JP2009077565A - Semiconductor device for switching power supply application - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor device for switching power supply which is improved in power supply efficiency in a stand by state and in operation with light load by reducing power consumption in a snubber circuit to reduce power consumption in a power circuit in a switching power supply. <P>SOLUTION: In a switching power supply device, a DC voltage applied to an input terminal 10, while being applied to the primary side of a transformer 11, is reduced to an output voltage Vo, which is a second DC voltage, by an output voltage generating circuit 14 arranged on the secondary side of the transformer 11 by the switching operation of a switching element 12, and then is output to an output terminal 15. During an intermittent operation under no load or light load, a switching control circuit 18 increases the number of oscillations in a switching-on period of intermittent oscillation to reduce power consumption in the snubber circuit 16 connected in parallel to a primary winding 11a of the transformer 11 to eventually improve the power supply efficiency under no load or light load. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a semiconductor device for switching power supply, and more particularly to a semiconductor device for switching power supply that can reduce power consumption at no load or light load.

  An example of a conventional semiconductor device for switching power supply is shown in FIG. Conventionally, as a kind of power supply device for electronic equipment, a DC input terminal 10 which is a commercial power supply is a primary side, an output terminal 15 which is a drive unit of the electronic equipment is a secondary side, and the primary side and the secondary side are connected. A switching power supply having a transformer (transformer) 11 and having a switching element including a power MOSFET and a switching control circuit 18 on the primary side is generally used. This switching power supply is inefficient at the time of no load or light load because the ratio of power consumption in the power supply circuit such as switching loss to input power increases. Therefore, in order to reduce switching loss and reduce power consumption in the power supply circuit, intermittent operation may be performed. By reducing the number of oscillations according to the load, the switching loss is reduced and the efficiency is improved. A conventional intermittent operation control method will be described below.

  For example, as the load current decreases, a delay circuit that delays the time when the switching element is turned on is connected to the winding that drives the switching element, thereby reducing the switching loss per hour of the switching element at the minimum load. There is a control method that greatly improves the efficiency at the minimum load (see Patent Document 1).

  Further, for example, there is a control method of an intermittent operation mode in which an operation state and a stop state are repeated in a switching operation when the switching power supply is at no load or light load (see Patent Document 2).

Further, in a general switching power supply, when the power MOSFET as a switching element is switched off at the time of high input, the potential on the primary winding side of the transformer of the power MOSFET jumps up (spike voltage), and the power MOSFET is destroyed. In order to prevent this, a circuit (snubber circuit) 16 for clamping the spike voltage is provided.
JP 2000-197360 A Japanese Patent No. 3598852

  In the conventional switching power supply semiconductor device, although the switching loss during the intermittent oscillation operation during standby or light load is reduced, power consumption in the snubber circuit is not taken into consideration.

  It is an object of the present invention to provide a semiconductor device for a switching power supply that reduces the power consumption in the snubber circuit, thereby further reducing the power consumption compared with the conventional example and improving the power supply efficiency during standby and light loads. It is the purpose.

  In order to achieve the above object, the present invention provides a switching control circuit, generates a light load switching signal at a predetermined period at light load, performs intermittent oscillation, and sets the number of oscillations in the on period of one intermittent oscillation. The power consumed by the snubber circuit is reduced by increasing the off period and controlling the off period.

  A semiconductor device for a switching power supply according to the present invention includes a transformer, a switching element for switching and driving a primary winding of a transformer connected to a voltage source that generates a first DC voltage, and a primary winding of the transformer. Connect one end of the resistor and the resistor connected in parallel to the high-voltage side of the capacitor, connect the cathode of the Zener diode to the opposite end of the parallel capacitor and resistor, and connect the anode of the Zener diode to the low-voltage side of the primary winding of the transformer An output voltage generation circuit which is connected to the connected snubber circuit and the secondary winding of the transformer, and generates and outputs a second DC voltage by rectifying and smoothing the secondary output voltage of the transformer And a switching control circuit that controls the operation of the switching element, and a switching power supply device that includes an output voltage error detection circuit that feeds back an output voltage error to the switching control circuit. Target semiconductor device for a switching power supply that.

