JP2012060815A - Integrated circuit device for controlling switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an integrated circuit device for controlling a switching power supply which discharges a residual charge from a bulk capacitor upon detecting an interruption of AC power input while suppressing an increase in the circuit scale of a discharge circuit.SOLUTION: An integrated circuit 10 for controlling a switching power supply includes: a state detection section 11 for detecting a state of interruption of AC voltage to a flyback transformer 5; a comparator 12 of an overcurrent protection circuit for detecting that a current flowing through a MOSFET 6 is equal to or higher than a set reference value; a discharge control section 13 for, when the sate detection section 11 detects a state of interruption of AC power input, controlling the MOSFET 6 on to discharge a residual accumulated charge in a bulk capacitor 4 connected to a primary winding of the flyback transformer 5; a PWM control section 14 for generating a PWM signal to the MOSFET 6; a PWM signal drive circuit 15 having a disable terminal dsbl; and a first switch circuit SW1.

Description

  The present invention relates to an integrated circuit device for controlling a switching power source that converts an AC voltage of an AC power source into a predetermined DC voltage by turning on and off a switching element connected to a primary winding of a flyback transformer.

  In a switching power supply that obtains a predetermined output voltage by connecting a flyback transformer to an AC power supply via a bridge rectifier circuit and turning on and off a switching element connected to the primary winding of the flyback transformer, When the AC power supply input is shut off in the load state, a high voltage may remain in the switching power supply device because the charge remaining in the bulk capacitor connected to the primary winding of the flyback transformer remains without being pulled out. There was a risk of electric shock. Therefore, from the viewpoint of safety, it is necessary to intentionally discharge the residual charge of the bulk capacitor and reduce its voltage value.

  FIG. 3 is a block diagram showing a discharge circuit in a conventional switching power supply device. Here, the transformer 222 also constitutes an auxiliary converter for supplying power to the integrated circuit 371. When the switching element 363 is turned on / off, power is supplied from the third winding of the transformer 222 to the PWM integrated circuit 371 (a portion surrounded by a broken line) via the power supply terminal VC 2, and the integrated circuit 371 is connected to the transformer 222. The power supply is supplied from the bulk filter capacitor (bulk capacitor) via the third winding or the resistor R303, and the PWM signal is supplied to the gate of the switching element 363. The power supply terminal VC2 is connected to a bulk filter capacitor not shown in FIG. 3 via a resistor R303 (see Patent Document 1 below).

  Once the AC power is cut off after normal operation, the potential of the power supply terminal VC2 starts to drop. When this voltage drops below 12V, the output of the operational amplifier 315 becomes H and the Q output of the flip-flop 337 becomes H ( High). At this time, in the PWM integrated circuit 371, the power supply from the power supply terminal VC2 does not drop to 12V unless the AC power is turned off. Therefore, if the switching power supply is operating normally before the AC power is turned off, the flip-flop The output of the node 339 is also H, and the output of the NAND gate 343 becomes L (low). Therefore, the diode 353 is killed, and the voltage VRs on the source side of the switching element 363 is directly input to the inverting input terminal of the operational amplifier 351.

  The operational amplifier 351 is a dedicated operational amplifier for bulk discharge control. When the output of the NAND gate 343 is L, the reference power supply 345 and the switching element 363 constitute a series regulator. Here, the magnitude of the drain current of the switching element 363 is controlled to a constant current so that the voltage VRs on the source side is always (almost) equal to the output voltage of the reference power supply 345. Thus, when the switching element 363 is turned on, the residual charge of the bulk filter capacitor is discharged through the primary winding of the transformer 222.

  Note that when the output of the NAND gate 343 becomes H, H is input to the inverting input terminal of the operational amplifier 351 via the diode 353, and the output of the operational amplifier 351 is always L. In that case, the output of the PWM control circuit 357 is pulled down by the resistor R355.

U.S. Pat. No. 5,999,429 (refer to columns 4-7, FIGS. 1-3)

In the conventional switching power supply described above, an operational amplifier dedicated to bulk discharge control, for example, the operational amplifier 351 is required.
Here, the operational amplifier 351 is configured to control the drain current during discharging of the bulk capacitor by controlling the gate voltage of the switching element 363 made of a power MOSFET. Since the operational amplifier is an analog circuit and requires a certain large layout area, there is a problem that an area of a circuit portion necessary for controlling the gate voltage of the switching element 363 increases in the integrated circuit.

  The present invention has been made in view of the above-described points, and is a switching power supply that detects an OFF of an AC input and discharges a residual charge from a bulk capacitor while suppressing an increase in circuit scale of the discharge circuit. An object is to provide an integrated circuit device for control.

