JP4918066B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply apparatus that supplies a stabilized DC voltage to a load by switching control means including current mode control means.

  A switching element such that a DC input voltage is switched at a predetermined cycle by a switching element such as a field effect transistor (FET), and a rectified and smoothed DC output voltage is supplied to a load, and the DC output voltage becomes a set value. Various configurations of switching power supply devices that control the on-period of these are already known. In addition, the switching power supply device controls the ON period of the switching element so that the DC output voltage becomes the set voltage. However, the phenomenon that the DC output voltage overshoots in a transient control state at the initial stage of startup. appear. Since the voltage due to this overshoot suddenly changes so as to exceed a predetermined voltage, there is a high possibility of adversely affecting a load such as a semiconductor integrated circuit. Accordingly, a configuration provided with a soft start circuit for suppressing the occurrence of this overshoot is often applied (see, for example, Patent Document 1). There is also known a switching power supply device provided with a current mode control means for detecting and feeding back a current flowing through a choke coil constituting a smoothing circuit (see, for example, Patent Document 2).

  FIG. 3 shows a main part of a conventional switching power supply device to which such a current mode control means and a soft start circuit are applied. In the figure, E is a DC power supply, Q1 is a switching element such as a field effect transistor, D1 is a diode, L1 is a choke coil, C1 and C2 are capacitors, R1 to R3 are resistors, RL is a load, and SW is a soft start. The switch, Vref is the reference power supply voltage, Vr is the reference voltage, Vin is the input voltage, Vout is the output voltage, V1 is the current detection voltage that detects the current flowing through the choke coil, V2 is the error voltage, V3 is the output detection voltage, and I1 is Load current, I2 is a choke coil current, 21 is a current-voltage converter (I / V), 22 is a current detector for detecting a current flowing through the choke coil by a current transformer, a choke coil secondary winding, etc., and 23 is an error amplifier , 24 is a comparator, 25 is an oscillator (OSC), 26 is a flip having a set terminal S, a reset terminal R, and an output terminal Q Show a drop.

  The soft start circuit has a configuration including a soft start switch SW, a resistor R3, and a capacitor C2. The current mode control circuit has a configuration including a current detector 22, a current-voltage converter 21, and a comparator 24. In this case, the switching element Q1 is turned on when the output terminal Q of the flip-flop 26 is at a high level (“1”). The oscillator 25 inputs an output signal having a constant period corresponding to the ON / OFF period of the switching element Q 1 to the set terminal S of the flip-flop 26. For example, when the soft start switch SW of the soft start circuit is turned on as illustrated, the capacitor C2 is short-circuited, and the reference voltage Vr input to the error amplifier 23 is 0V. Therefore, the error voltage V2 at the output of the error amplifier 23 becomes 0V, and the output signal of the comparator 24 continues to be at the high level “1”. The flip-flop 26 set by the output signal of the constant period of the oscillator 25 is reset by the output signal (“1”) of the comparator 24. Therefore, when the soft start switch SW is in the on state, the flip-flop 26 continues in the reset state, the switching element Q1 continues in the off state, and the output voltage Vout applied to the load RL becomes 0V. Therefore, the output detection voltage V3 input to the error amplifier 23 is also 0V. That is, the switching power supply device remains in a dormant state.

When the soft start switch SW is turned off, the capacitor C2 is charged from the reference power supply voltage Vref via the resistor R3, and the terminal voltage of the capacitor C2 is input as the reference voltage Vr of the error amplifier 23. In this case, the reference voltage Vr increases according to the CR time constant. An output detection voltage V3 obtained by dividing the output voltage Vout by the resistors R1 and R2 is input to the error amplifier 23, an error voltage V2 that is different from the reference voltage Vr is input to the comparator 24, and the choke coil is input by the current detector 22. The current flowing through L1 is detected, and the current detection voltage V1 converted by the current-voltage converter 21 is input to the comparator 24. When V1 ≧ V2, the flip-flop 26 is reset and the switching element Q1 is turned off. And As described above, the flip-flop 26 is set by the output signal of the oscillator 25, reset by the output signal of the comparator 24, and the switching element Q1 is turned on by the set output signal from the output terminal Q. When the switching element Q1 is turned on, a charging current for the capacitor C1 and a current I1 supplied to the load RL flow through the choke coil L1. Thereafter, when the switching element Q1 is turned off, a current due to the energy stored in the choke coil L1 flows to the capacitor C1 and the load RL via the diode D1. The output voltage Vout applied to the load RL gradually increases by turning off the soft start switch SW, and is controlled to a constant voltage determined by the reference voltage Vr.
JP 2000-102243 A JP 2002-281742 A

  The switching power supply device having the above-described configuration including the conventional soft start circuit and the current mode control circuit ideally has a smooth characteristic that the output voltage rises according to a predetermined time constant by the soft start circuit, and the current mode. The control circuit has a characteristic that suppresses fluctuations in the output voltage against changes in the load current. However, the conventional switching power supply device shown in FIG. 3 has a problem that the rising characteristics of the output voltage are not always smooth. FIG. 4 shows an outline of the operation characteristics of the switching power supply device shown in FIG. 4, and Vr, Vout, V1, V2, and V3 are a reference voltage, an output voltage, a current detection voltage, an error voltage, and an output, respectively. The outline characteristics of the detection voltage are shown. The horizontal axis indicates the same time t, but the vertical axis indicates the same or different voltage.

