JP2006050760A - Switching power supply circuit and control method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress power consumption under light load without producing harsh noise and accomplish cost reduction. <P>SOLUTION: The voltage obtained by rectifying and smoothing input alternating-current voltage is switched by a switching transistor Q1, and input to the primary winding Np of a voltage conversion transformer 3. The output alternating-current voltage obtained at the secondary winding Ns is rectified and smoothed to output direct-current voltage. Switching of the switching transistor Q1 is controlled by voltage generated at auxiliary winding Nb. When the frequency of a switching signal generated at the auxiliary winding Nb exceeds a predetermined value, the switching transistor Q1 is caused to perform intermittent operation so as to interrupt switching only for periods longer than the period of the frequency in audio frequency band. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an RCC (ringing choke converter) type switching power supply circuit and a control method therefor.

  A typical RCC switching power supply circuit is shown in FIG. In FIG. 3, 1 is a rectifier circuit for rectifying an input AC voltage, 2 is a switching control circuit, C1 and C2 are capacitors, R1 is a starting resistor, Q1 is a switching transistor, and D1 is a rectifier diode. Reference numeral 3 denotes a voltage conversion transformer, Np is a primary side winding, Ns is a secondary side winding, and Nb is an auxiliary winding.

  The frequency of the RCC switching power supply circuit changes depending on the current flowing through the secondary winding Ns, that is, the load on the secondary side, and the frequency increases as the load becomes lighter. Specifically, since either the primary current flowing through the primary winding Np or the secondary current flowing through the secondary winding Ns always operates, the primary current and the secondary current are Although it seems to flow continuously, when the load is light, the power supplied to the load is small, so the conduction time of the switching transistor Q1 on the primary side is shortened and the switching frequency is increased.

  When this switching frequency increases, the loss of the voltage conversion transformer 3 and the switching transistor Q1 increases, and the power consumption increases.

  Therefore, in order to solve such a problem, as shown in FIG. 4, a load state is detected by a load detection resistor R2 and a load detection circuit 7 provided on the secondary side, and a photodiode D2 and a phototransistor Q2 are formed. There has been proposed a method of transmitting the load detection signal to a pulse generation circuit 8 provided on the primary side by an insulating photocoupler (see, for example, Patent Document 1).

In this method, when the load is light, the switching control circuit 2 is controlled by the pulse generation circuit 8 that inputs the load state detection signal, and switching is performed intermittently, thereby reducing power consumption.
JP 2001-218461 A

  However, in the configuration shown in FIG. 4, it is necessary to provide the load detection resistor R2 and the load detection circuit 7 on the secondary side, the pulse generation circuit 8 on the primary side, and the photocoupler by the photodiode D2 and the phototransistor Q2. There was a disadvantage in cost.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a switching power supply circuit and a control method therefor that can reduce power consumption without generating a harsh sound at a light load and that is advantageous in terms of cost. .

  A switching power supply circuit according to a first aspect of the present invention includes a first rectifying / smoothing circuit for rectifying and smoothing an input AC voltage, and switching the output voltage of the first rectifying / smoothing circuit to a primary winding of a voltage conversion transformer. A switching element to be input; a second rectifying / smoothing circuit that rectifies and smoothes the output AC voltage obtained in the secondary winding of the voltage conversion transformer; and the voltage generated in the auxiliary winding of the voltage conversion transformer. A switching power supply circuit comprising a switching control circuit for controlling switching of a switching element, a switching frequency detection circuit for detecting a frequency of a switching voltage generated in the auxiliary winding, and a switching frequency detected by the switching frequency detection circuit Generates a standby pulse with a period longer than the frequency period of the audible frequency band Comprising a pulse generating circuit for controlling said switching control circuit Te, the switching control circuit for a period of time in the standby pulse is inputted and controls the switching element to off.

  According to a second aspect of the present invention, in the switching power supply circuit according to the first aspect, an activation resistor that applies the output voltage of the first rectifying and smoothing circuit as an activation voltage is connected to the gate of the switching element, and the pulse A switch is provided that opens the starting resistor when the generating circuit generates the standby pulse.

