JP2009177911A - Variable capacity circuit, error amplification circuit, and switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable capacity circuit which does not impart an adverse affect to the stability of a switching power supply, and an error amplification circuit and the switching power supply. <P>SOLUTION: The variable capacity circuit, the error amplification circuit and the switching power supply are connected to capacitors in parallel therewith before the capacitor CB is connected to the capacitor CA in parallel thereto after making both-side voltages of the capacitor CB equal to both-side voltages of the capacitor CA by using voltage followers VF1, VF2, and thereby the redistribution of an electric charge is not generated between the two capacitors. By this, even if a phase compensation capacity is switched, an error signal ErrorSignal is not generated, and the adverse affect is not imparted to the stability of the switching power supply. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an error amplifier used in a switching power supply and a control circuit thereof, and more particularly, an error capable of switching a capacitor for phase compensation so that the switching power supply can appropriately respond to both a continuous current mode and a discontinuous mode. The present invention relates to an amplifier circuit and a variable capacitance circuit that can be used therefor.

FIG. 6 is a block diagram showing a general configuration of the switching power supply. There are various types of switching power supplies such as an inverter and a converter. Here, a DC-DC converter is shown as an example.
The DC-DC converter shown in FIG. 6 is for voltage division for detecting a P-channel MOS transistor P1 as a switching element, an N-channel MOS transistor N1 as a synchronous rectifier, an inductor L, an output capacitor Co, and an output voltage Vout. Resistors R1, R2, a reference voltage source 10 that outputs a reference voltage Vref, an output voltage Vout that outputs an error signal by comparing the output voltage Vout divided by the resistors R1, R2 with the reference voltage Vref, and an output voltage Vout A resistor R3 connecting the divided voltage and the error amplifier circuit 20, an error signal Error Signal which is an output of the error amplifier circuit 20 is converted into a switching pulse having a duty ratio corresponding thereto, and a P-channel MOS according to the switching pulse Ton transistor P1 and N channel MOS ton run And a drive circuit 40 and the output terminal OUT to drive the static N1. A load 50 is connected to the output terminal. Vin is an input voltage to the DC-DC converter. With this configuration, the DC-DC converter controls on / off of the P-channel MOS transistor P1 and the N-channel MOS transistor N1 so that the divided voltage of the output voltage Vout by the resistors R1, R2 becomes equal to the reference voltage Vref. The N-channel MOS transistor N1 is complementarily turned on / off with respect to the P-channel MOS transistor P1 and may be replaced with a commutation diode.

Here, the error amplifying circuit 20 is connected to a phase compensation element composed of a resistor and a capacitor (capacitance). The phase compensation element adjusts the characteristics of the error amplifying circuit 20 so that the error amplifying circuit 20 functions to stably operate the DC-DC converter and to respond to disturbance at high speed.
FIG. 7 is a diagram illustrating an example in which the error amplifier circuit 20 is configured using the operational amplifier circuit 21 in the DC-DC converter of FIG. One end of the resistor R3 is connected to the inverting input terminal of the operational amplifier circuit 21, and the reference voltage Vref is input to the non-inverting input terminal. A first phase compensation element comprising a capacitor C1 is connected in parallel to both ends of a series circuit of resistors R1 and R3, and a series circuit of a resistor R4 and a capacitor C2 is provided between the output terminal and the inverting input terminal of the operational amplifier circuit 21. A second phase compensation element consisting of is connected. With this configuration, the operational amplifier circuit 21 outputs an error signal Error Signal that is a voltage obtained by amplifying the difference between the divided voltage by the resistors R1 and R2 and the reference voltage Vref. The duty conversion circuit 30 includes a PWM comparator 31 and an oscillation circuit 32. The signal Vosc output from the oscillation circuit 32 is a triangular wave or sawtooth wave having a constant period and a constant amplitude. The error signal Error Signal and the signal Vosc are input to the inverting input terminal and the non-inverting input terminal of the PWM comparator 31, respectively. The PWM comparator 31 compares the error signal Error Signal and the signal Vosc, and generates a switching pulse based on the magnitude relationship.

