JP5569954B2 - Frequency spreading circuit and frequency spreading method used for power supply - Google Patents

Description

  The present invention relates to a frequency spreading circuit and a frequency spreading method for spreading a switching frequency supplied to a switching circuit in a power supply.

  Switching power supplies are widely used as DC-DC converters as power supply circuits for supplying direct current power to electronic circuits. A switching power source generates not only conduction noise but also radiation noise by switching. There are various standards for EMI (Electric Magnetic Interference), and it is required to keep EMI within the range of those standards.

  In order to remove noise generated in the switching power supply, for example, a power supply line filter composed of a capacitor and an inductor is used. However, it is difficult to take sufficient EMI countermeasures only with the power supply line filter.

  When the frequency of the signal supplied to the switching circuit is constant, the generated noise has a peak at a specific frequency. That is, noise with a specific frequency appears strongly.

  Therefore, in order to reduce the peak noise of a specific frequency, a frequency spreading technique that randomly changes the frequency of the signal supplied to the switching circuit is used.

  Frequency spread (hereinafter spread is also referred to as dispersion) is an effective means of suppressing noise peaks, but it is not sufficient, and high performance is not required, but it should be used in combination with a power line filter. There are many. However, the filter characteristics of the power supply line filter are not flat, and when the frequency spread technique and the power supply line filter are used in combination, a peak may remain at the low band end or high band end of the diffusion. That is, there is a problem that it is necessary to correct the remaining high noise peak due to the characteristics of the filter inserted in the power supply line.

  Further, although a method of performing frequency spreading using a DSP (Digital Signal Processor) has been implemented, a control circuit using the DSP becomes expensive and is not suitable for application to a low-power power source.

  FIG. 10 is a block diagram showing the configuration of the switching power supply described in Patent Document 1. In FIG. The switching power supply shown in FIG. 10 includes an input terminal 101, an input circuit 102, a switch circuit 103, a transformer 104, a rectifying / smoothing circuit 105, and an output terminal 106, and supplies a constant DC power supply voltage to a load 107. The switch circuit 103 is composed of a switching element such as a MOS-FET and is connected to the primary winding of the transformer 104. The switch circuit 103 and the transformer 104 constitute an inverter. The rectifier circuit 105 rectifies and smoothes the output of the secondary winding of the transformer 104.

  The switching control circuit 109 includes a triangular wave oscillator 110, a comparator 111, and a driver 112. The voltage obtained by dividing the output voltage for the load 107 by the resistors R31 and R41 is input to the inverting input terminal of the comparator 111. The comparator 111 outputs a PWM signal whose pulse width changes by comparing the voltage input to the inverting input terminal with the triangular wave signal input to the non-inverting input terminal. The PWM signal is input to the switching element of the switch circuit 103 via the driver 112. The switching element is turned on / off by the PWM signal, and the output voltage is stabilized to a constant voltage.

The switching control circuit 109 has an oscillation capacitor terminal 113, an on time control terminal 114, and an off time control terminal 115. The oscillation capacitor terminal 113 is connected a capacitor C11 whose other end is grounded, the ON time control terminal 114 and the other end is connected to the on-time setting resistor R _on which is grounded and the other end to the off-time control terminal 115 Is connected to an off-time setting resistor _Roff . The inclination of the upward slope of the oscillation waveform (triangular wave signal) of the triangular wave oscillator 110 is determined by the product of the on-time setting resistance R _on and the capacitor C11, gradient of the descending slope of the triangular wave signal, the off-time setting resistor R _off and capacitor It is determined by the product of C11. In other words, it determines the charging current of the capacitor C11 at the outflow current _{I on} the on-time setting resistor _{R on,} a discharge current of the capacitor C11 is determined by the drain current _{I off} of off-time setting resistor _{R off.}

  A first weight resistor R11 for changing the frequency and duty is connected between the output of the spread signal generation circuit 116 and the on-time control terminal 114, and the output of the spread signal generation circuit 116 and the off-time control terminal 115 are connected to each other. A second weight resistor R21 for changing the frequency and duty is connected between them.

