JP6013263B2 - Soft start control circuit, semiconductor integrated circuit, and isolated switching power supply - Google Patents

Description

  The present invention relates to a soft start control circuit, a semiconductor integrated circuit, and an isolated switching power supply. More specifically, the present invention relates to a soft start control circuit for controlling a soft start operation of an isolated switching power supply, a semiconductor integrated circuit in which the soft start control circuit is formed on a semiconductor substrate, and an insulating type including the soft start control circuit. It relates to a switching power supply.

  Conventionally, the voltage obtained by rectifying the voltage input from the AC power source is converted to AC voltage by the switch and supplied to the primary side winding of the transformer. The AC voltage generated in the secondary side winding is rectified and smoothed. Thus, an insulated switching power supply that outputs a desired DC voltage is known.

  Some of these insulated switching power supplies have a burst mode in addition to the normal mode. In the normal mode, the switch is continuously turned on / off, whereas in the burst mode, the switching period in which the switch is turned on / off and the switching pause period in which the switch is not turned on / off are alternately repeated. Thereby, the power consumption of the switching power supply in the burst mode is reduced.

  In addition, in order to reduce an inrush current and an overshoot of the output voltage at the start-up of the isolated switching power supply, a start-up method called soft start that gradually increases the output voltage at the time of power-up is known. In the soft start operation, for example, a conduction time is gradually increased by using a PWM signal obtained by comparing a voltage of a capacitor charged by a constant current source (soft start voltage) and a triangular wave signal having a predetermined period. In this way, the on / off control of the switch is performed.

  Patent Document 1 describes a current resonance type switching power supply in which a soft start time is shorter than that at a normal start-up time when an operation of a switching pulse oscillator is started. Patent Document 2 describes a switching power supply in which a plurality of constant current sources are provided, and these are sequentially switched and used to shorten the overall soft start time.

JP 2009-189108 A JP 2007-244086 A

  By the way, when the power is activated by soft start under the burst mode setting, the soft start time is much shorter than that in the normal mode. That is, the speed at which the soft start voltage rises is much faster than in the normal mode. For this reason, the effect by the soft start operation of reducing the inrush current and the overshoot by gradually increasing the output voltage is reduced.

  For this reason, during the start-up of the power supply in which the output voltage rises, the output voltage is low and the current flowing through the load is large, and an excessive current flows through the transformer section of the insulated switching power supply. As a result, for example, in the case of a transformer squealing or an LLC current resonance type switching power supply which is a kind of current resonance type power supply, resonance may be lost.

  In order to solve this problem, it is conceivable to control the rising speed of the soft start voltage using a reference voltage based on the voltage of the primary side winding of the transformer unit. That is, when the reference voltage drops below a predetermined threshold voltage, it is conceivable to start the power supply by switching the rising speed of the soft start voltage to the rising speed of the soft start voltage in the normal mode.

  However, since the reference voltage is based on a voltage generated by an auxiliary winding or the like magnetically coupled to the primary winding, the reference voltage drops below a predetermined threshold voltage after the input from the AC power supply is interrupted. Takes a very long time. Therefore, if the input from the AC power supply is recovered after a short interruption, more precisely, if the input from the AC power supply recovers before the reference voltage drops to the predetermined threshold voltage, the soft start voltage There is a problem that the ascending speed does not switch.

  Therefore, the present invention provides a software that can reliably prevent an overcurrent from flowing in the transformer section when the power is started by the soft start in the burst mode regardless of the length of time that the input from the AC power supply is interrupted. An object is to provide a start control circuit, a semiconductor integrated circuit, and an isolated switching power supply.

A soft start control circuit according to one embodiment of the present invention includes:
A rectifying unit for rectifying an AC power source connected to an input terminal, a switching unit having a high-side switch and a low-side switch connected in series between the output of the rectifying unit and the ground, and one end of the high-side switch and the A transformer unit having a primary side winding connected to a switch connection point of a low side switch and a secondary side winding magnetically coupled to the primary side winding, and an alternating current generated at both ends of the secondary side winding A rectifying / smoothing unit that rectifies and smoothes a voltage and outputs a rectified and smoothed DC voltage to an output terminal, a capacitor unit having a capacitive element, a capacitor element of the capacitor unit, and a charge voltage of the capacitive element As the soft start voltage increases, the high side switch and the low size switch are set so that the conduction time becomes longer with a predetermined upper limit conduction time as an upper limit. A switch control circuit for controlling a complementary conductive state or blocking state switch, used in insulated switching power supply comprising,
In the burst mode in which the switch control circuit operates intermittently, when the monitor voltage based on the AC power supply drops below a predetermined input threshold voltage, the reference voltage based on the voltage of the primary winding becomes a predetermined output threshold voltage. Switching the rising speed of the soft start voltage from the first rising speed, which is the rising speed of the soft start voltage in the burst mode, to a second rising speed that is lower than the first rising speed until the first rising speed is reached. It is characterized by.

In the soft start control circuit,
The capacitor unit has a first capacitor element having one end connected to the capacitor voltage input terminal of the switch control circuit, a second capacitor terminal connected to the other end of the first capacitor element, and the other end grounded. And a capacitive element,
The soft start control circuit grounds a capacitive element connection point between the first capacitive element and the second capacitive element, thereby increasing the rising speed of the soft start voltage from the first rising speed to the first rising speed. You may make it switch to 2 raise speed.