FIG. 8 is a diagram illustrating an example in which the error amplifying circuit 20 is configured using a transconductance amplifier (Gm amplifier) 22 in the DC-DC converter of FIG. One end of the resistor R3 is connected to the minus (−) input terminal of the transconductance amplifier 22, and the reference voltage Vref is inputted to the plus (+) input terminal. Between the output terminal of the transconductance amplifier 22 and the ground potential, a phase compensation element / current / voltage change element comprising a series circuit of a resistor R5 and a capacitor C3 is connected. With this configuration, the transconductance amplifier 22 outputs a current corresponding to the difference between the voltage divided by the resistors R1 and R2 and the reference voltage Vref, and the current is passed through a series circuit of the resistor R5 and the capacitor C3 to generate a voltage signal. An error signal Error Signal is generated. The configuration of the duty conversion circuit 30 is the same as that in FIG.
In the switching power supply, there are two states, a continuous current mode in which current always flows through the inductor L and a non-continuous current mode. (If the inductor L is replaced with a transformer winding, the flyback type and forward type converters are also the same.) The current continuous mode is a case where the load is heavy to a certain extent, and the load current is not always flowing through the inductor L. It is in a state that cannot be covered. The current discontinuous mode corresponds to a light load state, and when a current is constantly passed through the inductor L, a current exceeding the current consumed by the load is supplied. Therefore, a part of the switching cycle is supplied with the inductor current set to zero. This is a state in which current and current consumption are balanced. Since a partly non-linear state exists in the current discontinuous mode, the loop characteristics of the switching power supply differ between the current continuous mode and the current discontinuous mode. Therefore, in order to keep the loop characteristics of the switching power supply appropriate, it is necessary to switch the characteristics of the error amplifier circuit 20 between the current continuous mode and the current discontinuous mode. As described above, since it is the phase compensation element that adjusts the characteristics of the error amplifier circuit 20, it is necessary to switch the phase compensation element between the current continuous mode and the current discontinuous mode.

Regarding an error amplifying circuit of a switching power supply, Patent Document 1 discloses a step-up / step-down switching regulator in which a plurality of phase compensation circuits are provided and switched according to input / output conditions. The step-up / step-down switching regulator disclosed in Patent Document 1 switches the phase compensation circuit between when the input voltage is low and the step-up operation is performed, and when the input voltage is high and the step-down operation is performed. In Patent Document 1, only a resistor is shown as a specific phase compensation element. However, when an error amplifying circuit including a phase compensation element or a control circuit for a switching power supply is realized by a semiconductor integrated circuit, the resistor is included in the semiconductor integrated circuit. However, since it occupies a large area and it is difficult to improve the accuracy of characteristic values such as temperature characteristics, there is a demand for switching the phase compensation element by switching capacitors.
Regarding the switching of the capacitor, for example, in Patent Document 2, in a variable gain operational amplifier in which an operational amplifier and a capacitor (capacitance in Patent Document 2) are combined, the value of the phase compensation capacitor is reduced so that the value of the phase compensation capacitor decreases as the gain increases. What switches a capacitor is disclosed. Such switching of the capacitor is performed by turning the switch on and off as shown in FIG. In FIG. 9, CA is a capacitor that is always connected to the circuit, CB is a capacitor that is connected or disconnected in parallel with the capacitor CA depending on the situation, terminals N1 and N2 are nodes of the circuit to which the capacitor CA is connected, and SW1 and SW2 are A switch such as a transistor and a terminal CNT are terminals to which a signal for controlling on / off of the switch is input from the outside. The terminals N1 and N2 correspond to, for example, the connection point between the resistor R4 and the capacitor C2 and the output terminal of the operational amplifier circuit 21 in FIG. According to the control signal from the terminal CNT, SW1 and SW2 are simultaneously turned on or simultaneously turned off to switch the capacitor (that is, switch the capacitance value).
Japanese Patent Laying-Open No. 2005-110468 JP 2000-201038 A

Since the capacitor is switched during the operation of the switching power supply, the voltage across the capacitor CA is not zero. In the circuit as shown in FIG. 9, when the capacitance value is increased, that is, when the capacitor CB is connected in parallel with the capacitor CA, a part of the charge of the capacitor CA is distributed to the capacitor CB, and the voltage across the capacitor CA changes. Resulting in. This means that the error signal Error Signal, which is the output of the error amplifier circuit 20, changes suddenly. For this reason, there arises a problem that the stability of the switching power supply is adversely affected, for example, the output of the switching power supply fluctuates.
An object of the present invention is to solve the above problems and provide a variable capacitance circuit, an error amplifier circuit, and a switching power supply that do not adversely affect the stability of the switching power supply.