  In the present invention, the power consumed by the snubber circuit per unit time is increased by increasing the number of oscillations in one switching on period and extending the switching off period in intermittent oscillation operation at no load or light load. And the power supply efficiency of the switching power supply circuit at the time of no load or light load can be improved.

(Embodiment 1)
Embodiment 1 of the present invention will be described below with reference to the drawings.

  FIG. 1 shows a schematic circuit configuration of a switching power supply apparatus according to an embodiment of the present invention. As shown in FIG. 1, the switching power supply according to the first embodiment is switched by a switching element 12 while applying a DC voltage applied to an input terminal 10 to a primary side of a transformer (transformer) 11. This is a switching power supply that drops to the output voltage Vo, which is the second DC voltage, and outputs it to the output terminal 15 by the output voltage generation circuit 14 provided on the secondary side of the transformer 11 by operation. In addition, a snubber circuit is provided in parallel with the primary winding 11a of the transformer 11 in order to prevent destruction of the switching element 12 due to a voltage jump on the switching element 12 side in the primary winding 11a of the transformer 11 as the switching element 12 is turned off. 16 is provided. Next, the feedback method will be described below. An output voltage error detection circuit 17 and a feedback circuit 13 are connected between a high potential side terminal and a low potential side terminal of the output voltage generation circuit 14. In FIG. 1, a photocoupler is used in the feedback circuit. The value of the current flowing through the output voltage error detection circuit 17 is proportional to the output voltage. A current proportional to the output voltage flows through the light emitting diode 13a of the photocoupler 13 inserted in series with the error detection circuit 17 of the output voltage, the light emitting diode 13a of the photocoupler 13 shines, and the output transistor 13b of the photocoupler 13 on the primary side is illuminated. A current flows and flows out from the feedback terminal of the switching adjustment circuit 19 of the switching element. The switching adjustment circuit 19 detects the current value flowing out from the feedback terminal, and controls increase / decrease in power supply to the secondary side due to the switching operation. When the supply power to the secondary side becomes excessive and the output voltage of the output voltage generation circuit 14 increases, an increase in the current flowing out from the feedback terminal is detected, and control is performed to decrease the supply power, and the supply is reversed When power is insufficient and the output voltage of the output voltage generation circuit 14 decreases, it is detected that the current flowing out from the feedback terminal decreases, and works to increase the supply power. As a method for controlling increase / decrease in the supplied power, a method for controlling the peak value of the current flowing through the primary winding 11a of the transformer 11, a method for controlling the number of oscillations per unit time by changing the frequency, and the like can be considered. . By repeating the above control method, the feedback current converges in a stable direction.

The above is a feedback method under normal load conditions, but when there is no load or light load, the switching method is switched to intermittent operation by the light load detection circuit in the switching adjustment circuit 19, and switching on in the switching control circuit 18 A period signal is generated, and the switching operation is performed only when the switching on period signal is on. When there is no load or light load, the switching control circuit 18 works in a direction to reduce the supplied power. When the supplied power reaches a certain value, the switching on period signal is turned off, the switching operation is stopped, and the off period is entered. . Since the supplied power becomes zero, the output voltage starts to decrease, the current flowing out from the feedback terminal decreases, and when it reaches a certain value, the switching on period signal is switched on again, and the switching control circuit 18 sends the switching signal to the power MOSFET. To start the switching operation and shift to the ON period. When power supply starts in the on period, the output power becomes excessive, so that the switching control circuit works in a direction to decrease the supplied power and shifts to the off period again. The above operation is repeated to perform intermittent oscillation operation. FIG. 8 shows the switching signal and the current waveform flowing through the primary winding 11a of the transformer 11 during the intermittent oscillation operation. FIG. 9 shows the waveform of intermittent oscillation at this time. FIG. 9 shows a waveform (a) of the current I _DS flowing from the primary winding of the transformer to the switching element in the switching power supply, the voltage V _DS on the switching element side of the primary winding, the capacitance C _SB and the resistance R of the snubber circuit. A waveform (b) of the voltage V _SB applied to _SB , a waveform (c) of the current I _RSB flowing through the resistor R _SB of the snubber circuit, and a waveform (d) of the power P _RSB consumed by the resistor R _SB of the snubber circuit is there.