In the soft start control circuit,
A line sense terminal to which the monitor voltage is input;
A divided voltage input terminal to which a divided voltage obtained by dividing the reference voltage is input;
A soft start time control terminal connected to the capacitor element connection point;
A set signal generator for outputting a set signal when the monitor voltage is less than the input threshold voltage;
A reset signal generator that outputs a reset signal when the divided voltage is higher than a voltage corresponding to the output threshold voltage;
When the set signal is received, the capacitive element connection point is electrically connected to ground, and when the reset signal is received, the soft start time control unit electrically isolates the capacitive element connection point from the ground;
May be provided.

In the soft start control circuit,
The monitor voltage may be a DC voltage obtained by rectifying the voltage of the AC power supply and dividing and smoothing the rectified voltage.

In the soft start control circuit,
The second rising speed may be a rising speed of the soft start voltage in a normal mode in which the switch control circuit operates continuously.

In the soft start control circuit,
The output threshold voltage may be an upper limit voltage of the reference voltage in the burst mode.

In the soft start control circuit,
The reference voltage may be a DC voltage obtained by rectifying and smoothing an AC voltage generated at both ends of the auxiliary winding magnetically coupled to the primary winding.

  A semiconductor integrated circuit according to one embodiment of the present invention is characterized in that the soft start control circuit of the present invention is formed over a predetermined semiconductor substrate.

  A semiconductor integrated circuit according to one embodiment of the present invention is characterized in that the soft start control circuit of the present invention and the switch control circuit are formed over a predetermined semiconductor substrate.

An insulated switching power supply according to one aspect of the present invention is provided.
A rectifying unit for rectifying the AC power source connected to the input terminal;
A switching unit having a high-side switch and a low-side switch connected in series between the output of the rectifying unit and the ground;
A transformer unit having a primary winding connected at one end to a switch connection point of the high-side switch and the low-side switch, and a secondary winding magnetically coupled to the primary winding;
A rectifying / smoothing unit that rectifies and smoothes the AC voltage generated at both ends of the secondary winding, and outputs the rectified and smoothed DC voltage to an output terminal;
A capacitor unit having a capacitive element;
The high-side switch and the low-side switch are charged so as to increase the conduction time with a predetermined upper limit conduction time as an upper limit as the soft start voltage, which is the charging voltage of the capacitance element, increases, while charging the capacitance element of the capacitor unit. A switch control circuit which controls the conduction state or the interruption state in a complementary manner;
A monitor voltage generator connected to the input terminal for generating a monitor voltage based on the AC power supply;
In the burst mode in which the switch control circuit operates intermittently, when the monitor voltage falls below a predetermined input threshold voltage, until the reference voltage based on the voltage of the primary winding reaches a predetermined output threshold voltage, A soft start control circuit that switches a rising speed of the soft start voltage from a first rising speed that is a rising speed of the soft start voltage in the burst mode to a second rising speed that is lower than the first rising speed; ,
It is characterized by providing.

In the insulated switching power supply,
The monitor voltage generation unit may rectify the voltage of the AC power supply and output a DC voltage obtained by dividing and smoothing the rectified voltage as the monitor voltage to the soft start control circuit.

In the insulated switching power supply,
The transformer unit further includes an auxiliary winding magnetically coupled to the primary winding,
The reference voltage may be a DC voltage obtained by rectifying and smoothing an AC voltage generated at both ends of the auxiliary winding.

In the insulated switching power supply,
A resonance capacitive element connected in series or in parallel to the primary winding may be further provided.

  In the present invention, in the burst mode, when the monitor voltage based on the AC power supply drops below a predetermined input threshold voltage, the soft start voltage is increased until the reference voltage based on the voltage of the primary winding reaches the predetermined output threshold voltage. The rising speed is switched from the first rising speed, which is the rising speed of the soft start voltage in the burst mode, to the second rising speed that is lower than the first rising speed. Thereby, even when the power supply is activated by the soft start in the burst mode, the effect of the soft start is not deteriorated, and it is possible to prevent an overcurrent from flowing through the transformer unit.

  In the present invention, since the soft start voltage rise speed is switched based on the monitor voltage based on the AC power supply, even if the input from the AC power supply is interrupted for a short time, the soft start voltage rise voltage is increased. It is possible to reliably switch from the rising speed of 1 to the second rising speed.

  Therefore, according to the present invention, it is possible to reliably prevent an overcurrent from flowing through the transformer section when the power is activated by the soft start in the burst mode regardless of the length of time that the input from the AC power supply is interrupted. .

1 is a schematic configuration diagram of an isolated switching power supply according to a first embodiment of the present invention. 1 is a circuit diagram of a soft start control circuit according to a first embodiment of the present invention. It is a timing chart for demonstrating operation | movement of the insulation type switching power supply which concerns on the 1st Embodiment of this invention. It is a schematic block diagram of a part of switch control circuit and capacitor | condenser part in the insulation type switching power supply which concerns on the 2nd Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
An insulated switching power supply 1 according to a first embodiment of the present invention will be described. FIG. 1 shows a schematic configuration diagram of an insulating switching power supply 1 according to the present embodiment.

  The insulation type switching power supply 1 is connected to input terminals 2a and 2b with an AC power supply (not shown), converts AC power input from the AC power supply into desired DC power, and outputs it from output terminals 7a and 7b. Switching power supply.

  As shown in FIG. 1, an insulating switching power supply 1 includes input terminals 2a and 2b, a rectifying unit 3, a switching unit 4, a transformer unit 5, a rectifying and smoothing unit 6, output terminals 7a and 7b, and a capacitor. Unit 8, switch control circuit 9, soft start control circuit 10, rectifying / smoothing unit 11, and monitor voltage generating unit 12.

  The isolated switching power supply 1 may include a power factor correction circuit (PFC, not shown) having an inductor between the output of the rectifying unit 3 and the high-side switch Q1.