In order to solve the above-described problem, the invention according to claim 1 includes a first capacitor, a second capacitor that can be connected or disconnected in parallel with the first capacitor, and the first capacitor. And a charge / discharge circuit that charges and discharges the second capacitor so that the voltage is equal to the voltage across both ends of the first capacitor, and before the second capacitor is connected in parallel to the first capacitor The voltage across the second capacitor is made equal to the voltage across the first capacitor by the charge / discharge circuit, and then the second capacitor is connected in parallel to the first capacitor. .
According to a second aspect of the present invention, in the first aspect of the invention, the charge / discharge circuit includes two voltage followers in which both ends of the first capacitor are respectively connected to inputs.
The invention according to claim 3 is the invention according to claim 1, wherein the charge / discharge circuit includes a voltage follower in which one end of the first capacitor is connected to an input, the other end of the first capacitor, and the first It has the connection which connects the end of 2 capacitors.
According to a fourth aspect of the present invention, there is provided an error amplifying circuit comprising the variable capacitance circuit according to any one of the first to third aspects of the present invention.

  According to a fifth aspect of the present invention, there is provided a switching power supply comprising the error amplifier circuit according to the fourth aspect of the invention.

  In the variable capacitance circuit, the error amplification circuit, and the switching power supply according to the present invention, before connecting the second capacitor in parallel with the first capacitor, the voltage across the second capacitor is set to the first capacitor by the charge / discharge circuit. Since it is equal to the voltage at both ends and then connected in parallel to the first capacitor, charge redistribution does not occur between the first and second capacitors. Therefore, even if the phase compensation capacitor is switched, the error signal Error Signal does not change, and the stability of the switching power supply is not adversely affected.

The variable capacitance circuit, error amplifier circuit, and switching power supply of the present invention will be described below with reference to the drawings.
FIG. 1 shows a first embodiment of a variable capacitance circuit according to the present invention. Portions common to those in FIG. 9 are denoted by the same reference numerals, and detailed description thereof is omitted. The variable capacitance circuit shown in FIG. 1 is obtained by adding voltage followers VF1, VF2, switches SW3, SW4 and an inverter INV1 to the variable capacitance circuit of FIG. The operation of this variable capacitance circuit is as follows. First, in a state in which the capacitor CB is disconnected from the capacitor CA, the control signal from the terminal CNT (also referred to as CNT) is L (Low), so that the switches SW1 and SW2 are turned off (cut off). It has become. On the other hand, an inverted signal H (High) of the control signal CNT is given to the switches SW3 and SW4 by the inverter INV1 and is turned on (conductive) (the switches SW1 to SW4 are turned on when the control signal is H, and turned off when the control signal is L). The same shall apply hereinafter.) At this time, the voltage followers VF1 and VF2 make the voltage across the capacitor CB equal to the voltage across the capacitor CA. That is, the voltage followers VF1 and VF2 are charge / discharge circuits for the capacitor CB. Further, since this operation, that is, the operation of charging / discharging the capacitor CB following the voltage across the capacitor CA is performed via the voltage followers VF1 and VF2, the terminal of the capacitor CA is not affected. When the capacitor CB is connected to the capacitor CA, the control signal CNT is set to H. As a result, the switches SW3 and SW4 are turned off, the switches SW1 and SW2 are turned on, and the capacitor CB is directly connected to the capacitor CA. At this time, since capacitors having the same voltage at both ends are connected in parallel, charge redistribution does not occur, the voltage at both ends does not change, and the circuit to which the capacitors CA and CB are connected is not disturbed. When the capacitor CB is disconnected again, the control signal is set to L. As shown in FIG. 2, the voltage followers VF1 and VF2 are impedance conversion elements in which the input voltage IN and the output voltage OUT are equal using the operational amplifier circuit OPA. The present invention uses this impedance conversion characteristic. Is. The configuration of the voltage follower of the embodiment shown below is the same.