  In the present invention, in order to solve the above problems, a transformer is connected to an AC power source via a bridge rectifier circuit, a switching element connected to one end of the primary winding of the transformer is turned on and off, and the AC power of the AC power source is An integrated circuit device for controlling a switching power supply that converts a voltage into a predetermined DC voltage is provided.

  The integrated circuit device includes a state detection unit that detects a state where the voltage of the AC power supply is cut off with respect to the transformer, and turns off the switching element when a current flowing through the switching element is equal to or higher than a set reference value When the overcurrent protection means for controlling the state and the state detection means detect the interruption state of the AC voltage, the switching element is controlled to be turned on and connected to the other end of the primary winding of the transformer And a discharge control means for discharging the accumulated charge remaining in the bulk capacitor.

  According to the present invention, a discharge control function for discharging a bulk capacitor can be configured in an integrated circuit device for switching power supply control with a small number of control components. In particular, it can be configured as an RC circuit that uses resistors and capacitors, and also functions as a comparator for overcurrent protection (OCP) of existing products, and controls the discharge current by switching elements that are configured with power MOSFETs, etc. can do.

It is a figure which shows the structure of the switching power supply device which concerns on embodiment of this invention. It is a wave form diagram for demonstrating the discharge operation in the switching power supply device of FIG. It is a block diagram which shows the discharge circuit in the conventional switching power supply device.

Hereinafter, the present invention will be described with reference to the drawings illustrating embodiments of a switching power supply device. FIG. 1 is a diagram showing a configuration of a switching power supply apparatus according to an embodiment of the present invention.
The AC power supply 1 is detachably connected to the AC input terminal CN1, and the AC power is supplied to the bridge rectifier circuit 3 via the input filter circuit 2. Here, the input filter circuit 2 includes a plurality of inductors L1 and L2, resistors R1 and R2, and a capacitor C1.

  Further, the primary winding 5P of the bulk capacitor 4 and the flyback transformer 5 is connected to the output side of the bridge rectifier circuit 3, and a switching element such as an N-channel MOSFET 6 connected in series to the primary winding 5P is connected. The source terminal is grounded via the current detection resistor Rs. A smoothing circuit 7 including a diode D4 and a smoothing capacitor C2 is connected to the secondary winding 5S of the flyback transformer 5, and both terminals of the smoothing capacitor C2 are connected to the DC output terminal CN2. A load is connected to the DC output terminal CN2.

  The integrated circuit 10 for controlling the switching power supply includes a VH terminal, a VCC terminal, an OUT terminal, an IS terminal, and the like. The anodes of the diodes D1 and D2 are connected to the AC line to which both poles of the AC power supply 1 are connected, and the cathodes are connected to each other and connected to the VH terminal of the integrated circuit 10 via the resistor R3.

  The gate terminal of the MOSFET 6 is connected to the OUT terminal of the integrated circuit 10 via a waveform shaping circuit 8 for blunting the gate drive pulse. The waveform shaping circuit 8 includes resistors R4 and R5 connected in series, and a diode D3 connected in parallel to one of the resistors R4. The pulse signal waveform supplied as the gate voltage Vg to the gate terminal of the MOSFET 6 is shaped by the waveform shaping circuit 8 so that the rise time is longer than the fall time. The connection point between the source terminal of the MOSFET 6 and the current detection resistor Rs is connected to the IS terminal of the integrated circuit 10.

  The integrated circuit 10 includes a state detection unit 11 that detects a state in which the AC voltage is interrupted with respect to the flyback transformer 5, and an overcurrent that controls the MOSFET 6 to be turned off when the current flowing through the MOSFET 6 is equal to or greater than a set reference value. The comparator 6 of the protection circuit (the reference voltage input to the non-inverting input terminal of the comparator 12 corresponds to the set reference value), and when the state detector 11 detects the AC voltage cutoff state, the MOSFET 6 The discharge controller 13 for discharging the accumulated charge remaining in the bulk capacitor 4 connected to the primary winding of the flyback transformer 5, the PWM controller 14 for generating the PWM signal for the MOSFET 6, disable The PWM signal drive circuit 15 having the terminal dsbl, the first switch circuit SW1, etc. It is configured. Since the overcurrent protection circuit other than the comparator 12 is not directly related to the present invention, the illustration thereof is omitted. The comparator 12 is for determining whether or not the current flowing through the MOSFET 6 is equal to or greater than a set reference value. When the comparator 12 detects an overcurrent, the protection circuit turns off the MOSFET 6 by a circuit (not shown). In addition, when the state detection part 11 detects the interruption | blocking state of an alternating voltage, the operation | movement in which a protection circuit directly turns off MOSFET6 is prohibited.