  As shown in FIG. 1, the insulating switching power supply 1 further includes a resonance capacitive element C4 and a resistor R1 connected in series between the primary winding W1 and the ground, and is a current resonance switching power supply. It may be configured as. The capacitive element C4 may be connected in series with the primary winding W1 between one end of the primary winding W1 and the switch connection point M. Alternatively, the capacitive element C4 may be connected in parallel to the primary side winding W1.

  The rectification unit 3 rectifies (half-wave rectification or full-wave rectification) the AC power supply connected to the input terminals 2a and 2b. And the rectifier 3 outputs the rectified input rectified voltage. The rectifying unit 3 is configured using, for example, a bridge diode. Note that the rectifying unit 3 may include a smoothing capacitor and rectify and smooth the AC power supply for output.

  As shown in FIG. 1, the switching unit 4 includes a high-side switch Q1 and a low-side switch Q2 connected in series between the output of the rectifying unit 3 and the ground. The high side switch Q1 and the low side switch Q2 are semiconductor switching elements such as an n-type MOSFET.

  As shown in FIG. 1, the transformer unit 5 includes a primary winding W1, a secondary winding W2, and an auxiliary winding W3. One end of the primary winding W1 is connected to a switch connection point M of the high side switch Q1 and the low side switch Q2. Secondary winding W2 and auxiliary winding W3 are magnetically coupled to primary winding W1.

  As shown in FIG. 1, the other end of the primary winding W1 is grounded via a series-connected capacitive element C4 and a resistor R1. Note that the other end of the primary winding W1 may be directly grounded without passing through the capacitive element C4 and the resistor R1.

  The rectifying / smoothing unit 6 rectifies and smoothes the AC voltage generated at both ends of the secondary winding W2, and outputs the rectified and smoothed DC voltage to the output terminals 7a and 7b. For example, as shown in FIG. 1, the rectifying / smoothing unit 6 is configured to include rectifying elements D1, D2 and a smoothing capacitive element C3. More specifically, in the rectifying element D1, an anode is connected to one end of the secondary winding W2, and a cathode is connected to the output terminal 7a. The rectifier element D2 has an anode connected to the other end of the secondary winding W2, and a cathode connected to the output terminal 7a. The capacitive element C3 has one end connected to the output terminal 7a and the other end connected to the output terminal 7b. The output terminal 7b is connected to the center tap of the secondary winding W2.

  Note that the rectifying / smoothing unit 6 is not limited to the above configuration as long as it can rectify and smooth the AC voltage generated at both ends of the secondary winding W2. In FIG. 1, only one rectifying / smoothing unit 6 is illustrated. However, the present invention is not limited to this, and two or more secondary windings W2 are provided in parallel, and each secondary winding W2 is provided. A rectifying / smoothing unit 6 may be provided.

  As shown in FIG. 1, the capacitor unit 8 includes capacitive elements C1 and C2 connected in series. One end of the capacitive element C <b> 1 is connected to the capacitor voltage input terminal SS of the switch control circuit 9. One end of the capacitive element C2 is connected to the other end of the capacitive element C1, and the other end is grounded.

  Note that the number of capacitor elements included in the capacitor unit 8 is not limited to two, and the capacitor unit 8 may include three or more capacitor elements.

  The switch control circuit 9 has a constant current source (not shown) connected to the capacitor voltage input terminal SS through the switch. During the soft start operation, the switch is turned on and the constant current source Capacitance elements C1 and C2 of the capacitor unit 8 are charged.

  In addition, the switch control circuit 9 switches the high-side switch Q1 and the low-side switch Q2 based on the charging voltage (soft-start voltage) of the capacitive elements C1 and C2, which is the voltage at the capacitor voltage input terminal SS, during the soft-start operation. Let

  More specifically, in the soft start operation, the switch control circuit 9 sets the high side so that the conduction time (on width) becomes longer with a predetermined upper limit conduction time (upper on width) as the upper limit as the soft start voltage becomes higher. The switch Q1 and the low-side switch Q2 are complementarily controlled to a conductive state or a cut-off state.

  Here, the term “complementarily” includes the case of switching with a so-called dead time. That is, including the time when both the high-side switch Q1 and the low-side switch Q2 are in the cut-off state, one switch is turned on and the other switch is cut-off.

  The switch control circuit 9 is configured as an IC chip, for example. That is, the switch control circuit 9 can be configured as a semiconductor integrated circuit formed on a predetermined semiconductor substrate.

  The soft start control circuit 10 controls the soft start operation. The detailed configuration and operation of the soft start control circuit 10 will be described later.

  The rectifying / smoothing unit 11 rectifies and smoothes the AC voltage generated at both ends of the auxiliary winding W3 and outputs the rectified and smoothed output to the soft start control circuit 10. For example, as shown in FIG. 1, the rectifying / smoothing unit 11 includes a rectifying element D3 having an anode connected to one end of the auxiliary winding W3 and a cathode connected to the soft start control circuit 10, and one end of the rectifying element D3. A smoothing capacitive element C5 is connected to the cathode and the other end is connected to the other end of the auxiliary winding W3.

  The monitor voltage generator 12 is connected to the input terminals 2 a and 2 b of the insulating switching power supply 1, generates a monitor voltage based on the AC power supply, and outputs the monitor voltage to the soft start control circuit 10.

  For example, as shown in FIG. 1, the monitor voltage generation unit 12 includes rectifying elements D4 and D5, resistors R4 and R5, and a smoothing capacitive element C6. Here, the rectifier element D4 and the rectifier element D5 have anodes connected to the input terminal 2a and the input terminal 2b, respectively, and cathodes connected to each other. Further, the resistor R4 and the resistor R5 are connected in series. The resistor R4 is connected to the cathodes of the rectifying elements D4 and D5, and the resistor R5 is grounded. One end of the capacitive element C6 is connected to the connection point between the resistor R4 and the resistor R5, and the other end is grounded.