If the output impedance of the terminal N1 or N2 in FIG. 1 is sufficiently low, that is, if the potential of the terminal does not fluctuate even when charge is supplied from the terminal to the capacitor CB, the voltage follower VF1 and the switches SW1, SW3, Alternatively, the voltage follower VF2 and the switches SW2 and SW4 may be omitted, and the terminal N1 or N2 and the terminal of the capacitor CB may be directly connected. In this case, the connection connecting the terminal N1 or N2 and the terminal of the capacitor CB becomes a part of the charge / discharge circuit for the capacitor CB. Example 2 with one voltage follower is shown in FIG. FIG. 3 shows a case where the output impedance of the terminal N2 is low, the voltage follower VF2 and the switches SW2 and SW4 of FIG. 1 are omitted, and the terminal N2 and the terminal of the capacitor CB are directly connected. In this case, the terminal N2 is constantly supplying or extracting charges for charging and discharging the capacitors CA and CB with respect to one end of the capacitors CA and CB.
FIG. 4 shows an embodiment of an error amplifier circuit using the operational amplifier circuit according to the present invention. This error amplifier circuit is applied in place of the operational amplifier circuit 20 in the DC-DC converter of FIGS. 6 and 7, and constitutes the switching power supply of the present invention. The error amplifying circuit of FIG. 4 will be described below, but the same reference numerals are given to portions common to FIG. 7, and detailed description thereof will be omitted.
The error amplifying circuit of FIG. 4 is configured such that capacitors C11 and C21 are respectively connected to or disconnected from the capacitors C1 and C2 constituting the phase compensation element in the arithmetic circuit 20 of FIG. 7 so that the capacitance value of the phase compensation element can be switched. Is. That is, the error amplifying circuit of FIG. 4 has capacitors C11, C21, switches SW11, SW31, SW12, SW32, voltage followers VF11, VF12 and inverters INV11, INV12 added to the arithmetic circuit 20 of FIG. Capacitance values are switched by control signals CONT1 and CONT2 respectively input to INV12. The switch SW31 is controlled by the control signal CONT1, and the switch SW11 is controlled to be turned on / off by the inverted signal of the control signal CONT1 by the inverter INV11. When the capacitor C11 is connected to the capacitor C1 in parallel, the control signal CONT1 is set to L SW11 is turned on and switch SW31 is turned off. When disconnecting the capacitor C11 from the capacitor C1, the control signal CONT1 is set to H, the switch SW11 is turned off, and the switch SW31 is turned on. When the control signal CONT1 is H, the capacitor C11 is supplied with electric charge from the output Vout of the DC-DC converter and the voltage follower VF11, and the voltage across the capacitor C11 is always equal to the voltage across the capacitor C1.

The switch SW32 is controlled by the control signal CONT2, and the switch SW12 is controlled to be turned on / off by the inverted signal of the control signal CONT2 by the inverter INV12. When the capacitor C21 is connected in parallel to the capacitor C2, the control signal CONT2 is set to L SW12 is turned on and switch SW32 is turned off. When disconnecting the capacitor C21 from the capacitor C2, the control signal CONT2 is set to H, the switch SW12 is turned off, and the switch SW32 is turned on. When the control signal CONT2 is H, the capacitor C12 is supplied with electric charges from the output of the operational amplifier circuit 21 and the voltage follower VF12, and the voltage across the capacitor C12 is always equal to the voltage across the capacitor C2. Here, since the output impedance of the output Vout of the DC-DC converter and the output of the operational amplifier circuit 21 is low, a voltage follower is not provided between these and the capacitors C11 and C21 (FIG. 3). Constitution). Of course, a voltage follower may be provided in the configuration of FIG. The control signals CONT1 and CONT2 may use the same signal.
FIG. 5 shows an embodiment of another error amplifier circuit using the operational amplifier circuit according to the present invention. This error amplifier circuit is applied in place of the operational amplifier circuit 20 in the DC-DC converter of FIGS. 6 and 8, and constitutes the switching power supply of the present invention. The error amplifier circuit of FIG. 5 will be described below, but the same reference numerals are given to portions common to FIG. 8, and detailed description thereof will be omitted.