A ramp signal that periodically changes via each of the first weight resistor R11 and the second weight resistor R21 is applied to the on-time control terminal 114 and the off-time control terminal 115 as a spread signal. The current I _on and the current I _off flowing out from the on-time control terminal 114 and the off-time control terminal 115 change according to the voltage level of the spread signal. As the current I _on and the current I _off change, the frequency and duty of the triangular wave signal output from the triangular wave oscillator 110 change, and the switching signal of the switch circuit 103 is spread on the frequency axis.

The frequency spread width ± Δf of the switching signal is the ratio (R11 / R _on ) of the first weighting resistor R11 for changing the frequency and duty and the on-time setting resistor R _on (R11 / R _on ) or the second for changing the frequency and duty. It is determined by the ratio (R21 / R _off ) between the weighting resistor R21 and the off-time setting resistor R _off . Further, the change rate of the duty of the switching signal is determined by the ratio (R11 / R21) of the first weighting resistor R11 and the second weighting resistor R21.

  As described above, the basic frequency of switching is dispersed in the frequency domain by changing the frequency of the triangular wave signal within a predetermined range. Note that harmonic components of the fundamental frequency are also dispersed in the frequency domain. By distributing the fundamental frequency of switching, noise peaks are made inconspicuous.

Japanese Patent Laying-Open No. 2005-160128 (paragraphs 0013-0021, FIG. 1)

  By using the circuit configuration described in Patent Document 1, an expensive component such as a DSP is not used, and only a relatively inexpensive component such as a resistor, a capacitor, a diode, and an operational amplifier (hereinafter referred to as an operational amplifier) is used. A frequency spread circuit having a high degree of freedom in changing the spread amount is realized.

  However, when the frequency spreading circuit described in Patent Document 1 is used in combination with a power supply line filter, there is a possibility that the noise spectrum is inclined and a high noise peak is generated. That is, there is a possibility that noise of a specific frequency appears.

  Accordingly, an object of the present invention is to provide a frequency spreading circuit and a frequency spreading method capable of preventing a noise spectrum from being inclined even when a power line filter is used.

The frequency spreading circuit used in the power source according to the present invention generates a triangular wave using the first resistor (R1) that charges and discharges the first capacitor (C1), and generates a triangular wave voltage and an output voltage of the power source. Is a frequency spreading circuit that spreads the frequency of the switching signal generated by comparing the two, and has an output terminal that supplies current to the first resistor (R1) via the second capacitor (C3), discharge path and is provided separately charging path and a second capacitor to a second capacitor (C3) (C3), the charging path between the output and the second capacitor is an oscillation unit an operational amplifier (C3) The second resistor (R3) is provided in the route, the discharge route is a route between the second capacitor (C3) and the operational amplifier, and the third resistor (R4) is provided in the route. ) Source noise filter is applied to a power supply provided, as a second resistor (R3) and a third resistor (R4), characterized Rukoto resistance were selected for resistance to cancel the peak noise due to power supply noise filter .

The frequency spreading method used for the power supply according to the present invention includes a triangular wave generated by using a first resistor (R1) that charges and discharges the first capacitor (C1) and an output voltage of the power supply. Is a frequency spreading method for spreading the frequency of the switching signal generated by comparing the currents to each other by supplying a current to the first resistor (R1) via the second capacitor (C3), and the second capacitor ( A charging path to C3) and a discharging path of the second capacitor (C3) are set separately, and a charging path that is a path between the output of the operational amplifier that is the oscillation unit and the second capacitor (C3) is set. A second resistor (R3) is provided, a third resistor (R4) is provided in the discharge path, which is a path between the second capacitor (C3) and the operational amplifier, and the second resistor (R3) and the third resistor As the resistance (R4) of Characterized by selecting the resistance of the resistance value for canceling the peak noise due to power supply noise filter provided in the source.

  According to the present invention, even if a power supply line filter is used, it is possible to prevent the noise spectrum from being inclined.

It is a block diagram which shows an example of the frequency spread circuit by this invention with power supply control IC. It is a circuit diagram which shows the structural example of an oscillation circuit. It is a wave form diagram which shows the example of waveforms, such as an output signal of a power supply control IC when the frequency spreading circuit is not provided. It is a wave form diagram which shows the example of waveforms, such as an output signal of a power supply control IC in case the frequency spreading circuit is provided. It is a wave form diagram which shows the peak value of noise. It is a wave form diagram which shows a flat noise spectrum. It is a wave form diagram which shows an example of the both-ends voltage of a capacitor | condenser, and the deviation of frequency spreading. It is a wave form diagram which shows an example of the both-ends voltage of a capacitor | condenser, and the deviation of frequency spreading. It is a block diagram which shows the principal part of the frequency spreading circuit by this invention. It is a block diagram which shows the structure of the switching power supply described in patent document 1. FIG.