  In the present invention, a delay circuit 20 for extending the on period of the intermittent oscillation switching signal is inserted between the power MOSFET 12 and the switching adjustment circuit 19 in the switching control circuit 18.

  In this case, the number of oscillations in one on period increases and the energy supplied to the secondary side increases, so that the off period until the next oscillation becomes longer after the switching element is turned off.

First, in FIG. 2, when the ON period becomes longer due to the delay circuit and the number of oscillations per one ON period increases (FIG. 2 shows the case where the number of oscillations is 3), the current flows through the primary winding 11a of the transformer 11. A current waveform (a), a switching signal (b), and a switching on period signal (c) through the delay circuit 20. The waveform of intermittent oscillation at this time is shown in FIG. FIG. 3 shows a waveform (a) of the current I _DS flowing from the primary winding of the transformer to the switching element in the switching power supply, the voltage V _DS on the switching element side of the primary winding, the capacitance C _SB and the resistance R of the snubber circuit. A waveform (b) of the voltage V _SB applied to _SB , a waveform (c) of the current I _RSB flowing through the resistor R _SB of the snubber circuit, and a waveform (d) of the power P _RSB consumed by the resistor R _SB of the snubber circuit is there. When the switching element is turned off, the voltage V _DS on the switching element side of the primary winding jumps by the reflected voltage V _OR on the secondary side. In the period (1) in which current flows on the secondary side, the reflected voltage V _OR is applied to the snubber circuit. Energy consumed by the resistor R _SB of the snubber circuit in this period (1),

It becomes. In addition, since the flyback power supply oscillates in a discontinuous mode, the voltage applied to the snubber circuit becomes zero after the secondary current is attenuated to zero, and the power stored in the snubber circuit capacitor C _SB is released. The power consumed by the snubber circuit resistance is

It becomes.

In the present invention, during intermittent oscillation of the switching power supply, a delay circuit that increases the on period is inserted to increase the number of oscillations per on period, and during the on period, oscillation is accumulated in the capacitor C _SB of the snubber circuit. The second oscillation occurs before all the power is released, and the off-period until the next oscillation after the on-period is lengthened to increase the capacity of the snubber circuit in ii) of the power loss in this snubber circuit. Reduce the power consumption stored in the.

  The formula for calculating power consumption is shown below.

First, calculate the voltage V _SB according to the capacitance C _SB and the resistance R _SB of the snubber circuit. In the period (1) in which current flows on the secondary side and power is consumed, V _SB becomes the reflected voltage V _OR . The energy consumed in this period (1) is the same as in the above equation i). During intermittent oscillation, the flyback power supply oscillates in discontinuous mode, so when the secondary current decays to zero, the reflected voltage applied to the snubber circuit also decays to zero, but is stored in the snubber circuit capacitance. In the period (2) during which power is released, current flows through the snubber circuit resistance, so the voltage V _SB applied to the snubber circuit resistance is expressed by the following equation iii).

  Here, when the number of oscillations in the on period per time is n, the power consumed by the snubber circuit in the on period is expressed by the following equation iv).

  In the above formula iv), when t> 0,

Therefore, as n increases, the power _PRBS consumed in the snubber circuit decreases.

  Note that the intermittent oscillation control at no load or light load has been explained by the control method that repeats the on period and the off period due to the hysteresis of the current to the feedback terminal, but by changing the frequency, oscillation per unit time A method of controlling the power supply by changing the number of times is also possible.

  Other intermittent oscillation control methods are also possible.

  Although the circuit in the figure is a flyback switching power supply circuit, a forward switching power supply circuit is also possible.

  As for the feedback circuit 13, a photocoupler is used in the figure, but other feedback methods are also possible.

(Embodiment 2)
Embodiment 2 of the present invention will be described below with reference to the drawings.

