JP2007236112A - Switching power supply circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power circuit capable of avoiding shutdown due to insufficient charging time of a floating power supply voltage, using a simple circuit configuration. <P>SOLUTION: The difference between an input voltage from a DC power supply V2 and a switching output voltage of FET1 charges a voltage clamped by a Zener voltage of a Zener diode D5 into a capacitor C6 of a backup power circuit 4. The electricity charged into a capacitor C1 is consumed by the switching operation of FET1; and when the voltage of the capacitor C1 is lower than a predetermined voltage, a diode D3 is turned on, and power is fed from the capacitor C6 to a gate drive circuit 2. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a switching power supply circuit mainly used for audio equipment.

  2. Description of the Related Art Conventionally, a switching power supply circuit shown in FIG. 2 is known as a switching power supply circuit that drives a switching element using a bootstrap / floating power supply.

  The switching power supply circuit shown in FIG. 2 includes DC power supplies V1 and V2, FET1, gate drive circuit 2, capacitors C1 and C2, diodes D1 and D2, and an inductor L1.

  Here, the positive electrode of the DC power supply V1 is connected to the anode of the diode D2, and the cathode of the diode D2 is connected to the load 3 via the capacitor C1 and the inductor L1. The positive electrode of the DC power source V2 is connected to the drain of the FET 1 that is a switching element, and the source of the FET 1 is connected to the load 3 via the inductor L1. The capacitor C1 is connected between the cathode of the diode D2 and the source of the FET1. The gate drive circuit 2 is connected to the gate and source of the FET 1 and to both ends of the capacitor C1. The diode D1 has a cathode connected to the source of the FET1, an anode connected to the negative electrode of the DC power source V1 and the DC power source V2, and is connected to the ground. The negative electrode of the DC power supply V1 and the negative electrode of the DC power supply V2 are connected to the load 3. The capacitor C2 is connected to both ends of the load 3.

  In the switching power supply circuit configured as described above, electricity charged in the capacitor C1 functioning as a floating power supply serves as a power supply for the gate drive circuit 2, and the FET 3 is turned on and off by the gate drive circuit 2, whereby the load 3 A stable voltage lower than the voltage output from the DC power supply V2 is output.

FIG. 3 shows a switching voltage waveform at point A shown in FIG. When the switching of the FET 1 is off, the potential at the point A is a potential obtained by subtracting the voltage drop due to the diode D1 from the ground potential. During this time, the diode D1 is turned on by the flyback voltage of the inductor L1, a current I _off flows toward the load 3, and the capacitor C1 is charged from the DC power source V1 via the diode D2.

When switching FET1 is on, the potential at the point A, a potential obtained by subtracting the voltage drop due to the on resistance of the FET1 from the input voltage V _in from the DC power supply V2. During this time, the current I _on flows toward the load 3 by the DC power source V2. At this time, the capacitor C1 is not charged.

  In the switching power supply circuit that uses the bootstrap system and drives the FET 1 with a floating power supply as shown in FIG. 2, the electricity charged in the capacitor C1 becomes the power source, and the capacitor C1 is charged with the switching operation of the FET 1. Electricity is consumed.

  For this reason, if the time ratio of the charging time to the capacitor C1 (the time when the switching of the FET1 is OFF) becomes small, the amount of electricity consumed for switching on becomes larger than the charged electricity, so the voltage of the capacitor C1 becomes It gradually decreases. When the voltage drops below a certain voltage, the gate drive circuit 2 detects this and stops driving the FET 1. For this reason, the switching power supply circuit is shut down. Therefore, in order not to shut down the switching power supply circuit, it is necessary to ensure a predetermined charging time during the switching operation.

  When the load 3 is a heavy load, the switching power supply circuit operates as a time ratio at which the FET 1 is turned on in order to keep the output voltage constant in response to a decrease in the input voltage from the DC power supply V2. Work to make it bigger. However, in order to secure the time ratio of the charging time (the time when the switching of the FET 1 is off), it is necessary to limit the maximum value of the on-time ratio in the switching operation of the FET 1 to a predetermined value. There are also restrictions on the ratio of the maximum output voltage to the voltage.

  For example, the power supply voltage utilization rate is 80%, the input voltage necessary to obtain a predetermined output voltage is increased, and parts having higher withstand voltage are required, resulting in higher costs.