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

  FIG. 1 is a block diagram showing an example of a frequency spreading circuit according to the present invention together with a power supply control IC. The frequency spreading circuit includes an oscillation circuit 10. The power supply control IC 20 illustrated in FIG. 1 includes an output terminal (Output terminal), a feedback terminal FB (hereinafter referred to as a terminal FB), a frequency setting external capacitor connection terminal CT (hereinafter referred to as a terminal CT), and a frequency setting. An external resistance connection terminal RT (hereinafter referred to as a terminal RT) is provided.

  For example, a PWM signal output from the output terminal is input to a switching circuit in a switching power supply (not shown). As an example, the configuration of the switching power supply includes a switch circuit, a transformer, and a rectifying / smoothing circuit as shown in FIG.

  A voltage obtained by dividing the output voltage of a switching power supply (not shown) by a resistor is input to the terminal FB. For example, a capacitor C1 whose other end is grounded is connected to the terminal CT, and a resistor R1 whose other end is grounded is connected to the terminal RT, for example.

  FIG. 2 is a circuit diagram illustrating a configuration example of the oscillation circuit 10. As shown in FIG. 2, in the oscillation circuit 10, the operational amplifier OP, the resistors R6 and R7 connected to the non-inverting input terminal of the operational amplifier OP, and the resistor connected between the output terminal and the non-inverting input terminal of the operational amplifier OP. R5 constitutes an oscillation unit.

  A resistor R3 and a resistor R4 are connected in parallel to the output terminal of the operational amplifier OP. Between the resistor R3 and, for example, the capacitor C3 whose other end is grounded, a diode D1 that passes a current flowing out from the output terminal of the operational amplifier OP is connected. Between the resistor R4 and the capacitor C3, the operational amplifier OP side A diode D2 is connected to pass the current flowing into the.

  Further, the current from the capacitor C3 flows to the terminal RT side via the capacitor C2 and the resistor R2.

  The power supply control IC 20 illustrated in FIG. 1 uses a repetitive voltage waveform of charging / discharging of the capacitor C1 in order to determine the operating frequency. Specifically, the operating frequency can be adjusted by the capacitor C1 and the resistor R1 that limits the charge / discharge current. That is, a triangular wave having a desired frequency is generated by the capacitor C1 and the resistor R1.

  The power supply control IC 20 has a built-in comparator. As shown in the waveform diagram of FIG. 3, the comparator compares the voltage input to the terminal FB with the triangular wave voltage to generate an output signal (PWM signal) output from the output terminal, and outputs the generated output signal. Supply to the switching circuit from the output terminal. FIG. 3 is a waveform diagram showing an example of the waveform of the output signal of the power supply control IC 20 when no frequency spreading circuit is provided.

  Next, the operation of the frequency spreading circuit (in the example shown in FIGS. 1 and 2, the oscillation circuit 10) will be described.

When the resistance value of the resistor R1 is increased to decrease the value of the current _IRT flowing through the terminal RT, the capacitor C1 and the charge / discharge speed are reduced. As a result, the switching frequency (frequency of the PWM signal) decreases. Further, when the resistance value of the resistor R1 is decreased and the value of the current _IRT is increased, the capacitor C1 and the charge / discharge speed are increased. As a result, the switching frequency increases.

In this embodiment, as the current flows _{I EXT} from the oscillation circuit 10 with the resistor R1, by adjusting the current _{I EXT,} to adjust the frequency of the output signal of the power supply control IC 20.

FIG. 4 is a waveform diagram showing a waveform example of an output signal of the power supply control IC 20 when a frequency spread circuit is provided. As shown in FIG. 4, when the output voltage V _OSC of the oscillation circuit 10 changes and the current I _EXT changes, the current I _RT ′ flowing through the resistor R1 changes and the switching frequency changes.