  FIG. 4 shows a schematic circuit configuration of a switching power supply device according to an embodiment of the present invention. As shown in FIG. 4, the switching power supply according to the second embodiment is switched by the switching element 12 while applying a DC voltage applied to the input terminal 10 to the primary side of a transformer (transformer) 11. This is a switching power supply that drops to the output voltage Vo, which is the second DC voltage, and outputs it to the output terminal 15 by the output voltage generation circuit 14 provided on the secondary side of the transformer 11 by operation. In addition, a snubber circuit is provided in parallel with the primary winding 11a of the transformer 11 in order to prevent destruction of the switching element 12 due to a voltage jump on the switching element 12 side in the primary winding 11a of the transformer 11 as the switching element 12 is turned off. 16 is provided. Next, the feedback method will be described below. An output voltage error detection circuit 17 and a feedback circuit 13 are connected between a high potential side terminal and a low potential side terminal of the output voltage generation circuit 14. In FIG. 4, a photocoupler is used in the feedback circuit. The value of the current flowing through the output voltage error detection circuit 17 is proportional to the output voltage. A current proportional to the output voltage flows through the light emitting diode 13a of the photocoupler 13 inserted in series with the error detection circuit 17 of the output voltage, the light emitting diode 13a of the photocoupler 13 shines, and the output transistor 13b of the photocoupler 13 on the primary side is illuminated. A current flows and flows out from the feedback terminal of the switching adjustment circuit 19 of the switching element. The switching adjustment circuit 19 detects the current value flowing out from the feedback terminal, and controls increase / decrease in power supply to the secondary side due to the switching operation. When the supply power to the secondary side becomes excessive and the output voltage of the output voltage generation circuit 14 increases, an increase in the current flowing out from the feedback terminal is detected, and control is performed to decrease the supply power, and the supply is reversed When power is insufficient and the output voltage of the output voltage generation circuit 14 decreases, it is detected that the current flowing out from the feedback terminal decreases, and works to increase the supply power. As a method for controlling increase / decrease in the supplied power, a method for controlling the peak value of the current flowing through the primary winding 11a of the transformer 11, a method for controlling the number of oscillations per unit time by changing the frequency, and the like can be considered. . The above control method is repeated to converge the feedback current in a stable direction.

The above is a feedback method under normal load conditions, but when there is no load or light load, the switching method is switched to intermittent operation by the light load detection circuit in the switching adjustment circuit 19, and switching on in the switching control circuit 18 A period signal is generated, and the switching operation is performed only when the switching on period signal is on. When there is no load or light load, the switching control circuit 18 works in a direction to reduce the supplied power. When the supplied power reaches a certain value, the switching on period signal is turned off, the switching operation is stopped, and the off period is entered. . Since the supplied power becomes zero, the output voltage starts to decrease, the current flowing out from the feedback terminal decreases, and when it reaches a certain value, the switching on period signal is switched on again, and the switching control circuit 18 sends the switching signal to the power MOSFET. To start the switching operation and shift to the ON period. When power supply starts in the on period, the output power becomes excessive, so that the switching control circuit works in a direction to decrease the supplied power and shifts to the off period again. The above operation is repeated to perform intermittent oscillation operation. FIG. 8 shows the switching signal and the current waveform flowing through the primary winding 11a of the transformer 11 during this intermittent oscillation operation. FIG. 9 shows the waveform of intermittent oscillation at this time. FIG. 9 shows a waveform (a) of the current I _DS flowing from the primary winding of the transformer to the switching element in the switching power supply, the voltage V _DS on the switching element side of the primary winding, the capacitance C _SB and the resistance R of the snubber circuit. A waveform (b) of the voltage V _SB applied to _SB , a waveform (c) of the current I _RSB flowing through the resistor R _SB of the snubber circuit, and a waveform (d) of the power P _RSB consumed by the resistor R _SB of the snubber circuit is there.

  In the present invention, the switching control circuit 18 includes a switching counter 21 that counts the number of times of switching when intermittent, and starts counting the number of times of switching when switching to the switching-on period during intermittent oscillation. It controls so that it does not shift to an off period until it reaches.

  In this case, if the set minimum number of oscillations is larger than the number of oscillations in one on-time when intermittent oscillation is performed by the hysteresis of the feedback current described above, it is supplied to the secondary side in one on period. Therefore, after switching to the off period, the off period until the next oscillation becomes longer.

  If the set minimum oscillation number is smaller than or equal to the number of oscillations during one on-time when the above-described feedback current hysteresis is performed by the hysteresis of the feedback current, the secondary side is turned on during the one on-period. There is no change in the energy supplied to or off period.