  With such a configuration, the monitor voltage generator 12 rectifies the voltage of the AC power supply, divides the rectified voltage with the resistors R4 and R5, and smoothes the divided voltage with the capacitive element C6. The obtained DC voltage is output to the soft start control circuit 10 as a monitor voltage.

  Note that the monitor voltage generation unit 12 may be configured not to monitor the input side voltage of the rectification unit 3 but to monitor the output side voltage of the rectification unit 3. In this case, the monitor voltage generator 12 divides the input rectified voltage output from the rectifier 3 as necessary and outputs the divided voltage to the soft start control circuit 10.

<Configuration of Soft Start Control Circuit 10>
As shown in FIG. 1, the soft start control circuit 10 includes, as terminals, a reference voltage input terminal VW, a divided voltage input terminal COMP, a soft start time control terminal SSC, a voltage input terminal Vin, and a power supply terminal KVc1. And a line sense terminal LSin.

  The reference voltage input terminal VW is connected to the auxiliary winding W3 via the rectifying / smoothing unit 11 and receives a reference voltage. The reference voltage is a DC voltage obtained by rectifying and smoothing the AC voltage generated at both ends of the auxiliary winding W3 by the rectifying and smoothing unit 11. The reference voltage may be a DC voltage output from the auxiliary power source. Here, the auxiliary power supply is a power supply that has a DC power supply and outputs a DC voltage corresponding to a voltage generated at both ends of the primary winding W1 to the reference voltage input terminal VW. Thus, the reference voltage is a voltage based on the voltage of the primary winding W1, and reflects the operation level (output voltage) of the isolated switching power supply 1.

  A divided voltage obtained by dividing the reference voltage by the resistors R2 and R3 is input to the divided voltage input terminal COMP. The soft start time control terminal SSC is connected to the capacitor element connection point N. The voltage input terminal Vin is a terminal to which the input rectified voltage output from the rectifying unit 3 is input. The power supply terminal KVc 1 is a terminal for supplying operating power to the switch control circuit 9. The line sense terminal LSin is connected to the output of the monitor voltage generator 12 and receives the monitor voltage.

  Next, the internal configuration of the soft start control circuit 10 according to the present embodiment will be described in detail with reference to FIG. FIG. 2 shows an example of a circuit diagram of the soft start control circuit 10.

  The soft start control circuit 10 includes a set signal generation unit 21, a reset signal generation unit 22, a comparator CMP3, an SR flip-flop FF2, a soft start time control unit 23, a comparator CMP4, a switch Q4, and a switch Q5, a gate voltage control unit 25, a level shift circuit 24, and a rectifier element D6 are provided.

  The set signal generation unit 21 is configured to output a set signal (H level signal) when the monitor voltage is less than the input threshold voltage Vth_LS. The set signal is an input voltage drop signal indicating that the input voltage from the AC power supply has fallen below a predetermined value.

  The set signal generation unit 21 includes, for example, a comparator CMP1 as shown in FIG. The comparator CMP1 includes an input terminal (−) connected to the line sense terminal LSin, an input terminal (+) to which the input threshold voltage Vth_LS is input, and the voltage at the input terminal (−) is the voltage at the input terminal (+). And an output terminal for outputting a set signal when it is smaller.

  The reset signal generation unit 22 is configured to output a reset signal (H level signal) when the divided voltage input to the divided voltage input terminal COMP is higher than the voltage Va corresponding to the output threshold voltage Vth_HL. Yes. The voltage Va corresponding to the output threshold voltage Vth_HL is a voltage obtained by dividing the output threshold voltage Vth_HL by the resistor R2 and the resistor R3. The reset signal is an output voltage rise completion signal indicating that the output voltage has reached the upper limit value in the burst mode.

  For example, as shown in FIG. 2, the reset signal generation unit 22 includes a comparator CMP2 and a NOT gate N1 connected to the output terminal of the comparator CMP2. The comparator CMP2 includes an input terminal (−) connected to the divided voltage input terminal COMP, an input terminal (+) to which the voltage Va is input, and the voltage at the input terminal (−) is the voltage at the input terminal (+). And an output terminal for outputting a reset signal when it is smaller.

  The comparator CMP3 has an input terminal (−) connected to the divided voltage input terminal COMP, an input terminal (+) to which a voltage Vb corresponding to the output threshold voltage Vth_LL is input, and a voltage at the input terminal (−). An output terminal that outputs an H level signal when the voltage is lower than the voltage of the input terminal (+), and outputs an L level signal otherwise. The voltage Vb corresponding to the output threshold voltage Vth_LL is a voltage obtained by dividing the output threshold voltage Vth_LL by the resistor R2 and the resistor R3.

  The output threshold voltage Vth_LL is lower than the output threshold voltage Vth_HL. For example, the output threshold voltage Vth_HL is the upper limit voltage of the reference voltage in the burst mode, and the output threshold voltage Vth_LL is the lower limit voltage of the reference voltage in the burst mode.

  The SR flip-flop FF2 includes a set input terminal S connected to the output terminal of the reset signal generation unit 22, a reset input terminal R connected to the output terminal of the comparator CMP3, and an output connected to the gate voltage control unit 25. Terminal Q.

  When an H level signal (reset signal) is input to the set input terminal S, the SR flip-flop FF2 outputs an H level burst control signal to the gate voltage control unit 25, and an H level signal is input to the reset input terminal R. In the case, the L level burst control signal is output to the gate voltage control unit 25.