That is, the switching frequency can be changed by changing the current _IEXT .

When the slope of I _EXT is close to a straight line, that is, when the increase / decrease of I _EXT is small, the fundamental frequency f _{SW of} switching spreads almost uniformly within a predetermined frequency range (variable band). That is, when the frequency spread is not performed, the noise peak is large as shown in FIG. 5, whereas when the frequency spread is performed, the noise spectrum is flat as shown in FIG. Become. In FIGS. 5 and 6, the horizontal axis indicates the frequency, and the vertical axis schematically indicates the noise voltage. Also, the noise peak value in FIG. 6 is smaller than the noise peak value in FIG.

However, if the power supply line filter is used only by evenly spreading the switching basic frequency f _SW in the variable band, the noise spectrum is inclined due to the characteristics of the power supply line filter, and a high noise peak is generated. there is a possibility. In other words, noise at a specific frequency may become significant.

  In this embodiment, even when a power line filter and a frequency spread circuit are combined, a frequency spread circuit that makes it possible to flatten the noise spectrum is realized.

  In the embodiment illustrated in FIG. 2, it is possible to adjust the noise spectrum by giving a bias to the diffusion amount in the variable band.

  Specifically, the direction in which the current flows is regulated so that the resistor R3 that limits the charging current to the capacitor C3 and the resistor R4 that limits the discharge current from the capacitor C3 can be used separately. Diodes D1 and D2 are installed. That is, the charging path of the capacitor C3 and the discharging path of the capacitor C3 are separated. Since the charging path and the discharging path of the capacitor C3 exist independently, by appropriately setting each of the resistance value of the resistor R3 and the resistance value of the resistor R4, that is, a resistor suitable for obtaining a desired noise spectrum. By selecting the resistors R3 and R4 having values, the oscillation output waveform of the oscillation circuit 10 can be set to a desired nonlinear waveform.

  Hereinafter, a specific example of the uneven diffusion amount will be described. FIGS. 7 and 8 are waveform diagrams showing an example of the voltage across the capacitor C3 and the frequency spread bias.

  As an example, let us consider a case where the resistance value of the resistor R3 is set to 0Ω and a resistor having a high resistance is selected as the resistor R4. In this case, since the charging speed is high and the discharging speed is slow, the voltage across the capacitor C3 is as shown in FIG. In the example shown in FIG. 7A, the period during which the voltage across the capacitor C3 is low is long, so that the switching frequency increases as shown in FIG. 7B. As a result, noise increases in a high frequency region.

  When a noise peak occurs at a low frequency due to the power supply noise filter, the overall noise spectrum by the power supply noise filter and the frequency spreading circuit is flattened by biasing the switching frequency as shown in FIG. Can be.

  Further, consider a case where the resistance value of the resistor R3 is set to a high resistance value, and a resistance value of 0Ω is selected as the resistor R4. In that case, since the charging speed is slow and the discharging speed is fast, the voltage across the capacitor C3 is as shown in FIG. In the example shown in FIG. 8A, since the period during which the voltage across the capacitor C3 is high is long, the switching frequency decreases as shown in FIG. 8B. As a result, noise increases in a low frequency region.

  When a noise peak occurs at a high frequency due to the power supply noise filter, the overall noise spectrum by the power supply noise filter and the frequency spreading circuit is flattened by biasing the switching frequency as shown in FIG. Can be.

  In order to simplify the explanation, in the examples shown in FIGS. 7 and 8, extreme values are selected as the resistance values of the resistors R3 and R4, but by appropriately selecting the resistance values of the resistors R3 and R4. The method of biasing the switching frequency can be set relatively freely. That is, by selecting the resistance values of the resistors R3 and R4, an appropriate spectrum in the frequency spreading circuit can be obtained in order to cancel out the noise peak caused by the power supply noise filter.

  As described above, in this embodiment, the total noise spectrum can be flattened even when the power supply line filter is used.

  In the above embodiment, the charging path of the capacitor C3 and the discharging path of the capacitor C3 are separated by the diodes D1 and D2, but a switching element such as a transistor is provided between the resistor R3 and the resistor R4 and the capacitor C3. And a charging path and a discharging path may be switched by a switching element.