  The PWM control unit 14 is connected to the control terminal ctrl of the drive circuit 15, and the output terminal of the drive circuit 15 is connected from the OUT terminal to the gate terminal of the MOSFET 6 via the waveform shaping circuit 8. When the output signal of the state detection unit 11 is supplied to the disable terminal dsbl and the AC signal is cut off, and the disable signal becomes H level, the drive circuit 15 becomes high impedance and the output becomes a PWM signal. Disable. The drive circuit 15 is connected to the VCC terminal and supplied with power from there.

  The first switch circuit SW1 is provided so as to connect the discharge controller 13 to a connection point between the output terminal of the drive circuit 15 and the OUT terminal. This switch circuit SW1 is ON / OFF controlled by an RS flip-flop circuit 11c, which will be described later, of the state detection unit 11, and is configured to be turned on when the Q output signal of the RS flip-flop circuit 11c is at the H level. Yes.

  The state detection unit 11 includes an AC input detection circuit 11a, a delay time setting timer 11b, and an RS flip-flop circuit 11c. Among these, when the AC power supply 1 is connected to the AC input detection circuit 11a, a full-wave rectified signal is input via the VH terminal as the terminal voltage of the capacitor C1 of the input filter circuit 2. In the AC input detection circuit 11a, it is compared with a reference signal level by a comparison circuit (not shown) to determine whether or not the AC power supply 1 is cut off, and the delay time setting timer 11b and the set input terminals (S of the flip-flop circuit 11c) ) And the detection signal AC_DET is output. That is, in the state where the AC power supply 1 is supplied, a waveform obtained by full-wave rectification of the AC power supply 1 (strictly speaking, a level-shifted waveform) is input to the VH terminal as described above. By comparing the voltage with the reference signal level, a pulse train having a frequency twice the frequency of the AC power supply 1 can be generated as an output of the comparison circuit inside the AC input detection circuit 11a. When the AC power supply 1 is cut off, the voltage input to the VH terminal is determined according to the charging voltage of the capacitor C1 at that time and does not change with the frequency of the AC power supply 1. Thus, the pulse train is not generated. Using this, the AC input detection circuit 11a determines that the AC power supply 1 is cut off when the pulse of the pulse train is not generated for a predetermined time, and uses the detection signal AC_DET as the output as an H level signal. In this case, it is assumed that the AC power supply 1 is connected, and the detection signal AC_DET is an L level signal.

  The delay time setting timer 11b starts a time measuring operation if the detection signal AC_DET is at the H level. After the set time has elapsed, the time-out signal T_O is supplied to the reset input terminal (R) of the RS flip-flop circuit 11c. Output. If the detection signal AC_DET is at L level, the delay time setting timer 11b is reset.

  The comparator 12 constituting the overcurrent protection circuit is a hysteresis comparator, and two reference voltages VrefL and VrefH are set. Here, the voltage signal VIS fed back from the IS terminal of the integrated circuit 10 is compared with the reference voltages VrefL and VrefH, and an overcurrent state can be detected from the current value flowing through the switching element.

  The discharge controller 13 includes an inverter circuit 13a, a NAND circuit 13b, an inverter circuit 13c, a constant current source 13d, a second switch circuit SW2, a third switch circuit SW3, and a capacitor C3. The output signal COMP of the comparator 12 becomes one input of the NAND circuit 13b. The Q output signal of the RS flip-flop circuit 11c is supplied to the other input of the NAND circuit 13b. The output terminal of the NAND circuit 13b is connected to the second switch circuit SW2 via the inverter circuit 13c. The constant current source 13d is connected to the capacitor C3 via the second switch circuit SW2, and when the output signal of the inverter circuit 13c is at the H level, the second switch circuit SW2 is turned on to turn on the capacitor C3. To supply a constant current. The other end of the capacitor C3 is grounded. The inverter circuit 13a is connected to the Q output terminal of the RS flip-flop circuit 11c, and on / off-controls the third switch circuit SW3 by the output signal. The third switch circuit SW3 is connected in parallel with the capacitor C3.

  The discharge controller 13 determines the charging / discharging timing of the capacitor C3 by the second switch circuit SW2 and the third switch circuit SW3. When the output signal of the inverter circuit 13a is at the H level, the third switch circuit SW3 It is turned on and controls to discharge the electric charge of the capacitor C3. Further, the capacitor C3 is used when the third switch circuit SW3 is turned off from the constant current source 13d at a timing (t1 described later) when the second switch circuit SW2 is turned on and the third switch circuit SW3 is turned off. Is charged. The discharge controller 13 has its output voltage Vo determined by the charging potential of the capacitor C3, and is supplied to the OUT terminal of the integrated circuit 10 via the first switch circuit SW1.