  When the soft start time control unit 23 receives the set signal from the set signal generation unit 21, the soft start time control unit 23 electrically connects the capacitive element connection point N of the capacitor unit 8 to the ground. Further, the soft start time control unit 23 is configured to electrically insulate the capacitive element connection point N of the capacitor unit 8 from the ground when receiving the reset signal from the reset signal generation unit 22.

  The soft start time control unit 23 includes, for example, a switch Q3 and an SR flip-flop FF1, as shown in FIG. The switch Q3 has one end connected to the soft start time control terminal SSC and the other end grounded. The switch Q3 is a semiconductor switching element such as an n-type MOSFET. In this case, the drain terminal of the n-type MOSFET is connected to the soft start time control terminal SSC, and the source terminal is connected to the ground.

  The SR flip-flop FF1 is connected to the set input terminal S connected to the output terminal SET_OUT of the set signal generator 21, the reset input terminal R connected to the output terminal RESET_OUT of the reset signal generator 22, and the gate terminal of the switch Q3. And an output terminal Q connected thereto.

  When the set signal is input to the set input terminal S, the SR flip-flop FF1 outputs an H level signal that makes the switch Q3 conductive. In addition, when a reset signal is input to the reset input terminal R, the SR flip-flop FF1 outputs an L level signal that turns off the switch Q3.

  The comparator CMP4 includes an input terminal (−) connected to the reference voltage input terminal VW, an input terminal (+) to which the threshold voltage Vth_SW is input, and the voltage at the input terminal (−) is the voltage at the input terminal (+). And an output terminal that outputs an H level signal when it is smaller, and outputs an L signal otherwise. The comparator CMP4 is preferably a hysteresis comparator in order to increase noise resistance. Further, the threshold voltage Vth_SW is lower than the output threshold voltage Vth_LL.

  The switch Q4 is a burst control switch, and one end is connected to the switch Q5 and the other end is connected to the power supply terminal KVc1. The switch Q4 is a semiconductor switching element such as a p-type MOSFET. In this case, the source terminal of the p-type MOSFET is connected to the switch Q5, and the drain terminal is connected to the power supply terminal KVc1.

  The switch Q5 has one end connected to the voltage input terminal Vin and the other end connected to one end of the switch Q4. The switch Q5 becomes conductive when it receives an H level signal from the comparator CMP4, and enters a cutoff state when it receives an L level signal. The switch Q5 is a semiconductor switching element such as an n-type MOSFET. In this case, the drain terminal of the n-type MOSFET is connected to the voltage input terminal Vin, and the source terminal is connected to one end of the switch Q4.

  The gate voltage control unit 25 controls the gate voltage so that the switch Q4 is intermittently turned on. Specifically, when receiving the H level burst control signal from the SR flip-flop FF2, the gate voltage control unit 25 turns off the switch Q4 and stops supplying the operating power to the switch control circuit 9. On the other hand, when receiving the L-level burst control signal from the SR flip-flop FF2, the gate voltage control unit 25 turns on the switch Q4 and supplies operating power to the switch control circuit 9.

  Moreover, when the gate voltage control unit 25 receives the set signal (input voltage drop signal) from the set signal generation unit 21, the gate voltage control unit 25 controls the gate voltage of the switch Q4 so that the switch Q4 is in the cut-off state. As a result, when the input from the AC power supply decreases, the supply of the operating power to the switch control circuit 9 is stopped, and the switch control circuit 9 can be prevented from malfunctioning due to insufficient operating power.

  The level shift circuit 24 has an input terminal connected to the reference voltage input terminal VW and an output terminal connected to a connection point between the switch Q4 and the switch Q5 via the rectifying element D6, and drops the reference voltage to a predetermined voltage. The level shift circuit 24 outputs, for example, a voltage obtained by shifting the reference voltage so as to become the operating voltage of the switch control circuit 9.

  The rectifier element D6 has an anode connected to the output terminal of the level shift circuit 24 and a cathode connected to a connection point between the switch Q4 and the switch Q5. Thus, when the input rectified voltage input to the voltage input terminal Vin is output from the power supply terminal KVc1, it is possible to prevent the input rectified voltage from flowing into the reference voltage input terminal VW.

  Note that the rectifying element D6 may be provided between the reference voltage input terminal VW and the level shift circuit 24. In this case, the anode of the rectifying element D6 is connected to the reference voltage input terminal VW, and the cathode is connected to the input terminal of the level shift circuit 24. Thus, the rectifying element D6 is connected in series to the level shift circuit 24 between the reference voltage input terminal VW and the connection point of the switches Q4 and Q5, and is directed from the reference voltage input terminal VW to the other end of the switch Q5. It is provided as a device for passing current.

  Further, the rectifying element D6 may be deleted. In this case, the output terminal of the level shift circuit 24 is connected to the connection point of the switches Q4 and Q5.

  By providing the set signal generation unit 21, the reset signal generation unit 22, and the soft start time control unit 23, the soft start control circuit 10 sets the switch Q3 in a conductive state when the monitor voltage becomes less than the input threshold voltage Vth_LS. When the soft start time control terminal SSC (capacitor element connection point N) is electrically connected to the ground and the reference voltage becomes higher than the output threshold voltage Vth_HL, the switch Q3 is turned off and the soft start time control terminal SSC ( The capacitive element connection point N) is electrically isolated from ground.

  In addition, by providing the reset signal generation unit 22, the comparator CMP3, the SR flip-flop FF2, the gate voltage control unit 25, and the switch Q4, the soft start control circuit 10 has a reference voltage of the output threshold voltage Vth_HL and the output threshold voltage Vth_LL. The switch control circuit 9 is operated intermittently so that the voltage is between. As a result, the insulating switching power supply 1 operates in the burst mode.