  The error amplifying circuit of FIG. 5 is configured such that the capacitance value of the phase compensation element can be switched by connecting or disconnecting the capacitor C31 to the capacitor C3 constituting the phase compensation element in the arithmetic circuit 20 of FIG. That is, the error amplifying circuit of FIG. 5 has a capacitor C31, switches SW13 and SW33, a voltage follower VF13, and an inverter INV13 added to the arithmetic circuit 20 of FIG. Switching is in progress. The switch SW13 is controlled by the control signal CONT3, and the switch SW33 is controlled to be turned on and off by the inverted signal of the control signal CONT3 by the inverter INV13. SW13 is turned on and switch SW33 is turned off. When disconnecting the capacitor C31 from the capacitor C3, the control signal CONT3 is set to L, the switch SW13 is turned off, and the switch SW33 is turned on. When the control signal CONT3 is L, the capacitor C31 is supplied with electric charges from the ground potential and the voltage follower VF13, and the voltage across the capacitor C31 is always equal to the voltage across the capacitor C1. Here, since the impedance of the ground potential is low, a voltage follower is not provided between the ground potential and the capacitor C32 but is directly connected (configuration in FIG. 3). Of course, a voltage follower may be provided in the configuration of FIG.

  In FIG. 5, IN connected to the minus (−) input terminal of the transconductance amplifier (Gm amplifier) 22 is a portion corresponding to one end of the resistor R3 shown in FIG.

1 is a first embodiment of a variable capacitance circuit according to the present invention. It is a figure which shows the structure of a voltage follower. It is a 2nd Example which concerns on the variable capacitance circuit which concerns on this invention. 1 is a first embodiment of an error amplifier circuit using an operational amplifier circuit according to the present invention. It is a 2nd Example of the error amplifier circuit using the operational amplifier circuit which concerns on this invention. It is a block diagram of the DC-DC converter shown as an example of the general structure of a switching power supply. FIG. 7 is a diagram illustrating an example in which an error amplifier circuit 20 is configured using an operational amplifier circuit 21 in the DC-DC converter of FIG. 6. FIG. 7 is a diagram illustrating an example in which an error amplifier circuit 20 is configured using a transconductance amplifier (Gm amplifier) in the DC-DC converter of FIG. 6. It is a figure which shows the structure of the conventional variable capacitance circuit.

Explanation of symbols

10 reference voltage source 20 error amplifier circuit 21 operational amplifier circuit 22 transconductance amplifier (Gm amplifier)
30 Duty conversion circuit 30
31 PWM comparator 32 Oscillation circuit 40 Drive circuit CA, CB, C1, C11, C2, C21, C3, C31 Capacitor Co Output capacitor
Error Signal Output from error amplifier circuit 20 (error signal)
INV1, INV2, INV3 Inverter L Inductance N1 N-channel MOS transistor P1 P-channel MOS transistor R1, R2, R3, R4, R5 Resistor SW1, SW12, SW12, SW2, SW3, SW31, SW32, SW33, SW4 Switch VF1, VF11, VF12, VF13, VF3 Voltage follower Vout Output voltage of switching power supply

Claims (5)

The first capacitor, the second capacitor that can be connected or disconnected in parallel with the first capacitor, and the second capacitor are charged and discharged so that the voltage is equal to the voltage across the first capacitor. A charge / discharge circuit,
Before connecting the second capacitor in parallel with the first capacitor, the voltage across the second capacitor is made equal to the voltage across the first capacitor by the charge / discharge circuit, and then the second capacitor The capacitor is connected in parallel with the first capacitor. 2. The variable capacitance circuit according to claim 1, wherein the charge / discharge circuit includes two voltage followers in which both ends of the first capacitor are respectively connected to inputs. The charge / discharge circuit includes a voltage follower in which one end of the first capacitor is connected to an input, and a connection connecting the other end of the first capacitor and one end of the second capacitor. Item 2. The variable capacitance circuit according to Item 1. An error amplifier circuit comprising the variable capacitance circuit according to claim 1. A switching power supply comprising the error amplifier circuit according to claim 4.

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