First, FIG. 5 shows a case where the number of oscillations in one on-time when the intermittent oscillation by the hysteresis of the feedback current is performed is 3 times or less and the set minimum number of oscillations is 3. Alternatively, when intermittent oscillation is performed with the above feedback current hysteresis, the number of oscillations in one on-time is three, and the minimum oscillation number set is three or less, one on The current waveform (a) of the transformer primary winding 11a, the switching signal (b), and the count number (c) in the switching counter 21 when intermittent oscillation is performed with the number of oscillations of 3 times are shown. FIG. The waveform of intermittent oscillation at this time is shown in FIG. FIG. 3 shows the waveform (a) of the current I _DS flowing from the primary winding of the transformer to the switching element in the switching power supply at this time, the voltage V _DS on the switching element side of the primary winding, and the capacitance C _{SB of the} snubber circuit. a resistor R _SB to such voltage V _SB of the waveform (b), the waveform of the current I _RSB flowing through the resistor R _SB of the snubber circuit (c), the power P _RSB waveform to be consumed by the resistor R _SB of the snubber circuit ( d). When the switching element is turned off, the voltage V _DS on the switching element side of the primary winding jumps by the reflected voltage V _OR on the secondary side. In the period (1) in which current flows on the secondary side, the reflected voltage V _OR is applied to the snubber circuit. Energy consumed by the resistor R _SB of the snubber circuit in this period (1),

It becomes. In addition, since the flyback power supply oscillates in a discontinuous mode, the voltage applied to the snubber circuit becomes zero after the secondary current is attenuated to zero, and the power stored in the snubber circuit capacitor C _SB is released. The power consumed by the snubber circuit resistance is

It becomes.

In the present invention, during intermittent oscillation of the switching power supply, the number of oscillations during one on period is fixed, and before the power accumulated in the capacitor C _SB of the snubber circuit is released during oscillation during the on period. The power consumption accumulated in the capacity of the snubber circuit among the power loss in this snubber circuit is reduced by adopting a configuration in which the next oscillation occurs and the off period until the next oscillation after the on period is lengthened. .

  The formula for calculating power consumption is shown below.

First, calculate the voltage V _SB according to the capacitance C _SB and the resistance R _SB of the snubber circuit. In the period (1) in which current flows on the secondary side and power is consumed, V _SB becomes the reflected voltage V _OR . The energy consumed in this period (1) is the same as in the above equation i). During intermittent oscillation, the flyback power supply oscillates in discontinuous mode, so when the secondary current decays to zero, the reflected voltage applied to the snubber circuit also decays to zero, but is stored in the snubber circuit capacitance. Since a current flows through the snubber circuit resistance during the period in which power is released, the voltage V _SB applied to the snubber circuit resistance is expressed by the following equation iii).

  Here, when the number of oscillations in the on period per time is n, the power consumed by the snubber circuit in the on period is expressed by the following equation iv).

  In the above formula iv), when t> 0,

Therefore, the power _PRBS consumed in the snubber circuit decreases as the number of oscillations n during the ON period increases. Assuming that the minimum oscillation number is set to n ′, n is always equal to or greater than n ′. Therefore, the power P _RBS consumed in the snubber circuit is equal to or less than P _RBS (n = n ′).

  Note that the intermittent oscillation control at no load or light load has been described with the control method that repeats the on period and off period due to the hysteresis of the current from the feedback terminal, but by changing the frequency, oscillation per unit time A method of controlling the power supply by changing the number of times is also possible.

  Other intermittent oscillation control methods are also possible.

  Although the circuit in the figure is a flyback switching power supply circuit, a forward switching power supply circuit is also possible.

  As for the feedback circuit 13, a photocoupler is used in the figure, but other feedback methods are also possible.

  The semiconductor device for a switching power supply according to the present invention is a switching element that controls switching driving in the switching power supply, and is particularly useful as a semiconductor device for a switching power supply that reduces power consumption during standby.