According to a third aspect of the present invention, there is provided a control method for a switching power supply circuit, wherein a voltage obtained by rectifying and smoothing an input AC voltage is switched by a switching element and input to a primary winding of a voltage conversion transformer. In the control method of the switching power supply circuit, the output AC voltage obtained on the line is rectified and smoothed to output a DC voltage, and the switching of the switching element is controlled by the voltage generated in the auxiliary winding of the voltage conversion transistor.
When the frequency of the switching voltage generated in the auxiliary winding of the voltage conversion transformer exceeds a predetermined value, the switching element is intermittently cut off for a period longer than the frequency period of the audible frequency band. .

  According to the present invention, since the intermittent operation is performed at a frequency lower than the audible frequency band, it is possible to reduce the power consumption at the time of light load without generating a harsh sound. In addition, since the load state is detected on the primary side of the voltage conversion transformer, there is no need for a load detection circuit on the secondary side or a photocoupler for transmitting the load state, which is advantageous in terms of cost.

  The switching power supply circuit of the RCC system has a high switching frequency at light load. In the present invention, however, the intermittent operation is performed at a frequency lower than the audible frequency band at this time, thereby reducing the power consumption without causing annoying noise. Plan. The intermittent operation is performed only on the primary side of the voltage conversion transformer. This will be described in detail below.

  FIG. 1 is a functional block diagram showing a switching power supply circuit according to an embodiment of the present invention. In FIG. 1, 1 is a rectifier circuit for rectifying an input AC voltage, 2 is a switching control circuit, C1 and C2 are capacitors, R1 is a starting resistor, Q1 is a switching transistor, and D1 is a rectifier diode. Reference numeral 3 denotes a voltage conversion transformer, Np is a primary side winding, Ns is a secondary side winding, and Nb is an auxiliary winding. The rectifier circuit 1 and the capacitor C1 constitute a first rectification / smoothing circuit, and the diode D1 and the capacitor C2 constitute a second rectification / smoothing circuit. 4 is a half-wave rectifier circuit for half-wave rectifying the voltage generated in the auxiliary winding Nb, 5 is a frequency detection circuit for detecting the frequency of the pulse obtained by rectification in the half-wave rectifier circuit 4, and 6 is this frequency detection circuit. This is a pulse generation circuit that controls the switching control circuit 2 by generating a standby pulse when the frequency detected by the circuit 5 exceeds a predetermined value.

  The input AC voltage is rectified and smoothed by the first rectifying and smoothing circuit and supplied to the starting resistor R1. When the gate of the switching transistor Q1 is biased by the starting resistor R1 and the transistor Q1 becomes conductive, a current flows in the primary winding Np of the voltage conversion transformer 3, and a voltage is generated in the auxiliary winding Nb at the same time. A voltage is also generated in the secondary winding Ns, but no current flows on the output side because the diode D1 is in the reverse direction.

  When a voltage is generated in the auxiliary winding Nb, the switching control circuit 2 operates to shut off the transistor Q1. When the transistor Q1 is cut off, the voltage conversion transformer 3 causes a current to flow from the winding start (black circle side) to the winding end in the secondary winding Ns so as to maintain the energy accumulated in the primary winding Np. D1 conducts, and the current is smoothed by the capacitor C2 to become the output voltage Vo. When all the energy accumulated in the voltage conversion transformer 3 is transferred to the secondary side, the current flowing through the diode D1 becomes zero, and the diode D1 is cut off. After that, the transistor Q1 is again biased and rapidly becomes conductive, and thereafter, the switching operation is performed by repeating these operations.

The current Ip flowing through the primary winding Np is expressed as follows: the voltage applied to the primary winding Np is Vi, the inductance of the primary winding Np is Lp, and the conduction time of the switching transistor Q1 is Ton.
Ip = Vi × Ton / Lp (1)
It becomes. The energy E stored in the voltage conversion transformer 3 is
E = 1/2 × Lp × Ip ² (2)
And the power P is f, where f is the switching frequency.
P = 1/2 × Lp × Ip ² · f (3)
It becomes. If the output voltage on the secondary side is Vo and the output current is Io, the output power is Vo × Io.
Vo × Io = P = 1/2 × Lp × Ip ² × f (4)
It becomes. Assuming that the conduction time and the cutoff time of the transistor Q1 are equal,
Ton = 1 / (2f) (5)
It becomes. When f is obtained from these relationships, f = Vi ² / (8 × Lp × Vo × Io) (6)
Thus, when the input voltage Vi and the output voltage Vo are constant, it can be seen that the switching frequency f is inversely proportional to the output current Io. Therefore, when the output current Io is small and the load is light, the switching frequency f is high.

