuit 21 Overload judgment comparator 22 Secondary output judgment comparator 26 Discharge MOSFET
30 Semiconductor device for switching power supply 70 Rectifier circuit 71 Input smoothing capacitor

Claims (4)

A transformer having a primary winding, a secondary winding, and an auxiliary winding;
Switching means for switching a DC voltage input to the primary winding and outputting a pulse voltage to the secondary winding and the auxiliary winding;
An output circuit that outputs a DC voltage obtained by rectifying and smoothing the pulse voltage of the secondary winding to a load;
A feedback circuit that outputs a feedback signal that feeds back an output voltage of the output circuit;
A control circuit for controlling the switching operation of the switching means based on the feedback signal;
A rectifier circuit for rectifying and smoothing the current pulse of the auxiliary winding;
When the auxiliary winding voltage output from the rectifier circuit decreases to the overload detection level in proportion to the output voltage, the control circuit is controlled according to the decrease in the auxiliary winding voltage, and the secondary winding With overload protection means to reduce the power supplied to
It said overload protection means, upon overload, have a overload discharge circuit for forcibly lowering said auxiliary winding voltage,
The overload discharge circuit includes a secondary side start detection circuit that detects that the output circuit has reached a rated voltage and started, an overload detection circuit that detects that the load of the output circuit is overload, Discharging means for reducing the auxiliary winding voltage by discharging . The switching power supply device according to claim 1, wherein the secondary side activation detection circuit detects a feedback signal . Overload detection circuit according to claim 1 Symbol mounting of the switching power supply for detecting a feedback signal. Overload discharge circuit, according to claim 1 Symbol mounting of the switching power supply auxiliary winding voltage is stopped and discharging is reduced to the overload detection level.

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