  When the soft start switch SW is turned off, the capacitor C2 is charged through the resistor R3 by the reference power supply voltage Vref, and the terminal voltage of the capacitor C2 is input to the error amplifier 23 as the reference voltage Vr. Therefore, as shown at the top of FIG. 4, the reference voltage Vr rises according to the CR time constant and becomes a reference voltage Vr having a value substantially similar to the reference power supply voltage Vref. Corresponding to the increase of the reference voltage Vr, the output voltage Vout (output detection voltage V3) also increases. The current I2 flowing through the choke coil L1 corresponding to the increase in the output voltage Vout (output detection voltage V3) is the sum of the current I1 flowing through the load RL and the charging current of the capacitor C1, and increases rapidly. V1 is a value exceeding a predetermined value. As the current mode control circuit, the output voltage Vout is controlled by shortening the ON period of the switching element Q1 even during the soft start period due to the flow of the current with the current detection voltage V1 exceeding a predetermined value. (Output detection voltage V3) is reduced.

  As the output voltage Vout (output detection voltage V3) decreases, the current I2 flowing through the choke coil L1 also decreases. In response to the change in the current I2, the current detection voltage V1 rapidly increases and then decreases. When the output voltage Vout (output detection voltage V3) reaches a predetermined value, the current I2 has a predetermined value, that is, the current detection voltage V1 has a predetermined value. As described above, in the conventional switching power supply device including the soft start circuit and the current mode control circuit, at the point a during the rising process of the output voltage Vout shown second from the top in FIG. By sudden change, the error voltage V2 also changes. Thus, a sudden change in the output voltage Vout applied to the load RL has a problem of adversely affecting a load with a low breakdown voltage such as a semiconductor integrated circuit.

  An object of the present invention is to prevent an output voltage applied to a load at the time of soft start of a switching power supply device from rising smoothly and to give an impact to the load due to a sudden change in the applied voltage.

  The switching power supply device according to the present invention provides an error amount obtained by inputting an output detection voltage and a reference voltage detected by an output voltage applied to a load to an error amplifier, and a current that is on / off controlled by a switching element. A current detection voltage obtained by detecting the current and converting it to a voltage, and comparing the error amount with the current detection voltage by a comparator. The switching power supply device includes a current mode control circuit that controls an ON period of the switching element corresponding to a difference between the switching element and a soft start circuit that gradually increases the reference voltage at the time of start-up. A series circuit of a resistor and a capacitor is connected between the comparator terminal and the error amplifier terminal for inputting the output detection voltage. Eteiru.

  By gradually increasing the reference voltage input to the error amplifier by the soft start circuit, the output voltage applied to the load is gradually increased, and the current flowing through the choke coil increases rapidly as the output voltage increases. Therefore, in the past, the output voltage temporarily decreased by shortening the ON period of the switching element, and then rapidly increased, but the current detection voltage input terminal of the comparator and the output of the error amplifier A resistor and a capacitor connected between the input terminal of the detection voltage superimpose a sudden change component of the current detection voltage on the output detection voltage, avoiding a temporary decrease in the output voltage due to shortening of the ON period of the switching element, Corresponding to the soft start characteristic by the soft start circuit, the output voltage can be increased smoothly.

  The switching power supply device of the present invention will be described with reference to FIG. 1. An output voltage V3 obtained by detecting an output voltage Vout applied to a load RL and a reference voltage Vr are input to an error amplifier 23 to obtain an error voltage V2. The current that is on / off controlled by the switching element Q1 is supplied to the load RL via the choke coil L1, and the current I2 flowing through the choke coil L1 is detected and converted to the current detection voltage V1 by the current-voltage converter 21. Then, the current detection voltage V1 and the error voltage V2 are input to the comparator 24 for comparison, and a current mode for controlling the ON period of the switching element Q1 corresponding to the difference between the error voltage V2 and the current detection voltage V1. A switching power supply including a control circuit and a soft start circuit for gradually increasing the reference voltage Vr input to the error amplifier 23 at the time of startup And having a configuration in which a series circuit of a resistor R4 and a capacitor C3 is connected between a terminal of the comparator 24 for inputting the current detection voltage V1 and a terminal of the error amplifier 23 for inputting the output detection voltage V3. ing.