  On the other hand, when the load 3 is a light load or when power returns to the switching power supply circuit depending on the load, the operation is performed with an on-pulse width much smaller than the theoretical value, that is, the operation in the discontinuous mode, and sufficient switching is performed. Although there is an off time, the charging time to the capacitor C1 due to the flyback voltage of the inductor L1 is insufficient, and the switching power supply circuit is shut down as described above. In order to avoid this, in addition to setting the minimum on-pulse width to a certain value or more, it is necessary to add a load resistance to flow an output load current of a certain value or more. This result leads to wasteful power consumption, leading to heat generation and a reduction in power supply efficiency.

As a technique for avoiding shutdown due to insufficient charging time of the floating power supply voltage as described above, there is one disclosed in Patent Document 1. Patent Document 1 describes that charging by a charge pump operation is performed when the bootstrap capacitor is not sufficiently charged.
JP 2004-304527 A

  However, the technique disclosed in Patent Document 1 requires a charge pump drive circuit and the like, which causes a problem that the circuit configuration becomes complicated.

  The present invention has been made in view of the above, and an object of the present invention is to provide a switching power supply circuit that avoids shutdown due to shortage of charging time of the floating power supply voltage with a simple circuit configuration.

  The switching power supply circuit of the present invention includes a DC power supply, a switching element, a gate drive circuit that controls the switching element, and a floating power supply that operates the gate drive circuit, and a DC input voltage from the DC power supply is received. In a switching power supply circuit configured to obtain a stable output voltage by being turned on and off by the switching element, the switching element is charged by a difference between the DC input voltage and the switching output voltage of the switching element, and the gate driving circuit drives the switching element. A backup power supply circuit for supplying a voltage to the gate drive circuit when the voltage of the floating power supply is lower than a predetermined voltage higher than a voltage for stopping the operation.

  The backup power supply circuit according to the switching power supply circuit of the present invention includes a Zener diode connected between the positive electrode of the DC power supply and the output terminal of the switching element, and a voltage clamped by the Zener voltage of the Zener diode. And a capacitor to be charged.

  The switching power supply circuit of the present invention includes a backup power supply circuit that is charged by the difference between the DC input voltage and the switching output voltage of the switching element and supplies a voltage to the gate drive circuit when the floating power supply drops below a predetermined voltage. With a simple circuit configuration, shutdown due to insufficient charging time of the floating power supply voltage can be avoided.

  The best mode for carrying out the switching power supply circuit of the present invention will be described below with reference to the drawings.

  FIG. 1 is a circuit diagram of a switching power supply circuit according to an embodiment of the present invention. The same or equivalent parts as those of the conventional switching power supply circuit shown in FIG.

  The switching power supply circuit of the present embodiment shown in FIG. 1 is obtained by adding a backup power supply circuit 4 and a diode D3 to the conventional switching power supply circuit shown in FIG.

  The backup power supply circuit 4 includes capacitors C5 and C6, a diode D4, a Zener diode D5, a resistor R7, and a transistor Q1.

  Here, the anode of the diode D4 is connected to the positive electrode of the DC power supply V2, the cathode of the diode D4 is connected to the collector of the transistor Q1, and is connected to the base of the transistor Q1 and the cathode of the Zener diode D5 via the resistor R7. Has been. The cathode of the Zener diode D5 is connected to the base of the transistor Q1, and the anode of the Zener diode D5 is connected to the source of the FET1. The capacitor C5 is connected between the anode of the Zener diode D5 and the cathode of the Zener diode D5. The capacitor C6 is connected between the emitter of the transistor Q1 and the anode of the Zener diode D5, and the negative electrode of the capacitor C6 is connected to the source of the FET1.

  The anode of the diode D3 is connected to the emitter of the transistor Q1, and the cathode of the diode D3 is connected to the gate drive circuit 2.

  In the switching power supply circuit configured as described above, electricity charged in the capacitor C1 functioning as a floating power supply serves as a power supply for the gate drive circuit 2, and the FET 3 is turned on and off by the gate drive circuit 2, whereby the load 3 A stable voltage lower than the voltage output from the DC power supply V2 is output.

  When the switching of the FET 1 is off, the diode D1 is turned on by the flyback voltage of the inductor L1, the current flows toward the load 3, and the capacitor C1 is charged from the DC power source V1 via the diode D2. When the switching of the FET 1 is on, a current flows toward the load 3 by the DC power source V2. At this time, the capacitor C1 is not charged.