The figure of the switching power supply circuit in Embodiment 1 of this invention In the switching power supply circuit according to the first embodiment of the present invention, (a) a current waveform diagram of the transformer primary winding 11a, (b) a waveform diagram of the switching signal, and (c) a switching on period through the on period delay circuit 20 Signal waveform diagram In the switching power supply circuit according to the first embodiment of the present invention, (a) the current waveform I _DS of the transformer primary winding 11a and the switching element 12 side of the transformer primary winding 11a when passing through the on-period delay circuit 20 (B) Waveform diagram of the voltage V _DS of the snubber circuit 16; (b) Waveform diagram of the voltage V _SB applied to the capacitor C _SB and the resistor R _SB of the snubber circuit 16; (c) Waveform diagram of the current flowing through the resistor R _SB of the snubber circuit 16; power waveform that is consumed by the resistor R _SB of the snubber circuit 16 Switching power supply circuit diagram according to the second embodiment of the present invention In the switching power supply circuit according to Embodiment 2 of the present invention, (a) a current waveform diagram of the transformer primary winding 11a during intermittent oscillation, (b) a waveform diagram of a switching signal, (c) a count number in the switching counter 21 Figure showing In the switching power supply circuit according to the second embodiment of the present invention, (a) the waveform of the current waveform I _DS of the transformer primary winding 11a and the voltage V _DS on the switching element 12 side of the transformer primary winding 11a during intermittent oscillation. , (B) a waveform diagram of the voltage V _SB applied to the capacitor C _SB and the resistor R _SB of the snubber circuit 16, (c) a waveform diagram of a current flowing through the resistor R _SB of the snubber circuit 16, and (d) a resistor R _{SB of the} snubber circuit 16. Of power consumption Diagram of switching power supply circuit in conventional example In the switching power supply circuit according to the conventional embodiment, (a) a current waveform diagram of the transformer primary winding 11a, (b) a waveform diagram of a switching signal, and (c) a waveform diagram of a switching on period signal. In the switching power supply circuit according to the conventional embodiment, (a) a waveform diagram of the current waveform I _DS of the transformer primary winding 11a and the voltage V _DS on the switching element 12 side of the transformer primary winding 11a, (b) a snubber circuit. 16 is a waveform diagram of the voltage V _SB applied to the capacitor C _SB and the resistor R _SB , (c) a current waveform diagram flowing through the resistor R _SB of the snubber circuit 16, and (d) a power waveform consumed by the resistor R _SB of the snubber circuit 16. Figure

Explanation of symbols

10 DC input terminal 11 Transformer
11a Primary winding of transformer 11b Secondary winding of transformer 12 Power MOSFET (switching element)
13 Photocoupler (Feedback circuit)
13a Photo-coupler light-emitting diode 13b Photo-coupler output transistor 14 Output voltage generation circuit 15 Output terminal 16 Snubber circuit 17 Output voltage error detection circuit 18 Switching control circuit 19 Switching adjustment circuit 20 On-period delay circuit 21 Switching counter

Claims (3)

A transformer,
A switching element for switching and driving a primary winding of the transformer connected to a voltage source for generating a first DC voltage;
In a circuit configuration in which a resistor and a capacitor connected in parallel are connected to a Zener diode, a capacitor connected in parallel to the high voltage side of the primary winding of the transformer and one end of the resistor are connected. A snubber circuit in which the cathode of the Zener diode is connected to the opposite end of the resistor, and the anode of the Zener diode is connected to the low voltage side of the primary winding of the transformer;
A switching power supply semiconductor device for controlling a switching power supply device comprising an output voltage generation circuit that generates and outputs a second DC voltage by rectifying and smoothing a secondary output voltage of the transformer. And
The semiconductor device for a switching power supply, comprising: a switching control circuit that performs an intermittent oscillation operation at a no load or a light load, and increases the number of oscillations in one switching on period of the intermittent oscillation operation. 2. The semiconductor device for a switching power supply according to claim 1, wherein the switching control circuit includes a delay circuit for extending a switching on period of an intermittent oscillation operation at no load or at a light load. The switching control circuit of the switching element has a switching counter that counts the number of oscillations in the switching on period of the intermittent oscillation operation at no load or light load, and sets the minimum number of oscillations in one switching on period The semiconductor device for a switching power supply according to claim 1.

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