  FIG. 1 is an explanatory diagram of Embodiment 1 of the present invention, where E is a DC power supply, Q1 is a switching element such as a field effect transistor, D1 is a diode, L1 is a choke coil, C1 to C3 are capacitors, and R1 to R4 are Resistor, RL is a semiconductor integrated circuit and various loads, SW is a soft start switch, Vref is a reference power supply voltage, Vr is a reference voltage, Vin is an input voltage, Vout is an output voltage, and V1 is a current detected through a choke coil. The detection voltage, V2 is the error voltage of the output of the error amplifier 23, V3 is the output detection voltage divided by the resistors R1 and R2, I1 is the load current supplied to the load RL, I2 is the capacitor C1 and the load via the choke coil L1 Choke coil current flowing through RL, 21 is a current-voltage converter (I / V), 22 is a secondary winding of a current transformer or choke coil Current detector for detecting current flowing through the choke coil L1, 23 an error amplifier, 24 a comparator, 25 an oscillator (OSC), 26 a flip-flop having a set terminal S, a reset terminal R, and an output terminal Q Show.

  By turning off the soft start switch SW, the reference voltage Vr is smoothly increased according to the CR time constant of the capacitor C2 and the resistor R3, and the ON period of the switching element Q1 of the switching power supply device is sequentially increased from 0 to output The voltage Vout is increased. The output voltage Vout causes a current to flow through the load RL and the capacitor C1. Further, when the switching element Q1 is turned off by the energy stored in the choke coil L1 due to the current that flows when the switching element Q1 is turned on, the current flows to the load RL and the capacitor C1. The current I2 flowing through the choke coil L1 is detected by the current detector 22, converted into the current detection voltage V1 by the current-voltage converter 21, and input to the comparator 24. The current detection voltage V1 converted by the current-voltage converter 21 is superimposed on the output detection voltage V3 via the resistor R4 and the capacitor C3 and input to the error amplifier 23. That is, the current detection voltage corresponding to the change in the current I2 flowing through the choke coil L1 is superimposed on the output detection voltage V3 by the voltage dividing resistors R1 and R2 and input to the error amplifier 23 via the resistor R4 and the capacitor C3. To do. Thereby, it is possible to avoid the shortening of the ON period of the switching element Q1 due to the sudden rise of the current I2 flowing through the choke coil L1 in the rising process of the output voltage Vout, and to smoothly increase the output voltage Vout.

  FIG. 2 is an explanatory diagram of an outline of the operation of the first embodiment of the present invention shown in FIG. 1, wherein Vr is a reference voltage, Vout is an output voltage, V1 is a current detection in which a current flowing through a choke coil is detected and converted into a voltage. Voltage, V2 is an error voltage of the output of the error amplifier 23, and V3 is an outline of the output detection voltage by the voltage dividing resistors R1, R2 in the soft start process. The reference voltage Vr input to the error amplifier 23 rises according to the CR time constant of the capacitor C2 and the resistor R3 by turning off the soft start switch SW as in the conventional example. As the reference voltage Vr increases, the output voltage Vout increases, the current I2 flowing through the choke coil L1 increases, the current detection voltage V1 also increases, and the change is output to the output detection voltage via the resistor R4 and the capacitor C3. The signal is superimposed on V3 and input to the error amplifier 23. As a result, even if the current flowing through the choke coil L1 suddenly increases, the output detection voltage V3 input to the error amplifier 23 also rises correspondingly, so that the error voltage V2 is almost maintained at a predetermined value, The output voltage Vout rises smoothly to the set value. That is, the rise of the output voltage Vout due to the soft start can be made smooth.

  The soft start circuit is not limited to the configuration of the resistor R3, the capacitor C2, and the soft start switch SW shown in FIG. 1, and various configurations already known are also applicable. A known switching control configuration can also be applied to a configuration including the oscillator 25 and the flip-flop 26 for controlling the switching element Q1 on and off.

It is explanatory drawing of Example 1 of this invention. It is operation | movement outline explanatory drawing of Example 1 of this invention. It is explanatory drawing of a prior art example. It is operation | movement outline explanatory drawing of a prior art example.

Explanation of symbols

21 Current-voltage converter (I / V)
22 Current detector 23 Error amplifier 24 Comparator 25 Oscillator (OSC)
26 flip-flop I1 load current I2 choke coil current E DC power supply Q1 switching element D1 diode L1 choke coil C1 to C3 capacitor L1 choke coil R1 to R4 resistance RL load SW soft start switch Vref reference power supply voltage Vin input voltage Vout output voltage V1 current Detection voltage V2 Error voltage V3 Output detection voltage

Claims (1)

An error amount obtained by inputting an output detection voltage and a reference voltage detected from the output voltage applied to the load to the error amplifier, and a current that is on / off controlled by the switching element are supplied to the load via the choke coil. The current detection voltage that is detected and converted into a voltage is compared by a comparator, and a current mode control circuit that controls the ON period of the switching element corresponding to the difference; In a switching power supply device including a soft start circuit for raising,
A switching power supply comprising a configuration in which a series circuit of a resistor and a capacitor is connected between a terminal of the comparator for inputting the current detection voltage and a terminal of the error amplifier for inputting the output detection voltage. apparatus.

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