  The switching frequency f is detected by half-wave rectifying the voltage waveform generated in the auxiliary winding Nb by the half-wave rectifier circuit 4 and taking it into the frequency detection circuit 5. When a frequency exceeding a predetermined frequency (a frequency corresponding to a light load) is detected, a standby pulse having a pulse width of 50 ms or more is generated by the pulse generation circuit 6 and input to the switching control circuit 2. While the standby pulse is generated, the switching control circuit 2 keeps the gate of the switching transistor Q1 at a low voltage level and stops switching. When the standby pulse disappears, the switching power supply circuit returns to normal operation. However, if the light load continues at that time, the switching frequency f rises, so that the frequency f is detected again by the switching frequency detection circuit 5. The standby pulse is generated by the pulse generation circuit 6 and the switching is stopped.

  As a result, since the switching transistor Q1 intermittently performs a switching operation at a light load with a high switching frequency, loss of the transistor Q1 and the like can be suppressed and power consumption can be reduced. Further, since the frequency of the intermittent operation is 20 Hz or less because the standby pulse is 50 ms and is lower than the audible frequency band (20 Hz to 20 kHz), no harsh sound is generated.

  FIG. 2 is a functional block diagram showing a switching power supply circuit according to Embodiment 2 of the present invention. Here, with respect to the switching power supply circuit of FIG. 1, a switch S1 is inserted in series with the starting resistor R1, and the switch S1 is controlled to be turned off by a standby pulse generated by the pulse generating circuit 6.

  In the second embodiment, while the standby pulse is generated by the pulse generation circuit 6, the switching control circuit 2 controls the gate of the transistor Q1 to a low voltage level to stop the switching and suppress the switching loss. S1 is turned off and no current flows through the switching control circuit 2 via the starting resistor R1. Thereby, power consumption can be further reduced.

It is a functional block diagram of the switching power supply circuit of Example 1 of this invention. It is a functional block diagram of the switching power supply circuit of Example 2 of the present invention. It is a functional block diagram of the conventional switching power supply circuit. It is a functional block diagram of another conventional switching power supply circuit.

Explanation of symbols

  1: rectifier circuit, 2: switching control circuit, 3: voltage conversion transformer, 4: half-wave rectifier circuit, 5: switching frequency detection circuit, 6: pulse generation circuit, 7: load detection circuit, 8: pulse generation circuit.

Claims (3)

A first rectifying / smoothing circuit for rectifying and smoothing an input AC voltage; a switching element for switching an output voltage of the first rectifying / smoothing circuit to be input to a primary winding of the voltage conversion transformer; A second rectifying / smoothing circuit for rectifying and smoothing the output AC voltage obtained in the next winding; and a switching control circuit for controlling switching of the switching element in accordance with a voltage generated in the auxiliary winding of the voltage conversion transformer. In the provided switching power supply circuit,
A switching frequency detection circuit for detecting the frequency of the switching voltage generated in the auxiliary winding, and a standby pulse having a period longer than the frequency period of the audible frequency band when the switching frequency detected by the switching frequency detection circuit exceeds a predetermined value And a pulse generation circuit for controlling the switching control circuit,
The switching control circuit is characterized in that the switching element is controlled to be cut off during a period in which the standby pulse is input. The switching power supply circuit according to claim 1,
A switch for connecting a starting resistor for applying the output voltage of the first rectifying and smoothing circuit as a starting voltage to the gate of the switching element, and for opening the starting resistor when the pulse generating circuit generates the standby pulse. A switching power supply circuit provided. A voltage obtained by rectifying and smoothing the input AC voltage is switched by a switching element and input to the primary winding of the voltage conversion transformer, and the output AC voltage obtained at the secondary winding of the voltage conversion transistor is rectified and smoothed to generate a DC voltage. And a switching power supply control method for controlling switching of the switching element by a voltage generated in an auxiliary winding of the voltage conversion transistor,
When the frequency of the switching voltage generated in the auxiliary winding of the voltage conversion transformer exceeds a predetermined value, the switching element is intermittently cut off for a period longer than the frequency period of the audible frequency band. Control method of switching power supply circuit.

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