  FIG. 9 is a block diagram showing the main part of the frequency spreading circuit according to the present invention. As shown in FIG. 9, the frequency spread circuit is generated by comparing the generated triangular wave voltage with the output voltage of the power source by generating a triangular wave using the resistor (R1) that charges and discharges the capacitor (C1). A frequency spreading circuit for spreading the frequency of the switching signal to be transmitted, having an output terminal 3 for supplying a current to the resistor (R1) via the capacitor (C3), and a charging path 1 and a capacitor to the capacitor (C3) The discharge path 2 of (C3) is provided separately.

  In the above embodiment, the following frequency spreading circuit is also disclosed.

(1) The charging path is a path between the output of the operational amplifier, which is an oscillation unit, and the capacitor (C3). A resistor (R3) is provided in the path, and the discharging path is a path between the capacitor (C3) and the operational amplifier. A frequency spread circuit in which a resistor (R4) is provided in the route.

(2) A frequency spreading circuit in which a diode (D1) that regulates the direction of current flow is provided in the charging path, and a diode (D2) that regulates the direction of current flow is provided in the discharge path.

(3) A frequency spreading circuit in which a switching element for switching a charging path and a discharging path is provided between the resistor (R3) and the resistor (R4) and the capacitor (C3).
(4) A frequency spreading circuit applied to a power supply provided with a power supply noise filter.

  With the configuration of the frequency spread circuit of (1) to (3) above, a frequency spread circuit capable of flattening the noise spectrum is obtained simply by using relatively inexpensive parts such as resistors, capacitors, operational amplifiers and diodes. be able to. Further, as described in (4) above, the noise spectrum can be flattened even when applied to a power supply provided with a power supply noise filter.

  The present invention can be widely applied to power supplies to which EMI countermeasures are applied.

1 Discharge path 2 Charge path 3 Output terminal 10 Oscillation circuit 20 Power control IC
FB feedback terminal FBCT
CT External frequency connection terminal for frequency setting RT External resistance connection terminal for frequency setting R1, R2, R3, R4, R5, R6, R7 Resistor C1, C2, C3 Capacitor D1, D2 Diode OP Operational Amplifier (Op Amp)

Claims (4)

The frequency of the switching signal generated by comparing the voltage of the generated triangular wave with the output voltage of the power supply using the first resistor (R1) that charges and discharges the first capacitor (C1). A frequency spreading circuit for spreading
An output terminal for supplying current to the first resistor (R1) via a second capacitor (C3);
A discharge path of the said second charging path of the capacitor (C3) the second capacitor (C3) is provided separately,
The charging path is a path between an output of an operational amplifier that is an oscillation unit and the second capacitor (C3), and a second resistor (R3) is provided in the path, and the discharging path is the second path. Of the capacitor (C3) and the operational amplifier, a third resistor (R4) is provided in the path,
Applied to power supply with power supply noise filter,
A frequency spread circuit used for a power supply, wherein a resistance having a resistance value that cancels peak noise caused by the power supply noise filter is selected as the second resistance (R3) and the third resistance (R4) . First diode (D1) is provided in a charging path for regulating the direction of current flow, the power of the second diode (D2) is according to claim 1, characterized in that arranged in the discharge path to regulate the direction of current flow Frequency spreading circuit used. During the second resistor (R3) and a third resistor and (R4) and second capacitor (C3), the power supply of claim 1, wherein the switching device to switch between a charging path discharge path is provided Frequency spreading circuit used. The frequency of the switching signal generated by comparing the voltage of the generated triangular wave with the output voltage of the power supply using the first resistor (R1) that charges and discharges the first capacitor (C1). A frequency spreading method for spreading
Supplying a current to the first resistor (R1) via a second capacitor (C3);
A discharge path of the said second charging path of the capacitor (C3) the second capacitor (C3) separately set,
A second resistor (R3) is provided in a charging path that is a path between the output of the operational amplifier that is an oscillation unit and the second capacitor (C3), and the second capacitor (C3), the operational amplifier, A third resistor (R4) is provided in the discharge path that is the path between
A frequency used for a power supply, wherein a resistance having a resistance value that cancels peak noise caused by a power supply noise filter provided in the power supply is selected as the second resistor (R3) and the third resistor (R4). Diffusion method.

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