  Further, by including the comparator CMP4, the switch Q5, the level shift circuit 24, and the rectifying element D6, the soft start control circuit 10 also has a function as a power supply circuit that supplies operating power to the switch control circuit 9. That is, the soft start control circuit 10 outputs the voltage input to the voltage input terminal Vin from the power supply terminal KVc1 to the power input terminal Vc1 of the switch control circuit 9 until the reference voltage reaches the threshold voltage Vth_SW. When the voltage reaches the threshold voltage Vth_SW, the voltage input to the reference voltage input terminal VW is output from the power supply terminal KVc1 to the power input terminal Vc1 of the switch control circuit 9.

  The soft start control circuit 10 can be configured as a semiconductor integrated circuit formed on a predetermined semiconductor substrate alone or together with the switch control circuit 9.

<Operation of Isolated Switching Power Supply 1>
Next, the operation of the insulating switching power supply 1 will be described with reference to FIG. FIG. 3 is a timing chart for explaining the operation of the isolated switching power supply 1 according to this embodiment.

  Before time t1, the isolated switching power supply 1 operates in a burst mode in which the switch control circuit 9 operates intermittently, and the reference voltage is controlled in a range between the output threshold voltage Vth_HL and the output threshold voltage Vth_LL. ing.

At time t _1, such as by connecting the stop of the AC power source, or an AC power source and the insulated switching power supply 1 is disconnected, is interrupted input from the AC power supply. As a result, the input rectified voltage output from the rectifier 3 becomes zero, and the monitor voltage starts to decrease according to the time constant determined by the monitor voltage generator 12. Further, the reference voltage starts to gradually decrease according to the time constant determined by the auxiliary winding W3 and the rectifying / smoothing unit 11.

Then, at time _{t 2, the} monitor voltage is less than the input threshold voltage Vth_LS. Thereby, the set signal generation unit 21 outputs a set signal (input voltage drop signal). When this input voltage drop signal is received, the SR flip-flop FF1 of the soft start time control unit 23 outputs an H level signal to the gate terminal of the switch Q3. Since the switch Q3 is turned on by the H level signal, the capacitive element connection point N is grounded. Therefore, the capacitance of the capacitor unit 8 viewed from the switch control circuit 9 is the capacitance of the capacitive element C1 that is larger than the combined capacitance of the capacitive element C1 and the capacitive element C2.

Then, at time t _3, the input from the AC power supply is restored. As a result, the monitor voltage is restored, and the set signal generator 21 stops outputting the set signal (input voltage drop signal). Further, since the input rectified voltage is supplied to the switch control circuit 9 and the switching unit 4 operates, the reference voltage starts to rise. The switch control circuit 9 connects the constant current source to the capacitor voltage input terminal SS and starts charging the capacitor unit 8. As a result, as shown in FIG. 3, the soft start voltage Vss starts to rise at a rising speed corresponding to the capacitance of the capacitive element C1.

Then, at time _{t 4,} the reference voltage reaches the output threshold voltage Vth_HL. Therefore, the reset signal generation unit 22 outputs a reset signal, and the SR flip-flop FF1 outputs an L level signal to the gate terminal of the switch Q3. Since the switch Q3 is cut off by receiving the L level signal, the capacitive element connection point N is electrically insulated from the ground. Therefore, the capacitance of the capacitor unit 8 as viewed from the switch control circuit 9 is a combined capacitance of the capacitive element C1 and the capacitive element C2, which is smaller than the capacitance of the capacitive element C1.

  When the reference voltage reaches the output threshold voltage Vth_HL, the gate voltage control unit 25 receives the H level burst control signal, turns off the switch Q4, and stops supplying the operating power to the switch control circuit 9. Therefore, the operation of the switching unit 4 stops and the reference voltage starts to decrease.

The operation of the insulated switching power supply 1 at time t ₄ later, as shown in FIG. 3, it is the same as the operation in the normal burst mode. That is, the switch control circuit 9 operates intermittently so that the reference voltage is between the output threshold voltage Vth_HL and the output threshold voltage Vth_LL. Further, since the capacitance of the capacitor unit 8 as viewed from the switch control circuit 9 is smaller than the capacitance of the capacitive element C1, the rising speed of the soft start voltage Vss is faster than the rising speed during the start-up. Therefore, the time t ₄ and later, the soft-start voltage Vss is increased at the same rate as the normal burst mode.

  In the state where the reference voltage is in the range between the output threshold voltage Vth_HL and the output threshold voltage Vth_LL, the output voltage of the isolated switching power supply 1 is sufficiently increased, so the soft start voltage Vss increases at the same speed as in the normal burst mode. However, the current Iw flowing through the transformer unit 5 does not become excessive as shown in FIG.

  As described above, in the soft start control circuit 10 according to the present embodiment, when the monitor voltage drops below the input threshold voltage Vth_LS in the burst mode, the soft start voltage increases until the reference voltage reaches the output threshold voltage Vth_HL. Are switched from the first rising speed, which is the rising speed of the soft start voltage in the burst mode, to the second rising speed that is lower than the first rising speed. This second rising speed is, for example, the rising speed of the soft start voltage in the normal mode in which the switch control circuit 9 operates continuously.

  The end of the period for switching the rising speed of the soft start voltage is not limited to when the reference voltage reaches the output threshold voltage Vth_HL, and may be, for example, when the reference voltage reaches the output threshold voltage Vth_LL. Generally speaking, the rising speed of the soft start voltage is switched to a lower speed than the rising speed in the normal burst mode until the reference voltage (and thus the output voltage) rises to such an extent that no overcurrent flows through the transformer unit 5. It only has to be.