  In general, a switching power supply device may be overloaded due to an unexpected accident or the like occurring on the load side, and the switching transistor may be thermally damaged or completely destroyed. In the overcurrent protection circuit, the drain current Id of the MOSFET 6 flowing through the current detection resistor Rs is detected by the overcurrent detection comparator 12 to which the voltage signal VIS is fed back, and compared with the reference voltage signal Vref set there, An output signal COMP is generated. Therefore, when the voltage signal VIS becomes larger than the reference voltage signal Vref, the output signal COMP becomes L level, and the overcurrent protection circuit determines that the drain current Id exceeds the allowable limit and immediately stops the PWM operation of the MOSFET 6 and turns off. It is a state.

Next, an operation for detecting the interruption state of the AC power supply and discharging the bulk capacitor 4 will be described.
FIG. 2 is a waveform diagram for explaining a discharge operation in the switching power supply device of FIG.

  FIG. 4A shows the detection signal AC_DET of the AC input detection circuit 11a. It is assumed that the detection signal AC_DET of the AC input detection circuit 11a becomes H level at the AC power supply cutoff timing t1, and the power supply cutoff state is detected.

  FIG. 2B shows the output signal COMP of the overcurrent detection comparator 12. When the detection signal AC_DET becomes H level at timing t1, the Q output signal of the RS flip-flop circuit 11c becomes H level. The Q output signal at H level is output from the state detection unit 11 to the first switch circuit SW1, the discharge control unit 13, and the drive circuit 15. FIG. 3C shows the on / off state of the first switch circuit SW1 (H is on, L is off, and the same is true for the states of SW2 and SW3 described later). When the input signal to the disable terminal dsbl becomes H, the output voltage VOUT of the drive circuit 15 becomes high impedance, and the PWM control unit 14 is disconnected from the OUT terminal. Further, the first switch circuit SW1 is turned on by the Q output signal at this time, whereby the discharge control unit 13 and the OUT terminal are connected.

  FIG. 2D shows an on / off state of the second switch circuit SW2. At timing t1, the output of the NAND circuit 13b becomes L and the output of the inverter circuit 13c becomes H, whereby the second switch circuit SW2 is turned on, and charging of the capacitor C3 by the charging current from the constant current source 13d is started. As a result, the voltage VOUT that determines the gate voltage Vg of the MOSFET 6 is increased with a constant slope. FIG. 4G shows a change in the gate voltage VOUT at the OUT terminal. When the gate voltage VOUT exceeds the threshold voltage Vth of the MOSFET 6 at the timing t2, the MOSFET 6 is turned on, the drain current Id flows through the current detection resistor Rs, and the discharge of the bulk capacitor 4 is started. Thus, the residual charge is gradually extracted from the bulk capacitor 4 after the timing t2.

  FIG. 2H shows a change in the drain current Id. The drain current Id flowing through the current detection resistor Rs during the discharge of the bulk capacitor 4 is limited by the hysteresis comparator 12. FIG. 1I shows the voltage signal VIS fed back to the IS terminal of the integrated circuit 10. The voltage signal VIS reaches the first threshold value VrefH of the hysteresis comparator 12 at timing t3. Therefore, the output signal COMP of the comparator 12 becomes L, whereby the output of the NAND circuit 13b becomes H, the output of the inverter circuit 13c becomes L, the second switch circuit SW2 is turned off, and the gate voltage Vg of the MOSFET 6 increases. After that, the discharge at a constant voltage continues.

Here, FIG. 2J shows a change in the voltage value of the bulk capacitor 4.
FIG. 2E shows a timer output T_O from the delay time setting timer 11b. The delay time setting timer 11b is set at the timing t1, and the timeout signal T_O becomes L level. Thereafter, when the set time elapses at timing t4, the timeout signal T_O becomes H level, and the RS flip-flop circuit 11c is reset. As a result, the output signal of the state detection unit 11 becomes L level, SW1 is turned off, the discharge control unit 13 is disconnected from the OUT terminal, and the drive circuit 15 is connected to the OUT terminal. FIG. 4F shows the on / off state of the third switch circuit SW3. The first switch circuit SW1 is turned off, the voltage of the OUT terminal is lowered by the drive circuit 15, and the third switch circuit SW3 of the discharge control unit 13 is turned on. As a result, the capacitor C3 enters a discharge state, and the output voltage Vo of the discharge control unit 13 decreases.