Here, with the on-off switching of the FET1, the potential of the negative electrode is connected point B of the capacitor C6, similarly to FIG. 3, between the input voltage V _in and Kurando potential from approximately the DC power supply V2 fluctuate. Then, the difference between the potential of the input voltage V _in and the point B from the DC power supply V2 (switching output voltage of the FET1), current flows through the diode D4 of the backup power supply circuit 4.

  In the backup power supply circuit 4, current flows through the noise absorbing capacitor C5 and the Zener diode D5 via the diode D4 and the resistor R7, and a Zener voltage is generated in the Zener diode D5. Further, a current also flows to the transistor Q1 and the capacitor C6 via the diode D4, and the capacitor C6 is charged with a voltage clamped by the Zener voltage of the Zener diode D5. Here, since the capacitor C6 functions as a backup power source, a capacitor having a sufficiently large capacitance is used.

  At the time of steady operation, the capacitor C1 (floating power supply) operates preferentially as the power supply for the gate drive circuit 2, but electricity charged in the capacitor C1 is consumed by the switching operation of the FET1, and the voltage of the capacitor C1 is predetermined. When the voltage drops below the voltage, the diode D3 is turned on and power is supplied from the capacitor C6 to the gate drive circuit 2. Here, the predetermined voltage for starting the power supply from the capacitor C6 is slightly higher than the voltage at which the gate drive circuit 2 stops driving the FET1.

  Thus, before the voltage across the capacitor C1 becomes lower than the voltage at which the gate driving circuit 2 stops driving the FET 1, the power is supplied to the gate driving circuit 2 by the capacitor C6, and the gate driving circuit 2 continues to operate. Continues the switching operation.

  That is, since the backup power supply circuit 4 works to back up the capacitor C1 (floating power supply), the switching power supply circuit can be prevented from shutting down even when the ON ratio becomes 95% or more, for example.

  In other words, even if the load fluctuation is large from light load to heavy load, a switching power supply circuit that sufficiently follows the fluctuation and does not shut down can be obtained. This is particularly effective as a power source for audio equipment that has a large difference between when sound is produced and when no sound is produced.

  In addition, the power supply voltage utilization rate is improved when the load is heavy, and a useless output load current can be reduced when the load is light, thereby improving the power supply efficiency.

  Furthermore, since the switching power supply circuit of the present embodiment is configured as described above, it can be realized with a simple circuit and at a low cost by using inexpensive general-purpose components.

  In the above description, the backup power supply circuit 4 is fed from the DC power supply V2. However, if the voltage across the capacitor C6 is insufficient with respect to the output voltage, it is higher than the DC power supply V2. By supplying the voltage V3 from a separate power source, the switching can be continued even if the switching-on ratio becomes 100%, that is, the off period disappears. Even if the switching is turned off, the power supply voltage of the gate drive circuit 2 is not lowered, and switching can be started at any time. That is, it can be switched in any state.

1 is a circuit diagram of a switching power supply circuit according to an embodiment of the present invention. It is a circuit diagram of the conventional switching power supply circuit. It is a figure which shows the switching voltage waveform of A point shown in FIG.

Explanation of symbols

1 FET
2 Gate drive circuit 3 Load 4 Backup power supply circuit
C1, C2, C5, C6 Capacitors D1, D2, D3, D4 Diode D5 Zener diode L1 Inductor Q1 Transistor R7 Resistors V1, V2 DC power supply

Claims (2)

A DC power supply, a switching element, a gate drive circuit that controls the switching element, and a floating power supply that operates the gate drive circuit. The DC input voltage from the DC power supply is turned on and off by the switching element to be stable. In a switching power supply circuit designed to obtain a high output voltage,
When the voltage of the floating power supply is lower than a predetermined voltage that is charged by the difference between the DC input voltage and the switching output voltage of the switching element, and the gate driving circuit stops driving the switching element, A switching power supply circuit comprising a backup power supply circuit for supplying a voltage to the gate drive circuit. The backup power supply circuit is
A Zener diode connected between the positive electrode of the DC power supply and the output terminal of the switching element;
The switching power supply circuit according to claim 1, further comprising: a capacitor charged with a voltage clamped by a Zener voltage of the Zener diode.

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