  As described above, in this embodiment, switching of the rising speed of the soft start voltage is performed by changing the capacitance of the capacitor unit 8 as viewed from the switch control circuit 9. That is, the soft start control circuit 10 grounds the capacitive element connection point between the capacitive element C1 and the capacitive element C2, so that the rising speed of the soft start voltage Vss is based on the combined capacitance of the capacitive element C1 and the capacitive element C2. The first rising speed is switched to the second rising speed based on the capacitance of the capacitive element C1.

As a result, the rising speed of the soft start voltage during the start-up of the isolated switching power supply 1 (in the case of FIG. 3, time t _{3 to} t ₄ ) is slower than the rising speed in the normal burst mode. Even when the power supply is activated by the above, it is possible to prevent an overcurrent from flowing through the transformer unit 5 without reducing the soft start effect. As a result, it is possible to prevent, for example, transformer squealing and resonance deviation of the LLC current resonance type switching power supply.

  Further, in the present embodiment, the switch Q3 is turned on based on the monitor voltage to switch the rising voltage of the soft start voltage to a lower rising voltage. The monitor voltage drops much earlier than the reference voltage when the input from the AC power supply is interrupted. For this reason, even when the input from the AC power supply is interrupted for a short time, the rising voltage of the soft start voltage can be reliably switched from the first rising speed to the second rising speed.

  Therefore, according to the present embodiment, it is possible to reliably prevent an overcurrent from flowing in the transformer section when performing power supply start-up by soft start in the burst mode regardless of the length of time that the input from the AC power supply is interrupted. it can.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. One of the differences between the second embodiment and the first embodiment is a method for switching the rising speed of the soft start voltage. In the first embodiment, the rising speed of the soft start voltage is switched by changing the capacitance of the capacitor unit 8 as viewed from the switch control circuit 9. On the other hand, in the second embodiment, the rising speed of the soft start voltage is switched by changing the number of constant current sources that charge the capacitive element of the capacitor unit 8. Hereinafter, the second embodiment will be described focusing on the differences from the first embodiment.

  In the second embodiment, the switch control circuit 9A has two constant current sources I1 and I2, as shown in FIG. The constant current sources I1 and I2 are electrically connected to the capacitor voltage input terminal SS via the switches SW1 and SW2, respectively. Further, the capacitor unit 8A includes a capacitive element C7.

  The switches SW1 and SW2 are composed of semiconductor switching elements such as n-type MOSFETs, for example.

  In the second embodiment, when the soft start voltage is increased at the first rate of increase, the capacitive element C7 is charged by the constant current sources I1 and I2 by making both the switches SW1 and SW2 conductive.

  On the other hand, when the soft start voltage is raised at a second rising speed that is lower than the first rising speed during power-on, only one of the switches SW1 and SW2 is turned on, thereby making the constant current sources I1 and I2 The capacitor C7 is charged by one of the above.

  The switches SW1 and SW2 are controlled based on the output signal of the SR flip-flop FF1 of the soft start control circuit 10, for example. In this case, the soft start control circuit 10 does not include the switch Q3, for example, and the output terminal Q of the SR flip-flop FF1 is connected to the soft start time control terminal SSC. The soft start time control terminal SSC is connected not to the capacitor element connection point N but to the signal input terminal of the switch control circuit 9A.

  When the switch control circuit 9A receives an H level signal via a terminal that receives the output of the SR flip-flop FF1, one of the switches SW1 and SW2 is turned on and the other is turned off. On the other hand, when the L level signal is received, the switch control circuit 9A makes both the switches SW1 and SW2 conductive.

  As described above, in the present embodiment, when the monitor voltage drops below the input threshold voltage Vth_LS in the burst mode, the number of constant current sources that charge the capacitive element of the capacitor unit 8A is reduced until the reference voltage reaches the output threshold voltage Vth_HL. By reducing it compared to the normal burst mode, the rising speed of the soft start voltage is switched from the first rising speed to the second rising speed that is lower than the first rising speed.

  Therefore, according to the second embodiment, it is possible to obtain the same effect as that of the first embodiment.

  As a modification of the second embodiment, the rising speed of the soft start voltage may be changed by changing the output current amount of the current source instead of the number of current sources.

  Further, the soft start control circuit according to the second embodiment can be configured as a semiconductor integrated circuit formed on a predetermined semiconductor substrate alone or together with the switch control circuit 9A.

  Based on the above description, those skilled in the art may be able to conceive additional effects and various modifications of the present invention, but the aspects of the present invention are not limited to the individual embodiments described above. . You may combine suitably the component covering different embodiment. Various additions, modifications, and partial deletions can be made without departing from the concept and spirit of the present invention derived from the contents defined in the claims and equivalents thereof.