When the voltage signal VIS reaches the low voltage side reference voltage VrefL of the comparator 12 at timing t5, the output signal COMP of the comparator 12 becomes H.
Since the time required for the discharge of the bulk capacitor 4 is secured by the time set in the delay time setting timer 11b, the discharge of the bulk capacitor 4 has been completed by the timing t5, and the PWM controller 14 at the timing t5. The integrated circuit 10 for controlling the switching power supply including is restored to the initial state.

  In the present embodiment, the drain current Id can be limited by controlling the gate voltage Vg of the MOSFET 6 during the bulk discharge using the existing comparator 12 constituting the overcurrent protection circuit. Therefore, the bulk discharge function can be integrated in the power supply control IC with a small number of control components.

  The discharge controller 13 generates a ramp voltage as the voltage VOUT that determines the gate voltage Vg of the MOSFET 6 by the constant current source 13d and the capacitor C3 when the bulk capacitor 4 is discharged. By controlling the gate voltage of the MOSFET 6 with this ramp voltage, the bulk capacitor 4 can be discharged while controlling the drain current.

  Further, since the delay time setting timer 11b built in the integrated circuit 10 sets the time from the detection of the interruption of the AC power supply to the resetting, the initial state may be restored after the discharge of the bulk capacitor 4 is completed. it can.

1 AC power supply 2 Input filter circuit 3 Bridge rectifier circuit 4 Bulk capacitor 5 Flyback transformer 6 Switching element (MOSFET)
DESCRIPTION OF SYMBOLS 7 Smoothing circuit 8 Waveform shaping circuit 10 Integrated circuit for switching power supply control 11 State detection part 11a AC input detection circuit 11b Delay time setting timer 11c RS flip-flop circuit 12 Comparator 13 Discharge control part 13a, 13c Inverter circuit 13b NAND (NAND) Circuit 13d Constant current source 14 PWM controller 15 Drive circuit C1, C3 capacitor C2 Smoothing capacitor CN1 AC input terminal CN2 DC output terminal D1-D4 Diode L1, L2 Inductor R1-R5 Resistance Rs Current detection resistance SW1-SW3 Switch circuit

Claims (7)

A switching power supply for connecting a transformer to an AC power supply via a bridge rectifier circuit, turning on and off a switching element connected to one end of the primary winding of the transformer, and converting the AC voltage of the AC power supply to a predetermined DC voltage In an integrated circuit device for control,
State detecting means for detecting a state in which the voltage of the AC power supply is cut off with respect to the transformer;
Overcurrent protection means for controlling the switching element to an off state when a current flowing through the switching element is equal to or higher than a set reference value;
When the interruption state of the AC voltage is detected by the state detection means, the switching element is controlled to be turned on to discharge the accumulated charge remaining in the bulk capacitor connected to the other end of the primary winding of the transformer. Discharge control means;
An integrated circuit device for controlling a switching power supply.   In the overcurrent protection unit, when the discharge current of the bulk capacitor flows to the switching element exceeding the reference value set in the overcurrent protection unit, the current value of the discharge current is controlled to a constant magnitude. The integrated circuit device for controlling a switching power supply according to claim 1.   3. The switching power supply control according to claim 2, wherein the overcurrent protection means includes a current value comparison circuit that compares a voltage signal fed back with a magnitude proportional to a current value flowing through the switching element with a reference voltage value. Integrated circuit device.   The discharge control means includes a constant current source, a capacitive element charged by the constant current source, and a switch circuit that controls charging / discharging of the capacitive element according to an output of the state detection means, and charging the capacitive element 2. The integrated circuit device for controlling a switching power supply according to claim 1, wherein the switching element is controlled by a voltage.   The state detection means includes a timer circuit that counts the interruption state of the AC voltage for a predetermined time, and after the AC voltage is interrupted and discharge of accumulated charge remaining in the bulk capacitor is started, after the predetermined time has elapsed, 2. An integrated circuit device for controlling a switching power supply according to claim 1, wherein the discharge control means is returned to the reset state.   The state detection means compares a signal representing a voltage obtained by full-wave rectifying the output of the AC power source with a reference signal level by a comparison circuit, and the AC voltage is in a cut-off state if a pulse is not generated for a predetermined time from the comparison circuit. 6. The integrated circuit device for controlling a switching power supply according to claim 1 or 5, characterized in that:   7. The integrated circuit device for switching power supply control according to claim 6, wherein the predetermined time is longer than a cycle of the AC power supply.

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