DESCRIPTION OF SYMBOLS 1 Isolated switching power supply 2a, 2b Input terminal 3 Rectification part 4 Switching part 5 Transformer part 6 Rectification smoothing part 7a, 7b Output terminal 8 Capacitor part 9 Switch control circuit 10 Soft start control circuit 11 Rectification smoothing part 12 Monitor voltage generation part 21 Set signal generator 22 Reset signal generator 23 Soft start time controller 24 Level shift circuit 25 Gate voltage controller C1, C2, C3, C4, C5, C6, C7 Capacitance elements CMP1, CMP2, CMP3, CMP4 Comparator D1, D2, D3, D4, D5, D6 Rectifier elements FF1, FF2 SR flip-flops I1, I2 Constant current source N1 NOT gate Q1 High side switch Q2, Low side switch Q3, Q4, Q5 Switch R1, R2, R3, R4, R5 Resistor SW1 , SW2 Switch W1 Primary winding W2 secondary winding W3 auxiliary winding M switch connection point N capacitance elements connected point

Claims (13)

A rectifying unit for rectifying an AC power source connected to an input terminal, a switching unit having a high-side switch and a low-side switch connected in series between the output of the rectifying unit and the ground, and one end of the high-side switch and the A transformer unit having a primary side winding connected to a switch connection point of a low side switch and a secondary side winding magnetically coupled to the primary side winding, and an alternating current generated at both ends of the secondary side winding A rectifying / smoothing unit that rectifies and smoothes a voltage and outputs a rectified and smoothed DC voltage to an output terminal, a capacitor unit having a capacitive element, a capacitor element of the capacitor unit, and a charge voltage of the capacitive element As the soft start voltage increases, the high side switch and the low size switch are set so that the conduction time becomes longer with a predetermined upper limit conduction time as an upper limit. A switch control circuit for controlling a complementary conductive state or blocking state switch, used in insulated switching power supply comprising,
In the burst mode in which the switch control circuit operates intermittently, when the monitor voltage based on the AC power supply drops below a predetermined input threshold voltage, the reference voltage based on the voltage of the primary winding becomes a predetermined output threshold voltage. Switching the rising speed of the soft start voltage from the first rising speed, which is the rising speed of the soft start voltage in the burst mode, to a second rising speed that is lower than the first rising speed until the first rising speed is reached. Soft start control circuit characterized by The capacitor unit has a first capacitor element having one end connected to the capacitor voltage input terminal of the switch control circuit, a second capacitor terminal connected to the other end of the first capacitor element, and the other end grounded. And a capacitive element,
The soft start control circuit grounds a capacitive element connection point between the first capacitive element and the second capacitive element, thereby increasing the rising speed of the soft start voltage from the first rising speed to the first rising speed. The soft start control circuit according to claim 1, wherein the soft start control circuit is switched to a rising speed of 2. A line sense terminal to which the monitor voltage is input;
A divided voltage input terminal to which a divided voltage obtained by dividing the reference voltage is input;
A soft start time control terminal connected to the capacitor element connection point;
A set signal generator for outputting a set signal when the monitor voltage is less than the input threshold voltage;
A reset signal generator that outputs a reset signal when the divided voltage is higher than a voltage corresponding to the output threshold voltage;
When the set signal is received, the capacitive element connection point is electrically connected to ground, and when the reset signal is received, the soft start time control unit electrically isolates the capacitive element connection point from the ground;
The soft start control circuit according to claim 2, further comprising:   The soft start control circuit according to any one of claims 1 to 3, wherein the monitor voltage is a DC voltage obtained by rectifying the voltage of the AC power supply and dividing and smoothing the rectified voltage.   5. The soft start control circuit according to claim 1, wherein the second rising speed is a rising speed of the soft start voltage in a normal mode in which the switch control circuit continuously operates. .   6. The soft start control circuit according to claim 1, wherein the output threshold voltage is an upper limit voltage of the reference voltage in the burst mode.   7. The reference voltage according to claim 1, wherein the reference voltage is a DC voltage obtained by rectifying and smoothing an AC voltage generated at both ends of an auxiliary winding magnetically coupled to the primary side winding. Soft start control circuit.   8. A semiconductor integrated circuit, wherein the soft start control circuit according to claim 1 is formed on a predetermined semiconductor substrate.   8. A semiconductor integrated circuit, wherein the soft start control circuit according to claim 1 and the switch control circuit are formed on a predetermined semiconductor substrate. A rectifying unit for rectifying the AC power source connected to the input terminal;
A switching unit having a high-side switch and a low-side switch connected in series between the output of the rectifying unit and the ground;
A transformer unit having a primary winding connected at one end to a switch connection point of the high-side switch and the low-side switch, and a secondary winding magnetically coupled to the primary winding;
A rectifying / smoothing unit that rectifies and smoothes the AC voltage generated at both ends of the secondary winding, and outputs the rectified and smoothed DC voltage to an output terminal;
A capacitor unit having a capacitive element;
The high-side switch and the low-side switch are charged so as to increase the conduction time with a predetermined upper limit conduction time as an upper limit as the soft start voltage, which is the charging voltage of the capacitance element, increases, while charging the capacitance element of the capacitor unit. A switch control circuit which controls the conduction state or the interruption state in a complementary manner;
A monitor voltage generator connected to the input terminal for generating a monitor voltage based on the AC power supply;
In the burst mode in which the switch control circuit operates intermittently, when the monitor voltage falls below a predetermined input threshold voltage, until the reference voltage based on the voltage of the primary winding reaches a predetermined output threshold voltage, A soft start control circuit that switches a rising speed of the soft start voltage from a first rising speed that is a rising speed of the soft start voltage in the burst mode to a second rising speed that is lower than the first rising speed; ,
An insulated switching power supply comprising:   The monitor voltage generation unit rectifies the voltage of the AC power supply and outputs a DC voltage obtained by dividing and smoothing the rectified voltage to the soft start control circuit as the monitor voltage. Item 11. The isolated switching power supply according to Item 10. The transformer unit further includes an auxiliary winding magnetically coupled to the primary winding,
The insulated switching power supply according to claim 10 or 11, wherein the reference voltage is a DC voltage obtained by rectifying and smoothing an AC voltage generated at both ends of the auxiliary winding.   The insulated switching power supply according to any one of claims 10 to 12, further comprising a resonance capacitive element connected in series or in parallel with the primary winding.

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